Rucaparib for the Treatment of Metastatic Castration-resistant Prostate Cancer Associated with a DNA Damage Repair Gene Alteration: Final Results from the Phase 2 TRITON2 Study

Background: Initial TRITON2 (NCT02952534) results demonstrated the efficacy of rucaparib 600 mg BID in patients with metastatic castration-resistant prostate cancer (mCRPC) associated with a BRCA1 or BRCA2 (BRCA) or other DNA damage repair (DDR) gene alteration. Objective: To present the final data from TRITON2. Design, setting, and participants: TRITON2 enrolled patients with mCRPC who had progressed on one or two lines of next-generation androgen receptor–directed therapy and one taxane-based chemotherapy. Outcome measurements and statistical analysis: The primary endpoint was objective response rate (ORR; as per the modified Response Evaluation Criteria in Solid Tumor Version 1.1/Prostate Cancer Clinical Trials Working Group 3 criteria in patients with measurable disease by independent radiology review [IRR]); prostate-specific antigen (PSA) response rate (≥50% decrease from baseline [PSA50]) was a key secondary endpoint. Results and limitations: As of July 27, 2021 (study closure), TRITON2 had enrolled 277 patients, grouped by mutated gene: BRCA (n = 172), ATM (n = 59), CDK12 (n = 15), CHEK2 (n = 7), PALB2 (n = 11), or other DDR gene (Other; n = 13). ORR by IRR was 46% (37/81) in the BRCA subgroup (95% confidence interval [CI], 35–57%), 100% (4/4) in the PALB2 subgroup (95% CI, 40–100%), and 25% (3/12) in the Other subgroup (95% CI, 5.5–57%). No patients within the ATM, CDK12, or CHEK2 subgroups had an objective response by IRR. PSA50 response rates (95% CI) in the BRCA, PALB2, ATM, CDK12, CHEK2, and Other subgroups were 53% (46–61%), 55% (23–83%), 3.4% (0.4–12), 6.7% (0.2–32%), 14% (0.4–58%), and 23% (5.0–54%), respectively. Conclusions: The final TRITON2 results confirm the clinical benefit and manageable safety profile of rucaparib in patients with mCRPC, including those with an alteration in BRCA or select non-BRCA DDR gene. Patient summary: Almost half of TRITON2 patients with BRCA-mutated metastatic castration-resistant prostate cancer had a complete or partial tumor size reduction with rucaparib; clinical benefits were also observed with other DNA damage repair gene alterations.