Reimagining Vaccines for Prostate Cancer: Back to the Future Eugene Shenderov1,2 and Emmanuel S

Published OnlineFirst July 31, 2020; DOI: 10.1158/1078-0432.CCR-20-2257

CLINICAL CANCER RESEARCH | CCR TRANSLATIONS

Reimagining Vaccines for Prostate Cancer: Back to the Future Eugene Shenderov1,2 and Emmanuel S. Antonarakis1,2

SUMMARY ◥ Given the modest clinical beneﬁts observed with immune DNA vaccine encoding the androgen receptor, called checkpoint blockade in advanced prostate cancer, there is pTVG-AR, in men with metastatic hormone-sensitive pros- a renewed interest in exploring other forms of immuno- tate cancer. therapy. Here, the authors report the use of a novel plasmid See related article by Kyriakopoulos et al., p. XXX

In this issue of Clinical Cancer Research, Kyriakopoulos and The pTVG-AR (MVI-118) vaccine discussed in the accompanying colleagues report the phase I results of a clinical trial using a article (1) comprises a plasmid encoding the AR-LBD cDNA posi- novel plasmid DNA vaccine encoding the human androgen recep- tioned downstream of a eukaryotic promoter element (Fig. 1, right). tor (AR; ref. 1). While the AR is a well-validated therapeutic target Patients are injected intradermally with the vaccine (with or without in prostate cancer, and one of the finest examples of oncogenic GM-CSF adjuvant), causing the naked DNA to act as a damage- addition in oncology, all prior therapeutic efforts have focused on associated molecular pattern (DAMP), thereby promoting APC acti- either decreasing the synthesis of its ligand (dihydrotestosterone) or vation and presentation of AR-LBD fragments leading to CD4 and inhibiting the AR ligand-binding domain (LBD) to switch off CD8 T-cell activation. This construct is similar to the group's previous its transcriptional activity. Here, for the first time, the AR is work describing another plasmid DNA vaccine, pTVG-HP, encoding reimagined not as a drug target but as a tumor-specificantigen the human PAP cDNA, which has shown activity in prior phase I against which an immunologic response can be generated, a novel and II trials (ref. 5; Fig. 1, right). In the current phase I study using concept in the field of prostate cancer therapeutics. pTVG-AR, patients were randomized into two different treatment Immunotherapy has a long history in prostate cancer. Indeed, the schedules, each with or without 200 mg of GM-CSF as a vaccine first (and only) FDA-approved therapeutic vaccine in oncology was the adjuvant. Patients received 10 total vaccinations with 100 mgof antigen-presenting cell (APC)-based immunotherapy, sipuleucel- pTVG-ARplasmid,eitherassixintradermal injections at 14-day T (2), targeting prostatic acid phosphatase (PAP), that was approved intervals and then quarterly for up to 12 months (arm 1, without in 2010 for patients with asymptomatic or minimally symptomatic GM-CSF and arm 3, with GM-CSF), or as two intradermal injec- metastatic castrate-resistant prostate cancer (mCRPC; Fig. 1, left). The tions at 14-day intervals and then quarterly for up to 12 months initial excitement generated by sipuleucel-T fizzled following a series of (arm 2, without GM-CSF and arm 4, with GM-CSF). As a positive attempts to develop other antigen-specific immunotherapies, includ- immunologic control, all subjects were immunized with tetanus ing a poxviral-based therapeutic vaccine targeting PSA that failed to toxoid prior to vaccination. improve outcomes in a pivotal randomized trial (ref. 3; Fig. 1, middle). The authors demonstrated the safety of the pTVG-AR vaccine when In the last 5 years, interest has shifted to examining the therapeutic given together with androgen deprivation therapy in men with met- potential of immune checkpoint blockade in men with mCRPC, astatic hormone-sensitive prostate cancer, reporting no grade 3–4 following some early anecdotes of clinical activity. Unfortunately, it adverse events and mostly minor grade 1–2 events, without toxicity has now become apparent that monotherapies targeting the CTLA-4 involving other organs that may also express AR (liver, heart, muscle, or PD-1/PD-L1 checkpoints do not lead to meaningful clinical or skin), except 1 patient who experienced grade 2 heart failure and responses in the majority of patients with advanced prostate cancer (4), came off trial. Serial vaccination resulted in an antigen-specific lytic and these agents are unlikely to have utility outside of the small fraction immune response in the majority of patients, as demonstrated by the of mismatch repair–deficient (or other DNA repair–deficient) prostate generation of AR-specific IFNg-secreting or granzyme B–secreting T cancer subsets. Thus, there has been a renewed interest in going “back cells (although this effect was not augmented by the use of GM-CSF to the future” to focus again on vaccine development with a fresh eye. adjuvant). Furthermore, pTVG-AR vaccination also elicited Th1 type immunity against a nontarget 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 1The Sidney Kimmel Comprehensive Cancer Center, Johns Hopkins University induce antigen spread. Most interestingly, patients who demonstrated 2 School of Medicine, Baltimore, Maryland. Bloomberg-Kimmel Institute AR-specific T-cell response (IFNg and/or granzyme B secretion), for Cancer Immunotherapy, Johns Hopkins University School of Medicine, showed a delayed time to castration-resistant progression compared Baltimore, Maryland. with those without antigen-specific lytic responses. This is important, Corresponding Author: Emmanuel S. Antonarakis, Johns Hopkins University and biologically plausible, because a common mechanism driving School of Medicine, Skip Viragh Building, 201 N. Broadway, Room 9129, fi Baltimore, MD 21287. Phone: 410-502-8341; Fax: 410-614-7287; E-mail: castration resistance is ampli cation or overexpression of AR, which [email protected] in this context would produce more antigen for the primed T cells to recognize. For this reason, and unlike the clinical development of other Clin Cancer Res 2020;26:1–3 prostate cancer immunotherapies, the pTVG-AR vaccine might be doi: 10.1158/1078-0432.CCR-20-2257 better suited for testing in the metastatic hormone-sensitive disease 2020 American Association for Cancer Research. setting.

AACRJournals.org | OF1

Shenderov and Antonarakis

Therapeutic prostate cancer vaccination

Cellular vaccines Viral vector vaccines DNA vaccines Sipuleucel-T PROSTVAC-VF pTVG-AR pTVG-HP Promoter Promoter AR-LBD gene PAP gene PAP GM-CSF Injected subcutaneously + PSA LFA3 ICAM1CD80 DNA plasmid DNA plasmid Fusion protein Monocytes +/- + GM-CSF GM-CSF Product Vaccinia (prime) and AR- injected fowl pox (boost) vaccine APC PAP APC intravenously LBD PAP product + GM-CSF Injected PAP intradermally PSA APC

PCa tumor cells PSA AR-LBD PSA PAP CD8 CD4 B cells Anti-AR-LBD Anti-PSA PAP AR-LBD Anti-PAP

ECM

Figure 1. Therapeutic prostate cancer (PCa) vaccines. Cellular vaccines exemplified by sipuleucel-T (left). Peripheral blood mononuclear cells are removed via leukapheresis from the patient, incubated ex vivo with a PAP–GM-CSF fusion protein, and then reinfused into the patient intravenously. PAP antigen is then presented by activated APCs to CD4 Th cells and CD8 cytotoxic T cells. Viral vector vaccines exemplified by ProstVac-VF (middle). Vaccine expresses PSA with costimulatory molecules (B7.1, LFA-3, and ICAM-1) in a vaccinia virus vector for priming, followed by six subsequent booster doses of fowl pox virus encoding the construct. It is injected subcutaneously with GM-CSF leading to direct uptake by skin-resident APCs, or infection and lysing of skin fibroblasts or epithelium producing debris for APCs to ingest. Either scenario promotes expression and presentation of PSA antigen in conjunction with costimulatory molecules by APCs, thus activating CD4 and CD8 T cells. Naked DNA vaccines exemplified by pTVG-HP and pTVG-AR (right). Plasmid encoding PAP (pTVG-HP) or the LBD of the AR (pTVG-AR) cDNA situated downstream of a eukaryotic promoter. Patients are injected intradermally with plasmid DNA (with or without GM-CSF adjuvant) causing the DNA to act as a DAMP, promoting APC activation and presentation of PAP or AR, leading to CD4 and CD8 activation. In addition, all vaccine types depicted are thought to promote subsequent CD4 Th-cell–mediated humoral immunity by B cells secreting physiologic antibodies against tumor proteins: AR, PSA, and PAP. The cellular (CD4 and CD8) and humoral (B-cell–produced antibodies) immune responses then act to kill prostate cancer tumor cells in the prostate gland and/or at metastatic sites. Killing is mediated via antibody-dependent mechanisms directed at AR, PAP, or PSA antigen cell surface expression or direct T-cell engagement to peptide–MHC complex containing PSA, PAP, or AR peptides via T-cell receptors.

A few limitations of the current study need to be acknowledged. of IFNg detection above baseline. Moreover, the AR is expressed on Because of the trial's limited sample size, only 7–8 patients were leukocytes, so further studies will need to report potential off-target evaluable for immune responses per arm, and arms were sometimes effects on leukocytes by examining pre- and postvaccination immune unbalanced for some disease characteristics (with significantly more cell subsets. patients in arms 2 and 4 having undergone prior prostatectomy). Only The advent of the pTVG-AR vaccine raises multiple important a subset of patients generated IFNg or granzyme B responses to AR or questions involving therapeutic DNA vaccination for prostate cancer. PSA, with limited patient overlap showing responses against both. Sipuleucel-T showed a survival benefit without decreasing PSA levels. Also, early-lytic responses at weeks 4–8 could not be evaluated for arms The current study was designed to assess time to first PSA elevation (as 2 and 4, due to lack of sample availability. The tetanus toxoid is an concurrent androgen deprivation prevented assessment of PSA antigen known to induce excellent T-cell–specific immune responses responses), but future studies will need to assess whether vaccination in humans, including by prior ELISPOT IFNg reports, but in the targeting the AR can decrease PSA levels. Furthermore, sipuleucel-T current study only a subset of patients had detectable responses, showed antibody responses consistent with a humoral response that perhaps due to preexisting immunity and an overly stringent definition correlated with survival, but neither the pTVG-AR nor the pTVG-HP

OF2 Clin Cancer Res; 26(19) October 1, 2020 CLINICAL CANCER RESEARCH

pTVG-AR Vaccine for Prostate Cancer

vaccinations elicited antigen-specific antibody responses (to AR or Disclosure of Potential Conflicts of Interest PAP, respectively) above baseline levels. It is unclear whether this is a E.S. Antonarakis reports grants and personal fees from Janssen, Sanofi, Dendreon, phenomenon related to DNA vaccination versus cellular/viral vacci- AstraZeneca, Bristol-Myers Squibb, Clovis, and Merck, personal fees from Astellas, fi nation, or tumor versus viral antigens. P zer, Amgen, Eli Lilly, and Bayer, and grants from Johnson & Johnson, Genentech, Novartis, and Tokai outside the submitted work, as well as a patent for an AR-V7 The current study raises the tantalizing possibility of designing a biomarker technology issued and licensed to Qiagen. No potential conflicts of interest therapeutic prostate cancer vaccine targeting the AR as a tumor- were disclosed by the other author. specific antigen. In light of the limitations of the current phase I study, important questions remain, but indications of AR-specific Acknowledgments T-cell responses, combined with possible tumor antigen spread This work was partially supported by National Institutes of Health Cancer Center through observed nontarget PSA-directed T-cell responses, are support grant P30 CA006973 (to E. Shenderov and E.S. Antonarakis), Prostate Cancer encouraging enough to warrant further exploration. It is likely that Foundation Young Investigator Award (to E. Shenderov), Department of Defense our understanding of these DNA-based therapeutic prostate cancer grant W81XWH-16-PCRP-CCRSA (to E.S. Antonarakis) and grant W81XWH-19-1- fi 0511 (to E. Shenderov), and the Bloomberg-Kimmel Institute for Cancer Immuno- vaccines will be further re ned in studies using pTVG-AR and therapy (to E. Shenderov and E.S. Antonarakis). pTVG-HR in combination with PD-1 checkpoint blockade, and such trials are currently underway in patients with recurrent and Received June 30, 2020; revised July 14, 2020; accepted July 29, 2020; published first advanced prostate cancer (NCT04090528 and NCT03600350). July 31, 2020.

References 1. Kyriakopoulos CE, Eickhoff JC, Ferrari AC, Schweizer MT, Wargowski metastatic castration-resistant prostate cancer. J Clin Oncol 2019;37: E, Olson BM, et al. Multicenter phase 1 trial of a DNA vaccine 1051–61. encoding the androgen receptor ligand binding domain (pTVG-AR, 4. Antonarakis ES, Piulats JM, Gross-Goupil M, Goh J, Ojamaa K, Hoimes CJ, et al. MVI-118) in patients with metastatic prostate cancer. Clin Cancer Res Pembrolizumab for treatment-refractory metastatic castration-resistant prostate 2020;26:xx–xxx. cancer: multicohort, open-label phase II KEYNOTE-199 study. J Clin Oncol 2019; 2. Kantoff PW, Higano CS, Shore ND, Berger ER, Small EJ, Penson DF, et al. 38:395–405. Sipuleucel-T immunotherapy for castration-resistant prostate cancer. N Engl J 5. McNeel DG, Eickhoff JC, Johnson LE, Roth AR, Perk TG, Fong L, et al. Phase II Med 2010;363:411–22. trial of a DNA vaccine encoding prostatic acid phosphatase (pTVG-HP 3. Gulley JL, Borre M, Vogelzang NJ, Ng S, Agarwal N, Parker CC, et al. [MVI-816]) in patients with progressive, nonmetastatic, castration-sensitive Phase III trial of PROSTVAC in asymptomatic or minimally symptomatic prostate cancer. J Clin Oncol 2019;37:3507–17.

AACRJournals.org Clin Cancer Res; 26(19) October 1, 2020 OF3

Reimagining Vaccines for Prostate Cancer: Back to the Future

Eugene Shenderov and Emmanuel S. Antonarakis

Clin Cancer Res Published OnlineFirst July 31, 2020.

Updated version Access the most recent version of this article at: doi:10.1158/1078-0432.CCR-20-2257

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