A Study of Talazoparib With or Without Enzalutamide in People With Prostate Cancer Who Have Previously Received Abiraterone Acetate

The purpose of this study is to find out whether talazoparib in combination with enzalutamide or talazoparib alone delays cancer progression in people with metastatic castration-resistant prostate cancer (mCRPC) who have homologous recombination repair (HRR) mutations and have previously received abiraterone acetate.

Inclusion Criteria • Willing and able to provide, or have a legally authorized representative provide, written informed consent and privacy authorization for the release of personal health information. A signed informed consent must be obtained before screening procedures are performed. NOTE: Privacy authorization may be either included in the informed consent or obtained separately. * Participants ≥ 18 years of age. * Are willing to be randomized into either study arm and adhere to the study protocol. * Ability to swallow study capsules and/or tablets whole. * Are willing to remain on study treatment and to continue undergoing study imaging despite PSA progression unless clinically deteriorating. * Histological or cytological proof of adenocarcinoma of the prostate without neuroendocrine differentiation or small cell features. * Presence of a pathogenic homologous recombination repair mutation in at least one of the following genes on tumor tissue or circulating tumor DNA testing: BRCA1, BRCA2, ATM (limited to 15% of enrolled participants), CDK12, CHEK2, PALB2, MLH1, NBN, ATR, FANCA, MRE11A, RAD51C. Assessment of HRR mutation status by germline or somatic testing. All testing must be per Clinical Laboratory Improvement Amendments (CLIA)-certified assay and may have occurred at any time prior to or at screening (not required to be completed within the screening window). * Metastatic castration-resistant prostate cancer (mCRPC) as demonstrated by one of the following: * Metastatic disease documented by conventional imaging: computed tomography (CT)/magnetic resonance imaging (MRI) chest/abdomen/pelvis and bone scan are required to be performed, but metastases do not need to be seen on both modalities. Measurable disease is not required. * Unequivocal prostate-specific membrane antigen (PSMA) positron emission tomography (PET) only defined metastatic disease with negative conventional imaging. PSMA PET imaging is not required to be performed, but may be used to document metastases when relevant. * Received prior abiraterone acetate with prednisone for mHSPC or locally advanced disease and on which progressed via a minimum of 2 rising PSA levels with a minimum of a 1-week interval between each determination or radiographic progression by any form of imaging. * Progressive disease at start of treatment and in the setting of medical or surgical castration as defined by 1 or more of the following 4 criteria: * PSA progression defined as 2 rising PSA levels, above an initial reference value, taken with a minimum of a 1-week interval. If PSA rise is the only indication of progression at start of study treatment, a minimum PSA of 1.0 ng/mL is required and all measured PSA values have to be considered to make a determination of progression. * Soft tissue disease progression as defined by RECIST 1.1. * Bone disease progression defined by PCWG3 with 2 or more new metastatic bone lesions on a whole-body radionuclide bone scan. * Appearance of newly identified, convincingly positive lesions consistent with metastatic prostate cancer on PSMA PET. * Surgically or medically castrated, with testosterone levels of \