Abstract 6807: Targeting EZH2 to overcome osimertinib resistant non-small cell lung cancer

Abstract Non-Small Cell Lung Cancer (NSCLC) is a continual issue across the United States as the leading cause of cancer-related mortalities. Within NSCLC, Epidermal Growth Factor Receptor (EGFR) mutations have emerged as a main genetic driver. Treatment for EGFR mutated lung cancers has evolved over the past two decades, starting with the EGFR tyrosine kinase inhibitors gefitinib and erlotinib, and more recently moving to osimertinib. These treatments are typically successful initially, but resistance to EGFR inhibitors is inevitable for many patients. Findings ways to overcome the myriads of resistance mechanisms that can occur is the key improving treatment outcomes. Our lab studies the Polycomb Repressive Complex 2 (PRC2) that contains the subunit Enhancer of Zeste Homology 2 (EZH2). PRC2 is an epigenetic methyltransferase that facilitates histone H3 lysine 27 tri-methylation (H3K27me3) to silence gene expression. EZH2 has been implicated in many cancers may potentially drive treatment resistance. Our hypothesis is that inhibition of EZH2 will overcome osimertinib resistance by blocking the oncogenic signaling driven by the transcription factor c-MYC. To test our hypothesis, PC9, PC9GR4, and HCC4006 were all made resistant to osimertinib with IC50s of 1µM and above. Following generation of acquired resistance, we found that treatment with the EZH2 inhibitor tazemetostat (EPZ6438) was able to resensitize cell lines to osimertinib. In PC9GR4OR this re-sensitization was accompanied by increased apoptosis. We next tested two cell lines, PC9GR4OR and HCC4006OR, as in vivo xenografts, and showed that combination of osimertinib and EPZ6438 led to significant tumor regression that was not achieved by either drug alone. Next, to understand mechanisms of resistance, we performed RNA sequencing on both parental and osimertinib resistant lines treated with vehicle, EPZ6438, osimertinib, or both. GSEA analyses showed that resistance mechanisms for the three lines were not the same. PC9OR upregulated cell cycle targets and c-MYC targets, HCC4006OR upregulated oxidative phosphorylation and c-MYC targets, and PC9GR4OR had an increase in cholesterol homeostasis. RT-qPCR analyses also showed that combination treatment increased antigen presentation (HLA-A) and lowered PD-L1 (CD274). This combination of changes may produce tumors that are more susceptible to immune surveillance, or that respond better to immune checkpoint inhibitors. Overall, this work highlights the ability of inhibiting EZH2 to overcome multiple osimertinib resistance mechanisms in human lung cancer cells and suggests that this drug combination may prolong the treatment benefit for many lung cancer patients. This work was funded by: Markey Women Strong, Markey STRONG Scholars Program through the American Cancer Society IRG-22-152-34 (HY), T32 CA165990 (DRP), R01 CA237643 + Cure Supplement (CFB, CMG), R01 HL170193 (CFB), P30 CA177558 (Markey Shared Resources) Citation Format: Christian M. Gosser, Xiulong Song, Dave-Preston Esoe, Avery R. Childress, Daniel R. Plaugher, Kassandra J. Naughton, Riya J. Patel, Ryan A. Goettl, Jinpeng Liu, Christine F. Brainson. Targeting EZH2 to overcome osimertinib resistant non-small cell lung cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 1 (Regular Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_1):Abstract nr 6807.