Immunotherapy-Related Autoimmune Diabetes Mellitus and Exogenous Insulin Antibody Syndrome in a Patient With Metastatic Oral Squamous Cell Carcinoma

Introduction: Exogenous insulin antibody syndrome (EIAS) is a rare condition characterized by wide glycemic excursions and recurrent hypoglycemia in the presence of high insulin antibody titers. It has been described in diabetic patients treated with exogenous insulin. Programmed death ligand 1 (PD-L1) inhibitors are known to cause autoimmune diabetes mellitus, but PD-L1-related EIAS has not yet been reported to our best knowledge. Case Description: A 63 years old Caucasian man with history of recurrent oral squamous cell cancer presented to emergency room with polyuria, polydipsia, nausea, and vomiting 3 months after initiation of immunotherapy (Durvalumab and cetuximab). He had no prior history of diabetes mellitus or hypoglycemia. He was admitted to hospital for management of diabetic ketoacidosis (Anion gap of 24 mEq/L, venous blood glucose of 805 mg/dL, Venous PH of 7.12, large urine ketone, A1C of 8.9%). After a brief hospital stay, the patient was discharged home on insulin glargine and metformin. His immunotherapy was resumed after hospital discharge. When the patient was seen by Endocrinologist in the clinic, metformin was discontinued and prandial insulin lispro was added. This basal-bolus insulin regimen improved his glycemic control initially. However, without significant changes in his lifestyle or medical condition, he developed worsening postprandial hyperglycemia and recurrent fasting hypoglycemia. Up-titration of his mealtime insulin did not lower postprandial hyperglycemia but possibly worsened fasting hypoglycemia. EIAS was suspected after reviewing his continuous glucose monitoring data. Further work up at this point revealed mildly elevated glutamic acid decarboxylase antibodies (5.9 units/mL, normal range 0.5 - 5.0) and markedly elevated insulin antibody level (77.0 µU/mL, normal range <5). His blood C-peptide was undetectable when his venous blood glucose was 252 mg/dl. In addition, his total insulin level (198 uU/mL) was much higher than his free insulin level (38 uU/mL) following an insulin lispro injection. The patient was diagnosed with EIAS. Switching insulin lispro to insulin aspart while he was on a different immunotherapy medication (Pembrolizumab) immediately reduced his average blood glucose and reduced his total daily insulin dosage by more than 50%. This improvement in glycemic control with insulin aspart only lasted for about 1 week. Unfortunately, the patient’s squamous cancer progressed on immunotherapy. He was referred to hospice care and passed away. Conclusion: Evaluation for EIAS would be reasonable in insulin-treated diabetic patients who develop wide glucose excursions and unexplained fasting hypoglycemia while on immunotherapy.