The purpose of this pre-approval access is to provide Niraparib plus Abiraterone Acetate (Nira/AA combination) in the treatment of (a) participant(s) with first line metastatic castration-resistant prostate cancer with specific Homologous Recombination Repair (HRR) gene alterations.

Inclusion Criteria:

• Diagnosed with metastatic castration-resistant prostate cancer (mCRPC)
• A confirmed homologous recombination repair (HRR) gene alteration as identified by a genomic assay through use of a validated test
• Eastern Cooperative Oncology Group Performance Score (ECOG PS) Grade of 0 or 1
• Willing to receive concomitant prednisone/ prednisolone 10 milligrams (mg) (not provided by Janssen) daily while receiving niraparib/abiraterone acetate (Nira/AA) combination treatment
• Willing to continue treatment with gonadotropin-releasing hormone (GnRH) analogue if not surgically castrated

Exclusion Criteria:

• Prior treatment with a Poly ADP-ribose Polymerase (PARP) inhibitor
• Prior systemic therapy for mCRPC (that is, taxane-based chemotherapy, androgen signaling inhibitors). Ongoing androgen deprivation therapy (ADT), prior systemic treatment (example, apalutamide, enzalutamide, darolutamide, docetaxel) for earlier disease settings (example, hormone sensitive disease, non-metastatic castration-resistant disease); and treatment recently initiated with abiraterone acetate for mCRPC less than or equal to (<=) 60 days is permitted
• Other treatment options available or participant eligible or able to participate in a clinical trial for first line mCRPC with HRR gene alterations
• Moderate-severe pain
• History or current diagnosis of myelodysplastic syndrome (MDS)/acute myeloid leukemia (AML)