This Study Will Explore Whether a Combination of the Investigational Drug Mevrometostat (PF-06821497) and Enzalutamide Will Work Better Than Taking Enzalutamide Alone in Participants With mCSPC Who Are ARPI naïve.

To join this study, you must be a man aged 18 or older with prostate cancer that has spread to other parts of your body. Your cancer must have been confirmed by tests, and your body must be generally strong enough to cope with the study treatments, as assessed by the doctors.

There are also reasons why you might not be able to join. For example, if you have other serious medical conditions, especially heart problems or untreated brain cancer. You also can't have had certain strong anti-cancer treatments, like chemotherapy, for your advanced prostate cancer before, though some hormone therapies are allowed. If you've recently been part of another drug study or had major surgery, that might also prevent you from joining.

The research team will carry out several tests to make sure that taking part in the study is both safe and appropriate for you.

If you decide to take part, you'll first have a screening phase where several tests are done to check if you're suitable. Once confirmed, you'll be randomly assigned to one of two groups: either you'll receive the new drug mevrometostat plus enzalutamide, or a placebo (dummy drug) plus enzalutamide. Neither you nor your doctor will know which group you are in.

You'll need to take the study medication regularly as instructed. There will be regular visits to the clinic for check-ups, blood tests, and scans to monitor your health and how your cancer is responding. This treatment phase will last for a period, followed by a safety follow-up period to track any side effects, and then a long-term follow-up to monitor your health over time. The exact number of visits and the full duration of your participation will be explained by the study team.