Acquired Resistance to Afatinib Mediated by EGFR T790M in Lung Adenocarcinoma Patients Harboring EGFR-KDD: A Case Report and Literature Review.

Epidermal growth factor receptor (EGFR)-kinase domain duplication (KDD) represents an atypical mutation distinct from classical EGFR mutations in lung adenocarcinoma (LUAD). Although first- and second-generation EGFR-tyrosine kinase inhibitors (TKIs) have demonstrated clinical activity in EGFR-KDD, the mechanisms of acquired resistance in this setting remain poorly characterized. Herein, we present a LUAD patient with EGFR-KDD who achieved a sustained initial response to afatinib lasting 67 months before developing acquired resistance. Re-biopsy with next-generation sequencing (NGS) uncovered EGFR T790M, accompanied by EGFR amplification and EGFR M766T. The patient was switched to firmonertinib, with subsequent tumor regression. We reviewed published EGFR-KDD cases that had both acquired resistance to first- or second-generation EGFR-TKIs and corresponding repeat biopsy findings. Five of the eleven cases harbored EGFR T790M, yielding a prevalence of 45%, which is similar to that seen in classical EGFR mutations. This case suggests that EGFR T790M mediates acquired resistance to first- and second-generation EGFR-TKIs in EGFR-KDD, mirroring the resistance pattern observed in classical EGFR mutations.