Author Manuscript Published OnlineFirst on July 31, 2020; DOI: 10.1158/1078-0432.CCR-20-2257 Author manuscripts have been peer reviewed and accepted for publication but have not yet been edited. Reimagining vaccines for prostate cancer: Back to the future Eugene Shenderov1,2, Emmanuel S. Antonarakis1,2# 1 The Sidney Kimmel Comprehensive Cancer Center, Johns Hopkins University School of Medicine, Baltimore, MD 2 Bloomberg-Kimmel Institute for Cancer Immunotherapy, Johns Hopkins University School of Medicine, Baltimore, MD # Corresponding Author: Emmanuel S. Antonarakis, M.D. Johns Hopkins Sidney Kimmel Cancer Center Viragh Building, 9th floor 201 N Broadway Baltimore, MD 21287 Email: [email protected] Running Title: pTVG-AR vaccine for prostate cancer Conflicts of Interest: ESA is a paid consultant/advisor to Janssen, Astellas, Sanofi, Dendreon, Pfizer, Amgen, Lilly, Bayer, AstraZeneca, Bristol-Myers Squibb, Clovis, and Merck; he has received research funding to his institution from Janssen, Johnson & Johnson, Sanofi, Dendreon, Genentech, Novartis, Tokai, Bristol Myers-Squibb, AstraZeneca, Clovis, and Merck; and he is the co-inventor of a biomarker technology that has been licensed to Qiagen. ES reports no conflicts. 1 Downloaded from clincancerres.aacrjournals.org on September 25, 2021. © 2020 American Association for Cancer Research. Author Manuscript Published OnlineFirst on July 31, 2020; DOI: 10.1158/1078-0432.CCR-20-2257 Author manuscripts have been peer reviewed and accepted for publication but have not yet been edited. Acknowledgments: This work was partially supported by National Institutes of Health Cancer Center Support Grant P30 CA006973 (E Shenderov, E Antonarakis), Prostate Cancer Foundation Young Investigator Award (E Shenderov), Department of Defense grant W81XWH-16-PCRP- CCRSA (E Antonarakis) and grant W81XWH-19-1-0511 (E Shenderov), and the Bloomberg- Kimmel Institute for Cancer Immunotherapy (E Shenderov, E Antonarakis). 2 Downloaded from clincancerres.aacrjournals.org on September 25, 2021. © 2020 American Association for Cancer Research. Author Manuscript Published OnlineFirst on July 31, 2020; DOI: 10.1158/1078-0432.CCR-20-2257 Author manuscripts have been peer reviewed and accepted for publication but have not yet been edited. SUMMARY Given the modest clinical benefits observed with immune checkpoint blockade in advanced prostate cancer, there is a renewed interest in exploring other forms of immunotherapy. Here the authors report the use of a novel plasmid DNA vaccine encoding the androgen receptor, called pTVG-AR, in men with metastatic hormone-sensitive prostate cancer. Main Text In this issue of Clinical Cancer Research, Kyriakopoulos and colleagues report the Phase 1 results of a clinical trial using a novel plasmid DNA vaccine encoding the human androgen receptor (AR)(1). While the AR is a well validated therapeutic target in prostate cancer, and one of the finest examples of oncogenic addition in oncology, all prior therapeutic efforts have focused on either decreasing the synthesis of its ligand (dihydrotestosterone) or inhibiting the AR ligand- binding domain (LBD) in order to switch off its transcriptional activity. Here, for the first time, the AR is reimagined not as a drug target but as a tumor-specific antigen against which an immunologic response can be generated, a novel concept in the field of prostate cancer therapeutics. Immunotherapy has a long history in prostate cancer. Indeed, the first (and only) FDA- approved therapeutic vaccine in oncology was the antigen-presenting cell–based immunotherapy sipuleucel-T(2), targeting prostatic acid phosphatase (PAP), that was approved in 2010 for patients with asymptomatic or minimally symptomatic metastatic castrate-resistant prostate cancer (mCRPC)[Figure 1,Left]. The initial excitement generated by sipuleucel-T fizzled following a series of attempts to develop other antigen-specific immunotherapies, including a poxviral- based therapeutic vaccine targeting prostate-specific antigen (PSA) that failed to improve outcomes in a pivotal randomized trial(3)[Figure 1,Middle]. In the last 5 years, interest has shifted to examining the therapeutic potential of immune checkpoint blockade in men with mCRPC, following some early anecdotes of clinical activity. Unfortunately, it has now become apparent that monotherapies targeting the CTLA-4 or PD-1/PD-L1 checkpoints do not lead to meaningful clinical responses in the majority of patients with advanced prostate cancer(4), and these agents are unlikely to have utility outside of the small fraction of mismatch-repair deficient (or other DNA repair-deficient) prostate cancer subsets. Thus, there has been a renewed interest in going “back to the future” in order to focus again on vaccine development with a fresh eye. 3 Downloaded from clincancerres.aacrjournals.org on September 25, 2021. © 2020 American Association for Cancer Research. Author Manuscript Published OnlineFirst on July 31, 2020; DOI: 10.1158/1078-0432.CCR-20-2257 Author manuscripts have been peer reviewed and accepted for publication but have not yet been edited. The pTVG-AR (MVI-118) vaccine discussed in the accompanying article (1) comprises a plasmid encoding the AR ligand-binding domain (AR-LBD) cDNA positioned downstream of a eukaryotic promoter element [Figure 1,Right]. Patients are injected intradermally with the vaccine (with or without GM-CSF adjuvant), causing the naked DNA to act as a damage-associated molecular pattern (DAMP), thereby promoting APC activation and presentation of AR-LBD fragments leading to CD4 and CD8 T cell activation. This construct is similar to the group’s previous work describing another plasmid DNA vaccine, pTVG-HP, encoding the human prostatic acid phosphatase (PAP) cDNA, that had shown activity in prior phase 1 and 2 trials(5) [Figure 1,Right]. In the present Phase 1 study using pTVG-AR, patients were randomized to two different treatment schedules, each with or without 200 µg of GM-CSF as a vaccine adjuvant. Patients received 10 total vaccinations with 100 µg of pTVG-AR plasmid, either as six intradermal injections at 14-day intervals and then quarterly for up to 12 months (arm 1: without GM-CSF; arm 3: with GM-CSF), or as two intradermal injections at 14-day intervals and then quarterly for up to 12 months (arm 2: without GM-CSF; arm 4: with GM-CSF). As a positive immunological control, all subjects were immunized with tetanus toxoid prior to vaccination. The authors demonstrated the safety of the pTVG-AR vaccine when given together with androgen deprivation therapy in men with metastatic hormone-sensitive prostate cancer, reporting no grade 3-4 adverse events and mostly minor grade 1-2 events, without toxicity involving other organs that may also express AR (liver, heart, muscle or skin) except one patient who experienced grade 2 heart failure and came off trial. Serial vaccination resulted in an antigen- specific lytic immune response in the majority of patients, as demonstrated by the generation of AR-specific IFNγ-secreting or granzyme B-secreting T cells (although this effect was not augmented by the use of GM-CSF adjuvant). Furthermore, pTVG-AR vaccination also elicited Th1-type immunity against a non-target antigen, PSA. Notably, PSA-directed T cell responses were only observed in patients who also generated AR-directed T cell responses, suggesting that vaccination was able to induce antigen spread. Most interestingly, patients who demonstrated AR-specific T cell response (IFNγ and/or granzyme B secretion), showed a delayed time to castration-resistant progression compared to those without antigen-specific lytic responses. This is important, and biologically plausible, because a common mechanism driving castration resistance is amplification or overexpression of AR, which in this context would produce more antigen for the primed T cells to recognize. For this reason, and unlike the clinical development of other prostate cancer immunotherapies, the pTVG-AR vaccine might be better suited for testing in the metastatic hormone-sensitive disease setting. 4 Downloaded from clincancerres.aacrjournals.org on September 25, 2021. © 2020 American Association for Cancer Research. Author Manuscript Published OnlineFirst on July 31, 2020; DOI: 10.1158/1078-0432.CCR-20-2257 Author manuscripts have been peer reviewed and accepted for publication but have not yet been edited. A few limitations of the current study need to be acknowledged. Because of the trial’s limited sample size, only 7-8 patients were evaluable for immune responses per arm, and arms were sometimes unbalanced for some disease characteristics (with significantly more patients in arms 2 and 4 having undergone prior prostatectomy). Only a subset of patients generated IFNγ or granzyme B responses to AR or PSA, with limited patient overlap showing responses against both. Also, early lytic responses at weeks 4-8 could not be evaluated for arms 2 and 4, due to lack of sample availability. The tetanus toxoid is an antigen known to induce excellent T cell- specific immune responses in humans, including by prior ELISPOT IFNγ reports, but in the present study only a subset of patients had detectable responses, perhaps due to preexisting immunity and an overly stringent definition of IFNγ detection above baseline. Moreover, the AR is expressed on leukocytes, so further studies will need to report potential off-target effects on leukocytes by examining pre- and post-vaccination immune cell subsets. The advent of the pTVG-AR vaccine raises multiple important