A Study to Learn About the Investigational Medicine Called PF-06821497 (Mevrometostat) in Men With mCRPC Who Were Previously Treated With Abiraterone Acetate for Prostate Cancer (MEVPRO-1).

Inclusion Criteria: * Histologically or cytologically confirmed adenocarcinoma of the prostate without small cell features. * Metastatic disease in bone documented on bone scan, or in soft tissue documented on CT/MRI scan. * Progressive disease in the setting of surgical or medical castration with evidence of disease progression on treatment with abiraterone acetate in the mCSPC setting or first line mCRPC setting is required. * Eastern Cooperate Oncology Group (ECOG) performance status 0 - 2, with life expectancy of at least 6 months as assessed by the investigator. Exclusion Criteria: * Any medical or psychiatric condition including recent (within the past year) or active suicidal ideation/behavior or laboratory abnormality that may make the participant inappropriate for the study. * Know history of active inflammatory gastrointestinal disease, chronic diarrhea, or previous gastric resection or lap-band surgery. * Clinically significant cardiovascular disease. * Known or suspected brain metastasis or active leptomeningeal disease or clinically significant history of seizure. * Prior treatment for prostate cancer at any stage with any cytotoxic chemotherapy, radioligand therapy (i.e. 177Lu-PSMA-617, radium 223), androgen receptor signaling inhibitors (ARSi) including enzalutamide, apalutamide, darolutamide, poly ADP-ribose polymerase (PARP) monotherapy, CDK4/6 inhibitors, or other systemic anti-cancer treatment, with the following exceptions: 1. Treatment with first-generation antiandrogen agents, if discontinued prior to first dose of study intervention. 2. Docetaxel treatment is allowed for mCSPC, as long as no signs of failure, or disease progression occurred during treatment or within 3 months of treatment completion. 3. Use of 5-alpha reductase inhibitors within 28 days of randomization. * Previous administration with an investigational product within 30 days or 5 half-lives preceding the first dose of study intervention (whichever is longer). * Inadequate organ function.