Abstract 5439: Characterization of the class 1 histone deacetylase inhibitor entinostat in EGFRi acquired resistance models and in combination with cetuximab in NSCLC
Authors: Chan Daniel C. , Witta Samir E. , Zheng Di , Franekova Veronika , Sui Xiaomei , Ordentlich Peter , Bunn Paul A. , Hirsch Fred R. 
Published: February 2011

Abstract
Background: Entinostat is a novel oral benzamide HDACi characterized by its selectivity for class 1 HDACs as well as a unique pharmacokinetic (PK) and pharmacodynamic (PD) profile. Previous results have demonstrated the ability of entinostat to synergize in vitro and in vivo with the small molecule EGFR inhibitors, gefitinib and erlotinib as well as EGFR directed antibody, cetuximab to inhibit growth of EGFRi resistant NSCLC cells. The mechanism for the combined efficacy may involve a) targeting of EGFR gene and protein expression b) down-regulation of EGFR-mediated signal transduction pathway c) alteration of the tumor phenotype to re-sensitize cancer cells to EGFRi.
Methods: The growth inhibitory effect of the combination of erlotinib and entinostat in H1975 and H1933 cells was determined using a 5-day 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay. In vivo efficacy of entinostat was tested in athymic nude mice bearing NSCLC xenografts, A549 and H460. Combined treatment of entinostat with cetuximab was studied in H460 xenografts
Results: We have extended our previous work examining the effect of entinostat on EGFRi resistance through testing of entinostat combined with erlotinib in cell line models of acquired resistance and evaluating the combination of entinostat with cetuximab in vivo in a xenograft model. Our results demonstrate that the combination of entinostat and erlotinib has enhanced activity in H1975 NSCLC cells which harbor the EGFRi T790M mutation. In addition the combination is more effective in H1933 NSCLC cells which have a met amplification. To further characterize the potential mechanism of action by which entinostat sensitizes NSCLC cells to EGFRi we have also tested entinostat in vivo in A549 and H460 xenograft models. Entinostat exhibits a dose response inhibition of tumor growth in both models and analysis of molecular markers is ongoing. In our previous experiments examining impact of entinostat on EGFRi resistance we reported on the combined efficacy of entinostat and cetuximab in the A549 NSCLC xenograft model. We have followed up on those initial findings through testing in a second xenograft model (H460) and will report those results.
Conclusions: Entinostat enhances the effect of EGFR inhibitors in models of acquired EGFR resistance. The results from these experiments support the rationale of combining HDACi and EGFRi (small molecules as well as antibodies) in clinical studies in NSCLC. Ongoing clinical trials testing these hypothesis include ENCORE-401, a randomized, placebo controlled phase 2 study of erlotinib +/− entinostat in NSCLC patients and a pilot study, ENCORE-403 which was initiated to test the effect of entinostat on resistance to the EGFRi erlotinib in NSCLC.
Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 5439.