Abstract
Osimertinib, a third-generation epidermal growth factor receptor (EGFR) tyrosine kinase inhibitor (TKI), shows great clinical activity in non-small cell lung cancer (NSCLC) patients with EGFR mutations regardless of T790M mutation at first-line chemotherapy. Previous studies demonstrated that there are few patients with initial resistance to osimertinib. Here, we describe a case to report the efficacy of afatinib in an EGFR-mutated NSCLC patient with early progression to first-line osimertinib treatment. A 68-year-old Japanese male was diagnosed with stage IVB lung adenocarcinoma with the EGFR L858R mutation in exon 21. Two months after the start of osimertinib, his tumor progressed at the initial response evaluation. Because he refused to receive cytotoxic chemotherapy, afatinib treatment was initiated. He was administered afatinib, and the tumor shrank. After five months of afatinib treatment, nevertheless the primary tumor was not enlarged, he experienced disease progression with leptomeningeal metastasis and passed away. To elucidate the resistance mechanisms of osimertinib in this patient, we performed next-generation sequencing (NGS) on tumor samples from pleural effusions after osimertinib failure. NGS revealed no specific gene mutations causing resistance to osimertinib except for the EGFR L858R mutation; however, his tumor had a relatively high tumor mutational burden. Afatinib is considered an option for EGFR-mutated patients with early progression to osimertinib.