Results of ENCORE 301, a randomized, phase II, double-blind, placebo-controlled study of exemestane with or without entinostat in postmenopausal women with locally recurrent or metastatic estrogen receptor-positive (ER+) breast cancer progressing on a nonsteroidal aromatase inhibitor (AI).

Abstract

268 Background: Hormonal therapy, including AIs, is the mainstay of ER+ breast cancer (BC) treatment; however, both acquired and intrinsic resistance limits its clinical benefit. Entinostat is a novel, oral, class I selective histone deacetylase inhibitor that has been shown to inhibit growth factor signaling pathways that mediate AI resistance. This study was designed to evaluate the impact of the addition of entinostat to exemestane therapy on progression-free survival (PFS). Methods: Postmenopausal women with ER+ advanced BC who had progressed on a non-steroidal AI were randomized to exemestane 25 mg daily + entinostat 5 mg or placebo weekly. Results: A total of 130 women were enrolled (66 exemestane+placebo; 64 exemestane+entinostat). All but 1 patient had Stage IV disease, and 82% had measurable disease. All patients had received prior hormonal therapy (1 prior line 42%; >1 prior line 58%), and 62% had received prior chemotherapy (33% in the advanced BC setting). Analysis of the intent-to-treat population showed that PFS was significantly (defined prospectively as p <0.10) longer with exemestane+entinostat than with exemestane+placebo (4.28 versus 2.27 months, respectively; hazard ratio [HR] = 0.73; p=0.06). Entinostat combined with exemestane was well-tolerated with the most frequent adverse events (AEs) consisting of fatigue, gastrointestinal disturbances, and hematologic abnormalities. AEs with a ≥20% higher incidence with exemestane+entinostat than with exemestane+placebo were fatigue (46% versus 26%, respectively) and uncomplicated neutropenia (25% versus 0%, respectively). The serious AE rate was similar for exemestane+entinostat (13%) and exemestane+placebo (12%). Conclusions: Exemestane+entinostat significantly prolonged the median PFS and reduced the risk of disease progression by 27% versus exemestane+placebo (HR = 0.73). In light of these positive data, a phase III evaluation of this combination is planned.