Advances in Targeting RET-Dependent Cancers

Home , ABL (gene), GDNF family of ligands, KIF5B, Pralsetinib, RET inhibitor, ROS1

Published OnlineFirst February 24, 2020; DOI: 10.1158/2159-8290.CD-19-1116

MINI REVIEW

Vivek Subbiah1,2,3 and Gilbert J. Cote4

ABSTRACT RET alterations have been characterized as oncogenic drivers in multiple cancers. The clinical validation of highly selective RET inhibitors demonstrates the utility of specific targeting of aberrantly activated RET in patients with cancers such as medullary thyroid cancer or non–small cell lung cancer. The remarkable responses observed have opened the field of RET-targeted inhibitors. In this review, we seek to focus on the impact of therapeutic RET targeting in cancers.

Significance: Successful clinical translation of selective RET inhibitors is poised to alter the therapeutic landscape of altered cancers. Questions that clearly need to be addressed relate to the ability to maintain long-term inhibition of tumor cell growth, how to prepare for the potential mechanisms of acquired resistance, and the development of next-generation selective RET inhibitors.

INTRODUCTION limited benefit from MKIs with secondary RET activity. MKIs such as vandetanib and cabozantinib are FDA-approved in Genome-driven precision oncology has altered the thera- the treatment of advanced MTC and have demonstrated activ- peutic landscape of multiple kinase-driven hematologic and ity in patients with RET fusion–positive NSCLC, but their solid malignancies. The use of imatinib in BCR–ABL fusion– response rates and duration of response are lower when com- positive chronic myelogenous leukemia (1) and the use of cri- pared with other selective kinase inhibitors for ALK or ROS1 zotinib in ALK fusion– and ROS1 fusion–positive non–small fusion–driven NSCLC (2, 3, 11, 13, 14). Development of selec- cell lung cancer (NSCLC) are prime examples of transforma- tive RET inhibitors is poised to change this paradigm. In this tive first-generation kinase inhibitor therapies (2, 3). Twenty- review, we focus on the impact of RET therapeutic targeting five years ago, inherited mutations inRET mutations were in cancers, within the context of a relatively short-term treat- identified as the cause of multiple endocrine neoplasia type 2 ment experience. Questions that need to be further addressed (MEN2; refs. 4–7), which was soon followed by the discovery of include the ability to maintain long-term inhibition of tumor somatic activating RET aberrations, genetic fusions, or muta- cell growth, and how to prepare for the potential mechanisms tions in diverse malignancies (8–10) warranting their choice as of acquired resistance. We also consider the need for devel- prospective therapeutic targets (Fig. 1). RET fusions are seen opment of second-generation selective RET inhibitors, and in NSCLC (2%) and papillary thyroid cancers (PTC; 10%–20%), finally the potential side effects associated with reduced RET whereas somatic (60%–90%) or germline (100%) RET mutations activity in tissues reliant on its expression. are seen in medullary thyroid cancer (MTC; refs. 11, 12). Although RET was one of the earliest genes to be cloned, and several multikinase inhibitors (MKI) have RET inhibi- THE DISCOVERY OF THE RET tory activity, patients with RET-driven malignancies, especially PROTO-ONCOGENE patients with RET fusion–positive NSCLC, have derived only By 1985, the search for human oncogenes was rapidly advancing. Approximately a dozen or so transforming genes, 1Department of Investigational Cancer Therapeutics, Division of Cancer most notably the RAS family members, had already been Medicine, The University of Texas MD Anderson Cancer Center, Houston, identified using a simple assay of transfecting human tumor 2 Texas. Division of Pediatrics, The University of Texas MD Anderson Can- DNA into NIH 3T3 cells. Serial passaging of the transformed cer Center, Houston, Texas. 3MD Anderson Cancer Network, Houston, Texas. 4Department of Endocrine Neoplasia and Hormonal Disorders, The NIH 3T3 cells allowed for these human oncogenes to be University of Texas MD Anderson Cancer Center, Houston, Texas. cloned through their association with human repetitive DNA Corresponding Author: Vivek Subbiah, The University of Texas MD Anderson sequences. Interestingly, the coincidence of using DNA iso- Cancer Center, 1515 Holcombe Boulevard, Houston, TX 77030. Phone: lated from a patient with T-cell lymphoma, 3215, led to the 713-563-1930; Fax: 713-792-0334; E-mail: [email protected] discovery of the RET oncogene (15). A single transformed Cancer Discov 2020;XX:XX–XX colony was ultimately expanded through secondary and ter- doi: 10.1158/2159-8290.CD-19-1116 tiary transfections, providing both confirmation of a trans- ©2020 American Association for Cancer Research. forming oncogene and a DNA source for characterization (15).

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Somatic RET fusions and mutations associated with oncogenesis

RET fusion partners RET mutations KIF5B** G10 S/C/R C620F/R/S E768D Y806C CCDC6** G533C C630R/Y L790F A883F Meningioma (5.6%) NCOA4** C609F/G/R/S/Y D631Y Y791F S891A TRIM33 C611F/G/S/Y/W C634F**/G/R/S/W/Y V804M** M918T** CUX1 C618F/R/S K666E V804L KIAA1217 FRMD4A KIAA1468 PTC (10% to 20%) PRKAR1A FKBP15 MTC (~60% to 90%) NCOA NSCLC (1% to 2%) Germline RET mutation GOLGA5 associated with oncogenesis TRIM24 TRIM27 Esophageal C515 KTN1 Melanoma (0.7%) adenocarcinoma (1.4%) C609 RFG9 and basal cell C611 ERC1 carcinoma (12.5%) Breast C618 HOOK3 carcinoma (0.2%) C620 PCM1 C630 AKAP13 E632 SPECC1L Gastric C634**(MEN2A) TBLXR1 adeno- V648 FGFR1OP carcinoma K666 EML4 CMML (0.7%) E768 EPHA5 L790 SQSTM1 Ureter urothelial Y791 PARD3 V804 Ovarian epithelial carcinoma (16.7%) PICALM R833 carcinoma (1.9%) AFAP1L2 M848 PPFIBP2 Colorectal adenocarcinoma (0.7%) A883 ACBD5 S891 MYH13 S904 M918**(MEN2B) **Most common fusions or mutations

Figure 1. Frequency and distribution of RET fusions and RET mutations across malignancies. Visual art © 2019 The University of Texas MD Anderson Cancer Center. Red text indicates the most prevalent RET-dependent malignancies. CMML, chronic myelomonocytic leukemia.

However, when the 3215 transforming DNA was compared with date, three general mechanisms of aberrant RET activation normal human DNA, a discontinuity was discovered, leading to have been reported in cancer: in-frame RET gene fusions (15, the hypothesis that the oncogene was derived from recombina- 16), targeted mutation of the RET gene itself (4–6), and finally tion of two unlinked segments. The authors proposed a mecha- aberrant overexpression of the RET gene (19, 20). What the nism of REarrangement during Transfection that ultimately led three mechanisms appear to share in common is the inap- to the naming of the RET oncogene. Molecular analysis of the propriate activation of the tyrosine kinase, most commonly RET transforming gene determined the fusion partner to be an in the complete absence of ligand. The multifunctional dock- upstream ring finger domain (originally unrelated to genes iden- ing sites at phosphotyrosine 1062 (pY1062) and pY1096 serve tified at the time) and a downstream transmembrane linked to a as the primary RET signaling hubs (reviewed in refs. 20, 21). tyrosine kinase domain (16). As other tyrosine kinases had previ- Activation of RAS–MAPK and PI3K–AKT signaling pathways ously been linked to oncogenic transformation, it was ultimately results from adaptor protein binding to these docking sites the gene encoding this domain that was given the name “RET (Fig. 2; refs. 22, 23). proto-oncogene.” Despite the belief that the RET oncogene was created through an experimental artifact, the same NIH 3T3 transformation assay was able to subsequently demonstrate the MECHANISMS OF RET ACTIVATION, frequent occurrence of RET gene fusions in papillary thyroid SIMILARITIES AND DIFFERENCES cancers (17) and to confirm the transforming ability of MEN2- Targeted mutation of RET results in aberrant activation associated RET-activating mutations (18). through three broad mechanisms—dimerization through the formation of intermolecular cysteine disulfide bonds, impacting of the ATP-binding domain, and finally enhance- WHY IS RET AN ONCOGENE? ment of the kinase domain activity (Fig. 2). Because germline In the decades that have passed since the discovery of RET, RET activating mutations occur in patients with hereditary much has been uncovered related to its role in cancer. To MEN2, we know that tumorigenesis is primarily limited to

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Normal RET signaling Oncogenic RET signaling

GDNF ligand Activating RET mutations GFRα1

C620/ C634

V804L/M P TK1 M918T P P TK2 P RET RAS RAF MEK ERK3

Dimeric RET fusions Organ development Tumorigenesis (e.g., KIF5B–RET, and tissue homeostasis CCDC6–RET )

Figure 2. Oncogenic RET signaling: RET is the signaling receptor for the glial cell–derived neurotrophic factor (GDNF) family of ligands. These ligands play a key role in organ development and tissue homeostasis. Targeted mutation of RET (e.g., RETM918T and RETV804M/L) results in aberrant activation through three broad mechanisms—dimerization through the formation of intermolecular cysteine disulfide bonds, impacting of the ATP-binding domain, and finally enhancement of the kinase domain activity. Chromosomal rearrangements (e.g.,KIF5B –RET and CCDC6–RET) produce hybrid proteins that fuse a partner protein that often contains a dimerization domain with the RET kinase domain. Visual art © 2019 The University of Texas MD Anderson Cancer Center. a subpopulation of cells of neuroendocrine origin, most beyond the examination of proliferative signaling, the cel- notably thyroid C cells and cells within the adrenal medulla. lular functions of RET fusion have not been extensively Although it has been argued that the specificity of tumor for- studied. It is also important to point out that key regulatory mation derives from the high level of RET expression in these mechanisms of RET inactivation, such as endocytosis and cell types, this is clearly an oversimplification.RET is known recruitment of membrane-associated ubiquitin ligases, do to be more broadly expressed and has clear roles in mediating not appear to affect the fusion proteins, which additionally progenitor stem-cell function. may enhance their oncogenicity (24, 25). Furthermore, when To date, germline RET fusions have not been observed, localized to the cytoplasm or nucleus, monomeric RET has only cell-specific somatic fusions. They were initially thought additional functions, such as regulation of ATF4, that could to be limited to thyroid follicular cell tumors, but have more certainly be affected (26). These circumstances seemingly pre- recently been identified in 1% to 2% of NSCLCs, and at a dict a greater oncogenic potential for RET fusions compared <1% frequency in a range of tumor types, including colorectal with activating mutations that remains to be fully addressed. cancer, breast cancer, and pancreatic cancer (11, 12). There Direct comparisons of RET fusions with full-length RET con- are numerous RET fusion partners, all of which appear to taining activating mutations are limited, but it is important provide dimerization domains (Fig. 1). A survey of data- to note that experimental inclusion of RET-activating muta- sets for cancer types within cBioPortal finds that approxi- tions into fusion constructs does not increase tumorigenicity mately 10% of reported RET mutations are fusions, with in xenograft models (27, 28). Given these differences, we are approximately 10% of those comprising tumor types other largely left to speculate on the precise impact of targeted than thyroid or lung. RET fusions are the most commonly inhibition on oncogenesis driven by these two mechanisms observed aberration in tumors that are not of neuroen- of aberrant RET activation. As a result, our understanding docrine origin. RET fusions are thought to be oncogenic of targeting RET-driven cancer continues to evolve largely for two reasons. First, fusion provides a mechanism to through clinical trials (27, 28). aberrantly express RET in a cell type where it is normally transcriptionally silent. Second, in all cases the extracellu- lar domain is replaced with a protein dimerization domain. DOES TARGETING RET WORK? The outcome is the production of an intracellular RET The original concept of therapeutically targeting RET tyrosine kinase domain capable of ligand-independent acti- largely stems from MTC studies. Activating germline muta- vation. Interestingly, despite the absence of a transmem- tions cause MTC, somatic activating mutations are found brane domain, the RET fusion proteins remain capable in sporadic MTC, and these same mutations are capable of MAPK pathway and PI3K–AKT activation. However, of inducing cellular transformation in the NIH 3T3 cell

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Table 1. ORRs, dose-reduction rates, and drug-discontinuation rates of MKIs versus selective RET inhibitors

ORRs Dose-reduction rate and drug-discontinuation ratesb

Drug Thyroid NSCLC Thyroid NSCLC Vandetanib (Caprelsa) 45% (MTC) 18%, 53% (Japan) DRR = 81/231 (35%) DRR = 10/19 (53%) DDR = 28/231 (12%) DDR = 4/19 (21%) Cabozantinib (Cabometyx/Cometriq) 28% (MTC) 28% DRR = 169/214 (79%) DRR = 19/26 (73%) DDR = 35/214 (16%) DDR = 2/26 (8%) Lenvatinib (Lenvima) 36% 16% DRR = 35/59 (59%) DRR = 16/25 (64%) DDR = 14/59 (24%) DDR = 5/25 (20%) Selpercatiniba (LOXO-292) 78% (RET+ thyroid 68% DRR = N/A cancer) DDR = 9/531 (1.7%) 59% (MTC) DRR = N/A 9/531 (1.7%) Pralsetiniba (BLU-667) 56% (MTC) 58% DRR = N/A DDR = 4% (276 patients)

Abbreviations: DDR, drug-discontinuation rate; DRR, dose-reduction rate; ORR, objective response rate. aClinical outcomes data on overall survival and progression-free survival emerging as clinical trials are ongoing. bRelatively high dose reduction and discontinuation rates in MKIs due to drug-related adverse events like hypertension, diarrhea, rash, fatigue, hand-foot syndrome, proteinuria, hypopigmentation, QT prolongation, thrombosis, and hemorrhage preclude effective long-term use of MKIs in thyroid cancer. assay. These facts ultimately led to the hypothesis that inferior pharmacokinetic properties, these MKIs prevented MTC cells were addicted to oncogenic RET and therefore potent RET inhibition. sensitive to targeted inhibition. The first proof-of-concept came when it was demonstrated that ribozyme-mediated cleavage of mutant RET mRNA inhibited MTC tumor cell SELECTIVE RET INHIBITORS growth in vitro (29). Subsequently, it was demonstrated that BLU-667 (pralsetinib) and LOXO-292 (selpercatinib) are overexpression of a dominant-negative RET was capable of two highly potent and selective RET inhibitors designed inhibiting human MTC cell line growth in vitro (30) and to offset the weaknesses of MKIs (refs. 11, 27, 28; Table 1). in xenograft models (31). Neither approach was clinically High potency and RET selectivity were confirmed in robust viable, however, thereby opening the door for small-molecule preclinical models using multiple in vitro and in vivo RET- inhibitors. dependent tumor models (27, 28). In addition, favorable pharmacokinetic properties, including high bioavailability, predictable exposure, and minimal potential for drug–drug MKIs interactions, were confirmed as well. On-target acquired Vandetanib/ZD6474 was among the first drugs demon- resistance with tyrosine kinase inhibitors (TKI) is always an strated to inhibit the activity of both oncogenic RET fusions issue in developmental therapeutics, specifically a mutation as well as RET activated through the M918T mutation (32). at the gatekeeper position that sterically hinders inhibi- However, as vandetanib is a repurposed VEGFR inhibitor, it tor binding, as seen in chronic myeloid leukemia (BCR– has never been definitively established whether its antitumor ABLT315I) or EGFR-mutant NSCLC (EGFRT790M). Similarly, actions (and those of similar multikinase-targeting drugs like the RET gatekeeper mutations V804L and V804M have cabozantinib, lenvatinib, and sorafenib) function primarily been described and have been shown as an acquired resist- through inhibition of RET, particularly as MKIs without RET ance mechanism to MKIs (28, 34). Interestingly, these gate- activity (axitinib) have been shown to be effective in RET- keeper aberrations have been seen as germline mutations in altered thyroid cancers (33). patients presenting with MTC, and although cabozantinib Vandetanib and cabozantinib are MKIs with nonselective and vandetanib do not cover these mutations adequately, RET activity that have been approved by the FDA for MTC. the selective RET inhibitors BLU-667 and LOXO-292 were They were originally designed to target other kinases, such specifically designed to inhibit these mutations in subna- as VEGFR2 and MET, but were repurposed because of the nomolar concentrations (27, 28, 34). discovery of their inhibitory actions on RET. This MKI activ- Following preclinical validation, early clinical studies with ity leads to significant and sometimes prohibitive “off-target” BLU-667 showed remarkable responses in MKI-naïve and clinical side effects such as nausea, diarrhea, rash, and hyper- MKI-refractory patients with RET-rearranged NSCLC and tension that limit use in some patients or limit the dose that patients with RET-mutant advanced MTC (27). Contempo- patients can tolerate, leading to drug discontinuation or dose raneously, utilizing rapid dose titration guided by real-time reduction (Table 1). Together with nonselectivity for RET and pharmacokinetic assessments to achieve meaningful clinical

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Targeting RET-Dependent Cancers MINI REVIEW exposures safely and rapidly, a patient with RETM918T-mutant in the subset of patients with brain metastases in the reg- MTC and an acquired RETV804M gatekeeper resistance muta- istrational dataset, selpercatinib treatment demonstrated a tion, previously treated with six MKI regimens, was treated central nervous system ORR of 91% (95% CI, 59%–100%). As with LOXO-292 and achieved a rapid tumor response (28). A of the data cutoff date of June 17, 2019, median duration of second patient with symptomatic brain metastases who was response (DOR) was 20.3 months (95% CI, 13.8–24.0) and treated similarly experienced rapid tumor regression in the median progression-free survival (PFS) was 18.4 months (95% brain metastases. The mature clinical data that we currently CI, 12.9–24.9; ref. 37). In a safety analysis of all 531 patients have for selective RET inhibitors is mainly from RET fusion–posi- enrolled to LIBRETTO-001, selpercatinib was well tolerated, tive NSCLC and RET-mutant MTC. Patients harboring RET- with only 9 patients (1.7%) discontinuing therapy due to fusion NSCLC or PTC respond regardless of fusion partner, treatment-related toxicity. and patients with RET-mutant MTC respond regardless of In the RET-mutant MTC registration dataset (38) consist- germline or somatic mutation, including patients harboring ing of the first 55 enrolled patients with prior cabozantinib germline V804M. Below we discuss the emerging clinical data and/or vandetanib therapy, selpercatinib treatment resulted for the selective RET inhibitors. in a 56% ORR (95% CI, 42%–70%). ORR was similar regardless of prior MKI therapy, and 53% of patients were previously treated with ≥2 prior MKI. As of the data cutoff date of PRALSETINIB (BLU-667) June 17, 2019, median DOR was not reached [95% CI, 11.1– ARROW, a phase I clinical trial designed to evaluate the not estimable (NE)] and median PFS was not reached (95% safety, tolerability, and efficacy of BLU-667 in multiple ascend- CI, 11.3–NE). Interestingly, in 76 cabozantinib/vandetanib ing doses in adults with RET-altered NSCLC, MTC, and other (MKI)–naïve patients with RET-mutant MTC, selpercatinib advanced solid tumors showed that the recommended phase treatment resulted in a 59% ORR (95% CI, 47%–70%; ref. 38). II dose was 400 mg every day. As per the data presented In 26 patients with RET fusion–positive thyroid cancer (PTC, at the American Society of Clinical Oncology 2019 annual Hürthle cell, poorly differentiated, and anaplastic) 62% ORR meeting, 48 patients with RET-fusion NSCLC were evaluable (95% CI, 41%–80%) was reached. Median DOR and PFS were for response assessment, including 35 patients previously not reached in the MKI-naïve or RET fusion–positive thyroid treated with platinum-based chemotherapy (35). Nearly all cancer cohorts, as the vast majority of patients continue to patients (90%) had radiographic tumor reductions. The objec- show tumor regression or remain progression-free (38). tive response rate (ORR) was 60% [one complete response and 20 partial responses (PR); all responses were confirmed], and the disease control rate (DCR) was 100% in the patients previ- MECHANISMS OF ON-TARGET AND ously treated with platinum-based chemotherapy. Remark- OFF-TARGET DRUG RESISTANCE ably, pralsetinib was highly active regardless of RET fusion The treatment of MTC with vandetanib represented the partner, including RET–KIF5B and RET–CCDC6. Thirty-two first attempt to target oncogenic RET. Despite a high DCR patients with RET-mutant MTC were evaluable for response (73%), resistance developed with a median PFS of 27.9 months assessment, including 16 patients previously treated with (39). With nearly a decade of clinical use, the mechanisms of the MKIs cabozantinib or vandetanib (36). The ORR was vandetanib resistance remain largely unknown (40). This has 63% (nine confirmed PRs, one PR pending confirmation) and largely been complicated by two factors. First, vandetanib is the DCR was 94%. On the basis of these data, pralsetinib has an MKI with antiangiogenic and anti-RET activity. As such, received two FDA breakthrough designations for NSCLC and both pathways could be subject to resistance mechanisms. MTC. In addition to NSCLC and MTC, in patients with RET Second, the tools to examine resistance mechanisms have fusion–positive PTC the ORR was 83% (5/6 patients) and PRs only recently begun to be applied to study RET-driven can- were noted in RET fusion–positive gastrointestinal malignan- cer. From clinical trial data, it is clear that both primary and cies (pancreatic cancer and cholangiocarcinoma) as well. acquired resistance mechanisms exist. Furthermore, similar to other oncogenic receptor tyrosine kinases, acquired resistance is expected to occur primarily through either target SELPERCATINIB (LOXO-292) modification or bypass signaling (41, 42). Indeed, the ear- LIBRETTO-001 is a global phase I/II trial of selpercatinib liest preclinical studies demonstrated that the RETV804M/L (LOXO-292) in RET-altered cancers. On the basis of early mutation functioned as a vandetanib gatekeeper, severely promising data, selpercatinib received FDA breakthrough limiting its efficacy (43). More recent studies have demon- designations for RET fusion–positive NSCLC, RET mutation– strated that V804M/L provides a gatekeeper function in positive MTC, and RET fusion–positive PTC. In the regis- oncogenic RET fusions, as well as limiting efficacy of other tration dataset consisting of the first 105 enrolled patients MKIs such as cabozantinib (44, 45). For this reason, the with RET fusion–positive NSCLC with prior platinum-based selective RET inhibitors have been screened and designed for chemotherapy, selpercatinib treatment resulted in a 68% activity against gatekeeper mutations. ORR [95% confidence interval (CI), 58%–76%; ref. 37]. These It is only recently that an acquired RETV804 mutation has patients received a median of three prior systemic regimens been demonstrated following resistance to multi-TKI treat- (55% previously treated with an anti–PD-1/PD-L1 antibody ment (28). The gatekeeper was detected in the circulating cell- and 48% previously treated with at least one MKI) and ORR free DNA (cfDNA) of a patient with MTC following treatment was similar regardless of prior therapy. Up to 50% of RET with consecutive TKIs over a 5-year time frame. In follow-up fusion–positive NSCLCs can metastasize to the brain, and studies examining a small cohort of patients with MTC

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MINI REVIEW Subbiah and Cote with RETM918T somatic driver mutations, 75% of patients in MTC cell lines (60). Development of an acquired NRASQ61K had detectable circulating RETV804M cfDNA after develop- mutation was also seen with prolonged treatment of NSCLC ment of resistance to vandetanib or cabozantinib (46), cells with ponatinib (53). suggesting that acquisition of a RET gatekeeper mutation is a common mechanism of resistance in MTC. Acquired RET gatekeeper mutations on treatment with MKIs and OTHER SELECTIVE RET INHIBITORS successful treatment with the selective RET inhibitor selp- IN DEVELOPMENT ercatinib has been reported for NSCLC as well (34). In the Multiple resistance mutations have been reported from LURET patient cohort of vandetanib-treated patients, only preclinical studies, including the gatekeeper mutation V804L a single case was observed to acquire a RET mutation after to cabozantinib and solvent front mutations (SFM) G810A/S development of resistance (47, 48). This patient, with a to vandetanib (61). Although the first-generation selective CCDC6–RET fusion, developed a novel RETS904F activation RET inhibitors are designed to cover the gatekeeper muta- loop mutation shown to reduce drug binding. Additional tion, they do not adequately cover the SFMs. Emergence of studies will be needed to clarify the frequency of acquired SFMs has been reported preclinically (62). Recently, RET RET mutation in these two cancer types and whether target G810R, S, and C SFMs as acquired resistance mechanisms modification is a less-favored mechanism of resistance for have been demonstrated in RET-aberrant patients who pro- oncogenic fusions. gressed on selective RET inhibitors (63). Second-generation Evidence also exists for bypass mechanisms of resistance RET inhibitors are in development that may have different to RET inhibition. Notably, resistance to cabozantinib has properties. TPX-0046, a next-generation RET inhibitor that is been associated with acquisition of METD1228V in NSCLC (49) structurally differentiated and potent against a broad range of and MET amplification in colorectal cancer (50). In support mutations, has currently entered first-in-human studies (Clini- of these findings, preclinical models have demonstrated that calTrials.gov Identifier: NCT04161391) after investigational new RET inhibition can be overcome through activation of MET drug–enabling preclinical studies. Other selective RET inhibi- or EGFR family members, as well as through activating RAS tors in development include BOS-172738 (NCT03780517) and mutations (51–53). TAS0953/HM06 (in preclinical development). In the near future, studies of acquired resistance in patients treated with selective RET inhibitors are expected, but as of CONCLUSION now, unlike with EGFR- and ALK-driven cancers, we do not yet have vast patient experience to draw upon (54). However, The considerable enthusiasm in the precision oncology what we have learned is that many of the discoveries of resist- of RET-dependent cancers including RET fusion–positive ance mechanisms initially made in preclinical studies found NSCLC, RET fusion–positive PTC, and RET-mutant MTC themselves duplicated in patient studies, and later addressed stems from the successful clinical trial results of selective by new clinical approaches. If we apply a similar approach to RET inhibitors and offers a tantalizing array of opportuni- treatment of RET-driven cancer, then identification of the ties in RET-dependent cancer research. The fewer off-target mechanism(s) of resistance becomes a critical step in provid- side effects, more effective control of growth, and sustained ing an effective treatment. First, it is clear that the current antitumor activity compared with MKIs opens up a new era generation of RET inhibitors provides the ability to use a of precision oncology in RET-driven cancers. Approval of higher targeting dose and is largely insensitive to acquisition these selective RET inhibitors will further the use of the drugs of gatekeeper mutations. Thus, these inhibitors provide a in the community. Next steps include clinical trials with logical treatment choice for TKI resistance in MTC, where these drugs earlier in the disease course, and combination acquired gatekeeper mutation frequency is high, or as a first- therapy trials. Current RET inhibitors are a proof of principle line therapy where the use of higher pharmacologic dosing for selective RET inhibition, but we need to know when we has proven more effective. Addressing resistance mediated will get the most benefit from this specific targeting, and we through bypass mechanisms presents a more difficult chal- clearly need to be prepared for addressing acquired resistance lenge, one that will become more common with next-gener- and escape from RET targeting. ation TKIs. Here the specific mechanism of resistance needs to be identified, and a targeted drug combination therapy is Disclosure of Potential Conflicts of Interest needed. There is considerable evidence demonstrating cross- V. Subbiah is a consultant/advisory board member at LOXO talk between RET and EGFR, with the activation of EGF Oncology/Eli Lilly, Helsinn, R-Pharma US, Incyte, QED, Novartis, known to trigger resistance to multikinase RET inhibitors. and Medimmune, and reports receiving commercial research grants Clinically, targeting both receptors has proven effective (55) from LOXO Oncology/Eli Lilly, Blueprint Medicines, Fujifilm, Phar- mamar, D3, Pfizer, Multivir, Amgen, AbbVie, Agensys, Boston Bio- and should be considered. A more generalized approach is medical, Idera, Turning Point Therapeutics, Exelixis, Inhibrx, Altum, the use of an mTOR inhibitor to target resistance mediated Medimmune, Dragonfly Therapeutics, Takeda, Roche/Genentech, through PI3K–AKT (56, 57). Phase I studies have demon- Novartis, Bayer, GSK, Nanocarrier, Berghealth, Incyte, and North- strated that the addition of everolimus is associated with a west Biotherapeutics. No potential conflicts of interest were disclosed greater response rate and longer PFS than vandetanib alone by the other author. in RET-driven tumors (58, 59). Whether a similar approach targeting MAPK pathway activation using MEK inhibitors Acknowledgments would be effective remains to be addressed, although at least This work was supported in part by The Cancer Prevention and preclinically the combination has been shown to be effective Research Institute of Texas (RP1100584), the Sheikh Khalifa Bin

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OF8 | CANCER DISCOVERY XXXX 2020 AACRJournals.org

Advances in Targeting RET-Dependent Cancers

Vivek Subbiah and Gilbert J. Cote

Cancer Discov Published OnlineFirst February 24, 2020.

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