Response to First-Line Osimertinib Treatment in Non–Small-Cell Lung Cancer With Coexisting G719A and Primary T790M Epidermal Growth Factor Receptor Mutations

Epidermal growth factor receptor (EGFR) tyrosine kinase inhibitors (TKIs) are a mainstay in the treatment of advanced unresectable non–small-cell lung cancer (NSCLC).1,2 Activating EGFR mutations are the primary predictive factors of EGFR-TKI therapy outcome.1 The most frequent and clinically significant EGFR mutations include exon 19 deletion (del) and exon 21 L858R, but in addition to these, exon 18 G719A mutations constitute a major part of uncommon mutations.3 EGFR exon 20 T790M mutations are the most frequent mechanism of acquired drug resistance to first- and second-generation EGFR-TKIs.