A Journey of Diagnosing a Case of Immune Checkpoint Inhibitor Pembrolizumab-Induced Diabetes Mellitus.

Autoimmune diabetes mellitus (DM) due to pembrolizumab is a rare but reported complication of immune checkpoint inhibitors (ICIs). It is often missed for a long initial period, leading to unnecessary admissions and poor glucose management. We report a case of a 72-year-old woman with prior history of gastrointestinal stromal tumor (GIST) and current diagnosis of squamous cell carcinoma (SCC) of the lung, who upon presentation at the emergency department with symptoms of encephalopathy, was diagnosed as having sodium glucose cotransporter 2 inhibitor (SGLT-2i)-induced diabetic ketoacidosis (DKA). Upon further investigation, we learned that this patient had multiple hospitalizations for recurrent DKA over 2 years after being managed on metformin and SGLT-2i. Biochemical testing helped confirm pembrolizumab-induced autoimmune diabetes with significantly elevated glutamic acid decarboxylase-65 (GAD65) autoantibodies and an undetectable C-peptide level. The patient has had clinical improvement with insulin therapy without further DKA episodes. She continued to be managed by outpatient endocrinology with improved glucose control. Altogether, this case demonstrates the importance of keeping a high vigilance for possible new onset of autoimmune endocrine diseases, such as diabetes, in patients treated with ICIs, to enable earlier diagnosis and prompt initiation of correct therapy.