Background: TALA is a poly(ADP-ribose) polymerase inhibitor (PARPi) approved as monotherapy for germline BRCA1/2-mutated HER2-negative advanced breast cancer. Clinical efficacy in metastatic castration-resistant prostate cancers (mCRPC) with alterations in DNA damage response (DDR) genes involved directly or indirectly in homologous recombination repair (HRR) has been demonstrated with some PARPi. Phase 3 study findings (de Bono et al. N Engl J Med, 2020;382:2091-2102) resulted in the approval of olaparib for mCRPC. ENZA is an androgen receptor (AR) inhibitor and established therapy for mCSPC. Since PARP activity has been shown to support AR function, PARP inhibition may increase sensitivity to AR-directed therapies. In addition, AR blockade downregulates HRR gene regulation, which has been hypothesized to induce a “BRCAness” phenotype. A Phase 2 study of TALA monotherapy (TALAPRO-1) demonstrated robust antitumor activity in men with heavily pretreated, HRR-mutated mCRPC. The Phase 3, double-blind, randomized trial TALAPRO-3 (NCT04821622) herein presented will compare the combination of TALA plus ENZA vs placebo plus ENZA in men with mCSPC with DDR/HRR alterations. Methods: Approximately 550 patients with mCSPC harboring DDR/HRR alterations will be randomized to TALA (0.5 mg once daily) plus ENZA (160 mg once daily) or placebo (once daily) plus ENZA (160 mg once daily). Patients will be stratified according to de novo mCSPC vs relapsed mCSPC, high-volume disease vs low-volume disease, where high-volume disease is defined as the presence of visceral metastases or ≥4 bone lesions with ≥1 beyond the vertebral bodies and pelvis and BRCA vs non-BRCA mutational status. Key eligibility criteria include age ≥18 years; histological diagnosis of prostate cancer; alterations in at least one of 12 DDR/HRR genes known to sensitize patients to PARPi (ATM, ATR, BRCA1, BRCA2, CDK12, CHEK2, FANCA, MLH1, MRE11A, NBN, PALB2, RAD51C); and metastatic disease (no brain metastases). Primary endpoint is rPFS (time to radiographic progression in soft tissue per RECIST 1.1 or in bone per PCWG3 criteria by investigator, or death). Secondary endpoints include overall survival, safety, and patient-reported outcomes. Patient recruitment is planned at approximately 285 sites in 27 countries, including the US and Europe, South America, South Africa, and Asia-Pacific. This study was approved by an Institutional Review Board. Clinical trial information: NCT04821622.