Phase II data for entinostat, a class 1 selective histone deacetylase inhibitor, in patients whose breast cancer is progressing on aromatase inhibitor therapy

1052 Background: Aromatase inhibitors (AI) block estrogen synthesis and are effective for the treatment of estrogen receptor (ER) positive breast cancer in postmenopausal women. Despite initial benefit, resistance to AIs and disease progression occur, at which point cytotoxic chemotherapy is often the remaining treatment option. Entinostat, an oral class 1 isoform-selective HDACi, has been shown in preclinical studies to restore sensitivity to acquired AI resistance. The goal of this study was to determine whether the addition of entinostat to AI therapy in patients currently progressing on AI therapy could restore AI sensitivity and prolong the clinical benefit. Methods: Postmenopausal women with ER+ breast cancer progressing on an AI after 3 months were enrolled. Additional entry criteria included measurable disease by RECIST and ≤1 chemotherapy for metastatic disease. Patients with rapidly-progressive breast cancer were excluded. Patients continued with the AI on which they progressed plus entinostat 5...