Genomic Characteristics Associated with Clinical Activity of Rucaparib in Patients (pts) with BRCA1 or BRCA2 (Brca)-Mutated Metastatic Castration-Resistant Prostate Cancer (Mcrpc).

178 Background: The phase 2 TRITON2 study (NCT02952534) is evaluating the PARP inhibitor rucaparib in pts with mCRPC and a deleterious germline or somatic alteration in BRCA ( BRCA pts) or 1 of 13 other DNA damage repair (DDR) genes. Methods: Eligible pts had progressed on 1–2 lines of androgen receptor-directed therapy and 1 taxane-based chemotherapy. Deleterious DDR gene alterations were identified from central testing of plasma and/or tissue samples by Foundation Medicine or from local testing. Results: As of Jul 2, 2019 (visit cutoff), 98 BRCA pts had received rucaparib; median (range) duration of follow-up was 13.0 (4.1–25.8) mo. Confirmed objective response rate (ORR) in BRCA pts (with investigator-assessed measurable disease) and PSA response rates (in pts with and without measurable disease) are shown in the Table. A biallelic alteration was observed in 81.4% (35/43) of BRCA pts for whom zygosity could be determined, and pts with homozygous BRCA loss (20.4%; 20/98) demonstrated high ORR (66.7% [95% CI, 38.4-88.2]; 10/15) and PSA response rates (75.0% [95% CI, 50.9-91.3]; 15/20). The safety profile in BRCA pts was consistent with the overall TRITON2 pt population. Conclusions: Results from TRITON2 showed antitumor activity with rucaparib in pts with mCRPC and a deleterious BRCA alteration. Responses were observed in patients with germline or somatic alterations in BRCA1 or BRCA2. The activity of rucaparib in these and additional genomic subgroups (e.g., based on zygosity, plasma genomic testing, or co-occurring alterations) will be reported. Clinical trial information: NCT02952534. [Table: see text]