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.

Due to slow accrual and feasibility concerns, the protocol was modified to single arm phase II study. All patients will receive definitive radiation therapy combined with ADT per institutional standards. In addition to concurrent ADT and radiation therapy, patients on this trial will also receive as follow: Pembrolizumab (17 cycles) combined with olaparib (the first three cycles). The remaining patients will be enrolled to Arm 1 only (i.e., Arm 2 usual care is closed to accrual upon Amendment 5 protocol).

Inclusion Criteria:

• Male participants with histologically confirmed adenocarcinoma of the prostate
• High-risk / very high-risk status per NCCN guidelines
• ECOG performance status 0 to 1
• Regional lymph nodes are allowed.
• Agree to use contraception during the treatment period and for at least 120 days after the last dose of study intervention and refrain from donating sperm during this period.
• Ability to understand and the willingness to sign a written informed consent document.
• Adequate organ and marrow function
• Prior pharmacologic androgen ablation for prostate cancer is allowed only if the onset of androgen ablation is ≤90 days prior to the date of registration
• Known history or current symptoms of cardiac disease, or history of treatment with cardiotoxic agents, should have a clinical risk assessment of cardiac function using the New York Heart Association Functional Classification.

Exclusion Criteria:

• Prior hormonal therapy with LHRH agonists (e.g., Lupron) and LHRH antagonists (e.g., Degarelix)for prostate cancer continuously for more than 90-days prior to study enrollment.
• Prior radiation to the prostate or pelvic nodes radiation.
• Previous or major surgery (colorectal anastomosis, total cystectomy, etc.).
• History of Ulcerative proctitis.
• Concurrent active, additional malignancy in the last 2 years.
• Prior systemic anti-cancer therapy including investigational agents within 4 weeks prior to randomization.
• Patients with M1 disease