LBA25 Final overall survival (OS) analysis from ARCHES: A phase III, randomized, double-blind, placebo (PBO)-controlled study of enzalutamide (ENZA) + androgen deprivation therapy (ADT) in men with metastatic hormone-sensitive prostate cancer (mHSPC).

Background: In ARCHES (NCT02677896), ENZA + ADT reduced risk of radiographic progression and improved secondary outcomes in men with mHSPC over PBO + ADT. Here we report the final OS (after 356 events), a key secondary endpoint in ARCHES and a critical benchmark of clinical efficacy, which was immature at the time of primary analysis. Method(s): Men with de novo or relapsed mHSPC (n=1150) were randomized 1:1 to ENZA (160 mg/day) + ADT or PBO + ADT, stratified by disease volume and prior docetaxel use. At the time of data cut-off and unblinding, 180 (31.3%) patients treated with PBO + ADT crossed over to open-label ENZA + ADT. A stratified log-rank test was performed at a two-sided significance level of 0.04 for OS based on an O'Brien-Fleming boundary. Kaplan-Meier estimates of OS and time to subsequent antineoplastic therapy (TTNAnti) were reported and hazard ratios (HRs) were estimated from a stratified Cox proportional hazards model. Safety was assessed via treatment-emergent adverse events. Result(s): Baseline characteristics were similar between treatment arms. As of the data cut-off of May 28, 2021, 397 (34.5%) patients remained on treatment, with a median follow-up of 44.6 months). Median treatment duration was 40.2 months on ENZA + ADT, 13.8 months on PBO + ADT, and 23.9 months for crossover patients. ENZA + ADT extended survival vs PBO + ADT (HR 0.66; 95% confidence interval 0.53, 0.81; p<0.0001) [Table] with similar results in most prespecified subgroups. ENZA + ADT continued to prolong TTNAnti vs PBO + ADT. The safety profile of ENZA + ADT vs PBO + ADT was consistent with findings from the primary analysis. Conclusion(s): This final analysis demonstrates that ENZA + ADT significantly prolongs survival in men with mHSPC and, together with the acceptable safety profile, supports the clinical benefit of ENZA + ADT in men with mHSPC. [Formula presented] Clinical trial identification: NCT02677896. Editorial acknowledgement: This study was funded by Astellas Pha