Pembrolizumab-Induced Insulin-Dependent Diabetes Mellitus in a Patient With Triple-Negative Breast Cancer: A Rare Immune-Related Adverse Event

A 68-year-old female patient with a background of triple-negative breast carcinoma on pembrolizumab with no history of diabetes presented to the emergency department with fatigue, polyuria, nausea, dizziness, shortness of breath, dry mouth, and increased thirst. She had recently received the third dose of the second cycle of neoadjuvant combination chemotherapy and immunotherapy (pembrolizumab/carboplatin/paclitaxel) and was due to receive the next dose. Initial assessment revealed hyperglycemia with ketosis without acidosis. The patient was treated with fluid resuscitation and insulin infusion under the diabetic ketoacidosis (DKA) guidelines of the hospital and was eventually transitioned to a basal-bolus insulin regimen, which was continued after discharge. Based on the temporal relationship between pembrolizumab therapy and the onset of diabetes, along with the patient's persistent insulin dependence, a diagnosis of immune checkpoint inhibitor-induced diabetes mellitus (ICI-DM) was established. The patient has clinically improved, chemotherapy and immunotherapy have been discontinued, and surgical intervention is planned. This case highlights the importance of recognizing ICI-DM as a rare immune-related adverse event in patients who receive immunotherapy with programmed cell death protein-1 (PD-1)/programmed cell death ligand-1 (PD-L1) inhibitors.