This study is testing if a new drug, mevrometostat (PF-06821497), works better with enzalutamide than enzalutamide alone for treating a type of prostate cancer called mCSPC (metastatic Castration-Sensitive Prostate Cancer). Being ARPI naïve means participants haven't used certain prostate cancer drugs before. The study is double-blind (neither participants nor doctors know who gets the real drug), placebo-controlled (some get a fake drug), and randomized (participants are chosen by chance for different treatments). Participants must have prostate cancer that has spread and meet specific health criteria. They shouldn't have serious heart problems or brain cancer. The study includes several phases: screening, treatment, and follow-up.

• The study will require multiple visits over a long period.
• Participants will be randomly assigned to different treatment groups.
• There may be risks involved, and some people may not be eligible due to health conditions or past treatments.