A Study of Niraparib in Combination With Abiraterone Acetate and Prednisone Versus Abiraterone Acetate and Prednisone for the Treatment of Participants With Deleterious Germline or Somatic Homologous Recombination Repair (HRR) Gene-Mutated Metastatic Castration-Sensitive Prostate Cancer (mCSPC)

The purpose of the study is to determine if the combination of niraparib with Abiraterone Acetate (AA) plus prednisone compared with AA plus prednisone in participants with deleterious germline or somatic Homologous Recombination Repair (HRR) gene-mutated Metastatic Castration-Sensitive Prostate Cancer (mCSPC) provides superior efficacy in improving radiographic progression-free survival (rPFS).

Prostate cancer is a heterogenous disease and recent genomic analyses have highlighted specific germline and somatic mutations and alternative driver growth signaling pathways in patients with metastatic disease. Abiraterone acetate plus prednisone (AAP) is an established standard of care for the treatment of participants with mCSPC and is included in widely accepted clinical treatment guidelines. Niraparib in combination with AAP has been approved for the treatment of BRCA-mutated Metastatic Castration-Resistant Prostate Cancer (mCRPC). Niraparib is an investigational agent in the Metastatic Castration-Sensitive Prostate Cancer (mCSPC) population. Whether the addition of niraparib to the AAP standard of care may improve initial disease control and long-term outcomes compared with AAP alone in a biomarker selected mCSPC population is being evaluated on this trial. The study will consist of 4 phases; a Prescreening Phase for biomarker evaluation for eligibility only, a Screening Phase, a Treatment Phase, and a Follow-up Phase. Efficacy evaluations include the following: tumor measurements by computed tomography (CT), magnetic resonance imaging (MRI; abdomen, chest, and pelvis), Technetium-99m (99mTc) bone scans, serum prostate sensitive antigen (PSA) evaluations, and patient reported outcomes (PROs). Safety evaluations include incidence of adverse events and clinical laboratory parameters.

Eligibility Criteria:
Inclusion criteria:

* Pathological diagnosis of prostate adenocarcinoma
* Must have appropriate deleterious homologous recombination repair (HRR) gene alteration
* Metastatic disease as documented by conventional imaging with computed tomography (CT) or magnetic resonance imaging (MRI) (for soft tissue lesions) or 99mTc bone scan (for bone lesions). Participants with a single bone lesion on Technetium-99m (99mTc) bone scan with no other non-nodal metastatic disease must have confirmation of bone metastasis by CT or MRI. Participants with lymph node-only disease are not eligible
* Androgen deprivation therapy (either medical or surgical castration) must have been started \>=14 days prior to randomization and participants be willing to continue androgen deprivation therapy (ADT) through the treatment phase
* Other allowed prior therapy for metastatic castration-sensitive prostate cancer (mCSPC): (a) maximum of 1 course of radiation and 1 surgical intervention for symptomatic control of prostate cancer (example, uncontrolled pain, impending spinal cord compression or obstructive symptoms). Participants with radiation or surgical interventions to all known sites of metastatic disease will be excluded from trial participation. Radiation must be completed prior to randomization (b) Up to a maximum of 6 months of ADT prior to randomization; (c) Up to a maximum of 45 days of abiraterone acetate + prednisone (AA-P) prior to randomization (d) Up to a maximum of 2 weeks of ketoconazole for prostate cancer prior to randomization

Exclusion criteria:

* Prior treatment with a poly (adenosine diphosphate-ribose) polymerase inhibitor (PARP inhibitor)
* History of adrenal dysfunction
* Long-term use of systemically administered corticosteroids (greater than \[\>\] 5 milligrams \[mg\] of prednisone or the equivalent) during the study is not allowed. Short-term use (\<=4 weeks, including taper) and locally administered steroids (for example, inhaled, topical, ophthalmic, and intra-articular) are allowed, if clinically indicated
* History or current diagnosis of myelodysplastic syndrome (MDS)/ acute myeloid leukemia (AML)