This clinical trial is focused on studying a type of prostate cancer known as Metastatic Castration-Resistant Prostate Cancer (mCRPC). This is a form of prostate cancer that continues to grow and spread even when the amount of male hormones in the body is reduced to very low levels. The study is testing a combination of two treatments: Pembrolizumab, also known by its code name MK-3475, and Enzalutamide. Pembrolizumab is a medication that helps the immune system fight cancer, while Enzalutamide is used to block the effects of male hormones that can promote cancer growth. The trial will compare the effects of these two drugs together against a combination of a placebo and Enzalutamide.

The purpose of the study is to see if the combination of Pembrolizumab and Enzalutamide can help patients live longer and slow down the progression of the cancer compared to the placebo and Enzalutamide. Participants in the study will receive either the combination of Pembrolizumab and Enzalutamide or the placebo and Enzalutamide. The study will monitor the participants over a period to assess their overall survival and the time it takes for the cancer to progress as seen on medical imaging tests.

Throughout the study, participants will receive regular check-ups and assessments to monitor their health and the effects of the treatment. The study aims to provide valuable information on the effectiveness of combining Pembrolizumab with Enzalutamide in treating this challenging form of prostate cancer. The trial is expected to continue until 2025, providing insights that could lead to improved treatment options for patients with mCRPC.