Background: Previous reports on the PROSPER trial have shown that enzalutamide (ENZA) plus androgen deprivation therapy (ADT) significantly improves metastasis-free survival and overall survival (OS) over placebo (PBO) in men with nonmetastatic castration-resistant prostate cancer (nmCRPC) and rapidly rising prostate-specific antigen (PSA) levels. (Hussain et al. N Engl J Med. 2018;378:2465-2474/Sternberg et al. N Engl J Med. 2020;382:2197-2206). To inform decision-making for specific patients, we report here a post hoc analysis of OS and safety in subgroups of PROSPER by age and region. Methods: PROSPER included men with nmCRPC and a PSA doubling time ≤ 10 months. Enrolled men continued ADT and were randomized 2:1 to ENZA 160 mg once daily vs PBO. We performed a multivariable analysis for OS, including age (≤ 70 yrs and > 70 yrs), geographic region, and other variables and further examined exposure-adjusted adverse events (AEs) by age and region. Results: Based on this post hoc analysis, OS benefit with ENZA treatment was similar across geographic regions (table) and for patients aged ≥ 70 yrs (hazard ratio [HR] 0.73; 95% CI 0.58-0.9) and those aged < 70 yrs (HR 0.72, 95% CI 0.5-1.04). In our multivariate analysis, 3 factors emerged as significantly impacting estimated OS: Eastern Cooperative Oncology Group (ECOG) performance status (1 vs 0; HR 1.7; 95% CI 1.4-2.1), log of PSA (HR 1.2; 95% CI 1.1-1.3), and use of subsequent therapy (yes vs no; HR 2.5; 95% CI 2.1-3.1). Overall safety was consistent between age groups and across geographic regions. The proportion of patients reporting any grade treatment-emergent AEs (TEAEs) related to ENZA use was similar between age groups but decreased with increasing age. Conclusions: In men with nmCRPC and rapidly rising PSA, ENZA plus ADT treatment reduced the risk of death, regardless of age or geographic location. Patients reported any grade TEAEs at a similar proportion in both arms. Variables impacting OS included ECOG status, log PSA, and subsequent therapy. Clinical trial information: NCT02003924

*Denotes statistically significant in this analysis.