First‐line osimertinib treatment in a patient with lung adenocarcinoma with coexisting epidermal growth factor receptor G719S and de novo T790M mutations

Osimertinib is the standard treatment for non‐small cell lung cancer (NSCLC) with an active epidermal growth factor receptor (EGFR) mutation and a T790M mutation—present in cases of acquired resistance. However, there have been no reports on the efficacy of osimertinib in patients with EGFR G719S and de novo T790M mutations. Here, we present the case of a 71‐year‐old woman who received first‐line osimertinib for lung adenocarcinoma with G719S and de novo T790M mutations. A partial response was observed after osimertinib initiation; however, the disease progressed 5 months after. Next‐generation sequencing using a rebiopsy sample from the brain metastases revealed no newly acquired resistance mutations, including EGFR C797S. From experience, the efficacy of osimertinib in NSCLC with G719S and T790M compound mutations may be poor. Therefore, optimal treatment for these cases should be determined.