In this case, the endogenous insulin secretion capacity was assessed not only by serum C-peptide but also by ΔCPR through a glucagon stimulation test, which more accurately confirmed that the endogenous insulin secretion capacity was normal. While ΔCPR and CPI remained within normal ranges, U-CPR was the only parameter that showed an abnormally high value. Despite this elevation, endogenous insulin secretion was preserved based on the results of the glucagon stimulation test. Interestingly, U-CPR levels gradually decreased after discontinuation of sacubitril/valsartan; however, blood glucose levels remained stable even after insulin therapy was stopped. This suggests that the decrease in U-CPR levels did not reflect a deterioration in endogenous insulin secretion. Instead, stable glycemic control was likely due to the resolution of glucotoxicity achieved through prior insulin therapy.

We reviewed previously reported cases in which the U-CPR levels increased during sacubitril/valsartan therapy [5,6,7, 9, 10]. Including the present case, Table 2 summarizes six such reports, detailing the number of cases, sacubitril/valsartan dose, changes in U‑CPR, other evaluation methods, and renal function. While many studies have assessed insulin secretory capacity using static indices such as serum C-peptide or CPI, few studies have additionally incorporated dynamic assessments such as the meal tolerance test (MMT). These dynamic approaches may provide supportive and potentially more reliable insights, particularly in situations where U‑CPR alone is difficult to interpret.

Among these methods, the glucagon stimulation test offers certain advantages. Specifically, it is simple to perform, can be completed in a short time, and exhibits good reproducibility [8]. Unlike the MMT, the glucagon stimulation test is less affected by individual variations in gastric emptying or nutrient absorption and allows for standardized, glucose-independent evaluation of pancreatic β-cell responsiveness [8]. Additionally, ΔCPR has been reported to correlate more strongly with pancreatic β-cell mass than do other markers such as fasting serum C-peptide, CPI, or immunoreactive insulin (IRI) [11].

A recent case report by Wang et al. [9] described a patient with type 2 diabetes who exhibited markedly elevated urinary C-peptide levels during sacubitril/valsartan therapy, including a rebound increase after rechallenge. While their report provided valuable observations, the assessment of insulin secretory capacity was based primarily on static markers such as serum CPR and CPI, with the glucagon stimulation test performed only after drug discontinuation. Urinary C-peptide levels in their case were calculated using spot urine samples that may be affected by hydration status or diurnal variation. In contrast, our case incorporated dynamic assessment using the glucagon stimulation test during sacubitril/valsartan administration and consistently used 24-h urine collections for U‑CPR throughout the study period. This methodological consistency strengthens the reliability of our findings and allows for a more physiologically integrated interpretation of insulin secretory capacity under the pharmacological influence of sacubitril/valsartan therapy.

A recent study by Haraguchi et al. highlighted that the glucagon-stimulated ΔCPR was significantly reduced in patients receiving GLP-1 receptor agonists, suggesting that incretin-based therapies may blunt the insulinotropic response to pharmacological stimulation and interfere with the accurate assessment of β-cell function [12]. Accordingly, the glucagon stimulation test may not be appropriate for evaluating the insulin secretory capacity in patients currently receiving GLP-1 receptor agonists. In the present case, the glucagon stimulation test was conducted without concurrent GLP-1 receptor agonist use and renal function was within the normal range. These conditions likely contributed to a more reliable assessment of the endogenous insulin secretory capacity under relatively controlled physiological conditions.

This table summarizes published case reports in which urinary C-peptide excretion (U-CPR) increased during sacubitril/valsartan treatment. For each study, the number of reported cases, sacubitril/valsartan dose, changes in U-CPR (before and after discontinuation of sacubitril/valsartan), other markers used to evaluate the insulin secretory capacity, and estimated glomerular filtration rate (eGFR) are listed. When multiple cases were presented, only one representative case with detailed information was included. “Not reported” indicates that the information was not provided in the original publication, and “described as > 30 mL/min/1.73 m²” reflects qualitative descriptions without specific values. The term “on ARNI” denotes measurements obtained during therapy with sacubitril/valsartan, a representative angiotensin receptor–neprilysin inhibitor (ARNI), whereas “off ARNI” denotes measurements obtained after discontinuation of the drug. Creatinine-corrected U-CPR values (µg/g·Cr) are marked with an asterisk (*). Abbreviations: CPR = C-peptide immunoreactivity; CPI = C-peptide index; IRI = immunoreactive insulin; MMT = meal tolerance test; ΔCPR = change in serum C-peptide after glucagon stimulation test; eGFR = estimated glomerular filtration rate; ARNI = angiotensin receptor neprilysin inhibitor.

Although many reports have focused on the elevation of urinary C-peptide levels associated with sacubitril/valsartan, Ishibashi et al. indicated that serum C-peptide levels and CPI can be significantly elevated after sacubitril/valsartan administration, even in the absence of changes in IRI or glucose levels [13]. This suggests that neprilysin inhibition may lead to an overestimation of the insulin secretory capacity when assessed using static markers. Compared to IRI, the glucagon stimulation test-derived ΔCPR offers several advantages in evaluating endogenous insulin secretion. As previously noted, C-peptide is not present in exogenous insulin preparations and is more metabolically stable with a longer half-life and reduced variability [1]. Moreover, ΔCPR reflects stimulated insulin secretion with relatively low intra-individual variability and is less affected by transient factors such as stress or recent food intake [14]. These characteristics suggest that ΔCPR may serve as a more reliable and specific indicator of β-cell function, particularly in patients treated with sacubitril/valsartan, as the glucagon stimulation test directly assesses β-cell responsiveness to pharmacological stimulation and is less influenced by alterations in C-peptide metabolism. This may provide a more physiologically valid estimate of endogenous insulin secretion in the clinical context. However, a potential limitation of this approach should be noted. Neprilysin inhibition has been reported to impair glucagon degradation, leading to an increase in circulating glucagon level [15]. Although the clinical relevance of this effect remains uncertain, it may influence the results of glucagon stimulation tests in patients receiving sacubitril/valsartan. In the present case, while the impact of sacubitril/valsartan on ΔCPR is uncertain, the observed ΔCPR response appeared to be consistent with the actual clinical course. The patient was able to discontinue insulin therapy without deterioration in glycemic control, possibly indicating preservation of endogenous insulin secretion. The potential effects of neprilysin inhibition warrant further investigation.

Although the precise mechanism remains unclear, it is hypothesized that neprilysin inhibition by sacubitrilat may impair the renal degradation of C-peptide, leading to increased urinary excretion. In our literature review, although direct evidence that the C-peptide is a substrate of neprilysin was lacking [16, 17], it was noted that the C-peptide shares several structural features with known neprilysin substrates [16,17,18]. The persistently elevated U-CPR level after discontinuation may be explained by delayed recovery of neprilysin activity, possibly due to tissue-level accumulation of sacubitrilat or the preferential degradation of higher-affinity substrates during enzymatic recovery. However, further mechanistic studies are required to elucidate this interaction.

A limitation of our study was the lack of baseline U-CPR data prior to sacubitril/valsartan initiation. This restricted our ability to definitively attribute elevated U-CPR levels to the drug. Moreover, post-discontinuation insulin secretory capacity was not reassessed by dynamic testing, and this may have limited the interpretation of U-CPR changes over time.

In this case, U-CPR was observed to be disproportionately elevated despite normal endogenous insulin secretory capacity based on ΔCPR. This case provides insights into the interpretation of insulin secretion markers and may be employed in similar situations.