Epidermal growth factor receptor (EGFR) T790M mutations are the most frequent mechanism of resistance to first- and second-generation tyrosine kinase inhibitors, and osimertinib is an effective treatment for patients with both EGFR-activating mutations and T790M resistance mutations. We describe the case of a 68-year-old woman with lung adenocarcinoma with G719S, S768I, and T790M mutations in which osimertinib treatment was unsuccessful. The patient died of disease progression one month after discontinuing osimertinib treatment. This case suggests that osimertinib may be ineffective for treating patients with uncommon mutations such as G719S when the patient has also acquired a T790M resistance mutation.
Keywords: EGFR exon 18G719S; S768I; T790M; osimertinib; uncommon mutation.