This Study Will Explore Whether a Combination of the Investigational Drug Mevrometostat (PF-06821497) and Enzalutamide Will Work Better Than Taking Enzalutamide Alone in Participants With mCSPC Who Are ARPI naïve.

Inclusion Criteria * Male participants aged ≥18 years (or the minimum age of consent in accordance with local regulations) at screening. * Histologically or cytologically confirmed adenocarcinoma of the prostate without small cell features. * Metastatic prostate cancer documented by positive bone scan (for bone disease) or metastatic lesion(s) on CT or MRI (for soft tissue/visceral disease). * Resolution of acute effects of any prior therapy to either baseline severity or CTCAE Grade ≤1 (except for AEs which do not constitute a safety risk in the investigator's judgement). * Participants must have ECOG PS 0 or 1. Exclusion Criteria * Any medical or psychiatric condition including recent (within the past year) or active suicidal ideation/behavior or laboratory abnormality that may increase the risk of study participation or, in the investigator's judgment, make the participant inappropriate for the study. * Clinically significant cardiovascular disease. * Known or suspected brain metastasis or active leptomeningeal disease. * Participants must be treatment naïve at the mCSPC stage, eg, participants cannot have received any cytotoxic chemotherapy with the following exceptions: Treatment with first-generation antiandrogen (ADT) agents is allowed for mCSPC. * Previous administration with an investigational product (drug or vaccine) within 30 days. * Use of 5-alpha reductase inhibitors is prohibited within 28 days of randomization. * Prior surgery from which the participant has not fully recovered at least 28 days prior to randomization * Current use or anticipated need for drugs that are known strong CYP3A4/5 inhibitors and inducers (with exception of enzalutamide as part of this study). * Inadequate organ function. * Known allergic or hypersensitivity reactions to mevrometostat or its excipients or to enzalutamide or its excipients.