SUITO index for evaluation of clinical islet transplantation

Abstract

The major endpoints for clinical islet transplantation for type 1 diabetes are insulin independence and reduction of hypoglycemic episodes. Both endpoints are influenced by patients' and physicians' preferences regarding the use of exogenous insulin. Therefore, development of an objective endpoint for assessing clinical islet transplantation is desirable. HOMA-beta score is useful in assessing functional β-cell mass. However, this score uses blood insulin levels that are influenced by exogenous insulin injection and therefore is not suitable for patients who receive exogenous insulin. For assessing functional β-cell mass for type 1 diabetic patients after islet transplantation, we created the Secretory Unit of Islet Transplant Objects (SUITO) index using fasting C-peptide and fasting glucose. The formula of the SUITO index is fasting C-peptide (ng/ml)/[fasting blood glucose − 63 (mg/dl)] × 1500. We demonstrated that, within 1 month of islet transplantation, an average SUITO index of >26 was an excellent predictor of achieving insulin independence. In addition, daily SUITO index scores correlated with a reduction of insulin dose and adversely correlated with blood glucose levels during an intravenous glucose tolerance test. Other important endpoints, reduction of hypoglycemic episodes and quality of life, also correlated with the SUITO index. Thus, the SUITO index is excellent for assessing important endpoints (insulin independence, reduction of hypoglycemia, improved quality of life) after allogeneic islet transplantation.

Figure 1

Relationship between fasting blood glucose levels and fasting C-peptide levels before and after islet transplantation. The angle of the regression line reflects functional islet mass.

Figure 2

The glucose disappearance rate and β-cell secretory capacity in IVGTT. The glucose disappearance rates (K-values) were assessed with SUITO index (A). The β-cell secretory capacity was evaluated by C-peptide responses calculated by the stimulated C-peptide levels minus the basal C-peptide levels in IVGTT (B). Both K-values and C-peptide responses were significantly correlated with the SUITO index (Spearman r = 0.571, p = 0.01 and r = 0.581, p = 0.01, respectively). The solid line shows a linear regression. IVGTT, intravenous glucose tolerance test; SUITO index, secretory unit of islet transplant objects index.