Abstract
Acquired resistance inevitably occurs after initial treatment with first-generation EGFR-tyrosine kinase inhibitors (TKIs). Several mechanisms have been identified, including EGFR T790M mutation and HER2 amplification. Herein, we present the case of a patient who progressed on first-generation EGFR-TKIs and developed EGFR T790M mutation, HER2 amplification, and HER2 mutation. The administration of single-agent osimertinib yielded an inconsistent response, with worsened pleural effusion and a reduction to lung metastases, but remarkably, a partial response was achieved after four weeks of treatment with combined osimertinib and afatinib, with grade 1 rash and grade 2 diarrhea. Our findings indicate an overlap of T790M, HER2 amplification, and HER2 mutation, which is rarely reported. Moreover, HER2 mutation was identified during the development of resistance, suggesting that HER2 mutation may cause resistance to first-generation EGFR-TKIs.