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 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 medical or surgical castration.

• ECOG performance status 0 or 1, with a life expectancy of ≥12 months as assessed bythe investigator.

Exclusion Criteria:

• Any medical or psychiatric condition including recent (within the past year) oractive suicidal ideation in the past year or suicidal behavior in the past 5 yearsor laboratory abnormality that make the participant inappropriate for the study.

• Known 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.

• Any history of myelodysplastic syndrome, acute myeloid leukemia, or any other priormalignancy with a few exceptions.

• Participants must be treatment naïve at the mCRPC stage, eg, no cytotoxicchemotherapy, radio-ligand therapy (i.e. 177Lu- PSMA-617), CDK4/6 inhibitors, 5-alpha reductase inhibitors for prostate cancer in any setting, androgen receptorsignaling inhibitors (ARSi) including enzalutamide, apalutamide, darolutamide, polyADP-ribose polymerase (PARP) monotherapy or other systemic anti-cancer treatmentwith 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, ordisease progression occurred during treatment or within 3 months of treatmentcompletion.

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

• Inadequate organ function.