Gefitinib response after acquisition of C797S following first-line osimertinib in EGFR L858R/E709K-mutated non-small cell lung cancer: A case report.

Osimertinib is the standard first-line therapy for epidermal growth factor receptor (EGFR)-mutated non-small cell lung cancer, but acquired resistance remains a major clinical issue. Here, we report a case of lung adenocarcinoma harboring compound EGFR L858R and E709K mutations that acquired C797S after progression on first-line osimertinib. Gefitinib achieved a durable partial response lasting approximately 15 months, despite E709K generally conferring reduced sensitivity to first-generation EGFR-tyrosine kinase inhibitors. This case highlights the importance of repeat molecular profiling and suggests gefitinib as a potential salvage option in selected C797S-positive cases.