A Novel Acquired Exon 20 EGFR M766Q Mutation in Lung Adenocarcinoma Mediates Osimertinib Resistance but is Sensitive to Neratinib and Poziotinib.

Introduction: Osimertinib is an effective third-generation tyrosine kinase inhibitor (TKI) for EGFR-mutant lung cancers. However, treatment for patients with acquired resistance to osimertinib remains challenging. We characterized a novel EGFR mutation in exon 20 that was acquired while on osimertinib.

Methods: A 79-year-old woman had disease progression during third-line treatment with osimertinib for an EGFR L858R/T790M-mutant lung cancer. Sequencing of circulating cell-free DNA showed EGFR L858R, an acquired novel EGFR M766Q mutation in exon 20, and no evidence of EGFR T790M. Homology modeling was performed to investigate the effects of M766Q on binding to osimertinib. L858R and L858R/M766Q mutations were retrovirally introduced into Ba/F3 and NIH/3T3 cells and evaluated for sensitivity to first-generation (erlotinib), second-generation (afatinib, neratinib, and poziotinib), and third-generation TKIs (osimertinib) by cell viability and colony-formation assays. EGFR-mediated signaling pathways were interrogated by western blotting.

Results: Modeling suggested that EGFR M766Q could disrupt osimertinib binding. L858R/M766Q double-mutant cells were 12-fold more resistant to osimertinib, and more than 250-fold more resistant to erlotinib and afatinib, as compared to L858R-mutant cells. In contrast, double-mutant cells remained sensitive to neratinib and poziotinib at clinically relevant doses (concentration that inhibits 50%, 4.3 and 1.3 nM, respectively). This was corroborated by the effects of the TKIs on colony formation and EGFR signaling.

Conclusions: Acquisition of EGFR M766Q exon 20 mutation is a novel mechanism of acquired resistance to osimertinib. EGFR-mutant lung cancers with an acquired EGFR M766Q mutation in the setting of osimertinib resistance may be sensitive to neratinib and poziotinib.