Oral glucose provokes a threefold to fourfold higher insulin response than an equivalent dose of glucose given intravenously. This is because the oral glucose causes a release of gut hormones, principally glucagon-like peptide 1 (GLP-1) and glucose dependent insulinotropic polypeptide (GIP1), that amplify the glucose-induced insulin release. This “incretin effect” is reduced in patients with type 2 diabetes. GLP-1 secretion (but not GIP1 secretion) is impaired in patients with type 2 diabetes and when GLP-1 is infused in patients with type 2 diabetes, it stimulates insulin secretion and lowers glucose levels. GLP-1, unlike the sulfonylureas, has only a modest insulin stimulatory effect at normoglycemic concentrations. This means that GLP-1 has a lower risk for hypoglycemia than the sulfonylureas.
In addition to its insulin stimulatory effect, GLP-1 also has a number of other pancreatic and extrapancreatic effects. It suppresses glucagon secretion and so may ameliorate the hyperglucagonemia that is present in people with diabetes and improve postprandial hyperglycemia. GLP-1 preserves islet integrity and reduces apoptotic cell death of human islet cells in culture. In mice, streptozotocin-induced apoptosis is significantly reduced by coadministration of exendin-4 or exenatide, a GLP-1 receptor agonist. GLP-1 acts on the stomach delaying gastric emptying; the importance of this effect on glucose lowering is illustrated by the observation that antagonizing the deceleration of gastric emptying markedly reduces the glucose lowering effect of GLP-1. GLP-1 receptors are present in the central nervous system, and intracerebroventricular administration of GLP-1 in wild type mice, but not in GLP-1 receptor knockout mice, inhibits feeding. Type 2 diabetic patients undergoing GLP-1 infusion are less hungry; it is unclear whether this is mainly due to a deceleration of gastric emptying or whether there is a central nervous system effect as well.
a. Exenatide - GLP-1 is rapidly proteolysed by dipeptidyl peptidase IV (DPP IV), and therefore for clinical effect would need to be administered as a continuous infusion. Exendin 4 or exenatide is a GLP-1 receptor agonist isolated from the saliva of the Gila Monster (a venomous lizard) that is more resistant to DPP IV action and, when given to type 2 diabetics by subcutaneous injection twice a day, lowers blood glucose and HbA1c levels. Exenatide appears to have the same effects as GLP-1 on glucagon suppression and gastric emptying. In clinical trials, adding exenatide therapy to patients with type 2 diabetes already taking metformin or a sulfonylurea, or both, further lowered the HbA1c value by 0.4% to 0.6% over a 30-week period. These patients also experienced a weight loss of 3-6 pounds. In an open label extension study up to 80 weeks, the HbA1c reduction was sustained and there was further weight loss (to a total loss of about 10 pounds). The main side effect was nausea, affecting over 40% of the patients. The nausea was dose-dependent and declined with time. The risk of hypoglycemia was higher in persons taking sulfonylureas. Exenatide is dispensed as two fixed-dose pens (5 mcg and 10 mcg). It is injected 60 minutes before breakfast and before dinner. Patients should be prescribed the 5 mcg pen for the first month and, if tolerated, the dose can then be increased to 10 mcg twice a day. The drug is less stable than insulin and needs to be refrigerated between injections.
b. Oral DPP IV inhibitors - These agents, which work by prolonging the action of endogenously released GLP-1, are in clinical trials for use in type 2 diabetes.