Abstract P6-14-01: A class I histone deacetylase inhibitor, entinostat, enhances lapatinib efficacy in both HER2-overexpressing inflammatory and non-inflammatory breast cancer cells through FOXO3-mediated Bim1 expression
Authors: Lee Jangsoon , Bartholomeusz Chandra , Hortobagyi Gabriel N , Ordentlich Peter , Ueno Naoto T 
Published: May 2015

Abstract
Background: Human epidermal growth factor receptor 2 (HER2) overexpression has been reported in 15%-20% of breast cancers and is associated with shorter survival and worse clinical outcome. Inflammatory breast cancer (IBC), a very aggressive subtype of advanced breast cancer, accounts for approximately 2% of all breast cancers and 8%-10% of all breast cancer-related deaths in the United States. Recent study represented that about 60% of IBCs overexpress HER2. Although there are effective HER2-targeted agents, patients with HER2+ breast cancer often have intrinsic and acquired resistance to the anti-HER2 agents that are currently approved by the U.S. Food and Drug Administration. Therefore, novel combination strategies are needed to treat HER2+ breast cancers that develop drug resistance. To this end, we investigated the combinational effect of entinostat, an oral isoform-selective histone deacetylase type I inhibitor, and lapatinib, a HER2/epidermal growth factor receptor dual tyrosine kinase inhibitor, in HER2+ inflammatory breast cancer (IBC) and non-IBC breast cancer cells. Methods: We assessed the combinational synergistic effect and its mechanism via CellTiter Blue assay, flow cytometry, anchorage-independent growth, quantitative real-time polymerase chain reaction, small interfering RNA, Western blotting, and mammary fat pad xenograft mouse models. Results: We found that compared with entinostat or lapatinib alone, the two drugs in combination synergistically inhibited tumor cell proliferation (P &lt; 0.001), reduced in vitro colony formation (P &lt; 0.05), and resulted in significant in vivo tumor shrinkage or growth inhibition in both IBC and non-IBC xenograft mouse models (SUM190 and BT474, P &lt; 0.001). The synergistic antitumor activity of the entinostat-lapatinib combination was due to downregulation of phosphorylated Akt, which induced the transcriptional activity of FOXO3, resulting in the induction of Bim1 (a BH3 domain-containing pro-apoptotic protein). Furthermore, entinostat sensitized trastuzumab/lapatinib-resistant HER2+ cells to the trastuzumab-lapatinib combination and enhanced the anti-proliferation effect compared with single-agent with lapatinib or combination treatment with lapatinib and trastuzumab. Conclusion: Taken together, our data provide evidence that entinostat has enhanced antitumor effect in combination with the HER2-targeted reagent lapatinib and results in the induction of apoptosis by FOXO3-mediated Bim1 expression. Our findings justify conducting a clinical trial (clinicaltrial.gov, NCT01434303) of combinational treatment with entinostat, lapatinib, and trastuzumab in patients with HER2+ IBC or non-IBC that is resistant to trastuzumab-based treatment.
Citation Format: Jangsoon Lee, Chandra Bartholomeusz, Gabriel N Hortobagyi, Peter Ordentlich, Naoto T Ueno. A class I histone deacetylase inhibitor, entinostat, enhances lapatinib efficacy in both HER2-overexpressing inflammatory and non-inflammatory breast cancer cells through FOXO3-mediated Bim1 expression [abstract]. In: Proceedings of the Thirty-Seventh Annual CTRC-AACR San Antonio Breast Cancer Symposium: 2014 Dec 9-13; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2015;75(9 Suppl):Abstract nr P6-14-01.