Background: Approved treatments for mCRPC (eg, enzalutamide and docetaxel) may increase programmed death ligand 1 (PD-L1) expression and facilitate neoantigen release. In phase 1b and 1/2 trials, pembro, an anti–PD-1 antibody, has produced antitumor responses in previously treated mCRPC as monotherapy and in combination with enzalutamide. Olaparib, a PARP inhibitor, has shown activity in mCRPC with DNA-repair defects. The nonrandomized, multicohort, open-label KEYNOTE-365 study (NCT02861573) will evaluate the safety and efficacy of pembro with olaparib (cohort A), docetaxel + prednisone (cohort B), or enzalutamide (cohort C) in mCRPC. Methods: Cohort allocation depends upon prior treatment: cohort A requires prior docetaxel (treatment with 1 other chemotherapy and ≤2 second-generation hormonal manipulations is allowed); cohort B requires prior abiraterone acetate or enzalutamide (but not both); cohort C requires prior abiraterone acetate. Additional eligibility criteria include confirmed prostate adenocarcinoma, disease progression (PD) ≤6 months before screening, ongoing androgen deprivation (serum testosterone < 50 ng/dL), and provision of nonirradiated tumor sample. Pembro 200 mg will be given every 3 weeks (Q3W) with either olaparib 400 mg twice daily (cohort A), docetaxel 75 mg/m² Q3W + prednisone 5 mg twice daily (cohort B), or enzalutamide 160 mg once daily (cohort C). Pembro treatment will continue for up to 35 cycles or until PD or unacceptable adverse events (AEs). Patients in cohort B may receive a maximum of 10 cycles of docetaxel + prednisone. Patients who discontinue 1 of 2 drugs in a combination because of a treatment-related AE may continue to receive the other drug until PD. Response will be evaluated by prostate-specific antigen (PSA) levels Q3W and by imaging Q9W for the first year and Q12W thereafter. Primary end points are safety and PSA response rate (decline of ≥50% from baseline twice ≥3 weeks apart). Secondary end points include time to PSA progression, progression-free survival, overall survival, and overall response rate. Enrollment will continue until 70 patients are enrolled for each cohort. Clinical trial information: NCT02861573