Abstract
Type 1 diabetes is a chronic autoimmune disease in which destruction of pancreatic beta-cells causes life-threatening metabolic dysregulation. Numerous approaches are envisioned for new therapies, but limitations of current clinical outcome measures are significant disincentives to development efforts. C-peptide, a direct byproduct of proinsulin processing, is a quantitative biomarker of beta-cell function that is not cleared by the liver and can be measured in the peripheral blood. Studies of quantitative measures of beta-cell function have established a predictive relationship between stimulated C-peptide as a measure of beta-cell function and clinical benefits. C-peptide levels at diagnosis are often high enough to afford glycemic control benefits associated with protection from end-organ complications of diabetes, and even lower levels offer protection from severe hypoglycemia in type 1 diabetes, as observed in large prospective cohort studies and interventional trials of islet transplantation. These observations support consideration of C-peptide not just as a biomarker of beta-cell function but also as a specific, sensitive, feasible, and clinically meaningful outcome defining beta-cell preservation or restoration for clinical trials of disease-modifying therapies. Regulatory acceptance of C-peptide as a validated surrogate for demonstration of efficacy would greatly facilitate development of disease-modifying therapies for type 1 diabetes.