Abstract
The non-small cell lung cancer with activating epidermal growth factor receptor (EGFR) mutation eventually acquires resistant to either first or second-generation EGFR tyrosine kinase inhibitor (TKI). As the following option, targeting EGFR T790M with third-generation EGFR TKI is now established as a standard treatment option. In this study, we are reporting the first case of resistance mechanism to the novel third-generation EGFR TKI, lazertinib, which showed promising clinical efficacy in phase 1-2 study. The patients showed resistance to the treatment by acquiring the additional EGFR C797S mutation in cis which is also confirmed from the patient-derived cell lines.