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

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