Entrectinib‐induced syndrome of inappropriate antidiuretic hormone secretion in a patient with ROS1‐rearranged non‐small cell lung cancer

A 75‐year‐old woman was referred to our hospital because of a productive cough and an abnormal shadow on chest radiography. She was diagnosed as having metastatic lung adenocarcinoma harbouring ROS proto‐oncogene 1 (ROS1). First‐line therapy was instituted with entrectinib 600 mg daily, and a gradual decrease in serum sodium level was noticed on day 6, which deteriorated to Grade 3 hyponatremia on day 12. Despite a partial therapeutic response to entrectinib, she developed fatigue and dizziness, so the drug was withdrawn. The clinical findings and laboratory workup were compatible with a diagnosis of syndrome of inappropriate antidiuretic hormone secretion (SIADH) due to entrectinib. The hyponatremia subsequently improved and entrectinib was resumed at a reduced dose of 400 mg daily, which has been continued to date, with no recurrence of SIADH.