This trial will evaluate whether the immune-sensitizing effects of immunotherapy (Pembrolizumab) and radiation with or without a PARP-inhibitor (Olaparib) will increase the effects of immunotherapy in men with high-risk localized prostate cancer.

Immunotherapy and PARP-inhibitor are known to have radio-sensitizing effects when combined with radiation therapy. In addition, the combination with PARP-inhibitor and radiation can increase neoantigen expression, cytotoxic lymphocyte infiltration within the tumor microenvironment and increased immune stimulating cytokine concentration. Thus, there is a potential synergy of combining immunotherapy and PARP-inhibitor. This is a phase 2 randomized 1:1 study. Subjects will be randomized to one arm (pembro + PARPi + standard of care therapy which is definitive radiation therapy combined with hormonal therapy) vs. another arm (pembro + standard of care therapy). All subjects will receive adjuvant immunotherapy for one year once they are done with definitive radiation treatment.