LAUSR

Introduction: Gefitinib is an epidermal growth factor receptor tyrosine kinase inhibitor (EGFR TKI) indicated for first-line treatment of advanced non-small cell lung cancer (NSCLC) bearing the sensitizing EGFR mutations. However, drug resistance is severely hindering its clinical efficacy. This study investigated the circumvention of gefitinib resistance by repurposing a non-oncology drug with a putative histone deacetylase (HDAC) inhibitory effect. Materials and Methods: A few clinically-approved drugs were identified by a computational drug repurposing tool called “DRUGSURV” and evaluated for HDAC inhibition. Flunarizine, indicated initially for migraine prophylaxis and vertigo treatment, was chosen for further investigation in several NSCLC cell lines exhibiting different gefitinib resistance mechanisms (H1975 - EGFR T790M; A549 - KRAS G12S; H1650 - PTEN loss; H1993 - MET amplification). Sulforhodamine B assay evaluated cell proliferation. Western blot analysis examined histone acetylation (HAC) and activation of EGFR downstream signaling. Flow cytometric assays investigated apoptosis and cell cycle effects. Chromatin immunoprecipitation explored the interaction of transcription factors to the promoter of the pro-apoptotic gene BIM. The circumvention of gefitinib resistance by flunarizine was further verified in a patient-derived tumor xenograft (PDX) from an EGFR TKI-refractory advanced-stage NSCLC patient. Results: Flunarizine was found to increase HAC in a dose- and time-dependent manner. Among the gefitinib-resistant NSCLC cell lines tested, flunarizine was shown to potentiate the anticancer effect of gefitinib the most in H1975 cells (harboring EGFR T790M). The apoptotic protein Bim was induced but the anti-apoptotic protein Bcl-2 was decreased by the gefitinib-flunarizine combination, presumably contributing to the circumvention of gefitinib resistance. Upregulation of Bim by flunarizine was associated with increased HAC and E2F1 binding to the BIM gene promoter. HDAC inhibitors (HDACIs) are known to exhibit pleiotropic cellular effects via histone-independent mechanisms. To this end, unlike the control HDACI vorinostat, flunarizine (tested at the resistance circumventing concentrations) did not appreciably affect acetylation of representative non-histone proteins (including α-tubulin and p53) relevant to drug resistance. Besides, flunarizine was found to increase E-cadherin but reduce vimentin expression which was associated with the inhibition of cancer cell migration and invasion. Flunarizine was further shown to significantly potentiate the antitumor effect of gefitinib in EGFR TKI-refractory PDX in vivo, but without any notable animal body weight loss during the treatment course. Conclusion: The findings advocate further clinical evaluation of the repurposing use of flunarizine to overcome gefitinib resistance. Citation Format: Kenneth K. To, William C. Cho. Circumvention of gefitinib resistance by repurposing flunarizine via histone deacetylase inhibition in lung cancer [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 379.