Andreas Herling

Pharmacological profile of lixisenatide: A new GLP-1 receptor agonist for the treatment of type 2 diabetes.

The glucagon-like peptide-1 (GLP-1) receptor represents an established therapeutic target in type 2 diabetes mellitus (T2DM). Agents that activate this receptor improve glucose tolerance alongside a low risk of hypoglycaemia, and have the potential to modify disease progression. Lixisenatide is a new potent and selective GLP-1 receptor agonist currently in development. The preclinical pharmacological profile of Lixisenatide suggests actions that are highly relevant to the long-term maintenance of glucose homeostasis. Lixisenatide protected Ins-1 cells (a rat-derived beta-cell line) from both lipid- and cytokine-induced apoptosis. More importantly, Lixisenatide also prevented lipotoxicity-induced insulin depletion in human islets and preserved insulin production, storage and pancreatic beta-cell function in vitro. Enhancement of insulin biosynthesis and pancreatic beta-cell volume could also be demonstrated in animal models of type 2 diabetes. The improvement of glucose-stimulated insulin secretion provided by Lixisenatide occurred in a strictly glucose-dependent manner. In animal models of diabetes, Lixisenatide improved basal blood glucose and HbA(1c) with a rapid onset and sustained duration of action, and prevented the deterioration of pancreatic responsiveness and glucose homeostasis. Lixisenatide also delayed gastric emptying and reduced food intake. The efficacy/safety profile of Lixisenatide is currently being studied further in an extensive ongoing Phase III clinical study programme. This article reviews the preclinical pharmacological profile of Lixisenatide.

Increased Energy Expenditure Contributes More to the Body Weight-Reducing Effect of Rimonabant than Reduced Food Intake in Candy-Fed Wistar Rats

The CB1 receptor antagonist, rimonabant, affects the endocannabinoid system and causes a sustained reduction in body weight (BW) despite the transient nature of the reduction in food intake. Therefore, in a multiple-dose study, female candy-fed Wistar rats were treated with rimonabant (10 mg/kg) and matched with pair-fed rats to distinguish between hypophagic action and hypothesized effects on energy expenditure. Within the first week of treatment, rimonabant reduced BW nearly to levels of standard rat chow-fed rats. Evaluation of energy balance (energy expenditure measured by indirect calorimetry in relation to metabolizable energy intake calculated by bomb calorimetry) revealed that increased energy expenditure based on increased fat oxidation contributed more to sustained BW reduction than reduced food intake. A mere food reduction through pair feeding did not result in comparable effects because animals reduced their energy expenditure to save energy stores. Because fat oxidation measured by indirect calorimetry increased immediately after dosing in the postprandial state, the acute effect of rimonabant on lipolysis was investigated in postprandial male rats. Rimonabant elevated free fatty acids postprandially, demonstrating an inherent pharmacological activity of rimonabant to induce lipolysis and not secondarily postabsorptively due to reduced food intake. We conclude that the weight-reducing effect of rimonabant was due to continuously elevated energy expenditure based on increased fat oxidation driven by lipolysis from fat tissue as long as fat stores were elevated. When the amount of endogenous fat stores declined, rimonabant-induced increased energy expenditure was maintained by a re-increase in food intake.

Guidelines for the care and use of laboratory animals

Blood is collected from laboratory animals for various scientific purposes, for example, to study the effects of a test drug on various constituents, such as hormones, substrates, or blood cells. In the field of pharmacokinetics and drug metabolism, blood samples are necessary for analytical determination of the drug and its metabolites. Blood is also needed for some in vitro assays using blood cells or defined plasma protein fractions.

Pharmacodynamic profile of a novel inhibitor of the hepatic glucose-6-phosphatase system

The glucose-6-phosphatase (G-6-Pase) system catalyzes the terminal enzymatic step of gluconeogenesis and glycogenolysis. Inhibition of the G-6-Pase system in the liver is expected to result in a reduction of hepatic glucose production irrespective of the relative contribution of gluconeogenesis or glycogenolysis to hepatic glucose output. In isolated perfused rat liver, S-3483, a derivative of chlorogenic acid, produced concentration-dependent inhibition of gluconeogenesis and glycogenolysis in a similar concentration range. In fed rats, glucagon-induced glycogenolysis resulted in hyperglycemia for nearly 2 h. Intravenous infusion of 50 mg . kg-1. h-1 S-3483 prevented the hyperglycemic peak and subsequently caused a further lowering of blood glucose. In 24-h starved rats, in which normoglycemia is maintained predominantly by gluconeogenesis, intravenous infusion of S-3483 resulted in a constant reduction of blood glucose levels. Intrahepatic concentrations of glucose-6-phosphate (G-6-P) and glycogen were significantly increased at the end of both in vivo studies. In contrast, lowering of blood glucose in starved rats by 3-mercaptopicolinic acid was accompanied by a reduction of G-6-P and glycogen. Our results demonstrate for the first time in vivo a pharmacologically induced suppression of hepatic G-6-P activity with subsequent changes in blood glucose levels.

Adenosine A1 receptors regulate lipolysis and lipogenesis in mouse adipose tissue-interactions with insulin.

Adenosine acting at adenosine A1 receptors is considered to be one major regulator of adipose tissue physiology. We have examined the role of adenosine and its interactions with insulin in adipose tissue by using A1R knock out (-/-) mice. Removal of endogenous adenosine with adenosine deaminase caused lipolysis in A1R (+/+), but not A1R (-/-) adipocytes. The adenosine analogue, 2-chloroadenosine, inhibited noradrenaline-stimulated lipolysis and cAMP accumulation in A1R (+/+), but not in A1R (-/-) adipocytes. Insulin reduces lipolysis and cAMP via another mechanism than adenosine and acted additively, but not synergistically, with adenosine. Plasma levels of free fatty acids, glycerol and triglycerides were significantly lower in A1R (+/+) than in A1R (-/-) mice after administration of an adenosine analogue. 2-chloroadenosine induced lipogenesis in presence of insulin in A1R (+/+), but not in A1R (-/-) adipocytes. There were no changes in mRNA levels for several genes involved in fat synthesis in adipose tissue between genotypes. Body weight was similar in young A1R (+/+) and A1R (-/-) mice, but old male A1R (-/-) mice were heavier than wild type controls. In conclusion, adenosine inhibits lipolysis via the adenosine A1 receptor and other adenosine receptors play no significant role. Adenosine and insulin mediate additive but not synergistic antilipolytic effects and 2-chloroadenosine stimulates lipogenesis via adenosine A1 receptors. Thus deletion of adenosine A1 receptors should increase lipolysis and decrease lipogenesis, but in fact an increased fat mass was observed, indicating that other actions of adenosine A1 receptors, possibly outside adipose tissue, are also important.

Alterations of carbohydrate and lipid intermediary metabolism during inhibition of glucose-6-phosphatase in rats

S 4048 (1-[2-(4-Chloro-phenyl)-cyclopropylmethoxy]-3, 4-dihydroxy-5-(3-imidazo[4, 5-b]pyridin-1-yl-3-phenyl-acryloyloxy)-cyclohexanecarboxylic acid), a derivative of chlorogenic acid, specifically inhibits the glucose-6-phosphate translocating component T1 of the glucose-6-phosphatase system. Its pharmacological effect was studied on carbohydrate and lipid parameters in rats. In starved and fed rats, S 4048 caused a dose-dependent reduction of blood glucose levels with a corresponding increase in hepatic and renal glycogen and glucose-6-phosphate. The major quantitative route of carbon flux in the liver during S 4048-induced inhibition of the glucose-6-phosphatase activity seemed to be glycogenesis. Plasma free fatty acids were increased secondarily due to the S 4048-induced hypoglycemia. Hepatic triglycerides were increased possibly due to increased re-esterification of the readily available free fatty acids. Glucose-6-phosphate translocase inhibitors may be useful for experimentally studying aspects of type 1 glycogen storage disease in laboratory animals as well as for the therapeutic modulation of inappropriately high rates of hepatic glucose production in type 2 diabetes.

Correlation between insulin resistance and intramyocellular lipid levels in rats

Increased intramyocellular lipid (IMCL) content has been proposed as biomarker for insulin resistance (IR). An inverse correlation between IMCL and insulin sensitivity (IS) was found in nonathletic humans, whereas in animal models only a few validation studies have been performed. The aim of this study was to investigate the interrelation between IS indices determined by the glucose clamp technique (glucose disposal (GD), exogenous glucose infusion rates (GIR)) and IMCL content in the tibialis (TIB) and the soleus (SOL) muscle obtained by magnetic resonance spectroscopy in different rat models of IR. Diet‐induced insulin‐resistant Wistar rats as well as genetic disease models (ZDF rats) were used. In both muscles, elevated IMCL correlated with an impaired IS in all models of IR. The correlation of IMCL with both parameters for IS was comparable in TIB and SOL. The best fit between IMCL and IS was obtained using TIB and GIR data (r = −0.69, P < 0.001). Diabetic male ZDF rats exhibited comparatively low IMCL levels due to their catabolic state: exclusion of this group improved r. In summary, IMCL, especially in TIB, is a valid biomarker for IS in various rat models of IR with the advantage of a fast repeatable noninvasive measurement in individual animals. Magn Reson Med 53:1275–1282, 2005.

Fatty acid and amino acid modulation of glucose cycling in isolated rat hepatocytes

We studied the influence of glucose/glucose 6-phosphate cycling on glycogen deposition from glucose in fasted-rat hepatocytes using S4048 and CP320626, specific inhibitors of glucose-6-phosphate translocase and glycogen phosphorylase respectively. The effect of amino acids and oleate was also examined. The following observations were made: (1) with glucose alone, net glycogen production was low. Inhibition of glucose-6-phosphate translocase increased intracellular glucose 6-phosphate (3-fold), glycogen accumulation (5-fold) without change in active (dephosphorylated) glycogen synthase (GSa) activity, and lactate production (4-fold). With both glucose 6-phosphate translocase and glycogen phosphorylase inhibited, glycogen deposition increased 8-fold and approached reported in vivo rates of glycogen deposition during the fasted-->fed transition. Addition of a physiological mixture of amino acids in the presence of glucose increased glycogen accumulation (4-fold) through activation of GS and inhibition of glucose-6-phosphatase flux. Addition of oleate with glucose present decreased glycolytic flux and increased the flux through glucose 6-phosphatase with no change in glycogen deposition. With glucose 6-phosphate translocase inhibited by S4048, oleate increased intracellular glucose 6-phosphate (3-fold) and net glycogen production (1.5-fold), without a major change in GSa activity. It is concluded that glucose cycling in hepatocytes prevents the net accumulation of glycogen from glucose. Amino acids activate GS and inhibit flux through glucose-6-phosphatase, while oleate inhibits glycolysis and stimulates glucose-6-phosphatase flux. Variation in glucose 6-phosphate does not always result in activity changes of GSa. Activation of glucose 6-phosphatase flux by fatty acids may contribute to the increased hepatic glucose production as seen in Type 2 diabetes.

The role of glucose 6‐phosphate in mediating the effects of glucokinase overexpression on hepatic glucose metabolism

Pharmacological activation or overexpression of glucokinase in hepatocytes stimulates glucose phosphorylation, glycolysis and glycogen synthesis. We used an inhibitor of glucose 6‐phosphate (Glc6P) hydrolysis, namely the chlorogenic derivative, 1‐[2‐(4‐chloro‐phenyl)‐cyclopropylmethoxy]‐3, 4‐dihydroxy‐5‐(3‐imidazo[4,5‐b]pyridin‐1‐yl‐3‐phenyl‐acryloyloxy)‐cyclohexanecarboxylic acid (also known as S4048), to determine the contribution of Glc6P concentration, as distinct from glucokinase protein or activity, to the control of glycolysis and glycogen synthesis by glucokinase overexpression. The validity of S4048 for testing the role of Glc6P was supported by its lack of effect on glucokinase binding and its nuclear/cytoplasmic distribution. The stimulation of glycolysis by glucokinase overexpression correlated strongly with glucose phosphorylation, whereas glycogen synthesis correlated strongly with Glc6P concentration. Metabolic control analysis was used to determine the sensitivity of glycogenic flux to glucokinase or Glc6P at varying glucose concentrations (5–20 mm). The concentration control coefficient of glucokinase on Glc6P (1.4–1.7) was relatively independent of glucose concentration, whereas the flux control coefficients of Glc6P (2.4–1.0) and glucokinase (3.7–1.8) on glycogen synthesis decreased with glucose concentration. The high sensitivity of glycogenic flux to Glc6P at low glucose concentration is consistent with covalent modification by Glc6P of both phosphorylase and glycogen synthase. The high control strength of glucokinase on glycogenic flux is explained by its concentration control coefficient on Glc6P and the high control strength of Glc6P on glycogen synthesis. It is suggested that the regulatory strength of pharmacological glucokinase activators on glycogen metabolism can be predicted from their effect on the Glc6P content.

Effects of AVE2268, a Substituted Glycopyranoside, on Urinary Glucose Excretion and Blood Glucose in Mice and Rats

AVE2268, a substituted glycopyranoside, is an orally active and selective inhibitor of sodium-dependent glucose transporter 2 (SGLT2; IC50 = 13 nmol/L). Investigation of the pharmacological profile of AVE2268 on urinary glucose excretion (UGE) and blood glucose after glucose challenge (po or Intraperitoneal) was performed in mice and rats. AVE2268 caused a dose-dependent increase of UGE in mice (ID30 = 79 +/- 8.1 mg/kg p.o.) and rats (ID30 = 39.8 +/- 4.0 mg/kg p.o.). AVE2268 in mice was more potent to decrease blood glucose ascent when glucose was given intraperitoneally (ID50 = 13.2 +/- 3.9 mg/ kg), compared to orally administered glucose (ID50 = 26.1 +/- 3.9 mg/kg), showing that AVE2268 has no effects on SGLT 1 in the gut in vivo, which is in accordance with ist very low affinity to the SGLT 1 in vitro (IC50 >10,000 nmol/L). During an oral glucose tolerance test, AVE2268 dose-dependently increased UGE, with subsequent decreases of AUC and blood glucose. A highly significant inverse correlation between AUC and UGE was found (p < 0.001). The increase in UGE is linked to the inhibition of SGLT2 only. This profile renders AVE2268 as a new antidiabetic drug for the treatment of type 2 diabetes.