A Study to Learn How PF-06821497 (Mevrometostat) Works in Men With Metastatic Castration-resistant Prostate Cancer.

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 medical or surgical castration. * ECOG performance status 0 or 1, with a life expectancy of ≥12 months as assessed by the investigator. Exclusion Criteria: * Any medical or psychiatric condition including recent (within the past year) or active suicidal ideation in the past year or suicidal behavior in the past 5 years or laboratory abnormality that make the participant inappropriate for the study. * Known 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. * Any history of myelodysplastic syndrome, acute myeloid leukemia, or any other prior malignancy with a few exceptions. * Participants must be treatment naïve at the mCRPC stage, eg, no cytotoxic chemotherapy, radio-ligand therapy (i.e. 177Lu- PSMA-617), CDK4/6 inhibitors, 5-alpha reductase inhibitors for prostate cancer in any setting, androgen receptor signaling inhibitors (ARSi) including enzalutamide, apalutamide, darolutamide, poly ADP-ribose polymerase (PARP) monotherapy or other systemic anti-cancer treatment with the following exceptions: 1. Treatment with first-generation antiandrogen (ADT) agents, estrogens, progestins, cyproterone acetate; 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. * Previous administration with an investigational product (drug or vaccine) within 30 days or 5 half-lives preceding the first dose of study intervention (whichever is longer). * Inadequate organ function.