Pembrolizumab-induced type 1 diabetes.

Introduction: Immunotherapy has a crucial role in the current treatment of multiple malignancies. Albeit described as rare, new onset autoimmune diabetes is a potentially life-threatening complication of programmed cell death-1 (PD-1) inhibitors, such as pembrolizumab, and its predisposing factors and pathological mechanism are yet to be clarified.

Case report: We present a case of a 72-year-old man with a high-grade bladder carcinoma undergoing pembrolizumab treatment. He had no personal or family history of diabetes mellitus but was diagnosed with primary hypothyroidism four months after starting pembrolizumab. Two years after starting pembrolizumab, he presented in the emergency department due to abdominal pain, anorexia, polydipsia, polyuria and vomiting over the preceding five days and he met criteria for severe diabetic ketoacidosis (DKA). Three days prior to his admission, he had received prednisolone therapy for suspected hypersensitivity related to a contrast-enhanced imaging that he performed.

Management & outcome: Prompt treatment for DKA was started, with transition to insulin basal-bolus therapy after DKA resolution, with progressive glycaemic stabilization. Further investigation revealed low C-peptide levels (0.07 ng/dL, with a fasting blood glucose of 288 mg/dL), HbA1c 9.2% and positive anti-IA2 antibodies, which allowed the diagnosis of new-onset autoimmune diabetes. Pembrolizumab was transiently suspended, and the patient resumed treatment after glycaemic profile optimization under multiple daily insulin administrations two months later.

Discussion: This case highlights the importance of clinical suspicion and glycaemic monitoring as an integral part of treatment protocols in patients on pembrolizumab and other immune checkpoint inhibitors. Additional research and investigation into the underlying mechanisms of this condition are necessary to identify potential screening tests for individuals at higher risk of developing DM and to guide the implementation of management and preventive strategies for ketoacidosis complication.