Repurposing of abemaciclib to overcome C797S-mediated osimertinib resistance in EGFR-mutant lung adenocarcinoma

Background Acquired resistance to osimertinib mediated by the EGFR C797S mutation remains a major therapeutic challenge in EGFR-mutant non–small cell lung cancer (NSCLC), with limited effective treatment options. This study explored whether abemaciclib, an FDA-approved CDK4/6 inhibitor, can overcome C797S-mediated osimertinib resistance in EGFR-mutant NSCLC. Methods A high-throughput screen of approximately 125,000 compounds was performed using Ba/F3 cells expressing EGFR triple mutations (L858R/T790M/C797S and Del19/T790M/C797S). The impact of abemaciclib on EGFR kinase activity was evaluated via in vitro kinase and Western blot assays. Apoptosis induction was assessed using flow cytometry. The activity of abemaciclib was evaluated using biochemical assays, cellular thermal shift assays (CETSA), molecular docking analyses, CRISPR-engineered isogenic models, xenografts, and patient-derived xenograft (PDX) models. Results Abemaciclib was identified as a potent inhibitor of EGFR triple-mutant cell growth and exhibited preferential activity toward mutant EGFR-dependent models. IL-3 rescue assays demonstrated that its antiproliferative effects were dependent on suppression of oncogenic EGFR signaling. Abemaciclib inhibited mutant EGFR kinase activity, reduced EGFR phosphorylation, and induced apoptotic cell death in osimertinib-resistant models while exhibiting substantially weaker activity toward wild-type EGFR. CETSA and molecular docking analyses supported direct engagement of mutant EGFR by abemaciclib. Importantly, abemaciclib significantly suppressed tumor growth in CRISPR-engineered isogenic xenografts and PDX models harboring EGFR C797S-mediated osimertinib resistance. Conclusions Abemaciclib selectively suppresses C797S-mutant EGFR signaling and demonstrates robust antitumor activity in preclinical models of osimertinib-resistant EGFR-mutant NSCLC. This breakthrough not only overcomes the existing therapeutic barrier but also demonstrates minimal interference with WT EGFR, offering a transformative and precise therapeutic strategy with significant translational and clinical potential. These findings support the therapeutic repurposing of abemaciclib as a potential treatment strategy for EGFR C797S-mediated resistance and provide a translational strategy for overcoming resistance-associated EGFR alterations using clinically available targeted agents.