A Phase 3, Randomized Open-label Study of Pembrolizumab (MK-3475) Plus Olaparib Versus Abiraterone Acetate or Enzalutamide in Participants With Metastatic Castration-resistant Prostate Cancer (mCRPC) Who Are Unselected for Homologous Recombination Repair Defects and Have Failed Prior Treatment With One Next-generation Hormonal Agent (NHA) and Chemotherapy (KEYLYNK-010)

The purpose of this study is to assess the efficacy and safety of the combination of the polyadenosine 5'-diphosphoribose poly (ADP-ribose) polymerase (PARP) inhibitor olaparib and pembrolizumab in the treatment of participants with mCRPC who have failed to respond to either abiraterone acetate or enzalutamide (but not both) and to chemotherapy. The primary study hypotheses are that the combination of pembrolizumab plus olaparib is superior to abiraterone acetate or enzalutamide with respect to: 1. Overall Survival (OS) and 2. Radiographic progression-free survival (rPFS) per Prostate Cancer Working Group (PCWG)-modified Response Evaluation Criteria in Solid Tumors Version 1.1 as assessed by blinded independent central review (BICR) As of Amendment 06, the Data Monitoring Committee (DMC) is no longer applicable. Participants still on treatment may have the option to continue receiving study intervention or SOC if they are deriving clinical benefit, until criteria for discontinuation are met. Participants who are still on study treatment and deriving clinical benefit will no longer have tumor response assessments by BICR. However, local tumor imaging assessments should continue per standard of care (SOC) schedule. In addition, electronic patient-reported outcome (ePRO) assessments will no longer be performed and biomarker samples will no longer be collected.