Onset of Type 1 Diabetes Mellitus Five Months After Pembrolizumab Therapy for Nodular Melanoma: A Case Report.

Pembrolizumab and other immune checkpoint inhibitors (ICIs) have revolutionized the treatment of melanoma. Nonetheless, they have been linked to endocrinopathies and other immune-related adverse events (irAEs). A rare but potentially fatal side effect, ICI-induced type 1 diabetes mellitus (ICI-DM), usually develops during or soon after therapy. We underscore the significance of long-term monitoring for irAEs by presenting a case of delayed-onset ICI-DM, discovered five months after stopping pembrolizumab. Our case involves a 47-year-old male diagnosed with nodular melanoma (Breslow thickness, 7 mm; mitotic index, 3), who received adjuvant pembrolizumab for 12 months. Five months after completing therapy, he presented with polyuria and polydipsia and was diagnosed with diabetic ketoacidosis (DKA). Laboratory findings included an HbA1c of 8.3%, low C-peptide (0.49 ng/mL), and negative diabetes autoantibodies. He was diagnosed with pembrolizumab-induced type 1 diabetes and managed with IV insulin in the hospital before being discharged on basal insulin (Tresiba). This case highlights the risk of delayed-onset ICI-DM, even after ICI discontinuation. Clinicians should maintain a high index of suspicion for metabolic complications in ICI-treated patients and provide lifelong monitoring for diabetes.