Exenatide does not evoke pancreatitis and attenuates chemically induced pancreatitis in normal and diabetic rodents

The risk of developing pancreatitis is elevated in type 2 diabetes and obesity. Cases of pancreatitis have been reported in type 2 diabetes patients treated with GLP-1 (GLP-1R) receptor agonists. To examine whether the GLP-1R agonist exenatide potentially induces or modulates pancreatitis, the effect of exenatide was evaluated in normal or diabetic rodents. Normal and diabetic rats received a single exenatide dose (0.072, 0.24, and 0.72 nmol/kg) or vehicle. Diabetic ob/ob or HF-STZ mice were infused with exenatide (1.2 and 7.2 nmol·kg(-1)·day(-1)) or vehicle for 4 wk. Post-exenatide treatment, pancreatitis was induced with caerulein (CRN) or sodium taurocholate (ST), and changes in plasma amylase and lipase were measured. In ob/ob mice, plasma cytokines (IL-1β, IL-2, IL-6, MCP-1, IFNγ, and TNFα) and pancreatitis-associated genes were assessed. Pancreata were weighed and examined histologically. Exenatide treatment alone did not modify plasma amylase or lipase in any models tested. Exenatide attenuated CRN-induced release of amylase and lipase in normal rats and ob/ob mice but did not modify the response to ST infusion. Plasma cytokines and pancreatic weight were unaffected by exenatide. Exenatide upregulated Reg3b but not Il6, Ccl2, Nfkb1, or Vamp8 expression. Histological analysis revealed that the highest doses of exenatide decreased CRN- or ST-induced acute inflammation, vacuolation, and acinar single cell necrosis in mice and rats, respectively. Ductal cell proliferation rates were low and similar across all groups of ob/ob mice. In conclusion, exenatide did not modify plasma amylase and lipase concentrations in rodents without pancreatitis and improved chemically induced pancreatitis in normal and diabetic rodents.

Glucagon-like peptide-1 and the exenatide analogue AC3174 improve cardiac function, cardiac remodeling, and survival in rats with chronic heart failure

BACKGROUND

Accumulating evidence suggests glucagon-like peptide-1 (GLP-1) exerts cardioprotective effects in animal models of myocardial infarction (MI). We hypothesized that chronic treatment with GLP-1 or the exenatide analog AC3174 would improve cardiac function, cardiac remodeling, insulin sensitivity, and exercise capacity (EC) in rats with MI-induced chronic heart failure (CHF) caused by coronary artery ligation.

METHODS

Two weeks post-MI, male Sprague-Dawley rats were treated with GLP-1 (2.5 or 25 pmol/kg/min), AC3174 (1.7 or 5 pmol/kg/min) or vehicle via subcutaneous infusion for 11 weeks. Cardiac function and morphology were assessed by echocardiography during treatment. Metabolic, hemodynamic, exercise-capacity, and body composition measurements were made at study end.

RESULTS

Compared with vehicle-treated rats with CHF, GLP-1 or AC3174 significantly improved cardiac function, including left ventricular (LV) ejection fraction, and end diastolic pressure. Cardiac dimensions also improved as evidenced by reduced LV end diastolic and systolic volumes and reduced left atrial volume. Vehicle-treated CHF rats exhibited fasting hyperglycemia and hyperinsulinemia. In contrast, GLP-1 or AC3174 normalized fasting plasma insulin and glucose levels. GLP-1 or AC3174 also significantly reduced body fat and fluid mass and improved exercise capacity and respiratory efficiency. Four of 16 vehicle control CHF rats died during the study compared with 1 of 44 rats treated with GLP-1 or AC3174. The cellular mechanism by which GLP-1 or AC3174 exert cardioprotective effects appears unrelated to changes in GLUT1 or GLUT4 translocation or expression.

CONCLUSIONS

Chronic treatment with either GLP-1 or AC3174 showed promising cardioprotective effects in a rat model of CHF. Hence, GLP-1 receptor agonists may represent a novel approach for the treatment of patients with CHF or cardiovascular disease associated with type 2 diabetes.

Insights into amylin-leptin synergy

Although the adipokine leptin is regarded as the prototypical long-term signal of energy balance, obese individuals are largely nonresponsive to exogenous leptin administration. Restoration of leptin responsiveness in obesity has been elusive despite a detailed understanding of the molecular mechanisms of leptin signaling. Recent translational research findings point to a potential therapeutic approach that incorporates amylin (a beta-cell hormone) and leptin agonism, with amylin restoring or enhancing leptin sensitivity. Here we hypothesize various physiological, neurobiological and molecular mechanisms that could mediate the interaction of these two neurohormonal signals and discuss several methodological challenges. Understanding how amylin agonism improves leptin function could point to general therapeutic strategies for combating leptin resistance and associated obesity.

Mechanisms of amylin/leptin synergy in rodent models

The present studies aimed to identify mechanisms contributing to amylin/leptin synergy in reducing body weight and adiposity. We reasoned that if amylin/leptin harnessed complementary neuronal pathways, then in the leptin-sensitive state, amylin should augment leptin signaling/binding and that in the absence of endogenous amylin, leptin signaling should be diminished. Amylin (50 microg/kg, ip) amplified low-dose leptin-stimulated (15 microg/kg, ip) phosphorylated signal transducer and activator of transcription-3 signaling within the arcuate nucleus (ARC) in lean rats. Amylin (50 microg/kg x d) or leptin (125 microg/kg x d) infusion to lean rats decreased 28-d food intake (14 and 10%, respectively), body weight (amylin by 4.3%, leptin by 4.9%), and epididymal fat (amylin by 19%, leptin by 37%). Amylin/leptin co-infusion additively decreased food intake (by 26%) and reduced body weight (by 15%) and epididymal fat (by 78%; all P < 0.05 vs. all groups) in a greater than mathematically additive manner, consistent with synergy. Amylin increased leptin binding within the ventromedial hypothalamus (VMN) by 35% and dorsomedial hypothalamus by 47% (both P < 0.05 vs. vehicle). Amylin/leptin similarly increased leptin binding in the VMN by 40% and ARC by 70% (P < 0.05 vs. vehicle). In amylin-deficient mice, hypothalamic leptin receptor mRNA expression was reduced by 50%, leptin-stimulated phosphorylated signal transducer and activator of transcription-3 within ARC and VMN was reduced by 40%, and responsiveness to leptin's (1 mg/kg x d for 28 d) weight-reducing effects was attenuated (all P < 0.05 vs. wild-type controls). We suggest that amylin/leptin's marked weight- and fat-reducing effects are due to activation of intrinsic synergistic neuronal signaling pathways and further point to the integrated neurohormonal therapeutic potential of amylin/leptin agonism in obesity.

Interaction of leptin and amylin in the long-term maintenance of weight loss in diet-induced obese rats

We have previously shown that combined amylin + leptin agonism elicits synergistic weight loss in diet-induced obese (DIO) rats. Here, we assessed the comparative efficacy of amylin, leptin, or amylin + leptin in the maintenance of amylin + leptin-mediated weight loss. DIO rats pretreated with the combination of rat amylin (50 microg/kg/day) and murine leptin (125 microg/kg/day) for 4 weeks were subsequently infused with either vehicle, amylin, leptin, or amylin + leptin for an additional 4 weeks. Food intake, body weight, body composition, plasma parameters, and the expression of key metabolic genes in liver and white adipose tissue (WAT) were assessed. Amylin + leptin treatment (weeks 0-4) reduced body weight to 87.5% of baseline. Rats subsequently maintained on vehicle or leptin regained all weight (to 104.2 and 101.2% of baseline, respectively), those maintained on amylin had partial weight regain (97.0%). By contrast, weight loss was largely maintained with continued amylin + leptin treatment (91.4%), associated with a 10% decrease in adiposity. Cumulative food intake (weeks 5-8) was reduced by amylin and amylin + leptin, but not by leptin alone. Amylin + leptin, but not amylin or leptin alone, reduced plasma triglycerides (by 55%), total cholesterol (by 19%), and insulin (by 57%) compared to vehicle. Amylin + leptin also reduced hepatic stearoyl-CoA desaturase-1 (Scd1) mRNA, and increased WAT mRNA levels of adiponectin, fatty acid synthase (Fasn), and lipoprotein lipase (Lpl). We conclude that, in DIO rats, maintenance of amylin + leptin-mediated weight loss requires continued treatment with both agonists, and is accompanied by sustained improvements in body composition, and indices of lipid metabolism and insulin sensitivity.

Davalintide (AC2307), a novel amylin-mimetic peptide: enhanced pharmacological properties over native amylin to reduce food intake and body weight

OBJECTIVE

The current set of studies describe the in vivo metabolic actions of the novel amylin-mimetic peptide davalintide (AC2307) in rodents and compares these effects with those of the native peptide.

RESEARCH DESIGN AND METHODS

The anti-obesity effects of davalintide were examined after intraperitoneal injection or sustained peripheral infusion through subcutaneously implanted osmotic pumps. The effect of davalintide on food intake after lesioning of the area postrema (AP) and neuronal activation as measured by c-Fos, were also investigated.

RESULTS

Similar to amylin, davalintide bound with high affinity to amylin, calcitonin and calcitonin gene-related peptide receptors. Acutely, davalintide displayed greater suppression of dark-cycle feeding and an extended duration of action compared with amylin (23 versus 6 h). Davalintide had no effect on locomotor activity or kaolin consumption at doses that decreased food intake. Davalintide-induced weight loss through infusion was dose dependent, durable up to 8 weeks, fat-specific and lean-sparing, and was associated with a shift in food preference away from high-fat (palatable) chow. Metabolic rate was maintained during active weight loss. Both davalintide and amylin failed to suppress food intake after lesioning of the AP and activated similar brain nuclei, with davalintide displaying an extended duration of c-Fos expression compared with amylin (8 versus 2 h).

CONCLUSION

Davalintide displayed enhanced in vivo metabolic activity over amylin while retaining the beneficial properties possessed by the native molecule. In vitro receptor binding, c-Fos expression and AP lesion studies suggest that the metabolic actions of davalintide and amylin occur through activation of similar neuronal pathways.

Amylin-mediated restoration of leptin responsiveness in diet-induced obesity: magnitude and mechanisms

Previously, we reported that combination treatment with rat amylin (100 microg/kg.d) and murine leptin (500 microg/kg.d) elicited greater inhibition of food intake and greater body weight loss in diet-induced obese rats than predicted by the sum of the monotherapy conditions, a finding consistent with amylin-induced restoration of leptin responsiveness. In the present study, a 3 x 4 factorial design was used to formally test for a synergistic interaction, using lower dose ranges of amylin (0, 10, and 50 microg/kg.d) and leptin (0, 5, 25, and 125 microg/kg.d), on food intake and body weight after 4 wk continuous infusion. Response surface methodology analysis revealed significant synergistic anorexigenic (P < 0.05) and body weight-lowering (P < 0.05) effects of amylin/leptin combination treatment, with up to 15% weight loss at doses considerably lower than previously reported. Pair-feeding (PF) experiments demonstrated that reduction of food intake was the predominant mechanism for amylin/leptin-mediated weight loss. However, fat loss was 2-fold greater in amylin/leptin-treated rats than PF controls. Furthermore, amylin/leptin-mediated weight loss was not accompanied by the counterregulatory decrease in energy expenditure and chronic shift toward carbohydrate (rather than fat) utilization observed with PF. Hepatic gene expression analyses revealed that 28 d treatment with amylin/leptin (but not PF) was associated with reduced expression of genes involved in hepatic lipogenesis (Scd1 and Fasn mRNA) and increased expression of genes involved in lipid utilization (Pck1 mRNA). We conclude that amylin/leptin interact synergistically to reduce body weight and adiposity in diet-induced obese rodents through a number of anorexigenic and metabolic effects.

Antiobesity effects of the beta-cell hormone amylin in combination with phentermine or sibutramine in diet-induced obese rats

OBJECTIVE

To characterize the interactive effects of amylin with phentermine or sibutramine on food intake, body weight/composition and gene expression in diet-induced obese (DIO) rats.

DESIGN

DIO rats were intraperitoneally injected with a single dose of amylin (10 microg kg(-1)) and/or phentermine (1 mg kg(-1)) or chronically infused with amylin (100 microg kg(-1) d(-1)) or vehicle with or without phentermine (0.5-10 mg kg(-1) d(-1)) or sibutramine (3 mg kg(-1) d(-1)) using two surgically implanted subcutaneous osmotic mini-pumps.

MEASUREMENTS

Twenty-four hour food intake, locomotor activity and components of meal microstructure (meal size, latency, duration and intermeal interval) were measured following acute administration (amylin, phentermine or amylin+phentermine). Body weight and composition (for amylin and/or sibutramine or phentermine) and metabolism-related gene mRNA expression in the liver (fatty acid synthase, stearoyl-CoA desaturase-1 and carnitine palmitoyltransferase-1) and brown fat (beta-adrenergic receptors and uncoupling protein-1) were measured (for amylin and/or phentermine) after sustained infusion (2 weeks).

RESULTS

Acute co-administration of amylin (10 microg kg(-1)) and phentermine (1 mg kg(-1)) reduced acute food intake (up to 19 h) more than either monotherapy. In two studies, sustained subcutaneous infusion of amylin for 2 weeks decreased cumulative food intake (22%) and vehicle-corrected body weight gain ( approximately 4-8%). Phentermine's anorexigenic (10-17%) and weight-reducing effects ( approximately 0-5%) were only evident at the highest dose tested (10 mg kg(-1) d(-1)). Combination of amylin (100 microg kg(-1) d(-1)) and phentermine reduced food intake (30-43%), body weight (8-12%) and adiposity to a greater extent than either monotherapy. Amylin prevented phentermine-induced reductions in UCP-1 mRNA in brown adipose tissue. When amylin+sibutramine were infused, mathematically additive decreases in food intake (up to 45%) and body weight (up to 12%) were evident. Similar to amylin+phentermine treatment, amylin+sibutramine mediated weight loss was attributable to significant reductions in fat mass.

CONCLUSIONS

Combined treatment of DIO rats with the pancreatic beta-cell hormone amylin and phentermine or sibutramine resulted in additive anorexigenic, weight- and fat-reducing effects.

Leptin responsiveness restored by amylin agonism in diet-induced obesity: evidence from nonclinical and clinical studies

Body weight is regulated by complex neurohormonal interactions between endocrine signals of long-term adiposity (e.g., leptin, a hypothalamic signal) and short-term satiety (e.g., amylin, a hindbrain signal). We report that concurrent peripheral administration of amylin and leptin elicits synergistic, fat-specific weight loss in leptin-resistant, diet-induced obese rats. Weight loss synergy was specific to amylin treatment, compared with other anorexigenic peptides, and dissociable from amylin's effect on food intake. The addition of leptin after amylin pretreatment elicited further weight loss, compared with either monotherapy condition. In a 24-week randomized, double-blind, clinical proof-of-concept study in overweight/obese subjects, coadministration of recombinant human leptin and the amylin analog pramlintide elicited 12.7% mean weight loss, significantly more than was observed with either treatment alone (P < 0.01). In obese rats, amylin pretreatment partially restored hypothalamic leptin signaling (pSTAT3 immunoreactivity) within the ventromedial, but not the arcuate nucleus and up-regulated basal and leptin-stimulated signaling in the hindbrain area postrema. These findings provide both nonclinical and clinical evidence that amylin agonism restored leptin responsiveness in diet-induced obesity, suggesting that integrated neurohormonal approaches to obesity pharmacotherapy may facilitate greater weight loss by harnessing naturally occurring synergies.

Pharmacokinetics and pharmacodynamics of exenatide following alternate routes of administration

Exenatide is a 39-amino acid peptide incretin mimetic approved for adjunctive treatment of type 2 diabetes. It shares several glucoregulatory activities with the mammalian hormone, glucagon-like peptide-1 (GLP-1). In clinical use, subcutaneous exenatide injections demonstrate glucoregulatory and weight loss effects with sustained plasma concentrations in the 50-100 pM range. We investigated the pharmacokinetics of exenatide in normoglycemic rats and biological activity in diabetic db/db mice after delivery to various epithelial surfaces of the intestinal and respiratory tracts. In rats, elimination kinetics were similar for all routes of administration (median k(e) 0.017 min(-1)). Bioavailability (versus intravenous administration) and C(max) per unit dose differed markedly. For gastrointestinal administration, sublingual administration invoked the highest bioavailability (0.37%); in db/db mice, potentially therapeutic concentrations were obtainable. In contrast, intraduodenal bioavailability was low (0.0053%). In regard to respiratory surfaces, bioavailability of intratracheal exenatide was up to 13.6%, and for nasal administration, 1.68%. Both routes of administration produced therapeutic plasma concentrations and glucose-lowering in db/db mice. At high doses, aerosolized exenatide also achieved effective concentrations and glucose-lowering. In summary, the intestinal tract seems to have limited potential as a route of exenatide administration, with sublingual being most promising. In contrast, the respiratory tract appears to be more viable, comparing favorably with the clinically approved subcutaneous route. Despite little optimization of the delivery formulation, exenatide bioavailability compared favorable to that of several commercially available bioactive peptides.

Combination therapy with amylin and peptide YY[3-36] in obese rodents: anorexigenic synergy and weight loss additivity

Circulating levels of the pancreatic beta-cell peptide hormone amylin and the gut peptide PYY[3-36] increase after nutrient ingestion. Both have been implicated as short-term signals of meal termination with anorexigenic and weight-reducing effects. However, their combined effects are unknown. We report that the combination of amylin and PYY[3-36] elicited greater anorexigenic and weight-reducing effects than either peptide alone. In high-fat-fed rats, a single ip injection of amylin (10 microg/kg) plus PYY[3-36] (1000 microg/kg) reduced food intake for 24 h (P < 0.05 vs. vehicle), whereas the anorexigenic effects of either PYY[3-36] or amylin alone began to diminish 6 h after injection. These anorexigenic effects were dissociable from changes in locomotor activity. Subcutaneous infusion of amylin plus PYY[3-36] for 14 d suppressed food intake and body weight to a greater extent than either agent alone in both rat and mouse diet-induced obesity (DIO) models (P < 0.05). In DIO-prone rats, 24-h metabolic rate was maintained despite weight loss, and amylin plus PYY[3-36] (but not monotherapy) increased 24-h fat oxidation (P < 0.05 vs. vehicle). Finally, a 4 x 3 factorial design was used to formally describe the interaction between amylin and PYY[3-36]. DIO-prone rats were treated with amylin (0, 4, 20, and 100 microg/kg.d) and PYY[3-36] (0, 200, 400 microg/kg.d) alone and in combination for 14 d. Statistical analyses revealed that food intake suppression with amylin plus PYY[3-36] treatment was synergistic, whereas body weight reduction was additive. Collectively, these observations highlight the importance of studying peptide hormones in combination and suggest that integrated neurohormonal approaches may hold promise as treatments for obesity.

Biological activity of AC3174, a peptide analog of exendin-4

Exenatide, the active ingredient of BYETTA (exenatide injection), is an incretin mimetic that has been developed for the treatment of patients with type 2 diabetes. Exenatide binds to and activates the known GLP-1 receptor with a potency comparable to that of the mammalian incretin GLP-1(7-36), thereby acting as a glucoregulatory agent. AC3174 is an analog of exenatide with leucine substituted for methionine at position 14, [Leu(14)]exendin-4. The purpose of these studies was to evaluate the glucoregulatory activity and pharmacokinetics of AC3174. In RINm5f cell membranes, the potency of AC3174 for the displacement of [(125)I]GLP-1 and activation of adenylate cyclase was similar to that of exenatide and GLP-1. In vivo, AC3174, administered as a single IP injection, significantly decreased plasma glucose concentration and glucose excursion following the administration of an oral glucose challenge in both non-diabetic (C57BL/6) and diabetic db/db mice (P<0.05 vs. vehicle-treated). The magnitude of glucose lowering of AC3174 was comparable to exenatide. The ED(50) values of AC3174 for glucose lowering (60 minute post-dose) were 1.2 microg/kg in db/db mice and 1.3 microg/kg in C57BL/6 mice. AC3174 has insulinotropic activity in vivo. Administration of AC3174 resulted in a 4-fold increase in insulin concentrations in normal mice following an IP glucose challenge. AC3174 was also shown to inhibit food intake and decrease gastric emptying in rodent models. AC3174 was stable in human plasma (>90% of parent peptide was present after 5 h of incubation). In rats, the in vivo half-life of AC3174 was 42-43 min following SC administration. In summary, AC3174 is an analog of exenatide that binds to the GLP-1 receptor in vitro and shares many of the biological and glucoregulatory activities of exenatide and GLP-1 in vivo.

Investigation of exenatide elimination and its in vivo and in vitro degradation

Exenatide is a 39 amino acid incretin mimetic for the treatment of type 2 diabetes, with glucoregulatory activity similar to glucagon-like peptide-1 (GLP-1). Exenatide is a poor substrate for the major route of GLP-1 degradation by dipeptidyl peptidase-IV, and displays enhanced pharmacokinetics and in vivo potency in rats relative to GLP-1. The kidney appears to be the major route of exenatide elimination in the rat. We further investigated the putative sites of exenatide degradation and excretion, and identified primary degradants. Plasma exenatide concentrations were elevated and sustained in renal-ligated rats, when compared to sham-operated controls. By contrast, exenatide elimination and degradation was not affected in rat models of hepatic dysfunction. In vitro, four primary cleavage sites after amino acids (AA)-15, -21, -22 and -34 were identified when exenatide was degraded by mouse kidney membranes. The primary cleavage sites of exenatide degradation by rat kidney membranes were after AA-14, -15, -21, and -22. In rabbit, monkey, and human, the primary cleavage sites were after AA-21 and -22. Exenatide was almost completely degraded into peptide fragments <3 AA by the kidney membranes of the species tested. The rates of exenatide degradation by rabbit, monkey and human kidney membranes in vitro were at least 15-fold slower than mouse and rat membranes. Exenatide (1-14), (1-15), (1-22), and (23-39) were not active as either agonists or antagonists to exenatide in vitro. Exenatide (15-39) and (16-39) had moderate-to-weak antagonist activity compared with the known antagonist, exenatide (9-39). In conclusion, the kidney appears to be the primary route of elimination and degradation of exenatide.

Antiobesity action of peripheral exenatide (exendin-4) in rodents: effects on food intake, body weight, metabolic status and side-effect measures

BACKGROUND

Exenatide (exendin-4) is an incretin mimetic currently marketed as an antidiabetic agent for patients with type 2 diabetes. In preclinical models, a reduction in body weight has also been shown in low-fat-fed, leptin receptor-deficient rodents.

OBJECTIVE

To more closely model the polygenic and environmental state of human obesity, we characterized the effect of exenatide on food intake and body weight in high-fat-fed, normal (those with an intact leptin signaling system) rodents. As glucagon-like peptide-1 receptor agonism has been found to elicit behaviors associated with visceral illness in rodents, we also examined the effect of peripheral exenatide on kaolin consumption and locomotor activity.

METHODS AND RESULTS

High-fat-fed C57BL/6 mice and Sprague-Dawley rats were treated with exenatide (3, 10 and 30 microg/kg/day) for 4 weeks via subcutaneously implanted osmotic pumps. Food intake and body weight were assessed weekly. At 4 weeks, body composition and plasma metabolic profiles were measured. Kaolin consumption and locomotor activity were measured in fasted Sprague-Dawley rats following a single intraperitoneal injection of exenatide (0.1-10 microg/kg). Exenatide treatment in mice and rats dose-dependently decreased food intake and body weight; significant reductions in body weight gain were observed throughout treatment at 10 and 30 microg/kg/day (P<0.05). Decreased body weight gain was associated with a significant decrease in fat mass (P<0.05) with sparing of lean tissue. Plasma cholesterol, triglycerides and insulin were also significantly reduced (P<0.05). Exenatide at 10 microg/kg significantly reduced food intake (P<0.05) but failed to induce kaolin intake. In general, locomotor activity was reduced at doses of exenatide that decreased food intake, although a slightly higher dose was required to produce significant changes in activity.

CONCLUSION

Systemic exenatide reduces body weight gain in normal, high-fat-fed rodents, a model that parallels human genetic variation and food consumption patterns, and may play a role in metabolic pathways mediating food intake.

Dose-response for glycaemic and metabolic changes 28 days after single injection of long-acting release exenatide in diabetic fatty Zucker rats

AIMS/HYPOTHESIS

Exenatide (exendin-4) injected subcutaneously twice daily reduces glycaemic deterioration in diabetic fatty Zucker (ZDF) rats and reduces HbA1c in humans with type 2 diabetes. Because tachyphylaxis may develop with continuous peptide exposure, we examined the activity of a long-acting-release (LAR) formulation of exenatide on HbA1c, insulin sensitivity and beta cell secretion in ZDF rats.

METHODS

Single subcutaneous injections of a poly-lactide-glycolide microsphere suspension (3% peptide) containing 0, 1, 10, 100, 1,000, 3,000 or 9,000 mug exenatide were administered to 9-week-old ZDF rats with matched initial HbA1c values (n=7 rats/group).

RESULTS

In contrast to the progressive 3.22+/-0.42% increase in HbA1c in control ZDF rats observed over 28 days, single exenatide-LAR injections dose-proportionally prevented such glycaemic deterioration (median effective dose 74 microg+/-0.1 log per rat; median effective concentration 52 pmol/l+/-0.06 log). Hyperinsulinaemic-euglycaemic clamp procedures incorporating an intraclamp glucose challenge performed 28 days after treatment revealed increases in beta cell response to the glucose challenge at lower exenatide-LAR doses, and up to a 2.1-fold increase in insulin sensitivity at higher exenatide-LAR doses.

CONCLUSIONS/INTERPRETATION

The finding that a single dose of exenatide-LAR enhanced glucose control for 28 days in the ZDF rat model of type 2 diabetes suggests that tachyphylaxis is unlikely to be a feature of exenatide-LAR preparations, and supports further clinical exploration.

Exenatide (exendin-4) improves insulin sensitivity and {beta}-cell mass in insulin-resistant obese fa/fa Zucker rats independent of glycemia and body weight

The effects of the incretin mimetic exenatide (exendin-4) on metabolic parameters, insulin sensitivity, and beta-cell mass were examined in nondiabetic, insulin-resistant obese fa/fa Zucker rats. After 6 wk of treatment, ad libitum-fed exenatide-treated (EX) and pair-fed vehicle control (PF) rats had comparable food intake, body weight, hemoglobin A(1c) (HbA(1c)), and fasting plasma concentrations of glucose, insulin, and lipids. Concurrent decreases in food intake and weight gain were observed in EX and PF rats, compared with ad libitum-fed vehicle control (CON) rats (P < 0.001). The increases in HbA(1c) and fasting plasma insulin concentrations that occur during the normal progression of this disease model were significantly reduced in EX and PF rats, compared with CON rats (P < 0.001). The insulin sensitivity index (ISI; glucose infusion rate to plasma insulin concentration) measured during a hyperinsulinemic euglycemic clamp was 224% higher in EX rats than CON rats (P < 0.001) and 61% higher in EX rats than PF rats (P < 0.004). The latter difference was despite comparable HbA(1c), fasting glucose, fasting insulin, total cholesterol, high-density lipoprotein, and daily food consumption between EX and PF animals. In the absence of exenatide, beta-cell mass was hyperbolically related to ISI (beta-cell mass * ISI was constant). Analogous to the disposition index, the beta-cell mass * ISI product was 63% greater in EX than PF rats (P < 0.05). Thus, exenatide increased beta-cell mass to a greater extent than would be expected in animals of comparable insulin resistance, suggesting a direct trophic effect on islet neogenesis in obese fa/fa rats independent of body weight and glycemia.

Effects of PYY[3-36] in rodent models of diabetes and obesity

BACKGROUND

Peptide YY (PYY) is a 36 amino-acid peptide secreted from ileal L cells following meals. The cleaved subpeptide PYY[3-36] is biologically active and may constitute the majority of circulating PYY-like immunoreactivity. The peptide family that includes PYY, pancreatic peptide and neuropeptide Y is noted for its orexigenic effect following intracerebroventricular administration.

OBJECTIVE

To investigate the effects of peripheral (intraperitoneal and chronic subcutaneous) infusions of PYY[3-36] on food intake, body weight and glycemic indices.

DESIGN/RESULTS

Food intake was measured in normal mice and in several rodent models of obesity and type II diabetes. In marked contrast to the reported central orexigenic effects, in the present study, PYY[3-36] acutely inhibited food intake by up to 45%, with an ED(50) of 12.5 microg/kg in fasted female NIH/Swiss mice. A 4-week infusion reduced weight gain in female ob/ob mice, without affecting the cumulative food intake. In diet-induced obese male mice, PYY[3-36] infusion reduced cumulative food intake, weight gain and epididymal fat weight (as a fraction of carcass) with similar ED(50)'s (466, 297 and 201 microg/kg/day, respectively) and prevented a diet-induced increase in HbA1c. Infusion at 100 microg/kg/day for 8 weeks in male fa/fa rats reduced the weight gain (288+/-11 vs 326+/-12 g in saline-infused controls; P<0.05), similar to effects in a pair-fed group. In female ob/ob and db/db mice, there was no acute effect of PYY[3-36] on plasma glucose concentrations. In male diabetic fatty Zucker rats, PYY[3-36] infused for 4 weeks reduced HbA1c and fructosamine (ED(50)'s 30 and 44 microg/kg/day).

CONCLUSION

Peripheral PYY[3-36] administration reduced the food intake, body weight gain and glycemic indices in diverse rodent models of metabolic disease of both sexes. These findings justify further exploration of the potential physiologic and therapeutic roles of PYY[3-36].

Pharmacology of exenatide (synthetic exendin-4) for the treatment of type 2 diabetes

New therapies for the long-term treatment of type 2 diabetes are needed to ameliorate declining pancreatic beta-cell function. Ideally, these therapies should lower fasting and post-prandial blood glucose, produce no hypoglycemia or weight gain, cause no other limiting side effects, and reduce cardiovascular complications. Exenatide (synthetic exendin-4) is a potential therapeutic which may fulfill these criteria. Dose-ranging studies have identified an optimal dose of 0.05 to 0.2 microgram/kg administered subcutaneously twice daily. Pharmacokinetic data support a pivotal study design which mitigates the transient nausea observed in early studies by including a dose initiation period of 1 month at 5 micrograms twice daily, followed by maintenance therapy at 10 micrograms twice daily. Ongoing studies suggest exenatide improves glycemic control through a combination of mechanisms discussed in this review.

Insulinotropic actions of exendin-4 and glucagon-like peptide-1 in vivo and in vitro

This study compares in vitro effects of exendin-4 and glucagon-like peptide (GLP)-1 on basal and glucose-stimulated insulin release from isolated rat islets and in vivo insulinotropic actions of exendin-4 and GLP-1 after an intravenous glucose challenge in rats. In static incubation of isolated islets, changing ambient glucose concentration from 3 mmol/L to 10 mmol/L stimulated insulin secretion 9.8 +/- 1.3-fold. The addition of exendin-4 or GLP-1 (1 nmol/L to 1 micromol/L) increased glucose-stimulated insulin secretion up to 5.8 +/- 1.6-fold and 3.3 +/- 1.0-fold, respectively, over basal rates (defined as no hormones added, 3 mmol/L glucose) and 19.6 +/- 2.3-fold and 15.0 +/- 3.1-fold at 10 mmol/L glucose. In dynamically perfused isolated islets exposed to 7.5 mmol/L glucose, insulin secretion increased 6.4 +/- 1.5-fold, and exendin-4 (20 nmol/L) or GLP-1 (20 nmol/L) increased this similarly by up to 13.5 +/- 2.8 and 12.7 +/- 3.9-fold,respectively. Anesthetized rats administered 5.7 mmol/kg intravenous glucose increased plasma insulin concentration 3.0-fold. Infusion of exendin-4 or GLP-1 increased this to a maximum of 7.6-fold and 5.3-fold, respectively. As with isolated islet studies, in vivo dose responses and concentration responses with exendin-4 and GLP-1 were bell-shaped. When insulinotropic effects were mapped onto the steady-state plasma concentrations associated with these infusion rates, both peptides exhibited bell-shaped concentration responses with peak insulinotropic effects occurring with plasma peptide concentrations of approximately 1 nmol/L in this model. In summary, exendin-4 and GLP-1 exhibited similar insulinotropic potencies (median effective dose [ED(50)]) when assessed on a concentration basis in in vitro and in vivo models, while exendin-4 exhibited greater efficacy (maximum response).

Cardiovascular actions of CRH and urocortin: an update

Urocortin is a potent regulator of cardiac function, with actions that are prolonged in experimental animals. These changes are mediated via binding to CRH receptors found in peripheral tissues. The diversity of actions of urocortin on behaviour, appetite, inflammation and the cardiovascular system suggest that this peptide may be an endogenous factor mediating actions previously attributed to CRH. The present review will focus on the recent understanding of mechanisms mediating the cardiovascular actions of urocortin and CRH reported to date.

Selective amylin inhibition of the glucagon response to arginine is extrinsic to the pancreas

Amylin, a peptide hormone from pancreatic beta-cells, is reported to inhibit insulin secretion in vitro and in vivo and to inhibit nutrient-stimulated glucagon secretion in vivo. However, it has been reported not to affect arginine-stimulated glucagon secretion in vitro. To resolve if the latter resulted from inactive peptide (a problem in the early literature), those experiments were repeated here with well-characterized peptide and found to be valid. In isolated perfused rat pancreas preparations, coperfusion with 1 nM amylin had no effect on arginine-, carbachol-, or vasoactive intestinal peptide-stimulated glucagon secretion. Amylin also had no effect on glucagon output stimulated by decreasing glucose concentration from 11 to 3.2 mM or on glucagon suppression caused by increasing glucose from 3.2 to 7 mM. Amylin at 100 nM had no effect in isolated islets in which glucagon secretion was stimulated by exposure to 10 mM arginine, even though glucagon secretion in the same preparation was inhibited by somatostatin. In anesthetized rats, amylin coinfusion had no effect on glucagon secretion stimulated by insulin-induced hypoglycemia. To reconcile reports of glucagon inhibition with the absence of effect in the experiments just described, anesthetized rats coinfused with rat amylin or with saline were exposed sequentially to intravenous L-arginine (during a euglycemic clamp) and then to hypoglycemia. Amylin inhibited arginine-induced, but not hypoglycemia-induced, glucagon secretion in the same animal. In conclusion, we newly identify a selective glucagonostatic effect of amylin that appears to be extrinsic to the isolated pancreas and may be centrally mediated.

Urocortin: A Novel Player in Cardiac Control

Urocortin is a potent regulator of cardiac function, with actions that are prolonged in experimental animals. These changes are mediated via binding to corticotropin-releasing factor receptors found in peripheral tissues. The effects of urocortin on behavior, appetite, inflammation, and the cardiovascular system suggest that this peptide may be an endogenous factor mediating actions previously attributed to corticotropin-releasing factor.

Regulation of corticotropin-releasing factor (CRF) messenger ribonucleic acid and CRF peptide in the amygdala: studies in primary amygdalar cultures

Amygdalar CRF has been implicated in the mediation of stress behaviors. The signal transduction pathways that regulate amygdalar CRF are not well understood. In this report, we have examined the effect of protein kinase A and C activators, dexamethasone, and interleukin 6 on CRF messenger RNA (mRNA) and CRF peptide expression in dissociated amygdalar cultures. The amygdala from E19 rat pups was dissected out bilaterally and dissociated in 0.25% trypsin for 10-15 min and plated. On day 17 in culture, CRF mRNA and peptide were measured following treatment with the following agents: forskolin, the phorbol ester-phorbol 12 myristate 13-acetate (TPA), dexamethasone, and interleukin-6 (IL6). Both forskolin and IL6, but not TPA, increased CRF mRNA in a time- and dose-dependent manner. Secretion and intracellular content of the CRF peptide also increased with both forskolin and IL6 treatment but not with TPA. Dexamethasone treatment did not alter the expression of CRF message or peptide. Transfection of the primary cultures with a rat CRF promoter-luciferase reporter construct followed by treatment with all four agents produced alterations in luciferase expression that were consistent with changes observed at the level of CRF mRNA and peptide. The results suggest that CRF regulation in the amygdala differs from that known to occur in the hypothalamus, and that elevation of IL6 levels within the central nervous system may directly act to stimulate CRF production and secretion from limbic structures such as the amygdala, to promote subsequent behavioral changes.