Background: The median overall survival for patients (pts) with metastatic castration-resistant prostate cancer (mCRPC) is short. Available agents may offer limited therapeutic benefit, but no molecularly stratified treatment has yet been approved for this heterogeneous disease. A sizable percentage of pts with mCRPC has loss of function aberrations in genes involved in homologous recombination repair (HRR) in tumor tissue, such as BRCA1/2 and ATM. These aberrations can confer sensitivity to poly(ADP-ribose) polymerase (PARP) inhibition. A Phase II study indicated that the oral PARP inhibitor olaparib (Lynparza) had antitumor activity in 33% of mCRPC pts who had progressed after new hormonal agent (NHA) treatment and chemotherapy, with a strikingly higher composite response rate in pts with a deleterious HRR gene aberration (HRRa) (88%; 14/16) vs pts without a HRRa (6%; 2/33) (Mateo et al.2015). The PROfound study evaluates olaparib efficacy and safety versus physician’s choice of either abiraterone acetate or enzalutamide, in pts with mCRPC and a HRRa (NCT02987543). Methods: To be eligible for this multinational, open-label, Phase III study, mCRPC pts must have progressed on prior NHA treatment and have a tumor HRRa in one of 15 genes, as confirmed by an HRR Assay (Foundation Medicine, Inc.). Cohort A (n = 240 approx) includes pts with mutations in BRCA1, BRCA2 or ATM, while pts with a mutation in 12 other HRR genes will be assigned to Cohort B (n = 100 approx). Pts will be randomized (2:1) to olaparib tablets (300 mg orally bid) or physician’s choice of either enzalutamide (160 mg orally od) or abiraterone acetate (1000 mg orally od with 5 mg bid prednisone) and treatment continued until radiographic progression (as assessed by blinded independent central review) or lack of treatment tolerability. The primary endpoint of radiographic progression-free survival (rPFS) will be assessed in Cohort A using RECIST 1.1 (soft tissue) and PCWG3 (bone) criteria. Key secondary efficacy endpoints include confirmed objective response rate, time to pain progression, overall survival (all Cohort A) and rPFS (both cohorts combined). Clinical trial information: NCT02987543