Alain Delcayre

Elucidating immunologic mechanisms of PROSTVAC cancer immunotherapy

BackgroundPROSTVAC®, an active immunotherapy currently studied for the treatment of metastatic castration-resistant prostate cancer (mCRPC), consists of a heterologous prime-boost regimen with two different poxvirus-based vectors to provoke productive immune responses against prostate specific antigen (PSA) as the target tumor antigen. A Phase 2 study of PROSTVAC immunotherapy showed significantly improved median overall survival by 8.5 months and is currently being validated in a global Phase 3 study (PROSPECT; NCT01322490). Here, preclinical models were explored to investigate the mechanism of action and immune signatures of anti-tumor efficacy with PROSTVAC immunotherapy with the goal to identify potential immune correlates of clinical benefit.MethodsPROSTVAC-induced immune responses and anti-tumor efficacy were studied in male BALB/c mice. Functionality of the induced T cell response was characterized by interferon-gamma (IFNγ) ELISPOT, cytotoxic degranulation, multi-cytokine intracellular staining, and in vivo T cell depletion. Tumor infiltrating lymphocytes (TILs) were evaluated phenotypically by flow cytometry.ResultsThe heterologous prime-boost regimen of the two PROSTVAC vectors significantly enhanced the magnitude and quality of activated PSA-specific CD4 and CD8 T cell responses compared to homologous, single vector regimens. PROSTVAC-activated CD4 and CD8 T cells were highly functional as evidenced by expression of activation markers, production of multiple cytokines, and amplified cytotoxic T cell activity. Importantly, PROSTVAC immunotherapy resulted in significant anti-tumor efficacy in a transplantable prostate cancer mouse model. Antigen-spreading occurred in PROSTVAC-treated animals that rejected PSA-expressing tumors, as shown by subsequent rejection of PSA-negative tumors. In vivo CD4 and CD8 depletion revealed that both T cell subsets contributed to anti-tumor efficacy. Characterization of TILs demonstrated that PROSTVAC immunotherapy greatly increased the intra-tumoral ratio of activated effector to regulatory T cells.ConclusionsPROSTVAC immunotherapy activates broad, highly functional T cell immunity to PSA and to endogenous tumor antigens via immune-mediated antigen spreading. These preclinical results further elucidate the mode of action of PROSTVAC immunotherapy and its potential causal relationship to extended overall survival as observed in the PROSTVAC Phase 2 study. The clinical validation is ongoing in the PROSPECT Phase 3 clinical study.

Abstract A21: Active immunotherapy with PROSTVAC® demonstrates potent antitumor efficacy in a mouse model of prostate cancer.

BN ImmunoTherapeutics (BNIT) specializes in developing novel active immunotherapies for cancer. These therapies use recombinant poxviruses engineered to express tumor-associated antigens (TAAs), with the intent of generating effective immune responses against the patients9 cancer. PROSTVAC® is a candidate product for the treatment of prostate cancer for which a global Phase III clinical trial (PROSPECT) was recently initiated. This product is composed of two different viral vectors derived from a recombinant vaccinia virus (PROSTVAC™-V) and a recombinant fowlpox virus (PROSTVAC™-F). Both vectors contain transgenes encoding prostate-specific antigen (PSA) and a triad of costimulatory molecules (B7-1, ICAM-1, and LFA-3), designated as TRICOM™. Patients are immunized using a prime-boost strategy consisting of an initial treatment with PROSTVAC™-V followed by repeated boosting with PROSTVAC™-F to maximize the immune responses against the PSA tumor-antigen. Here we show preclinical data characterizing PROSTVAC® activity in mice. Treatment with either PROSTVAC™-V or PROSTVAC™-F induced PSA-specific antibody and T cell responses; however, PSA-specific responses were further increased by the prime/boost strategy, particularly with respect to the frequency of responding CD8 T cells. These CD8 T cells produced IFN-gamma and degranulated in an antigen-specific manner. Furthermore, PROSTVAC® treatment resulted in strong efficacy in a mouse model of prostate cancer. In this model, treatment with PROSTVAC resulted in anti-tumor efficacy accompanied by a Th1-biased response against PSA. In contrast, growth of tumors in control mice induced only non-protective PSA-specific responses with strong Th2 bias. Overall, these animal studies help define the activity and mechanism of action of PROSTVAC® which is currently being evaluated in the clinic. R.B. Rountree and S.J. Mandl contributed equally to this work. Citation Format: Ryan B. Rountree, Stefanie J. Mandl, Joseph Cote, Tracy dela Cruz, Thierry Giffon, Evan Gordon, Susan P. Foy, John R. Lombardo, Erica Trent, Reiner Laus, Alain Delcayre. Active immunotherapy with PROSTVAC ® demonstrates potent antitumor efficacy in a mouse model of prostate cancer. [abstract]. In: Proceedings of the AACR Special Conference on Tumor Immunology: Multidisciplinary Science Driving Basic and Clinical Advances; Dec 2-5, 2012; Miami, FL. Philadelphia (PA): AACR; Cancer Res 2013;73(1 Suppl):Abstract nr A21.