A Study of Niraparib in Combination With Abiraterone Acetate and Prednisone Versus Abiraterone Acetate and Prednisone for Treatment of Participants With Metastatic Prostate Cancer

The purpose of this study is to evaluate the effectiveness of niraparib in combination with abiraterone acetate plus prednisone (AAP) compared to AAP and placebo.

Inclusion Criteria: * HRR gene alteration (as identified by the sponsor's required assays) as follows: 1. Cohort 1: positive for HRR gene alteration 2. Cohort 2: not positive for DRD (that is, HRR gene alteration) 3. Cohort 3: eligible by HRR status * Metastatic disease documented by positive bone scan or metastatic lesions on computed tomography (CT) or magnetic resonance imaging (MRI) * Metastatic prostate cancer in the setting of castrate levels of testosterone less than or equal to (\<=) 50 nanogram per deciliter (ng/dL) on a gonadotropin releasing hormone analog (GnRHa) or bilateral orchiectomy * Able to continue GnRHa during the study if not surgically castrate * Score of \<= 3 on the brief pain inventory-short form (BPI-SF) question number 3 (worst pain in last 24 hours) Exclusion Criteria: * Prior treatment with a poly (adenosine diphosphate \[ADP\]-ribose) polymerase (PARP) inhibitor * Systemic therapy (that is, novel second-generation AR-targeted therapy such as enzalutamide, apalutamide, or darolutamide; taxane-based chemotherapy, or more than 4 months of abiraterone acetate plus prednisone \[AAP\] prior to randomization) in the metastatic castration-resistant prostate cancer (mCRPC) setting; or AAP outside of the mCRPC setting * Symptomatic brain metastases * History or current diagnosis of myelodysplastic syndrome (MDS)/acute myeloid leukemia (AML) * Other prior malignancy (exceptions: adequately treated basal cell or squamous cell skin cancer, superficial bladder cancer, or any other cancer in situ currently in complete remission) \<= 2 years prior to randomization, or malignancy that currently requires active systemic therapy