SAT-619 A Rare Case of Late Onset Type 1 Diabetes Following Immune Checkpoint Inhibitor Therapy

Abstract Disclosure: L. Abdul Jabbar: None. B. Ataallah: None. Introduction: Type 1 diabetes mellitus (T1DM) is typically diagnosed in childhood, with a late-onset diagnosis being extremely rare. However, autoimmune diabetes can occur later in life, especially in the context of dysregulation of the immune system. Immune checkpoint inhibitors (ICIs), used for malignancies like triple-negative breast cancer (TNBC), have been linked to developing T1DM. This case highlights the importance of recognizing autoimmune diabetes in middle-aged adults undergoing ICI therapy. Case Report: A 46-year-old female with a past medical history of TNBC presented to clinic with pre-diabetic symtoms and A1c of 7% that was found incidently while receiving chemotherapy and steroids. She was initially, started on basal-bolus insulin with continuous glucose monitoring (CGM). Despite steroid discontinuation, her blood glucose remained uncontrolled. This prompted further testing for T1DM which showed a low C-peptide (<0.1 ng/mL), elevated glutamic acid decarboxylase (GAD) antibodies (>250 IU/mL), and Islet antigen 2 (IA-2) antibodies (8.7 IU/mL), confirming autoimmune T1DM. The patient was also found to have pembrolizumab-induced hypothyroidism (TSH:13.70 μIU/mL). Throughout her course of treatment, no adrenal insufficiency (AI) was noted, and her blood glucose level were later controlled with an insulin pump. Discussion: Late-onset T1DM is rare and may present with prior episodes of ketoacidosis. Autoantibodies, including GAD, IA-2, and C-peptide levels, help distinguish T1DM from type 2 diabetes and steroid-induced hyperglycemia. ICIs, such as pembrolizumab, are increasingly linked to endocrine-immune-mediated adverse effects, including thyroiditis and insulin-dependent diabetes. While ICIs enhance immune responses against tumors, they may also trigger pancreatic beta-cell destruction. However, it is unclear if this new-onset diabetes is related to the use of ICI as in this case. The incidence of ICI-induced diabetes is rare (∼1%) but is a critical consideration in oncology patients. In previous studies, it was found that 21% of individuals diagnosed with late-onset T1DM had a rapid insulin requirement over the first 3 years, due to severe endogenous insulin deficiency, confirming the diagnosis of T1DM. Treatment is similar to traditional T1DM, including insulin therapy and CGM. Early identification and genetic testing may help distinguish between autoimmune-mediated diabetes and monogenic diabetes. It can also prevent complications like ketoacidosis. Thus, screening for hyperglycemia, thyroid dysfunction, and AI is a crucial part of the initial workup. Presentation: Saturday, July 12, 2025