Phase I Study and Pilot Efficacy Analysis of Entinostat, a Novel Histone Deacetylase Inhibitor, in Chinese Postmenopausal Women with Hormone Receptor-Positive Metastatic Breast Cancer

Background: Previous clinical trials have demonstrated that entinostat in combination with exemestane had good tolerability and significant clinical efficacy in patients with advanced hormone receptor positive (HR+) and HER2 negative (HER2−) metastatic breast cancer (MBC) in the USA. However, no clinical trials have been conducted in Chinese populations. Objective: To investigate the safety, pharmacokinetics, and pilot efficacy of entinostat with or without exemestane in Chinese postmenopausal patients with locally advanced or metastatic HR+ /HER2− MBC. Patients and methods: Nineteen patients received entinostat for 4 weeks (dose-limiting toxicity (DLT) observation stage) at 3, 5, or 7 mg/week, with a “3+3” dose-escalation design and in combination with exemestane thereafter (extended treatment stage: entinostat, 3 or 5 mg/week; exemestane, 25 mg/day). An additional 21 patients were enrolled to assess the entinostat (5 mg) plus exemestane (25 mg) pharmacokinetic profile and potential efficacy. Results: The peak entinostat serum concentration and area under the curve increased dose proportionally, without significant interaction between entinostat and exemestane. Entinostat was well tolerated at all doses. The most common grade 3/4 adverse effects (AEs) included neutropenia (31.6%) and thrombocytopenia (15.8%). In the DLT observation stage, grade 3/4 AEs accounted for 16.7% in the 5 mg group with one suspicious DLT (G3 ventricular tachycardia) and 33.3% in the 7 mg group. In the extended treatment stage, 2/16 patients achieved partial response and three patients experienced stable disease (> 12 weeks). The median progression-free survival was 9.41 months for the additional 21 patients, who experienced grade 3/4 AEs of neutropenia (38%), thrombocytopenia (9.5%), anemia (9.5%), and fatigue (9.5%). Conclusion: Entinostat with exemestane showed reasonable safety, tolerability, and encouraging efficacy in Chinese patients with HR+/HER2− MBC. These results support further evaluation in a randomized, double-blind Phase III study with a weekly 5 mg entinostat dose in a Chinese population. Trial Registration: NCT02833155.