Abstract 2230: Entinostat potentiates chemotherapeutic efficacy in small cell lung cancer cells

Small cell lung cancer (SCLC) represents for 15% of all lung cancers. Although SCLC is initially highly sensitive to chemotherapy (CHT), almost all patients acquire resistance within the course of the disease. A promising class of compounds to overcome drug resistance comprises histone deacetylase inhibitors (HDACi), which have been shown to synergize with several CHT agents such as cisplatin. In this study, we addressed whether entinostat exhibits anti-cancer activity as single-therapy or in combination with CHT in SCLC. The efficacy of entinostat alone or in combination with CHT was determined using MTT-based viability assays and a mouse xenograft model. Synergistic drug interactions were evaluated using the combenefit software. Analysis of our previously published proteomic dataset comprised 1D annotation enrichment, DAVID functional annotation and ToppCluster analysis. Hypothetic mechanisms were also examined using ICP-MS and FACS-based cell cycle analysis. Class I HDAC inhibition by entinostat resulted in significantly stronger effects in neuroendocrine (NE) SCLC cell lines. Subsequent proteomic analysis revealed differentially expressed proteins and pathways that coincide with NE and non-NE characteristics. Since “platinum drug resistance” appeared in the corresponding pathway analysis, cell lines were exposed to cisplatin. A significant correlation was observed between entinostat and cisplatin responsiveness (r=0.8079, p=0.0003). With combination therapy, a strong synergism was detected in a subset of SCLC cell lines and validated in a mouse model. Importantly, similar effects were also found when entinostat was combined with etoposide, epirubicin or irinotecan. To address the underlying mechanism of the observed effects, another proteomic evaluation comparing cell lines with synergistic and additive features indicated changes in cell cycle regulation and DNA damage repair. Cell cycle analysis revealed similar distributions in control, entinostat and cisplatin conditions. However, synergistic cell lines displayed a shift from G1 to S-phase compared to additive cell lines upon combination treatment. Results from ICP-MS analysis revealed that although basal platinum levels were significantly higher in the synergistic subgroup, all SCLC cell lines showed increased intracellular platinum levels in the presence of entinostat. Here, we report that high NE expression profiles in SCLC cell lines are associated with increased sensitivity to HDAC inhibition and, moreover, that combinational therapy using entinostat and CHT results in strong synergism in a subset of SCLC cell lines featuring dysregulated cell cycle and DNA damage repair, according to proteomic analyses. Especially in SCLC, efficient therapeutic options in relapsed patients are missing, and deciphering the molecular basis of synergism between chemotherapy and entinostat might lead to more effective therapy for patients. Citation Format: Anna Schwendenwein, Kristiina Boettiger, Ildiko Kovacs, Nandor Barany, Christian Lang, Zsolt Megyesfalvi, Michael Grusch, Walter Berger, Christian Kowol, Melinda Rezeli, Konrad Hoetzenecker, Balazs Döme, Karin Schelch. Entinostat potentiates chemotherapeutic efficacy in small cell lung cancer cells [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 2230.