A Study of Niraparib Combination Therapies for the Treatment of Metastatic Castration-Resistant Prostate Cancer

The purpose of this study is to: a) establish the recommended phase 2 dose (RP2D) and to evaluate the antitumor activity and safety of niraparib combination therapies (Combinations 1 and 2) and b) to determine the relative bioavailability of niraparib and abiraterone acetate (AA) in combination (Combination 3) in participants with metastatic castration-resistant prostate cancer (mCRPC).

This multicenter study will evaluate safety and efficacy of niraparib in combination with other anti-cancer agents. Combination 1 will combine niraparib with the anti-programmed cell death protein (PD)-1 monoclonal antibody cetrelimab, in participants with mCRPC. Combination 1 has 2 parts: in Part 1 (dose selection), participants will be enrolled to establish RP2D doses of niraparib and cetrelimab; and Part 2 (dose expansion) will evaluate the combination therapy in an expanded number of participants into 2 cohorts (biomarker positive or biomarker negative). Combination 2 will combine niraparib with abiraterone acetate plus prednisone (AA-P) in mCRPC participants with DNA-repair gene defects (DRD). Combination 3 will evaluate the relative bioavailability (BA) of niraparib and AA in combination. In a pharmacokinetics (PK) assessment phase, niraparib and AA will be administered, and in an extension phase, niraparib and AA-P will be administered. Combinations 1 and 2 will have 5 phases: A Pre-screening Phase, a Screening Phase, a Treatment Phase, a Follow-up Phase, and a Long-term Extension (LTE) Phase; Combination 3 has 3 phases: A Screening Phase, A PK Assessment Phase, an Extension Phase (including LTE phase). Study evaluations will include efficacy, PK, PK/pharmacodynamics, biomarkers, safety and tolerability.

Eligibility Criteria:
Inclusion Criteria for Combination 3:

* Diagnosed with mCRPC, who in the opinion of the investigator may benefit from treatment in Combination 3 of this study
* Able to continue gonadotropin releasing hormone analogue (GnRHa) therapy during the study if not surgically castrate (that is, subjects who has not undergone bilateral orchiectomy).
* Eastern Cooperative Oncology Group Performance Status (ECOG PS) of less than or equal to (\<=) 1
* Toxicity associated with prior chemotherapy or radiotherapy has resolved to Grade \<= 1 (except alopecia or Grade \<= 2 neuropathy) at screening
* Participant must agree not to donate sperm while on study treatment, and for 3 months following the last dose of study treatment

Exclusion Criteria:

* History or current diagnosis of myelodysplastic syndrome (MDS)/acute myeloid leukemia (AML)
* Active malignancies (that is, progressing or requiring treatment change in the last 24 months) other than the disease being treated under study. The only allowed exceptions are: non-muscle invasive bladder cancer; skin cancer (non-melanoma or melanoma); breast cancer; malignancy that is considered cured with minimal risk of recurrence
* Active infection requiring systemic therapy
* Allergies, hypersensitivity, or intolerance to niraparib or the corresponding excipients

Combination 3:

* Symptomatic brain metastases
* Prior disease progression during combination treatment with AA and poly (adenosine diphosphate \[ADP\]-ribose) polymerase inhibitor (PARPi). Prior discontinuation of treatment with AA or PARPi due to AA- or PARPi-related toxicity