Pembrolizumab-Induced Checkpoint Inhibitor-Associated Diabetes Mellitus in Triple-Negative Breast Cancer: A Case Report

Immune checkpoint inhibitors (ICIs) have transformed the management of multiple malignancies but can lead to immune-related adverse events (irAEs) affecting endocrine organs. Diabetes mellitus secondary to ICIs is uncommon but potentially life-threatening. We report a case of new-onset insulin-dependent diabetes developing during pembrolizumab therapy in a patient treated for triple-negative breast cancer. The patient presented with severe hyperglycaemia and ketosis after several cycles of immunotherapy. Autoimmune diabetes antibodies were negative, and C-peptide levels indicated relative insulin deficiency. Glycaemic control remained challenging after discharge, with alternating hyperglycaemia and hypoglycaemia despite insulin therapy. This case highlights the variability in the presentation of checkpoint inhibitor-associated diabetes and the challenges in ongoing glycaemic management.