A case of osimertinib-resistant lung adenocarcinoma responded effectively to alternating therapy

We report a case of initial lung adenocarcinoma in which transformation to small cell lung carcinoma (SCLC) was observed after acquired resistance to the 3^rd generation epidermal growth factor receptor tyrosine kinase inhibitor (EGFR-TKI) osimertinib and alternating treatment between chemotherapy and osimertinib was effective. A 61-year-old woman with EGFR mutation positive stage IV lung adenocarcinoma was administered 1^st generation EGFR-TKI for 8 months as the first line therapy, then chemotherapy and 2^nd generation EGFR-TKI after progressive disease (PD). Four years after initial diagnosis, EGFR T790M was detected in a metastatic lesion of the right thoracic wall and osimertinib was prescribed. Although partial response (PR) was achieved, a new metastatic lesion appeared in the right pleurum near the diaphragm, in which SCLC characteristics were observed with elevation of pro-gastrin-releasing peptide (pro-GRP) at the time of PD under osimertinib. Osimertinib was discontinued and carboplatin plus irinotecan chemotherapy was chosen as the next treatment, leading to PR after 2 cycles. Subsequently, the right thoracic wall tumor harboring T790M and the right pleural tumor near the diaphragm showing transformation to SCLC exhibited opposite responses to therapy alternating between osimertinib and chemotherapy. It is concluded that extended disease control can be achieved by combining appropriate treatments according to the mechanisms of resistance inferred from precise genetic and pathological examination in real time.