A case of pembrolizumab-induced fulminant Type 1 diabetes mellitus in breast cancer.

Aim: The programmed cell death protein-1 (PD-1) inhibitor, pembrolizumab, can exert anti-tumor effects and induce immune-related adverse events. Here, we for the first time describe a patient with metastatic breast cancer who developed fulminant Type 1 diabetes mellitus (FT1DM) during pembrolizumab treatment. Case presentation: A 61-year-old woman received seven cycles of postoperative pembrolizumab combined with tegafur treatment, and developed sudden thirst, polyuria, polydipsia and weight loss. Her laboratory tests contributed to the diagnosis of diabetes ketoacidosis. Her fasting and 2-h postprandial C-peptide levels were both below 0.05 ng/ml, further supporting the diagnosis of FT1DM associated with pembrolizumab administration. Conclusion: Our report highlights the significance of autoimmune diabetes as a rapid and serious adverse event induced by PD-1 inhibitor therapies.