Background: Approximately 20% of patients with metastatic castration-resistant prostate cancer (mCRPC) harbor mutations in genes encoding DNA repair proteins (e.g. ATM, BRCA1, BRCA2, PALB2, RAD51, CHEK2, FANCA). These mutations impact base excision repair and alternative end-joining, which rely on the activity of poly(ADP-ribose) polymerase (PARP). Olaparib (olap) inhibits PARP and showed activity in a post hoc analysis in mCRPC patients with DNA repair defects (DRD). In a subgroup analysis of the NCI 9012 trial evaluating abiraterone (abi) +/- veliparib (a PARP inhibitor), patients with DRD in both treatment groups had a > 80% prostate-specific antigen (PSA) response rate and prolonged progression free survival (PFS) (13.8 mos vs. 7.8 mos; P = 0.01). Preclinical data suggests interactions between androgen signaling and PARP activity. These observations provide the rationale to evaluate the efficacy of olap, abi or the combination in patients with DRD. Methods: This is a prospective, randomized, open-label phase 2 clinical trial with a primary endpoint of objective PFS (radiographic + clinical). Secondary endpoints: objective disease and PSA response rates. Analysis of tumor specimens, circulating tumor cells/DNA will be performed. Eligible patients with progressive mCRPC, no prior mCRPC therapy with tumor loss of ATM, BRCA1 or BRCA2 (based on known germline loss, prior CLIA certified tumor analysis or new metastatic biopsy) will be stratified by germline vs. somatic mutational status and randomized 1:1:1 to abi (1000 mg daily) + prednisone (5 mg bid), olap (300 mg daily) or olap + abi + prednisone (same doses). Patients in single agent arms may cross over to opposite single agent therapy at progression. An exploratory cohort of patients with DNA repair defects besides ATM, BRCA1, BRCA2 may receive olap. Statistical analysis will provide median, 12-month, and 24-month product-limit estimates of PFS by treatment arm. The study is recruiting 60 randomized patients at sites in the US. The trial is coordinated by the Prostate Cancer Clinical Trials Consortium, LLC and funded by AstraZeneca. Clinical trial information: NCT03012321