Afatinib Overcomes Osimertinib Resistance via Egfr V804F Mutation in a Syngeneic Egfr-Mutant Lung Cancer Mouse Model

Osimertinib is the standard first-line treatment for advanced EGFR-mutant non-small cell lung cancer. However, acquired resistance invariably develops, and identifying novel resistance pathways is clinically important as a substantial portion remains undefined. We used an immunocompetent syngeneic lung cancer mouse model harboring an Egfr exon 19 deletion (mDEL tumors) to establish acquired resistance. Osimertinib-resistant tumors were generated in vivo using a drug-holiday/re-challenge protocol. The resistance mechanism was identified using Sanger sequencing, receptor tyrosine kinase arrays, and western blotting. Egfr V804F knock-in cells were generated using CRISPR/Cas9, and their sensitivity to osimertinib and afatinib was assessed in vitro and in vivo. We established two distinct osimertinib-resistant tumor lines in a syngeneic lung cancer mouse model (mDEL OsiR #1/#3). The analysis revealed an on-target secondary Egfr V804F mutation (corresponding to human EGFR V802F) in both tumor lines. Importantly, Egfr V804F knock-in cells demonstrated significant osimertinib resistance, with a 5.7-10.5-fold increase in in vitro IC₅₀ (mean, 8.2-fold), but remained highly sensitive to afatinib. Furthermore, afatinib effectively overcame osimertinib resistance in the V804F knock-in cell-derived mouse model. Consistently, afatinib treatment resulted in marked tumor shrinkage and suppression of EGFR signaling in the established mDEL OsiR #1/#3 in vivo. These findings establish secondary Egfr V804F/EGFR V802F as an on-target osimertinib resistance mechanism, providing a preclinical rationale for evaluating afatinib in biomarker-selected patients harboring this alteration.