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 withoutsmall cell features.

• Metastatic disease in bone documented on bone scan, or in soft tissue documented onCT/MRI scan.

• Progressive disease in the setting of surgical or medical castration.

• Eastern Cooperate Oncology Group (ECOG) performance status 0 - 2, with lifeexpectancy of at least 6 months as assessed by the investigator.

Exclusion Criteria:

• Any medical or psychiatric condition including recent (within the past year) oractive suicidal ideation/behavior or laboratory abnormality that may make theparticipant. inappropriate for the study.

• Clinically significant cardiovascular disease.

• Known or suspected brain metastasis or active leptomeningeal disease or clinicallysignificant history of seizure.

• Prior treatment for prostate cancer at any stage with any cytotoxic chemotherapy,radioligand therapy, androgen receptor signaling inhibitors (ARSi) includingenzalutamide, apalutamide, darolutamide, poly ADP-ribose polymerase (PARP)monotherapy or other systemic anti-cancer treatment. with the following exceptions:

1. Treatment with first-generation antiandrogen agents, if discontinued prior torandomization

2. Docetaxel treatment is allowed for mCSPC, as long as no signs of failure, ordisease progression occurred during treatment or within 3 months of treatmentcompletion.

• Previous administration with an investigational product within 30 days or 5half-lives preceding the first dose of study intervention (whichever is shorter).

• Inadequate organ function.