Abstract
Type 2 diabetes is characterized by the glucose intolerance, which is contributed to the peripheral (muscle, fat, and liver) insulin resistance as well as islet β-cell dysfunction. The glucose tolerance test is used in clinical practice and research to identify individuals with normal or impaired glucose tolerance and patients with type 2 diabetes. It is important to acknowledge that the improvement of insulin resistance and glycemic control, in addition to preservation of the pancreatic functions and morphology play an important role in the treatment of type 2 diabetes.
The aim of this study was to investigate the effects of a combined treatment of bovine insulin (an anti-diabetic agent with glucose lowering effect) nanoparticle (NP) capsule plus exendin-4 (a GLP-1 agonist) NP capsule via oral administration which could reduce the hyperglycemia, additively increase insulin secretion from the β-cells and strongly suppress glucagon secretion from the α-cells after glucose loading in a diabetic rat model.
The anti-diabetic effects of bovine insulin and/or exendin-4 in type 2 diabetic rat were examined in closed-loop study using TEM (transmission electron microscopy) and confocal microscopy to demonstrate the ability of absorption of insulin NPs and exendin-4 loaded NPs via the opening intestinal paracellular tight junction (TJ) and the uptake into the systemic circulation; then determined the hyperglycemia effect in an oral glucose tolerance test (OGTT) via measuring the blood glucose (BG) levels, bovine insulin and exendin-4 absorption, rat insulin and rat glucagon secretion.
The closed-loop and biodistribution results showed that the two active pharmaceutical ingredients (2APIs) NPs could enhance the opening TJ in duodenal segment of small intestine, thus improve the absorption of these agents via the intestinal paracellular route into the systemic circulation. In the OGTT study, insulin NPs in a combination with exendin-4
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NPs significantly lowered glucose level (60% reduction at 4 h after oral glucose loading) comparing with single API NPs treatment (40% in the treatment of insulin NPs and 23% in that of exendin-4 NPs). Moreover, the pharmacodynamic (PD) and pharmacokinetic (PK) profiles found that the combination of 2API NPs could significantly increase both bovine insulin and exendin-4 absorption (Cmax ~60μIU/mL at 2h, BAr: 21.2±0.4% and Cmax ~0.49 ng/mL at 2h, Bar: 16.7±1.2%, respectively); additively stimulate β-cells to secret insulin and effectively inhibit α-cells to produce glucagon.
The present study demonstrated that the co-administration of insulin NPs plus exendin-4 NPs was successful to enhance the opening TJ in paracellular pathway leading to improve the absorption of insulin and exendin-4 into the circulation system, additively increase insulin secretion and strongly suppress glucagon secretion via the improvement of pancreatic β-cell function and morphology, thus improving the glucose tolerance. These findings strongly support the rationale of a combined treatment of oral insulin NPs plus exendin-4 NPs in chronic treatment and clinical practice.

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