Temporal observation of endoscopic and histological findings of gastritis after administration of an immune checkpoint inhibitor: a case report.

A 71-year-old Japanese man was treated with 200 mg of pembrolizumab for lung adenocarcinoma with multiple bone metastases at the Department of Respiratory Medicine of Kameda General Hospital. After 19 treatment courses, he complained of epigastric pain before meals. Upper gastrointestinal endoscopy showed multiple erosions in the gastric antrum, and antacids were administered at follow-up. After 27 treatment courses, the patient underwent another endoscopy because of anorexia. The erosions were enlarged and had increased from the gastric antrum to the greater curvature of the body. Histological biopsy showed lymphocytic infiltration with a predominance of CD8-positive T cells. The patient had previously been treated for Helicobacter pylori infection, and we suspected drug-induced gastritis due to the administration of immune checkpoint inhibitors in the course of the disease. Pembrolizumab was discontinued, and the patient's symptoms gradually improved. Endoscopic examinations were performed 2, 5, and 9 months after discontinuation of pembrolizumab, and improvement in mucosal findings and decreased lymphocyte infiltration were confirmed each time. The patient has remained without any relapse of symptoms for more than 1 year after discontinuing treatment.