Glucagon-like peptide 1 and its derivatives in the treatment of diabetes

The incretin system: glucagon-like peptide-1 receptor agonists and dipeptidyl peptidase-4 inhibitors in type 2 diabetes

Glucagon-like peptide 1 (GLP-1) is a gut-derived incretin hormone that stimulates insulin and suppresses glucagon secretion, inhibits gastric emptying, and reduces appetite and food intake. Therapeutic approaches for enhancing incretin action include degradation-resistant GLP-1 receptor agonists (incretin mimetics), and inhibitors of dipeptidyl peptidase-4 (DPP-4) activity (incretin enhancers). Clinical trials with the incretin mimetic exenatide (two injections per day or long-acting release form once weekly) and liraglutide (one injection per day) show reductions in fasting and postprandial glucose concentrations, and haemoglobin A1c (HbA1c) (1-2%), associated with weight loss (2-5 kg). The most common adverse event associated with GLP-1 receptor agonists is mild nausea, which lessens over time. Orally administered DPP-4 inhibitors, such as sitagliptin and vildagliptin, reduce HbA1c by 0.5-1.0%, with few adverse events and no weight gain. These new classes of antidiabetic agents, and incretin mimetics and enhancers, also expand beta-cell mass in preclinical studies. However, long-term clinical studies are needed to determine the benefits of targeting the incretin axis for the treatment of type 2 diabetes.

How do different GLP-1 mimetics differ in their actions?

Glucagon-like peptide-1 (GLP-1) mimetics have been developed to overcome the pharmacokinetic limitations of GLP-1 for the treatment of type 2 diabetes. Their mechanisms of action and clinical effects appear particularly interesting because they target the main pathophysiologic mechanisms involved in type 2 diabetes. GLP-1 receptor agonists are more powerful and are particularly advantageous by their weight loss-inducing capacity, whereas dipeptidyl peptidase IV inhibitors exhibit a better tolerance profile. However, their ultimate role is still to be defined in the therapeutic strategy of type 2 diabetes.

Patient-reported outcomes in a trial of exenatide and insulin glargine for the treatment of type 2 diabetes

Background

Patient-reported measures can be used to examine whether drug differences other than clinical efficacy have an impact on outcomes that may be important to patients. Although exenatide and insulin glargine appear to have similar efficacy for treatment of type 2 diabetes, there are several differences between the two treatments that could influence outcomes from the patient's perspective. The purpose of the current study was to examine whether the two drugs were comparable as assessed by patient-reported outcomes using data from a clinical trial in which these injectable medications were added to pre-existing oral treatment regimens.

Methods

Patients were randomized to either twice daily exenatide or once daily insulin glargine during a 26-week international trial. At baseline and endpoint, five patient-reported outcome measures were administered: the Vitality Scale of the SF-36, The Diabetes Symptom Checklist – Revised (DSC-R), the EuroQol EQ-5D, the Treatment Flexibility Scale (TFS), and the Diabetes Treatment Satisfaction Questionnaire (DTSQ). Change from baseline to endpoint was analyzed within each treatment group. Group differences were examined with General linear models (GLMs), controlling for country and baseline scores.

Results

A total of 549 patients with type 2 diabetes were enrolled in the trial, and current analyses were conducted with data from the 455 per protocol patients (228 exenatide and 227 insulin glargine). The sample was primarily Caucasian (79.6%), with slightly more men (55.2%) than women, and with a mean age of 58.5 years. Paired t-tests found that both treatment groups demonstrated statistically significant baseline to endpoint change on several of the health outcomes instruments including the DSC-R, DTSQ, and the SF-36 Vitality subscale. GLMs found no statistically significant differences between groups in change on the health outcomes instruments.

Conclusion

This analysis found that both exenatide and insulin glargine were associated with significant improvements in patient-reported outcomes when added to oral medications among patients with type 2 diabetes. Despite an additional daily injection and a higher rate of gastrointestinal adverse events, treatment satisfaction in the exenatide group was comparable to that of the glargine group, possibly because of weight reduction observed in patients treated with exenatide.

Neurotrophic property of geniposide for inducing the neuronal differentiation of PC12 cells

The emerging data show that the insulinotrophic hormone glucagon-like peptide-1(GLP-1) and its agonist extendin-4 have neurotrophic function to inducing neuronal differentiation of PC12 cells and prevent neurons damage challenged by oxidative stress. Here, with the model of high throughput screen for GLP-1 receptor agonists, we screen and identify that geniposide is a novel agonist for GLP-1 receptor. Furthermore, geniposide induces the neuronal differentiation of PC12 cells with resulting neurites outgrowth; we also observe an increase in expression of growth-associated protein-43. U0126, a selective MEK inhibitor, prevents neurites out growth and phosphorylation of mitogen-activated kinase proteins in PC12 cells induced by geniposide. All these results show that activation of GLP-1 receptor by geniposide to induce the neuronal differentiation of PC12 cells involves in MAPK signaling cascade.

Exenatide: an incretin mimetic for the treatment of type 2 diabetes mellitus

BACKGROUND

Exenatide is a subcutaneously injected incretin mimetic. It is indicated as adjunctive therapy to improve glycemic control in patients with type 2 diabetes mellitus (T2DM) who are already receiving therapy with metformin, a sulfonylurea, or both but continue to have suboptimal glycemic control.

OBJECTIVE

This article reviews available information on the clinical pharmacology, comparative efficacy, tolerability, drug interactions, contraindications and precautions, dosage and administration, availability and storage, and cost of exenatide.

METHODS

MEDLINE (1966-April 2006) and Web of Science (1995-April 2006) were searched for original research and review articles published in the English language. The search terms used were exenatide, exendin-4, glucagon-Like peptide-1, GLP-1, and incretin mimetic. The reference lists of identified articles were also consulted, as was selected information from the package insert for exenatide. All relevant comparative efficacy studies that were available in published form were included in the review.

RESULTS

Naturally occurring incretins, such as glucagon-like peptide-1 (GLP-1), exhibit insulinotropic properties after release into the circulation from the gut. As a GLP-1 agonist, exenatide improves glucose homeostasis by mimicking the actions of naturally occurring GLP-1. It improves glycemic control by reducing fasting and postprandial glucose concentrations through a combination of known mechanisms, including glucose-dependent insulin secretion, restoration of first-phase insulin response, regulation of glucagon secretion, delaying gastric emptying, and decreasing food intake. Three Phase III comparative efficacy trials were identified that enrolled a total of 1,446 patients who received exenatide 5 pg SC BID, exenatide 10 mug SC BID, or placebo for 30 weeks in addition to their existing therapy with metformin, sulfonylurea, or both. In these trials, the addition of exenatide was associated with significant reductions in glycosylated hemoglobin (HbA(1c)) values (P < 0.001-P < 0.002), greater proportions of patients achieving an HbA(1c) <or=7%, significant decreases in fasting plasma glucose concentrations (P < 0.001-P < 0.005), and a dose-dependent progressive weight loss compared with placebo. Nausea (43.5%) was the most commonly reported adverse event in the combined exenatide groups. Other adverse events occurring in >10% of patients receiving exenatide were hypoglycemia (19.6%), diarrhea (12.8%), and vomiting (12.8%).

CONCLUSIONS

During clinical trials, exenatide added to existing metformin and/or sulfonylurea therapy in patients with T2DM reduced fasting and postprandial glucose concentrations, with improvements in HbA(1c) and modest weight loss. The main adverse effect associated with exenatide therapy was nausea.

Nuevas perspectivas en el tratamiento de la obesidad: el aparato digestivo como órgano endocrino

The gastrointestinal tract, besides digesting and processing nutrients, is now regarded as an endocrine organ able to modulate appetite, satiety, and carbohydrate metabolism. Several enteroendocrine cells produce numerous peptides codifying either orexigenic (ghrelin, orexins) or anorexigenic signals (pancreatic polypeptide, peptide YY, cholecystokinin, amylin, bombesin homologs, apolipoprotein A-IV, glucose-dependent insulinotropic polypeptide, glucagon-like peptide 1, oxyntomodulin), which interact in a complex network with other peripheral signals of energy balance and with different neuropeptides involved in the central control of appetite and energy homeostasis. The growing knowledge of the actions of these gastrointestinal peptides on appetite regulation and carbohydrate metabolism, and subsequent synthesis of analogs, particularly those derived from amylin and incretins, herald a new era in the therapy of 2 closely related diseases, obesity and type 2 diabetes mellitus.

Chronic inhibition of dipeptidyl peptidase-4 with a sitagliptin analog preserves pancreatic beta-cell mass and function in a rodent model of type 2 diabetes

Inhibitors of dipeptidyl peptidase-4 (DPP-4), a key regulator of the actions of incretin hormones, exert antihyperglycemic effects in type 2 diabetic patients. A major unanswered question concerns the potential ability of DPP-4 inhibition to have beneficial disease-modifying effects, specifically to attenuate loss of pancreatic beta-cell mass and function. Here, we investigated the effects of a potent and selective DPP-4 inhibitor, an analog of sitagliptin (des-fluoro-sitagliptin), on glycemic control and pancreatic beta-cell mass and function in a mouse model with defects in insulin sensitivity and secretion, namely high-fat diet (HFD) streptozotocin (STZ)-induced diabetic mice. Significant and dose-dependent correction of postprandial and fasting hyperglycemia, HbA(1c), and plasma triglyceride and free fatty acid levels were observed in HFD/STZ mice following 2-3 months of chronic therapy. Treatment with des-fluoro-sitagliptin dose dependently increased the number of insulin-positive beta-cells in islets, leading to the normalization of beta-cell mass and beta-cell-to-alpha-cell ratio. In addition, treatment of mice with des-fluoro-sitagliptin, but not glipizide, significantly increased islet insulin content and improved glucose-stimulated insulin secretion in isolated islets. These findings suggest that DPP-4 inhibitors may offer long-lasting efficacy in the treatment of type 2 diabetes by modifying the courses of the disease.

Therapeutic options in development for management of diabetes: pharmacologic agents and new technologies

OBJECTIVE

To review new pharmacologic therapies and technologies relevant to the management of diabetes and its complications.

METHODS

New treatment options for diabetes, made available through research efforts during the past 2 decades, are discussed.

RESULTS

Several new drugs and drug classes for the management of diabetes are under development, including the incretin mimetic agents (exenatide, dipeptidyl peptidase 4 inhibitors, and glucagon-like peptide 1 analogues), the amylin analogue pramlintide, the cannabinoid-1 receptor antagonist rimonabant, the mixed peroxisome proliferator-activated receptor agonists muraglitazar and tesaglitazar, the inhaled insulin preparation Exubera, and the insulin analogues (insulin glulisine and insulin detemir).

CONCLUSION

New drugs and technologic advances being made available will help achieve the goals of treating patients with diabetes to all the appropriate metabolic targets. Many other agents that act on fundamental abnormalities such as energy imbalance, inflammation, and vascular biologic conditions are in very early stages of development but are likely to become available during the next 5 to 10 years.

Applications of dipeptidyl peptidase IV inhibitors in diabetes mellitus

A number of alternative therapies for type 2 diabetes are currently under development that take advantage of the actions of the incretin hormones glucagon-like peptide-1 and glucose-dependent insulinotropic polypeptide on the pancreatic beta-cell. One such approach is based on the inhibition of dipeptidyl peptidase IV (DP IV), the major enzyme responsible for degrading the incretins in vivo. DP IV exhibits characteristics that have allowed the development of specific inhibitors with proven efficacy in improving glucose tolerance in animal models of diabetes and type 2 human diabetics. While enhancement of insulin secretion, resulting from blockade of incretin degradation, has been proposed to be the major mode of inhibitor action, there is also evidence that inhibition of gastric emptying, reduction in glucagon secretion and important effects on beta-cell differentiation, mitogenesis and survival, by the incretins and other DP IV-sensitive peptides, can potentially preserve beta-cell mass, and improve insulin secretory function and glucose handling in diabetics.

GIP(Lys16PAL) and GIP(Lys37PAL): novel long-acting acylated analogues of glucose-dependent insulinotropic polypeptide with improved antidiabetic potential

Glucose-dependent insulinotropic polypeptide (GIP) is a physiological insulin releasing peptide. We have developed two novel fatty acid derivatized GIP analogues, which bind to serum albumin and demonstrate enhanced duration of action in vivo. GIP(Lys(16)PAL) and GIP(Lys(37)PAL) were resistant to dipeptidyl peptidase IV (DPP IV) degradation. In vitro studies demonstrated that GIP analogues retained their ability to activate the GIP receptor through production of cAMP and to stimulate insulin secretion. Intraperitoneal administration of GIP analogues to obese diabetic (ob/ob) mice significantly decreased the glycemic excursion and elicited increased and prolonged insulin responses compared to native GIP. A protracted glucose-lowering effect was observed 24 h following GIP(Lys(37)PAL) administration. Once a day injection for 14 days decreased nonfasting glucose, improved glucose tolerance, and enhanced the insulin response to glucose. These data demonstrate that fatty acid derivatized GIP peptides represent a novel class of long-acting stable GIP analogues for therapy of type 2 diabetes.

Type 1 diabetes

Type 1 diabetes accounts for only about 5-10% of all cases of diabetes; however, its incidence continues to increase worldwide and it has serious short-term and long-term implications. The disorder has a strong genetic component, inherited mainly through the HLA complex, but the factors that trigger onset of clinical disease remain largely unknown. Management of type 1 diabetes is best undertaken in the context of a multidisciplinary health team and requires continuing attention to many aspects, including insulin administration, blood glucose monitoring, meal planning, and screening for comorbid conditions and diabetes-related complications. These complications consist of microvascular and macrovascular disease, which account for the major morbidity and mortality associated with type 1 diabetes. Newer treatment approaches have facilitated improved outcomes in terms of both glycaemic control and reduced risks for development of complications. Nonetheless, major challenges remain in the development of approaches to the prevention and management of type 1 diabetes and its complications.

Glucagon-like peptide 1 abolishes the postprandial rise in triglyceride concentrations and lowers levels of non-esterified fatty acids in humans

AIMS/HYPOTHESIS

Diabetic dyslipidaemia contributes to the excess morbidity and mortality in patients with type 2 diabetes. Exogenous glucagon-like peptide 1 (GLP-1) lowers postprandial glycaemia predominantly by slowing gastric emptying. Therefore, the effects of GLP-1 on postprandial lipid levels and gastric emptying were assessed.

METHODS

14 healthy male volunteers were studied with an i.v. infusion of GLP-1 (1.2 pmol kg(-1) min(-1)) or placebo over 390 min in the fasting state. A solid test meal was served and gastric emptying was determined using a (13)C-labelled sodium octanoate breath test. Venous blood was drawn frequently for measurement of glucose, insulin, C-peptide, glucagon, GLP-1, triglycerides and NEFA.

RESULTS

GLP-1 administration lowered fasting and postprandial glycaemia (p<0.0001). Gastric emptying was delayed by GLP-1 compared with placebo (p<0.0001). During GLP-1 administration, insulin secretory responses were higher in the fasting state but lower after meal ingestion. After meal ingestion, triglyceride plasma levels increased by 0.33+/-0.14 mmol/l in the placebo experiments (p<0.0001). In contrast, the postprandial increase in triglyceride levels was completely abolished by GLP-1 (change in triglycerides, -0.023+/-0.045 mmol/l; p<0.05). During GLP-1 infusion, plasma concentrations of NEFA were suppressed by 39% in the fasting state (p<0.01) and by 31+/-5% after meal ingestion (p<0.01).

CONCLUSIONS/INTERPRETATION

GLP-1 improves postprandial lipidaemia, presumably as a result of delayed gastric emptying and insulin-mediated inhibition of lipolysis. Thus, by lowering both glucose and lipid concentrations, GLP-1 administration may reduce the cardiovascular risk in patients with type 2 diabetes.

Exendin-4, a glucagon-like protein-1 (GLP-1) receptor agonist, reverses hepatic steatosis in ob/ob mice

Nonalcoholic fatty liver disease (NAFLD) represents a burgeoning problem in hepatology, and is associated with insulin resistance. Exendin-4 is a peptide agonist of the glucagon-like peptide (GLP) receptor that promotes insulin secretion. The aim of this study was to determine whether administration of Exendin-4 would reverse hepatic steatosis in ob/ob mice. Ob/ob mice, or their lean littermates, were treated with Exendin-4 [10 microg/kg or 20 microg/kg] for 60 days. Serum was collected for measurement of insulin, adiponectin, fasting glucose, lipids, and aminotransferase concentrations. Liver tissue was procured for histological examination, real-time RT-PCR analysis and assay for oxidative stress. Rat hepatocytes were isolated and treated with GLP-1. Ob/ob mice sustained a reduction in the net weight gained during Exendin-4 treatment. Serum glucose and hepatic steatosis was significantly reduced in Exendin-4 treated ob/ob mice. Exendin-4 improved insulin sensitivity in ob/ob mice, as calculated by the homeostasis model assessment. The measurement of thiobarbituric reactive substances as a marker of oxidative stress was significantly reduced in ob/ob-treated mice with Exendin-4. Finally, GLP-1-treated hepatocytes resulted in a significant increase in cAMP production as well as reduction in mRNA expression of stearoyl-CoA desaturase 1 and genes associated with fatty acid synthesis; the converse was true for genes associated with fatty acid oxidation. In conclusion, Exendin-4 appears to effectively reverse hepatic steatosis in ob/ob mice by improving insulin sensitivity. Our data suggest that GLP-1 proteins in liver have a novel direct effect on hepatocyte fat metabolism.

Role of glucagon-like peptide-1 in the pathogenesis and treatment of diabetes mellitus

Glucagon-like peptide-1 (GLP-1) is an incretin hormone secreted from enteroendocrine L cells in response to ingested nutrients. The first recognized and most important action of GLP-1 is the potentiation of glucose-stimulated insulin secretion in beta-cells, mediated by activation of its seven transmembrane domain G-protein-coupled receptor. In addition to its insulinotropic actions, GLP-1 exerts islet-trophic effects by stimulating replication and differentiation and by decreasing apoptosis of beta-cells. The GLP-1 receptor is expressed in a variety of other tissues important for carbohydrate metabolism, including pancreatic alpha-cells, hypothalamus and brainstem, and proximal intestinal tract. GLP-1 also appears to exert important actions in liver, muscle and fat. Thus, GLP-1 suppresses glucagon secretion, promotes satiety, delays gastric emptying and stimulates peripheral glucose uptake. The impaired GLP-1 secretion observed in type 2 diabetes suggests that GLP-1 plays a role in the pathogenesis of this disorder. Thus, because of its multiple actions, GLP-1 is an attractive therapeutic target for the treatment of type 2 diabetes, and major interest has resulted in the development of a variety of GLP-1 receptor agonists for this purpose. Ongoing clinical trials have shown promising results and the first analogs of GLP-1 are expected to be available in the near future.

Growth factors and beta cell replication

Recent studies have demonstrated that human islet allograft transplantation can be a successful therapeutic option in the treatment of patients with Type I diabetes. However, this impressive recent advance is accompanied by a very important constraint. There is a critical paucity of pancreatic islets or pancreatic beta cells for islet transplantation to become a large-scale therapeutic option in patients with diabetes. This has prompted many laboratories around the world to invigorate their efforts in finding ways for increasing the availability of beta cells or beta cell surrogates that potentially could be transplanted into patients with diabetes. The number of studies analyzing the mechanisms that govern beta cell proliferation and growth in physiological and pathological conditions has increased exponentially during the last decade. These studies exploring the role of growth factors, intracellular signaling molecules and cell cycle regulators constitute the substrate for future strategies aimed at expanding human beta cells in vitro and/or in vivo after transplantation. In this review, we describe the current knowledge on the effects of several beta cell growth factors that have been shown to increase beta cell proliferation and expand beta cell mass in vitro and/or in vivo and that they could be potentially deployed in an effort to increase the number of patients transplanted with islets. Furthermore, we also analyze in this review recent studies deciphering the relevance of these specific islet growth factors as physiological and pathophysiological regulators of beta cell proliferation and islet growth.

New therapeutic strategies for the treatment of type 2 diabetes mellitus based on incretins

Orally ingested glucose leads to a greater insulin response compared to intravenously administered glucose leading to identical postprandial plasma glucose excursions, a phenomenon referred to as the "incretin effect". The incretin effect comprises up to 60% of the postprandial insulin secretion and is diminished in type 2 diabetes. One of the very important gastrointestinal hormones promoting this effect is glucagon-like peptide 1 (GLP-1). It only stimulates insulin secretion and normalizes blood glucose in humans under hyperglycemic conditions, therefore it does not cause hypoglycemia. Other important physiological actions of GLP-1 are the inhibition of glucagon secretion and gastric emptying. It further acts as a neurotransmitter in the hypothalamus stimulating satiety. In vitro and animal data demonstrated that GLP-1 increases beta-cell mass by stimulating islet cell neogenesis and by inhibiting apoptosis of islets. In humans, the improvement of beta-cell function can be indirectly observed from the increased insulin secretory capacity after GLP-1 infusions. GLP-1 represents an attractive therapeutic principle for type 2 diabetes. However, native GLP-1 is degraded rapidly upon exogenous administration and is therefore not feasible for routine therapy. The first long-acting GLP-1 analog ("incretin mimetic") Exenatide (Byetta) has just been approved for type 2 diabetes therapy. Other compounds are being investigated in clinical trials (e.g. liraglutide, CJC1131). Dipeptidyl-peptidase IV inhibitors (DPP-IV inhibitors; e.g. Vildagliptin, Sitagliptin) that inhibit the enzyme responsible for incretin degradation are also under study.

New therapeutic options for the metabolic syndrome: what's next?

The metabolic syndrome (MSX), characterized by obesity, insulin resistance, dyslipidemia and hypertension, increases the risk of cardiovascular morbidity and mortality. It has recently been hypothesized that MSX and type 2 diabetes are caused by triglyceride and long-chain fatty acid accumulation in liver, muscle, pancreatic islets and selected brain areas. This lipocentric approach is integrated with analysis of inflammation associated with end-organ damage, including the vascular wall. Genes and proteins contributing to insulin resistance, beta cell dysfunction and vascular wall damage have been identified. Transcription factors and coactivators, including peroxisome proliferator-activated receptor gamma (PPARgamma) coactivator-1 are crucial in mediating insulin resistance and accelerating vascular wall inflammation, and represent promising therapeutic targets. New pharmacological strategies include dual PPARalpha/gamma agonists, drugs with pleiotropic effects or combination therapies.