Improvement of glucose tolerance in Zucker diabetic fatty rats by long-term treatment with the dipeptidyl peptidase inhibitor P32/98: comparison with and combination with rosiglitazone

Morphology of pancreatic islets: a time course of pre-diabetes in Zucker fatty rats

The Zucker fatty rat (fa/fa; ZR) is considered as a model for pre-diabetes, as characterised by a genetic defect in the leptin receptor, which results in hyperphagia, insulin resistance, hyperinsulinaemia, hyperlipoproteinaemia, and obesity. These animals become glucose intolerant but do not develop type 2 diabetes. As a consequence of increased adiposity and insulin resistance, the endocrine pancreas of ZR undergoes adaptive and compensatory changes. Measurements of the time course of the pathological changes by the histological analysis of the pancreatic islet in combination with metabolic parameters are an effective way to reveal disease progression. A loss in glucose tolerance occurs in ZR by 10 weeks of age and progressively worsens by 19 weeks of age. This process is accompanied by impaired islet histology, changes of beta-cell mass, and impaired islet function. The early expression of insulin resistance and glucose intolerance in ZR results in morphological and functional changes of pancreatic islets despite their capability to maintain normoglycaemia.

Scaffold-based discovery of indeglitazar, a PPAR pan-active anti-diabetic agent

In a search for more effective anti-diabetic treatment, we used a process coupling low-affinity biochemical screening with high-throughput co-crystallography in the design of a series of compounds that selectively modulate the activities of all three peroxisome proliferator-activated receptors (PPARs), PPARalpha, PPARgamma, and PPARdelta. Transcriptional transactivation assays were used to select compounds from this chemical series with a bias toward partial agonism toward PPARgamma, to circumvent the clinically observed side effects of full PPARgamma agonists. Co-crystallographic characterization of the lead molecule, indeglitazar, in complex with each of the 3 PPARs revealed the structural basis for its PPAR pan-activity and its partial agonistic response toward PPARgamma. Compared with full PPARgamma-agonists, indeglitazar is less potent in promoting adipocyte differentiation and only partially effective in stimulating adiponectin gene expression. Evaluation of the compound in vivo confirmed the reduced adiponectin response in animal models of obesity and diabetes while revealing strong beneficial effects on glucose, triglycerides, cholesterol, body weight, and other metabolic parameters. Indeglitazar has now progressed to Phase II clinical evaluations for Type 2 diabetes mellitus (T2DM).

Efficacy of the dipeptidyl peptidase IV inhibitor isoleucine thiazolidide (P32/98) in fatty Zucker rats with incipient and manifest impaired glucose tolerance

AIM

Incretin enhancers are a new class of antidiabetic drugs with promising therapeutic potential for type 2 diabetes. Therapeutic intervention in prediabetes is an attractive strategy for preventing or delaying diabetes onset. The aim of the present study was to investigate the therapeutic effects of incretin enhancement on incipient impaired glucose tolerance (iIGT) and manifest IGT (mIGT) using the dipeptidyl peptidase IV (DPP-4) inhibitor P32/98- and fatty Zucker rat (ZR, fa/fa) as a model.

METHODS

ZRs were classified into groups with iIGT and mIGT (n = 10 per group). P32/98 (21.61 mg/kg body weight) was administered orally to ZR with iIGT and mIGT once daily for 6 and 3 weeks respectively. Assessments included body weight, morning blood glucose and insulin, oral glucose tolerance test (oGTT; 2 g glucose/kg), plasma parameters and blood glucose day-night profile (DNP). In addition, glucose responsiveness of isolated islets and islet morphology were analysed.

RESULTS

P32/98 decreased non-fasting morning blood glucose more effectively in ZR with iIGT than in ZR with mIGT. Compared with study entry, P32/98 improved DNP of blood glucose in ZR with mIGT and nearly normalized DNP in ZR with iIGT. An acute bolus of inhibitor reduced glucose load during oGTT in rats chronically treated with placebo or P32/98. In contrast to placebo-treated rats, rats receiving long-term treatment with P32/98 required less insulin during oGTT. This effect was larger in rats with iIGT vs. rats with mIGT. In isolated pancreatic islets of ZR with mIGT, treatment with P32/98 decreased pancreatic insulin content and increased glucose responsiveness, while the beta-cell volume density was unaffected. P32/98 significantly reduced triglycerides and non-esterified fatty acids. Intestinal growth was comparable between inhibitor- and placebo-treated fatty rats.

CONCLUSIONS

Enhancement of incretin with the DPP-4 inhibitor P32/98 has therapeutic effects in hyperinsulinaemia, hyperglycaemia and IGT in ZR with iIGT and mIGT. Apparently, administration of P32/98 in ZR with iIGT results in more efficient beta-cell function, which is associated with less need for insulin to cope with deterioration of glucose tolerance. Importantly, P32/98 has a strong effect on dyslipidaemia in mIGT. P32/98 has no side effect on intestinal growth. Daily intake of P32/98 is a promising strategy for treatment of glucose intolerance and has the potential to prevent type 2 diabetes.

A review of islet of Langerhans degeneration in rodent models of type 2 diabetes

Type 2 diabetes mellitus (TTDM) is characterized by progressive loss of glucose control through multifactorial mechanisms. The search for an understanding of TTDM has relied on animal models since the realization of the importance of the pancreas in controlling plasma glucose concentration. Rodent models of TTDM are developed to express hyperglycemia and not islet degeneration per se. Degeneration of the islets of Langerhans with beta-cell loss is secondary to insulin resistance and is regarded as the more important lesion. Despite this, differences between models are seen in the development and progression of islet degeneration. Assessing the differences between the models is important to appreciate the various aspects of TTDM and understand their advantages as well as their deficiencies. Relevant animal models of TTDM provide opportunities to investigate important physiological and cell biological processes that may ultimately lead to development of targeted therapies. This article reviews the importance, advantages, and limitations of rodent models of TTDM in relation to the histopathological changes that characterize islet degeneration. Pathophysiological mechanisms that contribute to islet degeneration are also discussed and are placed into the context of changes in islet histological appearances.

Combination of the insulin sensitizer, pioglitazone, and the long-acting GLP-1 human analog, liraglutide, exerts potent synergistic glucose-lowering efficacy in severely diabetic ZDF rats

OBJECTIVE

Severe insulin resistance and impaired pancreatic beta-cell function are pathophysiological contributors to type 2 diabetes, and ideally, antihyperglycaemic strategies should address both.

RESEARCH DESIGN AND METHODS

Therapeutic benefits of combining the long-acting human glucagon-like peptide-1 (GLP-1) analog, liraglutide (0.4 mg/kg/day), with insulin sensitizer, pioglitazone (10 mg/kg/day), were assessed in severely diabetic Zucker diabetic fatty rats for 42 days. Impact on glycaemic control was assessed by glycated haemoglobin (HbA(1C)) at day 28 and by oral glucose tolerance test at day 42.

RESULTS

Liraglutide and pioglitazone synergistically improved glycaemic control as reflected by a marked decrease in HbA(1C) (liraglutide + pioglitazone: 4.8 +/- 0.3%; liraglutide: 8.8 +/- 0.6%; pioglitazone: 7.9 +/- 0.4%; vehicle: 9.7 +/- 0.3%) and improved oral glucose tolerance at day 42 (area under the curve; liraglutide + pioglitazone: 4244 +/- 445 mmol/l x min; liraglutide: 7164 +/- 187 mmol/l x min; pioglitazone: 7430 +/- 446 mmol/l x min; vehicle: 8093 +/- 139 mmol/l x min). A 24-h plasma glucose profile at day 38 was significantly decreased only in the liraglutide + pioglitazone group. In addition, 24-h insulin profile was significantly elevated only in the liraglutide + pioglitazone group. Liraglutide significantly decreased food intake alone and in combination with pioglitazone, while pioglitazone alone increased cumulated food intake. As a result, rats on liraglutide alone gained significantly less weight than vehicle-treated rats, whereas rats on pioglitazone alone gained significantly more body weight than vehicle-treated rats. However, combination therapy with liraglutide and pioglitazone caused the largest weight gain, probably reflecting marked improvement of energy balance because of reduction of glucosuria.

CONCLUSIONS

Combination therapy with insulinotropic GLP-1 agonist liraglutide and insulin sensitizer, pioglitazone, improves glycaemic control above and beyond what would be expected from additive effects of the two antidiabetic agents.

The role of alpha-cell dysregulation in fasting and postprandial hyperglycemia in type 2 diabetes and therapeutic implications

The hyperglycemic activity of pancreatic extracts was encountered some 80 yr ago during efforts to optimize methods for the purification of insulin. The hyperglycemic substance was named "glucagon," and it was subsequently determined that glucagon is a 29-amino acid peptide synthesized and released from pancreatic alpha-cells. This article begins with a brief overview of the discovery of glucagon and the contributions that somatostatin and a sensitive and selective assay for pancreatic (vs. gut) glucagon made to understanding the physiological and pathophysiological roles of glucagon. Studies utilizing these tools to establish the function of glucagon in normal nutrient homeostasis and to document a relative glucagon excess in type 2 diabetes mellitus (T2DM) and precursors thereof are then discussed. The evidence that glucagon excess contributes to the development and maintenance of fasting hyperglycemia and that failure to suppress glucagon secretion contributes to postprandial hyperglycemia is then reviewed. Although key human studies are emphasized, salient animal studies highlighting the importance of glucagon in normal and defective glucoregulation are also described. The past eight decades of research in this area have led to development of new therapeutic approaches to treating T2DM that have been shown to, or are expected to, improve glycemic control in patients with T2DM in part by improving alpha-cell function or by blocking glucagon action. Accordingly, this review ends with a discussion of the status and therapeutic potential of glucagon receptor antagonists, alpha-cell selective somatostatin agonists, glucagon-like peptide-1 agonists, and dipeptidyl peptidase-IV inhibitors. Our overall conclusions are that there is considerable evidence that relative hyperglucagonemia contributes to fasting and postprandial hyperglycemia in patients with T2DM, and there are several new and emerging pharmacotherapies that may improve glycemic control in part by ameliorating the hyperglycemic effects of this relative glucagon excess.

ROSIGLITAZONE, A PEROXISOME PROLIFERATOR-ACTIVATED RECEPTOR-? LIGAND, REDUCES INFARCTION VOLUME AND NEUROLOGICAL DEFICITS IN AN EMBOLIC MODEL OF STROKE

1. Stroke is accompanied by a robust inflammatory response, glutamate-mediated excitotoxicity, release of reactive oxygen species and apoptosis. Thiazolidinediones, which target the nuclear receptor peroxisome proliferator-activated receptor (PPAR)-g, have been reported recently to exhibit potent anti-inflammatory and anti-oxidant actions and inhibit both neural excitotoxicity and apoptosis. 2. The present study was conducted to determine whether rosiglitazone, a potent thiazolidinedione for PPAR-g, would show efficacy against the cerebral infarction and neurological dysfunctions induced by embolic middle cerebral artery (MCA) occlusion in the rat. 3. Focal ischaemic injury was induced by embolizing a preformed clot into the MCA. Rosiglitazone was dissolved in dimethyl sulphoxide and injected i.p. 1 h before MCA occlusion at doses of 0.33, 0.1, 0.3 or 1 mg/kg. In addition, 1 mg/kg rosiglitazone was used immediately or 4 h after embolization. Forty-eight hours after MCA occlusion, brains were removed, sectioned and stained with a 2% solution of 2,3,5-triphenyltetrazolum chloride and analysed using a commercial image-processing software program. 4. When rosiglitazone was administered 1 h before embolization, it significantly reduced infarct volume by 48.2, 68.4% and 70.3% at doses of 0.1, 0.3 and 1 mg/kg, respectively (P < 0.001). Administration of rosiglitazone (1 mg/kg) immediately or 4 h after stroke also reduced infarct volume by 67 and 50.8%, respectively (P < 0.001). Rosiglitazone-treated rats also demonstrated improved neurological functions. However, there were no statistically significant differences between control and treated groups in terms of brain oedema at 48 h after ischaemic injury. 5. The findings of the present study may support the idea of a potential benefit of thiazolidinediones in the management of ischaemic stroke.

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.

GLP-1 based therapy for type 2 diabetes

Type 2 diabetes mellitus is a major and growing health problem throughout the world. Current treatment approaches include diet, exercise, and a variety of pharmacological agents including insulin, biguanides, sulfonylureas and thiazolidinediones. New therapies are still needed to control metabolic abnormalities, and also to preserve beta-cell mass and to prevent loss of beta-cell function. Glucagon-like peptide 1 (GLP-1) is a drug candidate which potentially fulfils these conditions. GLP-1 is an incretin hormone secreted by intestinal L-cells in response to meal ingestion is a novel pharmacological target with multiple antihyperglycemic actions. GLP-1 glucoregulatory actions include glucose-dependent enhancement of insulin secretion, inhibition of glucagon secretion, slowing of gastric emptying and reduction of food intake. GLP-1 is rapidly inactivated by amino peptidase, dipeptidyl peptidase IV (DPP-IV) and the utility of DPP-IV inhibitors are also under investigation. There is a recent upsurge in the development of GLP-1 mimetics and DPP-IV inhibitors as potential therapy for type 2 diabetes. However, both the strategies are having their own advantages and limitations. The present review summarizes the concepts of GLP-1 based therapy for type 2 diabetes and the current preclinical and clinical development in GLP-1 mimetics and DPP-IV inhibitors. Further, the potential advantages and the limitations of both the strategies are discussed.