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 with evidenceof disease progression on treatment with abiraterone acetate in the mCSPC setting orfirst line mCRPC setting is required.

• 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.

• Know history of active inflammatory gastrointestinal disease, chronic diarrhea, orprevious gastric resection or lap-band surgery.

• 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 (i.e. 177Lu-PSMA-617, radium 223), androgen receptor signalinginhibitors (ARSi) including enzalutamide, apalutamide, darolutamide, poly ADP-ribosepolymerase (PARP) monotherapy or other systemic anti-cancer treatment, with thefollowing exceptions:

1. Treatment with first-generation antiandrogen agents, if discontinued prior tofirst dose of study intervention.

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.