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A B S T R A C T PurposeEntinostat is an oral isoform selective histone deacetylase inhibitor that targets resistance to hormonal therapies in estrogen receptor-positive (ERϩ) breast cancer. This randomized, placebocontrolled, phase II study evaluated entinostat combined with the aromatase inhibitor exemestane versus exemestane alone. Patients and MethodsPostmenopausal women with ERϩ advanced breast cancer progressing on a nonsteroidal aromatase inhibitor were randomly assigned to exemestane 25 mg daily plus entinostat 5 mg once per week (EE) or exemestane plus placebo (EP). The primary end point was progression-free survival (PFS). Blood was collected in a subset of patients for evaluation of protein lysine acetylation as a biomarker of entinostat activity. ResultsOne hundred thirty patients were randomly assigned (EE group, n ϭ 64; EP group, n ϭ 66). Based on intent-to-treat analysis, treatment with EE improved median PFS to 4.3 months versus 2.3 months with EP (hazard ratio [HR], 0.73; 95% CI, 0.50 to 1.07; one-sided P ϭ .055; two-sided P ϭ .11 [predefined significance level of .10, one-sided]). Median overall survival was an exploratory end point and improved to 28.1 months with EE versus 19.8 months with EP (HR, 0.59; 95% CI, 0.36 to 0.97; P ϭ .036). Fatigue and neutropenia were the most frequent grade 3/4 toxicities. Treatment discontinuation because of adverse events was higher in the EE group versus the EP group (11% v 2%). Protein lysine hyperacetylation in the EE biomarker subset was associated with prolonged PFS. ConclusionEntinostat added to exemestane is generally well tolerated and demonstrated activity in patients with ERϩ advanced breast cancer in this signal-finding phase II study. Acetylation changes may provide an opportunity to maximize clinical benefit with entinostat. Plans for a confirmatory study are underway.

Background:Histone deacetylases (HDACs) are crucial components of the oestrogen receptor (ER) transcriptional complex. Preclinically, HDAC inhibitors can reverse tamoxifen/aromatase inhibitor resistance in hormone receptor-positive breast cancer. This concept was examined in a phase II combination trial with correlative end points.Methods:Patients with ER-positive metastatic breast cancer progressing on endocrine therapy were treated with 400 mg of vorinostat daily for 3 of 4 weeks and 20 mg tamoxifen daily, continuously. Histone acetylation and HDAC2 expression in peripheral blood mononuclear cells were also evaluated.Results:In all, 43 patients (median age 56 years (31–71)) were treated, 25 (58%) received prior adjuvant tamoxifen, 29 (67%) failed one prior chemotherapy regimen, 42 (98%) progressed after one, and 23 (54%) after two aromatase inhibitors. The objective response rate by Response Evaluation Criteria in Solid Tumours criteria was 19% and the clinical benefit rate (response or stable disease >24 weeks) was 40%. The median response duration was 10.3 months (confidence interval: 8.1–12.4). Histone hyperacetylation and higher baseline HDAC2 levels correlated with response.Conclusion:The combination of vorinostat and tamoxifen is well tolerated and exhibits encouraging activity in reversing hormone resistance. Correlative studies suggest that HDAC2 expression is a predictive marker and histone hyperacetylation is a useful pharmacodynamic marker for the efficacy of this combination.

Name and URL of Registry: ClinicalTrials.gov. Registration number: NCT01177124.