Background: Histone deacetylase inhibitors (HDACi) prevent the emergence of drug tolerant clones and sensitize cells to a variety of anti-cancer therapies. We previously reported ENCORE 301, a randomized placebo controlled phase II study comparing exemestane with (EE) and without (EP) the HDACi entinostat met its primary endpoint of prolonging progression free survival (PFS) (4.3 months vs 2.3 months) and extending overall survival (OS) benefit (26.9 vs 19.8 months). In order to characterize and better understand the OS benefit, exploratory analyses were conducted. Methods: Hazard ratios (HR) for treatment were estimated from the Cox proportional hazards model, with EP serving as the reference treatment in the calculations. Inferential comparisons between treatment groups were made using the log-rank test. Results: Analysis of OS across multiple subsets of interest demonstrated a consistent benefit in EE group versus EP. Sensitivity analysis of baseline characteristics potentially impacting OS did not identify any factors that influenced the effect of EE on OS. Examination of treatments received during follow-up after discontinuation of EE and EP demonstrated balance between treatment group for patients receiving chemotherapy (48% EE: 44% EP), hormone therapy (37% EE: 35% EP), bisphosphonates (2% EE; 0% EP), radiation (6% EE; 5%EP), and surgery (0% EE: 2% EP). Compared to EP, OS was longer in EE group for patients whose first subsequent treatment was a hormone therapy (EE median not reached vs EP median 20.5 months; HR 0.65 [95% CI 0.26, 1.58]) or chemotherapy (EE median 26.9 months vs EP median 17.6 months; HR 0.51 [95% CI 0.25, 1.04]). Updated PFS and OS data will also be presented along with correlation analysis of PFS and OS. Conclusions: Analyses of baseline characteristics, subsequent treatment and subsets of prognostic factors in ENCORE 301 did not identify any contributing factors that account for the extended OS benefit in the EE treatment group. Long lasting effects of entinostat on progenitor cells, emergence of drug tolerant cells and or priming to subsequent therapies cannot be ruled out.