Androgen Ablation Therapy With or Without Niraparib After Radiation Therapy for the Treatment of High-Risk Localized or Locally Advanced Prostate Cancer

This study is testing whether adding a drug called niraparib (a PARP inhibitor that helps block cancer cell DNA repair) to standard hormone-blocking therapy (androgen deprivation therapy) improves outcomes for men with high-risk prostate cancer who are being treated with radiation. **You may be eligible if...** - You are male, 18 or older - You have high-risk localized or locally advanced prostate cancer (e.g., PSA over 20, aggressive tumor grade, or locally spread disease) - You are planned to receive radiation and hormone-blocking therapy as your primary treatment - You have not had prior surgery or radiation for prostate cancer - Your blood counts and organ function are within acceptable limits **You may NOT be eligible if...** - Your cancer has spread to distant organs - You have previously used a PARP inhibitor or had systemic prostate cancer treatment (other than hormone therapy started within 6 months) - You have a history of myelodysplastic syndrome (MDS) or acute myeloid leukemia (AML) - You have had a heart attack, stroke, or serious heart problems within 6 months - You have seizures or conditions that increase seizure risk - You are unable to swallow tablets - You have significant uncontrolled medical or psychiatric conditions Talk to your doctor to see if this trial is right for you.