This study demonstrated that alogliptin treatment significantly reduced postprandial levels of intestinally derived apo-B48–containing lipoproteins, which were induced by a conventional oral cookie loading test (28.5 g fat per person), and that alogliptin improved postprandial lipemia-induced endothelial dysfunction. Considering the significant association between the beneficial change in endothelial dysfunction and the decrease in TG-rich lipoproteins, cardiovascular risk, especially associated with postprandial lipemia, may be reduced with long-term treatment with alogliptin.
Our study with the oral cookie test showed that a greater increase in TG-rich lipoprotein, but not glucose, was correlated with postprandial FMD impairment in healthy volunteers. This study did not include people with impaired glucose tolerance and dyslipidemia. Our finding suggests that the impairment in endothelial dysfunction induced by postprandial lipemia is more common than that induced by postprandial hyperglycemia in the general population. As reported in other studies, postprandial hyperglycemia induces endothelial dysfunction, especially in patients with diabetes mellitus or glucose intolerance. In these patients, an increase in glucose is also associated with postprandial endothelial dysfunction . Furthermore, patients with diabetes mellitus often show dyslipidemia including postprandial hyperlipemia. Therefore, our results suggest that alogliptin therapy possibly improves glucose metabolism as well as postprandial hyperlipemia and postprandial endothelial dysfunction in patients with diabetes mellitus.
Previous studies showed that DPP-4 inhibitors such as vildagliptin and sitagliptin decrease postprandial TG, RLP-C, and apoB-48 levels after a fat-loading test in patients with type 2 diabetes [11, 12]; however, our study is the first to show that alogliptin reduces the postprandial increase in triglyceride-rich lipoproteins in non-obese nondiabetic subjects.
This study was not designed to examine the molecular mechanisms underlying the effect of alogliptin on postprandial hyperlipemia, but several mechanisms are possible. A study showed that GLP-1 influences intestinal TG absorption , potentially by inhibiting gastric lipase . Animal studies have shown that DPP-4 inhibition or GLP-1 receptor agonists significantly reduce intestinal secretion of TG, cholesterol, and apoB-48, suggesting that GLP-1 may directly regulate lipoprotein assembly or the secretion in enterocytes . As shown in our previous study, administration of ezetimibe, an inhibitor of cholesterol absorption, improves postprandial lipemia-induced endothelial dysfunction, mainly due to suppression of postprandial TG-rich lipoproteins . In our current study, the maximum decrease in FMD was significantly associated with the maximum change in TG, RLP-C, and apoB-48, but not glucose. Although further studies are needed to determine the extent to which decreased TG absorption and increased chylomicron clearance contribute to the alogliptin-induced reduction in postprandial lipid response, these findings support our concept that alogliptin markedly decreases the levels of postprandial TG-rich lipoproteins, resulting in prevention of postprandial lipemia-induced endothelial dysfunction.
Regarding another proposed mechanism of improvement in postprandial endothelial dysfunction, an increase in active GLP-1 after alogliptin administration may have direct favorable effects on vascular function. An experimental study reported that sitagliptin improves endothelial function and reduces proinflammatory cytokines and atherosclerosis in apoE-deficient mice . Another experimental study showed that a GLP-1 analog reduces oxidative stress in endothelial cells . A clinical study showed that vildagliptin improves endothelium-dependent vasodilatation as determined by plethysmography in patients with type 2 diabetes in a fasting state . Postprandial inflammation and oxidative stress, which are well known to affect the metabolism of nitric oxide and the release of vasoconstrictive mediators, result in endothelial dysfunction [5, 24]. In our study, we did not examine the effect of alogliptin on postprandial oxidative stress. We evaluated the levels of soluble VCAM-1 as a marker of vascular inflammation, but no significant difference was observed. Therefore, we cannot conclude whether the administration of alogliptin improves postprandial inflammation and oxidative stress. A previous study compared the effects of α-glucosidase inhibitors on postprandial glucose/lipid metabolism and endothelial dysfunction in patients with diabetes and showed that miglitol was better than voglibose regarding a greater reduction in triglyceride and a greater induction of GLP-1 . In addition, another study showed that a single dose of exenatide improves postprandial endothelial dysfunction in individuals with impaired glucose tolerance and recent-onset type 2 diabetes. These clinical studies indicate that GLP-1 has direct favorable effects on postprandial endothelial dysfunction . In our current study, no difference was observed between the alogliptin and control groups in glucose levels at 2 h, which was probably due to our use of healthy volunteers. Even though we did not compare alogliptin and other glycemic control agents in this study, a greater spike in GLP-1 after the fat-loading test and/or a greater secretion of GLP-1 after each meal for 1 week in the alogliptin group may have partly contributed to the protective effect on postprandial endothelial dysfunction.
A clinical report showed that sitagliptin treatment for 3 months increases adiponectin levels in patients with diabetes mellitus . This report also suggests that improvement of endothelial function by sitagliptin therapy is associated with the change in adiponectin. An experimental study also showed that sitagliptin significantly increases circulating levels of adiponectin in OLETF rats . Adiponectin has vasoprotective effects via regulation of endothelial nitric oxide synthase in vascular endothelial cells. Even though our study failed to show a significant increase in adiponectin levels after 1 week of treatment with alogliptin—probably owing to short-term administration—the long-term effect of alogliptin therapy on adiponectin levels needs to be elucidated.
There is mixed evidence for the benefits of improved glycemic control on cardiovascular events and mortality in patients with diabetes mellitus. The 10 years of primary follow-up from the landmark UK Prospective Diabetes Study (UKPDS)  and three recent outcome studies (the Action to Control Cardiovascular Risk in Diabetes [ACCORD] , Action in Diabetes and Vascular Disease: Preterax and Diamicron Modified Release Controlled Evaluation [ADVANCE] , and Veterans Affairs Diabetes Trial [VADT] ) all failed to demonstrate that intensive glycemic control reduces cardiovascular events and mortality. In contrast, recent studies showed that a DPP-4 inhibitor may reduce cardiovascular events in patients with diabetes . Experimental studies also showed the favorable action of DPP-4 on vascular cells via a GLP-1-independent mechanism . These data indicate that DPP-4 may have potential for preventing cardiovascular events beyond glycemic control; however, another group reported that vildagliptin has no protective effects on cardiac function in a rat model of post-myocardial infarction heart failure . Thus, the cardiovascular benefits of DPP-4 inhibitors beyond glycemic control should be clarified in a future study.
There are several important limitations of our study. First, this was an open-label study, and the number of participants enrolled in our study was small. Therefore, a degree of selection bias may have occurred. Second, no widely used method for assessing postprandial hyperlipemia has been established, and so various fat-loading tests, such as oral fat meal, fat cream intake, and intravenous fat load, have been used in previous studies. We used the cookie test, which provided sufficient information about glucose intolerance and postprandial hyperlipemia . Although the cookie provided a fixed amount of fat (28.5 g) per person, the amount of fat given per body surface area was not the same in this study. Therefore, the contribution of each person’s fat metabolism cannot be ruled out as an influential factor. A meal loading test using 30 g fat/m2 body surface area showed a greater increase in TG and RLP compared with that using a fixed amount of fat (28.5 g) per person, even in healthy volunteers . The effect of alogliptin on postprandial lipemia after the meal loading test with 30 g fat/m2 body surface area would also be informative, especially in patients with diabetes or dyslipidemia.
In conclusion, we demonstrated that inhibition of DPP-4 with alogliptin was effective for reducing postprandial elevation of TG-rich lipoproteins and the accompanying induction of postprandial endothelial dysfunction. Alogliptin may be a useful drug for reducing future cardiovascular disease by ameliorating endothelial dysfunction in the postprandial state, even in low-risk patients.