Impact of prior treatment setting on the efficacy of rucaparib vs physician’s choice in BRCA -mutated castration-resistant prostate cancer (mCRPC) in TRITON3.

5054 Background: Rucaparib significantly improved radiographic progression-free survival (rPFS) versus physician’s choice of docetaxel or an androgen-receptor pathway inhibitor (ARPI) (abiraterone or enzalutamide) in men with BRCA mutations who were chemotherapy-naïve in the mCRPC setting and had received one prior ARPI in the phase 3 TRITON3 trial (NCT02975934). This post-hoc analysis evaluated whether the timing of prior ARPI therapy impacted the efficacy of rucaparib. Methods: Patients were randomized 2:1 to rucaparib (600 mg BID) or physician’s choice of docetaxel or ARPI, after progression on 1 prior second-generation ARPI in any setting. The primary endpoint was rPFS. Outcomes were analyzed according to whether patients received the prior ARPI in the metastatic castration sensitive (mCSPC) or castration resistant (mCRPC) setting. Data cutoff was August 25, 2022. Results: Of the 302 BRCA1/2 patients, 68 received prior ARPI in the mCSPC setting (42 rucaparib; 26 physician’s choice) and 234 in the mCRPC setting (159 rucaparib; 75 physician’s choice). Baseline characteristics were generally similar. Prior treatments in the mCSPC and mCRPC groups, respectively, included abiraterone (73.5% vs 53.0%), enzalutamide (29.4% vs 48.3%), and docetaxel (17.6% vs 24.8%). Median rPFS favored rucaparib over physician’s choice in both groups (mCSPC: 13.6 vs 8.2 months; HR 0.60 95% CI, 0.32–1.15; mCRPC: 11.2 vs 5.8 months; HR 0.43 95% CI, 0.30–0.61). Median OS was similar between treatments in both settings (mCSPC: 23.2 vs 21.7 months; HR 0.81 95% CI, 0.47–1.41; mCRPC: 23.2 vs 21.0 months; HR 0.91 95% CI, 0.66–1.25). Treatment-related adverse events occurred at comparable rates (89.6% vs 86.1%). Conclusions: Rucaparib demonstrated efficacy in patients with BRCA-mutated mCRPC, regardless of whether prior ARPI was administered in the mCSPC or mCRPC setting. Clinical trial information: NCT02975934 .