A Phase 3, Randomized, Double-blind Trial of Pembrolizumab (MK-3475) Plus Enzalutamide Plus ADT Versus Placebo Plus Enzalutamide Plus ADT in Participants With Metastatic Hormone-Sensitive Prostate Cancer (mHSPC) (KEYNOTE-991)

This study will assess the efficacy and safety of pembrolizumab plus enzalutamide plus ADT versus placebo plus enzalutamide plus ADT in Chinese participants with mHSPC. The primary hypothesis is that in participants with mHSPC, the combination of pembrolizumab plus enzalutamide plus ADT is superior to placebo plus enzalutamide plus ADT with respect to 1) radiographic progression-free survival (rPFS) per Prostate Cancer Working Group (PCWG)-modified Response Evaluation Criteria in Solid Tumors (RECIST) 1.1 as assessed by blinded independent central review (BICR) and 2) overall survival (OS). As of Amendment 4, the study is being stopped for futility. All the prespecified interim analysis after interim analysis (IA1) and final analysis of the study described the statistical analysis plan (SAP) will not be performed. Safety analysis will be performed at the end of the study; there will be no further analyses for efficacy and electronic patient-reported outcome (ePRO) endpoints collected from participants beyond the IA1 cutoff date. All study participants will stop ongoing treatment with pembrolizumab/placebo. Exceptions may be requested for study participants who, in the assessment of their study physician, are benefitting from the combination of enzalutamide and pembrolizumab, after consulting with the Sponsor. All other study participants should be discontinued from study and be offered standard of care (SOC) treatment as deemed necessary by the Investigator. If enzalutamide as SOC is not accessible off study to the participant, central sourcing may continue. As of Amendment 04, disease progression will no longer be centrally verified, participants will only be assessed locally. As of Amendment 4, Second Course treatment is not an option for participants. There are currently no participants in the Second Course Phase.