Gastrointestinal (Colorectal and Anal)

GASTROINTESTINAL CANCER—COLORECTAL AND ANAL

3500 Oral Abstract Session

Final overall survival for the phase III KN177 study: Pembrolizumab versus chemotherapy in microsatellite instability-high/mismatch repair deficient (MSI-H/dMMR) metastatic colorectal cancer (mCRC).

Thierry Andre, Kai-Keen Shiu, Tae Won Kim, Benny Vittrup Jensen, Lars Henrik Jensen, Cornelis J. A. Punt, Denis Michel Smith, Rocio Garcia-Carbonero, Julia Alcaide, Peter Gibbs, Christelle De La Fouchardiere, Fernando Rivera, Elena Elez, Johanna C. Bendell, Dung T. Le, Takayuki Yoshino, Wen Yan Zhong, David R. Fogelman, Patricia Marinello, Luis A. Diaz; Sorbonne Universite� and Ho^pital- Saint Antoine, Paris, France; University College Hospital, NHS Foundation Trust, London, United King- dom; Asan Medical Center, University of Ulsan College of Medicine, Seoul, South Korea; Herlev and Gentofte Hospital, Herlev, Denmark; University Hospital of Southern Denmark, Vejle, Denmark; Depart- ment of Medical Oncology, Cancer Center Amsterdam, Amsterdam UMC, Amsterdam, Netherlands; Bordeaux University Hospital, Bordeaux, France; Hospital Universitario 12 de Octubre, Imas12, CNIO, UCM, Madrid, Spain; Hospital Regional Universitario de Malaga, Ma�laga, Spain; Western Health, St. Al- bans, Australia; Centre Le�on Be�rard, Lyon, France; Hospital Universitario Marque�s de Valdecilla, IDIV- AL, Santander, Spain; Vall dHebron Institute of Oncology, Barcelona, Spain; Sarah Cannon Research Institute/Tennessee Oncology PLLC, Nashville, TN; The Sidney Kimmel Comprehensive Cancer Center at Johns Hopkins, Baltimore, MD; National Cancer Center Hospital East, Kashiwa, Japan; MSD China, Shanghai, China; Merck & Co., Inc., Kenilworth, NJ; Memorial Sloan Kettering Cancer Center, New York, NY

Background: In the phase III, randomized open-label KEYNOTE-177 (NCT02563002) study 1L pembrolizumab (pembro) versus chemotherapy (chemo) provided superior progression-free survival (PFS) at second interim analysis (IA2) in patients (pts) with MSI-H/dMMR mCRC. The study continued to final analysis of overall survival (OS), planned after 190 OS events or 12 months after IA2, whichever occurred first. We present results of the final analysis of OS, 12 months after IA2. Methods: A total of 307 pts with MSI-H/dMMR mCRC and ECOG PS 0 or 1 were randomized 1:1 to 1L pembro 200 mg Q3W for up to 2y or investigator’s choice of mFOLFOX6 or FOLFIRI Q2W ± bevacizumab or cetuximab. Treat- ment continued until PD, unacceptable toxicity, pt/investigator decision to withdraw, or completion of 35 cycles (pembro only). Pts receiving chemo could crossover to pembro for up to 35 cycles after confirmed PD. Primary end points were OS and PFS (RECIST v1.1, central review). Secondary end points included ORR, duration of response (DOR) (RECIST v1.1, central review), and safety. For OS significance, the p-value had to meet a prespecified a of 0.0246 (one-sided). Sensitivity analyses to adjust for crossover effect were performed. Data cut-off for final analysis was Feb 19, 2021. Results: Median (range) study follow-up was 44.5 mo (36.0-60.3) with pembro vs 44.4 mo (36.2-58.6) with chemo. 56 (36%) pts crossed over from chemo to pembro, with 37 more receiving anti-PD-1/PD-L1 therapies off study (60% effective crossover rate in the ITT). The HR for OS favored pembro vs chemo with a trend toward reduction in the risk of death (HR 0.74; 95% CI, 0.53-1.03; P=0.0359; median not reached [NR] vs 36.7 mo); this difference did not reach statistical significance. Sensitivity analysis by the rank- preserving structure failure time model and inverse probability of censoring weighting showed OS HRs of 0.66 (95% CI 0.42-1.04) and 0.77 (95% CI 0.44-1.38), respectively. Pembro vs chemo met the prespecified criteria for PFS superiority at IA2. At final analysis, median PFS was 16.5 mo vs 8.2 mo (HR 0.59; 95% CI, 0.45-0.79), but was not formally tested per analysis plan. Confirmed ORR was 45.1% (20 CR, 49 PR) vs 33.1% (6 CR, 45 PR). Median (range) DOR was NR (2.3+ to 53.5+) vs 10.6 mo (2.8 to 48.3+), respectively. Treatment-related adverse events (TRAEs) occurred in 79.7% vs 98.6% of pts; 21.6% vs 66.4%, respectively, had grade $3 TRAEs. Conclusions: As 1L therapy for pts with MSI-H/dMMR mCRC, pembro vs chemo provides statistically superior PFS with fewer TRAEs, and is associated with a trend toward reduced mortality that did not meet statistical significance likely due to the high crossover rate from chemo to anti-PD1/PD-L1 therapies. Together these data confirm pembro as a new standard-of-care in the 1L for pts with MSI-H/dMMR mCRC. Clinical trial information: NCT02563002. Research Sponsor: Merck & Co., Inc.

3501 Oral Abstract Session

The randomized phase II study of FOLFOXIRI plus cetuximab versus FOLFOXIRI plus bevacizumab as the first-line treatment in metastatic colorectal cancer with RAS wild-type tumors: The DEEPER trial (JACCRO CC-13).

Akihito Tsuji, Hisatsugu Ohori, Tatsuro Yamaguchi, Masato Matsuura, Atsujiro Nishioka, Akitaka Makiyama, Shingo Noura, Mitsugu Kochi, Tamotsu Sagawa, Masahito Kotaka, Yutaro Kubota, Yu Sunakawa, Takashi Sekikawa, Masato Nakamura, Masahiro Takeuchi, Wataru Ichikawa, Masashi Fujii; Department of Medical Oncology, Kagawa University Hospital, Takamatsu, Japan; Department of Medi- cal Oncology, Japanese Red Cross Ishinomaki Hospital, Ishinomaki, Japan; Department of Surgery, To- kyo Metropolitan Cancer and Infectious Diseases Center Komagome Hospital, Tokyo, Japan; Department of Surgery, Kobe City Nishi-Kobe Medical Center, Kobe, Japan; Division of Hematology, Re- spiratory Medicine and Oncology, Saga University Hospital, Saga, Japan; Department of Hematology/ Oncology, Japan Community Healthcare Organization Kyushu Hospital, Kitakyushu, Japan; Department of Surgery, Toyonaka Municipal Hospital, Toyonaka, Japan; Department of Digestive Surgery, Nihon University Itabashi Hospital, Tokyo, Japan; Division of Gastroenterology, National Hospital Organization Hokkaido Cancer Center, Sapporo, Japan; Gastrointestinal Cancer Center, Sano Hospital, Kobe, Japan; Division of Medical Oncology, Showa University Hospital, Tokyo, Japan; Department of Clinical Oncolo- gy, St. Marianna University School of Medicine, Kawasaki, CA, Japan; Division of Clinical Oncology, De- partment of Internal Medicine, Showa University Fujigaoka Hospital, Yokohama, Japan; Aizawa Comprehensive Cancer Center, Aizawa Hospital, Matsumoto, Japan; Department of Clinical Medicine (Biostatistics), School of Pharmacy, Kitasato University, Tokyo, Japan; Division of Medical Oncology, Showa University Fujigaoka Hospital, Yokohama, Japan; Department of Digestive Surgery, Nihon Uni- versity School of Medicine, Tokyo, Japan

Background: Triplet regimens, FOLFOXIRI, combined with bevacizumab (bev) or panitumumab have been shown to be superior in terms of early tumor shrinkage (ETS) and depth of response (DpR) compared to doublet combinations in patients with RAS wild-type metastatic colorectal cancer (mCRC), in the TRIBE trial (N Engl J Med 2014) or the VOLFI trial (J Clin Oncol 2019), respectively. There have been few studies which directly compared cetuximab (cet) with bev when combined with triplet regimen. Therefore, we investigated the efficacy and safety of bev vs. cet in combination with FOLFOXIRI in previously untreated mCRC patients with RAS wild-type tumors. Methods: This trial was a randomized phase II trial to evaluate modified (m)-FOLFOXIRI (irinotecan 150mg/m2, oxaliplatin 85mg/m2, 5-FU 2400mg/m2) plus cet vs. bev as first-line treatment in terms of DpR during the entire course as the primary endpoint in 360 patients with RAS wild-type mCRC. The aim of the trial was to show that median DpR of cet arm was more than 12.5% higher than bev arm, with a power of 85% at a significance level of 0.05. Secondary endpoints included ETS rate at week 8, overall response rate (ORR), progression- free survival (PFS), overall survival (OS), secondary resection rate, and toxicity. Results: A total of 359 patients were enrolled between July 2015 and June 2019. For the full analysis set (median age 65y, 64% male, PS0/1: 91%/9%, left/right primary: 83%/17%), 173 and 175 patients were randomly assigned to the cet and bev arms, respectively. On the cutoff date of September 2020, the median number of cycles administered was 10 (range, 1-51) for the cet arm and 12 (range, 1-51) for the bev arm. Safety data was already reported at the ASCO GI symposium 2021 (J Clin Oncol 39, 2021 suppl 3; abstr 86). The primary endpoint was met (p =.001); 57.4%（ -15.0~100） for the cet arm versus 46.0% （-0.6~100）for the bev arm. As for primary tumor sidedness, median DpR were 60.3% versus 46.1% (p =.0007) in the left-side and 50.0% versus 41.2% (p =.46) in the right-side. The ETS rate and ORR as the secondary endpoints were 77.8% and 69.1% in the cet arm versus 74.6% and 71.7% in the bev arm, respectively, with no statistical significance. Although the survival data were immature, PFS and OS of both arms were 12.7 months (95%CI 11.5-14.0) and 37.6 months (95%CI 30.8 to 43.0), respectively. Conclusion: The mFOLFOXIRI plus cet has been shown to be significantly superior to the mFOLFOXIRI plus bev in terms of DpR as the primary endpoint in first-line treatment for RAS wild-type mCRC. Clinical trial information: NCT02515734. Clinical trial information: UMIN000018217. Research Sponsor: Merck Biopharma Co., Ltd.

3502 Oral Abstract Session

Randomized study to investigate FOLFOXIRI plus either bevacizumab or cetuximab as first- line treatment of BRAF V600E-mutant mCRC: The phase-II FIRE-4.5 study (AIO KRK-0116).

Sebastian Stintzing, Kathrin Heinrich, David Tougeron, Dominik Paul Modest, Ingo Schwaner, Jan Euker, Rudolf Pihusch, Martina Stauch, Florian Kaiser, Christoph Kahl, Meinolf Karthaus, Christian Mueller, Christof Burkart, Anke C. Reinacher-Schick, Stefan Kasper, Ludwig Fischer von Weikersthal, Beate Krammer-Steiner, Gerald W. Prager, Julien Taieb, Volker Heinemann; Medical Department, Divi- sion of Hematology, Oncology, and Tumor Immunology (CCM), Charite� Universita€tsmedizin Berlin, Ber- lin, Germany; Department of Medicine III, University Hospital, LMU Munich, Mu€nchen, Germany, Munich, Germany; Department of Gastroenterology, Poitiers University Hospital, Poitiers, France; De- partment of Medicine III, University Hospital, LMU Munich, Munich, Germany; Onkologische Schwer- punktpraxis, Berlin, Germany; Charite�–Universitaetsmedizin, Berlin, Germany; MVZ Praxis Pihusch, Rosenheim, Germany; Onkologische Schwerpunktpraxis Kronach, Kronach, Germany; VK&K Studien- zentrum, Landshut, Germany; Department for Hematology, Klinikum Magdeburg, Magdeburg, Germa- ny; Hematology, Oncology, and Palliative Medicine, Klinikum Neuperlach and Harlaching, Munich, Germany; Kliniken Essen Mitte, Essen, Germany; Schwarzwald-Baar-Klinikum Villingen-Schwennin- gen, Kirchentellinsfurt, Germany; St. Josef-Hospital Ruhr-University, Bochum, Germany; University Hospital Essen, Essen, Germany; Department of Oncology, Gesundheitszentrum St. Marien GmbH, Am- berg, Germany; Department Internal Medicine III, Klinikum Suedstadt, Rostock, Germany; Medical University of Vienna, Vienna, Austria; Ho^pital Europe�en Georges Pompidou, Paris, France; University Hospital Munich, LMU Munich, Munich, Germany

Background: FIRE-4.5 (AIO KRK-0116) compared FOLFOXIRI plus either cetuximab or bevacizumab in BRAF V600E-mutant metastatic colorectal cancer (mCRC) patients not treated for metastatic disease before. Methods: Within this 1:2 randomized, controlled, open-label phase-II study, patients received FOLFOXIRI every two weeks at the following schedule: irinotecan 150mg/m2 (30-90min, day 1), folinic acid 400mg/m2 (120min, day 1), oxaliplatin 85mg/m2 (120 min, day 1), followed by 5-fluorouracil 3,000 mg/m2, 48h. FOLFOXIRI was combined with either bevacizumab (arm A) at a dose of 5mg/kg body weight, every 2 weeks or cetuximab (arm B) at a loading dose of 400mg/m2 and subsequent weekly doses of 250mg/m2. FOLFOXIRI was applied for a maximum of 12 cycles before maintenance treatment was recommended. Primary endpoint was superiority of Arm B with respect to overall response rate (ORR) according to RECIST 1.1 criterions. Secondary endpoints included PFS, OS, and tolerability. Results: From November 2016 to December 2020 108 patients were randomized in 90 German and 10 French centers (35 arm A and 73 in arm B). No new or unexpected toxicities were observed. Primary endpoint was not met with an ORR of 66.7% and 52.0% (p =0.23) in the respective arms. Median PFS was significantly longer in arm A vs arm B (8.3 months vs 5.9 months; logrank p = 0.03; HR 1.8). While OS data is still immature, median OS time are comparable at the time of analysis. Patients with left- sided primary tumors had comparable results with either bevacizumab or cetuximab, whereas those with right-sided primary tumors showed a trend towards better efficacy of the bevacizumab combination. Updated results will be presented at the annual meeting. Conclusions: FIRE-4.5 is the first prospective and randomized study investigating efficacy of FOLFOXIRI combined with targeted therapy in the first-line treatment of BRAF V600E-mutant mCRC. FOLFOXIRI plus either bevacizumab or cetuximab have comparable efficacy with differential effects according to primary tumor sidedness supporting the heterogeneity of BRAF V600E-mutant subpopulation of mCRC. Clinical trial information: NCT04034459. Research Sponsor: MERCK.

3503 Oral Abstract Session

Maintenance therapy with 5-fluoruracil/leucovorin (5FU/LV) plus panitumumab (pmab) or 5FU/LV alone in RAS wildtype (WT) metastatic colorectal cancer (mCRC) - the PANAMA trial (AIO KRK 0212).

Dominik Paul Modest, Meinolf Karthaus, Stefan Fru€hauf, Ullrich Graeven, Lothar Mu€ller, Alexander Koenig, Ludwig Fischer Von Weikersthal, Karel Caca, Albrecht Kretzschmar, Eray Goekkurt, Siegfried Haas, Annika Kurreck, Arndt Stahler, Volker Heinemann, Swantje Held, Armin Jarosch, David Horst, Stefan Kasper, Sebastian Stintzing, Tanja Trarbach; Department of Medicine III, University Hospital, LMU Munich, Munich, Germany; Hematology, Oncology, and Palliative Medicine, Klinikum Neuperlach and Harlaching, Munich, Germany; Klinik Dr. Hancken, Stade, Germany; Kliniken Maria Hilf GmbH, Klinik fu€r Ha€matologie, Onkologie und Gastroenterologie, Mo€nchengladbach, Germany; Onkolgie Un- terEms, Leer, Germany; Department of Gastroenterology, Gastrointestinal Oncology and Endocrinology, University Medical Center Go€ttingen, Go€ttingen, Germany; Gesundheitszentrum St Marien, Amberg, Germany; Klinikum Ludwigsburg, Ludwigsburg, Germany; Practice for hematology and oncology, MVZ Mitte, Leipzig, Germany; Hematology Oncology Practice Eppendorf, and University Cancer Center Ham- burg, Hamburg, Germany; Clinics for Haematology, Oncology and Nephrology, Friedrich-Ebert Hospital, Neumuenster, Germany; Charite� University Medicine Berlin, Berlin, Germany; Medical Department, Munich, Germany; University Hospital Munich, LMU Munich, Munich, Germany; Clinassess GmbH, Le- verkusen, Germany; Charite� Medizinische Universitaet Berlin, Institute for Pathology, Berlin, Germany; University Hospital Essen, Essen, Germany; Medical Department, Division of Hematology, Oncology, and Tumor Immunology (CCM), Charite� Universita€tsmedizin Berlin, Berlin, Germany; Praxis fu€r inter- disziplina€re Onkologie, Denzlingen, Germany

Background: Planned discontinuation or stop-and-go use of oxaliplatin are established strategies in the systemic therapy of mCRC. Consequently, and irrespective of antibody use, 5FU/LV represents the standard backbone of most maintenance strategies. Unlike VEGF-targeted substances, there is limited evidence that EGFR-antibodies add efficacy to 5FU/LV maintenance in RAS wildtype (RAS WT) mCRC patients. Methods: Following induction therapy with six cycles of 5FU/LV, oxaliplatin (FOLFOX) and pmab, the trial randomized maintenance therapy with 5FU/LV plus pmab vs. 5FU/LV alone in a 1:1 fashion in patients (pts) with RAS WT mCRC. The primary endpoint was PFS (progression-free survival: time from randomization until progression or death). With 218 events needed for PFS, the trial was designed to demonstrate superiority of the 5FU/LV+ pmab arm vs. 5FU/LV alone with a hazard ratio (HR) of 0.75, power of 80% and a significance level of 10%. Secondary endpoints included overall survival (OS), objective response to induction- and maintenance therapy as well as quality of life. The trial is registered with ClinicalTrials.gov, NCT01991873. Results: The full analysis set consists of 248 pts (125 pts 5FU/LV + pmab and 123 pts 5FU/LV) who were randomized and received maintenance therapy. Me- dian age was 66 vs. 65 years, male patients were 69.6% vs. 63.4%, ECOG 0 was 56.8% vs. 60.2% in the respective trial arms (5FU/LV+ pmab vs. 5FU/LV). At data cut-off, with 218 events, PFS of maintenance therapy was improved with 5FU/LV+ pmab vs. 5FU/LV alone (8.8 (80% CI 7.6-10.2) months vs. 5.7 (80% CI 5.6-6.0) months, HR 0.72 (80%CI 0.60-0.85), p = 0.014). OS (event rate 54.4%) nu- merically favoured the 5FU/LV+ pmab arm (28.7 (95% CI 25.4-39.1) months) as compared to 5FU/LV alone (25.7 (95% CI 22.2-28.2) months), HR 0.84 (95% CI 0.60-1.18). Conclusion: In RAS WT mCRC, maintenance therapy with 5FU/LV+ pmab appears to be superior to 5FU/LV alone and should be regarded as standard of care maintenance regimen following induction therapy with FOLFOX plus pmab. Clinical trial information: NCT01991873. Research Sponsor: AMGEN, Arbeitsgemeinschaft In- ternistische Onkologie (AIO).

3504 Oral Abstract Session

Oral maintenance capecitabine versus active monitoring for patients with metastatic colorectal cancer (mCRC) who are stable or responding after 16 weeks of first-line treatment: Results from the randomized FOCUS4-N trial.

Richard Adams, David Fisher, Janet Graham, Jenny F. Seligmann, Matthew Seymour, Richard S. Kaplan, Emma Yates, Susan D Richman, Philip Quirke, Rachel Butler, Ewan Brown, Stephen Falk, Fiona Jane Collinson, Richard H. Wilson, Louise C. Brown, Tim Maughan, FOCUS4 Investigators; Car- diff University and Velindre Cancer Centre, Cardiff, United Kingdom; University College London, Lon- don, United Kingdom; Beatson West of Scotland Cancer Centre, Glasgow, United Kingdom; University of Leeds, Leeds, United Kingdom; Gastrointestinal Cancer Research Unit, Cookridge Hospital, Leeds, United Kingdom; Medical Research Council Clinical Trials Unit at UCL, London, United Kingdom; MRC Clinical Trials Unit at UCL, London, United Kingdom; Leeds Institute of Medical Research at St James’s, University of Leeds, Leeds, United Kingdom; SW Genomics Laboratory Hub, Bristol, United Kingdom; Western General Hospital, Edinburgh, United Kingdom; Bristol Oncology Centre, Bristol, United Kingdom; St James University Hospital, Felliscliffe Harrogate, United Kingdom; Institute of Cancer Sciences, University of Glasgow, Glasgow G61 1BD, Glasgow, United Kingdom; Headington, Oxford, United Kingdom

Background: There is extensive randomised evidence supporting the use of treatment breaks in mCRC, but breaks from treatment are not universally offered to patients despite reductions in toxicity, without detriment to OS. Prior trials have shown that the combination of Cp and bevacizumab extend PFS but not OS. FOCUS4-N explores oral maintenance Cp monotherapy in patients with disease control on first line therapy. Methods: FOCUS4 was a molecularly stratified trial programme registering patients with newly diagnosed mCRC from 88 hospitals in the UK. Whilst undergoing 16 wks of first line treatment, a sample of tumour was sent for laboratory testing to stratify their disease into molecular subtypes: MSI, BRAF, PIK3CA, TP53 and RAS mutations. For some molecular groups, a targeted therapy subtrial was available but entry into the FOCUS4-N trial was offered to those in whom a targeted subtrial was un- available. Patients were randomised 1:1 between maintenance Cp therapy or AM. The primary outcome was PFS assessed using 8-wkly RECIST reported CT scans with quality of life (using EQ5D 8 weekly) and OS as secondary outcomes. Toxicity and tolerability were assessed 4-wkly. On progression, from the nadir, patients recommenced first line treatment. Cox regression was used to assess efficacy by intention-to-treat (ITT) with adjustment for tumour location, WHO status, metastatic burden, first line treatment and biomarker subtype. Results: Between March 2014 and March 2020, 254 patients were randomised (127 to Cp and 127 to AM). Baseline characteristics were balanced between groups but event rates were higher than anticipated in the AM group and the final analysis was triggered early as a result of the COVID-19 pandemic halting recruitment. The table presents results for PFS and OS. Com- pliance with treatment was good with per-protocol analysis results very similar to ITT (PFS HR=0.38 (95% CI 0.28-0.51)). Toxicity from Cp v AM was as expected with G$2 fatigue (25% v 12%), diarrhoea (23% v 13%) and hand-foot syndrome (26% v 3%). Quality of life showed no statistically significant differences between the two arms. Conclusions: Despite strong evidence of prolongation of PFS with maintenance therapy, OS remains unaffected and FOCUS4-N provides additional evidence to support the use of treatment breaks as a safe management alternative for patients who are stable or responding well to first line treatment for mCRC. Cp without bevacizumab may be used to extend PFS, in the interval after 16 weeks of combination therapy. Clinical trial information: ISRCTN#90061546. Research Sponsor: Cancer Research UK, Other Government Agency.

Cp AM Cox regression adjusted Median (IQR) Median (IQR) HR (95% CI) Outcome Events/N survival time (mths) Events/N survival time (mths) p-value

PFS 117/127 3.88 (2.10-7.39) 122/127 1.87 (1.64-3.65) 0.40 (0.21-0.75) <0.0001 OS 99/127 13.9 (9.07-20.4) 90/127 13.3 (7.72-20.0) 0.87 (0.64-1.18) 0.37

3505 Oral Abstract Session

Trastuzumab deruxtecan (T-DXd; DS-8201) in patients (pts) with HER2-expressing metastatic colorectal cancer (mCRC): Final results from a phase 2, multicenter, open-label study (DESTINY-CRC01).

Takayuki Yoshino, Maria Di Bartolomeo, Kanwal Pratap Singh Raghav, Toshiki Masuishi, Fotios Loupakis, Hisato Kawakami, Kensei Yamaguchi, Tomohiro Nishina, Zev A. Wainberg, Elena Elez, Javier Rodriguez, Marwan Fakih, Fortunato Ciardiello, Kapil Saxena, Kojiro Kobayashi, Emarjola Bako, Yasuyuki Okuda, Gerold Meinhardt, Axel Grothey, Salvatore Siena; National Cancer Center Hospital East, Kashiwa, Japan; Fondazione IRCCS Istituto Nazionale Tumori, Milan, Italy; The University of Tex- as MD Anderson Cancer Center, Houston, TX; Aichi Cancer Center Hospital, Aichi, Japan; Oncology In- stitute Veneto IOV-IRCCS, Padua, Italy; Kindai University Hospital, Osaka, MN, Japan; The Cancer Institute Hospital of JFCR, Tokyo, Japan; National Hospital Organization Shikoku Cancer Center, Ma- tsuyama, Japan; UCLA Medical Center, Los Angeles, CA; Vall d’Hebron Institute of Oncology (VHIO), Medical Oncology, Vall d’Hebron University Hospital (HUVH), Barcelona, Spain; Clinica Universidad de Navarra, Navarra, Spain; City of Hope Comprehensive Medical Center, Duarte, CA; Universita� Degli Stu- di Della Campania L. Vanvitelli, Caserta, Italy; Daiichi Sankyo, Basking Ridge, NJ; Daiichi Sankyo, To- kyo, Japan; West Cancer Center, OneOncology, Germantown, TN; Department of Oncology and Hemato- Oncology, Universita� degli Studi di Milano, and Niguarda Cancer Center, Grande Ospedale Metropolita- no Niguarda, Milan, Italy

Background: T-DXd is an antibody–drug conjugate of a humanized anti-HER2 antibody bound to a topoisomerase I inhibitor by a cleavable linker. The primary analysis of DESTINY-CRC01 (DS8201-A-J203; NCT03384940), a phase 2, open-label, multicenter study of T-DXd in pts with HER2-expressing mCRC showed promising antitumor activity and a manageable safety profile (cohort A median follow-up [FU], 27.1 weeks; Siena S, ASCO 2020). We present updated longer-term efficacy and safety data. Methods: Pts had centrally confirmed HER2-expressing, RAS wild-type mCRC that progressed after $2 prior regimens. 6.4 mg/kg of T-DXd was administered every 3 weeks (Q3W) in 3 cohorts (A: HER2 IHC3+ or IHC2+/ISH+; B: IHC2+/ISHÀ ; C: IHC1+). The primary end point was confirmed objective response rate (ORR) by independent central review in cohort A. Secondary end points were disease control rate (DCR; CR + PR + SD), duration of response (DOR), progression-free survival (PFS), and overall survival (OS). Results: At data cutoff (Dec 28, 2020), 86 pts (A, 53; B, 15; C, 18) received T-DXd. Median age was 58.5 y (range, 27-79), 53.5% were male, and 90.7% had left colon or rectum cancer. Median prior regimens for metastatic disease was 4 (range, 2-11). All pts had prior irinotecan; 30.2% in cohort A had prior anti-HER2 therapy. Median (m) treatment duration (all pts) was 3.0 mo (95% CI, 2.1-4.1; cohort A, 5.1 mo [95% CI, 3.9-7.6]). In cohort A (median FU, 62.4 weeks), confirmed ORR was 45.3% (24/53 pts; 95% CI, 31.6-59.6), DCR was 83.0% (44/53 pts; 95% CI, 70.2-91.9), mDOR was 7.0 mo (95% CI, 5.8-9.5), mPFS was 6.9 mo (95% CI, 4.1-8.7) with 37 (69.8%) PFS events, and mOS was 15.5 mo (95% CI, 8.8-20.8) with 36 (67.9%) OS events. These results are consistent with the primary analysis. Confirmed ORR was 43.8% (7/16 pts; 95% CI, 19.8-70.1) in pts with prior anti-HER2 therapy, 57.5% (23/40 pts; 95% CI, 40.9-73.0) in pts with IHC3+ status, and 7.7% (1/13 pts; 95% CI, 0.2-36.0) in pts with IHC2+/ISH+ status. In cohorts B and C, mPFS was 2.1 mo (95% CI, 1.4-4.1) and 1.4 mo (95% CI, 1.3-2.1); mOS was 7.3 mo (95% CI, 3.0-NE) and 7.7 mo (95% CI, 2.2-13.9), respectively. Treatment-emergent adverse events (TEAEs) of grade (G) $3 occurred in 65.1% of pts (56/ 86); the most common TEAEs were hematologic and gastrointestinal. TEAEs leading to drug discontinuation occurred in 13 pts (15.1%). 8 pts (9.3%) had interstitial lung disease (ILD) adjudicated by an independent committee as related to T-DXd (4 G2; 1 G3; 3 G5). Conclusions: T-DXd at 6.4 mg/kg Q3W showed promising activity and durability with longer-term FU in this pt population. The safety profile was consistent with prior results; ILD continues to be recognized as an important identified risk that requires careful monitoring and intervention as needed. These results support continued exploration of T- DXd in pts with HER2-overexpressing mCRC. Clinical trial information: NCT03384940. Research Sponsor: Daiichi Sankyo, Pharmaceutical/Biotech Company.

3506 Oral Abstract Session

Phase II study of anti-EGFR rechallenge therapy with panitumumab driven by circulating tumor DNA molecular selection in metastatic colorectal cancer: The CHRONOS trial.

Andrea Sartore-Bianchi, Filippo Pietrantonio, Sara Lonardi, Benedetta Mussolin, Francesco Rua, Elisabetta Fenocchio, Alessio Amatu, Salvatore Corallo, Chiara Manai, Federica Tosi, Paolo Manca, Francesca Daniel, Valter Torri, Angelo Vanzulli, Giovanni Cappello, Caterina Marchio�, Anna Sapino, Silvia Marsoni, Salvatore Siena, Alberto Bardelli; Niguarda Cancer Center, Grande Ospedale Metropoli- tano Niguarda, University of Milano, Milan, Italy; Medical Oncology Department, Fondazione IRCCS Is- tituto Nazionale dei Tumori, Milan, Italy; Veneto Institute of Oncology (IOV)-IRCCS, Padua, Italy; Istituto di Candiolo, Fondazione del Piemonte per l’Oncologia, IRCCS, Candiolo, Italy; Multidisciplinary Outpatient Oncology Clinic, Candiolo Cancer Institute, FPO-IRCCS, Candiolo, Italy; Niguarda Cancer Center, Grande Ospedale Metropolitano Niguarda, Milan, Italy; Department of Medical Oncology, Fon- dazione IRCCS Istituto Nazionale dei Tumori, Milan, Italy; Mario Negri Institute for Pharmacological Research-IRCCS, Milan, Italy; Istituto di Candiolo, Fondazione del Piemonte per l’Oncologia, IRCCS, Candiolo, Italy, Italy; IFOM, FIRC Institute of Molecular Oncology, Milan, Italy; Candiolo Cancer Insti- tute-IRCCS, University of Torino, Torino, Italy

Background: Despite advances in molecular segmentation of metastatic colorectal cancer (mCRC), beyond RAS status therapeutic actionability remains confined to the limited subgroups of ERBB2 amplified, BRAF mutated and MSI-H patients. Optimization of available treatments is therefore warranted. Rechallenge with anti-EGFR monoclonal antibodies is often empirically used with some benefit as late- line therapy. We previously found that mutant RAS and EGFR ectodomain clones, which emerge in blood during EGFR blockade, decline upon antibody withdrawal leading to regain drug sensitivity. Based on this rationale, we designed CHRONOS, a multicenter phase II trial of anti-EGFR therapy rechallenge guided by monitoring of the mutational status of RAS, BRAF and EGFR in circulating tumor DNA (ctDNA). To our knowledge, this is the first interventional clinical trial of liquid biopsy for driving anti-EGFR rechallenge therapy in mCRC. Methods: Eligible patients were PS ECOG 0-2 RAS/BRAF WT mCRC having first achieved an objective response and then progression in any treatment line with an anti-EGFR antibody containing regimen, displaying RAS, BRAF and EGFR ectodomain WT status in ctDNA at molecular screening after progression to the last anti-EGFR-free regimen. Clonal evolution in ctDNA was analyzed by ddPCR and next generation sequencing. Panitumumab 6 mg/kg was administered IV every two weeks until progression. The primary endpoint was objective response rate (ORR) by RECIST version 1.1 with independent central review. 27 total patients and 6 responses were required to declare the study positive (power = 85%, type I error = 0.05). Results: Between Aug 19, 2019 and Nov 6, 2020 52 patients were screened by liquid biopsy and 36 (69%) were negative in ctDNA for RAS/BRAF/EGFR mutations. Of these, 27 patients were enrolled in 4 centers. Median age was 64 years (range: 42-80). PS ECOG was 0/50%, 1/46%, 2/4%. Previous anti-EGFR was administered in 1st line in 63%, 2nd in 15% and > 2nd in 22%. Median number of previous treatments was 3. The primary endpoint was met, with 8/27 partial responses (PR) observed (2 unconfirmed) (ORR = 30%, 95% CI: 12- 47%). Stable disease (SD) was obtained in 11/27 (40%, 95% CI: 24-59%), lasting > 4 months in 8/ 11. Disease control rate (PR plus SD > 4 months) was therefore obtained in 16/27 (59%, 95% CI: 41- 78%). Median progression-free survival was 16 weeks. Median duration of response was 17 weeks (1 ongoing). Maximal grade toxicity was G3, limited to dermatological and occurring in 19% of patients. ctDNA dynamics were studied in all patients. Conclusions: Liquid biopsy-driven rechallenge with anti- EGFR antibodies leads to further objective responses in one third of patients. Genotyping tumor DNA in the blood to direct therapy can be effectively incorporated in the management of advanced CRCs. Clinical trial information: 2016-002597-12. Research Sponsor: Associazione Italiana per la Ricerca sul Cancro (AIRC), Other Foundation, Amgen provided panitumumab.

3507 Oral Abstract Session

The TRUSTY study: A randomized phase 2/3 study of trifluridine/tipiracil plus bevacizumab versus irinotecan and fluoropyrimidine plus bevacizumab as second-line treatment in patients with metastatic colorectal cancer.

Yasutoshi Kuboki, Tetsuji Terazawa, Toshiki Masuishi, Masato Nakamura, Jun Watanabe, Hitoshi Ojima, Yudai Shinohara, Masahito Kotaka, Hiroki Hara, Takashi Ohta, Eiji Oki, Yu Sunakawa, Soichiro Ishihara, Hiroya Taniguchi, Takako Eguchi Nakajima, Satoshi Morita, Kuniaki Shirao, Takayuki Yoshino, TRUSTY Study Group; Department of Experimental Therapeutics, National Cancer Center Hospital East, Kashiwa, Chiba, Japan; Cancer Chemotherapy Center, Osaka Medical Collage, Osaka, Ja- pan; Aichi Cancer Center Hospital, Aichi, Japan; Aizawa Comprehensive Cancer Center, Aizawa Hospi- tal, Matsumoto, Japan; Department of Surgery, Gastroenterological Center, Yokohama City University Medical Center, Yokohama, Japan; Gastrointestinal Surgery, Gunma Prefectural Cancer Center, Gunma, Japan; Department of Hematology/Oncology, Japan Community Healthcare Organization Kyushu Hospi- tal, Fukuoka, Japan; Gastrointestinal Cancer Center, Sano Hospital, Kobe, Japan; Department of Gas- troenterology, Saitama Cancer Center, Saitama, Japan; Department of Clinical Oncology, Kansai Rosai Hospital, Amagasaki, Japan; Department of Surgery and Science, Graduate School of Medical Scien- ces, Kyushu University, Fukuoka, Japan; Department of Clinical Oncology, St. Marianna University School of Medicine, Kawasaki, CA, Japan; Department of Surgical Oncology, The University of Tokyo, Tokyo, Japan; Department of Gastroenterology and Gastrointestinal Oncology, National Cancer Center Hospital East, Kashiwa, Japan; Kyoto Innovation Center for Next Generation Clinical Trials and iPS Cell Therapy, Kyoto University Hospital, Kyoto, Japan; Department of Biomedical Statistics and Bioinfor- matics, Graduate School of Medicine, Kyoto University, Kyoto, Japan; Oita University Faculty of Medi- cine, Oita, Japan; National Cancer Center Hospital East, Kashiwa, Japan

Background: The efficacy of trifluridine/tipiracil (FTD/TPI) plus bevacizumab (BEV) as a later-line treatment for metastatic colorectal cancer (mCRC) has been demonstrated in clinical trials. Therefore, we conducted a randomized phase 2/3 study to determine whether FTD/TPI plus BEV is non-inferior to either FOLFIRI or S-1 and irinotecan plus BEV in terms of overall survival (OS) as second-line treatment in patients with mCRC. Methods: Patients with histologically confirmed mCRC who failed first-line doublet chemotherapy including fluoropyrimidine plus oxaliplatin with either BEV or an anti-EGFR antibody (in cases of RAS wild-type) were eligible. Patients were randomized to receive either FTD/TPI plus BEV (experimental group, BEV 5.0 mg/kg on days 1 and 15, FTD/TPI 35 mg/m2 twice daily on days 1–5 and 8–12 of each 28-day cycle) or either FOLFIRI or S-1 and irinotecan plus BEV (control group). The primary endpoint was the OS. The non-inferiority margin of a hazard ratio (HR) of 1.33 was based on the assumption of a median survival time of 19 months for the control (power 0.80, 1-sided alpha 0.025). The secondary endpoints were the progression-free survival (PFS), response rate (RR), disease control rate (DCR), time to treatment failure, time to post-study treatment failure, proportion of patients receiving post-study treatment, quality of life, and safety. Results: As a result of the interim analysis for futility, the study was terminated in July 2020, and 397 patients were finally enrolled at 65 institutions from October 2017. The baseline characteristics were similar between the groups. The median OS were 14.8 months in the FTD/TPI plus BEV group and 18.1 months in the control group [HR: 1.38; 95% confidence interval (CI): 0.99–1.93; p = 0.5920 for non-inferiority]; non-inferiority of FTD/TPI plus BEV was not demonstrated. The median PFS were 4.5 months in the FTD/TPI plus BEV group and 6.0 months in the control group (HR: 1.45; 95% CI: 1.14–1.84). The RR and DCR were 3.8% and 61.2% in the FTD/TPI plus BEV group, respectively, and 7.1% and 71.7% in the control group, respectively. The proportions of patients receiving post-study treatment in the FTD/TPI plus BEV and control groups were 59.9% and 52.3%, respectively. The main grade 3 or 4 adverse events in the FTD/TPI plus BEV and control groups were neutropenia (65.8% and 41.6%, respectively), diarrhea (1.5% and 7.1%, respectively), and grade 1 or 2 alopecia (3.6% and 24.9%, respectively). Conclusions: FTD/TPI plus BEV did not show non-inferiority to FOLFIRI or S-1 and irinotecan plus BEV as second-line treatment in patients with mCRC. Post hoc subgroup analyses are ongoing to investigate patients who likely benefit from FTD/TPI plus BEV. Clinical trial information: jRCTs031180122. Research Sponsor: Taiho Phar- maceutical Co, Ltd.

3508 Poster Discussion Session

CCTG CO.28 primary endpoint analysis: Neoadjuvant chemotherapy, excision and observation for early rectal cancer, the NEO trial.

Hagen Fritz Kennecke, Carl J Brown, Jonathan M. Loree, Husein Moloo, Derek J. Jonker, Manoj Raval, Reilly Musselman, Grace Ma, Antonio Caycedo-Marulanda, Val Simianu, Sunil Patel, Lacey D. Pitre, Ramzi Helewa, Vallerie Lynn Gordon, Katerina Neumann, Max Sherry, Dongsheng Tu, Christopher J. O’Callaghan; Providence Cancer Institute, Portland, OR; Providence Health-St. Paul’s Hospital, Van- couver, BC, Canada; BC Cancer, Vancouver, BC, Canada; The Ottawa Hospital, Ottawa, ON, Canada; Ot- tawa Hospital Research Institute, Ottawa, ON, Canada; Providence-St.Paul’s Hospital, Vancouver, BC, Canada; University of Ottawa, Ottawa, ON, Canada; Sudbury Health Services Network, Sudbury, ON, Canada; HSN, Sudbury, ON, Canada; Virginia Mason Medical Center, Seattle, WA; Queen’s University, Kingston, ON, Canada; Jurevinski Cancer Center, Hamilton, ON, Canada; University of Manitoba, Win- nipeg, MB, Canada; Cancer Care Manitoba, Winnipeg, MB, Canada; Nova Scotia Health, Halifax, NS, Canada; Queens University, Kingston, ON, Canada; Queen’s University, Canadian Cancer Trials Group, Kingston, ON, Canada

Background: CO.28 (NCT03259035) is a phase II study designed to determine if patients with cT1- T3a/bN0 rectal cancer can be treated with induction chemotherapy (FOLFOX/CAPOX) and organ-preserving surgery. Methods: Patients with MRI staged cT1-3a/bN0 tumors and no pathologic (p) high risk features received 6/4 cycles of FOLFOX/CAPOX, repeat sigmoidoscopy/pelvic MRI and subsequent Transanal Endoscopic Surgery (TES) in the absence of tumor progression. ypT0/T1N0 tumors were treated with observation while ypT2+ or ypN+ stage were recommended Total Mesorectal Excision (TME). The primary endpoint was protocol specified Organ Preservation Rate (psOPR = ypT0/T1N0, no p high risk features) and actual Organ Preservation Rate (aOPR = ypT0/T1N0 stage plus higher yp stage patients who declined TME surgery). The study would be considered negative with an psOPR of 50% or lower (H0) and as promising if it is 65% or higher (H1). Results: Between 08/2017 to 05/2020, 58 eligible patients were accrued in Canada and the United States, median age was 67 years, 71% male. All had well-moderately differentiated, non-mucinous rectal adenocarcinoma and median tumor height was 6 cm (range 0-18). Median follow-up was 15.4 months. Chemotherapy with FOLFOX (32) or CA- POX (26) was administered, 90% completed all planned cycles. A total of 56/58 (97%) proceeded to TES, while one patient was ineligible due to tumor progression (1.7%) and one declined. In the intention to treat analysis, the psOPR was 57% (95% CI 43-70%) while the aOPR was 79% (95% CI 67% to 89%) due to 13/23 declining recommended TME surgery. Of 10 patients who proceeded to recommended TME, a complete R0 TME was performed in 9/10, and no p residual carcinoma was found in 7/ 10. Crude loco-regional (LR) and distant recurrence rates were 3.5% (95% CI 0.4 to 12%) and 0%, respectively. A recurrence occurred in 1/13 patients who initially declined TME surgery. Conclusions: In select patients with early stage rectal cancer, three months of induction CAPOX/FOLFOX followed by TES resulted in a high OPR without the use of pelvic irradiation. The observed high rate of pathologic downstaging may point to high chemo-responsiveness in early rectal adenocarcinoma with no p high risk features. Further trials to evaluate this approach are justified and updated results will be presented. Clinical trial information: NCT03259035. Research Sponsor: CCTG (Canadian Cancer Trials Group), Other Foundation.

Eligible patients N = 58 100%

Gender Female/Male 17/41 29/71% Race White/Indigenous/Asian 48/6/4 83/10/7% Age Median 67 y (Range 31-83) cT T1/T2/T3a/b 8/37/13 14/64/22% cN N0 58 100% Transanal Endoscopic TEMS/TAMIS/Excision 26/27/3 45/47/5% Surgery (TES) TES Margin status R0/R1/R2 52/2/2 93/3.5/3.5% TES ypT T0/T1/T2/T3 20/14/20/2 36/25/36/3% TES ypN NX/N0 56 100%

3509 Poster Discussion Session

Survival and organ preservation according to clinical response after total neoadjuvant therapy in locally advanced rectal cancer patients: A secondary analysis from the organ preservation in rectal adenocarcinoma (OPRA) trial.

Hannah Thompson, Jin Ki Kim, Jonathan B. Yuval, Floris Verheij, Sujata Patil, Marc J Gollub, Abraham Jing-Ching Wu, Meghan Lee, Aram F Hezel, Jorge Marcet, Peter Cataldo, Blase N. Polite, Daniel Herzig, David Liska, Samuel Oommen, Charles Friel, Charles A. Ternent, Andrew L. Coveler, Steven R. Hunt, Julio Garcia-Aguilar; Colorectal Service, Department of Surgery, Memorial Sloan-Kettering Cancer Cen- ter, New York, NY; Department of Biostatistics, Memorial Sloan Kettering Cancer Center, New York, NY; NRG Oncology and Memorial Sloan Kettering Cancer Center, New York, NY; Memorial Sloan Kettering Cancer Center, New York, NY; Colorectal Service, Department of Surgery, Memorial Sloan Kettering Cancer Center, New York, NY; James P. Wilmot Cancer Institute, University of Rochester, Rochester, NY; Tampa General Hospital, Tampa, FL; University of Vermont, Burlington, VT; University of Chicago Medical Center, Chicago, IL; Oregon Health and Science University, Portland, OR; Cleveland Clinic, Cleveland, OH; John Muir Health, Walnut Creek, CA; University of Virginia, Charlottesville, VA; Colon & Rectal Surgery Inc., Omaha, NE; Seattle Cancer Care Alliance/University of Washington, Seattle, WA; Washington University School of Medicine in St. Louis, St. Louis, MO

Background: Clinical response following neoadjuvant therapy is paramount to identifying locally advanced rectal cancer (LARC) patients suitable for Watch and Wait (WW). A 3-tier schema was devised to stratify clinical response. Patients with a complete clinical response (cCR) are considered for WW, while those with an incomplete clinical response (iCR) are recommended for total mesorectal excision (TME). A near complete response (nCR) tier captures patients with significant, but not complete, response to be considered for WW. This schema’s efficacy has yet to be validated. We investigated survival and organ preservation (OP) rates based on this 3-tier clinical response assessment in patients with LARC who underwent total neoadjuvant therapy (TNT) in a prospective, multi-center clinical trial. Methods: Pa- tients with MRI stage II and III rectal adenocarcinoma were randomized to either induction chemotherapy (FOLFOX or CAPEOX) followed by chemoradiation or chemoradiation followed by consolidation chemotherapy (FOLFOX or CAPEOX). At 8+/-4 weeks following TNT, response on digital rectal and endoscopic examinations was evaluated by the 3-tier schema. The date of this restaging clinical response assessment was used as time zero. The endpoints of rate of OP, disease-free survival (DFS), TME-free DFS, and overall survival (OS) were evaluated using the Kaplan-Meier method with differences analyzed by the log-rank test. Results: Clinical response assessments were available for 294 patients. The median time to assessment after neoadjuvant therapy was 7.9 weeks. Based on the 3-tier schema, 124 patients were categorized as cCR, 113 as nCR, and 57 as iCR. Baseline age, sex, average distance from the anal verge, clinical T classification, and clinical N classification were similar between the response groups. The table shows the 3-year rates of OP, DFS, TME-free DFS, and OS. The median follow-up was 2.36 years. Of the patients with a nCR, the 3-year TME rate was 48% compared with 21% in the cCR group. Conclusions: The 3-tier clinical response assessment has prognostic implications for OP and DFS in patients with LARC who underwent TNT. In patients with a nCR, more than half achieved OP at 3 years. This information should be utilized to counsel patients regarding their expected outcomes. Clinical trial information: NCT02008656. Research Sponsor: U.S. National Institutes of Health.

3-year rates by 3-tier clinical response assessment. cCR (n=124) nCR (=113) iCR (n=57) Estimate 95% CI Estimate 95% CI Estimate 95% CI p-value

Organ Preservation 79% 72-87% 52% 42- 9% 4-21% <0.0001 63% Disease-Free Survival 84% 77-92% 76% 67- 52% 37-72% <0.0001 86% TME-Free Disease-Free 72% 64-81% 44% 35- 4% 1-14% <0.0001 Survival 55% Overall Survival 97% 93- 93% 87- 90% 79- 0.11 100% 99% 100% CI=Confidence Interval.

3510 Poster Discussion Session

A multicenter, randomized, phase III trial of short-term radiotherapy plus chemotherapy versus long-term chemoradiotherapy in locally advanced rectal cancer (STELLAR): The final reports.

Jing Jin, Yuan Tang, Chen Hu, Yong Cai, Yuan Zhu, Guanghui Cheng, Hongyan Zhang, Xin Wang, Suyu Zhu, Jun Wang, Gaofeng Li, Jialin Yang, Kuan Zhang, Yihebali Chi, Haitao Zhou, Lichun Wei, Wenling Wang, Shixin Liu, Yuanhong Gao, Ye-Xiong Li; Department of Radiation Oncology, National Cancer Cen- ter/National Clinical Research Center for Cancer/Cancer Hospital, Chinese Academy of Medical Scien- ces and Peking Union Medical College, Beijing, China; NRG Oncology Statistics and Data Management Center, Philadelphia, PA; Key Laboratory of Carcinogenesis and Translational Research (Ministry of Ed- ucation/Beijing), Department of Radiation Oncology, Peking University Cancer Hospital and Institute, Beijing, China; Department of Radiation Oncology, Zhejiang Provincial Cancer Hospital, Hangzhou, China; Department of Radiation Oncology, China-Japan Union Hospital, Jilin University, Changchun, China; Department of Radiation Oncology, Anhui Provincial Hospital, The First Affiliated Hospital of University of Science and Technology of China, Hefei, China; Department of Radiation Oncology, West China Hospital, Sichuan University, Chengdu, China; Department of Radiation Oncology, Hunan Cancer Hospital and Affiliated Cancer Hospital of Xiangya School of Medicine, Changsha, China; Department of Radiation Oncology, Hebei Provincial Cancer Hospital, Shijiazhuang, China; Department of Radiation Oncology Beijing Hospital, Beijing, China; Department of Radiation Oncology, Sichuan Provincial Can- cer Hospital, Chengdu, China; Department of Radiation Oncology, Qinghai Red Cross Hospital, Qinghai, China; National Cancer Center/National Clinical Research Center for Cancer/Cancer Hospital, Chinese Academy of Medical Sciences and Peking Union Medical College, Beijing, China; State Key Laboratory of Molecular Oncology and Department of Colorectal Surgery, National Cancer Center/National Clinical Research Center for Cancer/Cancer Hospital, Chinese Academy of Medical Sciences and Peking Union Medical College, Beijing, China; Department of Radiation Oncology, Guizhou Provincial Cancer Hospi- tal, Guiyang, China; Department of Radiation Oncology, Jilin Provincial Cancer Hospital, Changchun, China; Department of Radiation Oncology, Sun Yat-sen University Cancer Center, State Key Laboratory of Oncology in South China, Collaborative Innovation Center of Cancer Medicine, Guangzhou, China

Background: It’s presented the results of a phase III trial of short-term radiotherapy (SCRT) combined with chemotherapy versus long-term chemoradiotherapy (LCRT) in patients with locally advanced rectal cancer (LARC). Methods: Patients with distal or middle third, T3-T4 and/or N+ rectal adenocarcinomas diagnosed by MRI, were randomly assigned to experimental group or control group. In experimental group, patients received SCRT (25 Gy / 5 fractions / 5 days), followed by four courses of CAPOX. In control group, patients received LCRT (50 Gy / 25 fractions / 35 days with concurrent capecitabine). Surgery was recommended in both groups and performed 6-8 weeks after preoperative treatment. Two or six courses of CAPOX was pre- scribed as the postoperative chemotherapy in experimental group and control group, respectively. This trial was a multicenter, open-label, randomized, noninferior, phase III study, and all the patients were from 16 hospitals of China. The primary endpoint for this study was 3-year disease-free survival (DFS). Results: From Aug 30, 2015 to Aug 27, 2018, 599 patients were enrolled and entered random. Finally, 591 intention-to- treat (ITT) populations were included in the analysis, 298 patients assigned to SCRT followed by chemotherapy and 293 to CRT. For the experimental group and control group, cT3 and cT4 accounted for 82.3% vs. 84.6% and 15.4% vs. 12.3%, respectively, and approximately 85% were mrN positive (85.6% vs. 84.0%). As a whole, the completion and full-dose completion rates of preoperative treatment were 82.6% vs. 95.2% (p＜0.001) and 74.8% vs 93.2% (p＜0.001) in the experimental and control groups, respectively. Among the 465 patients who received surgery, 16.6% and 11.8% of them achieved pCR (p=0.134), respectively. Accounting for cCR after preoperative treatment, the total rate of pCR+cCR in experimental group was 22.5% and significantly higher than control group (12.6%, p=0.001). With median follow-up 35.0 months, the HR between experimental and control of DFS was 0.883, with 1-sided noninferiority p-value <0.001, so the noninferiority hypothesis was confirmed. The probability of DFS and OS at 3 years was 64.5% and 86.5% in the experimental group compared with 62.3% and 75.1% in control group. It’s observed the OS rate of the experimental group was significantly higher than that of the control group (p=0.036) and no significant difference in metastasis-free survival or loco-regional recurrence was observed. Conclusions: For LARC with high risk factors, SCRT combined with sequential chemotherapy was noninferior to CRT and could be used as an alternative to LCRT. Meanwhile, SCRT combined with chemotherapy presented a higher cCR+pCR and 3-year overall survival rates as compared with CRT. However, the long term results need to be further followed up. (ClinicalTrails No.: NCT02533271). Clinical trial information: NCT00833131. Re- search Sponsor: Collaborative Innovation Center for Cancer Medicine.

3511 Poster Discussion Session

Phase II study of preoperative (PREOP) chemoradiotherapy (CTRT) plus avelumab (AVE) in patients (PTS) with locally advanced rectal cancer (LARC): The AVANA study.

Lisa Salvatore, Maria Bensi, Salvatore Corallo, Francesca Bergamo, Ilaria Pellegrini, Cosimo Rasola, Beatrice Borelli, Emiliano Tamburini, Giovanni Randon, Sara Galuppo, Alessandra Boccaccino, Massimo Giuseppe Viola, Alessandra Auriemma, Elena Fea, Cecilia Barbara, Sara Bustreo, Valeria Smiroldo, Brunella Barbaro, Maria Antonietta Gambacorta, Giampaolo Tortora; Oncologia Medica, Com- prehensive Cancer Center, Fondazione Policlinico Universitario Agostino Gemelli–IRCCS, Universita� Cattolica del Sacro Cuore, Rome, Italy; Dipartimento di Oncologia Medica Fondazione IRCCS Istituto Nazionale dei Tumori, Milano, Italy; Department of Oncology, Oncology 1, Veneto Institute of Oncology IOV-IRCCS, Padua, Italy; Unit of Surgery, Oncology and Gastroenterology, University of Padua, Padua, Italy; Department of Translational Research and New Technologies in Medicine and Surgery, Unit of Medical Oncology 2, Azienda Ospedaliera Universitaria Pisana, Pisa, Italy; Oncology Department, Tri- case City Hospital, Tricase, Italy; Dipartimento di Oncologia Medica Fondazione IRCCS Istituto Nazio- nale dei Tumori, Milan, Italy; Radiation Oncology Dept-IOV-IRCCS Padova, Padua, Italy; Chirurgia, Pia Fondazione Panico, Tricase, Italy; Azienda Ospedaliera Universitaria Integrata Verona, Verona, Italy; S.Croce & Carle Teaching Hospital, Cuneo, Italy; Department of Oncology; Division of Medical Oncolo- gy, Livorno Hospital, Azienda USL Toscana Nord Ovest, Livorno, Italy; SSD ColoRectal Cancer Unit Di- partimento di Oncologia AOU Citta� della Salute e della Scienza di Torino, Turin, Italy; Medical Oncology Unit IRCCS Humanitas Research Hospital, Rozzano, Milan, Italy; Department of Bioimaging and Radiological Sciences, Fondazione Policlinico Universitario A. Gemelli IRCCS; Universita� Cattolica del Sacro Cuore, Rome, Italy; Diagnostic Imaging, Oncological Radiotherapy and Hematology, Fonda- zione Policlinico Universitario Agostino Gemelli IRCCS; Universita� Cattolica del Sacro Cuore, Rome, Italy

Background: Preop CTRT is considered the standard of care in the management of LARC. RT can induce antigen release from a low neoantigen-burden tumor (such as a mismatch repair proficient colorectal cancer) and activate dendritic cells leading to a CD8+ T lymphocyte-mediated anticancer immune response. In LARC patients, neoadjuvant CTRT increases PD-L1 expression in tumor cells, strongly suggesting a neoadjuvant combinatory strategy with RT and PD-1/PD-L1 pathway blockade. Based on such considerations, we have designed the AVANA study to investigate the role of Ave in combination with preop CTRT in LARC. Methods: This is an Italian multi-center, phase II study. Pts with resectable LARC, defined by the presence of at least one of the following features: cN+, cT4, high risk cT3, received standard preop CTRT (capecitabine 825 mg/sqm/bid 5 days/week+ 50.4 Gy in 28 fractions over 5.5 weeks) plus 6 cycles of Ave 10 mg/Kg every 2 weeks. Surgery with total mesorectal excision was performed at 8-10 weeks after the end of CTRT. The primary end-point was the pCR rate, defined as complete histological regression with no available tumor cells ypT0N0. Secondary end-points were R0 resection rate, tumor downstaging, local recurrence, sphincter preservation rate, progression-free survival, overall survival, safety profile, and the evaluation of exploratory predictive and/or prognostic biomarkers. Assuming as null hypothesis p0 a pCR rate of 15%, a significance level of 5% (one-side), and a power of 80%, a sample size of 101 pts was needed to detect an absolute increment of 10% in pCR rate (from 15% to 25%). The experimental regimen is considered for further studies if, in at least 22 pts, we observe a pCR. Results: From April 2019 to November 2020, a total of 101 resectable LARC pts were enrolled in 10 Italian Centers. The median age was 63 years (23-82), 62 (61.4%) pts were male, 93 (92%) had ECOG PS 0. At baseline, 94 (93%) and 16 (16%) pts had cN+ and cT4 LARC, respectively. All pts completed the induction phase. Out of 96 pts evaluable for pathological response, 22 (23%) pts achieved a pCR and 59 (61.5%) pts a major pathological response (a central review is ongoing). At this time, microsatellite status is available only in 39 pts, of which only one was instable. The rate of grade 3-4 non- immune and immune-related adverse events was 8% and 4%, respectively. Avelumab was early inter- rupted in 9 pts out 101, mainly due to toxicity. Conclusions: The combination of preop CTRT plus Ave showed a promising activity and a feasible safety profile. According to our statistical considerations, the experimental regimen will be considered for further studies. Updated results will be presented during the Congress. Sponsored by GONO and partially supported by Merck. EUDRACT 2017-003582-10. Clinical trial information: NCT03854799. Research Sponsor: Gruppo Oncologico del Nord Ovest (GONO Group), Pharmaceutical/Biotech Company.

3512 Poster Discussion Session

Phase II trial of neoadjuvant mFOLFOX 6 with panitumumab (P) in T3 rectal cancer with clear mesorectal fascia (MRF) and KRAS, NRAS, BRAF, PI3KCA wild type (4WT). GEMCAD 1601 PIER trial.

Carlos Fernandez-Martos, Carles Pericay, Joan Maurel, Ana Virgili, Jaume Capdevila, Javier Gallego, Ruth Vera, Nuria Rodriguez-Salas, Ferran Losa, Manuel Valladares, Ana Vivancos, Juan Ayuso, Monique Maas, Fernando Mart�ınez, Marcos Melian, Xabier Garc�ıa de Albe�niz; Hospital Quironsalud, Va- lencia, Spain; Corporacio Parc Tauli, Parc Taul�ı, Spain; Hospital Clinic Barcelona, Barcelona, Spain; Hospital Sant Pau, Barcelona, Spain; H. U. Vall d’Hebron, Barcelona, Spain; Hospital General Universi- tario de Elche, Elche, Spain; Hospital de Navarra, Pamplona, Spain; Medical Oncology Department, Hospital Universitario La Paz, Madrid, Spain; Hospital Sant Joan Desp�ı-Moise�s Broggi, Barcelona, Spain; Hospital Virgen del Roc�ıo, Sevilla, Spain; Cancer Genomics, Vall d’Hebron Institute of Oncology (VHIO), Barcelona, Spain; University Hospital Cl�ınic de Barcelona, Barcelona, Spain; Maastricht Uni- versity Medical Center, Maastricht, Netherlands; Instituto Valenciano de Oncolog�ıa, Valencia, Spain; Fundacio�n Instituto Valenciano de Oncolog�ıa (FIVO), Valencia, Spain; RTI Health Solutions, Barcelona, Spain

Background: Patients with advanced colorectal cancer with 4WT tumors achieve increased response rates with chemotherapy and anti-EGFR therapy as compared with chemotherapy alone. In clinically staged (c) T3 rectal cancer neoadjuvant oxaliplatin/fluoropyrimidine combination has shown to induce encouraging pathological complete response (pCR). We hypothesize that combining FOLFOX and P could improve outcomes in 4WT tumors. Methods: PIER was an investigator-initiated phase II, single- arm, multicentre clinical trial to evaluate the safety and efficacy of neoadjuvant mFOLFOX6 with P in pts < 75-y, with 4WT rectal cancer of the middle third staged as T3 by centrally-reviewed magnetic res- onance imaging (MRI) and clear MRF, who were candidate for a R0 resection with sphincter preservation surgery. Pts received 6 cycles and underwent re-staging with MRI and sigmoidoscopy. Pts without progression underwent total mesorectal excision 4 weeks after the last cycle. Patients with progression were treated with pre-op chemoradiotherapy. The primary endpoint was pCR. The study followed a 2- Stage Simon’s MiniMax design (P0 of 16%, P1 of 35%, alpha and beta of 0.1). The target sample size was 35 patients and if 9 or more achieved a pCR, the results would be compatible with efficacy. We present primary and early secondary endpoints. Results: Between 9/2017 and 6/2020, 90 patients were screened (56 excluded; 42 were excluded due to mutations, 12 were excluded due to discrepancies with central review of radiology) of whom 34 were enrolled. In the ITT population a pCR was observed in 11 pts (32.3%; [95% CI 17.39-50.53]) and a near-complete pathological response (Mandard 1+2) was observed 17 pts (52.9%). Clinical complete or near complete response was achieved in 50% and there were no progressions. R0 resection rate and pathological circumferential resection margin neg- were 100%. Full compliance with induction was 88%. Neoadjuvant G3/4 toxicity occurred in 54% and was consistent with FOLFOX/P safety profile. G3/4 postoperative related toxicity was 19% with one re- operation. Conclusions: The study met the threshold for efficacy. mFOLFOX6 with P as neoadjuvant therapy can be effective and safe without unexpected toxicities in mrT3, clear MRF and 4WT rectal cancer and resulted in a higher rate of pCR compared with our previous series (GEMCAD 0801; The Oncolo- gist 2014) in a similar molecular-unselected population. This study was funded by Amgen S.A. Clinical trial information: NCT03000374. Research Sponsor: AMGEN.

3513 Poster Discussion Session

Molecular correlates of clinical benefit in previously treated patients (pts) with BRAF V600E-mutant metastatic colorectal cancer (mCRC) from the BEACON study.

Scott Kopetz, Danielle A. Murphy, Jie Pu, Fortunato Ciardiello, Jayesh Desai, Axel Grothey, Eric Van Cutsem, Harpreet Singh Wasan, Rona Yaeger, Takayuki Yoshino, Amber C Donahue, Adele Golden, Ashwin Gollerkeri, Zhou Zhu, Josep Tabernero; MD Anderson Cancer Center, Houston, TX; Pfizer, La Jol- la, CA; University of Campania Luigi Vanvitelli, Naples, Italy; Peter MacCallum Cancer Centre, Mel- bourne, VIC, Australia; West Cancer Center, OneOncology, Germantown, TN; University Hospital Gasthuisberg and University of Leuven, Leuven, Belgium; Hammersmith Hospital, Division of Cancer, Imperial College London, London, United Kingdom; Memorial Sloan Kettering Cancer Center, New York, NY; National Cancer Center Hospital East, Kashiwa, Japan; Pfizer, Cambridge, MA; Pfizer, New York, NY; Vall d’Hebron University Hospital and Vall d’Hebron Institute of Oncology (VHIO), UVic-UCC, Barcelona, Spain

Background: Encorafenib + binimetinib + cetuximab (enco/bini/cetux; triplet) and enco + cetux (doublet) regimens improved overall survival and objective response rate vs standard of care in pts with previously treated BRAF V600E-mutant mCRC in the randomized phase 3 BEACON study. To identify molecular correlates of clinical outcome, we performed molecular profiling in tumors from pts in the study. Methods: Baseline tumor samples were retrospectively analyzed by whole-exome sequencing (WES) and whole transcriptome sequencing (WTS) using ImmunoID NeXT (Personalis, Menlo Park, CA, USA). BRAF-mutant (BM) and consensus molecular subtypes (CMS) were determined using published classifiers. Pathway activities were evaluated with gene set variation analysis. Objective tumor response was evaluated according to each subtype. Additional association and interaction analyses between molecular features and clinical outcomes by treatments are ongoing and will be presented. Results: Base- line tumor samples were analyzed by WES and/or WTS from 527 of 665 (79.2%) randomized pts. The biomarker analyses set is representative of the total pt population and had similar clinical outcomes. Of the 460 pts analyzed by WTS (165/224 [73.7%] in the triplet arm, 146/220 [66.4%] in the doublet arm, and 149/221 [67.4%] in the control arm), 84.6% were classified as either CMS1 (n = 225) or CMS4 (n = 164). The proportion of pts classified as BM1 was 32.2% (n = 148) and the majority (84.5%) of these were CMS4, whereas many of those classified as BM2 (67.8%, n = 312) were CMS1 (64.7%). In the BM1 and CMS4 tumors, expression of inflammatory response and epithelial mesenchymal transition genes were elevated, and expression of cell cycle genes was reduced. The response rate in pts with CMS4 and/or BM1 tumors was higher in the triplet arm (CMS4: 33.3% [95% CI: 21.7–46.7]; BM1: 33.3% [95% CI: 21.4–47.1]) compared with the doublet arm (CMS4: 19.2% [95% CI: 9.6–32.5]; BM1: 14.9% [95% CI: 6.2–28.3]). Conclusions: Molecular characteristics and biological properties observed in BRAF V600E-mutant mCRC suggest that a subset of pts with specific molecular features may derive greater clinical benefit from triplet than doublet therapy. Additionally, these findings support the utility of gaining further understanding of the biological landscape in BRAF- mutant mCRC to enable potential hypotheses for pt selection to improve clinical outcome in future studies. Clinical trial information: NCT02928224. Research Sponsor: Pfizer.

3514 Poster Discussion Session

Rarity of acquired mutations (MTs) after first-line therapy with anti-EGFR therapy (EGFRi).

Christine Megerdichian Parseghian, Ryan Sun, Stefania Napolitano, Van K. Morris, Jason Henry, Jason Willis, Eduardo Vilar Sanchez, Kanwal Pratap Singh Raghav, Agnes Ang, Scott Kopetz; The University of Texas MD Anderson Cancer Center, Houston, TX; University of Texas MD Anderson Cancer Center, Houston, TX; MD Anderson Hematology/Oncology Fellowship, Houston, TX; Departments of Gastroin- testinal Medical Oncology and Clinical Cancer Prevention, University of Texas MD Anderson Cancer Center, Houston, TX; Amgen Inc., Thousand Oaks, CA; Department of Gastrointestinal Medical Oncolo- gy, University of Texas MD Anderson Cancer Center, Houston, TX

Background: Colorectal cancers (CRC) lacking RAS MTs treated with EGFRi are thought to evolve by a re- petitive process of genetic diversification and clonal evolution. Acquired MTs in KRAS, NRAS, BRAF, MAP2K1, and EGFR are known mechanisms of acquired resistance in the EGFRi refractory population. However, the prevalence of MTs in the first line (1L) setting is not well established as most experience with EGFRi has been beyond the 1L setting. Methods: We analyzed paired plasma samples from RAS/ BRAF/EGFRWT mCRC patients (pts) enrolled in 3 large randomized phase 3 trials who had been treated with EGFRi and in whom paired baseline (BL) and time of progression (PRO) plasma samples had been collected for sequencing of ctDNA on a platform optimized for very low allele frequencies (PlasmaSe- lect-R™ and Resolution Bio™). Prevalence of MTs at BL and PRO from a 1L study (203; FOLFOX ± panitumumab) were compared with 2 studies in the third line setting (3L; 007; panitumumab + best supportive care [BSC] vs BSC; and 3L; 763; panitumumab vs. cetuximab), to assess the frequency of acquired resistance mutations via ctDNA analysis. Results: For pts with available paired plasma samples (n = 112 for 203; n = 89 for 007; n = 274 for 763), acquisition of at least one KRAS, NRAS, BRAF, MAP2K1, or EGFR MT was significantly less common in post-progression samples in the EGFR containing arms of the 1L 203 study compared to the 3L 763 and 007 studies (6.8% vs 50.4% vs 39.6%, respectively; p < 0.001). In the non EGFR containing arms of the 203 and 007 study, the rate of acquired MTs was 7.5% and 0%, respectively (p = 1). While this difference in the rate of acquired MTs between the EGFR and non EGFR containing arms was statistically significant for the 3L study (p < 0.001) it was not significant for the 1L study. Further, pts on both 3L studies treated with EGFRi who experienced CR, PR or SD acquired more MTs than those who had PD as best response (53.6% vs 33.3%, respectively; p < 0.001). This relationship was not significant in the 1L setting (7.7% vs 0%; p = 1). Subclonal MTs (rMAF < 25%) in KRAS, NRAS, EGFR, BRAF and MAP2K1 were present at BL in 129 pts (27%). Based on the hypothesis that EGFRi is selecting for rare existing mutated cells in the tumor, we would expect expansion of any preexisting subclones in the BL samples. However, in contrast to expectations, these subclones rarely expanded to become clonal at the time of progression (12.4%). Conclusions: In contrast to expectations, acquired KRAS, NRAS, BRAF, EGFR, or MAP2K1 MTs rarely develop after 1L therapy. While selective pressure appears to increase the frequency of acquired MTs in the 3L setting, preexisting subclonal MTs do not appear to be the dominant source of acquired MTs at progression, implying that there may also be a transient mutational process driving resistance rather than expansion of preexisting clones. These findings have significant implications for ongoing and planned EGFRi rechallenge studies. Research Sponsor: Amgen inc.

3515 Poster Discussion Session

Immune signatures to affect overall survival (OS) and response to bevacizumab (Bev) or cetuximab (Cet) in patients (pts) with metastatic colorectal cancer (mCRC) of CALGB/ SWOG 80405 (Alliance).

Federico Innocenti, Akram Yazdani, Xueping Qu, Fang-Shu Ou, Scott Van Buren, Omar Kabbarah, Charles David Blanke, Alan P. Venook, Heinz-Josef Lenz, Benjamin Garrett Vincent; University of North Carolina at Chapel Hill, Chapel Hill, NC; Genentech, San Francisco, CA; Mayo Clinic, Rochester, MN; Oric Pharmaceutical, South San Francisco, CA; Oregon Health and Science University, Portland, OR; University of California San Francisco, San Francisco, CA; USC Norris Comprehensive Cancer Center, Los Angeles, CA; UNC Lineberger Comprehensive Cancer Center, Chapel Hill, NC

Background: CALGB/SWOG 80405 was a randomized phase III trial in first-line mCRC patients treated with Bev, Cet, or both, plus chemotherapy. No difference in OS was found between Bev and Cet. We tested the effect of immune signatures on OS in all the three arms of the study and analyzed differences in OS between the Cet and Bev arms. Methods: 578 primary tumors were profiled by RNAseq. Immune signatures of TGF-b, cytotoxic T cells, wound healing, macrophages, lymphocytes, and INF-c, as well as relative frequencies of CD8+ T-cells, memory resting CD4+ T cells, memory activated CD4+ T cells, macrophages M1 and M2, and activated mast cells were measured. Multivariate Cox proportional hazard models were applied using elastic-net penalization with covariates (age, race, gender, all RAS and BRAF V600E mutations). For relevant signatures, optimal cut-offs for OS were calculated. Results: In all the three arms of the study, high expression of macrophages M2 (HR 6.81, 95% CI 3.56-30.16) and TGF-b (HR 1.37, 95% CI 1.03-2.10) conferred reduced OS compared to low expression; high expression of plasma cells (HR 0.52, 95% CI 0.27-0.83) and memory-activated CD4+ T cells (HR 0.34, 95% CI 0.10-0.65) conferred increased OS compared to low expression. Using optimal cut-offs from these 4 signatures, pts have been categorized as to whether they had either 4, 3, 2, 1, or 0 beneficial signatures associated with increased OS. In all arms of the study (N = 469, after accounting for covariates), the median (95% CI) OS decreased from 42.5 (35.8-47.8; N = 79), to 31.0 (28.8-34.4; N = 177), 25.2 (20.6-27.9; N = 144), and 17.0 (13.5-20.4; N = 69) months when the number of beneficial signatures decreased from 4, to 3, 2, and 0-1 (combined due to a low number of pts), respectively (p = 3.48e-11). In the Bev arm (N = 205), high expression of macrophages M2 conferred reduced OS compared to low expression (HR 6.6, 95% CI 2.7-67.1). In the Cet arm (N = 165), high expression of macrophages M2 conferred reduced OS compared to low expression (HR 4.3, 95% CI 2.1-79.8); high expression of plasma cells (HR 0.36, 95% CI 0.06-0.55) and memory activated CD4+ T cells (HR 0.37, 95% CI 0.03-0.98) conferred increased OS compared to low expression of either signatures. The plasma cell signature interacted with Bev and Cet on the OS of pts (interaction p = 0.009). Conclusions: Tumor immune signatures in mCRC pts are determinants of survival. In pts treated with Bev- and Cet- combination therapies that are standard of care, immune signatures affect response to therapy. These results, provide new markers for treatment selection and for the development of novel active combinations including immune checkpoint inhibitors. Support: U10CA180821, U10CA180882, U24CA196171; https://acknowledgments.alliancefound.org Research Sponsor: U.S. National Insti- tutes of Health.

3516 Poster Discussion Session

Examination of the tumor immune microenvironment (TIME) with multispectral immunofluorescence (m-IF): Association of markers with prognosis and bevacizumab (bev) benefit in NRG Oncology/NSABP C-08.

Katherine L. Pogue-Geile, Marion Joy, Ying Wang, Nan Song, Rim S. Kim, Greg Yothers, Carmen Joseph Allegra, Ashok Srinivasan, Melanie Finnigan, Samuel A. Jacobs, Soonmyung Paik, Jennifer Marie Marie Suga, Judith O. Hopkins, Nicholas J. DiBella, Naoyuki G. Saito, Peter C. Lucas, Norman Wolmark; NSABP/NRG Oncology, Pittsburgh, PA; University of Pittsburgh Department of Biostatistics, and NRG Oncology Statistics and Data Management Center, Pittsburgh, PA; NRG Oncology, and The University of Florida, Gainesville, FL; NRG Oncology/NSABP, and the Yonsei University College of Medicine, Seoul, PA; NSABP/NRG Oncology, and Kaiser Permanente NCI Community Oncology Research Pro- gram, Vallejo, CA; NSABP/NRG Oncology, and Novant Helath Forsyth Medical Center/Southeast Clini- cal Oncology Research Consortium, Winston Salem, NC; NSABP/NRG Oncology, Rocky Mt Cancer Ctrs, and Western States Cancer Research NCORP, Aurora, CO; NSABP/NRG Oncology, and the Melvin and Bren Simon Cancer Center, Indiana University School of Medicine, Indianapolis, IN; NSABP/NRG On- cology, and The UPMC Hillman Cancer Center, Pittsburgh, PA

Background: The purpose of this study was to quantify different molecules of TIME including T cells, macrophages, and immune checkpoint proteins (ICPs), and determine their association with clinical outcomes and treatment benefit in pts enrolled in C-08, which tested the efficacy of adding bev to 5-fluoruracil+leucovorin+oxaliplatin. Our pre-specified, NCTN-CCSC approved primary objective hypothesized that pts with more CD8 cells would have a better prognosis and receive benefit from bev. Methods: Tissue microarrays were used to assess TIME of 1,509 C-08 pts using m-IF and the Vectra Pathology System. Three m-IF panels were used to quantitatively assess T cells (CD3, CD8, CD45RO, FOXP3), macrophages (CD68, CD163), and ICPs (PD-1, PD-L1, CTLA4, TIM3, LAG3, OX40) in stromal and tumor (panCK) regions. The primary objective was to determine the association between overall survival (OS) and high (top 3rd) v low CD8 expression in both stromal and tumor regions. All markers were tested for associations with OS and recurrence-free interval (RFI) and with bev prediction using Cox models and median cut points. Results: Based on our pre-specified analysis, pts with high CD8 cells had better OS, HR=0.66 (95%CI: 0.49-0.88), p=0.005 but pts with high CD8 cells did not receive bev benefit. All T cells and double stained CD8/PD-1 were associated with better RFI. CD3, CD8, CD68, PD-1, PD- L1, and LAG3 cells were associated with better OS. PD-1 and CD8/PD-1 were associated with RFI in pts with deficient mismatch repair (dMMR) and proficient (p)MMR but TIM3, CD3/CD45RO and CD163 were only associated with RFI in dMMR. Association of CD8 cells with bev benefit (RFI) was seen in dMMR pts, HR 0.27 (95% CI: 0.1-0.73), p=.01 and OS, HR=0.27, (95% CI: 0.12-0.64), p=0.0028 but there was no significant interaction. Single staining CD8, PD-1, and double staining CD8/PD-1 cells were associated with bev benefit in dMMR pts but with bev harm in pMMR pts. Howev- er, pts with tumors having >1% of PD-1 and PD-L1 cells (n=197 including 76 dMMR, 100 pMMR, and 21 unknown), received significant bev benefit (int p=.0056). Conclusions: CD8 cells were associated with better OS but were not associated with bev benefit. All T cells and PD-1, PD-L1, and LAG3 cells, were associated with better prognosis in the entire cohort but when pts were stratified for MMR status differences in their association with prognosis and bev benefit emerged. PD-1, CD8, and CD8/PD-1 cells were associated with bev harm in pMMR but bev benefit in dMMR. A significant interaction for the association of high % PD-1 and PD-L1 with bev benefit regardless of MMR status may be a chance finding. However, VEGF has immunosuppressive effects and bev may block these effects in tumors with high PD-L1 and PD-1, regardless of MMR status. NCT: 00096278 PA DOH, U10CA-180868, -180822, -196067, Genentech, Sanofi; NSABP. Clinical trial information: 00096278. Research Sponsor: U.S. National Institutes of Health, Other Foundation, Other Government Agency, Pharmaceu- tical/Biotech Company.

3517 Poster Discussion Session

Early-onset stage II/III colorectal adenocarcinoma in the IDEA database: Treatment adherence, toxicities, and outcomes from adjuvant fluoropyrimidine and oxaliplatin.

Elisa Fontana, Jeffrey P. Meyers, Alberto F. Sobrero, Timothy Iveson, Anthony Frank Shields, Julien Taieb, Takayuki Yoshino, Ioannis Souglakos, Elizabeth Catherine Smyth, Florian Lordick, Markus H. Moehler, Andrea Harkin, Roberto Labianca, Jeffrey A. Meyerhardt, Thierry Andre, Takeharu Yamanaka, Ioannis Boukovinas, Axel Grothey, Irit Ben-Aharon, Qian Shi, Adolescent and Young Adult Gastrointestinal Cancer Task Force EORTC; Sarah Cannon Research Institute, United Kingdom, Lon- don, United Kingdom; Mayo Clinic, Rochester, MN; IRCCS Ospedale Policlinico San Martino, Genoa, It- aly; University Hospital Southampton NHS Foundation Trust, Southampton, United Kingdom; Karmanos Cancer Institute, Wayne State University, Detroit, MI; Ho^pital Europe�en Georges Pompidou, Paris, France; National Cancer Center Hospital East, Kashiwa, Japan; Department of Medical Oncology, University General Hospital of Heraklion, Heraklion, Greece; Cambridge University Hospitals NHS Foundation Trust, Cambridge, United Kingdom; University Cancer Center Leipzig, Leipzig, Germany; University Medical Center Mainz, I. Dept. of Internal Medicine, Mainz, Germany; CRUK, Glasgow, Unit- ed Kingdom; Cancer Center, ASST Papa Giovanni XXIII, Bergamo, Italy; Department of Medical Oncolo- gy, Dana-Farber Cancer Institute/Partners Cancer Care, Boston, MA; Sorbonne Universite� and Ho^pital- Saint Antoine, Paris, France; Department of Biostatistics, Yokohama City University School of Medi- cine, Yokohama, Japan; Bioclinic Thessaloniki Medical Oncology Unit, Athens, Greece; Division of Medical Oncology, Mayo Clinic, Rochester, MN; Division of Oncology, Rambam Health Care Center, Haifa, Israel

Background: Incidence of early-onset colorectal cancer (eoCRC, age < 50) is steadily increasing. Decisions on adjuvant treatment (adjTx) regimen and duration should consider tx adherence, toxicity (tox) and expected outcomes in a population with life-expectancy longer than late onset CRC (loCRC, age $ 50). Methods: Indi- vidual patient data from stage II/III patients (pts) from 6 randomised trials in the IDEA database were used to compare characteristics, tx adherence, and adverse events of eoCRC to loCRC. To reduce the confounder of non-cancer-related deaths due to age/co-morbidities, time-to-recurrence (TTR) and cancer-specific survival (CSS) were compared by stratified Gay k-sample test. 5-year cancer-specific mortality (CSM) rate were estimated by adjusted cumulative incidence function. 3-year relapse-free survival (RFS) rate were compared by stratified and adjusted COX models. Results: Out of 16,349 pts included, 1564 (9.6%) were eoCRC. Com- pared to loCRC, eoCRC had lower percent of male pts (51% vs 57%, p < 0.01) better performance status (PS0 86% vs 80%, p < 0.01), similar T stage distribution (% T1-3/T4: 76/24 vs 77/23, p = 0.97), higher rate of N2 disease (24% vs 22%, p < 0.01), more likely to complete pre-planned duration of adjTx (83.2% vs 78.2%, p < 0.01) and received a higher tx intensity especially with 6 month tx (mean oxaliplatin dose intensity 75% vs 72%, p < 0.01; capecitabine 85% vs 78%, p < 0.01; 5FU 85% vs 82% p < 0.01). Gastroin- testinal tox was more common in eoCRC (any grade nausea 58% vs 45%, p < 0.01; any grade vomiting 22% vs 16%, p < 0.01); haematological tox was more frequent in loCRC (62% vs, 69%, p = < 0.01); any grade neuropathy rate was similar (75%). Significant interaction was found between age and T stage for TTR (p = 0.04). Clinical outcome estimates and comparisons are shown in Table. Notably, high risk stage III (T4/N2) eoCRC had significantly lower 3-y RFS rate (54% vs 64%, HRadj 0.74, p < 0.01). Conclusions: eoCRC have better tx adherence than loCRC, as expected. While in high risk stage II and low risk stage III, cancer-specific outcomes are not different, in high risk stage III young age is negatively prognostic and associated with significantly higher relapse rate and risk of CRC death; this is despite a higher administered adjTx-intensity, suggesting a more aggressive disease biology. Clinical trial information: NCT00749450 (SCOT); NCT00646607 (TOSCA); NCT01150045 (CALGB/SWOG 80702); NCT00958737 (IDEA France); Re- search Sponsor: EORTC GI Group supported the pooled analyses, ACHIEVE supported by the Japanese Foun- dation for Multidisciplinary Treatment of Cancer (JFMC) and funded by Yakult Honsha Co, Ltd; CALGB/ SWOG 80702 was supported by grants (U10CA180821, U10CA180835, UG1C).

Adjusted Hazard Ratio (95% Confidence eoCRC loCRC Interval) p-value

3-y RFS, High risk Stage II 87.6 (84.1- 88.0 (86.8- 0.98 (0.72-1.34) 0.91 % 91.3) 89.2) Stage III T1-3 N1 81.6 (78.0- 84.0 (83.0- 0.99 (0.80-1.22) 0.90 85.3) 84.9) Stage III T4 or 54.5 (49.7- 64.5 (63.1- 0.74 (0.64-0.87) 0.0003 N2 59.9) 65.9) 5-y CSM % High risk Stage II 4.8 (2.9-7.8) 7.6 (6.6-8.7) 1.38 (0.84-2.27) 0.21 Stage III T1-3 N1 7.1 (5.1-9.8) 6.9 (6.3-7.5) 0.96 (0.70-1.30) 0.78 Stage III T4 or 23.9 (20.0- 20.7 (19.5- 0.81 (0.67-0.99) 0.040 N2 28.6) 21.9)

3518 Poster Discussion Session

Prevalence of fertility discussions between young adult colorectal cancer survivors and their providers.

Julia Stal, Serena Yi, Sally Cohen-Cutler, Phuong Gallagher, Afsaneh Barzi, Joel Milam, David R. Freyer, Heinz-Josef Lenz, Kimberly Ann Miller; University of Southern California, Los Angeles, CA; Children’s Hospital Los Angeles, Los Angeles, CA; Colon Club, Burbank, CA; USC Keck School of Medi- cine Norris Comprehensive Cancer Center, Los Angeles, CA; USC Norris Comprehensive Cancer Center, Los Angeles, CA

Background: Clinical guidelines indicate that oncologists should discuss potential treatment-induced infertility with patients with reproductive potential. Due to tumor location and use of multimodal therapies, young adults with colorectal cancer (CRC) are at heightened risk for treatment-related infertility. Methods: An online, cross-sectional survey was administered in collaboration with a national patient ad- vocacy organization for young adult CRC survivors (currently under age 50). Survivors were asked to indicate if a doctor had ever talked to them about potential problems with their ability to have children after treatment and if they banked eggs/embryos (females) or sperm (males) prior to their cancer therapy. Those who reported that they did not preserve fertility were asked to indicate why (not sure; I chose not to; I did not know this was an option; I wanted to, but could not afford it; and I wanted to, but my treatment would not allow it). Results: A total of 234 colon (N=86) or rectal (N=148) cancer survivors were included in the study (male [61.9%] and White [77.9%; table]). Most respondents were diagnosed with stage 2 cancer (55.8% colon, 61.6% rectal). Over half of male and female survivors reported that their doctor did not talk to them about problems with their ability to have children after treatment, and 75% did not bank eggs/embryos or sperm prior to their cancer therapy. Of those, over 20% endorsed I wanted to, but could not afford it’ and over 20% endorsed I did not know this was an option’. Conclu- sions: Most CRC survivors in this study reported never having a fertility discussion with their provider, suggesting that survivors are not receiving, or cannot recall, comprehensive and guideline-concordant cancer care. In addition, one-fifth were not aware of preservation options, suggesting potential healthcare and/or provider-level barriers to appropriate fertility counseling. Fertility preservation cost is another barrier to the appropriate delivery of care. Providers must ensure that patients receive timely fertility discussions covering options to preserve fertility to mitigate this late effect of cancer treatment to ensure optimal quality of life for CRC patients with reproductive potential. Research Sponsor: The Aflac Archie Bleyer Young Investigator Award in Adolescent and Young Adult Oncology from the Children's Oncology Group. Additional support was provided by the NCI Cancer Center Support Grant from the USC Norris Comprehensive Cancer Center.

Fertility preservation frequencies (N=234). Gender Male Female Has a doctor ever talked to you about problems with your ability to have children after your treatment? Yes 60 (41.38) 35 (41.18) No 81 (55.86) 49 (57.65) Not Sure 4 (2.76) 1 (1.18) Did you bank eggs/embryos (female; sperm, male) prior to your cancer therapy? Yes 30 (20.98) 19 (22.35) No 107 (74.83) 64 (75.29) Not Sure 6 (4.20) 2 (2.35) If no (did not bank eggs/embryos or sperm), I decided not to because. I wanted to, but my treatment would not allow it 9 (6.38) 4 (4.82) I wanted to, but could not afford it 31 (21.99) 19 (22.89) I did not know this was an option 33 (23.40) 18 (21.69) I chose not to 58 (41.13) 38 (45.78) Not sure 10 (7.09) 4 (4.82)

3519 Poster Discussion Session

Microbiome signature, global methylation and immune landscape in early onset colorectal cancer.

Ning Jin, Xiaokui Mo, Rebecca Hoyd, Ayse Selen Yilmaz, YunZhou Liu, Malvenderjit Jagjit Singh, Mitchell Muniak, Heather Hampel, Daniel Spakowicz; Univ of Wisconsin, Madison, WI; The Ohio State University, Center for Biostatistics, Columbus, OH; Division of Medical Oncology, Department of Inter- nal Medicine, The Ohio State University College of Medicine, Columbus, OH; OSU, Columbus, OH; The Ohio State University Wexner Medical Center, Columbus, OH; The Ohio State University, Columbus, OH; James Cancer Hospital, Columbus, OH; The Ohio State University Comprehensive Cancer Center, Columbus, OH; Division of Medical Oncology, Department of Internal Medicine & Department of Bio- medical Informatics, Ohio State University, Columbus, OH

Background: The incidence of colorectal cancer (CRC) in young adults ( < 50 years old) has been rapidly increasing by 2% per year since early 1990. Approximately 20% of early-onset (EO) CRC cases are due to germline gene mutations. However, the etiology of sporadic EO CRC remains poorly understood. Re- search suggests that environmental factors such as the Western diet (high in fat and low in fiber) may be associated with an increased incidence of sporadic EO CRC. The gut microbiota decompose and fer- ment dietary fibers to produce microbial metabolites, which play pivotal roles in maintaining the integri- ty of intestinal epithelium as well as the immune cell homeostasis. Also, these microbial metabolites may influence the host epigenome by altering either the activity of epigenetic enzymes or by modifying the availability of cofactors needed for epigenetic modifications. The aim of our research is to associate intratumoral microbiota with methylation pattern and immune cell composition in EO CRC. Methods: A total of 358 CRC cases, including 54 cases of EO CRC (age < 50 years) and 304 cases of late onset (LO) CRC (age $ 50 years), with matched methylation array (Infinium HM450), RNA-sequencing (Illu- mina HiSeq) from colon adenocarcinomas (COAD) and rectal adenocarcinomas (READ), and clinicopathological information of each patient, were extracted from the Cancer Genome Atlas (TCGA). We characterized and compared the intra-tumoral microbiota composition, tumor-infiltrating lymphocytes (TILs), and methylation profile between EO and LO CRC. Results: We found that there is a distinct microbial distribution, gene expression and methylation pattern in the EO CRC when compared with LO CRC. Non-human sequences from several kingdoms including bacteria, fungi and viruses were found and the incidences were consistent with reported values by other methods, e.g. Fusobacterium incidence. The EO CRC cases showed global hypomethylation, even though hypermethylation pattern is expected in the young chronological age group (known as Horvath’s clock). Pathway overrepresentation analysis of differentially expressed genes showed significant activation of p53 and pentose phosphate pathways and de novo nucleotide synthesis in EO CRC. Integration across datasets showed positive correlations between microbes and inflammasome pathway, positive correlation with regulatory T cells (Tregs), and negative correlations with CD4 memory T cells. Conclusions: These data suggest a mechanism by which the colorectal cancer-associated microbiota may be associated with epigenetic regulation and host immune response. Research Sponsor: Internal grant (Division of Internal Medicine Seed Grant) at OSU.

3520 Poster Session

HPV-mediated anal squamous cell carcinoma and precancerous lesions in HIV positive patients.

Omar Bushara, Brian Finkelman, Sam Weinberg, Katrina Krogh, Aparna Kalyan, Guang-Yu Yang; North- western University Feinberg School of Medicine, Chicago, IL; Department of Pathology, Northwestern University Feinberg School of Medicine, Chicago, IL; Northwestern University, Chicago, IL

Background: Anal cancer affects over 8,000 patients per year in the United States and the incidence is increasing. A significant risk factor for anal cancer and precancerous lesions is human papilloma virus (HPV), with up to 90% of cases being associated with HPV infection. Another emerging risk factor is HIV co-infection. In the present study, we further address if CD4 count is a significant factor for driving higher-grade HPV-mediated anal squamous lesions in HIV/HPV co-infection patients with a single insti- tution large cohort. Methods: A retrospective cohort of HPV-positive patients with anal biopsies was obtained from 2002-2020. Information on the grade of their anal lesion, HIV status, and CD4 count (cells/mm3) were collected. In patients with HIV, the most recent CD4 count up to one year prior to or one month after their biopsy was utilized in our analysis. Lesions were grouped into low grade squamous intraepithelial lesions (LSIL) and higher grade squamous intraepithelial lesions (HSIL), carcinoma in situ (CIS), or squamous cell carcinoma (SCC). The Center for Disease Control CD4 count levels to define HIV status were utilized in our sub-analyses. The distribution of lesion grades was compared between HIV-negative and -positive patients, and between HIV-negative and three subgroups of HIV-positive patients using the Fisher’s exact test. Results: Our cohort comprised of 3,354 total HPV-positive patients. 2,036 of these patients were HIV-negative and 1318 were HIV-positive. The proportion of higher grade lesions was significantly higher in HIV/HPV co-infected patients regardless of CD4 count (Table). The full cohort of HIV-positive patients had lower rates of LSIL (60.8% vs. 74.0%) and higher rates of higher-grade lesions (39.2% vs. 26.0%) (p<0.001) compared to HIV-negative patients. The distribution of lesion grades was also significantly different between HIV-negative patients and all HIV-positive patient subgroups, with all subgroups having higher rates of higher-grade lesions than HIV-negative patients (all p<0.001). Conclusions: Our data show that HIV-HPV co-infection is a risk factor for higher grade anal lesions, regardless of CD4 status. This suggests that CD4 count is not the only factor responsible for the increased risk of higher-grade anal lesions, as the groups of HIV-positive patients with CD4 counts between 200-499 and above 499 still had a higher rate of higher-grade lesions than HIV-negative patients. Further research into other HIV-related immunologic changes that increase risk for higher- grade HPV-driven anal lesions is warranted. Research Sponsor: None.

Distribution of anal lesions by HIV and CD4 status. LSIL HSIL/CIS/SCC Total N N % N % P Value

Full Cohort 3354 2309 68.8% 1045 31.2% HIV Negative 2036 1507 74.0% 529 26.0% HIV Positive 1318 802 60.8% 516 39.2% <0.001 CD4 < 200 170 79 46.5% 91 53.5% <0.001 CD4 200-499 492 296 60.2% 196 39.8% <0.001 CD4 > 499 656 427 65.1% 229 34.9% <0.001

3521 Poster Session

Final analysis of dose-finding and single-arm confirmatory study (phase I/II study) of definitive chemoradiotherapy (dCRT) with S-1/mitomycin-C (MMC) in patients (pts) with clinical (c) Stage II/III squamous cell carcinoma of the anal canal (SCCA): JCOG0903.

Yoshinori Ito, Tetsuya Hamaguchi, Atsuo Takashima, Junki Mizusawa, Yasuhiro Shimada, Manabu Shiozawa, Takeshi Kodaira, Masayuki Ohue, Makoto Kinouchi, Kohei Murata, Gen Iinuma, Fumihiko Fujita, Hirohisa Miura, Fumio Ishida, Yoshihisa Saida, Takahisa Matsuda, Hiroshi Katayama, Haruhiko Fukuda, Yukihide Kanemitsu; Department of Radiation Oncology, Showa University School of Medi- cine, Tokyo, Japan; Department of Gastroenterological Oncology, Saitama Medical University Interna- tional Medical Center, Saitama, Japan; Department of Gastrointestinal Oncology, National Cancer Center Hospital, Tokyo, Japan; JCOG Data Center/Operations Office, National Cancer Center Hospital, Tokyo, Japan; Clinical Oncology Division, Kochi Health Sciences Center, Kochi, Japan; Department of Gastrointestinal Surgery, Kanagawa Cancer Center, Yokohama, Japan; Department of Radiation Oncolo- gy, Aichi Cancer Center Hospital, Nagoya, Japan; Department of Gastroenterological Surgery, Osaka In- ternational Cancer Institute, Osaka, Japan; Department of Surgery, Miyagi Cancer Center, Miyagi Cancer Center, Miyagi, Japan; Department of Surgery, Suita Municipal Hospital, Suita, Japan; Depart- ment of Diagnostic Radiology, National Cancer Center Hospital, Tokyo, Japan; Department of Surgery, Kurume University School of Medicine, Kurume, Japan; Department of Lower Gastrointestinal Surgery, Kitasato University School of Medicine, Sagamihara, Japan; Digestive Disease Center, Showa University Northern Yokohama Hospital, Yokohama, Japan; Department of Surgery, Toho University Ohashi Hospi- tal, Tokyo, Japan; Endoscopy Division, National Cancer Center Hospital, Tokyo, Japan; Japan Clinical Oncology Group Data Center, National Cancer Center Hospital, Tokyo, Japan; Department of Colorectal Surgery, National Cancer Center Hospital, Tokyo, Japan

Background: dCRT with 5-FU/MMC is a standard treatment for cStage II/III SCCA. S-1 is an oral fluoropyrimidine and has a greater effect on radiosensitivity. We conducted this trial of dCRT with S-1/MMC to determine the recommended dose (RD) of S-1 in dose-finding (phase I) part and to evaluate the efficacy and safety in confirmatory (phase II) part for cStage II/III SCCA. We reported the RD of S-1 and the 3-year survival at 2019 Gastrointestinal Cancers Symposium. We report the final data after 5-year follow-up. Methods: Eligibility criteria included histologically proven SCCA, cStage II/III (UICC 6th), PS 0- 1, and age 20-80 years. dCRT consisted of MMC (10 mg/m2 on days 1, 29) and S-1 (60 mg/m2/d in level 0 and 80 mg/m2/d in level 1 on days 1-14, 29-42) with concurrent radiotherapy of 59.4 Gy/33fr. The dose-finding part adopted the 3+3 cohort design. The primary endpoint of confirmatory part was 3-year event-free survival (EFS). The sample size was 65 in the confirmatory part, with one-sided alpha of 5% and power of 80%, threshold and expected 3-year EFS as 60% and 75%. Key secondary endpoints were overall survival (OS) and progression-free survival (PFS) and colostomy-free survival (CFS), and adverse events. Final analysis was planned after 5-year follow-up for all pts. Results: From Feb/2010 to Mar/2015, 69 pts (3 in level 0 and 66 [7 in phase I and 59 in phase II] in level 1) were enrolled. Pts characteristics for level 1 were as follows: M/F, 12/54; Age, median 64 (range 33-80); cStage II/IIIA/ IIIB, 29/9/28. Three in level 1 were ineligible and 63 eligible assigned to level 1 were included in efficacy analysis. In the dose-finding part, RD of S-1 was determined as 80 mg/m2/d. The complete response rate was 81% (95% CI, 69.1-90.0%) on central review. With a median follow-up of 5.4 years, 3- and 5- year EFS was 65.0% (90% CI 54.1-73.9%) and 63.4% (95% CI 50.2-73.9%). 5-year OS, PFS, and CFS were 84.1% (95% CI 72.5-91.1%), 84.1% (95% CI 72.4-91.1%), and 73.0% (95% CI 60.2- 82.3%), respectively. In a univariable analysis, male sex (p = 0.045) prognosticated for poor OS and cT3 or T4 (p = 0.001), male sex (p = 0.019) and, PS 1 (p = 0.048) prognosticated for poor CFS. Nine (14.3%) of 63 pts at a dose level 1 developed recurrence or disease progression. Locoregional recurrence only and distant metastasis were observed in 1 pts (1.6%) and 8 pts (12.7%) respectively. Among all treated 65 pts, only 5 pts (7.7%) showed grade 3 late toxicities including jejunum obstruction, jejunum ulcer, proctitis, lower gastrointestinal hemorrhage, anal pain, radiation dermatitis, and ureteral stenosis. No Grade 4 or 5 late toxicities were observed. Conclusions: dCRT with S-1/MMC showed acceptable toxicities and favorable 5-year survival and could be a possible treatment option for pts with locally advanced SCCA. Clinical trial information: jRCTs031180002. Research Sponsor: 26-A-4, 29-A-3, 2020-J-3, Grants-in-Aid for Clinical Cancer Research (H23Gann-012) from the Ministry of Health.

3522 Poster Session

Association between cancer stem cell marker ALDH1 and clinical and morphological factors of colorectal cancer prognosis.

Elena A. Dzhenkova, Inna A. Novikova, Oleg I. Kit, Elena Yu. Zlatnik, Elena P. Ulianova, Anastasia O. Sitkovskaya, Oksana G. Shulgina, Yuriy A. Gevorkyan, Natalya V. Soldatkina, Vladimir E. Kolesnikov, Maksim K. Aleksandrovich, Andrey V. Dashkov, Sergey I. Poluektov, Dmitry O. Kaymakchi, Ellada A. Mirzoyan, Vladislav K. Pavlov, Maksim A. Vereshchak, Oksana V. Katelnitskaya, Dmitriy A. Savchenko, Roman E. Tolmakh; National Medical Research Centre for Oncology, Rostov-on-Don, Russian Federation

Background: Cancer stem cells (CSCs) capable of self-sustaining and multipotent differentiation are considered among the most important factors limiting treatment effectiveness. ALDH1 is a marker of colorectal cancer (CRC) CSCs; it is involved in cell differentiation and proliferation, determines resistance to alkylating chemotherapeutic agents, and also induces epithelial-mesenchymal transition (EMT), which increases the invasive and metastatic potential of tumors. The purpose of the study was to assess the association between the expression of the ALDH1 CSC marker in tissues of CRC of different stages and clinical and morphological factors of the disease prognosis. Methods: The study included 299 patients (aged 42-86 years, mean age 64.2±1.7) with stage II-IV CRC T1-4N0-2M0-1; histologically verified G1-G3 adenocarcinoma in all patients. Tissues of surgically removed tumors were studied with IHC analysis using mouse monoclonal anti-ALDH1 antibodies (clone B-5, Santa Cruz Biotechnolo- gy) diluted 1:1800 and the Reveal Polyvalent HRP-DAB Detection System. The percentage of cells positively stained for ALDH1 among all tumor cells was assessed. Statistical analysis was performed using the STATISTICA 13.0 program (StatSoftInc., USA). Results: Positive ALDH1+ expression was registered in 52.5% of all patients, negative expression – in 47.5%. Statistically significant association was observed between the ALDH1 expression and the CRC stage, since the ALDH1+ expression increased from stage II to stage IV (p = 0.003). The ALDH1 expression was statistically significantly associated with the depth of tumor invasion (p = 0.018) and the presence of distant metastases (p < 0.001). No significant relationship was observed between the ALDH1 expression and regional lymph node metastasis (p = 0.788). Statistically significant association was registered between the ALDH1 expression and the tumor grade (p < 0.001), perineural invasion (p = 0.010) and lymphocytic infiltration (p < 0.001). No significant relationship was observed between the tumor histological structure (p = 0.979), lymphovascular invasion (p = 0.772) and ALDH1 expression. Tumor site was not statistically significantly associated with ALDH1 expression (p = 0.349). Conclusions: The study demonstrated statistically significant association between the ALDH1 expression and clinical and morphological characteristics of CRC, determining invasive and metastatic potential of the tumor, and ALDH1 may be an independent prognostic factor and a new therapeutic target for the regulation of the progression process. Research Sponsor: None.

3523 Poster Session

Circulating tumor DNA-based genomic profiling of small bowel adenocarcinoma.

Pat Gulhati, Karan Pandya, Hiba I. Dada, Christopher R. Cogle, Jason S. Starr, Sujith R. Kalmadi, Fadi S. Braiteh, Leylah Drusbosky; Rutgers Cancer Institute of New Jersey, New Brunswick, NJ; Guar- dant Health, Inc., Redwood City, CA; Brio Ventures, Gainesville, FL; University of Florida Health Cancer Center, Jacksonville, FL; Ironwood Cancer/Rsrch Ctr, Chandler, AZ; Comprehensive Cancer Centers of Nevada, Las Vegas, NV

Background: Small bowel adenocarcinoma (SBA) is a rare malignancy, with lower incidence, later stage at diagnosis, and worse overall survival compared to other intestinal cancers, such as colorectal cancer (CRC). Since the majority of small bowel tumors are not accessible to endoscopic biopsy, comprehensive genomic profiling using circulating tumor DNA (ctDNA) may enable non-invasive detection of targetable genomic alterations (GA) in SBA patients. In this study, we characterize the ctDNA GA landscape in SBA. Methods: Analysis of 299 ctDNA samples prospectively collected from 265 SBA patients between 2017 to 2020 was performed using a 73 gene next generation sequencing panel (Guar- dant360). A subset of patients underwent longitudinal analysis of changes in GA associated with systemic therapy. Results: Of the 265 patients, 160 (60.3%) were male; the median age was 66 (range: 21-93 years). The most common GA identified in SBA patients included TP53 [58%], KRAS [44%], and APC [40%]. MSI was detected in 3.4% of SBA patients. When stratified by primary tumor location, APC, KRAS, TP53, PIK3CA, and ARID1A were the most common GA identified in both duodenal and jejunal adenocarcinomas. ERBB2, BRCA2 and CDK6 alterations were enriched in duodenal adenocarcinoma, while NOTCH and BRAF alterations were enriched in jejunal adenocarcinoma. The most common currently-targetable GA identified were ATM [18%], PIK3CA [17%], EGFR [15%], CDK4/6 [11%], BRAF [10%], and ERBB2 [10%]. Unique differences in GA between SBA and CRC were identified: i) the majority of ERBB2 alterations are mutations (89%) in the extracellular domain and kinase domain, not amplifications (11%); ii) the majority of BRAF alterations are non V600E mutations (69%) and amplifications (28%); iii) there is a significantly lower rate of APC mutations (40%). Alterations in DNA damage response pathway proteins, including ATM and BRCA 1/2, were identified in 30% of SBA patients. ATM alterations were more common in patients 365 years old. The most common mutations predicted to be related to clonal hematopoiesis of indeterminate potential were TP53, KRAS and GNAS. Longitudinal ctDNA analysis in 4 SBA patients revealed loss of mutations associated with therapeutic response (TP53 R342*, MAPK3 R189Q) and acquired mutations associated with therapeutic resistance (NF1 R1968*, MET S170N, RAF1 L613V). Conclusions: This study represents the first large-scale blood-based ctDNA genomic profiling of SBA. SBA represents a unique molecular entity with differences in frequency and types of GA compared to CRC. Variations in GA were noted based on anatomic origin within the small intestine. Longitudinal ctDNA monitoring revealed novel GA associated with therapeutic resistance. Identification of multiple targetable GA may facilitate clinical decision making and improve patient outcomes in SBA, especially when a tissue biopsy is not feasible or sufficient for comprehensive genomic profiling. Research Sponsor: None.

3524 Poster Session

Association of HER2 expression with pathologic features and prognosis in stage II and III colon cancer.

Zehua Wu, Huabin Hu, Yanhong Deng; Sixth Affiliated Hospital of Sun Yat-sen University, Guangzhou, China; Sun Yat-sen University, Guangzhou, China

Background: We examined the frequency, tumor characteristics, and prognostic impact of HER2 protein expression in stage II and III colon cancer after curative resection. Methods: Paraffin-embedded tumors from consecutive primary stage II and III colon cancer patients were analyzed for HER2 protein expression by immunohistochemistry between April, 2013 and May, 2020. HER2 determination of immunohistochemistry scores (0/1+/2+/3+) was according to HERACLES diagnostic criteria. Results: A total of 2088 stage II and III colon cancer patients were included (53.8% stage II, 46.3% stage III). HER2 scored positive (3+) was detected in 48(2.3%) tumors, and was correlated with younger age (P < 0.001), well/moderate differentiation (P = 0.026), proficient mismatch repair (pMMR) (P = 0.045) and KRAS wild-type (P < 0.001). HER2 scored positive (3+) was not significantly associated with disease- free survival (DFS) compared with HER2 scored negative (0/1+), neither in stage III patients (multivariable HR, 0.86; 95CI, 0.38 to 1.94; P = 0.717), nor in stage II patients (multivariable HR, 1.68; 95CI, 0.74 to 3.84; P = 0.218). In a separate analysis involving stage II patients without any high-risk factor (n = 741), those with HER2 scored positive (3+) tumors (n = 16) showed significantly reduced DFS (multivariable HR, 2.91; 95CI, 1.04 to 8.81; P = 0.041) compared with patients with HER2 scored negative (0/1+) tumors, independent of sex, age and MMR status. Conclusions: HER2 scored positive (3+) was independently associated with poor DFS in stage II colon cancer patients without high-risk factors. HER2 expression determination may help to judge the prognosis of those patients and guide adjuvant chemotherapy. Research Sponsor: None.

3525 Poster Session

Clinical utility of stool-based SDC2 methylation test for the detection and screening of colorectal cancer in a Chinese population.

Yanmei Liu, Weiguo Yin, Lei Peng, Xianshu Wang, Hongzhi Zou, Haibo Zhou; The Qingyuan People’s Hospital, Qingyuan, China; Creative Biosciences (Guangzhou) Co., Ltd., Guangzhou, China; Creative Bi- osciences (Guangzhou) Co., Ltd, Guangzhou, China

Background: In 2020, colorectal cancer (CRC) ranks second in incidence and third in mortality among all types of cancers in China based on data from GLOBOCAN. Moreover, the rates of incidence and mortality have been continuously rising over the past several decades. In addition to conventional methods for detection and screening of CRC such as gFOBT, FIT, and colonoscopy, a stool-based methylation test of human SDC2 gene was recently approved by National Medical Product Association (NMPA) of China. We hereby report the performance of this newly approved test in a hospital-based cohort of more than 10,000 patients in the real world of daily clinical practice. Methods: The methylation target, human SDC2 gene, was extracted from stool and purified via sequence-specific capture technology. The isolated DNA was further treated with bisulfite before it was subsequently amplified by quantitative methylation-specific polymerase chain reaction (qMSP) to analyze the fecal level of SDC2 methylation. Subjects were further examined by colonoscopy or CT imaging. Pathological examination would also be performed in cases deemed necessary. Results: A total of 11,284 individuals were tested using the novel SDC2 methylation test. Among them, 858 and 10,426 were tested positive and negative, respectively. Follow-up visits, treatment, and medical information were complete for 429 positive and 780 negative patients who were included in this final analysis. Positive predictive value (PPV) of CRC and adenomas was 36.4% (156/429) and 24.5% (105/429), respectively. Stratified analysis implies that SDC2 methylation level in CRC was significantly higher than those in adenoma and normal groups. In CRC, no significant correlation was observed between SDC2 methylation and clinicopathological features including gender and grade of dysplasia. In < 65 and $ 65 age groups, the CRC detection rate in males was higher than that in females-almost two times higher in $ 65 age group. Conclusions: The stool-based SDC2 methylation test demonstrated high accuracy in the detection of CRC and advanced adenomas. It is a sensitive and valid modality expected to play a significant role to aid diagnosis and screening of CRC and precancerous lesions in order to reduce the morbidity and mortality of this malig- nant disease. Research Sponsor: None.

3526 Poster Session

Patient-reported quality of life data from patients with pre-treated metastatic colorectal cancer receiving trifluridine/tipiracil: Interim results of the TALLISUR study.

Meinolf Karthaus, Albrecht Kretzschmar, Stefan Fuxius, Jorge Riera Knorrenschild, Florian Kaiser, Rolf Mahlberg, Manfred Welslau, Henning Pelz, Volker Heinemann, on behalf of the TALLISUR Study Group; Klinikum Neuperlach/Harlaching, Munich, Germany; Practice for hematology and oncology, MVZ Mitte, Leipzig, Germany; Tumor Center, Heidelberg, Germany; Universitatsklinikum, Giessen Und Marburg/Standort Marburg, Germany; VK&K Studienzentrum, Landshut, Germany; Department of In- ternal Medicine I, Klinikum Mutterhaus der Borromaeerinnen, Trier, Germany; Praxis Aschaffenburg, Aschaffenburg, Germany; Ambulantes Therapiezentrum fuer Haematologie und Onkologie, Offenburg, Germany; University Hospital Munich, LMU Munich, Munich, Germany

Background: Compared to placebo, trifluridine/tipiracil (FTD/TPI) significantly improved overall and progression-free survival in patients (pts) with pre-treated metastatic colorectal cancer (mCRC) in the phase III RECOURSE trial. Although time to deterioration of ECOG performance status (PS) from 0/1 to $ 2 was significantly longer in pts treated with FTD/TPI, health-related quality of life (HRQoL) was not formally assessed by direct means. Therefore, a two-arm trial with best supportive care (BSC) as appropriate comparative treatment was designed to specifically address the effect of FTD/TPI on HRQoL. Methods: In this prospective, multi-center, German, open-label, phase IV study, pts with pre-treated mCRC could choose between BSC or oral FTD/TPI (35 mg/m2bid on days 1-5 and 8-12 of each 28-day cycle). EORTC QLQ-C30 and EQ-5D-5L questionnaires were employed to assess HRQoL. Primary endpoint was the rate of responders with stabilized ( > -10 and < 10 scores) or improved ($ 10 scores) response (RR). Response was calculated as the mean score of the EORTC QLQ-C30 global health status/ QoL scale from the 2nd cycle until the end of treatment/ observation compared to the baseline score. Re- sults: Of 194 eligible pts, 185 pts chose treatment with FTD/TPI (median 3 cycles), while 9 pts decided to receive BSC only. Questionnaires from 109 pts receiving FTD/TPI and from 6 pts with BSC were evaluable for RR. The primary endpoint (RR) was 59.6% (95% CI 49.8 – 68.9) in FTD/TPI-treated pts and 50.0% (95% CI 11.8 – 88.2) in pts receiving BSC. Analysis of the extended follow-up period, demonstrated that RR was 67.0% (95% CI 57.3 – 75.7) in FTD/TPI-treated pts. In the FTD/TPI-group, median time to deterioration of HRQoL was 121 days (n = 61; 95% CI 87 – 151) according to EORTC QLQ-C30 and 119 days (n = 63; 95% CI 85 – 138) according to EQ-5D-5L. Conclusions: If pts can choose between treatment and BSC in late-stage CRC, the vast majority opts for treatment. According to the present results, FTD/TPI-treatment induced prolonged stabilization of HRQoL, a highly desired attribute of therapies for pts with late-stage cancer. Clinical trial information: No 2017-000292-83. Research Sponsor: Servier Pharma Germany.

3527 Poster Session

Globo H expression in metastatic colorectal cancer (CRC).

Priya Jayachandran, Yasmine Baca, Joanne Xiu, Jian Zhang, Francesca Battaglin, Hiroyuki Arai, Richard M. Goldberg, Benjamin Adam Weinberg, Emil Lou, Michael J. Hall, Moh’d M. Khushman, Davendra Sohal, Shivani Soni, Jingyuan Wang, Wu Zhang, Joshua Millstein, Wolfgang Michael Korn, Heinz-Josef Lenz; Division of Medical Oncology, USC Norris Comprehensive Cancer Center, Keck School of Medicine, Los Angeles, CA; Caris Life Sciences, Phoenix, AZ; West Virginia University Cancer Institute, Morgantown, WV; Georgetown University Hospital, Washington, DC; University of Minnesota School of Medicine, Minneapolis, MN; Fox Chase Cancer Center, Philadelphia, PA; Medical Oncology, The University of South Alabama, Mitchell Cancer Institute, Mobile, AL; Cleveland Clinic, Cleveland, OH; Department of Preventive Medicine, USC Norris Comprehensive Cancer Center, Keck School of Medicine, Los Angeles, CA

Background: Globo H is a carbohydrate antigen that is highly expressed on the cell surface of epithelial cancers but not in normal tissue, and has been reported to correlate with poor prognosis. An attractive therapeutic target, Globo H-targeted agents are being tested in early clinical trials (e.g., OBI-833, a Glo- bo H antigen conjugated to a mutated diphtheria toxin with potential antineoplastic activities, and OBI- 999, an antibody-drug conjugate (ADC) consisting of a Globo H monoclonal antibody with a synthetic antineoplastic agent). We aim to describe the molecular features associated with Globo H expression in CRC. Methods: A total of 7,604 CRC tumors were tested by Caris Life Sciences (Phoenix, AZ) by Next- Gen DNA and RNA sequencing. The expression of b3GalT5, FUT-1 and FUT-2 were evaluated as surro- gates for Globo H expression as they are the key enzymes in its biosynthesis. An average z-score of the 3 genes (GloboH) and of b3GalT5 (B3) alone were calculated; tumors with top quartile z-scores were considered expression-high (Q4) and bottom quartile, expression-low (Q1). QuantiSEQ was used to assess immune cell infiltration in the tumor microenvironment (TME). Statistical significance was determined using chi-square/Fisher-Exact and adjusted for multiple comparisons (q<0.05). Consensus molecular subtype (CMS) was developed using RNA seq data. Results: When the 3 genes were considered, Glo- boH-H tumors showed higher prevalence of CMS1 and CMS4 (23.8% vs. 12%; 38.7% vs. 29.4%) and lower prevalence of CMS2 (40% vs. 18.7%) compared to GloboH-L. Similar patterns of CMS distribution were seen for B3 alone. B3-H tumors were significantly more frequently TMB-H (>=10) (11.4% vs. 8.3%), PD-L1 positive (5.7% vs. 3.4%) and MSI-H/dMMR (8.3% vs. 5.5%). Strong positive associations were seen with mutations in BRAF, KRAS, RSPO3 fusion, and cMYC amplification with B3 alone and GloboH (all q<0.05). Anti-tumor CD4+ T cells and NK cells were increased in the TME with increased expression of GloboH and B3 (q<0.05). However, immune suppressive neutrophils and Tregs were also increased. Dendritic cells were negatively associated with B3 expression while endothelial cells and fibroblasts showed a positive association with GloboH and B3. Conclusions: The association with TMB-H, MSI-H, and PD-L1 status suggests that in some tumors Globo H may be a promising target for combination therapy with immune checkpoint inhibition. The association with different cell populations suggests manipulating the cellular balance in the TME as an approach to improve the efficacy of treatment. NK cell checkpoint inhibitors are in clinical trials and might be utilized in high Globo H cancers; treatments inducing DCs in tumors have been shown to enhance responses to BRAF and PD-L1 blockade and might be applicable in the context of Globo H immunotherapy to overcome Treg immune suppression. Anti-Globo H vaccines and ADCs might be particularly effective in BRAF and KRAS-mutant CRC patients. Research Sponsor: None.

3528 Poster Session

Use of circulating tumor DNA in colorectal cancer patients to assess tumor burden and response to therapy: An observational study.

Erin L. Symonds, Susanne Kartin Pedersen, Bernita Hui Li Yeo, Hiba Al Naji, Susan E. Byrne, Amitesh Chandra Roy, Graeme P. Young; Flinders University, Adelaide, SA, Australia; Clinical Geno- mics Pty Ltd, Sydney, NSW, Australia; Flinders Medical Centre, Flinders University, Adelaide, SA, Australia

Background: Residual disease after treatment for colorectal cancer (CRC) poses a risk for recurrence but imaging and CEA are limited in their capacity to detect residual disease. A simple test is needed for as- sessing treatment response. This study determined whether levels of methylated BCAT1/IKZF1 DNA in blood correlate with tumor burden and whether post-treatment levels inform efficacy of different treatments for CRC. Methods: Patients with primary CRC had blood collected prior to treatment (n = 282, 59.9% males, median age 68.5y). Cell free DNA (cfDNA) was extracted from plasma and assayed for methylation in BCAT1 and IKZF1. Detection of methylation in either gene deemed a sample positive; levels were expressed as %methylation (average methylation/average cfDNA). Positive patients had additional samples collected post-treatment for early stage CRC (surgery, n = 31), advanced/metastatic CRC (surgery + adjuvant chemotherapy, n = 15), and rectal cancer (neoadjuvant therapy, surgery +/- chemotherapy, n = 6), or following mid-therapy suspension of treatment in advanced CRC (n = 24). Tu- mor size was expressed as the maximum diameter of the primary (assessed surgically or by MRI). Re- sults: Pretreatment results increased with CRC stage. Positivity by stage was: I, 23.7% (14/59); II, 62.1% (54/87); III, 68.6% (70/102); IV, 85.3% (29/34). Level by stage: I, 0.0%; II, 0.06%; III, 0.07%; IV, 4.07%, p < 0.001). Pretreatment levels correlated significantly with tumor size (r = 0.372, p < 0.001). Post-treatment blood was collected a median 2.4mo (IQR 1.7-3.9) after therapy completion. Positivity decreased after completing treatment (Table), with 88.4% of cases (46/52) becoming ctDNA negative. All cases with complete treatment had a reduction in biomarker levels, whereas in those with incomplete therapy, 54.5% (12/22) remained positive and the pre- and post-treatment levels were not significantly different. Of those positive after treatment, 13 had a further blood sample: 8 had become ctDNA negative and all but one remained disease free. Five remained positive and all had further suspected or confirmed disease. Conclusions: Levels of methylated BCAT1 and IKZF1 DNA in blood correlated with tumor burden; levels became undetectable in the majority of patients following completion of planned curative intent treatment. The methylated ctDNA blood test aids monitoring of responses to therapy and identification of those cases with residual cancer who might benefit from ongoing therapy. Research Sponsor: Clinical Genomics, NHMRC Australia.

Post-treatment results No. that decreased Treatment No. positive (%) in ctDNA level (%) Surgery (n = 31) 3/31 (9.7%) 31/31 (100%) Surgery and complete adjuvant chemotherapy (n = 15) 3/15 (20.0%) 15/15 (100%) Total neoadjuvant radiotherapy, plus surgery (+/- complete 1/6 (16.7%) 6/6 (100%) adjuvant chemotherapy) (n = 6) Surgery, but incomplete or no adjuvant chemotherapy (n = 22) 12/22 (54.5%) 18/22 (81.8%)

3529 Poster Session

Profiling plasma angiogenesis factors after use of biologics in metastatic colorectal cancer (mCRC): Update results from GI-SCREEN CRC Ukit study.

Yu Sunakawa, Satoshi Yuki, Manabu Shiozawa, Toshiki Masuishi, Tomohiro Nishina, Hisateru Yasui, Takashi Ohta, Naoki Takahashi, Hironaga Satake, Akiyoshi Kanazawa, Masahiro Goto, Hideaki Bando, Hiroya Taniguchi, Yoshinaga Okugawa, Kentaro Yamazaki, Hiromichi Ebi, Yukiko Abe, Shogo Nomura, Chiharu Asano, Takayuki Yoshino; Department of Clinical Oncology, St. Marianna University School of Medicine, Kawasaki, Japan; Department of Gastroenterology and Hepatology, Hokkaido University Hos- pital, Sapporo, Japan; Department of Gastrointestinal Surgery, Kanagawa Cancer Center, Yokohama, Japan; Aichi Cancer Center Hospital, Aichi, Japan; National Hospital Organization Shikoku Cancer Cen- ter, Matsuyama, Japan; Department of Medical Oncology, Kobe City Medical Center General Hospital, Kobe, Japan; Department of Clinical Oncology, Kansai Rosai Hospital, Amagasaki, Japan; Department of Gastroenterology, Saitama Cancer Center, Saitama, Japan; Cancer Treatment Center, Kansai Medical University Hospital, Hirakata, Japan; Department of Surgery, Shimane Prefectural Central Hospital, Izumo, Japan; Cancer Chemotherapy Center, Osaka Medical Collage Hospital, Takatsuki, Japan; De- partment of Clinical Oncology, Aichi Cancer Center Hospital, Nagoya, Japan; Department of Gastroen- terology and Gastrointestinal Oncology, National Cancer Center Hospital East, Kashiwa, Japan; Division of Genomic Medicine, Mie University Hospital, Tsu, Japan; Division of Gastrointestinal Oncology, Shi- zuoka Cancer Center, Shizuoka, Japan; Division of Molecular Therapeutics, Aichi Cancer Center Re- search Institute/ Precision Medicine Center, Aichi Cancer Center, Nagoya, Japan; G&G Science Co., Ltd., Fukushima, Japan; Clinical Research Support Office, National Cancer Center Hospital East, Ka- shiwa, Japan; Clinical Research and Medical Innovation Center, Hokkaido University Hospital, Sapporo, Japan

Background: No predictive biomarkers have been validated to determine which patients (pts) with metastatic colorectal cancer (mCRC) benefit the most from angiogenesis inhibitors. Recent studies suggest that plasma angiogenesis factors and their dynamics may have some prognostic or predictive value. Methods: In this prospective longitudinal study, serial plasma sample collections were done at the time points of pre- and post- treatments in mCRC pts receiving biologics in either first- or second-line chemotherapy (chemo). Comprehen- sive measurements of 17 factors were performed by the multiplex assay with Luminex technology. Statistical analyses were conducted by using the Brunner-Munzel and Jonckheere-Terpstra test. Results: From Sep 2017 to Dec 2020, 789 plasma samples were collected from 498 enrolled pts [first-line chemo plus bevacizumab (BEV, n=102), first-line chemo plus anti-EGFR antibody (aEGFR, n=100), second-line chemo plus BEV (n=100), second-line FOLFIRI plus ramucirumab (RAM, n=99), second-line FOLFIRI plus aflibercept (AFL, n=85) and other treatment (n=7)]. 789 samples were evaluable for this analysis. In the analysis of first-line, level of VEGF-D was significantly higher in both post-BEV and post-aEGFR comparing with pre-first- line [pre-first-line; 264 pg/ml, post-first-line BEV; 354 pg/ml (p<0.001), post-first-line aEGFR; 380.5 pg/ml (p<0.001)], while PlGF was significantly higher only in post-BEV [pre-first-line; 6.7 pg/ml, post-first-line BEV; 23.4 pg/ml (p<0.001), post-first-line EGFR; 7.4 pg/ml (p=0.650)]. These dynamics were also observed in pts with paired samples (Table). In pts with paired samples who received second-line treatment, level of VEGF-A was significantly decreased in post-BEV, while it significantly increased in post-RAM and AFL. A significant elevation of VEGF-D level was observed in only post-RAM. PlGF level significantly increased in post all second-line angiogenesis inhibitors. In the distribution analysis of angiogenesis factors, there were no or weak correlations between VEGF-D and PlGF at the time points of all post-treatments (r = 0.09-0.26). Conclusions: Plasma levels of VEGF-D, VEGF-A and PlGF were independently changed by angiogenesis inhibitors as well as anti-EGFR therapy, suggesting the possibility of usefulness for selecting better biologics by measuring baseline angiogenesis-related factors in first- and second-line chemo. Clinical trial information: UMIN000028616. Research Sponsor: Japan Agency for Medical Research and Development.

First line Chemo plus BEV (n=46) Chemo plus aEGFR (n=34) Median, pg/mL Pre Post p Pre Post p

VEGF-D 263.5 361.0 0.054 341.0 447.0 0.037 PlGF 7.5 21.5 <0.001 8.8 5.9 0.121 VEGF-A 95.1 30.0 <0.001 224.5 107.0 0.009

Second line Chemo plus BEV (n=70) FOLFIRI plus RAM (n=51) FOLFIRI plus AFL (n=45) Median, pg/mL Pre Post p Pre Post p Pre Post p VEGF-D 327.5 342.5 0.362 301.0 732.0 <0.001 342.0 365.0 0.397 PlGF 17.6 21.6 0.006 24.1 158.0 <0.001 18.0 259.0 <0.001 VEGF-A 40.0 30.0 <0.001 30.7 804.0 <0.001 40.0 99.1 0.001

3530 Poster Session

The role of HPSE in BRAF V600E-mutant colorectal cancer.

Mengling Liu, Xiaojing Xu, Ke Peng, Pengcong Hou, Qing Liu, Yiyi Yu, Tianshu Liu; Department of Med- ical Oncology, Zhongshan Hospital, Fudan University, Shanghai, China; Department of Oncology, Zhongshan Hospital, Fudan University, Shanghai, China

Background: BRAF mutations occur in about 10% of colorectal cancer (CRC), more than 90% of which are BRAF V600E mutation. Patients (pts) with BRAF V600E mutation present poor prognosis. Complex molecular biological mechanisms in this population have not been well annotated. HPSE plays a multi- functional role in cell proliferation, invasion and angiogenesis in cancer. Here we identified differentially expressed genes (DEGs) between pts with and without BRAF V600E mutation, and then focused on the function of HPSE, one of top DEGs, in BRAF V600E-mutant CRC. Methods: Clinical and transcriptional data of pts with CRC from The Cancer Genome Atlas (TCGA, n = 525) database and GSE39582 dataset (n = 510) were analyzed to explore the top overlapped DEGs between BRAF V600E mutant and wild-type pts. Records and tumor samples of 172 pts with BRAF V600E-mutant CRC diagnosed at Zhongshan Hospital Fudan University between 6/2015-12/2018 were collected. The HPSE protein expression status of tumor samples was evaluated by immunohistochemistry staining. Moderate or strong staining in > 25% of tumor cells was interpreted as HPSE positive. Overall survival (OS) was analyzed using Kaplan-Meier Curves with Log-rank test and multivariable Cox regression. Next, lentiviral shRNA- based silencing of HPSE was performed in two BRAF V600E-mutant CRC cell lines (HT-29, RKO). The effect of HPSE on tumor growth was investigated through colony formation assays, cell cycle assays and subcutaneous xenograft models. Results: The top overlapped genes of the DEGs list included HPSE, TFF2, AXIN2, MLH1, RNF43, EPM2AIP1. Among them, HPSE had significantly high expression in BRAF V600E-mutant group. Of 172 pts with BRAF V600E mutation, 83 were identified as HPSE positive and 89 were negative. Two groups were generally well balanced on age (p = 0.096), gender (p = 1.000), location (p = 0.658), stage (p = 0.249) and MMR status (p = 0.129). HPSE positive pts had a significantly worse OS in comparison to HPSE negative pts (p = 0.037, median OS not reached). The multivariate analysis showed that HPSE positive was independently associated with inferior OS [HR 1.97 (95%CI: 1.02 – 3.80), p = 0.044). Silencing HPSE gene impaired colony formation activity significantly and arrested more cells in G0/G1 phase of BRAF V600E-mutant CRC cell lines. Tumor growth was inhibited apparently in HPSE-silencing xenograft models. Conclusions: Pts with BRAF V600E-mutant CRC had a high HPSE expression level, while HPSE protein expression was an independent prognostic factor for this population. The silencing of HPSE expression in BRAF V600E-mutant CRC cell lines inhibited cell proliferation and tumor growth in vitro and in vivo. HPSE may contribute to the poor prognosis of BRAF V600E-mutant CRC and might be a promising therapeutic target for this subtype of CRC. Research Sponsor: National Nature Science Foundation of China (81772511, 81602038).

3531 Poster Session

The impact of colorectal cancer screening on incidence and stage IV disease in the Netherlands.

Myrtle F Krul, Marloes AG Elferink, Niels FM Kok, Evelien Dekker, Iris Lansdorp-Vogelaar, Gerrit A. Meijer, Iris D Nagtegaal, Theo JM Ruers, Monique E van Leerdam, Koert FD Kuhlmann; Netherlands Cancer Institute, Amsterdam, Netherlands; Department of Research, Netherlands Comprehensive Can- cer Organisation, Utrecht, Netherlands; Amsterdam University Medical Center, Amsterdam, Nether- lands; Erasmus Medical Center, Rotterdam, Netherlands; Netherlands Cancer Institute, Department of Pathology, Amsterdam, Netherlands; Department of Pathology, Radboud Institute for Molecular Life Sciences, Radboud University Medical Center, Nijmegen, Netherlands

Background: Population-based screening for colorectal cancer (CRC) aims to decrease incidence and mortality due to precancerous polyp removal, early detection and early treatment of CRC. In the Nether- lands, phased introduction of a biennial fecal immunochemical hemoglobin test started in 2014 for individuals aged 55-75. This evaluation of the national data focuses on the initial effect of CRC screening on incidence and stage distribution and the impact on stage IV disease. Methods: All CRC patients diagnosed in the Netherlands between 2009 and 2018 were selected from the Netherlands Cancer Registry (NCR). Patients were linked to the Dutch national pathology registry (PALGA) to identify screen-detected tumors. Results: The NCR identified 137,717 CRC patients between 2009 and 2018. The incidence within screening age (55-75 yr) of all CRC stages showed an initial peak after introduction of screening in 2014, followed by a continuous decrease for all stages. CRC incidence outside the screening age did not show these explicit changes between 2009 and 2018. In 2018, the incidence of stage IV CRC within screening age was lower than the level at the start of the screening program. Stage distribution within screening age shifted towards earlier stages in the screening period (2014-2018) compared to the period before screening (2009-2012) (stage I: 31% vs. 18%, stage II: 22% vs. 26%, stage III: 29% vs. 31%, Stage IV: 18% vs. 25%, respectively). In the period 2014-2018 and within screening age, the ratio of screen-detected and symptom-detected tumors was highest in stage I (47%:53%) and lowest in stage IV (9%:91%). Screen-detected compared to symptom-detected stage IV patients diagnosed in the period 2014-2018 and within screening age had more frequently single organ metastases (74.5% vs 57.4%, p < 0.001), higher resection rate of the primary tumor (57.5% vs. 41.3%; p < 0.001) and higher local treatment rate of metastases (40.0% vs. 23.4% p < 0.001). The median overall survival of screen-detected stage IV patients was significantly longer than that of symptom-detected stage IV patients (31.0 months (95% CI: 27.7 – 34.3) vs. 15.0 months (95% CI: 14.5 – 15.5), p < 0.001). Conclusions: The initial results of the introduction of CRC screening in the Nether- lands showed a favorable trend on CRC incidence and stage distribution. Screen-detected patients with stage IV disease had less extensive disease, resulting in better treatment options and improved survival. Research Sponsor: None.

3532 Poster Session

Resectability, conversion and resections rates, and outcomes in RAS&BRAF wildtype (wt), RAS mutant (mt) and BRAFmt metastatic colorectal cancer (mCRC) subgroups in the prospective Finnish RAXO-study.

Pia J. Osterlund, Emerik Osterlund, Aki Uutela, Pa€ivi Halonen, Raija S. Kallio, Annika Ålgars, Tapio Salminen, Annamarja Lamminma€ki, Leena-Maija Soveri, Raija Ristama€ki, Kaisa I Lehtoma€ki, Hanna Stedt, Eetu Heerva€, Arno Nordin, Ali Ovissi, Jari Sundstro€m, Lasse Nieminen, Markus J Ma€kinen, Ari Ristima€ki, Helena Isoniemi, The RAXO study group; Tampere University Hospital and University of Tampere, Department of Oncology, Tampere, Finland; Uppsala University and Akademiska Sjukhuset, Department of Immunology, Genetics and Pathology, Uppsala, Sweden; Helsinki University Hospital and University of Helsinki, Department of Transplantation and Liver Surgery, Helsinki, Finland; Helsinki University Hospital and University of Helsinki, Department of Oncology, Helsinki, Finland; Oulu Univer- sity Hospital, Department of Oncology, Oulu, Finland; Turku University Hospital, Department of Oncolo- gy and Radiotherapy, Turku, Finland; Tampere University Hospital and Tampere University, Department of Oncology, Tampere, Finland; Kuopio University Hospital, Department of Oncology, Kuo- pio, Finland; Helsinki University Central Hospital and Hyvinka€a€ Home Care, Department of Palliative Care, Helsinki and Hyvinka€a€, Finland; Turku University Hospital and University of Turku, Department of Oncology, Turku, Finland; Helsinki University Hospital, Department of Radiology, Helsinki, Finland; Turku University Hospital and University of Turku, Department of Pathology, Turku, Finland; University of Oulu and Oulu University Hospital, Department of Pathology, Oulu, Finland; Helsinki University Cen- tral Hospital and University of Helsinki and Genome-Scale Biology, Research Programs Unit, University of Helsinki, Helsinki, Finland; Helsinki University Central Hospital and University of Helsinki, Depart- ment Transplantation and Liver Surgery, Helsinki, Finland

Background: Outcomes of metastasectomy varies with RAS and BRAF-status, but the effect on resectability, conversion and resection rates has not been extensively studied. Methods: The prospective Finn- ish RAXO study (NCT01531621) included 1086 patients 2011-2018 (Osterlund et al TLRHE 2021, Isoniemi et al BJS 2021) of which 906 were included in this secondary endpoint analysis. Excluded had missing KRAS/NRAS/BRAF-V600E test, were untreatable or had an atypical BRAF mutation. We studied repeated centralized resectability assessment, conversion and resectability rates in mCRC, and overall survival (OS) after resection and/or local ablative therapy (LAT) and systemic therapy. Results: In- cluded were 289 RAS&BRAFwt, 529 RASmt (overrepresented) and 88 BRAFmt, with median age 65.8/66.1/66.9 years. Demographics per RAS&BRAFwt, RASmt and BRAFmt showed significant differences in male proportion (68/61/39%), ECOG PS 2-3 groups (16/14/25%), primary tumour location (right colon 16/30/69%, left colon 47/34/17%, rectum 38/36/14%), but not for Charlson comorbidity index, BMI, resection of primary, synchronous presentation or adjuvant therapy (Bonferroni corrected Chi-square). Metastatic profile was different for liver (78/74/61% per RAS&BRAFwt, RASmt and BRAFmt), lung (24/35/28%) and peritoneal (15/15/32%) metastases, but not for lymph nodes or other sites, nor for number of metastatic sites (1 in 53/54/52%). Upfront resectability rates were different with 32/29/15% for RAS&BRAFwt, RASmt and BRAFmt, respectively, as were conversion rates with 16/13/7%, and resection/LAT rates with 45/37/17%, respectively. Kaplan-Meier median OS estimate in R0/1/2-resected and/or LAT group (n = 342) was 83/69/30 months for RAS&BRAFwt, RASmt and BRAFmt groups, respectively and 5-year OS-rates 67/60/24%, with Cox HR ref/1.53 (95% CI 1.04- 2.25)/3.11 (1.49-6.49). In the systemic therapy only (n = 564) OS was 29/21/15 months and 5-year OS-rates 11/6/2% respectively, with HR ref/1.43 (1.15-1.76)/2.34 (1.73-3.17). Resection/LAT patients had improved OS over systemic therapy only patients in all subgroups, HR 5.74 (3.90-8.44)/ 5.06 (3.92-6.55)/2.89 (1.43-5.86). Conclusions: There were significant differences in resectability, conversion and resection/LAT rates according to RAS&BRAFwt, RASmt and BRAFmt status. OS was also significantly longer for RAS&BRAFwt versus either mutant. Resected/LAT had better OS than systemic therapy alone patients in all subgroups. Clinical trial information: NCT01531621. Research Sponsor: The Finnish Cancer Foundation, Finska Lakaresallskapet, Competitive state research Helsinki, Tampere, Turku, Oulu, Kuopio and Satakunta hospitals, Research funds at Tampere and Helsinki Uni- versity Hospitals, Pharmaceutical/Biotech Company.

3533 Poster Session

A model combing an immune-related genes signature and an extracellular matrix-related genes signature in predicting prognosis of left- and right-sided colon cancer.

Min-Er Zhong, Du Cai, Dejun Fan, Wei Wang, Cheng-Hang Li, Ze-Ping Huang, Qiqi Zhu, Min-Yi Lv, Chuling Hu, Xiaojian Wu, Feng Gao; Department of Colorectal Surgery, The Sixth Affiliated Hospital, Sun Yat-sen University, Guangzhou, China; Department of Gastrointestinal Endoscopy, The Sixth Affili- ated Hospital, Sun Yat-sen University, Guangzhou, China; Biomedical Big Data Center, Department of Gynecology, Huzhou Maternity & Child Health Care Hospital, Huzhou, China

Background: Primary tumor sidedness has been found to be prognostic in colorectal cancer (CRC), with right-sided colon cancer (RCC) having a worse survival than left-sided tumors (LCC), even after control- ling for known negative prognostic factors. Our previous proteomic study identified differences in protein profiles between LCC and RCC. Immune-related proteins were found to be up-regulated in LCC while the differentially expressed proteins in RCC were mainly enriched in extracellular matrix-related proteins. Herein we aim to construct a prognostic prediction model for LCC and RCC patients by using immune-related genes (IRGs) and extracellular matrix-related genes (ECMGs). Methods: A total of 1,868 CRC patients with complete follow-up data from 1 training cohort (n = 562) and 3 independent validation cohorts (n = 622, n = 403, n = 281, respectively) were enrolled in our study. Tumors located in the splenic flexure, descending colon, sigmoid colon, and rectum are defined as LCC. In contrast, tumors located in the region from the hepatic flexure to the cecum are defined as RCC. The Least Absolute Shrinkage and Selection Operator (LASSO) algorithm was used to construct the multi-gene signatures. Univariate and multivariate analyses were used to test the prognostic value of these models. Results: Our biomarker discovery effort identified a 9-gene IRGs signature that significantly associated with poor DFS for LCC (HR = 3.46, 95%CI = 2.38-5.01, P < 0.001) and a 21-gene ECMGs signature associated with prognosis for RCC (HR = 4.53, 95%CI = 2.84-7.22, P < 0.001). For LCC, the IRGs signature was significantly correlated with worse prognosis in three independent validation cohort (Validation-1 cohort: HR = 2.08, 95%CI = 1.41-3.09, P < 0.001; Validation-2 cohort: HR = 2.19, 95%CI = 1.26- 3.81, P = 0.004; Validation-3 cohort: HR = 2.94, 95%CI = 1.53-5.63, P < 0.001). Similarly, the ECMGs signature also robustly predicted survival for RCC in three independent validation (Validation-1 cohort: HR = 1.86, 95%CI = 1.22-2.83, P = 0.003; Validation-2 cohort: HR = 1.96, 95%CI = 1.18- 3.26, P = 0.008; Validation-3 cohort: HR = 2.8, 95%CI = 1.27-6.17, P = 0.007). When compared with Oncotype DX, we found IRGs achieved an improved survival correlation in LCC (C-index, validation- 3 cohort: 0.75 vs 0.64) and ECMGs got a better survival correlation in RCC (C-index, validation-3 cohort: 0.74 vs 0.58). Conclusions: Combing a 9-gene IRGs signature for LCC and a 21-gene ECMGs signature for RCC, we established a prognostic model that can robustly stratify CRC patients into high- and low- risk groups of tumor recurrence and predict prognosis. Research Sponsor: National Natural Sci- ence Foundation of China (82002221, FG), Other Foundation.

3534 Poster Session

Automated computed tomography (CT)-derived skeletal muscle mass determination as a significant prognostic factor in colorectal cancer patients using deep neural network model.

Dongjin Seo, Han Sang Kim, Yu Rang Park; Yonsei University College of Medicine, Seoul, South Korea; Yonsei Cancer Center, Seoul, South Korea

Background: Although recent evidence suggests skeletal muscle depletion predicts the survival of patients with cancer, the retrieval and manual measurement of the computed tomography (CT) images hinder clinical application in routine clinical practice. The advent of recent deep learning applications enables highly accurate noninvasive longitudinal evaluation of skeletal muscle mass (SMM) changes. Here, we evaluated the prognostic impact of DNN-measured skeletal muscle changes in colorectal cancer (CRC) patients. Methods: A total of 6,196 newly diagnosed CRC patients were analyzed in the Yonsei Cancer Registry Database between Jan 1, 2010, and Sep 30, 2020. SMM is measured by the Skeletal muscle index (SMI). The formula used was: L3 skeletal muscle cross-sectional area (cm2)/height2 (m2). Patients’ SMI patterns were grouped by difference ratio of initial and last SMI. Patients were also classified by BMI pattern with the result of K-means clustering. Association of baseline SMI, baseline body mass index (BMI), SMI changes, BMI changes, and demographic factors with overall survival (OS) were evaluated. Univariate and multivariate analyses were conducted. Concordance (c) statistics were used to test the predictive accuracy of survival models. Results: Fully automated UNet architecture-based deep learning algorithms were applied for the third lumbar transverse CT detection, skeletal muscle segmentation, and skeletal muscle area quantification in CRC patients undergoing abdominal CT between at the time of diagnosis and one year after the diagnosis. Baseline BMI distribution was 28% obese, 26% overweight, 42% normal weight, and 4% underweight. Patients in all SMI categories varied widely in BMI. Changes in SMI were categorized into three groups: SMI increase (33%), steady (45%), and decrease (22%) group. Similarly, BMI changes were categorized into three groups: BMI increase (24%), BMI stable (57%), and BMI decrease (19%) group. Low baseline SMI, low baseline BMI, SMI decrease, and BMI decrease were independently prognostic of survival. Intriguingly, BMI and SMI changes had a different prognostic impact in men and women. For women, the SMI increase group (hazard ratio [HR], 0.4; 95% CI, 0.3-0.7; P= 0.001) was associated with longer OS, while the SMI decrease group (HR, 1.2; 95% CI, 0.6-2.2; P= 0.619) was not associated with shorter OS, both compared with SMI steady group. Conclusions: Automated CT-derived SMM depletion had a negative prognostic impact independent of BMI and age in CRC patients. A noninvasive automatic deep learning algorithm provides a unique opportunity to apply to routine clinical practice and understand how and when cachexia impacts cancer prognosis. Research Sponsor: A grant of the Korea Health Technology R&D Project through the Korea Health Industry Development Institute (KHIDI), funded by the Ministry of Health & Welfare, Korea.

3535 Poster Session

Genetic variants involved in the cGAS-STING pathway to predict outcome in patients (pts) with metastatic colorectal cancer (mCRC): Data from FIRE-3 and TRIBE trials.

Jingyuan Wang, Yi Xiao, Fotios Loupakis, Sebastian Stintzing, Hiroyuki Arai, Francesca Battaglin, Natsuko Kawanishi, Priya Jayachandran, Shivani Soni, Wu Zhang, Christoph Mancao, Chiara Cremolini, Volker Heinemann, Alfredo Falcone, Joshua Millstein, Heinz-Josef Lenz; Division of Medical Oncology, USC Norris Comprehensive Cancer Center, Keck School of Medicine, Los Angeles, CA; Department of Preventive Medicine, Norris Comprehensive Cancer Center, Keck School of Medicine, University of Southern California, Los Angeles, CA; Oncology Institute Veneto IOV-IRCCS, Padua, Italy; Medical De- partment, Division of Hematology, Oncology, and Tumor Immunology (CCM), Charite� Universita€tsmedizin Berlin, Berlin, Germany; Chiba Cancer Center, Chiba, Japan; USC Keck School of Medicine, Los Angeles, CA; F. Hoffmann-La Roche Ltd., Basel, Switzerland; Department of Translational Research and New Technologies in Medicine and Surgery, Unit of Medical Oncology 2, Azienda Ospedaliera Uni- versitaria Pisana, Pisa, Italy; University Hospital Munich, LMU Munich, Munich, Germany; Azienda Ospedaliera Universitaria Pisana, Pisa, Italy; Department of Preventive Medicine, USC Norris Compre- hensive Cancer Center, Keck School of Medicine, Los Angeles, CA; USC Norris Comprehensive Cancer Center, Los Angeles, CA

Background: The intracellular DNA sensor stimulator of interferon genes (STING) plays a vital role in anti-tumor immune responses by recognition of self-DNA from tumors and by-products of genomic instability. Activation of STING was reported to enhance cetuximab mediated natural killer cell activation and dendritic cell maturation. Previous reports suggested that polymorphisms of cGAS-STING can affect innate immune response. Therefore, we hypothesized that genetic variants in the cGAS-STING pathway may predict first-line treatment outcome in mCRC pts treated with bevacizumab/cetuximab- based chemotherapy. Methods: Genomic DNA from blood samples of pts enrolled in two independent randomized trials, FIRE-3 (cetuximab arm, n = 129; bevacizumab arm, n = 107) and TRIBE (bevacizumab arm, n = 215), was genotyped through the OncoArray, a customized array manufactured by Illumi- na including approximately 530K SNP markers. The impact on outcome of 7 selected SNPs in 3 genes involved in the STING pathway (cGAS, STING, IFNB1) was analyzed. Results: In the Cetuximab cohort, pts with STING rs1131769 any T allele (N = 29) showed significantly shorter overall survival (36.3 vs 56.07 months) compared to carriers of C/C (N = 95) in both univariate (hazard ratio [HR] = 2.08; 95% confidence interval [CI]: 1.06-4.07; p = 0.003) and multivariate (HR = 2.98; 95%CI 1.35-6.6; p = 0.00848) analysis; Pts carrying IFNB1 rs1051922 any A allele (N = 68) showed significant shorter progression-free survival (10.23 vs 14.1 months) than carriers of G/G (n = 59) in both univariate (HR = 1.87; 95%CI 1.26-2.78; p = 0.00163) and multivariate (HR = 2.03; 95%CI 1.25-3.3; p = 0.004) analysis. No association were observed in the bevacizumab cohort of TRIBE and FIRE-3. Conclusions: Our study demonstrates for the first time that STING and IFNB1 polymorphisms could predict outcomes of Cetuximab-based treatment in mCRC patients; These finding may provide insight for the combination of STING agonist and anti-EGFR treatment in mCRC patients. Research Sponsor: National Cancer Insti- tute (grant number P30CA014089), The Gloria Borges WunderGlo Foundation-The Wunder Project, Dhont Family Foundation, San Pedro Peninsula Cancer Guild, Daniel Butler Research Fund and Call to Cure Fund.

3536 Poster Session

Multimodal circulating tumor DNA (ctDNA) colorectal neoplasia detection assay for asymptomatic and early-stage colorectal cancer (CRC).

Jeeyun Lee, Hee C Kim, Seung Tae Kim, Yupeng He, Paul Sample, Yoshiaki Nakamura, Victoria M. Raymond, Ariel Jaimovich, AmirAli Talasaz; Division of Hematology-Oncology, Department of Medicine, Samsung Medical Center, Sungkyunkwan University School of Medicine, Seoul, South Korea; Depart- ment of Surgery, Samsung Medical Center, Sungkyunkwan University School of Medicine, Seoul, South Korea; Guardant Health, Inc., Redwood City, CA; Department of Gastroenterology and Gastrointestinal Oncology, National Cancer Center Hospital East, Kashiwa, Japan

Background: To improve average risk CRC screening compliance, additional options are needed, especially options that address patient and provider reported barriers such as time and convenience. LU- NAR-2 is a multimodal blood-based colorectal neoplasia detection assay incorporating ctDNA assessment of somatic mutations and tumor derived methylation and fragmentomic patterns, aimed to maximize sensitivity for early stage CRC detection. We evaluated this test in a large patient cohort with newly diagnosed CRC. Methods: Individuals diagnosed with CRC between 2013-2016 consented to provide blood samples prior to surgical resection. Those treated with neoadjuvant chemotherapy were excluded. Isolated plasma samples (median 3mL from EDTA) from 434 individuals were analyzed with LUNAR-2 (Guardant Health, USA) and included in the analysis. Median age at CRC diagnosis was 63 years (range 28 - 89) and 41% were female. Control samples were from 271 age-matched cancer free individuals. ctDNA detected and ctDNA not detected results were generated by a model trained on a separate sample set (N=614) from both cancer free individuals and those with CRC. Calling threshold was determined based on this held-out set to target 90% specificity. ctDNA results and clinical characteristics were correlated. Results: Overall CRC sensitivity was 91% (393/434), with high sensitivity across all stages; 88% Stage I/II, 93% Stage III (Table). Specificity was 94% (255/271). There was no difference in sensitivity when excluding those with early (<45 years) or late (>84 years) onset CRC (90% sensitivity; 388/429; p=0.95; 88% Stage I/II, 93% Stage III). There were no differences in sensitivity for asymptomatic CRC (88%) compared to symptomatic CRC (91%; p=0.4; Table). However, higher cell-free DNA tumor fractions were observed in the symptomatic cohort. Sensitivity for detection of right-sided and left-sided CRC was similar (93% vs. 90%; p=0.5; Table). Conclusions: In this large early-stage CRC cohort, multimodal ctDNA assessment has high sensitivity for CRC detection with high specificity. Equivalent sensitivity in the asymptomatic cohort suggests this test will have clinically meaningful performance in an average risk screening population. A prospective registrational study is ongoing to evaluate the test in an average risk CRC screening cohort. Research Sponsor: Guardant Health.

Cohort demographics. ctDNA Detected Cohort Overall Cohort (N = 393) (N = 434) (N; %)

Stage I / II 239 211; 88% III 195 182; 93% Presentation at CRC diagnosis Asymptomatic 139 123; 88% Symptomatic 233 212; 91% Unknown 62 58; 94% Location of Primary Tumor Right-Sided 83 77; 93% Left-Sided 334 302; 90% Transverse 17 14; 82%

3537 Poster Session

Comprehensive characterization of neurotransmitters and neuronal signaling (NT) pathway alterations in colorectal cancer (CRC).

Francesca Battaglin, Joanne Xiu, Jia Zeng, Yasmine Baca, Priya Jayachandran, Natsuko Kawanishi, Hiroyuki Arai, Krutika Deshpande, Richard M. Goldberg, A. Craig Lockhart, Jimmy J. Hwang, Andreas Seeber, Wu Zhang, Shannon M. Mumenthaler, Anthony Frank Shields, John Marshall, Wolfgang Michael Korn, Josh Neman, Heinz-Josef Lenz; Division of Medical Oncology, USC Norris Com- prehensive Cancer Center, Keck School of Medicine, Los Angeles, CA; Caris Life Sciences, Phoenix, AZ; Departments of Neurological Surgery, Physiology & Neuroscience, Norris Comprehensive Cancer Cen- ter, Keck School of Medicine, University of Southern California, Los Angeles, CA; West Virginia Univer- sity Cancer Institute, Morgantown, WV; University of Miami Sylvester Comprehensive Cancer Center, Miami, FL; Levine Cancer Institute, Charlotte, NC; Department of Internal Medicine V (Hematology and Oncology), Medical University of Innsbruck, Comprehensive Cancer Center Innsbruck, Innsbruck, Aus- tria; Lawrence J. Ellison Institute for Transformative Medicine, Norris Comprehensive Cancer Center, Keck School of Medicine, University of Southern California, Los Angeles, CA; Karmanos Cancer Insti- tute, Wayne State University, Detroit, MI; Ruesch Center for the Cure of Gastrointestinal Cancers, Lom- bardi Comprehensive Cancer Center, Georgetown University, Washington, DC

Background: Aberrant NT signaling has been shown to activate uncontrolled proliferation and dissemi- nation in several gastrointestinal cancer types. Neurotransmitters have been shown to affect endothelial cells and immune cells in the tumor microenvironment to promote tumor progression. We previously showed that single nucleotide polymorphisms in the dopamine and GABA pathways are associated with outcome in patients with metastatic CRC receiving first-line treatment. Here we further evaluated the distribution and molecular context of NT pathway alterations in CRC. Methods: A total of 7,595 CRC tumors tested at Caris Life Sciences (Phoenix, AZ) with NextGen Sequencing on DNA (Next Seq, 592 genes or NovaSeq, WES) and RNA (NovaSeq, WTS) were analyzed. ssGSEA (single-sample gene set enrichment analysis) was used to calculate pathway enrichment scores (ES) of 7 NT gene sets (GABA, nic- otinic, muscarinic, dopamine (DA), reelin, glial cell line-derived neurotrophic factor and neurotrophins). X2/Fisher-Exact was used for comparison and significance was determined as p-value adjusted for multiple comparison of (q) < 0.05. Results: ES based on sample sites showed a substantial heterogeneity in NT enrichment. Notably, when compared to primary tumors, all 7 gene sets were significantly enriched in brain metastases (mets; ES ratio 1.14-1.55), while abdomen, liver, and peritoneal mets displayed significant decreases in most NT gene sets. DA was enriched in ovarian and lung mets (ES ratio: 1.18 and 1.09, respectively), the latter also showing increased neurotrophins ES (1.06) (all q < 0.05). When investigating primary tumors grouped according to overall ES by unsupervised clustering, right-sided and CMS4 CRCs were more prevalent in the high ES cluster compared to the low ES cluster (32 vs 29%, P = 0.02 and 46 vs 30%, P < 0.001, respectively). In addition, tumors in the high ES cluster showed lower prevalence of TMB-H ($ 10mt/MB) (7 vs 10%), MSI-H (6 vs 10%) and PD-L1 (2 vs 6%), while higher CNA rates were noted in 9 genes (all q < 0.05). High ES tumors showed significant positive associations with microenvironment infiltration of B cells, T cells (NK, CD4+ and CD8+ T cells, but not Treg), M2 Macrophages, Myeloid Dendritic Cell, Neutrophils, and an inverse association with M1 Macrophages, regardless of MSI status (q < 0.05). Conclusions: This is the first and most extensive molecular profiling study to investigate NT signaling pathway alterations in CRC. Our data show a distinct distribution of pathway enrichment according to metastatic site, distinct molecular features in high vs low ES clusters in primary tumors (including CMS subtypes, TMB, MSI and PD-L1 rates), and differential immune cell infiltration. These findings support the role of NT signaling in the metastatic spread of CRC and modulation of tumor immune microenvironment. Research Sponsor: NCI P30CA014089, Gloria Borges WunderGlo Foundation, Dhont Family Foundation, Victoria and Philip Wilson Research Fund, San Pedro Peninsula Cancer Guild, Daniel Butler Research Fund.

3538 Poster Session

KMT2C as a positive predictor for treatment of immune checkpoint inhibitor and correlation with immune infiltrates in colorectal cancer (CRC).

Ling Zhang, Jianping Song, Yiting Wang, Yaoxu Chen; The First Affiliated Hospital of Nanchang Univer- sity, Nanchang, China; First Hospital of Nanchang City, Nanchang, China; The Medical Department, 3D Medicines Inc., Shanghai, China

Background: Lysine Methyltransferase 2C (KMT2C), a member of the myeloid/lymphoid or mixed-line- age leukemia (MLL) family, possesses histone methylation activity and is involved in transcriptional co- activation. Present study has shown that KMT2C is positive correlated with better efficacy of Immune checkpoint inhibitor (ICI) in NSCLC. However, the role of KMT2C in treatment of ICI on colorectal cancer (CRC) is still unknown. Methods: NGS (Next Generation Sequencing) was performed on 1628 CRC patients. TMB of these patients were analyzed. A public accessible cohort (Samstein2018) with data from 130 CRC patients were used to investigate the correlation between KMT2C mutation and efficacy of ICI. WES and survival data of TCGA database (1099 CRC) was used to analyze prognostic effect of KMT2C mutation. Furthermore, CIBERSORT was used to analyze the tumor-infiltrating immune cells present in COAD（colon adenocarcinoma, 404 patients）from TCGA database. Results: Among 1628 CRC patient, 230(14.1%) had KMT2C mutation. TMB was positive correlated with KMT2C mutation (Mut vs. WT, 30.75 vs. 7.26 mut/Mb, p < 0.0001). The Samstein2018 cohort showed that KMT2C mutations (15.4%, 20/130) were significantly associated with better OS (Mut vs. WT, 11.5 vs. 7.5 month, HR = 0.29; 95% CI, 0.1-0.81; P = 0.012), and a higher TMB was also observed in KMT2C-Mut group (p = 1.98e-08). In TCGA, no association between KMT2C mutation and OS was observed (P = 0.23), suggesting that was not prognostic factor. Moreover, we analyzed the relationship between KMT2C mutation and immune cell infiltration through CRC TCGA database. The results showed, in COAD, KMT2C mutation was positively correlated with the abundance of CD8+ T cells (P = 0.0014), B cells (P = 0.014), M1 macrophages (P = 0.015), neutrophil (P = 0.0019) and NK cells (P = 0.043), and negatively correlated with Treg cells (p = 0.0063). Conclusions: KMT2C has an impact on the immune microenvironment and may be used as a potential positive predictor for treatment of ICI on CRC patients. The role of KMT2C in immunotherapy warrant further studies. Research Sponsor: None.

3539 Poster Session

Multi-omics analysis to reveal the role of gut microbiome–associated serum metabolites in the detection of colorectal cancer and adenoma.

Feng Chen, Xudong Dai, Wei Cui; State Key Laboratory of Molecular Oncology, Department of Clinical Laboratory, National Cancer Center/National Clinical Research Center for Cancer/Cancer Hospital, Chi- nese Academy of Medical Sciences and Peking Union Medical College, Beijing, China; Precogify Phar- maceutical Co., Ltd, Beijing, Beijing, China

Background: Gut microbiome and their metabolites have been revealed to be associated with the initiation and progression of colorectal cancer (CRC), and gut microbiome produced or associated metabolites could enter the circulation system. Methods: Integrated analysis of untargeted serum metabolomics by liquid chromatography–mass spectrometry (LS-MS) and metagenome sequencing of fecal samples derived from matched individuals were used to profile serum metabolites that are both significantly affected by CRC and co-related with the gut microbiome. Targeted LC-MS was further used to test the ability of these metabolites for discriminating CRC and adenoma from healthy individuals. Results: More than 300 gut microbiome–associated serum metabolites with significantly altered abundance in both colorectal carcinoma (CRC) and adenoma patients have been identified. A panel of eight gut microbiome–associated serum metabolites (GMSM panel) was established and accurately discriminated CRC and adenoma from normal population. The GMSM panel–based CRC and colorectal adenoma prediction model yielded an area under the curve (AUC) of 0.94 (95% confidence interval: 0.90–0.99) and AUC of 0.91 (sensitivity 82%, specificity 91%) in the training and validation set respectively. This GMSM model shows significantly superior performance to the clinical marker carcinoembryonic antigen (CEA) (AUC 0.72), and more importantly, it suggests promising diagnostic potential in detecting adenoma (AUC = 0.81) and early-stage CRC (AUC = 0.91). Conclusions: Our results indicate that gut microbiome reprogramming in CRC patients is associated with the alterations of the serum metabolome, and these gut microbiome–associated serum metabolites has potential application for the detection of earlier CRC and adenoma. Research Sponsor: CAMS Innovation Fund for Medical Sciences (CIFMS), Other Foundation, Pharmaceutical/Biotech Company.

3540 Poster Session

Serial circulating tumor DNA analysis to assess recurrence risk, benefit of adjuvant therapy, growth rate and early relapse detection in stage III colorectal cancer patients.

Tenna V Vesterman Henriksen, Noelia Tarazona, Amanda Frydendahl, Thomas Reinert, Juan Antonio Carbonell-Asins, Shruti Sharma, Derrick Renner, Desamparados Roda, Marisol Huerta, Susana Rosello�, Kåre Andersson Gotschalck, Lene H. Iversen, Uffe S. Lve, Ole Thorlacius-Ussing, Himanshu Sethi, Alexey Aleshin, Andres Cervantes, Claus Lindbjerg Andersen; Department of Molecular Medicine, Aar- hus University Hospital, Aarhus, Denmark; Biomedical Research Institute INCLIVA, University of Valen- cia, Valencia, Spain; Department of Molecular and Clinical Medicine, Aarhus University Hospital, Aarhus, Denmark; Department of Medical Oncology, INCLIVA Biomedical Research Institute, University of Valencia, Instituto de Salud Carlos III, CIBERONC, Valencia, Spain; Natera, Inc., San Carlos, CA; De- partment of Medical Oncology, INCLIVA Biomedical Research Institute, University of Valencia, Valen- cia, Spain; Department of Surgery, Regional Hospital Randers, Randers, Denmark; Department of Surgery, Aarhus University Hospital, Aarhus, Denmark; Department of Surgery, Regionshospitalet Vi- borg, Viborg, Denmark; Department of Gastrointestinal Surgery, Aalborg University Hospital, Aalborg, Denmark

Background: Challenges in the postoperative management of stage III colorectal cancer include: 1) selection of high-risk patients for adjuvant chemotherapy (ACT), 2) lack of markers to assess ACT efficacy, 3) assessment of recurrence risk after ACT, and 4) lack of markers to guide treatment decisions for high-risk patients e.g. additional therapy or intensified surveillance. Circulating tumor DNA (ctDNA) is a promising marker with potential to mitigate the challenges. Here we used serial ctDNA measurements to assess the correlation between recurrence and ctDNA detection: postoperative, during and after ACT, and during surveillance; and to assess growth rates of metachronous metastases. Uniquely, we also used concurrent CT scans and ctDNA measurements to compare the sensitivity for detecting recurrence. Methods: Stage III CRC patients treated with curative intent at Danish and Spanish hospitals in 2014-2019 were recruited (n = 166). Blood samples (n = 1227) were collected prior to and immediately after surgery, and every third month for up to 36 months. Per patient 16 personal mutations were used to quantify plasma ctDNA (Signatera, bespoke mPCR NGS assay). Results: Detection of ctDNA was a strong recurrence predictor, both postoperatively (HR 7.2, 95% CI 3.8-13.8, P< 0.001), directly after ACT (HR = 18.2, 95% CI 7.1-46, P < 0.001), and when measured serially after end of treatment (HR = 41, 95% CI 16-100, P < 0.001). The recurrence rate of postoperative ctDNA positive patients treated with ACT was 80% (16/20). Patients who stayed ctDNA positive during ACT all recurred. Serial post-treatment ctDNA measurements revealed exponential growth for all recurrence patients following either a SLOW (26%-increase/month) or a FAST (126%-increase/month) pattern (P < 0.001). From ctDNA detection to radiologic recurrence, ctDNA levels of FAST patients increased by a median 117- fold, and up to 554-fold. The 3-year overall survival was 43% for FAST patients and 100% for SLOW and non-recurrence patients (HR = 41.3, 95% CI 7.5-228, P < 0.001). Coinciding CT scans and ctDNA measurements (n = 113 patients, 235 coinciding events, median 2 per patient) showed a high agreement (92%) and ctDNA either detected residual disease before the CT scan (n = 7 patients) or at the same time (n = 14 patients). The median lead-time was 7.5 months. Conclusions: The study confirmed the prognostic power of serial postoperative ctDNA analysis. Moreover, it provided novel analyses demonstrating that ctDNA is more sensitive for recurrence detection than CT scans and can be used for tumor growth rate assessments. The difference between FAST and SLOW growing tumors suggest that growth rates could guide whom to start on systemic therapy rapidly and whom to send for diagnostic imaging. Altogether, the study highlights many potential utilities of ctDNA in guiding clinical decision- making. Research Sponsor: Danish Cancer Society, Other Foundation, Pharmaceutical/Biotech Company.

3541 Poster Session

The effect of primary tumor location on second- or later-line treatment with anti-EGFR antibodies in patients with metastatic colorectal cancer: A single-center cohort study.

Anita Archwamety, Naravat Poungvarin, Charuwan Akewanlop, Krittiya Korphaisarn; Division of Medical Oncology, Faculty of Medicine Siriraj Hospital, Bangkok, Thailand; Department of Clinical Pathology, Faculty of Medicine Siriraj Hospital, Mahidol University, Bangkok, Thailand; Faculty of Medicine Siriraj Hospital, Mahidol University, Bangkok, Thailand; Faculty of Medicine Siriraj Hospital, Mahidol Univer- sity, Bangkoknoi, Thailand

Background: The guideline recommends anti-EGFR monoclonal antibodies (anti-EGFR Ab) as first-line treatment only for patients with left-sided RAS wild type (RASwt) metastatic colorectal cancer (mCRC). However, there are no recommendations on tumor sidedness in subsequent lines. This study aimed to evaluate the effect of primary tumor location on second- or later-line treatment outcomes in patients with KRASwt mCRC. Methods: Medical records of patients diagnosed with mCRC at Siriraj Hospital between 2008 and 2019 were retrospectively reviewed. Patients with KRASwt who received anti-EGFR Ab in second- or later-line treatment were included. The impact of tumor sidedness on progression-free survival (PFS) was determined using the Kaplan-Meier method and compared using the log-rank test. Results: Of 671 patients who had data on KRAS analysis, 396 patients (59%) had KRASwt. Of these, 210 patients received anti-EGFR Ab in second- or later-line treatment. Twenty-nine percent of patients (60 out of 210) had extended RAS analysis. Thirty patients (14%) had right-sided tumors, while 180 patients (86%) had left-sided tumors. Sixty-nine percent of patients (146 of 210) were treated in the third line, while 19% and 12% were treated in the second and the fourth line, respectively. Single- agent irinotecan was the most commonly used chemotherapy backbone (92%). Patients with right- sided tumors had non-significantly inferior PFS compared with patients with left-sided tumors (median PFS was 4.7 months, 95% CI 0.8–8.7 vs. 6 months, 95% CI 4.6–7.3; p = 0.55). Subgroup analysis on the impact of primary tumor location showed no difference in PFS when stratified by treatment lines. Conclusions: This study demonstrated that tumor sidedness has no impact on treatment outcomes in patients treated with anti-EGFR Ab in second- or later-line treatment. Therefore, there is not enough evidence to use tumor sidedness for treatment selection in these settings. A multi-center retrospective study is ongoing. Research Sponsor: None.

3542 Poster Session

Multi-omics characterization of left-right colorectal cancer.

John Marshall, Takayuki Yoshino, Sun Young Rha, David N. Church, Anelisa Kruschewsky Coutinho, Carlos Alberto Sampaio-Filho, David James Gallagher, Jesu�s Garc�ıa-Foncillas, Silvia von der Heyde, Hartmut Juhl, Jonathan Woodsmith, David J. Kerr; Georgetown University, Washington, DC; National Cancer Center Hospital East, Kashiwa, Japan; Division of Medical Oncology, Department of Internal Medicine, Yonsei Cancer Center, Yonsei University College of Medicine, Seoul, South Korea; University of Oxford, Oxford, United Kingdom; Clinica AMO, Salvador, Brazil; Cl�ınica AMO, Salvador, Brazil; St. James’ Hospital, Dublin, Ireland; Oncology Department and Translational Oncology Division, University Hospital Fundacion Jimenez Diaz, Madrid, Spain; Indivumed, Hamburg, Germany; Indivumed GmbH, Hamburg, Germany

Background: Right (R) vs left (L) sided colorectal cancers are clinically distinguishable based on prognosis and response to certain therapies, but as of yet, limited data have emerged to explain these differences. The science of molecular testing has evolved rapidly. Enabled by improved technologies and computing power, it is now feasible to obtain to systematic multi-omic datasets covering DNA, RNA, proteins, phospho-proteins and metabolomics on large numbers of patients. Multi-omic analysis can further define disease specific subgroups but pre-analytic quality of the tissues (ischemia time) and comparison to normal tissue controls is paramount to optimize results. Methods: Following informed consent, 450 colorectal cancer primary tumors and paired normal tissues were collected following an SOP to minimize ischemia time, and were analyzed using comprehensive genomics, transcriptomics, proteomics, phosphoproteomics, morphology and annual clinical information. Right (C18.0,2,3) and left (C18.6,7) CRC tumors, normal tissue were compared using machine learning tools to unravel the molecular mechanisms that underpin these clinically distinguishable phenotypes as well as correlating with known genomic metrics such MSI and KRAS mutation status. Results: Through leveraging the tumor and paired normal patient samples, systematic differences between left and right tumor samples were observed including specific molecular events associated with these anatomical differences. The detailed results will be presented at the meeting. Conclusions: Progress in precision medicine requires the inclusion of multi-omics which in turn requires changes to our current SOPs of tissue collection. The ability to define molecular distinctions such as between R and L colon cancer will permit the rapid discovery of clinically useful prognostic and predictive markers, dramatically adding to our fundamental understanding to colon cancer biology. Future work will focus on the discovery of novel targets and signatures, creating innovative tools that depict multi-omic results for clinicians. Research Sponsor: Indi- vumed GmbH.

3543 Poster Session

Genome-wide analysis indicating cancer associated fibroblast (CAF) impacts on colorectal cancer (CRC) prognosis via immunosuppression.

Yu-feng Chen, Xiaojian Wu, Feng Gao; Department of Colorectal Surgery, The Sixth Affiliated Hospital, Sun Yat-sen University, Guangzhou, China

Background: Cancer associated fibroblast (CAF) in tumor microenvironment is associated with poor prognosis and chemo-resistance in multiple solid tumors, however, there is lack of universal measure of CAF in colorectal cancer (CRC). The aim of this study was to assess fibroblast-signature for predicting outcome and analyze relevant mechanism. Methods: A dataset including 316 CRC patients without adjuvant chemotherapy was used as the discovery cohort for the identification of prognostic fibroblast-related genes (FRGs). A total of 1,352 CRC patients were then divided into one training cohort (n = 461) and two validation cohorts (n = 338, n = 553, respectively) for the construction of fibroblast-related gene signature (FRGS) and the verification of its prognostic value in stage II/III CRC patients. Functional annotation and analysis were performed to reveal the relevant mechanism. Results: A 11-gene signature was derived, which was prognostic for stage II/III CRC patients in two validation cohort (Validation-1 cohort: HR = 1.90, 95%CI = 1.16-3.12, P< 0.01; Validation-2 cohort: HR = 1.95, 95%CI = 1.39-2.73, P< 0.001). High CAF risk was correlated with worse prognosis in CRC patients without adjuvant chemotherapy (HR = 3.63, 95%CI = 2.24-5.88, P< 0.001), but not in patients who received adjuvant chemotherapy (P= 0.154). Similar trends were found in Validation-1 cohort. After integrated with clinical characteristics, FRGS was confirmed as an independent prognostic factor after adjusted for TNM stage of tumor in multivariate analysis (Training cohort: HR = 3.19, 95%CI = 1.88-5.41, P< 0.001; Valida- tion-1 cohort: HR = 5.00, 95%CI = 1.58-15.85, P= 0.007; Validation-2 cohort: HR = 2.99, 95%CI = 1.44-6.21, P= 0.003). Furthermore, enrichment analysis found that anti-tumor immune response was suppressed in the high CAF risk group. Conclusions: The 11-gene FRGS had independent prognostic value for CRC patients, as well as in prediction of benefit from chemotherapy. CAF in tumor microenvironment might impact on the prognosis of CRC patients via inhibiting immune response. Research Sponsor: National Natural Science Foundation of China (NSFC) (No. 82003197).

3544 Poster Session

NTRK fusion positive colorectal cancer as a unique subset of CRC with high tumor mutation burden and microsatellite instability.

Hui WANG, Qiuxiang Ou, Xue Wu, Misako Nagasaka, Sai-Hong Ignatius Ou, Yang Shao; Department of Medical Oncology, Beijing Hospital, National Center of Gerontology; Institute of Geriatric Medicine, Chinese Academy of Medical Sciences, Beijing, China; Nanjing Geneseeq Technology Inc., Nanjing, China; Barbara Ann Karmanos Cancer Institute, Detroit, MI; Chao Family Comprehensive Cancer Cen- ter, University of California Irvine, Orange, CA; Geneseeq Technology Inc, Nanjing, ON, China

Background: Neurotrophin receptor tyrosine kinase (NTRK) gene fusions are rare but actionable oncogenic drivers that are present in a wide variety of solid tumors. This study aims to identify the frequency and the clinicopathologic and genetic features of NTRK-driven colorectal cancers (CRC). Methods: Co- lonic and rectal tumor DNA specimen from colorectal cancer patients submitted for molecular profiling at a CLIA-certified genomics laboratory in China that performed NTRK1/2/3 fusion detection by hybridization-based targeted next generation sequencing (NGS) were retrospectively reviewed. Patients’ demographic, clinical characteristics, and treatment history were retrieved from the database for further evaluation. Results: A total of 2,519 unique Chinese colorectal cancer cases were profiled from April 2016 to May 2020, and 17 NTRK+ fusion events were identified (0.7%, 17/2,519) consisting of 14 cases of NTRK1+ and 3 cases of NTRK3+ fusions. Furthermore, thirteen out of 17 NTRK+ CRC tumors (76%) were microsatellite instability-high (MSI-H) tumors, a much higher rate than that of the molecularly unselected CRC population (8%) or NTRK+ non-CRC tumors ( < 1%). NTRK+ CRC patients also had increased tumor mutation burden (median TMB = 65 mut/MB) compared to that of non-NTRK+ CRC (median TMB = 7.7 mut/MB) or NTRK+ non-CRC tumors (median TMB = 4 mut/MB). POLE/ POLD1 mutations were also enriched in NTRK+ CRC (8/17, 47%) relative to molecularly unstratified CRC patients (8%) with over half carrying concurrent POLE and POLD1 mutations. TPM3 was the most common fusion partner of NTRK1 (78%, N = 14), followed by LMNA and TRP. Three NTRK3+ CRC were identified (ETV6-NTRK3, RUNX1-NTRK3, CSNK1G1-NTRK3). RNF43 (71%) was the most frequently mutated gene and the aberrations of RNF43 and ARID1 were significantly enriched in MSI-positive NTRK+ tumors as compared to the MSS NTRK+ subgroup. TP53 (53%) and APC (35%) aberrations frequently co-occurred with NTRK fusions, whereas the majority of the NTRK+ cohort were RAS/BRAFwildtype, except in one case that an oncogenic KRAS Q61R variant co-occurred with RUNX1-NTRK3. Conclusions: NTRK+ colorectal cancer is rare. In addition to the absence of canonical driver mutations, NTRK+ tumors demonstrated increased tumor mutation burden, higher frequency of microsatellite instability, and an enrichment of POLE/POLD1 mutations relative to molecularly unselected CRC population. Research Sponsor: None.

3545 Poster Session

A panel of DNA methylation markers for the classification of consensus molecular subtypes 2 and 3 in patients with colorectal cancer.

Inge van Den Berg, Marcel Smid, Robert R.J. Coebergh van den Braak, Mark A van de Wiel, Carolien H. M. Van Deurzen, Vanja de Weerd, John W.M. Martens, Jan N.M. IJzermans, Saskia M Wilting; Erasmus MC, Rotterdam, Netherlands; Department of Medical Oncology, Erasmus MC Cancer Institute, Erasmus University Medical Center, Rotterdam, Netherlands; Erasmus University Medical Center, Rotterdam, Netherlands; Department of Epidemiology and Biostatistics and Department of Mathematics, VU University Medical Center, Amsterdam, Netherlands; Department of Pathology, Eras- mus MC, Rotterdam, Netherlands

Background: Consensus molecular subtypes (CMSs) can guide precision treatment of colorectal cancer (CRC). Currently available assays can identify CMS1 and CMS4 cases well, while a dedicated test to distinguish CMS2 and 3 is lacking. This study aimed to identify a panel of methylation markers to distinguish between CMS2 and 3 in patients with CRC. Methods: Fresh-frozen tumor tissue of 239 patients with stage I-III CRC was included. CMS classification was performed on RNA-seq data using the single- sample-prediction parameter from the CMSclassifier package. Methylation profiles were obtained using the Infinium HumanMethylation450 BeadChip. We performed adaptive group-regularised logistic ridge-regression with post-hoc group-weighted elastic net marker selection to build prediction models for classification of CMS2 and CMS3 based on 15, 10 or 5 markers. Data from TCGAwas used for validation. Results: Overall methylation profiles differed between CMS2 and CMS3. Group-regularisation of the probes was done based on their location either relative to a CpG island or relative to a gene present in the CMS classifier resulting in two different prediction models and subsequently different marker panels. For both panels, even when using only 5 markers, sensitivity, specificity, and accuracy were > 90%. Validation showed comparable performances. Conclusions: Our highly sensitive and specific methylation marker panel can be used to distinguish CMS2 and 3. This enables development of a qPCR DNA methylation assay in patients with CRC to provide a specific and non-invasive classification tool. Research Sponsor: None.

3546 Poster Session

Clinical performance of methylation-based liquid biopsy test COLVERA after optimization of test interpretation rules.

Zivjena Vucetic, Naima Loayza, Susanne Kartin Pedersen, Missy Tuck, Lawrence Charles LaPointe; Clinical Genomics, Bridgewater, NJ; Clinical Genomics Technologies Pty Ltd., North Ryde, Australia; Clinical Genomics Technologies Pty Ltd, North Ryde, Australia

Background: Clinical guidelines recommend surveillance for patients who complete primary treatment for colorectal cancer (CRC) with the aim of detecting recurrence when amenable to curative intent treatment. Currently recommended surveillance protocols, including imaging and CEA have limitations both in sensitivity and specificity, thus novel methods that detect circulating tumor DNA (ctDNA) have been introduced into clinical practice. COLVERA is a laboratory-developed, real-time PCR test that detects DNA methylation of BCAT1 and IKZF1 genes. These two genes are hypermethylated in 95% of CRC tissue and COLVERA showed improved sensitivity for detection of recurrent disease in comparison to CEA in several clinical populations. The current study evaluated the impact of optimizing the assay’s qualita- tive reporting method on actionability and clinical performance for recurrence detection in CRC surveillance setting. Methods: Two previously described cohorts of CRC patients (N=322 and N=144) who completed primary treatment and were undergoing surveillance were evaluated. Imaging and blood collections were performed at, or adjacent to, a standard of care visit. cfDNA was extracted from whole blood, bisulphite-treated and assayed in triplicates for BCAT1/IKZF methylation. Previously, any positive replicate of either target gene was reported as COLVERA detected. In the current study, COLVERA is detected when at least one replicate of IKZF1 or multiple replicates of either IKZF1 and/or BCAT1 are present. Sensitivity, specificity, and diagnostic odds ratio (DOR) for CRC recurrence detection from a single time-point blood sample was determined using radiological imaging as clinical reference standard. Results: In the first cohort (N=322), overall COLVERA test positivity was 6.5% (21/322) with a sensitivity of 59.3% (95% CI: 38.8 - 77.6) and specificity of 98.3% (96.1 - 99.5) for detecting recurrence at a time-point adjacent to imaging, representing improved specificity, from 91.5% (87.7 - 94.4%), with minimal decrease in sensitivity, from 63.0% (42.4 – 80.6). Similarly, in the second cohort (N=144) sensitivity was 62% (47.2 -75.4), compared to 66.0% (57.1 – 69.3) under the prior interpretation method, while specificity was 92.6% (85.3-97), compared to 90.4% (84.7 - 94.7). A high DOR of 84 (26 - 272) (previously 18 (7.6 – 44.4)) indicates that the revised COLVERA interpretation method is clinically more informative and differentiates with greater accuracy patients with and without the disease. Conclusions: This change in the COLVERA interpretation rule resulted in optimized clinical specificity with minimal impact on sensitivity. For an assay intended to aid in surveillance and early recurrence detection, improved accuracy allows the physician to have increased confidence in making actionable decisions based on test result, including further imaging or treatment. Research Sponsor: Clinical Genomics Pathology, Inc.

3547 Poster Session

Effect of Medicaid expansion on incidence of early-onset colorectal cancer incidence among Hispanics.

Shafia Rahman, Riya Jayesh Patel, Jianyou Liu, Ana Acuna-Villaorduna, Mimi Kim, Sanjay Goel; Albert Einstein College of Medicine/Montefiore Medical Center, New York, NY; Jacobi Medical Center, Bronx, NY; Department of Epidemiology and Population Heath, Albert Einstein College of Medicine, New York, NY; Montefiore Medical Center/Albert Einstein College of Medicine, Bronx, NY; Department of Epidemi- ology and Population Heath, Albert Einstein College of Medicine, Bronx, NY; Albert Einstein College of Medicine, The Bronx, NY

Background: Early onset colorectal cancer (EO-CRC, age < 50 years) is an emerging public health crisis; especially in Hispanics. Access to healthcare is critical for timely detection and is tied to medical insur- ance. In 2010, the Affordable Care Act allowed for expansion of Medicaid eligibility across the country, however, states were permitted to opt out by the US Supreme Court ruling of 2012, which created an unintended experiment in the healthcare market. We evaluated the effects of Medicaid expansion on the incidence of EO-CRC among Hispanics with the hypothesis that it would lead to an increase in incidence and early detection EO-CRC. Methods: The National Cancer Data Base was used to collect data on newly diagnosed Hispanics with EO-CRC (40-49 years), across all stages, from 2010-2017. Data for 21 expansion states (ES) that expanded Medicaid in 2014, and 16 non expansion (NES) states was analyzed. The yearly state-wise Hispanic population was collected from U.S Census Bureau for 2010-17. Incidence was computed as number of new cases of CRC divided by size of the state’s Hispanic population. Segmented Poisson generalized linear mixed effects model was used to analyze rate of change in yearly incidence of EOCRC before and after 2014, in ES and NES. Results: Average annual incidence (AI) of EO-CRC in Hispanics was 6/100,000 and 8/100,000 pre and post expansion, in ES, and 8/ 100,000 and 9/100,000 pre and post 2014 in the NES, respectively. Increase in AI of EO-CRC was 3.6% per year (2010-14) (95% CI: -0.1% to 7.4%), and 9.8% (2014-17) (95% CI: 5.2% to 14.7%) for ES states; and 6.4% (2010-14) (95% CI: 2.1% to 10.8%), and 1% (2014-17) (95% CI: -3.8% to 6.1%) in NES. ES showed greater change in EO-CRC incidence post expansion (2014) vs. pre-expansion, as compared to NES (p=0.078) table. There was no difference in stage at diagnosis between pre- and pos- expansion periods between ES and NES. Conclusions: Increase in incidence of EO-CRC in ES is likely due to greater access to health care due to Medicaid coverage as compared to NES. Other potential factor is migration of Medicaid eligible persons from NES to ES. However we need data past 2017 to confirm the current trend. Research Sponsor: None.

Rate of change in yearly incidence of early onset colorectal cancer. Pre- 2014 Post 2014 p=0.078 Non-expansion states 6.4% per year 1% per year (95% CI: 2.1% to 10.8%) (95% CI: -3.8% to 6.1%) Expansion states 3.6% per year 9.8% per year (95% CI: -0.1% to 7.4%) (95% CI: 5.2% to 14.7%)

3548 Poster Session

PMC: A more precise classifier of POLE mutations to identify candidates for immune therapy.

Fadl Zeineddine, Benjamin Garmezy, Timothy A. Yap, John Paul Y.C. Shen; University of Texas MD An- derson Cancer Center, Houston, TX; The University of Texas MD Anderson Cancer Center, Houston, TX; UCSD Moores Cancer Center, La Jolla, CA

Background: Specific somatic mutations in DNA polymerase epsilon (POLE) can cause a hypermutant phenotype with tumor mutation burden (TMB) in excess of 100 mutations per megabase. It has been reported that POLE mutant tumors are enriched in response to immune therapy and this association is being tested in multiple active clinical trials. However, most POLE mutations are passenger mutations and have no pathogenic role. Current methods to classify POLE mutations are limited in both accuracy and completeness, which could lead to inappropriate use of immune agents in tumor such as MSS CRC, where response rate is 5% or less. Here we present a new classifier, POLE Mutation Classifier or PMC, based on the unique trinucleotide mutation signature caused by selective loss of the proofreading function (LOP) of POLE. Methods: cBioPortal was queried to identify all tumors with POLE mutation. TMB was calculated for each, additionally, trinucleotide mutation signatures were obtained for all POLE mutant tumors in TCGA. Using OncoKB to identify a gold standard of 12 functional POLE mutations (n = 98 tumors) a POLE mutational signature was created. A combination of mutational signature, amino acid location, and TMB was used to classify each POLE variant. Results: Among all 48035 unique tumors the overall frequency of POLE mutations was 2.5% (n = 1184), however only 9.2% (n = 110) were determined to cause the selective LOP. The incidence of LOP POLE mutation was highest in uterine carcinoma and CRC, these tumors also had the highest ratio of LOP to passenger mutations. In a pan-cancer analysis the overall survival of LOP POLE patients was significantly better than those with passenger mutations (not-yet-reached vs. 51 mo, HR = 4.4, p < 0.0001). A similar analysis performed using the polyphen-2 classifier to identify functional POLE mutations did not show a difference in overall survival (HR = 1.0, p-value = 0.57). To further validate the improved specificity of the PMC classifier TMB was used as a surrogate marker, using the PMC classifier 98% of tumors with LOP showed hyper- mutation (TMB > 20mut/Mb), vs. 53% called functional by polyphen-2. A retrospective analysis of MD Anderson CRC patients identified 25 patients with LOP POLE mutation, who had improved OS relative to 267 CRC patients with passenger POLE mutation (not-yet-reached vs. 70 mo, HR:4.2, p = 0.028). Four metastatic CRC patients with LOP POLE mutation were treated with immune therapy (nivolumab, or ipilimumab/nivolumab) in 2nd or 3rd line, all four achieved objective response and remain on therapy (mean time on treatment 15 mo). Conclusions: The PMC classifier specifically identifies mutations in POLE that cause loss of the proofreading function, outperforming both manually curated databases and machine learning-based methods. Clinical trials that use POLE mutation as a selection criteria for immune therapy should be restricted to just those POLE mutations that cause LOP. Research Sponsor: U.S. National Institutes of Health, Other Government Agency, U.S. National Institutes of Health.

3549 Poster Session

Are current family-history based colorectal cancer screening guidelines adequate for early detection and potential prevention of young-onset cases?

Y. Nancy You, Miguel A. Rodriguez-Bigas, George J. Chang, Brian K. Bednarski, John Michael Skibber, Maureen E Mork, Julie B Moskowitz, Sa Thi Nguyen, Selvi Thirumurthi, Patrick M. Lynch, Scott Kopetz, Eduardo Vilar Sanchez; Department of Colon and Rectal Surgery, University of Texas MD Anderson Can- cer Center, Houston, TX; Clinical Cancer Genetics Program, University of Texas MD Anderson Cancer Center, Houston, TX; Department of Gastroenterology, University of Texas MD Anderson Cancer Center, Houston, TX; Department of Gastrointestinal Medical Oncology, University of Texas MD Anderson Can- cer Center, Houston, TX; Departments of Gastrointestinal Medical Oncology and Clinical Cancer Preven- tion, University of Texas MD Anderson Cancer Center, Houston, TX

Background: Strategies to detect and prevent young-onset colorectal cancer (YOCRC, diagnosed under age 50) are critical. Established high-risk screening guidelines (SGs) aim to detect/prevent YOCRCs arising from hereditary syndromes. For non-hereditary YOCRCs, average-risk screening is being considered at an earlier age, but family history (FH)-based increased-risk screening has been poorly studied. We aimed to define the proportion of non-hereditary YOCRC with a FH, and to determine whether existing SGs could have detected/prevented these cases. Methods: 394 consecutive YOCRC patients pre- senting for surgical resection were reviewed for tumor MMR status, pedigree and genetic testing. Those with known/suspected hereditary syndrome (by phenotype, MMR status, and/or germline mutation) were excluded (N = 65). Pedigrees (N = 329) were analyzed for first- or second-degree relatives (FDR, SDR) with CRC and the ages of diagnosis. The gap between the recommended age for FH-based CRC screening and the age of YOCRC diagnosis was calculated. Results: 89 (27%) non-hereditary YOCRC patients had a FH of CRC. The median age of diagnosis was 45; the tumors were mostly from the distal colon (22%) and rectum (60%), and stage III (48%) and IV (27%). Twenty-one (24%) patients had 22 FDRs with CRCs diagnosed at age 64 (median); and 71 (80%) patients had 92 SDRs with CRCs diagnosed at age 65 (median). Thirteen (15%) had a FH of YOCRC. The existing SGs consider 39 patients (44%) at increased-risk, and the remaining, average-risk (Table). Screening would have begun prior to the YOCRC diagnoses in 28 (31% [or 46, 52%]) patients. But YOCRC diagnosis preceded the recommended screening age in the remaining 61(69% [or 43, 48%]) patients by a median of 5.3 [or 3.9] years (Table). Conclusions: FH is found in 27% of the non-hereditary YOCRC patients; 15% has a FH of YOCRC. In nearly half of the patients, YOCRC was diagnosed several years earlier than the recommended age for FH-based screening, even assuming perfect SG adoption and starting average risk screening at age 45. Refining existing FH-based SGs can potentially be impactful. Research Sponsor: University of MD Anderson Cancer Center Clinical Innovator Award.

YOCRCs Diagnosed AFTER YOCRC diagnosed PRECEDING/ YOCRCs Meeting Recommended AT Endorsed Criteria Screening Recommended Screening FH-based Screening Recommendation Criteria (Age at Diagnosis) by (N; % of 89) Age (N; Median Gap) age (N; median gap)

Increased-risk At least 1 39 (44%) 26 13 8.4 years - 3.5 years Start at age 40 y or 10 y before earliest FDR at < 60 or >2 SDR at any NCCN 20 (22%) 16 CRC age MSTF ACS ACG Start at age 40 y FDR at >60 MSTF 16 (18%) 12 ACS Start at age 50 y SDR at < 50 NCCN 3 (4%) 0 Average-risk NCCN 50 (56%) 2 48 Start at age 50 y [or 45y] MSTF 0.6 years -5.9 years ACS [or 20 [or 30 ACG 2.8 years] -4.1 years] y = years; NCCN = National Comprehensive Cancer Network; MSTK = Multi-society Task Force on CRC; ACS = American Cancer Society; ACG = American College of Gastroenterology.

3550 Poster Session

A nationwide analysis of outcomes and healthcare utilization in HIV versus non-HIV patients with colon cancer.

Syed Ali Amir Sherazi, Ikechukwu Achebe, Yaseen Alkaddoumi, Jennifer C Asotibe, Bashar Attar, Shweta Gupta, Michael Russell Mullane; Department of Internal Medicine, John H. Stroger Jr. Hospital of Cook County, Chicago, IL; John H. Stroger, Jr. Hospital of Cook County, Chicago, IL; John H. Stroger, Jr. Hospital of Cook County, Darien, IL; John H. Stroger Jr. Hospital of Cook County, Chicago, IL

Background: The incidence and mortality of Colon cancer (CC) is reportedly similar in HIV and non-HIV populations with same screening guidelines. The pathogenesis of HIV-CC may be multifactorial; related to chronic inflammation from AIDS colopathy, 2-fold increase in risk of polyps, smoking, elevation in proinflammatory cytokines, decrease in adiponectin, activation of b-catenin signaling pathway all of which may promote neoplastic growth of colonic mucosa. With increasing survival in HIV due to effective antiretroviral therapy, non-AIDS defining cancers are rising as population ages. We attempted to compare demographics and outcomes of HIV and non-HIV-CC patients in a national database. Methods: Healthcare Cost and Utilization Project (HCUP) National Inpatient Sample (NIS) was queried to identify all HIV and non-HIV-CC admissions. The groups were compared for socio-demographic differences, medical comorbidities, inpatient mortality, length of stay (LOS) and hospital charges (THC). Secondary outcomes studied included rates of sepsis, septic shock (SS), Anemia, transfusions, GI bleeding, colostomy rates, Acute Kidney Injury (AKI) and protein energy malnutrition (PEM). Statistics were performed using t-test, univariate and multinomial logistic regression. Results: A total 895 HIV-CC and 514,840 non-HIV-CC admissions were identified. HIV-CC were younger (mean age 56.3 vs 67.3 years, p < 0.001) with 76% < 65 years old compared to 40% in non-HIV-CC. HIV-CC were more likely male (75.4% vs 50.5%, p < 0.001), African Americans (AA) (43% vs 14%, p < 0.001) and Hispanic (19% vs 9%, p < 0.001), were more likely from lowest income quartile zip codes (44% vs 28%, p < 0.001) from the Northeast region of US (27% vs 19%, p < 0.001) and on Medicaid (30% vs 10%, p < 0.001). HIV-CC had significantly lower rates of medical comorbidities (hypertension, diabetes, obesity, dyslipidemia, heart failure, all p < 0.05) compared to non-HIV-CC. The odds of adjusted inpatient mortality were significantly lower in HIV-CC (aOR = 0.46 CI = 0.24-0.87, p = 0.018), however HIV-CC had longer mean LOS (8.17 vs 6.66 days, p < 0.01) and higher mean THC ($88,305 vs $76,317, p = 0.051). Cancer pain and PEM were significantly higher in the HIV-CC group, but other secondary outcomes were similar. Conclusions: HIV-CC patients were significantly younger and minorities with significantly lower all-cause mortality compared to non-HIV-CC. The lower mortality may be explained by younger age, treatment of teaching hospitals for HIV-CC and lower incidence of medical comorbidities which may be driving mortality higher in non-HIV-CC. However, healthcare utilization of HIV-CC was higher with over $10 million in extra charges in 3 years compared to non-HIV-CC. The young age of HIV- CC compared to non-HIV suggests a need for studies to evaluate the role of starting colon-cancer screening at a younger age in the HIV population. Research Sponsor: None.

3551 Poster Session

Investigating intra-tumor microbes, blood microbes, and CEA for development of non- invasive biomarkers in colorectal cancer.

Pannaga G. Malalur, Xiaokui Mo, Rebecca Hoyd, John L. Hays, David Paul Carbone, Daniel Spakowicz; The Ohio State University/Wexner Medical Center, Columbus, OH; The Ohio State University, Center for Biostatistics, Columbus, OH; Division of Medical Oncology, Department of Internal Medicine, The Ohio State University College of Medicine, Columbus, OH; The Ohio State University Wexner Medical Center, Columbus, OH; The Ohio State University Comprehensive Cancer Center, Columbus, OH; Division of Medical Oncology, Department of Internal Medicine & Department of Biomedical Informatics, Ohio State University, Columbus, OH

Background: The development of non-invasive biomarkers has the potential to revolutionize clinical care for colorectal cancer (CRC) patients. The presence of bacteria in CRC tumor biopsies has been shown to contribute to CRC development. In a previous study, our group showed some intra-tumor microbes in CRC tumor biopsies correlated with overall survival in CRC patients. However, the correlations between microbes in tumor vs blood, and between non-invasive serum marker carcinoembryonic antigen (CEA) and microbes are unknown. We hypothesize that tumor microbes will also be found in blood, and that CEA will correlate with certain microbes. Methods: We obtained RNA-seq data from CRC tumor biopsies from patients treated at The Ohio State University Comprehensive Cancer Center as part of the Oncology Research Information Exchange Network (ORIEN). Reads were aligned to human and exogenous ge- nomes using TopHat2 and Kraken2/Bracken, respectively. RNA-seq from CRC tumor biopsies as well as peripheral blood at the Cancer Genome Atlas (TCGA) consortium were processed by the same method. Results: The analyzed ORIEN cohort included 93 CRC patients with an age range from 30-83 years, 60.2% male, 87.1% adenocarcinoma, and 47.3% with metastatic CRC. The TGCA cohort included 495 CRC patients with an age range from 31-90 years, 53.3% male, 85.1% adenocarcinoma, and 15.5% with metastatic CRC. Over fifteen exogenous phyla (including bacteria, viruses, fungi) were observed in both ORIEN and TCGA cohorts. Several of the samples were dominated by viral sequences while others by bacteria, suggesting considerable tumor microbiome heterogeneity. Evaluation of the fraction of microbes in tumor and blood showed that nearly all the microbes found in blood (97.6%) were also observable in tumor in the TCGA cohort. Microbial abundances of various taxa, including Fu- sobacterium, significantly correlated between blood and tumor. Several bacteria including members of the genera Bacillus and Staphylococcus were positively associated with tumor stage (metastatic vs non- metastatic), but microbial relative abundances were not correlated with the location of tumor in colon (right, left, transverse colon). Certain microbial species from the ORIEN cohort were found to positively correlate with CEA, (including from the genera Fusobacterium, Lactobacillus, Pseudomonas, Vibrio, Clostridium) and these associations remained when adjusted for alcohol and smoking by multivariate analysis. Conclusions: Nearly all the microbes found in blood were found in tumor and abundances of various taxa were significantly correlated, suggesting that blood-based cancer microbiome analysis has great potential. Serum CEA has a low diagnostic ability when used alone, but combining this with blood microbiome could improve diagnostic/prognostic utility as a non-invasive biomarker. Research Sponsor: Award Number UL1TR002733.

3552 Poster Session

Consensus molecular subtypes and RAS status as biomarker of treatment intensity with fluoropyrimidine, bevacizumab, and irinotecan in metastatic colorectal cancer (XELAVIRI, AIO KRK 0110).

Arndt Stahler, Volker Heinemann, Veronika Schuster, Annabel Helga Sophie Alig, Laura Elisabeth Fischer, Lena Weiss, Kathrin Heinrich, Annika Kurreck, Ivan Jelas, Jobst C. von Einem, Clemens Giessen-Jung, Ludwig Fischer von Weikersthal, Ursula Vehling-Kaiser, Thomas Decker, Jens Neumann, Thomas Kirchner, Andreas Jung, Joerg Kumbrink, Sebastian Stintzing, Dominik Paul Modest; Medical Department, Division of Hematology, Oncology and Tumor Immunology (CCM), Charite� Universitaets- medizin Berlin, Berlin, Germany; University Hospital Munich, LMU Munich, Munich, Germany; Depart- ment of Medicine III, University Hospital, LMU Munich, Munich, Germany; Medical Department, Divison of Hematology, Oncology, and Tumor Immunology (CCM), Charite� Universitaetsmedizin, Berlin, Germany; Department of Medicine III, University Hospital, LMU Munich, Mu€nchen, Germany, Munich, Germany; Charite� University Medicine Berlin, Berlin, Germany; Charite� Comprehensive Cancer Center, Berlin, Germany; Department of Medicine III, University Hospital, Munich, Germany; Department of Oncology, Gesundheitszentrum St. Marien GmbH, Amberg, Germany; Practice for Medical Oncology, Landshut, Germany; Onkologie Ravensburg, Ravensburg, Germany; Department of Pathology, Universi- ty of Munich, Munich, Germany; DKTK German Cancer Consortium, Heidelberg, Germany; Medical De- partment, Division of Hematology, Oncology, and Tumor Immunology (CCM), Charite� Universita€tsmedizin Berlin, Berlin, Germany

Background: Prognostic biomarkers beside RAS/BRAF status are necessary to identify metastatic colorectal cancer (mCRC) patients who benefit from combined (COMB) versus sequential (SEQ) treatment with fluoropyrimidine, bevacizumab and irinotecan (randomized phase III XELAVIRI trial). Methods: mRNA was extracted from formalin-fixed paraffin embedded (FFPE) tumor tissue of 337 patients, gene expression was measured by the Nanostring PanCancer Progression Panel. Consensus molecular subtypes (CMS) classification was re-derived using a multinomial regression model. Data of Guinney et al. (Nat. Med. 2015. 21:1350-6) and FIRE-3 served as training and validation set. RAS/BRAF MUT were assessed by pyrosequencing. Median overall (OS) and progression free survival (PFS), hazard ratios (HR) and 95% confidence interval (CI) were estimated by Kaplan-Meier method and univariate Cox regression. Results: The multinomial regression model employed in the present analysis correctly predicted CMS labels in 98.3 % of the original Guinney- and 100.0 % of FIRE-3 population. In XELAVIRI, CMS subgroups were predicted as follows: CMS1: n = 62 (18.4 %); CMS2: n = 174 (51.6 %); CMS3: n = 9 (2.7 %); CMS4: n = 92 (27.3 %). A general prognostic impact of CMS was not observed when all patients were analysed. In RAS/BRAF WT mCRC patients, substantial benefit of COMB versus SEQ treatment was shown for OS and PFS in CMS2 and CMS4, but not in CMS1. Conversely, OS was significantly longer for COMB treatment in patients with RAS MUT and CMS1 mCRC, while SEQ treatment was not inferior in RAS MUT and CMS2 or CMS4 subgroups (see TABLE). Additional data for overall response rates, early tumor shrinkage and sidedness might be presented at the meeting. Conclusions: This retrospective analysis of XELAVIRI suggests that CMS may serve as biomarker that predicts response to initially combined versus less intensive sequential chemotherapy in patients with RAS/BRAF WT mCRC. Research Sponsor: Roche.

Outcome of XELAVIRI patients according to molecular subtype (CMS) and stratification for RAS status.

CMS1 CMS2 CMS3* CMS4 n = 56 n = 161 n = 8 n = 87

RAS/BRAFWT RAS MUT RAS/BRAFWT RAS MUT RAS/BRAFWT RAS MUT RAS/BRAFWT RAS MUT n = 26 n = 30 n = 74 n = 87 n = 5 n = 3 n = 37 n = 50

COMB SEQ COMB SEQ COMB SEQ COMB SEQ SEQ SEQ COMB SEQ COMB SEQ n = n = n = n = n = n = n = n = COMB n = COMB n = n = n = n = n = Outcome 13 13 12 18 41 33 42 45 n = 2 3 n = 1 2 17 20 26 24

OS, median 31.4 31.4 21.0 14.1 29.9 27.3 25.3 21.3 n/a 28.5 18.7 19.8 23.5 HR 1.18 0.43 0.56 0.95 0.53 1.24 (95% CI) (0.45 – 3.11) (0.20 – 0.95) (0.33 – 0.96) (0.61 – 1.50) (0.26 – 1.08) (0.66 – 2.33) p 0.74 0.038 0.036 0.84 0.079 0.50 PFS, 12.7 8.5 9.4 6.5 13.4 7.8 10.4 10.3 13.0 8.5 10.6 6.2 median HR 0.71 0.54 0.48 1.15 0.54 0.84 (95% CI) (0.31 – 1.65) (0.24 – 1.22) (0.29 – 0.79) (0.74 – 1.78) (0.27 – 1.07) (0.46 – 1.56) p 0.43 0.14 0.004 0.53 0.078 0.59

* Outcome not estimated owing to the limited sample size.

3553 Poster Session

FOLFOX plus panitumumab or FOLFOX alone as additive therapy following R0/1 resection of RAS wild-type colorectal cancer liver metastases: The PARLIM trial (AIO KRK 0314).

Dominik Paul Modest, Meinolf Karthaus, Stefan Kasper, Nicolas Moosmann, Verena Keitel, Alexander Kiani, Ludwig Fischer von Weikersthal, Jens Uhlig, Lutz Jacobasch, Martin Fuchs, Florian Kaiser, Christian A. Lerchenmuller, Christian Junghanss, Swantje Held, Kathrin Heinrich, Andreas Jung, Arndt Stahler, Sebastian Stintzing, Volker Heinemann; Department of Medicine III, University Hospital, LMU Munich, Munich, Germany; Hematology, Oncology, and Palliative Medicine, Klinikum Neuperlach and Harlaching, Munich, Germany; Universita€tsklinikum Essen (Ao€R), Essen, Germany; Krankenhaus Barmherzige Bru€der, Dept. Hematology and Oncology, Regensburg, Germany; University of Du€sseldorf, Du€sseldorf, Germany; Klinikum Bayreuth GmbH, Medizinische Klinik IV, Bayreuth, Germany; Depart- ment of Oncology, Gesundheitszentrum St. Marien GmbH, Amberg, Germany; Medical Practice for He- matology and Oncology Muldental, Naunhof, Germany; Onkologische Schwerpunktpraxis, Dresden, Germany; Department of Gastroenterology, Hepatology and Gastrointestinal Oncology, Klinikum Bogen- hausen, Munich, Germany; VK&K Studienzentrum, Landshut, Germany; Group Practice of Hematology and Oncology, Muenster, Germany; Department of Hematology and Oncology, University Hospital, Ros- tock, Germany; ClinAssess GmbH, Leverkusen, Germany; Department of Medicine III, University Hospi- tal, LMU Munich, Mu€nchen, Germany, Munich, Germany; Department of Pathology, University of Munich, Munich, Germany; Medical Department, Munich, Germany; Medical Department, Division of Hematology, Oncology, and Tumor Immunology (CCM), Charite� Universita€tsmedizin Berlin, Berlin, Ger- many; University Hospital Munich, LMU Munich, Munich, Germany

Background: This trial investigates the addition of panitumumab to chemotherapy with fluorouracil/ folinic acid and oxaliplatin (FOLFOX) in a 2:1 randomized, controlled, open label, phase II trial in RAS wild-type colorectal cancer patients with R0/1-resected liver metastases. Methods: The primary endpoint was progression-free survival (PFS) two years after randomisation. The experimental arm (12 wks of biweekly mFOLFOX6 plus panitumumab followed by 12 wks of panitumumab alone) was considered active if the 2-year-PFS rate was $65%. Based on historical data, a 2-year-PFS rate of 50% was estimated in the control arm (12 wks of biweekly FOLFOX). The trial was performed with a power of 80% and an alpha of 0.05. Secondary endpoints included overall survival (OS) and toxicity. The trial is registered with ClinicalTrials.gov, NCT01384994. Results: The full analysis set consists of 70 patients (pts) in the experimental arm and 36 pts in the control arm. The 2-year-PFS rate was 34.3% with FOLFOX plus panitumumab and failed to meet the primary endpoint. The 2-year-PFS rate in the control arm was 25%. In the experimental arm, a more favourable outcome was observed with regard to PFS (HR: 0.72, 95%CI 0.45-1.17; P = 0.18) and OS (HR: 0.76 (95% CI 0.34-1.71, P = 0.51) which did, however, not reach the level of significance. Updated data including toxicity and subgroup analyses might be presented at the meeting. Conclusions: The PARLIM trial clearly failed to demonstrate a PFS rate of 65% after resection of colorectal liver metastases 2 years after randomisation, potentially indicating that the generally high frequency of recurrence and the choice of primary endpoint did not correspond in this study population. However, a trend for improved PFS and OS by the addition of panitumumab to 12 wks of FOLFOX followed by 12 wks panitumumab maintenance therapy may support future trials with ant- EGFR antibodies in this specific treatment setting. Clinical trial information: NCT01384994. Research Sponsor: AMGEN, University of Munich (LMU).

3554 Poster Session

Impact of a metastatic site on circulating tumor DNA (ctDNA) analysis in patients (pts) with metastatic colorectal cancer (mCRC).

Hideaki Bando, Yoshiaki Nakamura, Hiroya Taniguchi, Manabu Shiozawa, Hisateru Yasui, Taito Esaki, Takashi Ohta, Tadamichi Denda, Taroh Satoh, Kentaro Yamazaki, Yu Sunakawa, Takeshi Kato, Masahiro Goto, Satoshi Yuki, Tomohiro Nishina, Eiji Oki, Eiji Shinozaki, Nobuhisa Matsuhashi, Masayuki Hata, Takayuki Yoshino; Department of Clinical Oncology, Aichi Cancer Center Hospital, Na- goya, Japan; Department of Gastroenterology and Gastrointestinal Oncology, National Cancer Center Hospital East, Kashiwa, Japan; Department of Gastrointestinal Surgery, Kanagawa Cancer Center, Yo- kohama, Japan; Department of Medical Oncology, Kobe City Medical Center General Hospital, Kobe, Japan; Department of Gastrointestinal and Medical Oncology, National Hospital Organization Kyushu Cancer Center, Fukuoka, Japan; Department of Clinical Oncology, Kansai Rosai Hospital, Amagasaki, Japan; Division of Gastroenterology, Chiba Cancer Center, Chiba, Japan; Department of Frontier Sci- ence for Cancer and Chemotherapy, Osaka University Graduate School of Medicine, Osaka, Japan; Divi- sion of Gastrointestinal Oncology, Shizuoka Cancer Center, Shizuoka, Japan; Department of Clinical Oncology, St. Marianna University School of Medicine, Kawasaki, Japan; Department of Surgery, Na- tional Hospital Organization Osaka National Hospital, Osaka, Japan; Cancer Chemotherapy Center, Osa- ka Medical Collage Hospital, Takatsuki, Japan; Department of Gastroenterology and Hepatology, Hokkaido University Hospital, Sapporo, Japan; National Hospital Organization Shikoku Cancer Center, Matsuyama, Japan; Department of Surgery and Science, Graduate School of Medical Sciences, Kyushu University, Fukuoka, Japan; Department of Gastrointestinal Oncology, Cancer Institute Hospital of Jap- anese Foundation for Cancer Research, Tokyo, Japan; Department of Surgical Oncology, Gifu University Graduate School of Medicine, Gifu, Japan; Translational Research Support Section, National Cancer Center Hospital East, Kashiwa, Japan

Background: ctDNA genotyping has been used as an alternative to tissue genotyping for precision oncology. In mCRC, although low plasma RAS variant allelic frequencies (VAFs) and low concordance with tissue RAS tests have been reported in pts with lung-only metastasis (mets) (Kagawa Y. et al., Clin Cancer Res 2021), the association of ctDNA identified using the next-generation sequencing method with metastatic sites is still unknown. Methods: We investigated the association between metastatic site and ctDNA detection by using the Guardant360 (G360), a ctDNA assay which detects 74 gene alterations including mutations with the 95% of limit of detection of 0.2%, in mCRC pts with single organ mets in pts who had not received anti- EGFR therapy in the SCRUM-Japan GOZILA study. We also evaluated the correlations between the size/number of mets by CT and detected VAFs. Results: Of 1187 mCRC pts enrolled in GOZILA, 138 pts (49 with liver- only, 15 with lymph node-only, 27 with peritoneum-only, and 47 with lung-only mets) were eligible for this study. The concordance of RAS/BRAF status between G360 and tissue in-vitro diagnostic tests were 93.9% in liver-only, 80.0% in lymph node-only, 56.0% in peritoneum-only, and 65.9% in lung-only mets. The median maximum VAF (maxVAF) corresponding to the highest ctDNA fraction and the median numbers of detected variants were 23.1% and 5 in liver-only, 6.0% and 5 in lymph node-only, 0.4% and 3 in peritoneum- only, and 0.4% and 3 in lung-only (all P<0.001, Kruskal-Wallis test). A few pts with liver-only (2.0%) and lymph node-only mets (13.3%) had a maxVAF of <0.2%, but maxVAF was more frequently <0.2% in pts with lung-only (27.7%) or peritoneum-only mets (29.6%), especially in those with lung-only mets <20 mm as the longest diameter and <20 lesions (69.2%) or with peritoneum-only mets <20 mm as the longest diameter (87.5%). Conclusions: Lung-only and peritoneum-only mets had significantly lower maxVAF and lower numbers of detected variants, suggesting lower detections of subclonal variants. To ensure the sufficient clinical performance in G360 assay, inclusion of pts with lung-only mets $20 mm of longest diameter and/or $20 lesions, and $20 mm of the longest diameter in pts with peritoneum-only mets may be required. Re- search Sponsor: None.

Liver-only Lymph node-only Peritoneum-only Lung-only (N=49) (N=15) (N=27) (N=47) maxVAF (%) (median, 23.1 (0.04, 80.4) 6.0 (0, 45.5) 0.4 (0, 31.6) 0.4 (0, 10.9) (range)) RAS/BRAF VAF (%) 10.4 (0.06, 44.3) 9.1 (1.0, 36.8) 0.6 (0.09, 0.4 (0.08, 7.6) (median, 31.6) (range)) maxVAF< 0.2 (%) 2.0 (1/49) 13.3 (2/15) 29.6 (8/27) 27.7 (13/47) Number of detected variants 5 (0, 52) 5 (0, 23) 3 (0, 32) 3 (0, 7) (median, (range)) Longest diameter (median, 41.0 (11.5, 25.2 (12.0, 18.3 (4.9, 19.2 (7.5, (range)) 149.8) 53.0) 67.6) 56.0) Lesion numbers (median, 5 (1, 82) 3 (1, 35) 2 (1, 16) 7 (1,100<) (range))

3555 Poster Session

Pertuzumab plus trastuzumab and real-world standard of care (SOC) for patients (pts) with treatment refractory metastatic colorectal cancer (mCRC) with HER2 (ERBB2) amplification (amp) confirmed by tumor tissue or ctDNA analysis (TRIUMPH, EPOC1602).

Wataru Okamoto, Yoshiaki Nakamura, Takeshi Kato, Taito Esaki, Masato Komoda, Ken Kato, Yoshito Komatsu, Toshiki Masuishi, Tomohiro Nishina, Kentaro Sawada, Hiroya Taniguchi, Nozomu Fuse, Shogo Nomura, Makoto Fukui, Steven R. Olsen, Justin Iver Odegaard, Akihiro Sato, Satoshi Fujii, Atsushi Ohtsu, Takayuki Yoshino; Cancer Treatment Center, Hiroshima University Hospital, Hiroshima, Japan; Department of Gastroenterology and Gastrointestinal Oncology, National Cancer Center Hospital East, Kashiwa, Japan; Department of Surgery, National Hospital Organization Osaka National Hospital, Osaka, Japan; Department of Gastrointestinal and Medical Oncology, National Hospital Organization Kyushu Cancer Center, Fukuoka, Japan; Department of Head and Neck Medical Oncology, National Cancer Center Hospital, Tokyo, Japan; Department of Cancer Chemotherapy, Hokkaido University Hos- pital Cancer Center, Sapporo, Japan; Department of Clinical Oncology, Aichi Cancer Center Hospital, Nagoya, Japan; National Hospital Organization Shikoku Cancer Center, Matsuyama, Japan; Division of Cancer Center, Hokkaido University Hospital, Sapporo, Japan; Clinical Research Support Office, Na- tional Cancer Center Hospital East, Kashiwa, Japan; Biostatistics Division, Center for Research Admin- istration and Support, National Cancer Center, Kashiwa, Japan; Guardant Health AMEA, Minato-Ku, Tokyo, CA, Japan; Guardant Health, Inc., Redwood City, CA; Department of Molecular Pathology, Yoko- hama City University Graduate School of Medicine., Yokohama, Japan; National Cancer Center Hospital East, Kashiwa, Japan

Background: HER2 amp occurs in 1-4% of mCRC pts. Two single arm phase 2 studies, HERACLES and MyPathway, showed efficacy for dual HER2-targeted therapy in pts with RAS wild type (RAS wt) mCRC with HER2 amp detected in tumor tissue; however, efficacy for pts prospectively enrolled with HER2 amp identified in ctDNA is unknown. Furthermore, the efficacy of real-world non-HER2-targeted SOC for HER2 amplified RASwt mCRC pts is not clear. Methods: We conducted a phase 2 trial to evaluate the efficacy of pertuzumab (P) plus trastuzumab (T) in RASwt mCRC pts with HER2 amp centrally confirmed by tissue (IHC and/or FISH) and/or ctDNA (Guardant360) who had progressed on SOC including EGFR blockade. Pts received intravenous P (840 mg loading dose followed by 420 mg) and T (8 mg/kg loading dose followed by 6 mg/kg) every 3 weeks. The primary endpoint was confirmed objective response rate (ORR) by investigator assessment, analyzed for two primary populations: pts with HER2 amp in tissue (tissue+) or in ctDNA (ctDNA+). Efficacy of real-world non-HER2-targeted SOC for HER2 amplified RASwt mCRC pts was prospectively assessed in a concurrent registry: the SCRUM-Japan registry. Results: Among 75 pts screened, concordance of HER2 amp between tissue and ctDNA was 83%. The primary endpoint was met in each cohort of TRIUMPH, with confirmed ORR of 30% (95% CI 14- 50%) in 27 tissue+ pts and 28% (12-49%) in 25 ctDNA+ pts. In contrast, ORR in first salvage SOC after EGFR blockade was 0% (0.0-24.7%) in the real-world cohort. Median progression free and overall survival were 4.0 months (1.4-5.6) and 10.1 months (4.5-16.5) in the tissue+ pts and 3.1 months (1.4-5.6) and 8.8 months (4.3-12.9) in the ctDNA+ pts. One pt withdrew due to an adverse event (grade 3 decreased ejection fraction), but no treatment related deaths occurred. In exploratory analyses, pts without ctDNA mutations of RAS/BRAFV600/PIK3CA/HER2 were more likely to respond to P+T than those with a ctDNA mutation in at least one of these genes (ORR 44% vs. 0% in tissue+ and 37% vs. 0% in ctDNA+). Decreased ctDNA fraction and HER2 plasma copy number at 3 weeks after treatment initiation corresponded to P+T response. At least one actionable alteration emerged after progression in 16 (62%) of 26 pts with ctDNA results at both baseline and progression. Among 5 pts who achieved response and had ctDNA results at both time points, 4 pts acquired actionable alteration at progression. Conclusions: We demonstrate promising efficacy and safety of P+T for RASwt mCRC pts with HER2 amp in either tumor tissue or ctDNA. Our results show that complete ctDNA genotyping identifies pts most likely to benefit from dual HER2 blockade and can be used to monitor response and detect actionable resistance biomarkers. Clinical trial information: UMIN000027887 and UMIN000028058. Research Sponsor: Japan Agency for Medical Research and Development.

3556 Poster Session

Repeat sequential oxaliplatin-based chemotherapy (FLOX) and nivolumab versus FLOX alone as first-line treatment of microsatellite-stable (MSS) metastatic colorectal cancer (mCRC): Initial results from the randomized METIMMOX study.

Anne Hansen Ree, Hanne Hamre, Christian Kersten, Eva Hofsli, Marianne Grnlie Guren, Halfdan Sorbye, Christin Johansen, Anne Negård, Kjersti Flatmark, Sebastian Meltzer; Akershus University Hos- pital, Lorenskog, Norway; Srlandet Hospital Trust, Kristiansand, Norway; Department of Oncology, St. Olavs Hospital, Trondheim, Norway; Department of Oncology, Oslo University Hospital, Oslo, Norway; Haukeland University Hospital, Bergen, Norway; Oslo University Hospital, Oslo, Norway

Background: Immune checkpoint blockade (ICB) has revolutionized patient outcome for the small mCRC subgroup with highly immunogenic disease. The majority of mCRC cases, however, are MSS without innate ICB susceptibility. In our ongoing METIMMOX study, we hypothesize that MSS mCRC can be transformed into an immunogenic condition by short-course oxaliplatin-based therapy (FLOX), enabling patients with unresectable, previously untreated metastases to obtain durable disease control when adding ICB therapy. Here we present the protocol-planned interim analysis. Methods: Eligibility criteria include infradiaphragmatic metastasis and C-reactive protein < 60 mg/L. At analysis 15 Janu- ary 2021, 54 patients stratified according to primary tumor sidedness and mutational status and evaluable for the primary end point (progression-free survival; PFS) had been randomly assigned to a standard-of-care schedule of 8 FLOX cycles Q2W (control arm) or repeat sequential 2 FLOX cycles and 2 nivolumab cycles (240 mg Q2W) to a total of 8 cycles (experimental arm), for both arms before treatment break until disease progression and reintroduction of a new treatment sequence. Radiologic response assessment is every 8 weeks. Safety, tolerability, objective response rate, and duration of response are among secondary end points. Results: At median follow-up of 6.4 (range, 0.5-20) months, patients were well balanced between the treatment arms with regard to the predefined strata and single-organ or multiple-organ metastases. Median PFS for the entire groups of control and experimental arm patients was 5.6 (range, 0.5-15; n = 26) and 6.6 (range, 0.5-20; n = 28) months, respectively. The number of FLOX-related CTCAE grade 3 or higher adverse events, including 2 deaths after initial FLOX administration, was comparable in the two arms. Twelve immune-related grade 3-4 adverse events (no new safety signals) were recorded. In the experimental arm, 4 (16%) patients, all RAS/ BRAF-mutant cases, had experienced complete response and 9 (32%) patients had ongoing objective response at 8 months. The control arm cases had 0 with complete response and 6 (23%) with ongoing objective response at 8 months, 1 of whom had proceeded to curative-intent liver surgery. Conclusions: MSS mCRC patients may hold the opportunity of ICB responsiveness evoked by short-course oxaliplatin-based chemotherapy. The search for predictive biomarkers of ICB responsiveness is ongoing in the specifically designed METIMMOX correlative study program. Clinical trial information: NCT03388190. Research Sponsor: Norwegian Cancer Society Grant 182496, Pharmaceutical/Biotech Company.

3557 Poster Session

Impact of BRAF mutations on prognosis and immunotherapy response in microsatellite instability/mismatch repair deficient metastatic colorectal cancer: A systematic review and meta-analysis.

Robin Park, Lae�rcio Lopes da Silva, Sunggon Lee, Anwaar Saeed; MetroWest Medical Center, Framing- ham, MA; Korea University, Seoul, South Korea; Kansas University Cancer Center, Kansas City, KS

Background: Mismatch repair deficient/microsatellite instability high (dMMR/MSI-H) colorectal cancer (CRC) defines a molecular subtype with distinct clinicopathologic characteristics including an excellent response to immunotherapy. Although BRAF mutations are established as a negative prognostic marker in CRC, whether they retain their negative prognostic impact in or alter the response to immunotherapy in dMMR/MSI-H CRC remains unknown. Herein, we present a systematic review and meta-analysis of the impact of BRAF mutations on the overall survival (OS) and immune checkpoint inhibitor (ICI) response in dMMR/MSI-H CRC. Methods: Studies published from inception to 26 January 2021 were searched in PubMed, Embase, and major conference proceedings (AACR, ASCO, and ESMO). Eligible studies included the following: 1) observational studies reporting outcomes based on BRAF mutation status in dMMR/MSI-H CRC patients and 2) experimental studies of ICI reporting outcomes based on BRAF mutation status in dMMR/MSI-H CRC patients. A summary hazard ratio (HR) was calculated for OS in BRAF mutated (BRAFmut) vs. BRAF wild type (BRAFwt) patients (pts) with the random effects meta-analysis (REM). A summary odds ratio (OR) was calculated for objective response rate (ORR) in BRAFmut vs. BRAFwt pts treated with ICI with the REM. Results: Database search conducted according to PRISMA guidelines found 4221 studies in total. Initial screening identified 30 studies and after full- text review, 9 studies (N = 4158 pts) were included for the meta-analysis of prognosis (analysis A) and 3 studies (N = 178 pts) were included for the meta-analysis of ICI response (analysis B). The outcome measures are summarized in the table below. Analysis A showed that in stage I-IV dMMR/MSI-H CRC pts, BRAFmut was associated with worse OS than BRAFwt (HR 1.57, 1.23-1.99). The heterogeneity was low (I2 = 21%). Subgroup analysis showed no significant difference in the prognostic impact of BRAF mutation status between stage IV only and stage I-IV CRC pts. Analysis B showed no difference in ORR (OR 1.04, 0.48-2.25) between BRAFmut vs. BRAFwt dMMR/MSI-H pts who received ICI. The heterogeneity was low (I2 = 0%). Conclusions: BRAF mutations retain their negative prognostic impact in dMMR/MSI-H stage I-IV and stage IV CRC but are not associated with differential ICI response. Limi- tations include the following: analysis A was based on retrospective studies; also, the impact of BRAF status on the survival outcome of ICI could not be assessed due to limited number of studies. Research Sponsor: None.

BRAF mutated vs. BRAF wild type Number of Studies (Pts) Outcome

Overall Survival (I-IV) 9 (N = 4158) HR 1.57 [1.23-1.99]* Overall Survival (IV only) 3 (N = 3247) HR 2.02 [1.20-3.42] Objective Response Rate to ICI 3 (N = 178) OR 1.04 [0.48-2.25]**

*Lower HR favors BRAFmut **Higher OR favors BRAFmut.

3558 Poster Session

Phase II study of panitumumab monotherapy in chemotherapy-nave frail or elderly patients with unresectable, RAS wild type colorectal cancer: OGSG 1602, survival update data.

Shingo Noura, Takeshi Kato, Tetsuji Terazawa, Masahiro Goto, Katsuya Ohta, Hironaga Satake, Yoshinori Kagawa, Hisato Kawakami, Hiroko Hasegawa, Kazuhiro Yanagihara, Tatsushi Shingai, Ken Nakata, Masahito Kotaka, Masayuki Hiraki, Ken Konishi, Shiro Nakae, Daisuke Sakai, Yukinori Kurokawa, Toshio Shimokawa, Taroh Satoh, Osaka Gastrointestinal Cancer Chemotherapy Study Group (OGSG); Department of Surgery, Toyonaka Municipal Hospital, Toyonaka, Japan; Department of Sur- gery, National Hospital Organization Osaka National Hospital, Osaka, Japan; Cancer Chemotherapy Center, Osaka Medical Collage, Osaka, Japan; Cancer Chemotherapy Center, Osaka Medical Collage Hospital, Takatsuki, Japan; Department of Gastroenterological surgery, Higashiosaka City Medical Cen- ter, Higashiosaka, Japan; Kobe City Medical Center General Hospital, Kobe City, Japan; Kansai Rosai Hospital, Amagasaki, Japan; Kindai University Hospital, Osaka, MN, Japan; Department of Gastroenter- ology and Hepatology, National Hospital Organization Osaka National Hospital, Osaka, Japan; Depart- ment of Medical Oncology, Kansai Electric Power Hospital, Osaka, Japan; Department of Surgery, Osaka Saiseikai Senri Hospital, Suita-Shi, Japan; Sakai City Medical Center, Sakai, Japan; Gastrointes- tinal Cancer Center, Sano Hospital, Kobe, Japan; Dana-Farber Cancer Institute/Harvard Medical School, Boston, MA; Hyogo Prefectural Nishinomiya Hospital, Nishinomiya, Japan; Department of Med- ical Oncology, Mimihara General Hospital, Sakai, Japan; Osaka University Hospital, Osaka, Japan; De- partment of Gastroenterological Surgery, Osaka University, Graduate School of Medicine, Suita City, Osaka, Japan; Department of Medical Data Science, Graduate School of Medicine, Wakayama Medical University, Wakayama, Japan; Department of Frontier Science for Cancer and Chemotherapy, Osaka University Graduate School of Medicine, Osaka, Japan

Background: We previously reported the result of the phase II OGSG1602 study in which single agent of panitumumab (Pmab) demonstrated 76.5% and 50% of disease control rate (DCR), primary endpoint, and response rate (RR), respectively, in chemotherapy-nave frail or elderly patietns (pts) with wild-type (wt) RAS unresectable colorectal cancer (CRC). Here, we reports the survival analysis including overall survival (OS) and progression free survival (PFS) in terms of sidedness and early tumor shrinkage (ETS). Methods: Thirty-six pts aged $76 years, or $65 considered unsuitable for intensive chemotherapy were enrolled and received Pmab 6 mg/kg intravenously every 2 weeks. Primary tumors located in the cecum to transverse colon were coded as right-sided tumors (RST), while tumors located from the splenic flexure to rectum were considered left-sided tumors (LST). Early tumor shrinkage (ETS) was determined as tumor reduction of 20% at week 8 compared to baseline. Results: Of total of 36 enrolled pts, 34 pts were included in the efficacy analysis, with pts with LST vs. RST being 26 vs. 8 cases, while pts who achieved ETS (ETS+) vs. those who did not achieve ETS (ETS-) being 15 vs. 19 cases. Among the evaluable 34 pts, the median PFS (mPFS) and median OS (mOS) were 6.0 months (95% Confidence Interval [CI]: 5.4-10.0) and 17.5 months (95%CI, 13.8-24.3), respectively, with the median follow-up of 17.0 months. For PFS, there were no significant differences between pts with LST vs. RST [6.6 months (95%CI, 5.4-11.5) vs. 4.9 months (95%CI, 1.9-NA), p = 0.120] but between pts with ETS+ vs. ETS- [10.4 months (95%CI, 7.4-NA) vs. months (95%CI, 2.1-7.9), p = 0.001]. Furthermore, OS was significantly different either in pts with LST vs. RST [19.3 months (95%CI, 14.2-NA) vs. 12.3 (95%CI, 9.9- NA), p = 0.043] or in pts with ETS+ vs. ETS- [ months (n = 15, 95%CI, 19.6-NA) vs. 10.1 months (n = 19, 95%CI, 6.8-21.8), p < 0.001]. Conclusions: Pmab monotherapy showed the favolable OS in the frail or elderly pts with RAS wt, unresectable CRC. Our data also confirmed the prognostic value of sidedness as well as predictive value of ETS in this setting. Clinical trial information: UMIN000024528. Research Sponsor: OGSG and Takeda Pharmaceutical Company Limited.

PFS and OS in subgroup analysis. n mPFS (months) mOS (months)

All patients 34 6.0 17.5 95% CI (5.4-10.0) 95% CI (13.8-24.3) LST vs. RST 26 vs. 8 6.6 vs 4.9 19.3 vs 12.3 p = 0.120 (HR 0.518) p = 0.043 (HR 0.413) ETS+ vs. ETS- 15 vs. 19 10.4 vs 3.6 34.8 vs 10.1 p = 0.001 (HR 0.282) p < 0.001 (HR 0.184) HR; hazard ratio.

3559 Poster Session

Phase Ib/II open-label, randomized evaluation of atezolizumab (atezo) + Imprime PGG (Imprime) + bevacizumab (bev) vs regorafenib (rego) in MORPHEUS: Microsatellite-stable (MSS) metastatic colorectal cancer (mCRC).

Marwan Fakih, James M. Cleary, Yong Sang Hong, Tae-You Kim, Rachael A Safyan, Simon Allen, Lorna Bailey, Edward Cha, Christelle Lenain, Danny Lu, Jochen Schulze, Colby S. Shemesh, Stefan Zimmermann; City of Hope Comprehensive Medical Center, Duarte, CA; Dana Farber Cancer Institute, Boston, MA; Asan Medical Center, University of Ulsan College of Medicine, Seoul, South Korea; Seoul National University Hospital, Seoul, South Korea; Columbia University Irving Medical Center, New York, NY; Genentech, Inc., South San Francisco, CA; Roche Products Limited, Welwyn, United King- dom; F. Hoffmann-La Roche, Ltd., Basel, Switzerland; Hoffmann-La Roche Limited, Mississauga, ON, Canada; Immuno-Oncology Service, Department of Oncology, University Hospital of Lausanne, Lau- sanne, Switzerland

Background: The MORPHEUS platform consists of multiple, global, open-label, randomized Phase Ib/II trials designed to identify early efficacy and safety signals of treatment (tx) combinations across cancers. Here, atezo (anti-PD-L1) was tested with Imprime and bev (anti-VEGF) for MSS mCRC, a poorly immunogenic cancer generally resistant to checkpoint inhibitors. Imprime acts as a pathogen-associated molecular pattern that, when bound to anti-b glucan antibodies (ABA), activates the innate immune system with the potential to 1) promote priming and expansion of tumor-specific T cells, 2) promote M2-M1 macrophage polarization and 3) enhance the immunomodulatory effects of atezo and bev. Therefore, we hypothesized that atezo + Imprime + bev would induce an antitumor response beyond that of rego, a standard-of-care multikinase inhibitor, in patients (pts) with MSS mCRC. Methods: Pts with MSS mCRC unselected for the Imprime-specific biomarker (ABA) and refractory to 1-2 prior lines of standard therapy received atezo (1200 mg IV every 3 weeks [q3w]) + Imprime (4 mg/kg IV on Days 1, 8, 15) + bev (7.5 mg/kg IV q3w) or control tx with rego (160 mg orally days 1-21; dose escalation to 160 mg during Cycle 1 allowed per institutional guidelines). The primary endpoint was objective response rate (ORR; investigator-assessed RECIST 1.1); secondary endpoints included disease control rate (DCR; response or stable disease $ 12 weeks), progression-free survival (PFS), overall survival (OS) and safety. Results: Pts were followed-up for $18 wk. 15 pts received atezo + Imprime + bev and 13 received rego. Grade (Gr) 3/4 tx-related adverse events (TRAEs) were seen in 13% of atezo + Imprime + bev and 62% of rego pts. No Gr 5 AEs occurred in atezo + Imprime + bev pts and 1 (8%) was reported in a rego pt. One pt in each arm (7% vs 8%, respectively) withdrew from tx due to a TRAE. No radiological responses were seen in either arm. Five pts (33%) receiving atezo + Imprime + bev and 8 (62%) receiving rego had stable disease as best response. DCR was 13% with atezo + Imprime + bev and 23% with rego. Median PFS was 1.5 mo (95% CI: 1.4, 2.8) and 2.8 mo (95% CI: 1.6, 3.1), and median OS was 5.7 mo (95% CI: 4.4, 10.5) and 10.2 mo (95% CI: 4.8, NE) with atezo + Imprime + bev and rego, respectively. There was no apparent correlation between baseline PD-L1 expression or CD8+ lymphocyte tumor infiltration and clinical benefit. Further, the systemic exposure of atezo, Imprime and bev and immunogenicity of atezo and bev are in line with previous clinical experience. Additional biomarker, pharmacokinetics and anti-drug antibody data will be shown. Conclusions: Atezo + Imprime + bev was well tolerated; toxicities were consistent with the safety profiles of the individual agents. No efficacy signal was identified with atezo + Imprime + bev in pts with MSS refractory mCRC. Clinical trial information: NCT03555149. Research Sponsor: F. Hoffmann-La Roche, Ltd.

3560 Poster Session

Single-arm, phase 2 study of regorafenib plus nivolumab in patients with mismatch repair- proficient (pMMR)/microsatellite stable (MSS) colorectal cancer (CRC).

Marwan Fakih, Kanwal Pratap Singh Raghav, David Z. Chang, Johanna C. Bendell, Timothy Larson, Allen Lee Cohn, Timothy K. Huyck, David Cosgrove, Joseph A. Fiorillo, Lawrence E. Garbo, Shruthi Ravimohan, Von Potter, David D’Adamo, Neelesh Sharma, Ying A. Wang, Sabine Coppieters, Matthias Herpers, Carolina Soares Viana de Oliveira, Andrew Scott Paulson; City of Hope Comprehensive Cancer Center, Duarte, CA; Department of Gastrointestinal Medical Oncology, The University of Texas MD An- derson Cancer Center, Houston, TX; Sarah Cannon Research Institute/Tennessee Oncology, Nashville, TN; Minnesota Oncology/The US Oncology Network, Minneapolis, MN; Rocky Mountain Cancer Center, Denver, CO; Nebraska Cancer Specialists, Omaha, NE; Division of Medical Oncology, Vancouver Cancer Center, Compass Oncology, Vancouver, WA; Willamette Valley Cancer Institute, Eugene, OR; New York Oncology Hematology, Albany, NY; Bristol-Myers Squibb, Lawrenceville, NJ; Bristol-Myers Squibb, New Haven, CT; Bayer HealthCare Pharmaceuticals, Whippany, NJ; Bayer HealthCare Pharmaceuti- cals, Cambridge, MA; Bayer AG, Diegem, Belgium; ClinStat GmbH, Cologne, Germany; Texas Oncology/ The US Oncology Network, Dallas, TX

Background: The role of immunotherapy in the treatment of pMMR/MSS metastatic CRC is not established. A Japanese phase 1b trial in this setting showed the combination of regorafenib (multikinase inhibitor with immunomodulatory activity) plus nivolumab (anti PD-1) had encouraging activity and manageable safety (Fukuoka, 2020). This study further assessed the safety and efficacy of this combination. Methods: Patients (pts) from the US aged $18 years who progressed on/were intolerant to standard chemotherapy were enrolled. Regorafenib was given orally, once daily in 28-day (D) cycles (21D on/7D off) plus IV nivolumab 480 mg on D1. Regorafenib starting dose was 80 mg; if well tolerated, it could be escalated to 120 mg in Cycle 2. Primary endpoint was overall response rate (ORR; RECIST 1.1); secondary aims included disease control rate (DCR), overall survival (OS), progression-free survival (PFS), and safety (NCI-CTCAE v5.0 grade). Biomarker analysis was exploratory. Results: 70 pts (59% male) started treatment. At baseline, median age was 57 years (range 34–85), ECOG PS 0/1 was 51%/ 49%, 67% had liver metastases (mets), and the primary tumor site was right-sided colon in 36% and rectum in 17%. Median number of cycles was 3.0 (range 1–13); 41% of pts escalated regorafenib to 120 mg. Five pts (7.1%) had a partial response (PR) lasting $16 weeks (wks) and 22 (31.4%) had stable disease (SD); pts without liver mets had a higher ORR (21.7%). In pts with tumor samples (n = 40), higher baseline expression (IHC) of cytotoxic T cells (CD3+/CD8+/GranzymeB+), Tregs (FoxP3+), and macrophages (CD68+) trended with clinical benefit (PR/SD $16 wks/PFS); pts with liver mets had lower expression. Lower plasma levels of biomarkers of vascular biology (e.g. VEGF-D, Ang-2, VWF) trended with longer PFS. Grade (Gr) 3 treatment-emergent adverse events (TEAEs) occurred in 53% of pts and Gr 4 in 10%. Three pts had a Gr 5 TEAE: n = 1 related to the combination (sepsis); n = 1 related to nivolumab only by investigator (sepsis); n = 1 unrelated to treatment (respiratory failure). Most common Gr 3/4 TEAEs: maculopapular rash (14%), fatigue (7%), pneumonia (6%), increased bilirubin (6%). Con- clusions: Combination treatment with regorafenib (up to 120 mg/day) and nivolumab (480 mg every 28D) has manageable safety. Efficacy of this combination in the North American population did not em- ulate results in the Japanese population. Absence of liver mets and expression of specific biomarkers indicate a better response and may warrant further analysis. Clinical trial information: NCT04126733. Research Sponsor: Bayer, Bristol Myers-Squibb, Ono.

No liver mets Liver mets All patients n = 23 n = 47 N = 70

ORR, n (%) 5 (21.7) 0 5 (7.1) PR, n (%) 5 (21.7) 0 5 (7.1) SD, n (%) 8 (34.8) 14 (29.8) 22 (31.4) Progressive disease, n (%) 9 (39.1) 27 (57.4) 36 (51.4) Not evaluable, n (%) 1 (4.3) 6 (12.8) 7 (10.0) DCR at 40 wks, n (%) 13 (56.5) 14 (29.8) 27 (38.6) Median duration of SD, wks 30 21 30 Median PFS, wks 15 8 8 Median OS, wks 52 47 52

3561 Poster Session

Concurrent BRAFV600E and BRCA mutations in microsatellite stable (MSS) metastatic colorectal cancer (mCRC): Prevalence and case series of mCRC (pts) with prolonged overall survival (OS).

Timothy Lewis Cannon, Jamie Randall, Ethan Sokol, Sonja Alexander, Raymond Couric Wadlow, Daniel Barnett, Danny Rayes, John F. Deeken, Halla Nimeiri, Kimberly McGregor; Inova Schar Cancer Insti- tute, Fairfax, VA; Inova Fairfax Hospital, Fairfax, VA; Cancer Genomics Research, Foundation Medicine, Cambridge, MA; FMI, Annapolis, MD; Inova, Fairfax, VA; Virginia Commonwealth Universiry, Richmond, VA; University of Virginia Medical School, Charlottesville, VA; Foundation Medicine, Cambridge, MA

Background: BRAF V600E+, MSS mCRC patients comprise up to 10% of advanced CRC. They have a poor prognosis with median survivals typically <1 year. Despite use of multi-agent first-line chemotherapy regimens and combination targeted therapies, outcomes are still poor. In our Institutional Molecular Tumor Board database, we identified 3 consecutive mCRC pts with MSS/BRAF V600E who also had a BRCA1 or BRCA2 co-mutation and had prolonged overall survival. Prior studies suggested that BRCA mutations are uncommon in CRC and we queried the Foundation Medicine (FM) genomic database to evaluate the prevalence of these cases as well as those with co-mutations in other homologous recombination genes. Methods: 36,966 CRC pts were sequenced by FMI using hybrid capture comprehensive genomic profiling (CGP) to evaluate all classes of genomic alterations (GA) for pathogenic BRAF mutations and/or a mutation in BRCA1/2 or a co-mutation in other homologous recombination (HR) genes (BARD1, CDK12, FANCL, PALB2, ATM, RAD54L, CHEK2, BRAF, BRIP1, RAD51D, RAD51C, RAD51B, CHEK1). Selected cohort analysis were BRAF V600E co-mutated with BRCA1 and BRCA2, separated into MSI-H and MSS cohorts. The clinicopathological features and genomic loss of heterozy- gosity (gLOH) of those with a BRAF V600E and a BRCA1/BRCA2 mutation are described along with 3 consecutive cases of CRC patients, identified through the Inova Schar Cancer Institute (ISCI) molecular tumor board (MTB) registry, whom had prolonged OS. Results: Of 36,966 colorectal cancer pts, 6.6% were BRAF V600E+ and 1.5% had any co-occurring HR gene mutation(s) with 0.6% having co-mutations of BRAF V600E and BRCA1/2.BRCA co-mutations were higher in MSI-High BRAF V600E, however 24.1% of these occurred in MSS BRAF V600E. BRCA1 co-mutated was more commonly associated with MSS BRAF V600E and was associated with a higher gLOH than MSI-H BRAF V600E (18.7% vs 2.8%; p <0.001 ). In our institutional MTB database, (3/241;1.2%) CRC patients were MSS, BRAF V600E+ with BRCA1 or BRCA2 co-mutations, one confirmed germline and 2 somatic in origin, and had average gLOH of 21.4% with overall survivals of 72+(alive), 17+(alive), and 30 months, respectively. Conclusions: Co-existence of BRAF V600E/BRCA1/2 may represent a unique subset of advanced MSS CRC that may have a better prognosis and represent an opportunity to test novel targeted therapies. Larger prospective clinical validation trials in this subset is warranted. Research Sponsor: None.

Summary of colorectal cancer with comutation of BRAFV600E and HR variants. All CRC Cases MSI-High MSS MSI-unknown (N= 36,966) (N=707) (N=32141) (N= 3172)

# any BRAF mutation 707 2431 255 # of BRAF V600E 663 1610 175 BRAF V600E + any HR gene mut 384 158 21 BRAF V600E + BRCA1 or BRCA2 165 55 8 BRCA1 22 27 2 BRCA2 126 28 2 BRCA1 and BRCA2 17 0 1 2.80 18.7 NR Average gLOH BRCA2 2.02 9.5 NR Average gLOH BRCA1/2 2.84 NR NR

3563 Poster Session

Increased neutrophil infiltration and lower prevalence of tumor mutation burden and microsatellite instability are hallmarks of RAS mutant colorectal cancers.

Emil Lou, Yasmine Baca, Joanne Xiu, Andrew Nelson, Subbaya Subramanian, Mohamed E. Salem, Muhammad Shaalan Beg, Elisa Fontana, Maria Diab, Philip Agop Philip, Richard M. Goldberg, Ritu Pandey, Tobias Arkenau, Weijing Sun, Heinz-Josef Lenz, Anthony Frank Shields, Wafik S. El-Deiry, Wolfgang Michael Korn; University of Minnesota School of Medicine, Minneapolis, MN; Caris Life Sci- ences, Phoenix, AZ; University of Minnesota, Minneapolis, MN; Levine Cancer Institute, Atrium Health, Charlotte, NC; University of Texas Southwestern Medical Center, Dallas, TX; Sarah Cannon Research In- stitute, United Kingdom, London, United Kingdom; Emory University, Atlanta, GA; Karmanos Cancer Institute, Detroit, MI; West Virginia University Cancer Institute, Morgantown, WV; Arizona Cancer Cen- ter, Tucson, AZ; Sarah Cannon Research Institute UK Limited, London, United Kingdom; University of Kansas Medical Center, Kansas City, KS; USC Norris Comprehensive Cancer Center, Los Angeles, CA; Karmanos Cancer Institute, Wayne State University, Detroit, MI; Cancer Center at Brown University, Providence, RI

Background: The tumor microenvironment (TME) of colorectal cancers (CRC) is modulated by oncogenic drivers such as KRAS. The TME comprises a broad landscape of immune infiltration. How tumor genomics associates with the immune cell landscape is less known. We aim to characterize immune cell types in RAS wild-type (WT) and mutant (MT) CRC, and to examine the prevalence of immuno-oncologic (IO) biomarkers (e.g. tumor mutation burden (TMB), PD-L1, MSI-H/dMMR) in these tumors. We performed genomic and transcriptomic analysis to confirm associations of mutant RAS with immune infiltration of the TME conducive to metastasis vs. potential response to immunotherapies. Methods: A total of 7,801 CRC were analyzed using next-generation sequencing on DNA (NextSeq, 592 Genes and WES, NovaSEQ), RNA (NovaSeq, whole transcriptome equencing) and IHC (Caris Life Sciences, Phoe- nix, AZ). MSI/MMR was tested by FA, IHC and NGS. TMB-H was based on a cut-off of > 10 mutations per MB). Immune cell fraction was calculated by QuantiSeq (Finotello 2019, Genome Medicine). Sig- nificance was determined by X2 and Fisher-Exact and p adjusted for multiple comparisons (q) was <0.05. Results: Mutant KRAS was seen in 48% of mCRC tumors; NRAS in 3.7%, HRAS in 0.1%. The distribution was similar in patients < or >= than 50 yrs. In MSS tumors, there was a significantly higher neutrophil infiltration in KRAS MT (median cell fraction 6.6% vs. 5.9%) and NRAS MT (6.9%) overall and also when individual codons were studied. B cells, M2 macrophages, CD8+ T cells, dendritic cells and fibroblasts were lower in KRAS mutant tumors; B cells and M1 macrophages are lower in NRAS (q<0.05). dMMR/MSI-H was significantly more prevalent in RAS WT (9.1%) than in KRAS (2.9%) or NRAS MT (1.8%) tumors, and highest in HRAS MT tumors (60%, q<0.05).TMB-H was more prevalent in RAS WT (11%) than KRAS (5.8%) or NRAS (5.1%) MT, and highest in HRAS MT tumors (70%, all q<0.05). In MSS tumors, KRAS MT tumors showed more TMB-H than WT (3.1% vs. 2.1%, q<0.05), especially in KRAS non 12/13/61 mutations (5.5%, vs. 2.1%, q<0.05) and G12C (4.4%, p<0.05). PD-L1 expression was studied: in MSS tumors, KRAS-G12D (10.4%) and G13 MT (11.8%) showed higher mutation rates than RAS WT tumors (q<0.05). Conclusions: KRAS & NRAS mutations are associated with increased neutrophil abundance, with codon specific differences, while HRAS shows no difference. Overall CD8+ T cells and B cells are less abundant in KRAS & NRAS mutants; substantial variability was seen amongst different protein changes. RAS mutations were more prevalent overall than generally reported, but did not vary by age. These results demonstrate significant differences in the TME of RAS mutant CRC that identify variable susceptibilities to immuno-oncologic agents, and provide further detailed characterization of heterogeneity between RAS variants, at the molecular as well as immunogenic levels. Research Sponsor: None.

3564 Poster Session

LEAP-005: A phase 2 multicohort study of lenvatinib plus pembrolizumab in patients with previously treated selected solid tumors—Results from the colorectal cancer cohort.

Carlos A. Gomez-Roca, Eduardo Yanez, Seock-Ah Im, Eduardo Castanon Alvarez, He�le�ne Senellart, Mark Doherty, Javier Garcia-Corbacho, Juanita Suzanne Lopez, Bristi Basu, Corinne Maurice-Dror, Sanjeev Singh Gill, Razi Ghori, Peter Kubiak, Fan Jin, Kevin Glen Norwood, Hyun Cheol Cheol Chung; Institut Claudius Regaud, Toulouse, France; Oncology-Hematology Unit, Department of Internal Medi- cine, School of Medicine, Universidad de la Frontera, Temuco, Chile; Department of Internal Medicine, Seoul National University Hospital, Seoul, South Korea; Cl�ınica Universitaria de Navarra, Pamplona, Spain; Institut de Cance�rologie de l’Ouest, Centre Rene� Gauducheau ICO, Saint-Herblain, France; Sun- nybrook Health Sciences Centre, University of Toronto, Toronto, ON, Canada; Department of Medical Oncology (Hospital Clinic)/Translational Genomics and Targeted Therapies in Solid Tumors (IDIBAPs), Barcelona, Spain; The Royal Marsden Foundation Trust and the Institute of Cancer Research, London, United Kingdom; Department of Oncology, University of Cambridge, Cambridge, United Kingdom; Rambam Health Care Campus, Division of Oncology, Haifa, Israel; The Alfred Hospital, Melbourne, VIC, Australia; Merck & Co., Inc., Kenilworth, NJ; Eisai Inc., Woodcliff Lake, NJ; Division of Medical Oncolo- gy, Department of Internal Medicine, Yonsei Cancer Center, Yonsei University College of Medicine, Seoul, South Korea

Background: Pembrolizumab (pembro), an anti-PD-1 antibody, is approved for the treatment of patients (pts) with unresectable or metastatic microsatellite instability-high (MSI-H) or mismatch repair (MMR) deficient colorectal cancer, both as first-line treatment and after progression following treatment with fluoropyrimidine, oxaliplatin, and irinotecan. The combination of lenvatinib, a multiple receptor tyrosine kinase inhibitor, and anti-PD-1 treatment showed synergistic antitumor activity in preclinical models. LEAP-005 (NCT03797326) is evaluating the efficacy and safety of lenvatinib plus pembro in pts with previously treated advanced solid tumors. We present findings from the colorectal cancer cohort. Meth- ods: In this nonrandomized, open-label, phase 2 study, adult pts (aged $18 y) with histologically/cyto- logically documented metastatic and/or unresectable colorectal cancer, non–MSI-H/pMMR tumor per local determination, previous treatment with oxaliplatin and irinotecan in separate lines of therapy, measurable disease per RECIST v1.1, ECOG PS of 0–1, and a tissue sample evaluable for PD-L1 expression were eligible. Pts received lenvatinib 20 mg QD plus pembro 200 mg Q3W for up to 35 cycles of pembro (~2 y) or until confirmed disease progression, unacceptable toxicity, or withdrawal of consent. Treatment with lenvatinib could continue beyond 2 y in pts with clinical benefit. Primary endpoints were ORR (per RECIST v1.1 by blinded independent central review) and safety. Secondary endpoints included disease control rate (DCR), duration of response (DOR), PFS, and OS. Tumor imaging was performed Q9W from treatment initiation for 54 wks, then Q12W to week 102, and Q24W thereafter. Results: 32 pts with colorectal cancer received treatment with lenvatinib plus pembro (median age, 56 y [range, 36-77]; male, 81%; 3L, 91%); median time from first dose to data cutoff (April 10, 2020) was 10.6 mo (range, 5.9-13.1). ORR was 22% (95% CI, 9–40; table). Grade 3–5 treatment-related AEs occurred in 16 (50%) pts. Treatment-related AEs led to treatment discontinuation in 3 pts (grade 2 ischemic stroke [n = 1], grade 3 increased liver transaminases [n = 1], grade 5 intestinal perforation [n = 1]). Conclusions: In pts with previously treated advanced non–MSI-H/pMMR colorectal cancer, lenvatinib plus pembro demonstrated promising antitumor activity and a manageable safety profile. Enrollment in the colorectal cohort was expanded to 100 pts. Clinical trial information: NCT03797326. Research Sponsor: Eisai Inc., Woodcliff Lake, NJ, USA, and Merck Sharp & Dohme Corp., a subsidiary of Merck & Co., Inc., Kenilworth, NJ, USA.

Efficacy results. Outcome N = 32 ORR (CR+PR), % (95% CI) 22 (9–40) DCR (CR+PR+SDa), % (95% CI) 47 (29–65) DOR, median (range), mos NR (2.1+–10.4+) PFS, median (95% CI), mos 2.3 (2.0–5.2) OS, median (95% CI), mos 7.5 (3.9–NR)

NR, not reached. aConfirmation was not required for best overall response of SD, but a final visit response of SD or better must have occurred $6 wks after starting study treatment.

3565 Poster Session

Circulating tumor derived cell-free DNA (ctDNA) to predict recurrence of metastatic colorectal cancer (mCRC) following curative intent surgery or radiation.

Bryant Chee, Faaiz Ibrahim, Mikaela Esquivel, Emily E. Van Seventer, Joy X. Jarnagin, Li Zhang, Jin Hyun Ju, Kristin Sedgwick Price, Victoria M. Raymond, Carlos U. Corvera, Kenzo Hirose, Eric K. Nakakura, Katherine Van Loon, Ryan Bruce Corcoran, Aparna Raj Parikh, Chloe Evelyn Atreya; Universi- ty of California San Francisco, Helen Diller Family Comprehensive Cancer Center, San Francisco, CA; Massachusetts General Hospital, Boston, MA; Guardant Health, Inc., Redwood City, CA; University of California San Francisco, San Francisco, CA

Background: Over half of patients (pts) with oligometastatic CRC treated with curative intent surgery or radiotherapy experience cancer recurrence with or without adjuvant chemotherapy. ctDNA detection post-definitive treatment could identify high risk pts for additional intervention to eliminate molecular residual disease. Here we report results of a prospective observational study aiming to determine ctDNA detection rates using a sensitive liquid biopsy and to correlate post-procedure ctDNA detection (post- ctDNA (+)) with radiographic mCRC recurrence. Methods: Pts with mCRC intending to undergo a curative intent procedure were prospectively recruited at two US sites. ctDNA was collected pre-procedure, 3 weeks post-procedure, and at multiple structured follow-up timepoints. The presence of ctDNA was evaluated using a plasma-only integrated genomic and epigenomic assay (Guardant Reveal, Guardant Health). A bioinformatic classifier was applied to differentiate tumor derived versus non-tumor derived cell-free DNA. Results: Among 52 enrolled pts, post-ctDNA data is available for 45 pts (87%), with a median of 4 (range 1-10) timepoints per pt. The sample analysis failure rate was 1% (2/217). As of 1/ 1/2021, the radiographic recurrence rate was 60% with a median follow-up time of 50 (range 4-192) weeks. 23 of 25 pts with post-ctDNA(+) have had recurrence (Positive Predictive Value [PPV], 92%). On average, ctDNA was detected 28 weeks before radiographic recurrence (mean 12 vs. 40 weeks, respectively). The two pts with post-ctDNA(+) but no recurrence have > 3 years follow-up; one pt received adjuvant chemotherapy and cleared ctDNA. With a median event-free follow-up time of 97 (range 4- 192) weeks, 4 of 20 pts with no post-ctDNA detection (-) have recurred (Negative Predictive Value, 80%). 3 of 4 pts with recurrence despite post-ctDNA(-) also were pre-ctDNA(-). We observed a sensitivity of 85% and a specificity of 89% for the ctDNA assay. The median time to radiographic recurrence was 36 wks for ctDNA(+) vs. not reached for ctDNA(-) (Hazard Ratio, 7.7; 95% CI, 2.6-22.5; P <.001). Conclusions: In mCRC pts undergoing curative intent surgery or radiotherapy, detection of ctDNA post-procedure had a high PPV for cancer recurrence, with a median lead time of 6 months compared to surveillance imaging. Thus, ctDNA holds promise as a biomarker for pt enrollment on clinical trials and as an endpoint for monitoring of response to experimental therapies in this oligometastatic CRC population. Research Sponsor: Guardant Health.

3566 Poster Session

RAMucirumab in combination with TAS102 versus TAS102 monotherapy in metastatic colorectal cancer: Safety results from the phase IIb part of the RAMTAS phase II/III trial of the German AIO (AIO-KRK-0316).

Stefan Kasper, Thorsten Oliver Goetze, Sebastian Stintzing, Ralf Dieter Hofheinz, Marianne Sinn, Tobias Nicolaas Dechow, Thomas Jens Ettrich, Verena Keitel, Ullrich Graeven, Ludwig Fischer von Weikersthal, Alexander Kolov, Thomas Edelmann, Alexander Stein, Tanja Trarbach, Sabine Junge, Claudia Pauligk, Isabel Virchow, Jens T Siveke, Salah-Eddin Al-Batran, Martin H. Schuler; University Hospital Essen, West German Cancer Center, Essen, Germany; Institute of Clinical Cancer Research (IKF) at Krankenhaus Nordwest, UCT-University Cancer Center, Frankfurt, Germany; Medical Depart- ment, Division of Hematology, Oncology, and Tumor Immunology (CCM), Charite� Universita€tsmedizin Berlin, Berlin, Germany; University Hospital Mannheim, Mannheim, Germany; Universita€tsklinikum Hamburg-Eppendorf II. Medizinische Klinik, Hamburg, Germany; Onkologie Ravensburg, Ravensburg, Germany; Ulm University Hospital, Department of Internal Medicine I, Ulm, Germany; University of Du€sseldorf, Klinik fu€r Gastroenterologie, Hepatologie und Infektiologie Gastroonkologisches Studien- zentrum, Du€sseldorf, Germany; Kliniken Maria Hilf GmbH, Klinik fu€r Ha€matologie, Onkologie und Gas- troenterologie, Mo€nchengladbach, Germany; MVZ Gesundheitszentrum St. Marien GmbH, Amberg, Germany; VIDIA Christliche Kliniken Karlsruhe Medizinische Klinik 2, Karlsruhe, Germany; MedCenter Nordsachsen, Schkeuditz, Germany; Ha€matologisch-Onkologische Praxis Eppendorf (HOPE), Facharzt- zentrum Eppendorf, Hamburg, Germany; Klinikum Wilhelmshaven, Wilhelmshaven, Germany; Institut fu€r Klinische Krebsforschung IKF GmbH am Krankenhaus Nordwest, Frankfurt, Germany; Institute of Clinical Cancer Research (IKF) GmbH at Krankenhaus Nordwest, Frankfurt, Germany

Background: Patients (pts) with mCRC progressing on standard chemotherapy have limited therapeutic options. Trifluridine/tipiracil (TAS102) significantly improved survival in patients with refractory mCRC. Ramucirumab (ram) is approved in combination with FOLFIRI for second-line treatment. There is a strong rationale to evaluate the efficacy and safety of ram in combination with TAS102 in pts with refractory mCRC. Methods: This is a randomized, open-label, multicenter, starting as phase IIb and extended to a phase III study in pts with advanced mCRC. Eligible pts were randomized to receive either ram (8 mg/kg on d1+15, q4w) and TAS102 (35 mg/m2 on d1-5 and d8-12, q4w, arm A) or TAS102 alone (arm B). The primary endpoint is overall survival. A total of 145 pts were enrolled into phase IIb. Here, we present the data from an interim safety analysis comprising the first 80 treated pts. The trial was extended to phase III including a total of 426 pts. Enrolment of additional 281 pts started in Dec 2020. The trial endpoints remained unchanged. Results: Pts (40 arm A, 40 arm B) received a median of 2.5 treatment cycles in arm A and 2 cycles in arm B; 31 pts in treatment arm A and 32 pts in arm B discontinued participation prematurely, mainly due to cancer progression. Most patients developed adverse events (AEs): grade 3 AEs were observed in 28 pts (70%) in arm A (24 treatment-related) and 27 pts (67%, 17 treatment-related) in arm B. More grade 4 AEs were seen in arm A (13 pts, 32.5%) than in arm B (5 pts, 12.5%). In total, 46 Serious AEs (SAEs) occurred, 27 in arm A (10 treatment-related) and 19 in Arm B (2 treatment-related). Five SAEs (3 in arm A, 2 in arm B) had a fatal outcome (one in arm A treatment-related). Within the analyzed population, no SUSAR occurred. Conclusions: This safety analysis demonstrates a minor increase in AEs in the experimental arm but no unexpected events. There were no excessive toxicity or unacceptable risks. In summary, a favorable risk-benefit-profile was confirmed. Based on these safety results and the ongoing need for efficient treatment in the tested population, the trial was extended to phase III. Clinical trial information: NCT03520946. Research Sponsor: Lilly.

3567 Poster Session

Neoadjuvant chemotherapy to improve colon cancer survival in resectable metastatic colon cancer: A real world NCDB data analysis.

Saurabh Parasramka, Aasems Jacob, Quan Chen, Bin Huang, Zhonglin Hao; University of Kentucky, Lexington, KY; Markey Cancer Center, Lexington, KY; Kentucky Cancer Registry, Lexington, KY

Background: According to the SEER database, approximately 21% of colon cancer patients have synchronous metastatic disease at presentation with a five-year survival of only 14%. Liver is by far the most common site of metastasis. For resectable and borderline resectable metastatic lesions after conversion to surgical resection, five-year survival ranges between 40-70% in different series. Survival advantage of neoadjuvant chemotherapy is not clear. We present here an updated analysis of effect of different variables on survival of 3,247 patients from the National Cancer Database (NCDB) treated from 2010-2015. Methods: Adults 20 years or older with primary colon cancer and single organ metastatic disease either in the liver and/or lung at diagnosis were identified. All patients had received surgery to the primary site, resection of the distant site and chemotherapy within 1 year of diagnosis. Patients were categorized into 2 cohorts based on whether they received chemotherapy in the pre-operative/peri-operative setting (neoadjuvant chemotherapy –NAC) or post-operative setting (adjuvant chemotherapy AC). Descriptive analysis, Kaplan-Meier plots, Log-Rank tests and Cox regression models for multivariate survival analyses were performed. To assess uncertainty of estimates, a sensitive analysis was also performed based on the intention to treat principle by including additional surgery only and chemotherapy only cases. Results: A total of 3,247 patients with colon cancer with liver or lung metastases were identified. A large majority 2,527 patients (77.8%) received AC. 54.5% were males and 45.5% females. On multivariate analysis, patients who received NAC had overall survival (OS) advantage with hazard ratio (HR) 0.86 (0.75-0.98). Clinical factors associated with worse survival included age > 75 HR 1.31; positive margin status with R1 HR 1.49 or R2 HR 2.33; Comorbidity index $ 2 HR 1.68; positive KRAS status HR 1.20; N2 disease HR1.95; ; having liver metastasis compared to lung HR 1.65 ;. Factors associated with improved survival were CEA less than 30 ng/ml at diagnosis and left sided tumor with HR of 0.64 (0.56-0.72) and 0.75(0.67-0.84) respectively. Conclusions: Metastatic colon cancer with single organ liver or lung lesions benefit from neoadjuvant chemotherapy based on our analysis of the real-world data. The survival advantage in this setting has not been shown before. Re- search Sponsor: None.

3568 Poster Session

Influence of dietary insulin scores on survival in patients with metastatic colorectal cancer (mCRC): Findings from CALGB (Alliance) 80405.

Katherine DiNardo, Chao Ma, Fang-Shu Ou, Chen Yuan, Brendan John Guercio, Vicente Morales- Oyarvide, Erin Van Blarigan, Donna Niedzwiecki, I-Wen Chang, Heinz-Josef Lenz, Charles David Blanke, Alan P. Venook, Robert J. Mayer, Charles S. Fuchs, Federico Innocenti, Andrew B. Nixon, Richard M. Goldberg, Eileen Mary O’Reilly, Jeffrey A. Meyerhardt, Kimmie Ng; Brigham and Women’s Hospital, Boston, MA; Dana-Farber/Partners CancerCare, Boston, MA; Mayo Clinic, Rochester, MN; Dana-Farber Cancer Institute, Boston, MA; Memorial Sloan Kettering Cancer Center, New York, NY; UT Southwestern, Dallas, CO; University of California San Francisco, San Francisco, CA; Duke University Medical Center, Department of Biostatistics and Bioinformatics, Durham, NC; Southeastern Medical Oncology, Goldsboro, NC; USC Norris Comprehensive Cancer Center, Los Angeles, CA; Oregon Health and Science University, Portland, OR; Yale Cancer Center and Smilow Cancer Hospital, New Haven, CT; University of North Carolina at Chapel Hill, Chapel Hill, NC; Duke University Medical Center, Dur- ham, NC; West Virginia University Cancer Institute, Morgantown, WV; Department of Medical Oncology, Dana-Farber Cancer Institute/Partners Cancer Care, Boston, MA

Background: Diets inducing an elevated insulin response have been associated with increased recurrence and mortality in patients with non-metastatic colorectal cancer, but it remains unknown if post- prandial hyperinsulinemia also affects progression and mortality in mCRC patients. The goal of this study was to assess the influence of dietary insulin load (DIL) and dietary insulin index (DII) on survival of mCRC patients. Methods: This was a prospective cohort study of 1,177 patients with previously untreated mCRC enrolled in a phase III trial of systemic chemotherapy plus biologics who reported dietary intake within one month after chemotherapy initiation. DIL was calculated as a function of food insulin index and the energy content of individual foods reported on a food frequency questionnaire. DII was calculated by dividing DIL by total energy intake. The primary endpoint was overall survival (OS). Sec- ondary endpoints were progression-free survival (PFS) and treatment-related adverse events (TRAEs). The primary statistical test was a test for trend, which was performed using the median value for each quintile of dietary insulin score as a continuous variable. Cox proportional hazards regression was used to adjust for potential confounders including assigned treatment arm, known prognostic factors, comorbidities, body mass index, and physical activity. Results: Higher DIL was significantly associated with worse OS (ptrend = 0.04); patients in the highest quintile survived 34.1 months, compared to 27.7 months in the lowest quintile (Cox hazard ratio [HR] 1.22, 95% confidence interval [CI] 0.99 - 1.51). Higher DII was non-significantly associated with worse OS (HR 1.18, 95% CI 0.94 - 1.48, ptrend = 0.09). There was no significant association between dietary insulin scores and PFS. The influence of dietary insulin scores on survival did not differ significantly by various molecular markers involved in the insulin signaling pathway, including C-peptide, adiponectin, IGF-1, IGFBP-3, and IGFBP-7. Higher dietary insulin scores were significantly associated with greater risk of any TRAE. Those with a DIL greater than the median had a 75.4% rate of any TRAE, compared to 70.8% in those with a DIL less than or equal to the median (HR 1.19, 95% CI 1.03 - 1.38, p=0.02); the most significant associations were with neutropenia (HR 1.30, 95% CI 1.05 - 1.61, p=0.01) and diarrhea (HR 1.43, 95% CI 1.00 - 2.06, p=0.05). Conclusions: Higher DIL was significantly associated with worse OS, and both higher DIL and DII were significantly associated with increased TRAEs, in patients with previously untreated mCRC. These findings may inform future dietary recommendations for patients with mCRC. Further investiga- tion into the molecular mechanisms underlying these associations is warranted. Clinical trial information: NCT00265850. Research Sponsor: U.S. National Institutes of Health, Pharmaceutical/ Biotech Company.

3570 Poster Session

Exploratory biomarker findings from cohort 2 of MODUL: An adaptable, phase 2, signal- seeking trial of fluoropyrimidine + bevacizumab ± atezolizumab maintenance therapy for BRAFwt metastatic colorectal cancer.

Josep Tabernero, Axel Grothey, Dirk Arnold, Michel Ducreux, Peter J. O’Dwyer, Maurizio Perdicchio, Alexander Abbas, Meghna Das Thakur, Natsumi Irahara, Anila Tahiri, Hans-Joachim Schmoll, Eric Van Cutsem; Vall d’Hebron University Hospital and Institute of Oncology (VHIO), IOB-Quiron, UVic-UCC, Barcelona, Spain; West Cancer Center, Germantown, TN; Asklepios Tumorzentrum Hamburg, AK Alto- na, Hamburg, Germany; Gustave Roussy, Universite� Paris-Saclay, Villejuif, France; Abramson Cancer Center, University of Pennsylvania, Philadelphia, PA; F. Hoffmann-La Roche Ltd., Basel, Switzerland; Genentech Inc., South San Francisco, CA; Martin-Luther-University, Halle, Germany; University Hospi- tals Gasthuisberg, Leuven and KU Leuven, Leuven, Belgium

Background: MODUL is an adaptable, phase 2, signal-seeking trial testing novel agents as first-line therapy for predefined subgroups of patients with metastatic colorectal cancer (mCRC). Previously reported findings demonstrated that adding atezolizumab to fluoropyrimidine (FP)/bevacizumab as first-line maintenance treatment after induction with FOLFOX + bevacizumab did not improve efficacy outcomes in BRAFwt mCRC. Given these efficacy results, exploratory assessments on tumour samples were conducted to provide insights into factors that might affect efficacy of maintenance treatment and provide guidance on appropriate therapeutic strategies for BRAFwt mCRC. Methods: In patients with BRAFwt tumours (Cohort 2), experimental treatment was FP/bevacizumab + atezolizumab. Primary efficacy endpoint: progression-free survival (PFS). Overall survival (OS) was a secondary endpoint. Archival tissue samples from 104 patients were analysed by immunohistochemistry (IHC) at HistoGeneX (PD-L1; CD8/ GrB/FoxP3). SP142 antibody was used for PD-L1 IHC analysis, which evaluated PD-L1low (IC 0–1) vs PD-L1high (IC > 1) in correlation with PFS and OS in the control and experimental arms. CD8/GrB/ FoxP3 triplex staining was also performed to evaluate potential correlations with efficacy. Results: 445 patients with BRAFwt mCRC were randomised (2:1 ratio) to maintenance treatment in Cohort 2. Archi- val samples from 104 patients were analysed: FP/bevacizumab + atezolizumab (n = 82); FP/bevacizumab (n = 22). The biomarker evaluable population (BEP) for PD-L1 was n = 81 for FP/bevacizumab + atezolizumab [PD-L1low n = 35 (43%); PD-L1high n = 46 (57%)] and n = 22 for FP/bevacizumab [PD- L1low n = 16 (72%); PD-L1high n = 6 (28%)]. The BEP for CD8/GrB was n = 50 for FP/bevacizumab + atezolizumab and n = 16 for FP/bevacizumab. No difference in PFS or OS was observed in the FP/bevacizumab + atezolizumab vs FP/bevacizumab arms for PD-L1high [PFS: HR = 1.5 (95% CI 0.45À 4.8), p = 0.51; OS: HR = 1.3 (95% CI 0.38À 4.1), p = 0.71] or PD-L1low [PFS: HR = 0.92 (95% CI 0.47À 1.8), p = 0.81; OS: HR = 0.78 (95% CI 0.4À 1.5), p = 0.48]. Similar results were observed with CD8/GrBhigh [PFS: HR = 0.73 (95% CI 0.27À 2.0), p = 0.55; OS: HR = 0.66 (95% CI 0.24À 1.8), p = 0.44], CD8/GrBlow [PFS: HR = 1.0 (95% CI 0.42–2.5), p = 0.96; OS: HR = 0.73 (95% CI 0.3–1.8), p = 0.5], FoxP3high [PFS: HR = 0.97 (95% CI 0.37À 2.5), p = 0.95; OS: HR = 0.95 (95% CI 0.36À 2.5), p = 0.91] and FoxP3low [PFS: HR = 0.73 (95% CI 0.29À 1.9), p = 0.53; OS: HR = 0.5 (95% CI 0.19À 1.3), p = 0.18]. Conclusions: These findings suggest that PD-L1, CD8/GrB and FoxP3 might not be predictive biomarkers in BRAFwt mCRC. Further analyses are needed to further evaluate potential predictive and prognostic factors of response in this setting. Clinical trial information: NCT02291289. Research Sponsor: F. Hoffmann-La Roche Ltd.

3571 Poster Session

Treatment responses and disease dynamics in patients with untreated metastatic colorectal cancer receiving bevacizumab-based sequential versus combination chemotherapy: Analysis of a phase 3 trial (AIO KRK0110, XELAVIRI study).

Annika Kurreck, Volker Heinemann, Ludwig Fischer von Weikersthal, Thomas Decker, Florian Kaiser, Jens Uhlig, Michael Schenk, Jens Freiberg-Richter, Bettina Peuser, Claudio Denzlinger, Ullrich Graeven, Kathrin Heinrich, Swantje Held, Arndt Stahler, Annabel Helga Sophie Alig, Ivan Jelas, Jobst C. von Einem, Sebastian Stintzing, Clemens Giessen-Jung, Dominik Paul Modest; Charite� Univer- sity Medicine Berlin, Berlin, Germany; University Hospital Munich, LMU Munich, Munich, Germany; Department of Oncology, Gesundheitszentrum St. Marien GmbH, Amberg, Germany; Onkologie Ravens- burg, Ravensburg, Germany; VK&K Studienzentrum, Landshut, Germany; Medical Practice for Hema- tology and Oncology Muldental, Naunhof, Germany; Hospital Barmherzige Bru€der Regensburg, Regensburg, Germany; Oncological Practice, Dresden, Germany; Oncological Practice, Leipzig, Germa- ny; Marienhospital Stuttgart, Stuttgart, Stuttgart, Germany; Kliniken Maria Hilf GmbH, Klinik fu€r Ha€matologie, Onkologie und Gastroenterologie, Mo€nchengladbach, Germany; Department of Medicine III, University Hospital, LMU Munich, Mu€nchen, Germany, Munich, Germany; ClinAssess GmbH, Le- verkusen, Germany; Medical Department, Munich, Germany; Medical Department, Divison of Hematol- ogy, Oncology, and Tumor Immunology (CCM), Charite� Universitaetsmedizin, Berlin, Germany; Charite� Comprehensive Cancer Center, Berlin, Germany; Department of Medicine III, University Hospital, Mu- nich, Germany; Medical Department, Division of Hematology, Oncology, and Tumor Immunology (CCM), Charite� Universita€tsmedizin Berlin, Berlin, Germany; Department of Medicine III, University Hospital, LMU Munich, Munich, Germany

Background: Early response parameters such as early tumor shrinkage (ETS), depth of response (DpR), and time to DpR represent exploratory endpoints that may serve as early efficacy endpoints and potential predictors of long-term outcome. We analyzed the association of these endpoints with bevacizumab-based sequential (initial fluoropyrimidines) versus combination (initial fluoropyrimidines plus irinotecan) chemotherapy within a randomized phase III trial. Methods: DpR (change from baseline to smallest tumor diameter), ETS ($20% reduction in tumor diameter at first reassessment), and time to DpR (study randomization to DpR-image) were analyzed in the XELAVIRI-trial. Moreover, progression- free survival (PFS) and overall survival (OS) were evaluated with ETS as stratification parameter (ETS vs. no ETS) according to treatment arm, molecular subgroup, and sex. Results: 370 patients were available for analysis of early treatment response parameters. A higher rate of ETS (60.9% vs. 43.5%; p = 0.001) and significantly greater DpR (-40.0% vs. -24.7%; p < 0.001) were observed in the initial combination compared to the sequential therapy arm, respectively. The improvement of ETS and DpR was pronounced in the subpopulation of RAS/BRAF wildtype patients. Male in contrast to female patients significantly benefitted from initial combination treatment in terms of median DpR (male: -40.0% vs. -22.2%; p < 0.001; female: -34.0% vs. -24.4%; p = 0.13) and rate of ETS (male: 64.8% vs. 40.2%; p < 0.001; female: 52.5% vs. 49.3%; p = 0.73). Achievement of ETS correlated with improved survival irrespective of treatment arm (PFS: p < 0.001; OS: p = 0.012) and molecular subgroup (PFS: p < 0.001; OS: p < 0.001). Whereas the survival benefit in male patients achieving ETS was statistically significant (PFS: p < 0.001, HR 0.532 (0.409-0.692); OS: p < 0.001, HR 0.574 (0.437-0.756)), there were no significant differences in PFS (p = 0.107) and OS (p = 0.965) of female patients depending on ETS. Conclusions: In the XELAVIRI trial, initial irinotecan-based combination therapy with bevacizumab improves ETS and DpR in mCRC patients. Improvement in early response parameters appears pronounced in patients with RAS/BRAF wildtype tumors suggesting a high sensitivity to irinotecan- based treatment. ETS was predictive of PFS and OS regardless of treatment arm. This finding was rather driven by male than female patients, potentially indicating that ETS might be less predictive of long- term outcome in an elderly, female population. Research Sponsor: Arbeitsgemeinschaft Internistische Onkologie.

3572 Poster Session

Serial circulating tumor DNA (ctDNA) monitoring in metastatic colorectal cancer (mCRC) reveals dynamic profile of actionable alterations.

Jonathan M. Loree, Jason Henry, Kanwal Pratap Singh Raghav, Christine Megerdichian Parseghian, Kimberly Banks, Victoria M. Raymond, Rebecca Nagy, Chuck Hensel, John H. Strickler, Ryan Bruce Corcoran, Michael J. Overman, AmirAli Talasaz, Scott Kopetz; BC Cancer, Vancouver, BC, Canada; MD Anderson Hematology/Oncology Fellowship, Houston, TX; The University of Texas MD Anderson Cancer Center, Houston, TX; Guardant Health, Inc., Redwood City, CA; Duke University School of Medicine, Durham, NC; Massachusetts General Hospital, Boston, MA; Department of Gastrointestinal Medical Oncology, University of Texas MD Anderson Cancer Center, Houston, TX

Background: Serial ctDNA can measure dynamic changes in disease burden over time, however utility of serial profiling to detect changes in actionable alterations remains unclear. Methods: We evaluated 501 patients with $3 serial Guardant360 assays performed between 09/2016 and 11/2020 and compared MSI, fusion, amplification and single nucleotide variant (SNV) detection over time. This comprised 2147 assays with a median of 4 assays per patient (min 3, max 18) occurring an average of 163 days apart (+/- SD of 147 days). Maximum detected variant allele frequency in samples (maxVAF) was assessed for relation to changes in detected alterations as a surrogate for tumor volume. Results: Among 406 patients with assays assessable for MSI-status, 17 (4.2%) had MSI detected. New MSI detection on a subsequent assay always occurred with a rising maxVAF (3/3) that was also $0.7%, while loss of detectable MSI between assays always associated with falling maxVAFs (7/7) with 6/7 occurring when maxVAF fell below 0.4%. Fusions were noted in 9/501 (2%) patients. Among 3 patients who lost a detectable fusion, maxVAF decreased in 1 patient and changed #0.2% between assays in 2, while 2/3 patients with new fusions had rising maxVAFs and 1 patient had a falling maxVAF. Amplifications were detected in 242/501 patients (48%). While most genes had highly variable amplification detection between assays (9% serially detected), ERBB2 amplifications were more consistent and serially detected in 39% of detected cases (P < 0.0001). New detection of amplifications occurred more commonly in cases with rising maxVAF (OR 11.70, 95% CI 7.61-18.00, P < 0.0001) and loss of detectable amplifications occurred more between samples with falling maxVAF (OR 12.37, 95% CI 8.35-18.66, P < 0.0001). Change in maxVAF correlated with change in number of detected amplifications (r = 0.62, P < 0.0001), but only partially explained changes seen (R2= 0.39). Between serial assays, SNVs changed a median of 0 variants (IQR -1 to 1), however some patients had significant changes (max gain 21/max loss 18). Among 1646 serial time points, 454 (28%) had no change in SNVs, 674 (41%) gained SNVs, and 518 (31%) lost SNVs on subsequent assays. Gains were more common in samples with rising maxVAF (OR 7.76, 95% CI 6.18-9.73, P < 0.0001) while losses were more common when maxVAF fell (OR 6.90, 95% CI 5.47-8.66, P < 0.0001). The correlation between maxVAF change and SNV change was significant (r = 0.29, P < 0.0001), but minimally explained SNV changes (R2= 0.086) and was a much weaker association than noted for amplification changes. Conclusions: We noted significant differences in detection of actionable alterations across serial ctDNA assays. Increased ctDNA volume (higher maxVAF) due to tumor progression may explain some variation over time, but variability also occurs outside these changes, likely reflecting clonal evolution following therapy. Research Sponsor: None.

3573 Poster Session

Efficacy and safety of vactosertib and pembrolizumab combination in patients with previously treated microsatellite stable metastatic colorectal cancer.

Tae Won Kim, Keun Wook Lee, Joong Bae Ahn, Jin Lee, Jiyeon Ryu, Bitna Oh, Chan-Young Ock, Sunjin Hwang, Ki Baik Hahm, Seong-Jin Kim, Young Suk Park; Asan Medical Center, University of Ulsan Col- lege of Medicine, Seoul, South Korea; Seoul National University Bundang Hospital, Seoul National Uni- versity College of Medicine, Seongnam, South Korea; Department of Internal Medicine, Yonsei Cancer Center, Yonsei University College of Medicine, Seoul, South Korea; MedPacto, Inc., Seoul, South Ko- rea; Medpacto, Seoul, South Korea; MedPacto, Inc., Suwon, South Korea; Division of Hematology-On- cology, Department of Medicine, Samsung Medical Center, Sungkyunkwan University School of Medicine, Seoul, South Korea

Background: Microsatellite stable metastatic colorectal cancer (MSS mCRC) represents a high unmet need since there are currently no approved immunotherapy options. Since the inhibition of the trans- forming growth factor-b (TGF-b) pathway is known to contribute to the enhancement of immunotherapy efficacy, here, we report the results of vactosertib, a potent and selective TGF-b receptor I kinase inhibitor, combined with pembrolizumab in patients with MSS mCRC. Methods: In this phase 2, open label trial, patients have received vactosertib (300 mg BID, 5 days on / 2 days off) and pembrolizumab (200 mg, every 3 weeks) until confirmed progressive disease (PD), unacceptable toxicity or consent withdrawal. Patients with histologically confirmed mCRC who had disease progression after treatment with all available therapies including fluoropyrimidine and oxaliplatin or irinotecan were enrolled. Eligible patients were $19 years old, had ECOG status #1, and had no prior exposure to immunotherapy. The objectives were to evaluate the safety and efficacy (objective response rate (ORR) per RECIST v1.1). Re- sults: Thirty-three patients with MSS mCRC were enrolled. Median age was 60 (range 33-72), 55% were male, median number of previous lines of chemotherapy was 3 (range 1-7), and 82% were consensus molecular subtype (CMS) 4. The ORR was 15.2% including 5 partial responses (PRs), 7 stable diseases, and 17 PDs as best overall responses; 12 patients remain on treatment. Among 5 patients with PR, 3 patients had confirmed PR and median duration of response was not reached yet. As of 04 Jan 2021, the most common treatment related adverse events (AEs) were increased amylase (21.2%), pru- ritus (21.2%), rash (21.2%), and increased lipase (18.2%). There were 3 treatment-related SAEs reported; drug induced pneumonitis (3%), nausea (3%), and vomiting (3%). Conclusions: The combination treatment with vactosertib and pembrolizumab showed favorable safety profile with promising efficacy in patients with MSS mCRC. Updated data including pharmacodynamic markers will be presented at the meeting. Clinical trial information: NCT03724851. Research Sponsor: MedPacto.

3574 Poster Session

Safety and pharmacokinetic analysis of UGT1A1 genotype-guided dosing of irinotecan.

Emma C. Hulshof, Mirjam de With, Femke M. de Man, Geert-Jan Creemers, Birgit ALM Deiman, Jesse J. Swen, Saskia Houterman, Stijn L.W. Koolen, Marjan Laven, Saskia Luelmo, Ron H.N. van Schaik, Henk-jan Guchelaar, Ron H.J. Mathijssen, Hans Gelderblom, Maarten J. Deenen; Department of Clinical Pharmacy, Catharina Hospital, Eindhoven, Netherlands; Department of Medical Oncology, Erasmus University Medical Center, Rotterdam, Netherlands; Department of Medical Oncology, Cathar- ina Hospital, Eindhoven, Netherlands; Department of Molecular Biology, Catharina Hospital, Eindho- ven, Netherlands; Department of Clinical Pharmacy and Toxicology, Leiden University Medical Center, Leiden, Netherlands; Department of Education and Research, Catharina Hospital, Eindhoven, Nether- lands; Department of Medical Oncology, Leiden University Medical Center, Leiden, Netherlands; De- partment of Clinical Chemistry, Erasmus Medical Center, Rotterdam, Netherlands; Department of Clinical Pharmacy & Toxicology, Leiden University Medical Center, Leiden, Netherlands; Department of Medical Oncology, Erasmus Medical Center, Rotterdam, Netherlands

Background: Irinotecan is commonly used in the treatment of advanced colorectal and pancreatic cancer. The polymorphisms UGT1A1*28 (7 TA repeats) and UGT1A1*93 (SNP -3156G > A) are significantly associated with increased systemic exposure of irinotecan’s active metabolite SN-38 and subsequently severe irinotecan-associated adverse-events (AEs) including (febrile) neutropenia and diarrhea. Severe AEs may lead to hospitalization, loss of quality of life, treatment delay and/or treatment discontinuation. Nonetheless, prospective genetic screening is not yet routinely performed. The aim of this study was to determine the safety and pharmacokinetics of UGT1A1 genotype-guided dosing of irinotecan in UGT1A1 poor metabolizers (PMs), i.e. UGT1A1 *28/*28 and/or UGT1A1*93/*93 individuals, in order to reduce the incidence of severe irinotecan-associated AEs. Methods: A prospective, multicenter, non-randomized study was conducted in patients intended to be treated with irinotecan at a dose of $ 180 mg/m2 or 450-600 mg flat dose. All patients were pre-therapeutically genotyped for UGT1A1*28 and UGT1A1*93. In UGT1A1 PMs, an initial 30% dose reduction in the first cycle was applied followed by further individual dose titration based on neutrophil count and clinical tolerability. The primary endpoint was the incidence of febrile neutropenia in the first 2 cycles of irinotecan treatment. UGT1A1 PMs were compared to 1] historical control patients, i.e. homozygous polymorphic patients treated with full dose therapy identified from systematic literature review and to 2] UGT1A1 non- PMs treated with standard dose therapy. In addition, systemic SN-38 exposure (AUC0-500h) of reduced dosing in the UGT1A1 PM cohort was compared to a standard dosed irinotecan patient cohort [doi: 10.1200/JCO.2000.18.1.195] by an independent T-test. Results: A total of 349 patients were pre- therapeutically genotyped and included for analysis. Thirty-one (8.9%) patients were UGT1A1 PM, in whom an initial median 30% dose reduction was applied. The incidence of febrile neutropenia in this group was 6.5% compared to 24% in historical controls (n = 50) (p = 0.042) and comparable with the incidence (4.1%; p = 0.632) in UGT1A1 non-PMs treated with full dose therapy. The systemic exposure of SN-38 of reduced dosing in UGT1A1 PMs (n = 17) was comparable to the systemic exposure of the standard dosed irinotecan patient cohort (n = 46) with a relative difference of +24% (p = 0.054) with a geometric mean (CV) of SN-38 AUC0-500h of 391 (43.7%) versus 298 (75.3%) ng*h/mL, respectively. Conclusions: UGT1A1 genotype-guided dosing significantly reduces the incidence of febrile neutropenia in UGT1A1 PM patients treated with irinotecan. In addition, systemic drug exposure remained adequate, despite the 30% dose reduction. Therefore, UGT1A1 genotype-guided dosing of irinotecan should be considered standard of care in order to improve the individual patient safety. Clinical trial information: Trial NL6270 (NTR6612). Research Sponsor: Catharina Research Foundation.

3575 Poster Session

Characteristics of patients (pts) and prognostic factors across treatment lines (TL) in metastatic colorectal cancer (mCRC): An analysis from the Analysis and Research in Cancers of the Digestive System (ARCAD) database.

Jean-Baptiste Bachet, Benoist Chibaudel, Manel Rakez, Richard M. Goldberg, Niall C. Tebbutt, Eric Van Cutsem, Daniel G. Haller, J. Randolph Randolph Hecht, Robert J. Mayer, Stuart M. Lichtman, Al Benson, Alberto F. Sobrero, Josep Tabernero, Richard Adams, John Raymond Zalcberg, Axel Grothey, Takayuki Yoshino, Qian Shi, Aimery De Gramont; Pitie�-Salpe^trie�re Hospital, Paris, France; Franco-Brit- ish Institute, Levallois-Perret, France; Statistical Unit, ARCAD Foundation, Levallois-Perret, France; West Virginia University Cancer Institute, Morgantown, WV; Olivia Newton-John Cancer, Wellness and Research Centre, Austin Health, Heidelberg, VIC, Australia; University Hospitals Gasthuisberg, Leuven and KU Leuven, Leuven, Belgium; Abramson Cancer Center of the University of Pennsylvania, Philadel- phia, PA; David Geffen School of Medicine, University of California, Los Angeles, CA; Dana-Farber Can- cer Institute, Boston, MA; Memorial Sloan Kettering Cancer Center, New York, NY; Northwestern Medicine, Chicago, IL; IRCCS Ospedale Policlinico San Martino, Genoa, Italy; Vall d’Hebron University Hospital and Vall d’Hebron Institute of Oncology (VHIO), UVic-UCC, Barcelona, Spain; Cardiff Universi- ty and Velindre Cancer Centre, Cardiff, United Kingdom; Peter MacCallum Cancer Centre, Melbourne, Australia; West Cancer Center, OneOncology, Germantown, TN; National Cancer Center Hospital East, Kashiwa, Japan; Mayo Clinic, Rochester, MN

Background: Pts with mCRC frequently receive $1 sequential treatment TL. Approximately 50%-60% of pts receive second-line (L2) and 20%-30% third-line (L3) regimens in routine practice. We investigated the pts clinical/tumor characteristics and their prognostic impact across TL. Methods: Data from 37,560 pts enrolled in 48 randomized trials (34 in first (L1), 9 in L2, and 5 in L3) were analyzed. Can- didate variables (VAR) measured at enrollment were sex, age, body mass index, performance status (PS), bilirubin, hemoglobin (Hb), platelets (Pl), derived white blood cells-to-absolute neutrophil counts ratio (WBC/ANC), lactate dehydrogenase (LDH), alkaline phosphatase (ALP), primary tumor location, and number and location of metastatic sites (MS). Missing data were imputed. VAR with significant value at all TL were selected to construct a prognostic score of overall survival (OS) in training set (TS, n=30,050; 80%). For each TL, the score was calculated as the sum on the estimations of the VAR’ coef- ficients from the common multivariate model; Cox’s model was used to define risk groups. The discrimi- nation capability was assessed using the Harrell’s C-index. External validation was done in the validation set (VS, n=7,510; 20%). Results: A total of 26,974 pts in L1, 7,693 pts in L2 and 2,893 pts in L3 were analyzed. The following characteristics increased continuously over TL: $2 MS (57%, 72%, 82%), lung metastases (50%, 74%, 91%), lymph nodes metastases (51%, 61%, 80%), KRAS mutation (37%, 47%, 51%) and elevated ALP (46%, 52%, 61%). BRAF mutation decreased (9%, 7%, 5%). In L1 vs L3 trials, 70% vs 89% of patients had primary tumor resection, 10% vs 80% had at least one metastasectomy and 31% vs 78% had a late metachronous (>12 months) metastasis. 7 independent VAR were retained in the prognostic score (PS, Hb, Pl, WBC/ANC, LDH, ALP, and the number of MS); four pt groups with significantly different prognoses were defined (table). This score remained valid when excluding pts with PS 2. Third-line oral drugs (vs placebo) and subsequent line (L2/L1 or L3/ L2) were effective in all prognostic groups. Conclusions: Clinical/tumor pt characteristics significantly varied over subsequent TL in patients included in randomized trials. The same prognostic model using practical clinical and biological variables can be used in all TL. Research Sponsor: ARCAD.

L1* L2* L3* Score class TS VS TS VS TS VS

1 25.7 [25.3–26.3] 27.0 [26.0–27.7] 17.1 [16.5–17.7] 17.4 [16.1–18.6] 10.1 [9.7–11.1] 10.2 [8.9–12.5] 2 18.8 [18.4–19.3] 18.4 [17.6–19.4] 11.4 [10.8–11.8] 12.0 [11.2–13.0] 6.0 [5.6–6.3] 5.6 [4.9–6.2] 3 13.8 [13.4–14.3] 14.1 [13.2–15.0] 8.2 [7.7–8.7] 8.3 [7.7–9.2] 4.2 [3.8–4.6] 3.9 [3.3–4.9] 4 10.3 [9.8–10.7] 11.0 [10.3–12.0] 5.2 [4.7–5.7] 5.0 [4.3–6.1] 2.7 [2.4–3.2] 3.1 [2.7–4.2] 1-yr OS 0.65 0.65 0.66 0.66 0.69 0.68 C-index

*median OS [95% CI], months.

3577 Poster Session

Liquid biopsy-driven anti-EGFR rechallenge in patients with metastatic colorectal cancer.

Stefano Mariani, Marco Puzzoni, Nicole Liscia, Valentino Impera, Andrea Pretta, Simona Tolu, Anna Grazia Pireddu, Mara Persano, Francesca Musio, Clelia Donisi, Giovanna Pinna, Eleonora Lai, Laura Demurtas, Pina Ziranu, Valeria Pusceddu, Marina Pisano, Grazia Palomba, Milena Casula, Giuseppe Palmieri, Mario Scartozzi; University of Cagliari, Cagliari, Italy; Medical Oncology Depart- ment, University Hospital, University of Cagliari, Cagliari, Italy; Gastrointestinal Unit, Department of Medical Oncology, Institut Jules Bordet-Universite� Libre de Bruxelles (ULB), Brussels, Belgium; Medi- cal Oncology Department, University Hospital, University of Cagliari, Monserrato, Italy; Medical Oncolo- gy Unit, University Hospital and University of Cagliari, Cagliari, Italy; Unit of Cancer Genetics, National Research Council (CNR), Sassari, Italy; Institute of Biomolecular Chemistry (ICB), CNR, Sassari, Italy; ICB-CNR, Cancer Genetics Unit, Sassari, Italy

Background: The rechallenge with EGFR inhibitors represents an emerging strategy for anti-EGFR pre- treated patients with RAS wild type colorectal cancer (CRC). Unfortunately definitive selection criteria for anti-EGFR rechallenge in this setting are lacking. Very recently RAS wild type status on circulating tumor DNA (ct-DNA) at the time of rechallenge along with already known clinical criteria emerged as a potential watershed for this strategy. In the present study we explored liquid biopsy-driven anti-EGFR rechallenge strategy in the clinical practice for patients with metastatic colorectal cancer. Methods: Ct- DNA from RAS and BRAF wild type metastatic CRC patients previously treated with an anti-EGFR containing therapy was analyzed for RAS/BRAF mutations with the aim to evaluate the rechallenge strategy with anti-EGFR. The ct-DNA was analyzed for RAS-BRAF mutations using pyro-sequencing (PyroMark Q24 MDx Workstation) and nucleotide sequencing (Genetic Analyzer ABI3130) assays. Real-time PCR (Idylla) and droplet digital PCR (QX200 System) were performed to confirm the RAS-BRAF mutation status. Several clinical variables including previous response to anti EGFR containing therapy, tumor sidedness and anti-EGFR free interval were evaluated in relation to outcome. Tumor response evaluation was performed according to RECIST 1.1. Differences between categorical variables were evaluated using the Fisher’s exact test. Survival probability over time was estimated by the Kaplan–Meier method. Significant differences in the probability of survival between the strata were evaluated by log-rank test. Results: Twenty patients were included in the study. All patients were tested for RAS-BRAF mutations in ct-DNA. Fourteen patients (70%) showed a RAS-BRAF WT molecular profile, six patients (30%) showed a KRAS mutation. All the patients with ct-DNA RAS-BRAF WT profile underwent rechallenge with anti-EGFR. In details 11 patients (78.6%) underwent irinotecan+ cetuximab treatment, whereas 3 patients (21.4%) underwent panitumumab monotherapy. As for the outcome results to the rechallenge strategy, the median OS was 7 months (95% CI 5.0 to 13.0), the median PFS was 3 months (95% CI 2.0 to 6.0), the ORR was 27.3% with a DCR of 54.5%. Among the clinical variables evaluated as puta- tive predictive/prognostic factors, previous response to anti-EGFR treatment was related to a not statistically significant improved OS (12 months vs 5 months HR:0.19 p: 0.06) and to a statistically significant improved ORR (75% vs 0% p:0.03). Conclusions: The rechallenge strategy with anti-EGFR confirmed to be feasible in clinical practice. The clinical outcome resulted consistent with the literature data. In addition to the molecular selection through the analysis of ct-DNA for RAS, previous response to anti EGFR treatment is confirmed as a prospective selection criteria for this therapeutic option. Re- search Sponsor: None.

3578 Poster Session

Final results from the CAVE (cetuximab rechallenge plus avelumab) mCRC phase II trial: Skin toxicity as a predictor of clinical activity.

Giulia Martini, Stefania Napolitano, Vincenzo Famiglietti, Filippo G. De Braud, Marinella Terminiello, Carola Borrelli, Pietro Paolo Vitiello, Antonio Avallone, Nicola Normanno, Evaristo Maiello, Alfredo Falcone, Giuseppe Santabarbara, Carmine Pinto, Daniele Santini, Alessandra Di Liello, Daniela Renato, Lucia Esposito, Francesca Marrone, Teresa Troiani, Davide Ciardiello; Medical Oncology, Universita� degli Studi della Campania "Luigi Vanvitelli", Naples, Italy; Universita� Degli Studi Della Campania, Luigi Vanvitelli, Naples, Italy; AOU Oncoematologia Universita Della Campania Luigi Vanvitelli, Naples, Italy; Medical Oncology Department, ENETS Center of Excellence, Fondazione IRCCS Istituto Nazionale dei Tumori and Oncology and Hemato-oncology Department, University of Milan, Milan, Italy; Medical On- cology, Department of Precision Medicine, School of Medicine, "Luigi Vanvitelli" University of Campa- nia, Naples, Italy; Istituto Nazionale Tumori Fondazione G. Pascale, Naples, Italy; Istituto Nazionale Tumori, IRCCS, Fondazione G.Pascale, Naples, Italy; Oncology Unit, Foundation IRCSS Casa Sollievo della Sofferenza, San Giovanni Rotondo, Italy; Azienda Ospedaliera Universitaria Pisana, Pisa, Italy; Oncologia Medica, Azienda Ospedaliera di Rilievo Nazionale S. G. Moscati, Avellino, Avellino, Italy; Medical Oncology Unit, Clinical Cancer Center, AUSL-IRCCS Reggio Emilia, Reggio Emilia, Italy; De- partment of Medical Oncology, Campus Bio-Medico University of Rome, Rome, Italy; Universita� degli Studi della Campania Luigi Vanvitelli, Naples, Italy; University of Campania "Luigi Vanvitelli", Naples, Italy; Medical Oncology Universita� degli Studi della Campania "Luigi Vanvitelli", Naples, Italy

Background: Promising antitumor activity of so called rechallenge treatment with anti-epidermal growth factor receptor (EGFR) drugs in patients with RAS wild type (RAS WT) metastatic colorectal cancer (mCRC) has been recently reported. Beside the absence of resistance mutations at plasma circulating tumor DNA (ctDNA) analysis, no biomarkers of response to anti-EGFR rechallenge strategy have been identified. Methods: We conducted the single arm phase II CAVE mCRC trial to evaluate the combination of cetuximab as rechallenge plus avelumab treatment in 77 RAS WT mCRC patients, with complete (CR) or partial response (PR) to first line chemotherapy plus anti-EGFR drugs, who developed acquired resistance and received a subsequent line of therapy. A post-hoc baseline analysis of circulating tumor DNA (ctDNA) for KRAS, NRAS, BRAF and EGFR-S492R mutations was performed for 67/77 (87%) patients. The correlation of skin toxicity (ST) and other clinical variables with OS, PFS and response rate (RR) was assessed. Results: Cetuximab plus avelumab provided in the intention to treat population (ITT) median overall survival (mOS) of 11.6 months [95% Confidence Interval (CI), 8.4-14.8] and median PFS (mPFS) of 3.6 months (95% CI, 3.2-4.1) with a manageable toxicity profile. Thirty-three (42.9%) patients experienced grade 2-3 ST with mOS of 17.8 months (CI 95% 14.9-20.7), whereas 44 (57.1%) patients with grade 0-1 ST exhibited mOS of 8.2 months (CI 95% 5.6-11), (HR 0.51, CI 95% 0.29-0.89, P = 0.019). mPFS was 4.6 months (CI 95% 3.5-5.8) in patients with grade 2-3 ST, compared to 3.4 months (CI 95% 2.8-4.1) in patients with grade 0-1 ST (HR 0.49, CI 95% 0.3-0.8, P = 0.004). Grade 2-3 ST and baseline RAS/BRAF/EGFR WT ctDNA were the only variables with statistically significant effect on OS both at univariate and multivariate analyses. ST, number of metastatic sites #2, surgery of primary tumor and RAS/BRAF/EGFR WT ctDNA were associated with an improvement in PFS only at univariate analysis. In the 33 patients with grade 2-3 ST, 1 (3%) CR, 2 (6.1%) PR and 24 (72.7%) stable disease (SD) were observed, with disease control rate (DCR) of 81.8%. In the 44 patients with grade 0-1 ST 0 CR, 3 (6.8%) PR, 20 (45.5%) SD, with 52.3% DCR were reported. Conclu- sions: Cetuximab plus avelumab is effective and well tolerated as rechallenge treatment in mCRC. ST is a clinical biomarker for the identification of RAS/BRAF mCRC patients that could benefit from anti- EGFR rechallenge. Clinical trial information: NCT04561336. Research Sponsor: Regione Campania I-Cure Research Project.

3579 Poster Session

Circulating tumor DNA and circumferential resection margin as key prognostic indicators for survival in rectal cancer.

Mia Shepherdson, Erin L. Symonds, Susan Byrne, Kirsten Gormly, Christos Stelios Karapetis, Sina Vatandoust, Graeme P. Young, Amitesh Chandra Roy; Flinders University, Adelaide, SA, Australia; Flin- ders Centre for Innovation in Cancer, Bedford Park, Australia; Dr Jones & Partners, Stepney, SA, Austra- lia; Flinders Medical Centre, Flinders University, Adelaide, SA, Australia

Background: Recurrence of colorectal cancer has been linked to the presence of epigenetic circulating tumour DNA (ctDNA) in patient plasma after surgery. The prognostic significance of ctDNA prior to treatment remains unknown. This study investigated the correlation between pre-treatment ctDNA and current radiological (MRI) prognostic markers in patients with rectal cancer. Methods: Forty-two patients with rectal cancer were enrolled, with all having staging MRI prior to treatment. Plasma was taken for ctDNA at diagnosis. The presence of either methylated branched-chain amino acid transaminase 1 (BCAT1) or Ikaros family zinc finger (IKZF1) in cell-free DNA from plasma was deemed a positive ctDNA result. Correlation of MRI prognostic indicators and ctDNA results was assessed with chi-square tests. Univariable and multivariable cox regression analyses were performed to determine variables associated with overall survival (OS). Results: Mean age was 64.4 years (SD 12.5) and majority were male (30/42, 71.4%). 11, 13, 9 & 9 patients had stages I, II, III, IV respectively. Patients had a minimum follow-up of 36 months. Thirty-three (78.6%) patients received neoadjuvant chemoradiotherapy. 29 (69.0%) patients underwent surgical resection. 3-year survival rate was 64% in the overall group. 67% (n=28/42) of patients were positive for the methylated ctDNA at diagnosis. 11 out of 12 patients with a positive circumferential resection margin (CRM +) were ctDNA positive (p=0.03). Univariable analysis showed that prognostic indicators for OS were presence of extramural venous invasion (EMVI) (HR 3.0, 95% CI 1.1-8.4), CRM+ (HR 12.2, 95%CI 3.9-37.6), metastatic disease (HR 7.32, 95% CI 2.63-20.37) and ctDNA% methylation (HR 1.1, 95% CI 1.04-1.13) (Table 1). The presence of CRM+ and a positive ctDNA had a HR of 20.5 (95% CI 5.1-82.3). With multivariable analysis, including adjustment for age and EMVI, only CRM+/ctDNA+ variable was an independent predictor for poor survival (HR 20.2, 95% CI 4.5-90.9). Conclusions: In rectal cancer, almost all patients with CRM involvement have ctDNA, and these patients had the worst prognosis. Future studies with longitudinal ctDNA assessment pre and post treatment could potentially inform prognosis and help tailor patients’ treatment. Research Spon- sor: Clinical Genomics and Cancer Australia.

Univariable analysis for overall survival. Variable Hazard ratio (95% CI) P value

Age 0.99 (0.96-1.03) 0.78 Lower rectal (vs mid and upper) 1.79 (0.65-4.96) 0.26 T4 (vs T1, T2, T3) 2.49 (0.78-7.91) 0.12 EMVI (vs absent) 3.00 (1.07-8.38) 0.04 N1/2 (vs N0) (excluding tumour deposits/N1c) 1.10 (0.38-3.23) 0.86 Tumour deposits / N1c (vs no tumour deposits) 0.83 (0.20-3.52) 0.80 M1 (vs M0) 7.32 (2.63-20.37) <0.01 CRM involved (vs not involved) 12.16 (3.93-37.63) <0.01 ctDNA % methylation 1.09 (1.04-1.13) <0.01 ctDNA+/CRM+ (vs both negative or only 1 positive) 20.51 (5.11-82.34) <0.01

3580 Poster Session

Parameters associated with outcomes in pretreated MSI/dMMR metastatic colorectal cancer (mCRC) treated with immune checkpoint inhibitors (ICI): Subgroup analysis of a prospective cohort.

Raphael Colle, Marine Cachanado, Alexandra Rousseau, Magali Svrcek, Yves Menu, Romain Cohen, Thierry Andre; Sorbonne Universite�, Department of Medical Oncology, Saint-Antoine Hospital, AP-HP, Paris, France; URC-Est, Ho^pital Saint-Antoine, Sorbonne Universite�, Saint-Antoine Hospital, AP-HP, Paris, France; Unite� de Recherche Clinique de l’Est Parisien (URC-Est), Sorbonne Universite�, Ho^pital Saint Antoine, APHP, Paris, France; Pathology Department, Sorbonne Universite�, Saint-Antoine Hospi- tal, APHP, Paris, France; Sorbonne University, Department of Radiology, Saint-Antoine, AP-HP, F- 75012, Paris, France; Sorbonne University, Department of Medical Oncology, Saint-Antoine, AP-HP, F-75012, Paris, France

Background: Immune checkpoint inhibitors (ICI) have demonstrated efficacy in patients (pts) with MSI/ dMMR mCRC. We aimed to evaluate clinical, pathological and molecular factors associated with progression-free survival (PFS) and overall survival (OS) in ICI-treated pretreated mCRC patients (pts). Methods: Pts are drawn from a prospective cohort of all patients treated with ICI for MSI/dMMR mCRC at Saint-Antoine Hospital, Paris, France. All MSI/dMMR mCRC pts with disease progression after $ 1 prior systemic treatment (fluoropyrimidine and oxaliplatin or irinotecan ± targeted therapy) were included. MSI/dMMR status was centrally reviewed. Lynch syndrome or sporadic status was determined according to MMR gene germline mutational testing, MLH1 methylation status and BRAFV600E mutation. PFS and objective response rate (ORR) were assessed using iRECIST criteria. The impact of Lynch syndrome on PFS was analyzed apart from the multivariate analysis due to the interaction with the BRAFV600E mutation status. Results: Of 130 included pts, 66 received anti-PD1 alone, 1 anti-PDL1 alone and 63 anti-PD1 plus anti-CTLA4. 71% have had at least 2 prior lines of treatment. 33 patients (25%) have BRAFV600Emutation (mt) and 49 (38%) RASmt. The ORR for the whole population was 62.8 % IC95% [53.8; 71.1]. Median follow-up was 21.0 months, median PFS and OS were not reached. Results of PFS unadjusted and adjusted analysis are displayed in the table. BRAFV600E and RAS mutation were not associated with PFS and OS in multivariate analyses. After adjustment for the treatment type, Hazard Ratio (HR) for PFS between patients with proven Lynch syndrome (N=44) and patients with proven sporadic tumors (n= 44) was 0.57 (95%IC 0.26 -1.26). Conclusions: In this cohort, main known clinical, pathological and molecular factors do not influence the efficacy of ICI in pre- treated MSI/dMMR mCRC. Research Sponsor: None.

PFS Unadjusted HR [IC95%] Sexe Female vs. Male N=125 1.41 [0.74; 2.67] p=0.29 Adjusted HR [IC95%]

Age at first injection (years) 125 1.02 [1.00; 1.05] 0.07 KRAS/NRAS-mutated versus wild-type 125 0.63 [0.31; 1.28] 0.20 0.61 [0.26; 1.47] 0.27 BRAFV600E-mutated vs. wild-type 125 1.98 [1.02; 3.84] 0.0422 1.20 [0.52; 2.76] 0.67 Mucinous Yes vs. No 125 1.83 [0.97; 3.45] 0.06 1.90 [0.96; 3.76] 0.07 Localization Left vs. Right 125 0.83 [0.41; 1.68] 0.61 Treatment type Bitherapy vs. Monotherapy 125 0.35 [0.17; 0.68] 0.0023 0.40 [0.20; 0.80] 0.0097 Number of prior treatment lines >=2 vs. 1 125 0.51 [0.27; 0.98] 0.0425 0.57 [0.28; 1.15] 0.12 ECOG performance score >=1 vs. 0 125 1.72 [0.87; 3.40] 0.12

3581 Poster Session

The role of PP2A variants to predict outcome in patients (pts) with metastatic colorectal cancer (mCRC): Data from FIRE-3 and TRIBE trials.

Jingyuan Wang, Joshua Millstein, Fotios Loupakis, Sebastian Stintzing, Hiroyuki Arai, Francesca Battaglin, Natsuko Kawanishi, Priya Jayachandran, Shivani Soni, Wu Zhang, Christoph Mancao, Chiara Cremolini, Volker Heinemann, Alfredo Falcone, Heinz-Josef Lenz; Division of Medical Oncology, USC Norris Comprehensive Cancer Center, Keck School of Medicine, Los Angeles, CA; Department of Pre- ventive Medicine, USC Norris Comprehensive Cancer Center, Keck School of Medicine, Los Angeles, CA; Oncology Institute Veneto IOV-IRCCS, Padua, Italy; Medical Department, Division of Hematology, Oncology, and Tumor Immunology (CCM), Charite� Universita€tsmedizin Berlin, Berlin, Germany; Chiba Cancer Center, Chiba, Japan; USC Keck School of Medicine, Los Angeles, CA; F. Hoffmann-La Roche Ltd., Basel, Switzerland; Department of Translational Research and New Technologies in Medicine and Surgery, Unit of Medical Oncology 2, Azienda Ospedaliera Universitaria Pisana, Pisa, Italy; University Hospital Munich, LMU Munich, Munich, Germany; Azienda Ospedaliera Universitaria Pisana, Pisa, Ita- ly; USC Norris Comprehensive Cancer Center, Los Angeles, CA

Background: Protein phosphatase 2A (PP2A) is a serine/threonine phosphatase with functions that counter-balance kinase-mediated phosphorylation throughout cell signaling networks. PP2A was reported to upregulate the angiogenesis, while negatively regulate the pathways downstream of receptor tyrosine kinases at multiple nodes. Previous studies showed PP2A variants were associated with the increased risk of cancer. Therefore, we hypothesized that PP2A variants may predict first-line treatment outcome in mCRC pts treated with bevacizumab (bev)/cetuximab (cet)-based chemotherapy. Methods: Genomic DNA from blood samples of pts enrolled in two independent randomized trials, TRIBE (bev arm, n=215, as discovery cohort) and FIRE-3 (bev arm, n=107, as validation cohort; cet arm, n=129, as control cohort), was genotyped through the OncoArray, a customized array manufactured by Illumina including approximately 530K SNP markers. The impact on outcome of 17 selected SNPs in 3 main PP2A core subunits (PPP2CA, PPP2R1B, PPP2R1A), one phosphatase activator (PPP2R4) and 2 en- dogenous inhibitors (TIPRL, CIP2A) was analyzed. Results: In the discovery cohort, pts with PPP2R4 rs2541164 A/A (N=16) showed significantly shorter overall survival (15.3 vs 27.3 months) compared to carriers of any G allele (N=198) in both univariate (hazard ratio [HR]=1.8; 95% confidence interval [CI]: 1.1-3.1; p=0.02) and multivariate (HR=2.4; 95%CI: 1.4-4.4; p=0.006) analysis. These data were validated in the FIRE-3 bev cohort in both univariate (A/A vs. Any G: 17.3 vs 39.9 months, HR=2.8, 95%CI: 1.4-5.9, p=0.004) and multivariate (HR=4.3, 95%CI: 1.5-12.2, p=0.0095) analysis. Conversely, pts carrying CIP2A rs13069780 C/C (N=24) only showed significantly longer progression-free survival (17.7 vs 12.3 months) than carriers of any T allele (n=105) in the FIRE-3 cet cohort in both univariate (HR=0.6; 95%CI 0.4-0.99; p=0.04) and multivariate (HR=0.5; 95%CI 0.3-0.94; p=0.02) analysis, but no association were observed in the bev cohort of TRIBE and FIRE-3. Conclusions: Our study demonstrated for the first time that PPP2R4 polymorphisms could predict outcomes of bev- based treatment in mCRC patients; Meanwhile CIP2A polymorphism could predict outcomes of cet- based treatment in mCRC patients. These findings support a possible role of the PP2A variants in con- tributing to resistance to anti-VEGF/EGFR treatment. Research Sponsor: National Cancer Institute (grant number P30CA014089), The Gloria Borges WunderGlo Foundation-The Wunder Project, Dhont Family Foundation, San Pedro Peninsula Cancer Guild, Daniel Butler Research Fund and Call to Cure Fund.

3582 Poster Session

The prognostic impact of RAS and TP53 mutation according to primary tumor location in colorectal liver metastases.

Yongjun Cha, Bun Kim, Seung Jae Roh, Moon Ki Choi, Dong Woon Lee, Sung-Sik Han, Seong Hoon Kim, Sang-Jae Park, Ji Yeon Baek, Sung Chan Park, Hee Jin Chang, Jae Hwan Oh; National Cancer Cen- ter, Goyang, South Korea; Center for Colorectal Cancer, Research Institute and Hospital, National Can- cer Center, Goyang, South Korea

Background: Somatic gene mutations have been suggested to impact survival following resection of colorectal liver metastases (CRLM). However, most studies included a selected population with known mutation data and did not employ homogeneous methods. This study aimed to determine the prognostic impact of somatic gene mutations and microsatellite instability (MSI) in CRLM using a standardized protocol and assess their survival effects according to primary tumor location. Methods: A total of 568 patients who underwent resection of CRLM during 2001-2014 were identified from a prospectively maintained registry of the National Cancer Center. MassARRAY based mutation profiling of cancer-related genes (KRAS, NRAS, HRAS, BRAF, PIK3CA, MET, PTEN, APC, TP53)/MSI analysis was made in primary tumors from 538 (94.7%)/526 (92.6%) patients. Results: Primary tumor locations were: right colon for 51 (9.0%); transverse colon for 42 (7.4%); left colon for 238 (34.5%); rectum for 279 (49.1%) patients. Right sided tumors were associated shorter overall survival (OS) after liver resection compared to left colon primary tumors (5-year OS, 31.4% vs. 54.0% [P = 0.011]). Mutation frequencies were: 45.9% for RAS ; 2.4% for BRAF ; 8.4% for PIK3CA ; 0.2% for PTEN ; 0.4% for MET ; 12.1% for APC ; 24.3% for TP53. RAS (5-year OS, 40.8% vs. 55.7% [P = 0.001], PIK3CA (5-year OS, 31.1% vs. 50.5% [P = 0.027]), and TP53 mutation (5-year OS, 42.7% vs. 50.8% [P = 0.035]) were associated with worse OS after liver resection. On multivariable analyses, RAS (hazard rato [HR] 1.27; P = 0.033) and TP53 mutation (HR 1.35; P = 0.014) were significantly associated with poor OS after adjustment for covariates. Co-mutation in RAS/TP53 (12.4%) was associated with the worst oncologic outcome (HR 1.81; P <.001). Notably, while the negative prognostic impact of RAS mutation did not differ significantly according to primary tumor location, the adverse effect of TP53 mutation was limited to rectal cancer (interaction P = 0.002). In this study, MSI-high (2.3%) was not associated with survival. Conclusions: Both RAS and TP53 mutation are associated with worse survival following CRLM resection. In contrast to RAS mutation, the negative prognostic impact of TP53 mutation appears to be limited to CRLM from the rectal origin. Research Sponsor: NCC-1910210.

3583 Poster Session

Overall survival (OS) with encorafenib (enco) + cetuximab (cetux) in BEACON CRC: Effect of prior therapy for BRAF V600E-mutant metastatic colorectal cancer (mCRC).

Scott Kopetz, Dan Aderka, Axel Grothey, Eric Van Cutsem, Rona Yaeger, Harpreet Singh Wasan, Takayuki Yoshino, Jayesh Desai, Fortunato Ciardiello, Ashwin Gollerkeri, Adele Golden, Michelle L. Edwards, Josep Tabernero; MD Anderson Cancer Center, Houston, TX; Sheba Medical Center, Tel- Hashomer, Israel; West Cancer Center, OneOncology, Germantown, TN; University Hospital Gasthuis- berg and University of Leuven, Leuven, Belgium; Memorial Sloan Kettering Cancer Center, New York, NY; Hammersmith Hospital, Division of Cancer, Imperial College London, London, United Kingdom; National Cancer Center Hospital East, Kashiwa, Japan; Peter MacCallum Cancer Centre, Melbourne, VIC, Australia; University of Campania Luigi Vanvitelli, Naples, Italy; Pfizer, New York, NY; Pfizer, Cam- bridge, MA; Vall d’Hebron University Hospital and Vall d’Hebron Institute of Oncology (VHIO), UVic- UCC, Barcelona, Spain

Background: Enco + cetux (doublet) has been approved in the US, EU, and Japan for the treatment of BRAF V600E-mutant mCRC after progression on 1–2 prior regimens. In the BEACON CRC study (NCT02928224), median OS (95% CI) with the doublet was 9.3 months (8.0–11.3) compared with 5.9 months (5.1–7.1) with cetux + irinotecan or FOLFIRI (control) in patients (pts) with BRAF V600E- mutant mCRC (HR 0.61 [95% CI: 0.5–0.8]). This post-hoc analysis investigates OS by prior therapies to the doublet treatment in pts with BRAF V600E-mutant mCRC from the BEACON CRC study. Meth- ods: OS of pts treated with the doublet or control were compared according to prior treatment with bevacizumab, oxaliplatin, or FOLFOXIRI and duration of prior anticancer therapy (ACT). Results: The proportion of pts in the doublet and control arms who received prior bevacizumab were 64% and 55%, respectively. Of pts who had one prior therapy, 95% and 88% received prior oxaliplatin and 20% and 14% received prior FOLFOXIRI, respectively. OS by prior treatment in the doublet and control arms is shown in the table. In the doublet arm, pts who had bevacizumab < 4 months before start of study treatment had a median OS of 8.3 months (95% CI: 6.2–11.2); those who had bevacizumab $4 months pri- or had a median OS of 10.7 (95% CI: 7.5–17.7). Within each treatment arm, OS was similar regardless of prior treatment with oxaliplatin or FOLFOXIRI. The duration of prior ACT was similar across study arms, ranging from 5.6–5.8 months for the first line of ACT. Conclusions: In the BEACON CRC study, pts treated with the doublet for BRAF V600E-mutant mCRC demonstrated similar OS regardless of prior therapies or duration of prior therapy use. This exploratory post-hoc analysis provides data that reflect the prior treatment landscape clinicians may face when deciding subsequent treatment regimens for pts with BRAF V600E-mutant mCRC. Clinical trial information: NCT02928224. Research Sponsor: Pfizer.

Doublet Control Events/subjects Median OS (95% CI), Events/subjects Median OS (95% CI), Prior treatment (%) mos (%) mos HR (95% CI)

Prior bevacizumab 67/111 (60) 8.3 (6.2–11.2) 78/103 (76) 5.1 (4.0–6.4) 0.53 < 4 months 16/29 (55) 10.7 (7.5–17.7) 15/19 (79) 4.4 (2.0–11.6) (0.4–0.7) $4 months 45/80 (56) 9.4 (7.6–16.5) 64/99 (65) 7.4 (5.6–9.5) 0.47 No prior (0.2–1.0) 0.71 (0.5–1.1) Prior oxaliplatin* Yes 81/138 (59) 9.7 (8.3–12.3) 88/128 (69) 6.4 (5.2–8.0) 0.60 No 3/8 (38) 8.4 (3.6–NR) 10/17 (59) 6.5 (3.2–NR) (0.4–0.8) 0.73 (0.2–2.7) Prior FOLFOXIRI* Yes 18/29 (62) 9.4 (5.3–17.7) 13/21 (62) 4.6 (2.1–NR) 0.63 No 66/117 (56) 10.7 (8.3–12.6) 85/124 (68) 6.5 (5.6–8.8) (0.3–1.3) 0.59 (0.4–0.8) Duration of prior ACT* 46/72 (64) 8.8 (7.6–10.7) 55/78 (71) 5.8 (4.8–7.3) 0.61 #6 months 38/74 (51) 11.3 (8.4–17.7) 43/67 (64) 6.5 (4.8–11.3) (0.4–0.9) > 6 months 0.61 (0.4–0.9) *Based on pts with only 1 line of prior ACT. NR, not reached.

3584 Poster Session

A phase 1 first-in-human study of the anti-LAG-3 antibody MK4280 (favezelimab) plus pembrolizumab in previously treated, advanced microsatellite stable colorectal cancer.

Elena Garralda, Ammar Sukari, Nehal J. Lakhani, Amita Patnaik, Yanyan Lou, Seock-Ah Im, Talia Golan, Ravit Geva, Martin Wermke, Maria De Miguel, John Palcza, Sujata Jha, Marya F. Chaney, Jane Anne Healy, Gerald Steven Falchook; Vall d’Hebron Institute of Oncology (VHIO), Medical Oncolo- gy, Vall d’Hebron University Hospital (HUVH), Barcelona, Spain; Karmanos Cancer Center, Detroit, MI; START Midwest, Grand Rapids, MI; START, San Antonio, TX; Mayo Clinic, Jacksonville, FL; Seoul Na- tional University Hospital, Seoul National University College of Medicine, Seoul, South Korea; Sheba Medical Center and Tel Aviv University, Tel Aviv, Israel; Tel Aviv Sourasky Medical Center, Tel Aviv Uni- versity, Tel Aviv, Israel; Universitaetsklinikum Carl Gustav Carus, Technical University, Dresden, Ger- many; START-HM Sanchinarro, Madrid, Spain; Merck & Co., Inc., Kenilworth, NJ; Sarah Cannon Research Institute at HealthONE, Denver, CO

Background: Patients (pts) with microsatellite stable (MSS) metastatic colorectal cancer (mCRC) that progressed on $2 prior therapies have limited treatment options, with median OS ranging from 6-9 months (mo). In the dose-escalation phase of this first-in-human multicohort study (NCT02720068), the anti-lymphocyte activation gene (LAG)-3 antibody favezelimab (fave) was well tolerated alone and with pembrolizumab (pembro) across all dose levels (Lakhani, SITC, 2018, abstract O26). Here, we evaluate the safety and efficacy of fave alone or in combination with pembro in pts with advanced MSS CRC from the dose confirmation phase. Methods: Eligible pts with MSS PD-1/PD-L1-treatment-nave mCRC that progressed on prior standard-of-care (3L+) were enrolled (cohort A) to receive the RP2D of 800 mg fave alone (Arm 1), 800 mg fave + 200 mg pembro (Arm 2C), or 800 mg fave + 200 mg pembro (MK-4280A) co-formulation (Arm 5), all Q3W. Treatment continued for 35 cycles or until progression, unacceptable toxicity, or investigator/pt decision. Pts with confirmed progression per irRECIST v1.1 on fave alone could crossover to 800 mg fave + pembro. Safety was assessed in all treated pts; efficacy in the full analysis set (FAS) of all treated pts with baseline scan. Objectives included safety (primary), ORR (RECIST v1.1 by investigator [secondary]), and DOR, PFS, and OS (exploratory). Interim analysis data cut-off was: Oct. 23, 2020. Results: A total of 20 pts received fave (Arm 1); 89 pts (including 9 crossover) received fave + pembro (Arms 2C+5); 12 pts (Arm 1) and 36 pts (Arms 2C+5), had PD- L1 CPS $1 tumors. At data cut-off, median follow-up was 5.8 months (mo) in Arm 1 and 6.2 mo in Arms 2C+5. Treatment-related adverse events (TRAEs) were 65% with fave (Arm 1) and 65.2% with fave + pembro (Arms 2C+5). Grade $3 TRAEs were 15% (Arm 1), and 20% [Arms 2C+5]). No grade 5 TRAEs were reported. Common TRAEs ($15%) included fatigue (20.0%), nausea (15%) with fave, and fatigue (16.9%) with fave + pembro. Confirmed ORR was 6.3% (4PR, 1CR) with fave + pembro (Arms 2C+5). No pt receiving fave alone responded. In Arms 2C+5, median DOR was 10.6 mo (range, 5.6-12.7). ORR, OS and PFS by PD-L1 status are reported in the Table. Conclusions: Favezelimab alone or in combination with pembrolizumab had a manageable safety profile, with no treatment-related deaths. Promising antitumor activity was observed with combination therapy, including with MK- 4280A, compared with monotherapy most notably in pts with PD-L1 CPS $1 tumors. Clinical trial information: NCT02720068. Research Sponsor: Merck & Co., Inc.

Fave + pembroa,b Total N = 80c CPS $1N = 36 CPS < 1N = 35

ORR, n (%) 5 (6.3) 4 (11.1) 1 (2.9) OS, median, mo (95% CI) 8.3 (5.5-12.9) 12.7 (4.5-NR) 6.7 (3.8-11.0) 12-mo OS rate, % 40.8 50.6 29.5 PFS, median, mo (95% CI) 2.1 (1.9-2.2) 2.2 (1.8-4.2) 2.0 (1.9-2.1) 6-mo PFS rate, % 16.2 25.4 9.1 aIncludes MK-4280A; bMissing PD-L1 status (n = 9); cFAS; NR, not reached.

3585 Poster Session

Differential impact of different TP53 gain-of-function mutations on overall survival of patients with metastatic colorectal cancer: Results from a large integrated healthcare system.

Minggui Pan, Chen Jiang, Pamela Tse, Elaine Chung, Aleyda Solorzano, Wenwei Hu, Thach-Giao Truong, Amit Arora, Tilak Kumar Sundaresan, Jennifer Marie Marie Suga, Laurel A. Habel, Sachdev P. Thomas; Kaiser Permanente, Dept of Medical Oncology, Santa Clara, CA; Kaiser Permanente, Division of Research, Oakland, CA; Kaiser Permanente, Oakland, CA; Rutgers Cancer Institute of New Jersey, New Brunswick, NJ; Kaiser Permanente, Dept of Medical Oncology, Vallejo, CA; Kaiser Permanente, Fremont, CA; San Francisco Medical Center, Kaiser Permanente Northern California, San Francisco, CA; NSABP/NRG Oncology, and Kaiser Permanente NCI Community Oncology Research Program, Val- lejo, CA

Background: TP53 mutation is present in approximately 50% of metastatic colorectal cancer (CRC). The spectrum of the TP53 mutations is extremely broad including approximately 80% missense mutations. Several missense mutations have been found to possess gain-of-function (GOF) properties in cell line and animal studies, however, confirmation of the concept of GOF in human malignancies is still lacking. Methods: We investigated the impact of TP53 GOF mutations in patients with metastatic CRC using the NGS data within Kaiser Permanente Northern California (KPNC), a large integrated healthcare system. Results: From November 2017 to January 2021, genomic profiling by StrataNGS was performed on 8658 patients, with 1056 patients being metastatic CRC, among whom 740 patients harbored a TP53 mutation (TP53mut) and 316 patients had wild-type TP53 (TP53wt). Ras (KRAS and NRAS) and BRAF mutation appropriately discriminated the overall survival (OS) of patient populations with either TP53wt or TP53mut, confirming the validity of our dataset. We identified seven GOF TP53 mutations (R175H, R248W, R248Q, R249S, R273H, R273L, R282W) in these CRC patients. We show that different GOF mutation differentially impacts the OS. Patients whose CRC harbored TP53mut R248W, R249S, and R282W (poor prognostic TP53mut, N = 47) had significantly worse OS versus patients whose CRC harbored TP53mut R248Q, R175H, R273H and R273L (N = 160, median OS 29.4 vs 44.2 months, HR 0.47, p = 0.007). The OS of the poor prognostic TP53mut patients was also significantly inferior compared to patients whose CRC harbored all other TP53 mutations (N = 1099, median OS 50.1 months, HR 0.55, p = 0.01) or TP53wt (N = 316, median OS 47,5 months, HR 0.54, p = 0.01). The demographics and the percent of Ras, BRAF, and PI3KCA mutations were similar except that the patients with the poor prognostic TP53mut had significantly higher percent of Ras mutation compared to the rest of the GOF TP53mut patients (p = 0.035). When compared to R248Q alone, R248W confers worse OS (median OS 36.3 vs 63.2 months, p = 0.05). Conclusions: Our data suggest that different TP53 GOF mutations are associated with very different clinical outcomes. Additional studies identifying specific TP53 GOF mutations that impact outcomes may provide further insight for drug development and clinical trial design. Research Sponsor: None.

3586 Poster Session

Mucinous colorectal cancer: Disease characteristics, treatment outcomes and the impact of metastasectomy.

Darren Cowzer, Emily Harrold, Jane Sze Yin Sui, Mairi Lucas, Helen M Fenlon, Karen C Redmond, Donna Eaton, John B Conneely, Gerry P McEntee, Ann E Brannigan, Conor J Shields, Jurgen Mulsow, Catherine Margaret Kelly, Megan Greally, John McCaffrey; Mater Misericordiae University Hospital, Dublin, Ireland; Mater Private Hospital, Dublin, Ireland

Background: Mucinous colorectal cancer (CRC) differs from adenocarcinoma with regard to clinical and histological features and is reported to have inferior outcomes when compared to non-mucinous CRC. This study aims to evaluate the clinical features and outcomes of patients with mucinous CRC at our in- stitution. Methods: Medical records of patients with CRC that were referred to medical oncology between September 1999 and September 2018 were retrospectively reviewed. Mucinous histology was defined as those containing > 50% mucin identified on histology specimens. Statistical analysis was performed using Prism V9.0. Results: We identified 1,115 patients with CRC that were referred to medical oncology during this period. The tumours of 81 (7.3%) patients were classified as mucinous. Median age was 65 (28-94 years) and 45 (55.5%) were male. Forty-one patients (51%) had right sided tumours, 27 (33%) had left sided tumours and 13 (16%) had rectal tumours. Twenty-three (28.4%) had de novo metastatic disease. Eleven of 24 patients (46%) with stage II disease relapsed and 18 of 33 (55%) of those with stage III disease relapsed. Radiological surveillance identified 20/29 (69%) of relapsed disease, 5 (17%) were symptomatic and 4 (14%) had a rise in CEA. Median follow up for patients with stage II disease was 53 months and 3 year and 5-year disease free survival (DFS) was equal in both groups at 60.9%. For stage III disease 3- and 5-year DFS was 58.1% and 48.4% respectively with a median follow up of 43 months. In the metastatic setting, we observed no significant difference in overall survival (OS) between left and right sided tumours (p = 0.550). Median OS for pts with stage IV mucinous CRC who received any treatment was 25 months. Metastasectomy was performed in 25/52 (48%) patients and was associated with a significant improvement in OS, 23 vs 51 months (p < 0.005, HR 0.4). Conclusions: Mucinous CRC has been associated with inferior responses to treatment and worse overall outcomes compared to non-mucinous histologies. Survival in advanced-stage disease in our cohort is higher than what has been reported in the literature. With an effective multi-disciplinary approach and the increasing use of metastasectomy as a treatment option, survival in the advanced disease setting may be comparable to non-mucinous CRC. Research Sponsor: None.

3587 Poster Session

Young-onset colorectal cancer treatment side effects: Infertility, sexual dysfunction, and quality-of-life outcomes.

Laura Diane Porter, Ronit Yarden, Kim Lynn Newcomer, Negeen Fathi, N2Y Advisory Board; Colorectal Cancer Alliance, Washington, DC

Background: Colorectal cancer is the third-most commonly diagnosed cancer and the second-leading cause of cancer death in men and women combined in the United States. Young-onset colorectal cancer refers to individuals diagnosed under the age of 50. In recent years, the incidence has increased by 2.2% annually in individuals younger than 50 years and 1% in individuals 50-64, in contrast to a 3.3% decrease in adults 65 years and older. Young-onset (YO) CRC patients and survivors face unique clinical challenges with fertility and sexual dysfunctions, but this risk is not well quantified. There is limited data and public discussion on the long-term effects of colorectal cancer treatments on fertility and sexual dysfunction and the long-term impact on the quality of life. Methods: To explore the unique challenges and unmet needs of the young-adult patient population, a cross-sectional study was conducted. Colorectal cancer patients and survivors (N = 884) diagnosed between the ages of 20 to 50 years old (median age 42 ± 7.0) completed an online questionnaire based on established instruments EORTC- QOL-30, EORTC-CR-29, and EORTC-SHC-22. Results: Thirty-one percent of respondents stated that a medical professional spoke to them about fertility preservation at the time of diagnosis and during treatment. Only 31% were referred to a reproductive endocrinologist, even though 37% of women and 16% of men reported that treatment left them infertile or sterile. Among survey respondents, 12% of women had an egg retrieval procedure, and 36% of men had their sperm preserved prior to the start of treatment. Fifty-three percent of women reported treatment led to premature menopause. Sixty-five percent of respondents suffer from some level of sexual dysfunction due to treatment. In patients who received radiation therapy, women were 12% less likely than men to have discussed sexual side effects with the provider before treatment. Patients who have an ostomy reported more severe sexual dysfunction (17.8%). Rectal cancer patients were 2.5 times more likely than those with colon cancer to report severe dysfunction after their treatment. More than 25% of the respondents said they would have considered alternative treatment if they would have known the risks of sexual dysfunction. Conclusions: Our survey demonstrates inadequate communications between patients and providers about the irreversible fertility and sexual effects of colorectal cancer treatments. Younger patients and survivors face unique long-term challenges and require further information about fertility preservation options and emotional support regarding their sexuality post-treatment. Other studies are needed to assess the physical and psychological side effects endured by young-onset CRC patients and survivors. Research Sponsor: None.

3588 Poster Session

Dietary fat in relation to overall and progression-free survival among patients (pts) with advanced or metastatic colorectal cancer (CRC): Data from CALGB 80405 (Alliance).

Erin Van Blarigan, Chao Ma, Fang-Shu Ou, Alan P. Venook, Kimmie Ng, Donna Niedzwiecki, Edward L. Giovannucci, Heinz-Josef Lenz, Federico Innocenti, James Edward Shaw, Blase N. Polite, Howard S. Hochster, Richard M. Goldberg, Robert J. Mayer, Eileen Mary O’Reilly, Charles S. Fuchs, Jeffrey A. Meyerhardt; University of California San Francisco, San Francisco, CA; Dana-Farber/Partners Cancer- Care, Boston, MA; Mayo Clinic, Rochester, MN; Dana-Farber Cancer Institute, Boston, MA; Duke Uni- versity Medical Center, Department of Biostatistics and Bioinformatics, Durham, NC; Harvard T.H. Chan School of Public Health, Harvard Medical School, Brigham and Women’s Hospital, Boston, MA; USC Norris Comprehensive Cancer Center, Los Angeles, CA; University of North Carolina at Chapel Hill, Chapel Hill, NC; Medstar Washington Hospital Center, Washington, DC; University of Chicago Medical Center, Chicago, IL; Rutgers Cancer Institute, New Brunswick, NJ; West Virginia University Cancer In- stitute, Morgantown, WV; Memorial Sloan Kettering Cancer Center, New York, NY; Yale Cancer Center and Smilow Cancer Hospital, New Haven, CT; Department of Medical Oncology, Dana-Farber Cancer In- stitute/Partners Cancer Care, Boston, MA

Background: Growing data suggest dietary factors are associated with survival among pts with non-metastatic CRC. However, data on diet and survival among pts with advanced or metastatic disease are very limited. Methods: We prospectively examined dietary fat intake assessed at initiation of treatment for advanced or metastatic CRC in relation to OS and PFS. This analysis was conducted among 1,149 pts in the CALGB 80405 randomized controlled trial who completed a validated food frequency questionnaire. We examined intakes of saturated, monounsaturated, and polyunsaturated (total n-3, long-chain n-3, and total n-6) fats as well as animal and vegetable fats. Based on data from non-metastatic CRC and other cancers, we hypothesized that higher intakes of long-chain n-3 fatty acids and vegetable fats would be associated with longer OS and PFS and higher intakes of saturated fat and animal fat would be associated with shorter OS and PFS. We used Cox proportional hazards regression to estimate adjusted hazard ratios (HR) and 95% confidence intervals (CI). Results: Over a median follow-up of 6.1 years [y; interquartile range (IQR): 5.3, 7.2 y], we observed 974 deaths and 103 events of progression without death during follow-up. Participants in this analysis had a median age of 59 y (IQR: 51 to 67 y); 41% were female and 86% identified as white. We observed no statistically significant associations between any type of dietary fat and OS. However, vegetable fat was non-linearly associated with longer PFS (HR comparing 4th to 1st quartile: 0.78; 95% CI: 0.64, 0.96; p-trend: 0.10). We also observed a linear association between continuous saturated fat and PFS (HR per 5% kcal/d: 1.21; 95% CI: 1.03, 1.42; p-value: 0.02), perhaps driven by pts with high saturated fat intake. Conclusions: We observed no statistically significant associations between types of dietary fat and OS among pts with advanced or metastatic CRC. However, a healthy diet that includes vegetable fat and is modest in saturated fat may be associated with longer PFS. Future studies to replicate these findings and examine diet in relation to cancer survival in racially/ethnically diverse populations are needed. Support: K07CA197077, U10CA180821, U10CA180882, https://acknowledgments.alliancefound.org. Clinical trial information: NCT00265850. Research Sponsor: U.S. National Institutes of Health, Pharmaceutical/ Biotech Company.

3589 Poster Session

Assessment of HER2 (ERBB2) amplification (HER2amp) using blood-based circulating tumor DNA (ctDNA) next generation sequencing (NGS) and correlation with tissue-based testing in metastatic colorectal cancer (mCRC).

Kanwal Pratap Singh Raghav, Yoshiaki Nakamura, Silvia Marsoni, John H. Strickler, Rona Yaeger, Aakash Tushar Shah, Wataru Okamoto, Giovanni Crisafulli, Rebecca Nagy, Victoria M. Raymond, Mark Routbort, Salvatore Siena, Ryan Bruce Corcoran, Alberto Bardelli, Scott Kopetz, Takayuki Yoshino; The University of Texas MD Anderson Cancer Center, Houston, TX; Department of Gastroenterology and Gas- trointestinal Oncology, National Cancer Center Hospital East, Kashiwa, Japan; Istituto di Candiolo, Fon- dazione del Piemonte per l’Oncologia, IRCCS, Candiolo, Italy; Duke University School of Medicine, Durham, NC; Memorial Sloan Kettering Cancer Center, New York, NY; Baylor College of Medicine, Houston, TX; BB/TR Support Section, Clinical Research Support Office, National Cancer Center Hospi- tal East, Kashiwa, Japan; Department of Oncology, University of Torino, Candiolo Cancer Institute, FPO-IRCCS, Candiolo, Italy; Guardant Health, Inc., Redwood City, CA; Grande Ospedale Metropolitano Niguarda and Universita� degli Studi di Milano, Milan, Italy; Massachusetts General Hospital, Boston, MA; Institute for Cancer Research and Treatment, Candiolo, Italy; Department of Gastrointestinal Medi- cal Oncology, University of Texas MD Anderson Cancer Center, Houston, TX; National Cancer Center Hospital East, Kashiwa, Japan

Background: HER2 amplified mCRC has emerged as a unique clinical subset, characterized by resistance to anti-EGFR therapy and response to anti-HER2 strategies. Accurate identification and quantification of HER2amp has predictive value for efficacy of anti-HER2 therapies and appropriate patient selection. Despite availability and use of various tumor tissue-based and blood-based assays for detecting HER2amp, data on cross-performance of these platforms are lacking. Methods: Leveraging a multicenter international consortium (Italy, Japan and USA), we generated a large cohort (N = 353) of mCRC patients (pts), tested for HER2amp using both tissue and blood. Tissue testing was done using immunohistochemistry (IHC), in-situ hybridization (ISH) and (NGS). ctDNA NGS was performed using CLIA-certified Guardant360 ctDNA assay, capable of detecting HER2 copy number (CN) variations. The primary endpoint was to correlate HER2 gene CNs in tissue (tCN) and plasma (pCN). Descriptive statistics, spearman correlation (r) and Fisher’s exact test were used. Results: Baseline tumors characteristics included right-sided primary in 234 (23%), proficient mismatch repair in 264 (98%) and RAS/BRAF wild type (WT) genotype in 194 (67%) pts. Tissue testing was done by IHC, ISH and NGS in 76%, 64% and 74% pts, respectively. A total of 177 pts had HER2amp detected by at least one test: 116 (66%), 157 (89%) and 96 (54%) of which had tissue +, ctDNA +, and both tissue and ctDNA + disease, respectively. Discordant cases consisted of 20 (6%) with positivity in tumor only and 61 (17%) in ctDNA only. Sensitivity, specificity, positive and negative predictive values of ctDNA assay (vis-a�-vis tissue) were 83%, 74%, 61% and 90% respectively. Among HER2amp pts, median (range) HER2/CEP17 (ISH) ratio, tCN and pCN were 5.2 (2–12), 11.6 (2–700) and 3.5 (2–122), respectively. The pCN showed strong correlation with ISH ratio (r = 0.69) and tCN (r = 0.68) (P < 0.001). Median pCN differed significantly between pts with HER2 IHC 3+ (12.0), 2+ (2.2) and 0/1+ (2.0) tumors (P < 0.001). High HER2amp (pCN > 4.0) appeared to be enriched with tissue + cases (69% vs 8% [OR 24.6, P < 0.001]), tumor tissue HER2 + status (IHC3+ [75%] vs IHC2+ISH+ [50%] vs IHC2+/ISH- or IHC0/1+ [12%], P < 0.001), HER2 tCN > 6 (79% vs 31% [OR 8.7, P < 0.001]) and RAS/BRAF WT tumors (41% vs 17% [OR 3.5, P = 0.064) but not left sidedness (41% vs 38%; OR 1.1; P = 0.82). Conclu- sions: In this large diverse cohort of mCRC, we demonstrated correlation of HER2 tCN and pCN obtained by tissue-based and blood-based ctDNA assay. Further prospective efforts are needed to standardize this cross-platform quantification of HER2amp to facilitate robust clinical application of HER2 therapies. This effort shows the value of strategic international partnership in furthering research for rare cancer subsets. Research Sponsor: None.

3590 Poster Session

XELOX or mFOLFOX6 chemotherapy combined with resection of primary lesion versus chemotherapy alone for colon cancer with unresectable metastases: A randomized clinical trial.

Weijian Guo, Xinxiang Li, Mingzhu Huang, Ya’nan Yang, Qingguo Li, Chenchen Wang, Lei Liang, Xiaodong Zhu, Wen Zhang, Zhiyu Chen, Wenhua Li, Xiaowei Zhang, Xiaoying Zhao, Lixin Qiu, Qirong Geng, Xuedan Sheng; Department of Medical Oncology, Fudan University Shanghai Cancer Center; De- partment of Oncology, Shanghai Medical College, Fudan University, Shanghai, China; Fudan University Shanghai Cancer Center, Shanghai, China; Department of Colorectal Surgery, Fudan University Shang- hai Cancer Center, Shanghai, China; Department of Medical Oncology, Fudan University Shanghai Can- cer Center, Department of Oncology, Shanghai Medical College, Fudan University, Shanghai, China; Department of Medical Oncology, Fudan University Shanghai Cancer Center; Department of Oncology, Shanghai Medical College, Fudan University, Shangai, China; Department of Medical Oncology, Fudan University Shanghai Cancer Center, Shanghai, China

Background: It is still controversial for colon cancer patients with unresectable metastases whether to resect the primary tumor when there are no symptoms of primary lesion. Methods: This is an open-label, single-center, prospective, randomized, controlled phase II trial. Colon cancer patients aged 18-80 years with unresectable metastases at enrollment will be randomly allocated to either resection group (group A) or chemotherapy group (group B), and stratified by tumor response and number of organ metastases, after receiving induction chemotherapy with 4 cycles of XELOX or 6 cycles of mFOLFOX6, excluding those with disease progression, lesions radically resectable, or primary lesion unresectable. Patients in group A received resection of primary lesion and then continued chemotherapy, and patients in group B just continued chemotherapy, both up to 4 cycles of XELOX or 6 cycles of mFOLFOX6, and capecitabine maintenance afterwards. If progression occurs 3 months after discontinuation of oxaliplatin and toxicity has recovered to grade I, the original regimen can be applied again. The primary endpoint was TFS (time to failure of strategy, defined as the time from randomization to secondary progression in patients received re-introduce of the induction chemotherapy regimen, or to first progression in patients without re-introduce of the original regimen). The secondary endpoints included progression-free survival (PFS, the time from randomization to first progression), overall survival (OS, the time from enrollment to death), and adverse events (AEs). Efficacy data were analyzed on an intention- to-treat (ITT) basis. This study is registered with ClinicalTrials.gov, number NCT02291744. Results: Between April, 2015, and July, 2020, 140 patients were enrolled, and 54 patients withdrew due to colon obstruction (16), perforation (1), disease progression (22), death (1), radical resection (3), or other reasons (11). Finally, 86 patients were randomized into group A (n = 42) or group B (n = 44). The median TFS was 143 days (95%CI: 104.9-181.1) in group A, and 196 days (95%CI: 96.5-295.5) in group B (HR:0.930 95%CI:0.589-1.468, p = 0.755). The median PFS was 147 days (95%CI: 105.7- 188.3) in group A, and 206 days(95%CI:180.9-231.1) in group B (HR:0.831, 95%CI:0.522-1.323, p = 0.436). The median OS was 530 days (95%CI: 308.9-751.1) in group A, and 779 days (95%CI:626.3-931.7) in group B (HR:0.948 95%CI:0.554-1.622, p = 0.845). The incidence of treatment-related AEs was similar between two groups. Conclusions: Resection of primary tumor after induction chemotherapy could not bring survival benefits. It’s not recommended for patients without symptoms of primary lesion to receive primary tumor resection, but it also requires individualized treatment as colon obstruction or perforation occurred in some patients. Clinical trial information: NCT02291744. Research Sponsor: Foundation from Fudan University Shanghai Cancer Center.

3591 Poster Session

Association of plasma adiponectin with tumor infiltrating lymphocytes and survival in patients with stage III colon cancer (NCCTG N0147; Alliance).

Frank A. Sinicrope, Qian Shi, Thomas C. Smyrk, Richard M. Goldberg, Steven J. Cohen, Sharlene Gill, Morton S. Kahlenberg, Suresh Nair, Anthony Frank Shields, Balkrishna N. Jahagirdar, Sawyer B. Jacobson, Nathan R. Foster, Michael N. Pollak, Steven R Alberts; Mayo Clinic, Rochester, MN; Mayo Clinic Cancer Center, Rochester, MN; West Virginia University Cancer Institute, Morgantown, WV; Jef- ferson Health System/Abington Memorial Hospital, Abington, PA; BC Cancer, Vancouver, BC, Canada; Surgical Oncology Associates of South Texas, San Antonio, TX; Lehigh Valley Health Network, Allen- town, PA; Karmanos Cancer Institute, Wayne State University, Detroit, MI; Region’s Hospital and Health Partners, Edina, MN; Division of Biomedical Statistics and Informatics, Mayo Clinic, Rochester, MN; McGill University, Jewish General Hospital Stroll Cancer Prevention Centre, Montre�al, QC, Canada

Background: Adiponectin is a peptide hormone exclusively secreted by adipocytes that plays a role in immune regulation and in the host inflammatory response to cancer. We examined postsurgical adiponectin levels in relationship to tumor infiltrating lymphocytes (TILs), clinicopathological features, vitamin D status, and patient survival in participants in a phase 3 trial of adjuvant chemotherapy. Methods: Plasma adiponectin and 25-hydroxyvitamin D [25(OH)D] were analyzed by radioimmunoassay in 600 patients with stage III colon carcinoma who received adjuvant FOLFOX +/- cetuximab. TIL densities were determined at light microscopy in routine histopathological sections. The associations between adiponectin and 25(OH)D, TILs, other factors were evaluated by Fisher’s Exact, Chi-squared, t-test, and Kruskal- Wallis tests where appropriate. The association between adiponectin or 25(OH)D with disease-free survival (DFS), time to recurrence (TTR) and overall survival (OS) were evaluated by multivariable Cox regression, adjusting for body mass index (BMI), race, T, N stage, performance status, tumor location, TILs, BRAF/KRAS, and mismatch repair status. Results: A statistically significant and inverse association between adiponectin level and BMI was observed with lower levels found with obesity (BMI > 30 kg/m2) [p < 0.001]. The level of adiponectin was significantly lower in men vs women (p < 0.001), in blacks vs whites or Asians (p < 0.032), and in patients with fewer regional lymph node metastases (N1 vs N2 stage, p = 0.011). A significantly lower level of adiponectin was found in patients whose tumors had high vs low TIL densities (p = 0.040), but was unrelated to 25(OH)D. Insufficiency of 25(OH)D ( < 30 ng/ml) was detected in 291 (49%) of patients and was not associated with TILs. By multivariable analysis, adiponectin was not associated significantly with patient DFS (HRadj= 0.98, 95% CI 0.74- 1.29, padj= 0.88) nor with OS nor time-to-recurrence (TTR). TIL densities were significantly prognostic, but 25(OH)D was not (DFS: HRadj= 1.12, 95% CI 0.85-1.47, padj= 0.44). No significant interaction was observed for adiponectin with TILs for the association with DFS. Conclusions: Lower adiponectin levels were associated with significantly increased TIL densities in colon cancers, indicating an enhanced anti-tumor immune response. In contrast to TILs, adiponectin was not independently associated with patient outcome. Nearly one-half of stage III patients were vitamin D insufficient, although 25(OH)D was not prognostic. Research Sponsor: U.S. National Institutes of Health.

3592 Poster Session

Young-onset colorectal: Emotional and psychosocial effects on patients, survivors, and caregivers.

Laura Diane Porter, Ronit Yarden, Kim Lynn Newcomer, N2Y Advisory Board; Colorectal Cancer Alli- ance, Washington, DC

Background: Colorectal cancer is the third-most commonly diagnosed cancer and the second-leading cause of cancer death in men and women combined in the United States. Young-onset colorectal cancer refers to individuals diagnosed under the age of 50. In recent years, the incidence has increased by 2.2% annually in individuals younger than 50 years, and 1% in individuals 50-64, in contrast to a 3.3% decrease in adults 65 years and older. The Colorectal Cancer Alliance launched the Never Too Young Survey and the Caregiver Survey to assess and better understand the unmet needs of the young- onset population and their caregivers. Methods: A cross-sectional study, conducted in the form of an online survey, was launched to better understand the experiences around YO-CRC patients and caregivers. YO-CRC patients and survivors (N = 885) and caregivers (N = 204) completed an online questionnaire that was based on established instruments including PROMIS, EORTC-QOL-30, and EORTC-CR-29. The final survey instrument and study plan were reviewed and approved by the Aspire Inc. Institutional Review Board. Results: Nearly 75% of patients/survivors shared that they have been concerned about their mental health, and 64% responded that they have needed help for their depression. Further, 67% of caregivers surveyed responded that they were also concerned about their mental health, and 68% responded that they needed help with their depression. Seventy-one percent of caregivers often felt sad- ness, and 30% indicated that they had lost hope. Emotional exhaustion was reported by 77% of caregivers, whether they were providing round-the-clock care or caregiving from a distance. The effect was more pronounced in the patient/survivor cohort, with 95% indicating that emotional exhaustion im- pacted their lives. As a result, 71% of caregivers and 29% of patients/survivors indicated that they had withdrawn from other people. These results indicate the emotional toll that colorectal cancer has on patients/survivors and caregivers and their need for further resources. Conclusions: The Colorectal Cancer Alliance is committed to meeting these needs and providing resources that support patients, survivors and caregivers. Information and services may assist the caregiver in helping the patient make decisions, including shifting roles and routines in response to changing demands of YO-CRC. Further studies should investigate psychological well-being and support strategies. Research Sponsor: None.

3593 Poster Session

Doublet (FOLFOX or FOLFIRI) versus triplet (FOLFOXIRI) backbone chemotherapy regimen as first-line treatment of metastatic colorectal cancer: A meta-analysis and systematic review.

Vishal Jindal, Ruby Gupta, Kamal Kant Sahu, Mandeep Singh Rahi, Michael J. Stender, Ishmael A. Jaiyesimi; Beaumont Health, Department of Hematology and Oncology, Oakland University William Beaumont School of Medicine, Royal Oak, MI; St. Vincent Hospital, Worcester, MA; Bridgeport Hospi- tal, Bridgeport, CT

Background: Doublet chemotherapy FOLFOX and FOLFIRI are standard for first-line treatment of metastatic colorectal cancer (mCRC). Recently, use of triplet chemotherapy FOLFOXIRI has shown an increased anti-cancer activity but still there is uncertainty regarding first line backbone chemotherapy. Therefore, we conducted this metanalysis to determine the efficacy, safety and outcome of triplet vs doublet chemotherapy. Methods: The study protocol was published at PROSPERO (CRD42020166745) and prepared as per PRISMA guidelines. Total 10 studies were included, with sample size of 1536 participants in triplet arm and 1535 participants in doublet arm. The primary outcome is Response rate (RR) and secondary outcomes are Progression-free survival (PFS), Overall survival (OS), post chemotherapy radical (R0) surgical resection rate of metastases. Quantitative synthesis was performed using R statistical package. Dichotomous outcomes were summarized using odds ratio (OR) and time to event data was summarized using hazard ratio (HR). Results: A total of 678 articles were retrieved. The Medline article search gave a result of 271 article, Embase 296, the Cochrane Li- brary 100 and Clinical tral.gov 11, when searched through April 2020. Total 10 studies were included. All the studies were randomized, open-label, multicenter study. Out of the 10 trials 5 each were phase II and phase III studies. The pooled odds ratio for RR was 1.66 (95% CI 1.42 to 1.93) and PFS was (HR, 0.70; 95% CI, 0.63–0.78) in favor of triplet chemotherapy. There was significant improvement in radical resection (R0) of metastases (OR 1.59; 95% CI, 1.27–1.98) in triplet arm. Triplet arm was also associated with increased toxicity especially neurological events 2.51(0.88-7.16), diarrhea 2.40(1.74-3.31), neutropenia, 2.23(1.71-2.90) and thrombocytopenia 1.94(1.05-3.59). Conclu- sions: Findings in this meta-analysis showed that FOLFOXIRI significantly improves the PFS, RR, OS, and R0 resection rate of overall metastases over the doublet chemotherapy. The incidence of fatal adverse events was found to be more in triplet chemotherapy compared to doublet therapy. Therefore, we concluded that with moderate evidence FOLFOXIRI provide clinically meaningful efficacy benefit at cost of increased toxicity. Research Sponsor: None.

Odds ratio (OR)/Hazard ratio(HR) Number of Number of I2 Value Outcome (95% Confidence interval) studies participants (%)

OS HR - 0.81(0.73-0.89) 7 2235 0 PFS HR - 0.70(0.63-0.78) 8 2568 19 RR OR -1.66 (1.42-1.93) 10 2815 0 R0 Resection Rate OR- 1.59 (1.27-1.98) 10 2834 0 Neurological OR- 2.51(0.88-7.16) 9 2578 44 events Diarrhea OR- 2.40(1.74-3.31) 9 2578 54 Neutropenia OR- 2.23(1.71-2.90) 9 2578 0 Thrombocytopenia OR- 1.94(1.05-3.59) 7 2149 69 Mucositis OR- 1.68(1.35-2.10) 9 2498 91

3594 Poster Session

A novel clinical tool to estimate risk of false negative KRAS mutation in circulating tumor DNA testing.

Stefania Napolitano, Ryan Sun, Aparna Raj Parikh, Jason Henry, Christine Megerdichian Parseghian, Jason Willis, Kanwal Pratap Singh Raghav, Van K. Morris, Arvind Dasari, Michael J. Overman, Rajyalakshmi Luthra, Ryan Bruce Corcoran, Scott Kopetz; Universita� Degli Studi Della Campania, Luigi Vanvitelli, Naples, Italy; The University of Texas MD Anderson Cancer Center, Houston, TX; University of California San Francisco, San Francisco, CA; MD Anderson Hematology/Oncology Fellowship, Hous- ton, TX; Department of Hematopathology, University of Texas MD Anderson Cancer Center, Houston, TX; Massachusetts General Hospital, Boston, MA; Department of Gastrointestinal Medical Oncology, University of Texas MD Anderson Cancer Center, Houston, TX

Background: Recently, in metastatic colorectal cancer (mCRC), the detection of RAS mutations by circulating tumor (ct) DNA has recently emerged as a valid and non-invasive alternative approach, overall showing a high concordance with the standard tissue genotyping, giving information on response to EG- FRi treatment and resistant mechanisms. However, RAS mutations may be missed due to low levels of any ctDNA in the blood (false-negative), and it has been difficult to distinguish this from patients without a RAS mutation in the tumor (true-negative). We propose a methodology that can be applied to multi-gene ctDNA testing panels to accurately distinguish true- and false-negative tests. Methods: 357 subjects with tissue and multi-panel ctDNA testing from MD Anderson (MDACC) were used as a training dataset and 295 subjects from Massachusetts General Hospital (MGH) dataset as the testing dataset. CtDNA panels contained between 65 and 70 genes, allowing evaluation of tumor ctDNA shedding from variant allele fraction (VAF) levels in the plasma from other genes (such as APC and TP53). Based on the relationship between KRAS and the VAFs of other gene, we established a Bayesian model providing a posterior probability of false negative in the ctDNA test, using thresholds of < 5% (low), 5-15% (medium), and > 15% (high). This model was validated on the MGH database. Results: Across both cohorts, 431 patients were ctDNA wild type for KRAS. Of those, 29 had tissue documenting a KRAS mutation for a false negative rate of 8%. The model provides the posterior probability that a KRAS mutation is in- deed present in the tissue given the observed values of allele frequencies for other mutated genes in the plasma. In the validation cohort, a predicted low false negative had no false negatives (0/62, 95% CI 0%-5.8%), while a predicted medium false negative rate was associated with 3% false negative (1/32, 95% CI 0%-16%). In contrast, a high predicted false negative rate was associated with 5% false negative (5/100, 95% CI 1.6%-11%). The results demonstrate the ability of our tool to discriminate between subjects with true negative and false negatives, as a higher proportion of false negatives are observed at higher posterior probabilities. Conclusions: In conclusion, our approach provides increased confidence in KRAS ctDNA mutation testing in clinical practice, thereby facilitating the identification patients who will benefit from EGFR inhibition while reducing the risk of false negative tests. Extension of this methodology to NRAS and BRAF is possible, with clinical application enabled by a freely available online tool. Research Sponsor: None.

3595 Poster Session

Impact of time to treatment initiation on real-world (RW) outcomes in metastatic colorectal cancer (mCRC) in the United States.

Olumide B. Gbolahan, Darryl Alan Outlaw, Neda Hashemi, Ravi Kumar Paluri, Grant Richard Williams; Indiana University School of Medicine, Indianapolis, IN; UAB Hematology Oncology Fellowship, Bir- mingham, AL; University of New Mexico, Albuquerque, NM; University of Alabama at Birmingham, Bir- mingham, AL; University of Alabama Birmingham, Birmingham, AL

Background: The COVID-19 pandemic caused disruptions in cancer care delivery and forced oncologists to make recommendations about safely delaying initiation of therapy. Compared to the adjuvant setting, information about the impact of time to treatment initiation on outcomes in the palliative setting for CRCis scarce. We sought to determine the median time to initiation of systemic therapy (TIT) in mCRC in the US pre-pandemic, and to assess the impact of TIT on survival outcomes. Methods: We retrospectively analyzed de-identified data of patients (pnts) with mCRC in the Flatiron Health nationwide EHR- derived database (metastatic diagnosis dates 01/2013 - 04/202000. Demographics, treatments (tx), and outcomes were collected. TIT, the period between diagnosis and initiation of first-line systemic therapy was split into 3 categories (I: < 2 weeks, II: 2- < 4 weeks, and III: 4-8 weeks). Overall survival (OS) was defined from time of diagnosis to time of death. Post-chemotherapy survival (PCS) was time from initiation of first-line therapy to death. Adjusted and unadjusted multinomial logistic regression were used to evaluate the association of demographics and clinical factors with TIT. PCS and OS were estimated using Kaplan-Meier curves. Adjusted (demographics and clinical factors) Cox proportional hazard models were used to estimate the effect of TIT groups on PCS and OS. Category II was control group. Results: 7,108 pnts with mCRC who received at least one line of tx were identified. 16% (N = 1132), 34% (N = 2406), and 50% (N = 3570) were in TIT categories I-III. The mean age at diagnosis was 63.4 years, with no significant difference in age (P = 0.6) among categories. Median TIT was 28 days. Multinomial logistic regression showed that compared to TIT II, Hispanic pnts were more likely than Whites to receive chemotherapy in 4-8 weeks (OR 1.4, 95% CI 1.12- 1.7, P= 0.0022). Females were more likely to receive treatment in 4-8 weeks (OR 1.14, 95% CI 1.03- 1.27, P= 0.01). Pnts without documented KRAS testing were more likely to receive tx within 2 weeks (OR 1.3, 95% CI 1.05- 1.48, P= 0.01). Median RW OS favored group III (I: 18.1, II: 22.6, III: 26.9, P< 0.0001). Adjusted Cox regression analysis suggested that Blacks had a higher hazard of death compared to Whites, (HR, 1.14. 95% CI 1.03 -1.27, P = 0.01) Also, compared to TIT of 2-4 weeks, TIT < 2 weeks was associated with lower RW PCS (HR, 1.22, 95% CI 1.11- 1.33, P= 0.0001), and RW OS (HR, 1.25, 95% CI 1.14- 1.37, P= <0.0001). In contrast, TIT 4-8 weeks was associated with higher RW PCS (HR, 0.81, 95% CI 0.75- 0.87, P= 0.0001) and RW OS (HR, 0.78, 95% CI 0.72- 0.83 P= <0.0001. Conclusions: This RW analysis suggests that pre-pandemic, 50% of patients with mCRC who receive first-line therapy were treated within 4 weeks of diagnosis. We observed disparities in TIT. Paradoxically, RW survival increased with TIT, with the best outcomes reported in those treated in 4-8 weeks. Research Sponsor: None.

3596 Poster Session

Real-world survival outcomes associated with completion of adjuvant chemotherapy for stage III colon cancer.

Jemma Megan Boyle, Angela Kuryba, Thomas E Cowling, Jan van der Meulen, Nicola S Fearnhead, Kate Walker, Michael Braun, Ajay Aggarwal; London School of Hygiene and Tropical Medicine, London, United Kingdom; Royal College of Surgeons of England, London, United Kingdom; London School of Hygiene & Tropical Medicine, London, United Kingdom; Clinical Effectiveness Unit, The Royal College of Surgeons of England, London, United Kingdom; Department of Colorectal Surgery, Cambridge, Unit- ed Kingdom; The Christie NHS Foundation Trust, Manchester, United Kingdom; Guy’s and St Thomas’ NHS Foundation Trust, London, United Kingdom

Background: The optimal duration of adjuvant combination chemotherapy administered to patients with stage III colon cancer is debated. Our study assessed the effect of completed chemotherapy cycles on 3-year colon cancer-specific mortality, as well as the effect of dose reduction and early discontinuation of oxaliplatin in patients with 100% completion, within a real-world population. Methods: 4,147 patients undergoing major resection between 01 June 2014 and 30 April 2017 with pathological stage III colon cancer in the English National Health Service were identified. Chemotherapy data were obtained from linked administrative hospital records and a national chemotherapy dataset. Patients were stratified according to completion of < 50% ( < 6 FOLFOX cycles or < 4 CAPOX cycles), 50-92% (6-11 FOL- FOX cycles or 4-7 CAPOX cycles) or 100% of cycles (12 FOLFOX cycles or 8 CAPOX cycles). Competing-risk regression analysis for 3-year colon cancer-specific death was performed with adjustment for patient, tumour and hospital-level characteristics to estimate subdistribution hazard ratios (sHR) as a measure of relative risk. Results: Patients included within our study were less fit and had increased rates of high-risk disease (T4 and/or N2 pathological staging) compared to the IDEA study. For FOLFOX, the 3-year cumulative incidence of colon cancer-specific death in patients completing 100% of cycles was 15.1% (95% CI, 12.8% to 17.6%), 18.2% (95% CI, 15.3% to 21.3%) for 50-92% of cycles and 26.4% (95% CI, 20.6% to 32.5%) for < 50% of cycles. For CAPOX, this was 12.0% (95% CI, 10.2% to 14.0%) for 100% completion of cycles, 18.2% (95% CI, 15.6% to 21.0%) for 50-92% of cycles, and 19.8% (95% CI, 15.8% to 24.1%) for < 50% cycles. Compared to 100% completion of FOLFOX cycles, colon cancer-specific death was higher in patients recorded as completing < 50% (sHR 2.17; 95% CI, 1.56 to 3.03; P = < 0.001) and 50-92% of FOLFOX cycles (sHR 1.40; 95% CI, 1.09 to 1.78; P = 0.007). Compared to 100% completion of CAPOX cycles, colon cancer-specific death was higher in patients recorded as completing < 50% (sHR 2.02; 95% CI 1.53 to 2.67; P< 0.001) and 50-92% of CAPOX cycles (sHR 1.63; 95% CI 1.27 to 2.10; P< 0.001). Dose reduction and early discontinuation of oxaliplatin did not have a statistically significant effect on mortality. Con- clusions: Patients within the real world setting were more likely to have poor prognostic factors. Those who completed adjuvant chemotherapy for stage III colon cancer had improved survival rates regardless of dose reduction or early discontinuation of oxaliplatin. Research Sponsor: None.

3597 Poster Session

Clinicopathological and molecular characteristics of early-onset stage III colon adenocarcinoma: An analysis of 25 studies with 35,713 patients in the Adjuvant Colon Cancer End Points (ACCENT) database.

Zhaohui Jin, Jesse G. Dixon, Hiral Parekh, Frank A. Sinicrope, Greg Yothers, Daniel G. Haller, Hans Schmoll, Aimery De Gramont, Rachel Kerr, Julien Taieb, Eric Van Cutsem, Chris Twelves, Leonard B. Saltz, Naohiro Tomita, Takayuki Yoshino, Thierry Andre, Amit Mahipal, Richard M. Goldberg, Thomas J. George, Qian Shi; Division of Medical Oncology, Mayo Clinic, Rochester, MN; Department of Health Science Research, Mayo Clinic, Rochester, MN; Cancer Specialist of North Florida, Jacksonville, FL; Mayo Clinic, Rochester, MN; University of Pittsburgh Department of Biostatistics, and NRG Oncolo- gy Statistics and Data Management Center, Pittsburgh, PA; Abramson Cancer Center of the University of Pennsylvania, Philadelphia, PA; Martin Luther University, Halle, Germany; Franco-British Institute, Levallois-Perret, France; University of Oxford, Oxford, United Kingdom; Ho^pital Europe�en Georges Pompidou, Paris, France; University Hospitals Gasthuisberg, Leuven and KU Leuven, Leuven, Belgium; St. James’s Hospital and The University of Leeds, Leeds, United Kingdom; Department of Colorectal Oncology, Memorial Sloan Kettering Cancer Center, New York, NY; Division of Lower GI Surgery, Depart- ment of Surgery, Hyogo College of Medicine, Hyogo, Japan; National Cancer Center Hospital East, Ka- shiwa, Japan; Sorbonne Universite� and Ho^pital-Saint Antoine, Paris, France; West Virginia University Cancer Institute, Morgantown, WV; University of Florida/UF Health Cancer Center, Gainesville, FL

Background: Colon cancer (CC) incidence and mortality have decreased since the 1970s, but the incidence in young adults (20-49 years) is increasing. There are limited data suggesting that, as a group, patients with early onset CRC (eoCC) may have different phenotypic characteristics compared to those with late onset CRC (loCC, age $ 50 years). Methods: Individual patient data on 35,713 subjects with stage III CC from 25 randomized studies (recruiting between 1987 and 2009) in the ACCENT database were pooled. The distributions of demographics, clinicopathological features, biomarkers, and outcome data were summarized by age group. Overall survival (OS), disease-free survival (DFS), recurrence free rate (RFR), and survival after recurrence (SAR) were assessed by Kaplan-Meier curves and Cox models stratified by treatment arms within studies, adjusting for gender, race, body mass index, performance status, disease stage, grade, risk group, number of lymph nodes examined, disease sidedness and molecular markers. Results: Using a 5% difference between age groups as the clinically meaningful cutoff, patients with stage III eoCC (n = 6246) had similar distributions according to gender, race, PS, risk group, tumor sidedness and T/N stage compared to those with loCC (n = 29467). Patients with eoCC were significantly less likely to be overweight (30.2% vs 36.2%) but more commonly had $ 12 lymph nodes resected (69.5% vs 58.7%). The eoCC tumors were more frequently mismatch repair deficient (16.4% vs 11.5%), and less likely to have BRAFV600E (5.6% vs 14.0%), suggesting a higher frequency of Lynch syndrome in eoCC. In univariate analysis, patients with stage III eoCC had significantly better OS, DFS, and SAR; the difference between 3-year DFS and RFR strongly suggests the OS/DFS difference between these the eoCC and loCC may be due to increased competing risks and comorbidities in patients with loCC. In multivariate analysis, age at onset lost its prognostic value when outcome was adjusted for molecular markers. The clear relation between age of onset and KRAS/BRAF status was confirmed in the interaction analysis. Conclusions: Tumor biology was an important determinant of prognosis regardless of patient age. In multivariate analysis age of onset was not a statistically significant determinant of outcome. Research Sponsor: None.

Multivariate analysis with molecular Univariate Analysis markers eoCC loCC Hazard Ratio 95% CI Adjusted Hazard Ratio 95% CI 5-y OS,% 76.0 71.6 0.80 0.76-0.85** 0.92 0.80-1.05 3-y DFS, % 69.4 68.2 0.89 0.85-0.93** 0.94 0.83-1.06 3-y RFR, % 70.2 70.4 1.00 0.95-1.05 1.07 0.95-1.22 median SAR, months 20.4 17.5 0.85 0.80-0.90** 1.01 0.89-1.16

**p < 0.001.

3598 Poster Session

Clinical efficacy and safety of early adjuvant chemotherapy for stage III colon cancer: Short-term outcomes of a multicenter, randomized, open-label, phase 3 trial.

Jun Seok Park, Soo Yeun Park, Gyu-Seog Choi, Hye Jin Kim, Jong Gwang Kim, Byung Woog Kang, In Kyu Lee, Yoon Suk Lee, Sohyun Kim, Seong Kyu Baek, Gyung Mo Son, Ki Beom Bae, Ji Yeon Kim, Kyung-ha Lee; Colorectal Cancer Center, Kyungpook National University Medical Center, Daegu, South Korea; Kyungpook National University Medical Center, Daegu, South Korea; Department of Oncology/ Hematology, Kyungpook National University Medical Center, Kyungpook National University School of Medicine, Daegu, South Korea; Department of Surgery, Seoul St. Mary’s Hospital, College of Medicine, The Catholic University of Korea, Seoul, South Korea; Department of Surgery, College of Medicine, Yeungnam University, Daegu, South Korea; Department of Surgery, School of Medicine, Dongsan Medi- cal Center, Keimyung University, Daegu, South Korea; Department of Surgery, Busan National Universi- ty, Busan, South Korea; Inje University Pusan Paik Hospital, Busan, South Korea; Chungnam National University Hospital, Chungnam National University College of Medicine, Dajeon, South Korea; Depart- ment of Surgery, Chungnam National University Hospital, Chungnam National University College of Medicine, Dajeon, South Korea

Background: Adjuvant chemotherapy (AC) is recommended to commence within 8 weeks since after surgical resection of stage II or III colon cancer. Results of many retrospective studies showed inferior survival outcomes following delay of AC delay. Moreover, preclinical studies showed that the progression of disseminated cancer cells is profound during the postoperative period. This study is the first prospective trial to evaluate early (# 14 days postoperative) AC for patients (pts) with stage III colon cancer. Meth- ods: This study is a prospective, multicenter, randomized phase III trial. Pts with pathological stage III colon cancer were enrolled and randomized 1:1 to early AC (starting AC # 14 days after surgery) or conventional AC (starting AC > 14 days after surgery). Pts were recommended to receive 12 cycles of FOL- FOX-6 for AC. The primary endpoint was disease-free survival. The secondary endpoints were overall survival, adverse events, surgical complication during AC, and patient-reported outcomes (quality of life) during 1 year after surgery. Herein, safety data, chemotherapy delivery, and quality of life are presented. Results: This study randomized 443 pts either early AC arm (221pts) or early AC arm (222 pts) to the during September 2011 to March 2020. 380 pts who received at least one cycle of FOLFOX-6 were included in the safety analysis (192 and 188 in the early and conventional AC arms, respectively). The baseline characteristics of the two groups were well-balanced except for the interval from the surgery to the initial AC. The early and conventional AC arms started their first chemotherapy at median of 13 (4-43 days) and 29 (17-53 days) after surgery (p < 0.001), respectively. No significant differences were seen in the median chemotherapy cycles, AC completion, and relative oxaliplatin dose intensity between groups. AC Completion without any change of dose or schedule delay was seen in 18% and 20% in early and conventional AC arms respectively, while dose reduction or delay was 65% and 61%, respectively. Toxicities of grade 3 or more were seen in 28% in both groups. One patient in the early AC arm underwent an emergent operation for anastomotic leakage on the second day of 5-fluorouracil infusion (postoperative day 14). However, the surgical complication was not seen in any other patient. The scores of the European Organization for Research and Treatment of Cancer Quality of Life core 30 questionnaire were similar in both arms at baseline (before starting AC), and 1 month, 3 months, 6 months, and 12 months after surgery. Conclusions: Early AC was safe and did not increase either chemotherapy- related adverse events or surgery-related complications during treatment. Moreover early AC did not reduce the quality of life of the pts during 1 year after surgery. This study continues to follow-up the patients for survival outcomes. Clinical trial information: NCT01460589. Research Sponsor: None.

3599 Poster Session

Patient-specific meta-analysis of 3 validation studies of the 12-gene colon cancer recurrence score assay for recurrence risk assessment after surgery with or without 5FU and oxaliplatin.

Greg Yothers, Alan P. Venook, Takeharu Yamanaka, Yan Lin, Michael Crager, Calvin Y. Chao, Frederick L Baehner, Takayuki Yoshino; NSABP, NRG Oncology and the University of Pittsburgh, Pitts- burgh, PA; University of California San Francisco, San Francisco, CA; Department of Biostatistics, Yoko- hama City University School of Medicine, Yokohama, Japan; NRG Oncology, University of Pittsburgh, Pittsburgh, PA; Genomic Health Inc, an Exact Sciences Corporation, Redwood City, CA; Exact Sciences Corporation, Redwood City, CA; National Cancer Center Hospital East, Kashiwa, Japan

Background: The 12-gene Oncotype DX Colon Recurrence Score assay is a clinically validated genomic assay that evaluates recurrence risks in stage II and stage III colon cancer patients independent of clinical-pathologic features. Improved colon cancer care has reduced recurrence rates since the late 1990’s. Methods: Pre-specified patient-specific meta-analysis methods were used to estimate 1-, 3- and 5-year recurrence risk combining the 12-gene colon recurrence score (RS) validation studies CALGB 9581, NSABP C-07 and SUNRISE. Cox models had effects for RS result, number of nodes examined (<12 or $ 12), T-stage, MMR status, and stage (II, IIIAB or IIIC). Baseline cumulative hazard estimates used the latest two studies to reflect current medical practice. Estimates for surgery, surger- y+5FU and surgery+5FU+oxaliplatin treatment were provided by integrating stage-specific 5FU hazard ratios from a meta-analysis of the QUASAR study (2007) and a pooled analysis of NSABP studies (Wil- kinson 2010), and oxaliplatin treatment effect estimates from NSABP C-07. Recurrence risk with 5FU alone was not estimated for MMR-deficient patients due to expected lack of 5FU efficacy in these patients (Sargent 2010). Results: In the overall population of 2,179 patients, 55%, 32% and 13% were Stage II, IIIA/B and IIIC, 63% had $12 nodes examined, 90% were T3, and 88% were MMR proficient. Median RS result was 31 (IQR 23–39). RS result and each clinical-pathologic factor contributed independent prognostic information (meta-analysis Wald tests, all p<.001). Risk estimates are generally lower than previous RS report risk estimates. For patients with pathological stage II, T3, MMR-proficient tumors with $12 nodes examined, approximately 40% are expected to have 5-year recurrence risk #10% with surgery alone based on the distribution of RS results. The table shows example 5-year recurrence risk estimates for specific RS results and clinical-pathologic characteristics. Conclusions: The new recurrence risk estimates provide more patient-specific information reflecting more current medical practice than previous reports using RS result, allowing better, more individualized treatment decisions. Research Sponsor: None.

Example 5-year recurrence risk estimates (95% confidence intervals). T- Nodes MMR RS stage ex. status Stage result Surgery alone Surgery+5FU Surgery+5FU+oxali T3 $ 12 Proficient II 10 7% (5%, 9%) 5% (4%, 7%) 4% (3%, 6%) 30 10% (8%, 13%) 8% (6%, 10%) 7% (5%, 9%) 55 17% (13%, 13% (10%, 11% (7%, 15%) 22%) 18%) Proficient IIIAB 10 15% (11%, 9% (7%, 13%) 8% (5%, 11%) 20%) 30 22% (17%, 15% (11%, 12% (8%, 17%) 29%) 19%) 55 34% (26%, 23% (17%, 19% (13%, 27%) 44%) 31%) Deficient IIIAB 10 9% (5%, 13%) À 4% (3%, 8%) 30 13% (8%, 20%) À 7% (4%, 11%) 55 20% (13%, À 11% (6%, 18%) 31%) 56% of patients in the meta-analysis population were T3 with $12 nodes examined.

3600 Poster Session

Prognostic value of baseline and early changes of circulating-free (cf) and circulating tumor (ct) DNA in the neoadjuvant (NA) setting of early stage colon cancer (CC).

Giacomo Bregni, Chiara Senti, Caroline Vandeputte, Elena Acedo Reina, Paraskevas Gkolfakis, Jean- Luc Van Laethem, Philippe Vergauwe, Marc Van Den Eynde, Jos Janssens, Gauthier Demolin, Stephane Holbrechts, Marylene Clausse, Thierry De Grez, Lionel A. D’Hondt, Karen Paula Geboes, Tatiana Besse- Hammer, Francoise Rothe, Patrick Flamen, Alain Hendlisz, Francesco Sclafani; Institut Jules Bordet- Universite� Libre de Bruxelles (ULB), Brussels, Belgium; Erasme Hospital, Brussels, Belgium; AZ Groe- ninge, Kortrijk, Belgium; Cliniques Universitaires St-Luc, Brussels, Belgium; AZ Turnhout, Turnhout, Belgium; St. Joseph’s Community Health Centre, Lie�ge, Belgium; CHU Ambroise Pare�, Mons, Belgium; Clinique St-Luc Bouge, Namur, Belgium; CHR Namur, Namur, Belgium; Cliniques Universitaires UCL de Mont-Godinne, Yvoir, Belgium; UZ Gent, Gent, Belgium; CHU Brugmann, Brussels, Belgium

Background: ctDNA is an indicator of minimal residual disease and negative prognostic factor in stage II-III CC treated with surgery +/- adjuvant chemotherapy (CT). No study, however, has ever analysed the prognostic value of this biomarker in CC patients (pts) treated with NACT. We sought to evaluate the prognostic value of baseline and early changes of cf/ctDNA in stage II-III CC pts who were treated with one cycle of NA FOLFOX CT followed by surgery +/- adjuvant FOLFOX CT in the PePiTA trial. Methods: PePiTA was a multicentre, single-arm, prospective phase II trial testing in vivo tumour chemosensitivity of early stage CC (as assessed by 18F-FDG PET/CT-based metabolic response to one cycle of NA FOL- FOX) and its association with long-term outcome (NCT00994864). Plasma samples were prospectively collected at baseline, 2 weeks after one cycle of NA FOLFOX CT, and before surgery. NPY and WIF1 were selected as universal methylation markers for ctDNA and analysed with digital droplet (dd)PCR technology. Data from ddPCR were processed with the QuantaSoft v1.6 software (Bio-Rad). Survival outcome measures were 5-year disease-free survival (DFS) and 6-year overall survival (OS). ROC curve analyses, Kaplan-Meier method, cox proportional hazards models and log-rank tests were used. Statisti- cal analyses were carried out with SPSS v25.0 (SPSS Inc.). Results: 80 pts were included (44 ypStage I-II and 36 ypStage III). After a median follow-up of 52.5 months, 5-year DFS and 6-year OS were 68% (95%CI 52-84) and 84% (95%CI 74-94), respectively. Pts with high ($1600 ng/ml) baseline cfDNA had worse 6-year OS (HR 6.45, 95%CI 1.61-25.84; p = 0.008). Early changes of cfDNA after one cycle of NA FOLFOX CT failed to predict survival (HR DFS 0.96, 95%CI 0.38-2.43; p = 0.92; HR OS 0.62, 95%CI 0.16-2.50; p = 0.50). At baseline, 25 out of 60 (42%) ctDNA-assessable patients were positive. Detectable ctDNA at baseline (HR DFS 2.06, 95%CI 0.65-6.49; p = 0.22; HR OS 3.11, 95%CI 0.57-16.99; p = 0.19) or at any timepoint before surgery (HR DFS 1.65, 95%CI 0.54-5.04; p = 0.38; HR OS 2.80, 95%CI 0.54-14.44; p = 0.22) was not significantly associated with survival. A trend toward a significant association between ctDNA increase at surgery and 5-year DFS was found (HR 3.66, 95%CI 0.81-16.44; p = 0.09). Data on the correlation between early changes of cf/ctDNA and 18F- FDG PET/CT-based metabolic response will be presented at the meeting. Conclusions: For the first time, we have shown that baseline cfDNA may predict survival outcome in early stage CC pts treated with NACT. Pending confirmation in larger series, testing for cfDNA at baseline could help select high-risk pts who may benefit from FOxTROT-like, NACT treatment strategies. While analysis of ctDNA in this setting did not appear useful to predict prognosis, these results might be secondary to the small sample size. Research Sponsor: Fondation Les Amis de Bordet.

3601 Poster Session

Association of suboptimal lymph node yield with inferior survival in resected stage 1 colon cancer patients.

Alexander C Chacon, Alexa D. Melucci, Nicholas A. Ullman, Paul Burchard, Anthony S. Casabianca, Alexandra Reitz, David A. Swift, Vasileios Tsagkalidis, Jeffrey M. Switchenko, Subir Goyal, Darren R. Carpizo, Mihir Maheshkumar Shah; University of Rochester Medical Center, Rochester, NY; University of Rochester, Rochester, NY; Emory University, Atlanta, GA; Emory University, Department of Biostatis- tics and Bioinformatics, Atlanta, GA; Winship Cancer Institute and Rollins School of Public Health at Emory University, Atlanta, GA; Emory University School of Medicine, Atlanta, GA

Background: A minimum of 12 lymph nodes are required during colectomy to accurately stage colon cancer. Prior studies in stage II colon cancer patients demonstrate association of inadequate lymph node examination (LNE) with worse overall survival (OS). No large-scale analogous studies related to LNE have been completed in stage I colon cancer patients. We evaluated patients with stage I colon cancer to determine the association between lymph node yield and OS. Methods: We reviewed the Na- tional Cancer Database between 2004-2015 to identify patients with pathologic stage I colon cancer (pT1N0 or pT2N0) who underwent definitive surgical resection. Patients who received radiation therapy or had missing values were excluded. Clinical and demographic characteristics were analyzed. Based on LNE, patients were stratified into 4 cohorts (LNE, 0-5, 6-11, 12-19, 20+) and 2 cohorts (0-11, 12+). Univariable and multivariable analyses were performed to identify variables associated with OS. Kaplan-Meier survival curves were computed to compare the cohorts. Results: We included 81,909 patients for analyses. Median age at diagnosis was 69. A majority were female (51.1%), white (83.8%), received care in a community cancer program (59.5%), and had a Charlson-Deyo score of 0 (66.6%). Only 0.7% of patients had a margin positive resection with a 2.5cm median tumor size. Patients were similarly split between pT1 and pT2. Suboptimal LNE was noted in 27.8% of patients. Patients with LNE were distributed - 10.7% (0-5), 17.1% (6-11), 43.4% (12-19) and 28.9% (20+). Postoperative 30-day mortality was 1.9%. 521 (0.7%) received systemic therapy. Ten-year survival in patients with 0-5 LNE was 52.8% compared to 60.1% with 20+ LNE. On multivariable analyses, patients aged $ 69, male sex, increasing tumor size (quartile), pT2 staging and a higher Charlson-Deyo score independently predicted worse OS (p < 0.001). LNE categories were significantly associated with OS (p < 0.001) (Table). On regrouping into 0-11 and 12+ LNE groups, 0-11 LNE group predicted worse OS (HR 1.22, p < 0.001). On multivariable analysis, the above variables continued to show similar association with OS (p < 0.001). Conclusions: Our study demonstrates that lymph node yield is associated with overall survival in patients with stage 1 colon cancer undergoing surgical resection. Furthermore, patients with suboptimal lymph node yield are associated with an inferior overall survival compared to those with optimal lymph node yield. Moreover, this study finds that a large number of patients ( > 25%) continue to have suboptimal lymph node yields. Future efforts should focus on improving the lymph node yield with optimal efforts by the surgeon and pathologist. Future studies should examine the role of systemic therapy in patients with inadequate lymph node yield. Research Sponsor: None.

LNE HR (95% CI) P-value

0-5 1.50 (1.43-1.58) <.001 6-11 1.25 (1.20-1.30) <.001 12-19 1.15 (1.11-1.19) <.001 20+ - -

3602 Poster Session

Phase I study of transarterial chemoembolization of lung metastases.

Franz Edward Boas, Nancy E. Kemeny, Constantinos T. Sofocleous, Randy Yeh, Vanessa R Thompson, Meier Hsu, Chaya S. Moskowitz, Etay Ziv, Hooman Yarmohammadi, Achiude Bendet, Stephen Barnett Solomon; Memorial Sloan Kettering Cancer Center, New York, NY; Sloan Kettering Institute, New York, NY; Memorial Sloan-Kettering Cancer Center, New York, NY

Background: Lung chemoembolization (via the bronchial or pulmonary artery) is a new treatment option for unresectable and unablatable lung metastases. Methods: 10 patients with unresectable and unablatable lung, endobronchial, or mediastinal metastases, who failed systemic chemotherapy, were enrolled in this single center, single arm, phase I trial. Pulmonary and bronchial angiography was performed in all patients, to determine the blood supply to the lung metastases. Based on the angiographic findings, bronchial or pulmonary artery chemoembolization was performed, using a lipiodol / mitomycin emulsion, followed by spherical particles. Technical success, safety, efficacy, and pharmacokinetics were evaluated. Wilcoxon signed-rank test was used to compare change in size of treated versus untreated tumors. Results: On angiography, all patients had lung metastases that were hypervascular compared to normal lung. 90% of patients had lung metastases supplied by the bronchial artery, and 10% were supplied by the pulmonary artery. Technical success rate of intra-tumoral drug delivery was 100% (95% CI: 76-100%). There were no severe adverse events, and all patients met criteria for discharge 4 hours post procedure. Response rate of treated lesions was 10% by RECIST and 40% by PERCIST. Treated tumors were mostly stable to decreased in size after chemoembolization (median change in size: 0%; IQR: -11% to 2%; mean: -4%), and untreated tumors were mostly increased in size (median change in size: 10%; IQR: 0% to 17%; mean 9%; p= 0.02). Intra-tumoral lipiodol retention at 4-6 weeks was correlated with decreased tumor size and metabolic activity. Pharmacokinetics showed that 45% of the mitomycin dose underwent burst release in 2 minutes, and 55% of the dose was retained intratumorally with a half-life > 5 hours. Initial tumor-to-plasma ratio of mitomycin concentration was 380. Half-life of intratumoral lipiodol retention was 16 days. In vitro experiments showed 50% emulsion separation in 6.2 days, and 50% drug release in 7.1 hours. Conclusions: Lung chemoembolization can safely treat lung, mediastinal, and endobronchial metastases, with minimal systemic toxicity. High intratumoral drug concentrations after chemoembolization can overcome chemoresistance. Clinical trial information: NCT04200417. Research Sponsor: Brockman Medical Research Foundation, Society of Interventional Oncology.

3603 Poster Session

Long-term outcome of a phase III trial on neoadjuvant chemoradiation with capecitabine and irinotecan in patients with locally advanced rectal cancer: Updated results of the CinClare trial.

Ji Zhu, Xinchen Sun, Anwen Liu, Yaqun Zhu, Tao Zhang, Luying Liu, Jianhui Jia, Shisheng Tan, Junxin Wu, Xin Wang, Juying Zhou, Jialin Yang, Chen Zhang, Hongyan Zhang, Xinjia He, Gang Cai, Wei Zhang, Sanjun Cai, Zhen Zhang; Fudan University Cancer Center, Shanghai, China; Jiangsu Provincial People’s Hospital (The First Affiliated Hospital of Nanjing Medical University), Nanjing, China; 2nd Affiliated Hospital of Nanchang University, Nanchang, China; Department of Radiotherapy and Oncology, Second Affiliated Hospital of Soochow University, Suzhou, China; The First Affiliated Hospital of Chongqing Medical University, Chongqing, China; Zhejiang Cancer Hospital, Hangzhou, China; Liaoning Cancer Hospital and Institute, Shenyang, China; Guizhou Provincial People’s Hospital, Guiyang, China; Fujian Cancer Hospital, Fuzhou, China; Department of Radiation Oncology, West China Hospital, Sichuan Uni- versity, Chengdu, China; First Affiliated Hospital of Soochow University, Suzhou, China; Department of Radiation Oncology, Sichuan Provincial Cancer Hospital, Chengdu, China; Ningbo No.2 Hospital, Ning- bo, China; Department of Radiation Oncology, Anhui Provincial Hospital, The First Affiliated Hospital of University of Science and Technology of China, Hefei, China; The Affiliated Hospital of Qingdao Univer- sity, Qingddao, China; Ruijin Hospital, Shanghai, China; Fudan University, Shanghai, China; Depart- ment of Colorectal Surgery, Fudan University Shanghai Cancer Center, Shanghai, China; Department of Radiation Oncology, Fudan University Shanghai Cancer Center, Shanghai, China

Background: Adding UGT1A1-guided irinotecan to capecitabine-based neoadjuvant chemoradiotherapy (CRT) significantly increased the pathological complete response (pCR) rate nearly doubling [J Clin On- col. 2020 Dec 20;38(36):4231-4239]. Here, results of long-term outcome are reported. Methods: Eli- gible patients with clinical stage II/III rectal adenocarcinoma, UGT1A1 genotype *1*1 or *1*28 were randomized to the control group: pelvic radiation of 50 Gy/25 fractions with concurrent capecitabine, followed by a cycle of oxaliplatin and capecitabine; or the experimental group: radiation with capecitabine combined with weekly irinotecan 80 mg/m2 for patients with *1*1 or 65 mg/m2 for patients with *1*28, followed by a cycle of irinotecan and capecitabine. Surgery was scheduled for 8 weeks after completion of CRT. Five cycles of adjuvant XELOX chemotherapy were administered regardless of the pathologic result. Patients were stratified by UGT1A1 genotype (*1*1 vs. *1*28) clinical T stage (cT3 vs. cT4) and tumor distance from the anal verge (#5 cm vs. > 5 cm). The primary end point of pCR was reached. Survival time was calculated from the date of randomization to the date of event or the last follow-up. Secondary endpoints were defined as local failure for local control (LC), tumor recurrence or death from any cause for disease-free survival (DFS), and death from any cause for overall survival (OS). Results: Of the 360 patients initially enrolled, 356 were evaluated as the modified intention-to-treat population (n = 178 in both groups). A total of 311 patients underwent surgery and pCR was achieved in 80 patients, another 10 patients undergo a watch-and-wait approach after achieving cCR. With a median follow-up time of 48 months (Q25-Q75, 41-55 months), 57 deaths (33 and 24), 17 local failures (11 and 6) and 69 distant metastases (37 and 32) were observed, respectively. Overall, the 4y LC rate were 93% and 96% in control and experimental groups, with estimated LC HR of 0.53 (95% confidence interval [CI], 0.20-1.43), the 4y DFS rates were 69% and 74% (HR = 0.74, 95% CI 0.49-1.10), and the 4y OS were 80% and 85%, (HR = 0.70, 95% CI 0.42-1.19), respectively. In the subgroup analysis, irinotecan showed a significant improvement in DFS (HR = 0.77, 95% CI 0.61-0.98) and OS (HR = 0.71, 95% CI 0.51-0.98) in UGT1A1 *1*1 patients. Conclusions: The addition of irinotecan to standard capecitabine-based CRT had a tendency towards improving LC, DFS, and OS, but without reaching statistical significance. UGT1A1 *1*1 patients seem to benefit the most from irinotecan. Mo- lecular studies and subsequent therapies should be considered. Clinical trial information: NCT02605265. Research Sponsor: National Natural Science Foundation of China, Natural Science Foundation of Shanghai.

3604 Poster Session

Impact of radiotherapy for local control in T3 N0 rectal cancer managed with total mesorectal excision: A systematic review and meta-analysis.

Jesus C. Fabregas, Andrew Bang, Michael Tjong, Michael Kucharczyk; Billings Clinic Cancer Center, Billings, MT; Harvard TH Chan School of Public Health, Boston, MA; Princess Margaret Cancer Centre, Toronto, ON, Canada; Nova Scotia Cancer Center, Halifax, NS, Canada

Background: Total mesorectal excision (TME) significantly improved rectal cancer outcomes. Radiother- apy (RT) is recommended for T3N0 rectal cancers, though benefit has not been demonstrated in combination with TME for this specific population. This meta-analysis could provide evidence to ameliorate toxicities from treatment. Methods: Randomized clinical trials and observational studies published until October 18, 2020 were identified via PubMed and Embase. Objective: To determine whether RT decreased the risk of local recurrence (LR) in T3N0 rectal cancer managed with TME. Study Selection and Extraction: Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines were observed for literature search, extraction, and screening. Studies with LR data specific to T3N0M0 rectal cancer, treated with and without RT, were included. Reviews, non-English articles, and non-TME studies were excluded. Newcastle Ottawa Scale (NOS) evaluated quality. Meta-analysis was done with a random-effects model. Main outcome: Meta-analysis of the relative risk of LR was conducted. Results: 7,246 studies were screened, 134 full-text studies assessed for eligibility, 5 studies were included in the final analysis. No randomized data reported results specific to our study population. Five retrospective cohort studies involving 932 participants reported LR outcomes. The median follow-up ranged from 38.4 months up to 71 months. Four studies took place in Asia (797 participants) and one in North America (135 participants) (Table). Quality according to NOS ranged from 7–9. The estimated average relative risk for LR at 5 years was 0.63 (95% CI 0.31–1.29; I2=41.8%) when RT was used. Conclusions: This meta-analysis’ supports that there is no clear benefit to LR with the addition of RT in T3N0 patients with rectal adenocarcinoma undergoing TME. As meta-analysis was limited to retrospective cohort studies, there is concern for bias. Registration Prospero number CRD42020216058. Research Sponsor: None.

Characteristics of included studies.

Median Accrual N Follow-up Trial Country Time Design Participants Population Intervention Comparator (m) Outcomes

Delaney et al. 2002 USA 1980–2001 Retrospective 135 pT3NXM0 adenoca, Neoadj RT + TME TME 41 5yr LR Cohort <8cm from AV 5yr OS Kim et al. 2010 South 1996–2004 Retrospective 151 pT3N0 adenoca TME + Adj RT + TME + Adj 78 5yr LR Korea Cohort Adj Ctx Ctx 5yr OS Lin et al. 2019 China 2010–2014 Retrospective 272 cT3N0M0 adenoca Neoadj RT + TME TME 38.4–46.3 2yr LR Cohort + Adj Ctx ± Adj CRT 3yr OS + Ctx Peng et al. 2019 China 2005–2015 Retrospective 121 pT3N0M0 adenoca, TME + Adj CRT ± TME + Adj 56.4–57.1 3yr 5yr Cohort <7cm from AV Adj Ctx Ctx LR 3yr 5yr OS Baek et al. 2020 Korea 2003–2012 Retrospective 365 pT3N0M0 adenoca, TME + Adj CRT TME ± Adj 71 5yr LR Cohort negative margins Ctx 5yr OS

AAdj Ctx: Adjuvant Chemotherapy. CRT: chemoradiation.

3605 Poster Session

H101 treatment of hepatic metastasis of colorectal cancer with recombinant human adenovirus 5 injection: A phase I clinical trial-TROJAN 021.

Yang He, Jianhua Chen, Zhongzheng Zhu, Song Gao, Hui Wang, Wei Mao, Hong Qian, Weixing Liu, Xianling Guo, Huanlong Qing, Qing Xu; Shanghai Tenth People’s Hospital, Tongji University, Shanghai, China; Department of Oncology, Shanghai Tenth People’s Hospital, Tongji University, Shanghai, China; Department of Oncology, Shanghai Tenth People’s Hospital,Tongji University, Shanghai, China

Background: Recombinant human adenovirus serotype 5 injection (H101), obtained through genetic engineering to delete the E1B domain and part of the E3 domain and then selectively replicated in tumor cells, has been approved in 2005 for the local treatment of nasopharyngeal carcinoma and head and neck cancers. This trial aimed to evaluate the safety and the preliminary treatment efficacy of H101 combined with standard treatments in patients with liver metastases from colorectal cancer. Methods: In this phase 1, dose-escalation trial (ChiCTR1900027922), 17-75 years old colorectal cancer patients with unresectable liver metastases that failed to first-line therapy were included at The Tenth People’s Hospital Affiliated to Tongji University between 2018.9 and 2020.12. All patients received H101 combined with standard therapy (bevacizumab + mFOLFOX6/FOLFIRI). Ultra- sound-guided injection of H101 into the liver metastases was performed for all patients, with one of the following doses: 5�1011 vp/injection for the low, 1�1012 vp/injection for the moderate, 2�1012 vp/injection for the moderate-high, and 3�1012 vp/injection for the high dose groups. Intravenous infusion of bevacizumab (5 mg/kg) and administration of standard-dose mFOLFOX6 or FOLFIRI within 48 h after the intratumor injection was performed. The primary endpoint of the trial was the maximum tolerated dose (MTD). The secondary endpoints included safety, tumor responses, and tumor marker CEA. The adverse events (AEs) were monitored according to the NCI Common Terminology Criteria for Adverse Events (CTCAE, Version 3.0) and evaluated within 1 week after injection. Imaging examinations were performed for all patients to evaluate the tumor responses, according to the irRE- CIST. Results: Finally, 8 patients were included; the numbers of patients in the 4 groups were 1, 3, 3, and 1, respectively. MTD was not observed in this study. No grade >4 AEs were observed. The major AE included fatigue (5/8), fever (4/8), shiver (3/8), abdominal pain (3/8), and night sweat (3/8). The tumor responses included partial response in one patient (moderate dose group), stable disease in 6 patients (1, 1, 3, and 1 in the low, moderate, moderate-high, and high dose groups, respectively), and progressive disease in 1 patient (moderate group). No dose-effect response was found. The tumor marker CEA was reduced in 6 of the 8 patients, including 1 (1/1), 3 (3/3), 2 (2/3), and 0 patients in the low, moderate, moderate-high, and high-dose groups, respectively. Conclusions: The safety of H101 combined with standard therapy for liver metastases from colorectal cancer is acceptable, which also shows certain preliminary efficacy. The phase 2 trial is now ongoing. Clinical trial information: ChiCTR1900027922. Research Sponsor: Shanghai Science and Technology Commission Industry-University-Research Medical Project (No: 18DZ1910102).

DLT. Low dose Moderate dose Moderate-high dose High dose Total DLT (n=1) (n=3) (n=3) (n=1) (n=8)

Fatigue 0 3 1 1 5 Fever 0 2 1 1 4 Shiver 0 2 0 1 3 Abdominal pain 0 0 2 1 3 Night sweat 0 1 1 1 3

3606 Poster Session

Accurate early-stage colorectal cancer detection through analysis of cell-free circulating tumor DNA (ctDNA) methylation patterns.

James M. Kinross, Pol Canal-Noguer, Marko Chersicola, Primoz� Knap, Marko Bitenc, Alexandre Perera- Lluna, Michael H. A. Roehrl, Kristi Kruusmaa; Section of Biosurgery & Surgical Technology, Depart- ment of Surgery & Cancer, Imperial College London, London, United Kingdom; Universal Diagnostics S.L., Sevilla, Spain; Geneplanet D.O.O., Ljubljana, Slovenia; Universitat Polite�cnica de Catalunya, Bar- celona, Spain; Memorial Sloan Kettering Cancer Center, Department of Pathology, Human Oncology and Pathogenesis Program, New York, NY

Background: Colorectal cancer (CRC) screening programs suffer from poor uptake and biomarkers have limited diagnostic accuracy. The measurement of the methylation status of tumor-derived cell-free DNA in plasma may address these challenges. We used a targeted methylation panel, tumor-derived signal deduction and machine learning algorithm to refine a blood test for the detection of early-stage CRC. Methods: This was a prospective, international multicenter observational cohort study. Plasma samples were collected either prior to a scheduled colonoscopy as part of standard colorectal cancer screening or prior to colonic surgery for primary CRC. Differentially methylated regions (DMRs) were initially selected by analyzing CRC and control tissue samples with whole genome bisulfite sequencing. A targeted sequencing assay was designed to capture these DMRs in plasma ctDNA. Individual sequencing reads were evaluated for cancer-specific methylation signal and scores calculated for each DMR in a sample. A panel of methylation scores originating from 203 DMRs was used in a prediction model build- ing and validated in a test cohort of patients. Results: Calculated scores were used to train a machine learning model on 68 ctDNA samples from 18 early stage (I-II) and 16 late-stage (III-IV) CRC patients and 34 age, BMI, gender and country of origin matched neoplasia-free controls (median age 63 [50- 74], mean BMI 27 [19.5-37], female 50%, Spanish and Ukrainian population, distal cancers 50%). This model was then applied to an independent set of subjects from Spanish, Ukraine and Germany, including 36 stage I-IV cancer patients (median age 61.5 [55-82], BMI 28 [16-39], female 47%, 42% of the tumors were distal) and 159 age and sex matched controls. 87 of the control patients had a negative colonoscopy finding (cNEG), 19 had hyperplastic polyps (HP), 37 had small non-advanced adenomas (NAA) and 16 were diagnosed with other benign gastrointestinal diseases (GID). The model correctly classified 92% (33/36) of CRC patients. Sensitivity per cancer stage ranged from 83% (5/6) for stage I, 92% (11/12) for stage II, 92% (12/13) for stage III to 100% (5/5) for stage IV. Specificity of the model was 97% (154/159), with 100% (37/37) NAA, 94% (15/16) GID, 95% (18/19) HP and 97% cNEG patients correctly identified. Lesion location, gender, BMI, age and country of origin were not significantly correlated to prediction outcome. Conclusions: Methylation sequencing data analyzed using read-wise scoring approach combined with a machine-learning algorithm is highly diagnostic for early-stage (I-II) CRCs (89% sensitivity at 97% specificity). This method could serve as the basis for a highly accurate and minimally invasive blood-based CRC screening test with significant implications for the clinical utility of ctDNA in early-stage cancer detection. Research Sponsor: Universal Diagnostics S.L.

3607 Poster Session

A phase II study of capecitabine plus concomitant radiation therapy followed by durvalumab (MEDI4736) as preoperative treatment in rectal cancer: PANDORA study first- stage.

Stefano Tamberi, Elisa Grassi, Jody Corbelli, Giorgio Papiani, Maria aurelia Barbera, Chiara Zingaretti, Chiara Carli Moretti, Isacco Montroni, Elisabetta Petracci, Dora Caruso, Sofia Nosseir, Oriana Nanni, Giovanni Luca Frassineti, Maria Di Bartolomeo, Marina Marzola, Andrea Bonetti, Fabio Gelsomino, Carmine Pinto, Davide Tassinari, Giampaolo Ugolini; Ospedale per Gli Infermi, Faenza, Italy; Degli In- fermi Hospital, Faenza, Italy; Unita� di Biostatistica e Sperimentazioni Cliniche, IRCCS/IRST Meldola, Ravenna, Italy; Degli Infermi Hospital, Faenza, Italy, Faenza, Italy; Biostatistics and Clinical Trials, Isti- tuto Scientifico Romagnolo per lo Studio e la Cura dei Tumori (IRST) IRCCS, Meldola, Italy; Department of Pathology, Ravenna Hospital, Ravenna, Italy; Istituto Scientifico Romagnolo per lo Studio e la Cura dei Tumori (IRST) IRCCS, Meldola, Italy; Medical Oncology Unit, Istituto Scientifico Romagnolo per lo Studio e la Cura dei Tumori (IRST) IRCCS, Meldola, Italy; Fondazione IRCCS Istituto Nazionale Tumori, Milan, Italy; University of Ferrara, Ferrara, Italy; Department of Medical Oncology Mater Salutis Hospi- tal, Legnago, Italy; University Hospital of Modena, Modena, Italy; Medical Oncology Unit, Clinical Can- cer Center, AUSL-IRCCS Reggio Emilia, Reggio Emilia, Italy; Department of Oncology, Rimini, Italy

Background: The combination of capecitabine plus long course radiotherapy (RT) is the standard preoperative therapy in cT3-4 cN+ rectal cancer. Pathologic Complete remission (pCR) can be considered as surrogate end point of efficacy of treatment in terms of disease free survival (DFS). Preclinical data points heavily toward a strong synergy between RT and immune treatments. Methods: This is a prospective phase II, open label, single arm, multi-centre study, conducted with support from AstraZeneca, in patient with locally advanced rectal cancer who receive concomitant CT/RT therapy (825 mg/m2 twice daily capecitabine every day and 5040 cGy radiotherapy for 5 days per week for 5 weeks) followed by durvalumab (1500 mg Q4W for 3 administrations). Surgery is performed after 10-12 weeks from neoadjuvant therapy. The primary endpoint is pCR rate after at least 1 cycle of durvalumab. The sample size has been estimated by using the optimal Simon’s two-stage design. If more than 4 complete responses are observed in the first 19 enrolled patients, 36 additional patients will be accrued for a total of 55 evaluable patients. Results: Between November 2019 and July 2020, 20 patients were accrued and 19 were evaluable for study objectives, concluding the first stage of the trial. Baseline characteristics of the first 19 evaluable patients enrolled are listed in the table. All patients received 3 infusions of durvalumab; 18 patients underwent surgery after a median of 13 weeks from CHT/RT end. Five complete pathological responses (ypT0N0) were observed, allowing to proceed to the second stage. About toxicity, four patients had Grade 3-4 adverse events (AE); the most frequent G3-4 AE related to the neoadjuvant therapy were anemia (n=1), diarrhea (n=2) and neuthropenia (n=2). No grade 3 and 4 adverse events related to Durvalumab treatment were observed. Eight patients had G1-2 AE related to durvalumab, the most common being asthenia (n=2) and nausea (n=2). Conclusions: At the end of study’s first stage the preoperative treatment with radiotherapy plus capecitabine followed by durvalumab showed a safe toxicity profile and promising activity in terms of pCR rate. The second part of the trial is ongoing, and the accrual is under completion (44 patients enrolled as of 10 February 2021). Clinical trial information: NCT04083365. Research Sponsor: None.

Characteristic n (%)

Gender Female 13 (68.4) Male 6 (31.6) Age Median 63 years Range 35–81 years ECOG PS 0 14 (73.7) DRE result 1 5 (26.3) Palpable 10 (66.7) Not palbable 5 (33.3) Clinical T stage Not evaluable 4 (21) cTx 1 (5.3) cT3 13 (68.4) Clinical N stage cT4 5 (26.3) cN0 4 (21.0) cN1 11 (57.9) EMVI cN2 4 (21.1) Positive 5 (27.8) Negative 13 (72.2) Not evaluable 1 (5.3) ECOG PS: Eastern Cooperative Oncology Group performance status; DRE: digital rectal exploration; cT: clinical tumor; cN: clinical node; T stage: tumor stage; N stage: nodal stage; EMVI: extramural vascular invasion.

3608 Poster Session

Minimal residual disease by circulating tumor DNA analysis for colorectal cancer patients receiving radical surgery: An initial report from CIRCULATE-Japan.

Hiroki Yukami, Yoshiaki Nakamura, Jun Watanabe, Masahito Kotaka, Kentaro Yamazaki, Keiji Hirata, Yasunori Emi, Mitsuru Yokota, Kentaro Kato, Tatsuro Yamaguchi, Masataka Ikeda, Alexey Aleshin, Daisuke Kotani, Saori Mishima, Hiromichi Shirasu, Eiji Oki, Ichiro Takemasa, Takeshi Kato, Hiroya Taniguchi, Takayuki Yoshino; Department of Gastroenterology and Gastrointestinal Oncology, National Cancer Center Hospital East, Kashiwa, Japan; Department of Surgery, Gastroenterological Center, Yoko- hama City University Medical Center, Yokohama, Japan; Gastrointestinal Cancer Center, Sano Hospital, Kobe, Japan; Division of Gastrointestinal Oncology, Shizuoka Cancer Center, Shizuoka, Japan; Depart- ment of Surgery 1, University of Occupational & Environmental Health, Fukuoka, Japan; Department of Surgery, Saiseikai Fukuoka General Hospital, Fukuoka, Japan; Department of General Surgery, Kurashi- ki Central Hospital, Okayama, Japan; Department of Surgery, Teine-Keijinkai Hospital, Sapporo, Japan; Department of Surgery, Tokyo Metropolitan Cancer and Infectious Diseases Center Komagome Hospital, Tokyo, Japan; Division of Lower Gastrointestinal, Department of Surgery, Nishinomiya, Japan; Natera, Inc., San Carlos, CA; Division of Clinical Oncology, Shizuoka Cancer Center, Shizuoka, Japan; Depart- ment of Surgery and Science, Graduate School of Medical Sciences, Kyushu University, Fukuoka, Ja- pan; Department of Surgery, Surgical Oncology and Science, Sapporo Medical University, Sapporo, Japan; Department of Colorectal Surgery, National Hospital Organization Osaka National Hospital, Osa- ka, Japan

Background: Circulating tumor DNA (ctDNA) analysis can be used to predict the risk of recurrence by detecting molecular residual disease (MRD) in patients with colorectal cancer (CRC). We are conducting a prospective observational study to monitor MRD status in patients with clinical stage II–IV or relapsed CRC amenable to radical surgical resection (GALAXY study), as part of the CIRCULATE-Japan, a nationwide ctDNA-guided precision adjuvant therapy project. Methods: Analysis of ctDNA is being performed at pre- and post-surgery timepoints and will continue periodically for up to 2 years using Signatera, a personalized, tumor-informed ctDNA assay that is designed to track 16 patient-specific somatic variants based on whole-exome sequencing of tumor tissue. The association of peri-operative ctDNA status with clinicopathological characteristics was investigated. Results: As of January 13, 2021, 941 patients have been enrolled in the GALAXY study, of which 400 patients had their pre-operative ctDNA status evaluated. Of the 400 patients, baseline ctDNA was detected in 92% (367/400) of the patients: consisting of 35 patients with pathological stage (pStage) I, 135 with pStage II, 152 with pStage III, and 78 with pStage IV or relapsed disease (pStage IV/R). Patient-specific Signatera assays targeting 16 variants were designed for 100% of the patients. Out of the 6400 designed variants 99.3% passed quality control in the plasma analysis and produced the final results. Among 4425 genes selected for 400 patients, 3330 genes were selected for only one patient, while TP53 was the most commonly selected in 113 patients (28%). Median ctDNA levels, measured in mean tumor molecules per mL of plasma and ctDNA detection rate, stratified by stage are presented in table. Positive ctDNA status post-surgery was significantly associated with advanced pStage, pT and pN, and lymphovascular invasion. Of the 13 patients with recurrence, 10 were detected with a positive ctDNA at 4-weeks post-surgery, before confirmation of recurrence by the radiological imaging. Conclusions: Preoperative ctDNA detection rates were observed to be in >90% in patients with pStage II–III by personalized ctDNA assay based on unique somatic variants, specific to each patient. ctDNA- based MRD detected post-surgery (4W) was significantly associated with certain known clinicopathological factors for recurrence with ctDNA positivity associated with a very short-term of recurrence. Clinical trial information: 000039205. Research Spon- sor: Japan Agency for Medical Research and Development. ctDNA levels and detection rates pre- and post-surgery (4 weeks). Pre-surgery Post-surgery (4 weeks) pStage Median ctDNA (MTM/mL) ctDNA detection rate Median ctDNA (MTM/mL) ctDNA detection rate

Stage I 0.73 80% 0.92 6% Stage II 3.66 96% 0.72 6% Stage III 4.54 94% 0.46 25% Stage IV/R 27.07 86% 1.81 32%

3609 Poster Session

Pharmacologic ascorbate enhances the therapeutic index of ATM-inhibitor based chemoradiation for colorectal cancer.

Cameron Callaghan, Ibrahim Abukhiran, Richard VanRheeden, Michael Petronek, Kranti Mapuskar, Md Yousuf Ali, Amanda Kalen, Samuel Rodman, Steven Neema Seyedin, Joseph J. Cullen, Mitchell Coleman, John Michael Buatti, Prabhat Goswami, Bryan G. Allen, Douglas Spitz, Joseph Michael Caster; University of Iowa Hospitals and Clinics, Iowa City, IA; Department of Pathology, Iowa City, IA; UIHC Stead Family Department of Pediatrics, Division of Medical Genetics and Genomics, Iowa City, IA; UIHC Free Radical and Radiation Biology, Iowa City, IA; Department of Orthopedics and Rehabilita- tion, Iowa City, IA; University of Iowa Hospital and Clinics, Iowa City, IA; The University of Iowa, Iowa City, IA; University of Iowa Hospitals and Clinics, Department of Radiation Oncology, Iowa City, IA

Background: Ataxia telangectasia mutated protein (ATM) is one of the key sensors of DNA damage and specific inhibitors of ATM are potent radiosensitizers. However, their clinical utility with radiation (RT) is limited because they lack tissue specificity and increase normal tissue injury. Pharmacologic (high dose) ascorbate (P-AscH-) selectively increases oxidative stress in tumors while functioning as a donor antioxidant and reducing RT damage in normal tissues. We hypothesized that P-AscH- could enhance the therapeutic index of ATM-inhibitor based chemoradiation (CRT) for colorectal cancer (CRC) by simultaneously enhancing efficacy and reducing RT bowel injury. Methods: Human HCT116, SW480, and HT29 and murine CT26 and MC38 CRC models were used. Clonogenic survival was assessed following single-fraction RT (2-8 Gy) +/- P-AscH- (5 pM/cell) +/- veliparib (PARP), VE821 (ATR), or KU60019 (ATM). Catalase expression was induced using HCT116 cells expressing a doxycycline inducible catalase transgene. DNA double strand breaks (DSBs) were quantified using neutral comet assays 0-24 hours post RT. Cell cycle phases were assessed using flow cytometry. ATM and pATM localization were assessed using IF. Jejunal toxicity was assessed using IHC in fixed tissues following single fraction (10 Gy) whole abdominal RT in c57blj/6 mice. Tumor growth delay was assessed following RT (5 Gy x 3) +/- drug treatment in unilateral flank tumors. Results: Veliparib, VE821, and KU60019 were potent radiosensitizers in HCT116, SW480, HT29, MC38, and CT26 CRC tumor models and P-AscH- further reduced clonogenic survival with DRIs in all lines except for HT29. In contrast, P-AscH- enhanced survival of cultured HUVEC and FHs-74 cells exposed to RT. Enhanced cell kill with - - P-AscH is H202 mediated as it is completely attenuated by inducible catalase expression. P-AscH significantly increased the number of DNA DSBs in tumors after RT in vitro. Despite the increase in DNA DSBs, P-AscH-significantly decreased nuclear localization of activated P-ATM after RT and significantly decreased the fraction of cells in G2/M phases of the cell cycle. In vivo, RT + P-AscH- + KU60019 induced more tumor growth delay/clearance than all other combinations in unilateral MC38 or HCT116 flank tumors. Finally, P-AscH- significantly reduced loss of jejunal crypt cell density, epithelial architecture, and markers of lipid and protein oxidation following whole abdominal RT. Conclusions: P-AscH- selectively enhances the efficacy of ATM-based CRT in CRC tumor models while simultaneously decreasing RT-mediated small bowel toxicity. In tumors, P-AscH- enhances DNA DSBs by stimulating an H202 flux and prevents activation of DNA repair pathways and cell cycle checkpoints by inhibiting RT-induced activation of ATM. Selective radioprotectors like P-AscH- could facilitate the clinical trans- lation ATM inhibitors as radiosensitizers. Research Sponsor: RSNA Resident seed grant #RR1914.

3610 Poster Session

Evaluating longitudinal toxicity of cetuximab in patients with metastatic colorectal cancer (mCRC): A pooled analysis from 1,302 patients in the ARCAD database.

Guilherme Lopes, Jane Yeojeong So, Katrin Marie Sjoquist, Curtis L. Olswold, Eric Van Cutsem, Carsten Bokemeyer, Richard Adams, Benoist Chibaudel, Axel Grothey, Takayuki Yoshino, Aimery De Gramont, Qian Shi, Christophe Tournigand, John Raymond Zalcberg; Mayo Clinic, Rochester, MN; Auckland City Hospital, Auckland, New Zealand; NHMRC Clinical Trials Centre, The University of Sydney, Sydney, Australia; University Hospitals Gasthuisberg, Leuven and KU Leuven, Leuven, Belgium; University Medical Center Hamburg-Eppendorf, Hamburg, Germany; Cardiff University and Velindre Cancer Cen- tre, Cardiff, United Kingdom; Franco-British Institute, Levallois-Perret, France; West Cancer Center, OneOncology, Germantown, TN; National Cancer Center Hospital East, Kashiwa, Japan; Ho^pital Henri Mondor, Assistance Publique-Ho^pitaux de Paris, Cre�teil, France; Peter MacCallum Cancer Centre, Mel- bourne, Australia

Background: Chronic lower grade adverse events (AE) can negatively affect a patient’s quality of life but it is difficult to capture using a traditional toxicity reporting approach. A novel AE reporting method was recently developed to describe, summarize, and present longitudinal AE profiles(Lopes et al, 2021). We leveraged this method to describe and compare the AE profiles of doublet chemotherapy (DC) + Cetuxi- mab and DC alone in mCRC patients. Methods: This AE reporting method utilizes two additional AE metrics to complement the maximum (max) toxicity grade usually reported in clinical trials. Onset time indicates the time period in which max grade for an AE occurred for the first time, defined here as early (i.e. within first 42 days) and late (i.e. after the 42nd day). AE Load (AEL) indicates the overall severity of an AE in the entire treatment. AEL varies from 0 to 1. Higher AEL indicates a worse overall severity of that AE over time. AEL is the key metric for describing chronic AE. We included patients receiving DC + cetuximab (n = 738) and DC alone (n = 564 [ref group]) from two randomized first-line trials in the AR- CAD database. Diarrhea, rash, hand-foot syndrome (HFS), fatigue, anorexia, and mucositis were examined and adjusted for backbone (FOLFOX vs. FOLFIRI), ECOG PS, sex, site location, dose reduction, and treatment length. Results: For rash, DC + cetuximab had a higher risk of G3+ (21% vs. 0.5%; odds ratio {OR} [95% confidence interval {CI}] = 50 [16,157], p < 0.001), increased overall severity over the entire treatment (AEL = 0.257 vs. 0.069; Adjusted difference in means–Mdiff [95% CI] = 0.22 [0.21,0.23], p < 0.001), and increased risk of early onset (67% vs. 33%; OR [95% CI] = 4.3 [2.7,6.7], p < 0.001). DC + cetuximab also had higher AEL for rash across max grades (p < 0.001 within G1, G2, and G3+). For HFS, DC + cetuximab had a higher risk of G3+ (OR [95% CI] = 6.0 [2.5,14], p < 0.001), increased overall severity (AEL = 0.139 vs. 0.087; Mdiff [95% CI] = 0.03 [0.03,0.04], p < 0.001), and slightly earlier onset (12.4 vs. 13.9 weeks; Mdiff, weeks [95% CI] = -4.9 [-0.80,-9.0], p = 0.021). Within each max grade, DC + cetuximab did not have higher AEL of HFS. No associations were found for diarrhea, fatigue, anorexia, or mucositis. Conclusions: The addition of cetuximab is associated with higher grade, more persistent, and more immediate rash. The higher severity in HFS with the addition of cetuximab appears to be related to higher grade but not chronic HFS. This method may be useful to describe different strategies, e.g. intermittent cetuximab. It provided a comprehensive view of acute and chronic toxicity profiles supporting its potential interest as new metrics in clinical trials. Lopes GS, Tournigand C, Olswold CL, et al. Adverse event load, onset, and maximum grade: A novel method of reporting adverse events in cancer clinical trials. Clinical Trials. 2021;18(1):51-60 Research Sponsor: None.

3611 Poster Session

Influence of preoperative chemoradiation on tumor-infiltrating lymphocytes in locally advanced rectal cancer: The STAR-01 cohort.

Francesca Negri, Letizia Gnetti, Lorena Bottarelli, Nicoletta Campanini, Maria Emanuela Negru, Francesca Bergamo, Salvatore Siena, Michela Frisinghelli, Marija Petric, Germana Chiaulon, Valeria Smiroldo, Domenico C. Corsi, Giulia Anna Carmen Vita, Enrico Maria Silini, Cinzia Azzoni, Federica Gaiani, Luigi Laghi, Gian Luigi de’Angelis, Luca Boni, Carlo Aschele; University Hospital of Parma, Par- ma, Italy; Ospedale Sant’Andrea, La Spezia, Italy; Department of Oncology, Oncology 1, Veneto Insti- tute of Oncology IOV-IRCCS, Padua, Italy; Grande Ospedale Metropolitano Niguarda and Universita� degli Studi di Milano, Milan, Italy; Oncology Unit S. Chiara Hospital, Trento, Italy; Central Regional Hospital, Bolzano, Italy; Azienda Ospedaliera-Universitaria Santa Maria della Misericordia, Udine, Italy; Humanitas Clinical and Research Center, IRCCS, Rozzano, Italy; UOC Oncologia Medica San Giovanni Calibita Fatebenefratelli, Rome, Italy; IRCCS CROB, Rionero in Volture (PZ), Italy; Unita� Operativa Epi- demiologia Clinica IRCCS Ospedale Policlinico San Martino, Genoa, Italy; Medical Oncology Unit, Gen- eral Hospital, La Spezia, Italy

Background: Preoperative chemoradiotherapy may increase antitumor immunity through enhancing T- cell activation and tumor infiltration. These effects could possibly sensitize tumors to immunotherapies, including checkpoint inhibitors. We explored whether preoperative chemoradiation for locally advanced rectal cancer induces immunologic changes and if the post-operative biological parameters are associated with tumor regression grade (TRG sec. Ryan –AJCC Eight ed.). Methods: The multicenter STAR-01 study compared a standard preoperative chemoradiotherapy regimen (50.4 Gy in 28 daily fractions with concomitant infused fluorouracil at the dose of 225 mg/m2/d) with the same regimen plus oxaliplatin given weekly at the dose of 60 mg/m2 in patients with locally advanced rectal cancers. Paired pre- and post-operative specimens were available for 58 patients from this trial and were analyzed by immunohistochemistry. The immunoistochemical analysis was performed with a panel of immune cells and associated factors as CD3, CD20, CD4/CD8, PD1. The pattern of tumor infiltrating lymphocytes (TILs) and related infiltrating lymphocytes (RILs) was also evaluated. Response to pre-operative chemoradiotherapy was assessed according to TRG. Results: After therapy we observed a decreased CD4/CD8 ratio (p < 0.001) and reduced expression level of CD20 (p < 0.001). The expression level of CD3+ and PD- 1+ cells after therapy did not change significantly. The relative increase of lymphocytes CD8+ inside CD4/CD8 ratio evaluated on post-operative samples was significantly associated with TRG 0 (p < 0.001). Conclusions: Our data suggest that chemoradiation may induce an enrichment of CD8+ T lymphocytes and this translates in better response to chemoradiation. The new frontier of best treatment could be the use of specific immune cells (T lymphocytes) to trigger the system’s immune response against disease. Research Sponsor: SNUPI Onlus.

3612 Poster Session

Impact of the COVID-19 pandemic in treating gastrointestinal (GI) cancer patients receiving systemic anticancer treatment (SACT): The Guy’s Cancer Centre experience.

Jose Roca, Ailsa Sita-Lumsden, Eirini Tsotra, Charalampos Gousis, Beth Russell, Charlotte Moss, Victoria Harris, Kasia Owczarczyk, Julien De Naurois, Nicholas R. Maisey, Sarah Ngan, Debashis Sarker, Kiruthikah Thillai, Vasiliki Michalarea, Asad Qureshi, Imran Petkar, Anne Sagar Rigg, Mieke Van Hemelrijck, Paul J. Ross, Saoirse Dolly; Guy’s and St Thomas’ NHS Foundation Trust, London, United Kingdom; Department of Translational Oncology and Urology Research, London, United King- dom; Department of Translational Oncology and Urology Research, King’s College London, London, United Kingdom; London Research Institute, Milton Keynes, United Kingdom; Guys Hospital, London, United Kingdom; King’s College Hospital, Institute of Liver Studies, London, United Kingdom; Guy’s and St Thomas’s NHS Foundation Trust, London, United Kingdom; Guy’s and St Thomas NHS Founda- tion Trust, London, United Kingdom; Guys and St Thomas NHS Foundation Trust, London, United Kingdom; Guy’s Hospital, London, United Kingdom

Background: The COVID-19 pandemic has hugely affected the spectrum of cancer care. Worldwide healthcare systems have rapidly reorganized cancer services to ensure patients continue to receive es- sential care whilst optimizing the use of systemic anti-cancer treatments (SACT) and minimizing exposure to SARS-CoV-2 infection. Our study aimed to identify the outcome of patients with gastrointestinal (GI) cancers in our Cancer Centre during the pandemic compared to the same period in 2019. Methods: Retrospective analysis of all GI patients receiving any SACT at Guy’s Cancer Centre from the 1 March- 31 May 2020 and 2019. Demographic data (age, ethnicity, socio-economic status (SES), Performance status, cancer and SACT characteristics (type, intent and treatment-line) were collected during both periods. Also we collated the number of COVID-19 infections confirmed by PCR and severity defined by the WHO classification. Patients with clinical or radiological diagnosis were excluded. Results: 567 patients received SACT in 2019 and 417 patients in 2020 (26.4% less). No differences were observed in the demographic or tumour type characteristics. Commonest cancers in both periods were similar: colorectal (47.1, 47%), oesophago-gastric (29, 27.6%), pancreatic-biliary and NET tumours (23.8, 25.4%). However, there were a higher proportion of patients with advanced disease treated in 2020 (70.3% versus 55.2%). Treatment intent was similar in both years: radical (3.5 vs 3.8%), adjuvant (18.2% vs 17.3%), neoadjuvant (15.3% vs 12.7%) and palliative (63% vs 66.2%). There was an increase in the proportion of patients treated in the palliative first line setting (63.8% in 2020 vs 47.6% in 2019) and a reduction in the proportion of third or more treatment (8.7% versus 16.2%) mainly in the colorectal patients. Of 417 GI patients receiving SACT, 14 (3.35%) were diagnosed with COVID-19 infection. Of these, 64.3% were male, 92.9% were low SES and 35.7% were of Caucasian ethnicity. Colorectal cancer was the commonest (57.1%) tumour-type in the COVID-positive group and 57.1% had advanced disease. All the patients that died from COVID-19 were male. 13 patients were on chemotherapy and 1 was on targeted/biological treatment. None was in immunotherapy (n=4). Only one patient was neutropenic (grade 1). 8 patients (57.1%) had severe infection and there were 3 (21.4%) COVID-related deaths. Conclusions: Our study shows the delivery of SACT through the COVID-19 pandemic is relatively safe with low COVID-related mortality rate. It also reflects how we tailored the delivery of anti-cancer treatments to reduce the possible detrimental myelo-suppressive toxicities that could potentially put GI patients at higher risk of severe SARS-CoV-2 infection. This is crucial data that can inform anti-cancer treatment decision making during the protracted COVID-19 pandemic. Research Sponsor: None.

3613 Poster Session

Multi-omics longitudinal analyses in stages I to III CRC patients: Surveillance liquid biopsy test to predict early recurrence and enable risk-stratified postoperative CRC management.

Xuanhui Liu, Yani Zhang, Xiurui Zhu, Sheeno P. Thyparambil, Wei-Li Liao, Xiao-bin Zheng, Jin You, Ashiq Masood, Zhen Li, Gabriel Yang, Xiaoming Yao, Shiying Hao, Robert Heaton, James Schilling, Karl G. Sylvester, Jiayu Liao, Feng Gao, Ping Lan, Xuefeng Ling, Xiaojian Wu; Guangdong Provincial Key Laboratory of Colorectal and Pelvic Floor Diseases, The Sixth Affiliated Hospital of Sun Yat-sen Uni- versity, Guangzhou, China; mProbe Inc, Rockville, MD 20850, Rockville, MD; University of California, Riverside, Riverside, CA 92521, Riverside, CA; Rush University, Chicago; Stanford University Medical Center, Stanford University, Stanford; Stanford University Medical Center, Stanford University, Stan- ford, CA; Stanford University Medical Center, Stanford University, Palo Alto, CA; University of Califor- nia, Riverside, Riverside, CA 92521, Riverside; Department of Colorectal Surgery, The Sixth Affiliated Hospital, Sun Yat-sen University, Guangzhou, China

Background: One-third of colorectal cancer (CRC) recurs following curative surgery and chemotherapy. Accordingly, novel methods are needed to predict recurrence to enable clinical course mitigating strategies. Serial monitoring of plasma by mass spectrometry (MS) and multi-omics modeling (MMO) of CRC relapse chronology provide the framework for liquid biopsy test development to supersede existing imaging modalities such as CT scans according to relapse related pathologies. We hypothesized that plasma MS and MMO analysis of relapse related pathologies can deconvolute high risk stratification for CRC recurrence within the cancer continuum of care pre/post-surgery and/or pre/post adjuvant chemotherapy (ACT). Methods: 189 CRC patients (Stage I-III) underwent one of three treatment modalities: Modality 1 (Surgery followed by ACT), Modality 2 (Surgery only), Modality 3 (Neoadjuvant chemotherapy followed by surgery and ACT). Plasma samples (n = 441) were collected from patients before surgery, 30 days post-op, and every 3 months until death or month 24 whichever came first. The MMO approach was used to analyze biological features encompassing native peptides, proteins, metabolites, lipids, and ce- ramides. MMO panels were developed comprising the significantly perturbed features as per the treatment modalities. These panels were used to predict relapse from plasma collected pre-op, 30-day post- op or after adjuvant chemotherapy. CEA levels were monitored in parallel. Results: Follow-up data was available for 135 patients (Stage I-III) and 25/135 had evidence of radiological recurrence. Irrespective of the treatment modality, longitudinal follow-up using the MMO panel was able to predict disease recurrence greater than 7 months before clinical progression was confirmed by CT scan. There was no significant correlation between longitudinal CEA levels and recurrence status, hence CEA levels alone did not provide any lead time advantage over the MMO panel or radiological surveillance. Kaplan-Meier (KM) survival analysis revealed that patients that were MMO panel positive had a poor survival irrespective of treatment modalities used: Modality 1 (HR = 6.2, p value = 0.003, test immediately post-surgery and immediately before ACT; HR = 31.6, p value = 0.01, test immediately after ACT); Modality 2 (HR = 11.2; p value = 0.01, test immediately after-surgery); Modality 3 (HR > 40, p value = 0.08, test immediately after neo-ACT and before-surgery; HR > 40, p value = 0.004, test immediately after-surgery). Conclusions: The MMO panel predicts CRC recurrence several months prior to detection by conventional CT scans, thus providing opportunity for alternative therapeutic strategies much earlier in the disease course. Research Sponsor: National Natural Science Foundation of China (No.81972212)， Guang- dong Natural Science Foundation of China (No. 2019A1515010063).

TPS3614 Poster Session

A randomized phase III study of immune checkpoint inhibition with chemotherapy in treatment-naive metastatic anal cancer patients: A trial of the ECOG-ACRIN cancer research group (EA2176).

Marc Thomas Roth, Paul J. Catalano, Kristen Keon Ciombor, Al Bowen Benson, Xin Yao, Rona Yaeger, Mohamed E. Salem, Van K. Morris, David H. Henry, Jennifer G. Whisenant, Peter J. O’Dwyer, Cathy Eng; Vanderbilt-Ingram Cancer Center, Nashville, TN; Dana-Farber Cancer Institute, Boston, MA; Northwestern Medicine, Chicago, IL; Fox Valley Hem Onc, Appleton, WI; Memorial Sloan Kettering Can- cer Center, New York, NY; Levine Cancer Institute, Atrium Health, Charlotte, NC; The University of Tex- as MD Anderson Cancer Center, Houston, TX; University of Pennsylvania, Pennsylvania Hospital, Philadelphia, PA

Background: Anal cancer is growing in annual incidence globally and human papillomavirus (HPV) remains the predominant risk factor underlying its development. Due to its relative rarity, clinical trials in anal cancer have historically been difficult to conduct and treatment options for metastatic disease remain limited. Carboplatin/paclitaxel (CP) was compared to cisplatin/5-fluorouracil (historical standard of care) in a recent randomized phase II clinical trial (InterAACT; EA2133) in treatment-nave metastatic anal cancer, finding that response rates were equivocal, but that overall survival (OS) was significantly longer in the CP arm (20 months vs 12.3 months, p = 0.014). Additionally, reduced grade 3/4 toxicities were seen in the CP arm. NCI9673, a single-arm phase II study, established safety and efficacy of nivolumab in previously-treated metastatic anal cancer. Progression-free survival (PFS) was 4.1 months (95% CI 3.0-7.9) and OS was 11.5 months (95% CI 7.1-not estimable). Multiple randomized trials in lung cancer have demonstrated efficacy of platinum-based chemotherapy combined with checkpoint inhibitors. Together these studies form the rationale behind combining CP and nivolumab in treatment- nave metastatic anal cancer. Methods: EA2176 (NCT04444921) is the first NCTN phase III randomized clinical trial in treatment-nave metastatic anal cancer. Stratification factors include HIV status and history of chemoradiation for curative intent. Patients will be randomized to carboplatin (AUC = 5, Day 1) plus paclitaxel (80mg/m2, Days 1, 8, 15) +/- nivolumab 240mg IV (Cycle 1 = Days 1, 15; Cycle $2 = Day 1, 480mg) q 28-days until disease progression or treatment intolerance. CP will be given for up to 6 cycles, while nivolumab will be continued as maintenance for up to 2 years. The primary endpoint is PFS. Secondary objectives include OS, response rate, and toxicity. Goal enrollment is 205 patients and the study continues accrual. This sample size will provide 80% power at a two-sided a of 0.05 to detect a 4.8-month improvement in PFS assuming 8 months in the control arm. Novel correlative studies include sequential quantitative tumor-derived cell-free HPV ctDNA levels (serotypes 16 and 18; Sysmex-Inostics SafeSEQ NGS assay). Correlative funding provided in part by the Farrah Fawcett Foun- dation and Sysmex Inostics, Inc. Clinical trial information: NCT04444921. Research Sponsor: NCI/ CTEP, Other Foundation, Pharmaceutical/Biotech Company.

TPS3615 Poster Session

Total neoadjuvant treatment versus standard chemoradiation to increase the sphincter preservation rate for distal locally advanced rectal cancer (TESS).

Weiwei Xiao, Xiaojun Wu, Peiqiang Cai, YeZhong Zhuang, Xiaozhong Wang, Shoumin Bai, Qiaoxuan Wang, YiJing Ye, Qing Liu, Min Liu, Shuang Liu, ZhiFan Zeng, Zhi-Zhong Pan, Yuanhong Gao, Gong Chen; Department of Radiation Oncology, Sun Yat-sen University Cancer Center, State Key Laboratory of Oncology in South China, Collaborative Innovation Center of Cancer Medicine, Guangzhou, China; Sun Yat-sen University Cancer Center, Guangzhou, China; Shantou Cancer Hospital, Shantou, China; Shantou Central Hospital, Shantou, China; Sun Yat-sen Memorial Hospital, Guangzhou, China; Zhong- shan People’s Hospital, Zhongshan, China; Department of Medical Statistics and Epidemiology, School of Public Health, Sun Yat-sen University, Guangzhou, China; The Sun Yat-Sen University Cancer Cen- ter, Guangzhou, China; Department of Radiation Oncology, Sun Yat-sen University Cancer Center, State Key Laboratory of Oncology in South China, Collaborative Innovation Center of Cancer Medicine, Guang- zhou, China, Guangzhou, China; Department of Colorectal Surgery, Sun Yat-Sen University Cancer Cen- tre, Guangzhou, China

Background: Standard treatment of rectal cancer is neoadjuvant capecitabine chemotherapy with radiotherapy, followed by total mesorectal excision (TME). Total neoadjuvant treatment (TNT), a new concept, suggests organ preservation as an alternative to rectal excision in good responders after neoadjuvant chemoradiotherapy to decrease surgical morbidity and increase quality of life. RAPIDO and PRODIGE-23 trials showed that TNT strategy could improve the pathological complete response (pCR) rateand reduce the risk of distant metastasis. The objective of this trial is to increase the proportion of sphincter preservation rate for distal rectal cancer patients by optimizing tumor response, by using TNT regimen as compared to conventional chemoradiotherapy. TESS (clinicalTrials.gov, NCT03840239), a prospective, open label, multicenter, randomized phase 2 study, is underway. Meth- ods: Main inclusion criteria include: cT3-4aNany or cTanyN+ rectal adenocarcinoma aged 18-70y; ECOG performance 0-1; distance#5cm from anal verge. 168 patients will be randomized 1:1. Patients in the TNT group will receive 2 cycles of neoadjuvant chemotherapy Capeox (capecitabine + oxalipla- tion) before, during and after radiotherapy 50Gy/25 fractions, before TME (or other treatment decisions, such as watch and wait) and adjuvant chemotherapy capecitabine 2 cycles. Patients in the standard treatment group will receive neoadjuvant radiotherapy 50Gy/25 fractions combined with capecitabine 5 weeks before TME (or other treatment decisions, such as watch and wait), and adjuvant chemotherapy Capeox 6 cycles. Primary endpoint is the rate of sphincter preservation rate (absence of stoma). Second- ary endpoints include: Ratio of sphincter preservation strategy; pathological complete response rate and tumor regression grade distribution; acute toxicity; surgical complications; long-term anal function; late toxicity; ECOG standard score; disease-free survival; overall survival. First site opened in January 24, 2019. Clinical trial information: NCT03840239. Research Sponsor: the 5010 Clinical Research Foundation of Sun Yat-sen University.

TPS3616 Poster Session

PERSPECTIVE: Tepotinib + cetuximab in patients (pts) with RAS/BRAF wild-type left-sided metastatic colorectal cancer (mCRC) and acquired resistance to anti-EGFR antibody therapy due to MET amplification (METamp).

Tanios S. Bekaii-Saab, Eric Van Cutsem, Antonio Cubillo, Caroline Petorin-Lesens, Nuria Rodriguez- Salas, Kanwal Pratap Singh Raghav, Olivier Dupuis, Carlos Lo�pez-Lo�pez, Christophe Tournigand, Nicolas Isambert, Khalid Abubaker, Karl-Maria Schumacher, Karin Berghoff, Soetkin Vlassak, Gordon Otto, Josep Tabernero; Mayo Clinic, Phoenix, AZ; UZ Leuven, Leuven, Belgium; Hospital Universitario Sanchinarro (Madrid)-Claro Campo (HIOCC), Madrid, Spain; CHU Estaing, Clermont-Ferrand, France; Hospital Universitario La Paz, Madrid, Spain; MD Anderson Cancer Center, Houston, TX; Clinique Victor Hugo, Le Mans, France; Hospital Universitario Marque�s de Valdecilla, Santander, Spain; CHU Ho^pital Henri Mondor, Creteil, France; Service d’Oncologie me�dicale, CLCC Georges-Francois Leclerc, Dijon Cedex, France; Clinical Biomarkers and Companion Diagnostics, Merck KGaA, Darmstadt, Germany; Global Clinical Development, Merck KGaA, Darmstadt, Germany; Global Patient Safety, Merck KGaA, Darmstadt, Germany; Global Medical Affairs, Merck KGaA, Darmstadt, Germany; Vall d’Hebron Univer- sity Hospital and Vall d’Hebron Institute of Oncology (VHIO), UVic-UCC, Barcelona, Spain

Background: METamp is a secondary, or co-driving, genetic change in pts with mCRC and acquired resistance to anti-EGFR therapy, which can contribute to disease progression. In EGFR-resistant pts with mCRC and METamp, MET inhibition + an anti-EGFR agent may achieve disease control by targeting emerging MET pathway activation and maintaining EGFR pathway inhibition. Tepotinib is an oral, once-daily, highly selective, potent MET tyrosine kinase inhibitor (TKI), recently approved in the US for NSCLC harboring MET exon 14 skipping. Tepotinib + gefitinib demonstrated improved outcomes in pts with EGFR-mutant METamp NSCLC and acquired EGFR TKI resistance vs chemotherapy (INSIGHT: NCT01982955). In these pts, progression-free survival (PFS) was 16.6 vs 4.2 months (HR = 0.13; 90% CI: 0.04, 0.43) and overall survival (OS) was 37.3 vs 13.1 months (HR = 0.08; 90% CI: 0.01, 0.51). In pts with mCRC and acquired resistance to anti-EGFR antibody therapy due to METamp, tepotinib + anti-EGFR antibody cetuximab may be active and provide an effective therapeutic option. Meth- ods: This Phase II, multicenter, single-arm, open-label study will assess preliminary safety and tolerability, antitumor activity, and explore pharmacokinetic (PK) profiles of tepotinib + cetuximab in pts with RAS/BRAF wild-type left-sided mCRC and acquired resistance to anti-EGFR antibody-targeted therapy due to METamp (NCT04515394). A safety run-in (6–12 pts) will evaluate the recommended Phase II dose of tepotinib to be used in combination with cetuximab (endpoint: dose-limiting toxicities). Enrollment is based on a confirmed advanced left-sided CRC diagnosis (RAS/BRAF wild-type), documented previous anti-EGFR therapy and acquired resistance on most recent anti-EGFR antibody and METamp confirmed by liquid and/or tissue biopsy. Pts must be $18 years old, have ECOG PS of 0/1 and normal organ function. The study will screen sufficient pts to account for setting-specific heteroge- necity in reported METamp incidence. Approximately 42 pts are planned to receive study treatment: ~22 in Cohort A (second-line, outside US) and 20 in Cohort B ($third-line, US only). Primary endpoint: investigator-assessed objective response (RECIST 1.1). Secondary endpoints are investigator-assessed duration of response (DoR), PFS (RECIST 1.1) and OS, tolerability and safety (NCI-CTCAE v5.0), and cetuximab immunogenicity (measured by antidrug antibody assays at the start and end of treatment). Additional endpoints include assessment of tepotinib and cetuximab PK profiles, and expression of biomarkers of resistance (from blood and/or tissue samples). Retrospective assessment of best overall response, DoR and PFS by an independent review committee may be conducted. No formal statistical hypothesis will be tested in this exploratory study. Clinical trial information: NCT04515394. Research Sponsor: Merck KGaA, Darmstadt, Germany.

TPS3618 Poster Session

Colorectal Cancer Metastatic dMMR Immuno-Therapy (COMMIT) Study: A randomized phase III study of atezolizumab (atezo) monotherapy versus mFOLFOX6/bevacizumab/atezo in the first-line treatment of patients (pts) with deficient DNA mismatch repair (dMMR) or microsatellite instability high (MSI-H) metastatic colorectal cancer (mCRC)—NRG-GI004/ SWOG-S1610.

Michael J. Overman, Greg Yothers, Samuel A. Jacobs, Hanna Kelly Sanoff, Deirdre Jill Cohen, Katherine A Guthrie, Norah Lynn Henry, Patricia A. Ganz, Scott Kopetz, Peter C. Lucas, Charles David Blanke, Norman Wolmark, Howard S. Hochster, Thomas J. George, Caio Max Sao Pedro Rocha Lima; University of Texas MD Anderson Cancer Center, and SWOG, Houston, TX; University of Pittsburgh De- partment of Biostatistics, and NRG Oncology Statistics and Data Management Center, Pittsburgh, PA; NSABP/NRG Oncology, Pittsburgh, PA; UNC Lineberger Comprehensive Cancer Center, University of North Carolina at Chapel Hill and Alliance, Chapel Hill, NC; Icahn School of Medicine at Mount Sinai and ECOG-ACRIN, New York, NY; Fred Hutchinson Cancer Research Center, and SWOG Statistics and Data Management Center, Seattle, WA; Department of Internal Medicine, University of Michigan Medi- cal School and SWOG, Ann Arbor, MI; NRG Oncology, and UCLA Jonsson Comprehensive Cancer Center at UCLA, UCLA Fielding School of Public Health, Los Angeles, CA; NRG Oncology and Department of Gastrointestinal Medical Oncology, University of Texas MD Anderson Cancer Ctr, Houston, TX; NRG On- cology, and University of Pittsburgh School of Medicine, UPMC Hillman Cancer Center, Pittsburgh, PA; SWOG Cancer Research Network Group Chair’s Office, Oregon Health and Science University Knight Cancer Institute, Portland, OR; NSABP/NRG Oncology, and The UPMC Hillman Cancer Center, Pitts- burgh, PA; NRG Oncology, and Rutgers Cancer Institute of New Jersey, New Brunswick, NJ; NRG On- cology, and The University of Florida Health Cancer Center, Gainesville, FL; NRG Oncology, and Wake Forest University Baptist Medical Center, Winston-Salem, NC

Background: The superiority of inhibition of programmed cell death-1 (PD-1) pathway in dMMR/MSI-H over chemotherapy with either anti-vascular endothelial growth factor receptor (VEGFr) or anti- epitheli- al growth factor receptor (EGFr) antibodies in mCRC has been demonstrated in a phase III trial (N Engl J Med 2020; 383:2207). However, more patients had progressive disease as the best response in the anti-PD1 monotherapy arm (29.4% vs. 12.3%) with mean progression-free survival (PFS) of 13.7 months. Preclinical models have demonstrated synergistic interactions between FOLFOX, anti-VEGF, and anti-PD-1. We hypothesize that the dMMR/MSI-H mCRC patients may be more effectively treated by the combination of PD-1 pathway blockade and mFOLFOX6/bevacizumab (bev) rather than with anti-PD-L1 therapy (atezo) alone. Methods: Initially a three-arm study, the mFOLFOX6/bev arm was closed to new enrollment on 6-4-20 due to emerging data; the redesigned COMMIT trial was reactivated on 1/29/2021 as a prospective phase III open-label trial that randomizes (1:1) mCRC dMMR/MSI-H pts (N=211) to either atezo monotherapy or mFOLFOX6/bev+atezo combination. Stratification factors include BRAFV600E status, metastatic site, and prior adjuvant CRC therapy. Primary endpoint is PFS as assessed by site investigator. Secondary endpoints include overall survival (OS), objective response rate (RECIST v1.1), safety profile, disease control rate, duration of response, and centrally-reviewed PFS. Health-related quality of life is an exploratory objective. Archived tumor tissue and blood samples will be collected for correlative studies. Key inclusion criteria are: mCRC without prior chemotherapy for advanced disease; dMMR tumor determined by local CLIA-certified IHC assay (MLH1/MSH2/MSH6/ PMS2) or MSI-H by local CLIA-certified PCR or NGS panel; and measurable disease per RECIST. Clini- cal trial: NCT02997228. Support: U10CA180868, -180822, -180888, -180819, UG1CA189867, U24CA196067; Genentech, Inc. Clinical trial information: NCT02997228. Research Sponsor: U.S. National Institutes of Health, Pharmaceutical/Biotech Company.

TPS3619 Poster Session

BREAKWATER: Randomized phase 3 study of encorafenib (enco) + cetuximab (cetux) ± chemotherapy for first-line (1L) treatment (tx) of BRAF V600E-mutant (BRAFV600E) metastatic colorectal cancer (mCRC).

Scott Kopetz, Axel Grothey, Rona Yaeger, Fortunato Ciardiello, Jayesh Desai, Tae Won Kim, Tim Maughan, Eric Van Cutsem, Harpreet Singh Wasan, Takayuki Yoshino, Michelle L. Edwards, Adele Golden, Ashwin Gollerkeri, Josep Tabernero; MD Anderson Cancer Center, Houston, TX; West Cancer Center, OneOncology, Germantown, TN; Memorial Sloan Kettering Cancer Center, New York, NY; Uni- versity of Campania Luigi Vanvitelli, Naples, Italy; Peter MacCallum Cancer Centre, Melbourne, VIC, Australia; Asan Medical Center, University of Ulsan College of Medicine, Seoul, South Korea; MRC Ox- ford Institute for Radiation Oncology, University of Oxford, Oxford, United Kingdom; University Hospital Gasthuisberg and University of Leuven, Leuven, Belgium; Hammersmith Hospital, Division of Cancer, Imperial College London, London, United Kingdom; National Cancer Center Hospital East, Kashiwa, Ja- pan; Pfizer, New York, NY; Pfizer, Cambridge, MA; Vall d’Hebron University Hospital and Vall d’Hebron Institute of Oncology (VHIO), UVic-UCC, Barcelona, Spain

Background: Approximately 10% of patients (pts) with mCRC have BRAF mutations (mostly V600E). 1L tx options for BRAFV600E mCRC are limited to cytotoxic chemotherapy ± anti-VEGF or anti-EGFR, or immune checkpoint inhibitors in pts with MSI-H tumors. In Europe, Japan, and USA, the combination of BRAF inhibitor enco + EGFR inhibitor cetux is approved for tx of BRAFV600E mCRC after prior therapy. In BEACON CRC, enco + cetux resulted in a median overall survival (OS) of 9.3 months (95% confidence interval [CI]: 8.0–11.3) and an objective response rate (ORR) of 19.5% (95% CI: 14.5%–25.4%) in previously treated pts with BRAFV600E mCRC (median follow-up: 12.8 months); 57.4% of pts had grade 3/4 adverse events (AEs); 9% discontinued due to AEs. Given the poor prognosis of pts with BRAFV600E mCRC and based on the efficacy and tolerability of enco + cetux from BEA- CON CRC, BREAKWATER will evaluate efficacy and safety of enco + cetux ± chemotherapy in tx-naive pts with BRAFV600E mCRC. Methods: BREAKWATER is an open-label, global, multicenter, randomized, phase 3 study with a safety lead-in (SLI). Approximately 60 and 870 pts will be enrolled in the SLI and phase 3 parts of the study, respectively. Pts must have BRAFV600E mCRC (determined using tumor tissue or blood); ECOG performance status 0/1; and adequate bone marrow, hepatic, and renal function. Pts in the SLI must have evaluable disease (RECIST v1.1) and have received # 1 prior tx regimen; those previously treated with a BRAF or EGFR inhibitor, or both oxaliplatin and irinotecan, will be excluded. Pts in the phase 3 study must have measurable disease and be tx naive for metastatic disease. Study tx and endpoints are shown in the table. Enrollment began on 6 January 2021. Clinical trial information: NCT04607421. Research Sponsor: Pfizer.

SLI Phase 3

Tx* Enco 300 mg QD + Arm A cetux 500 mg/m2 + mFOLFOX6 Enco 300 mg QD + cetux 500 mg/m2 or Arm B Enco 300 mg QD + cetux 500 mg/m2 + mFOLFOX6 or FOLFIRI (depending on SLI) Enco 300 mg QD + Control (± bevacizumab) mFOLFOX6 cetux 500 mg/m2 + FOLFIRI or FOLFOXIRI or FOLFIRI or CAPOX (21-day cycle; oxaliplatin, Q3W; capecitabine, BID Days 1–14) Endpoints Primary Incidence of dose-limiting toxicities Progression-free survival (PFS; by blinded independent central review [BICR]) (arm A vs control; arm B vs control) Secondary Incidence/severity of AEs, ORR, duration of Key: OS (arm A vs control; arm B vs control) response (DOR), PFS, time to response (TTR), OS, Other: ORR, DOR, PFS (arm A vs arm B by BICR; arm pharmacokinetic (PK) parameters, drug–drug A vs control; arm B vs control; arm A vs arm B by interaction of enco with irinotecan/oxaliplatin investigator), OS (arm A vs arm B), TTR, progression after next tx line, incidence/severity of AEs, pt-reported outcomes, PK parameters, MSI status, BRAF V600E variant allele fraction

*All 28-day cycles except CAPOX; Q2W.

TPS3620 Poster Session

Trastuzumab deruxtecan in patients with HER2-overexpressing locally advanced, unresectable, or metastatic colorectal cancer (mCRC): A randomized, multicenter, phase 2 study (DESTINY-CRC02).

Kanwal Pratap Singh Raghav, Takayuki Yoshino, Rosine Guimbaud, Ian Chau, Marc Van Den Eynde, Joan Maurel, Jeanne Tie, Tae Won Kim, Kun-Huei Yeh, Daniel Barrios, Kojiro Kobayashi, Emarjola Bako, Mehreteab Aregay, Gerold Meinhardt, Salvatore Siena; The University of Texas MD Anderson Can- cer Center, Houston, TX; National Cancer Center Hospital East, Kashiwa, Japan; CHU Toulouse, Tou- louse, France; The Royal Marsden Hospital NHS Foundation Trust, London and Sutton, United Kingdom; Cliniques Universitaires St-Luc, Brussels, Belgium; Hospital Clinic Barcelona, Barcelona, Spain; Peter MacCallum Cancer Centre, Melbourne, Australia; Asan Medical Center, University of Ulsan College of Medicine, Seoul, South Korea; National Taiwan University Hospital, Taipei, Taiwan; Daiichi Sankyo, Basking Ridge, NJ; Grande Ospedale Metropolitano Niguarda and Universita� degli Studi di Mi- lano, Milan, Italy

Background: Trastuzumab deruxtecan (T-DXd) is an antibody–drug conjugate consisting of an anti- HER2 antibody (trastuzumab) linked to a potent topoisomerase I inhibitor (DXd). T-DXd has been approved to treat HER2-positive metastatic breast cancer (United States, Japan, Europe) and advanced gastric cancer (United States, Japan). It is currently being evaluated in other solid tumor types including colorectal cancer. The phase 2 DESTINY-CRC01 study included patients with RAS wild-type mCRC, with median 4 (range, 2-11) prior lines of therapy. Preliminary results in patients with HER2- overexpressing (IHC 3+ or IHC 2+/ISH+) mCRC showed T-DXd treatment (6.4 mg/kg intravenously [IV] every 3 weeks [Q3W]) resulted in a confirmed objective response rate (ORR) of 45.3% (24/53; 95% CI, 31.6%-59.6%) and a median progression-free survival (PFS) of 6.9 months (95% CI, 4.1 months-not evaluable; Siena J Clin Oncol. 2020;38[15]:4000). Activity was also seen in patients treated with prior anti-HER2 therapy. Although 5.4-mg/kg and 6.4-mg/kg doses of T-DXd have shown clinical efficacy in multiple cancer indications, the lower dose has not yet been tested in patients with HER2-overexpressing mCRC. Preliminary data also suggest T-DXd may be active in RAS mutant mCRC, unlike other anti- HER2 therapies. The DESTINY-CRC02 study aims to determine efficacy and safety of T-DXd in patients with HER2-overexpressing, RAS wild-type or mutant mCRC at 5.4-mg/kg and 6.4-mg/kg doses. Meth- ods: DESTINY-CRC02 (NCT04744831) is a multicenter, randomized, double-blind, 2-arm, parallel phase 2 study that will be conducted in 2 stages. Eligible patients ($18 years; $20 years in Japan, Tai- wan, and Korea) will have HER2-overexpressing (IHC 3+ or IHC 2+/ISH+) locally advanced, unresectable or metastatic CRC and have previously received chemotherapy, anti-EGFR therapy, anti-VEGF treatment, and/or anti–PD-1/PD-L1 therapy, as clinically indicated. Prior anti-HER2 therapy will be allowed. In stage 1, patients will be randomly assigned 1:1 to receive T-DXd IV Q3W at a dose of 5.4 mg/ kg (n = 40; arm 1) or 6.4 mg/kg (n = 40; arm 2). Randomization will be stratified by ECOG PS (0 or 1), HER2 status (IHC 3+ or IHC 2+/ISH+), and RAS status (wild-type or mutant). After stage 1 enrollment is complete, eligible patients in stage 2 (n = 40) will receive T-DXd 5.4 mg/kg until disease progression or other treatment discontinuation criteria are met. The study is actively enrolling and aims to enroll 120 patients across 60 sites. The primary objective is to assess efficacy of T-DXd at the 5.4-mg/kg and 6.4-mg/kg doses, with a primary end point of confirmed ORR by blinded independent central review. Secondary end points include investigator-assessed ORR, PFS, duration of response, disease control rate, clinical benefit rate, overall survival, pharmacokinetics, and safety. Clinical trial information: NCT04744831. Research Sponsor: Daiichi Sankyo, Inc, Pharmaceutical/Biotech Company.

TPS3621 Poster Session

JCOG1805 (PanDRa-BD study): A randomized controlled study of adjuvant chemotherapy for stage II colorectal cancer patients at high-risk of developing recurrence according to T-stage and three selected pathological factors (Pn, DR, and BD).

Megumi Ishiguro, Hideki Ueno, Atsuo Takashima, Junki Mizusawa, Keita Sasaki, Hiroshi Katayama, Tetsuya Hamaguchi, Shunsuke Tsukamoto, Yukihide Kanemitsu; Department of Chemotherapy and On- cosurgery, Tokyo Medical and Dental University, Tokyo, Japan; National Defense Medical College, De- partment of Surgery, Saitama, Japan; Department of Gastrointestinal Oncology, National Cancer Center Hospital, Tokyo, Japan; JCOG Data Center, National Cancer Center, Tokyo, Japan; JCOG Operations Of- fice, National Cancer Center Hospital, Tokyo, Japan; Department of Gastroenterological Oncology, Sai- tama Medical University International Medical Center, Saitama, Japan; Department of Colorectal Surgery, National Cancer Center Hospital, Tokyo, Japan

Background: Adjuvant chemotherapy for stage II colorectal cancer (CRC) still remains controversial. Al- though the NCCN and ESMO guidelines recommend adjuvant chemotherapy for patients with high-risk features, the survival benefit has not been confirmed. We reviewed the evidence levels for prognostic values of risk factors, because lack of their robustness is a major source of uncertainty regarding the optimal indication of adjuvant chemotherapy. Consequently, on top of the T-stage, three pathological factors—perineural invasion (Pn), tumor budding (BD), and desmoplastic reaction (DR)—were selected as robust risk factors of recurrence. Among the conventional factors, the prognostic value of Pn had been well validated in a multicenter study conducted by the Japanese Society for Cancer of the Colon and Rectum (JSCCR; Am J Surg Path 2013), but others were deemed suboptimal in terms of the prognostic value. BD and DR are novel tumor- and stroma-factors, respectively, associated with cancer microenvironment at the tumor front. According to the JSCCR and ITBCC 2016 criteria, tumors are graded as BD1, BD2, or BD3. The DR heterogeneity is categorized into Mature, Intermediate, and Immature patterns based on site-specific products of cancer-associated fibroblasts—keloid-like collagen and myxoid stroma. According to a recent prospective multicenter study, BD and DR characterization represent a higher level of prognostic value than other conventional factors (SACURA trial; J Clin Oncol 2019, Br J Cancer 2021). Based on the four selected risk factors, we can exclude the patient group with favorable prognosis (i.e., > 90% of 5-year RFS), which accounts for approximately 40% of the total population, resulting in enabling us to identify the concentrated population of high risk of developing recurrence. Methods: The Japan Clinical Oncology Group (JCOG) launched a randomized controlled phase III trial to evaluate the superiority of adjuvant chemotherapy in terms of relapse-free survival (RFS) over observation only in stage II CRC patients aged 20–80 years having one or more of the following risk factors: pathological T4, Pn, BD3, and non-Mature DR. Patients are randomised, in a 1:1:1 ratio, to [A] observation, [B] capecitabine monotherapy for 6 months, or [C] capecitabine and oxaliplatin (CAPOX) for 3 months. A total of 1680 patients will be accrued from 54 Japanese institutions assuming 3-year RFS with [A] to be 82% and expected 5% increase in 3-year RFS for [B] and [C] with one-sided alpha of 2.5% and power of 80% for each pair comparison. Patient enrollment was started in January 2020 and 170 patients have been enrolled until January 2021. This trial has been registered at Japan Registry of Clinical Trials as jRCTs031190186. Clinical trial information: jRCTs031190186. Research Sponsor: Japan Agency for Medical Research and Development.

TPS3622 Poster Session

Phase II/III study of Circulating tumOr DNA as a predictive BiomaRker in Adjuvant chemotherapy in patients with stage II colon cancer: NRG-GI005 (COBRA).

Van K. Morris, Greg Yothers, Scott Kopetz, Samuel A. Jacobs, Peter C. Lucas, Atif Iqbal, Patrick M Boland, Dustin A. Deming, Aaron James Scott, Howard John Lim, Norman Wolmark, Thomas J. George; The University of Texas MD Anderson Cancer Center, Houston, TX; University of Pittsburgh Department of Biostatistics, and NRG Oncology Statistics and Data Management Center, Pittsburgh, PA; Depart- ment of Gastrointestinal Medical Oncology, University of Texas MD Anderson Cancer Center, Houston, TX; NSABP/NRG Oncology, Pittsburgh, PA; NSABP/NRG Oncology, and The UPMC Hillman Cancer Center, Pittsburgh, PA; NRG Oncology, and Baylor College of Medicine, Houston, TX; Rutgers Cancer Institute of New Jersey, New Brunswick, NJ; University of Wisconsin Carbone Cancer Center, and ECOG-ACRIN, Madison, WI; Banner-University of Arizona Cancer Center, Division of Hematology and Oncology, Tucson, AZ; British Columbia Cancer Vancouver, and CCTG Co-Chair, Vancouver, BC, Cana- da; University of Florida/UF Health Cancer Center, Gainesville, FL

Background: There are currently no validated predictive biomarkers for stage II resected colon cancer (CC) after adjuvant chemotherapy. However, circulating tumor DNA (ctDNA) that is shed into the blood- stream represents a highly specific and sensitive approach for identifying microscopic or residual tumor cells. For patients (pts) with CC, the detection of ctDNA is associated with persistent disease after resection and may outperform traditional clinical and pathological features as a prognostic factor to assess risk for recurrence. We hypothesize that for pts whose stage II colon cancer has been resected and who have no traditional high-risk features, a positive ctDNA status may identify those who will benefit from adjuvant chemotherapy. Methods: In this prospective phase II/III clinical trial, pts (N = 1,408) with resected stage II CC without traditional high-risk features and whom the evaluating oncologist deems suitable for no adjuvant chemotherapy will be randomized 1:1 into 2 arms: standard-of-care/observation (Arm A), or prospective testing for ctDNA (Arm B). Postoperative blood will be analyzed for ctDNA with the GuardantHealth LUNAR panel, covering CC-relevant mutations and CC-specific methylation profiling. Pts in Arm B with ctDNA detected will be treated with 6 months of adjuvant (FOLFOX) chemotherapy. For all pts in Arm A, ctDNA status will be analyzed retrospectively at the time of endpoint analysis. The primary endpoints are clearance of ctDNA with adjuvant chemotherapy (phase II) and recurrence-free survival (RFS) for ctDNA-detected pts treated with or without adjuvant chemotherapy (phase III). Secondary endpoints will include time-to-event outcomes (OS, RFS, TTR) by ctDNA marker status and treatment, prevalence of detectable ctDNA in stage II CC, and rates of compliance with assigned intervention. Archived normal and matched tumor and blood samples will be collected for exploratory correlative research. The trial is actively accruing towards the phase II endpoint in North America. NCT#: 04068103. Support: U10-CA-180868, -180822; UG1CA-189867; GuardantHealth. Clinical trial information: NCT04068103. Research Sponsor: U.S. National Institutes of Health, Phar- maceutical/Biotech Company.

TPS3623 Poster Session

Neoadjuvant chemoradiotherapy with sequential ipilimumab and nivolumab in rectal cancer (CHINOREC): A prospective randomized, open-label, multicenter, phase II clinical trial.

Johannes Laengle, Irene Kuehrer, Dietmar Pils, Julijan Kabiljo, Judith Stift, Friedrich Herbst, Bernhard Dauser, Matthias Monschein, Peter Razek, Stefanie Haegele, Wolfgang Huller, Reinhold Fuegger, Hans Geinitz, Andreas L. Petzer, Clemens Bitterman, Friedrich Laengle, Dietmar Tamandl, Joachim Widder, Rainer Schmid, Michael Bergmann; Division of Visceral Surgery, Department of General Surgery, Com- prehensive Cancer Center Vienna, Medical University of Vienna, Vienna, Austria; Department of Pathol- ogy, Comprehensive Cancer Center Vienna, Medical University of Vienna, Vienna, Austria; Department of Surgery, Hospital of St. John of God, Vienna, Austria; Department of Surgery, Clinic Floridsdorf, Vien- na, Austria; Department of Pathology, Clinic Floridsdorf, Vienna, Austria; Institute of Pathology, State Hospital Wiener Neustadt, Wiener Neustadt, Austria; Department of General and Visceral Surgery, Con- gregational Hospital Linz, Sisters of Mercy, Linz, Austria; Department of Radiation Oncology, Congrega- tional Hospital Linz, Sisters of Mercy, Linz, Austria; Department of Internal Medicine I, Congregational Hospital Linz, Sisters of Mercy, Linz, Austria; Department of Surgery, State Hospital Wiener Neustadt, Wiener Neustadt, Austria; Division of General and Pediatric Radiology, Department of Biomedical Imag- ing and Image-guided Therapy, Comprehensive Cancer Center Vienna, Medical University of Vienna, Vi- enna, Austria; Department of Radiation Oncology, Comprehensive Cancer Center Vienna, Medical University of Vienna, Vienna, Austria

Background: Immune checkpoint inhibitors (ICI), such as ipilimumab (anti-cytotoxic T-lymphocyte-associated protein 4) or nivolumab (anti-programmed cell death protein 1) have been proven to be an effective strategy in solid cancers. However, ICI seem not to be effective in microsatellite stable (MSS) cancers. As they might lack an immunogenic priming, radiotherapy (RT) is capable to induce an immunogenic cell death (ICD) and subsequently an immunogenic tumor immune microenvironment (TIME). Thus, RT might restore the susceptibility of MSS tumors to ICI and consequently leading to an effective anti-tumor immune response. Methods: This is a prospective, randomized, open-label, multicenter, phase II investigator-initiated clinical trial (IIT), including patients with locally advanced rectal cancer (LARC). Patients receive either neoadjuvant chemoradiotherapy (CRT) alone (50 Gy in 2 Gy fractions over 25 working days + capecitabine 1650 mg/m2/d PO) or in combination with ipilimumab (1 mg/kg IV on day 7) and nivolumab (3 mg/kg IV on day 14, 28 and 42). Patients will undergo surgery within 10- 12 weeks post CRT. The primary endpoint is incidence of treatment-emergent adverse events (AEs) assessed by the Clavien-Dindo classification of surgical complications and the common terminology criteria of adverse events (CTCAE). Secondary objectives are radiographic and pathological therapy response. Serial liquid (plasma, serum and peripheral blood mononuclear cells) and tissue biopsies will be taken before, during and after neoadjuvant treatment. Genomic, transcriptomic, epigenomic and proteomic pattern of liquid and tissue biopsies, as well as the immune cell infiltrate of resected specimen, will be correlated with therapy response and clinical outcome. Currently 8 of planned 80 patients have been enrolled. Registration numbers: NCT no. NCT04124601, EudraCT no. 2019-003865-17. Clinical trial information: NCT04124601. Research Sponsor: This is an investigator-initiated trial (IIT), which received a research grant and the study medications from Bristol-Myers Squibb (BMS).