This study looks at how well a drug called **Saruparib** works compared to a **placebo** (a pill with no medicine) in men with high-risk prostate cancer who have a special gene change called **BRCA mutation**. The study involves about 700 men who will be split into two groups. One group gets Saruparib and the other gets a placebo, along with regular prostate cancer treatments like **radiation therapy (RT)** and **androgen deprivation therapy (ADT)**, which lowers certain hormones. - Participants will be monitored for survival until the study ends. - Men must have a BRCA1 or BRCA2 gene mutation and have received radiation and hormone therapy. - Some risks include side effects from Saruparib and the need to follow certain lifestyle restrictions. Men must provide a sample of their tumor and have a specific type of scan to confirm their cancer status. They should not have certain health conditions or be taking specific medications that could interfere with the study. Participants should not father children during the study and for six months after. This study is important to see if Saruparib helps patients live longer without the cancer spreading.