The Effect of Sacubitril/Valsartan on Urinary C-Peptide Excretion and Endogenous Insulin Secretory Capacity in a Patient with Type 2 Diabetes: A Case Report

Background The evaluation of endogenous insulin secretory capacity is important in the selection of diabetes treatment. C-peptide, which is secreted in equivalent amounts as insulin, is a versatile test for this evaluation. Urinary C-peptide is widely used because it is less invasive. Sacubitril/valsartan, used to treat hypertension and chronic heart failure, has been reported to increase urinary C-peptide levels; however, its effect on endogenous insulin secretion remains unknown. In this report, we present a case in which insulin secretory capacity was evaluated in addition to urinary C-peptide levels in a patient receiving sacubitril/valsartan. Case presentation A male patient in his 50s with type 2 diabetes and hypertension, without renal dysfunction, was treated with sacubitril/valsartan. The results of the glucagon stimulation test showed a C-peptide change of 2.28, and the C-peptide index on the same day was 1.25, indicating normal endogenous insulin secretory capacity. In contrast, 24-h urinary C-peptide excretion was abnormally high at 615.2 µg/day. After discontinuation of sacubitril/valsartan, urinary C-peptide excretion decreased over time (615.2 to 369.0 µg/day), but blood glucose levels did not increase during this period. Conclusions Twenty-four-hour urinary C-peptide excretion was abnormally high during and immediately after discontinuation of sacubitril/valsartan and did not correlate with endogenous insulin secretory capacity. Twenty-four-hour urinary C-peptide excretion may overestimate endogenous insulin secretory capacity and interfere with appropriate treatment selection. In patients taking sacubitril/valsartan, other endogenous insulin secretory capacities should be assessed, such as a glucagon stimulation test rather than 24-h urinary C-peptide excretion.