Evidence for C-Peptide as a Validated Surrogate to Predict Clinical Benefits in Trials of Disease-Modifying Therapies for Type 1 Diabetes

Type 1 diabetes is a chronic autoimmune disease in which destruction of pancreatic β-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 β-cell function that is not cleared by the liver and can be measured in the peripheral blood. Studies of quantitative measures of β-cell function have established a predictive relationship between stimulated C-peptide as a measure of β-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 β-cell function but also as a specific, sensitive, feasible, and clinically meaningful outcome defining β-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.

• Mixed-meal stimulated C-peptide level has been established as a biomarker of endogenous β-cell function and is used clinically to monitor disease progression in type 1 diabetes.

• Large prospective cohort studies and interventional trials of islet transplantation demonstrate the relationship between stimulated C-peptide levels and clinical benefits.

• A recent meta-analysis of interventional clinical trials aimed at preserving β-cell function in those recently diagnosed further supports stimulated C-peptide level as a validated surrogate end point for clinical trials of disease-modifying therapies in type 1 diabetes.