Phase 2 Study of Fulvestrant With and Without Entinostat in Postmenopausal Women With ER+ Advanced Breast Cancer

Entinostat (SNDX-275) inhibits mechanisms of resistance to hormone therapy in breast cancer (BC) cells, thereby prolonging sensitivity of the cells to fulvestrant. Preclinical data has demonstrated that entinostat can enhance fulvestrant sensitivity in hormone receptor-positive BC in animal models. Thus, it is hypothesized that the addition of entinostat to fulvestrant will provide clinical benefit to patients with locally advanced or metastatic BC when compared to fulvestrant plus placebo. Preliminary data from Phase 2 Study SNDX-275-0301 suggest patients with higher levels of protein lysine acetylation who receive entinostat with exemestane potentially have improved clinical outcomes (e.g., PFS, OS) when compared to patients with lower levels of protein lysine acetylation. Thus, it is hypothesized that patients exposed to entinostat and who demonstrate an elevated level of protein lysine acetylation will have an improved efficacy outcome.