Background: Despite new treatments, metastatic castration resistant prostate cancer (mCRPC) remains ultimately lethal. Upwards of 25% of mCRPC tumors have alterations in homologous recombination DNA repair (HRD) genes, most frequently BRCA2. Retrospective data suggest addition of the DNA damaging agent carboplatin to standard docetaxel for patients with mCRPC whose tumors have biallelic inactivation of BRCA2 may be effective. Methods: In this prospective, pilot single-arm phase 2 study, we assess response to the combination of docetaxel 60mg/m² and carboplatin AUC 5 IV q21 days in patients with mCRPC whose tumors have evidence of biallelic inactivation of BRCA1, BRCA2 or ATM and have progressed after any prior treatment, including prior docetaxel and/or PARP inhibitor (PARPi). The primary endpoint is rate of ≥50% PSA decline from baseline (PSA₅₀). With H0 of PSA₅₀ of 26%, 14 patients will provide 80% power to conclude H1 of PSA₅₀ of 60% based on Simon's 2-stage design with 1-sided α = 5%. Secondary endpoints include PSA₃₀, response duration, time to progression and correlative studies. Biallelic inactivation of other HRD-related genes were included at investigators’ discretion. Results: The study opened in Jan 2016 and has enrolled 8/14 (57%) of patients plus 5 patients with other HRD-related genes. 6/13 (46%) treated pts had grade 3 expected AEs, and no grade 4-5 AEs were observed. To date, 7/8 (88%) of pts with biallelic inactivation of BRCA1, BRCA2 and ATM (Table) and 10/13 (77%) of all pts achieved a PSA₅₀ (other HRD-related genes include CDK12, CHD1, MRE11A and PALB2). Updated results will be reported at final presentation. Conclusions: Addition of carboplatin to docetaxel appears to be well-tolerated and effective for patients with mCRPC whose tumors have biallelic inactivation in selected HRD genes such as BRCA2, including those with prior treatment with PARP inhibitor. Clinical trial information: NCT02598895

*still on treatment