Your treatment will begin on Day 1 after completing all required tests and procedures.

You will be randomly assigned to receive either the study medicine mevrometostat combined with enzalutamide, or a placebo combined with enzalutamide. Neither you nor your doctor will know which treatment you receive.

You must continue your androgen deprivation therapy (a treatment that lowers male hormones) throughout the entire study. This therapy must have been started before entering the trial.

You will take enzalutamide capsules by mouth every day. The dose is 40 mg soft capsules.

You will also take mevrometostat tablets or placebo tablets by mouth every day. These tablets are taken orally.

You must take your medications as instructed throughout the treatment period.

If you previously received certain hormone treatments such as estrogens, cyproterone acetate, or first-generation antiandrogens, these must be stopped before starting the trial medication.

You will attend regular study visits at the trial site.

During these visits, blood samples will be taken before you take your daily dose to measure the levels of the study medicine in your blood.

Blood samples will also be taken after you take your dose at selected visits.

Blood tests will be performed to measure your prostate-specific antigen levels, which is a marker used to monitor prostate cancer.

Additional blood tests will be conducted to check for circulating tumor DNA, which are cancer-related genetic materials in your blood.

You will undergo imaging scans at scheduled intervals to assess how your cancer is responding to treatment.

These scans may include CT scans or MRI scans to check for soft tissue or organ involvement.

You will also have bone scans to monitor any cancer spread to your bones.

The scans will be reviewed by independent experts who do not know which treatment you are receiving.

Your health will be closely monitored throughout the study for any side effects or adverse reactions.

Electrocardiograms (heart rhythm tests) will be performed at selected visits to monitor your heart function.

Laboratory tests will be conducted regularly to check your blood counts, liver function, kidney function, and other health indicators.

Any side effects will be assessed and graded according to standard medical criteria.

You will be asked to complete questionnaires about your symptoms and quality of life at regular intervals.

These questionnaires will ask about pain levels using the Brief Pain Inventory.

You will complete questions about fatigue using the Brief Fatigue Inventory.

Additional questionnaires will assess your overall well-being, daily functioning, and prostate cancer-specific symptoms.

You will also complete a general health status questionnaire.

You will continue taking the study medications daily as long as your cancer does not progress and you do not experience unacceptable side effects.

Your doctor will monitor for signs of disease progression through regular scans, blood tests, and physical examinations.

The study will track the time until your cancer shows signs of progression, until it becomes resistant to hormone therapy, or until you require additional cancer treatment.

Your treatment in the study will end if your cancer progresses, if you experience unacceptable side effects, or if you decide to stop participating.

If your disease progresses or becomes resistant to hormone therapy, your doctor will discuss other treatment options with you.

The study team will continue to collect information about your health and survival status even after you stop taking the study medication.