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Purpose: Deleterious germline/somatic homologous recombination-repair mutations (HRRm) are present in ~25% of metastatic castration resistant prostate cancer (mCRPC) patients. Preclinically, PARP-Inhibition demonstrated synergism with ARP-targeted therapy. This trial evaluated efficacy of ARP-Inhibitor versus PARP-Inhibitor versus combination as first-line therapy in mCRPC patients with HRRm., Patients and Methods: BRCAAway is a biomarker pre-selected, randomized, phase-2 trial. Patients with BRCA1/2 and/or ATM alterations were randomized 1:1:1 to Arm1: Abiraterone (1000mg)/prednisone (Abi/pred) (5mg), Arm2: Olaparib (Ola) (300mg), or Arm3: Abiraterone/prednisone + Olaparib (Abi/pred+Ola). Single-agent arms could cross over at progression. Exploratory Arm4 patients with other HRRm received Olaparib alone. The primary endpoint was progression-free survival (PFS), and secondary endpoints were objective response, PSA response, and safety., Results: 61/165 eligible patients had BRCA1/2 or ATM mutations: Median age: 67 (IQR 62-73) years. Mutations: BRCA1 n=3, BRCA2 n=46, ATM n=11, multiple n= 1; 33 germline, 28 somatic. Median PFS (95% CI): Abi/pred, 8.6 months (m) (2.9, 17), Ola, 14 m (8.4, 20), Abi/pred+Ola, 39 m (22, NR). There were no G4/5 AEs. 8/19 on Abi/pred crossed over to Ola, and 8/21 vice versa: Median PFS (95% CI) from crossover: Ola-after-Abi/pred, 8.3 m (5.5, 15); Abi/pred-after-Ola, 7.2 m (2.8, NR). Median PFS (95% CI) from randomization: Ola-after-Abi/pred, 16 m (7.8, 25), Abi/pred-after-Ola, 16 m (11, NR). 17/165 patients with other HRRm received olaparib: Median PFS (95% CI): 5.5 m (2, 11)., Conclusions: In mCRPC patients with BRCA1/2 or ATM HRRm, abiraterone/prednisone + olaparib was well tolerated and demonstrated longer PFS versus either agent alone or sequentially.