Glucagon challenge in the rat: A robust method for the in vivo assessment of Glycogen phosphorlyase inhibitor efficacy

Pharmacokinetics of glucagon after intravenous, intraperitoneal and subcutaneous administration in a pig model

Introduction

There is increasing scientific evidence to substantiate using low‐dose glucagon as a supplement to insulin therapy in artificial pancreata for diabetes mellitus type 1. The delivery of both these hormones intraperitoneally would mimic normal physiology. However, our knowledge of the pharmacological properties of glucagon after intraperitoneal administration is limited. This study compared the pharmacokinetics of glucagon after intraperitoneal, subcutaneous and intravenous administration and the pharmacodynamic effects of glucagon on glucose metabolism after intraperitoneal and subcutaneous administration in a pig model.

Materials and methods

Twelve pigs were included. Glucagon was administered intraperitoneally, subcutaneously and intravenously in a randomised order. Arterial samples were collected every 2–10 min for 150 min to determine plasma glucagon and blood glucose concentrations.

Results

The bioavailability of glucagon was significantly lower after intraperitoneal compared with subcutaneous administration with a median difference (95% confidence interval) of 13% (4–22). The effect of glucagon on glucose metabolism was equal after intraperitoneal and subcutaneous administration.

Conclusions

Intraperitoneal glucagon administration resulted in lower systemic glucagon exposure than subcutaneous administration without loss of efficiency. We interpret this as evidence of a major first‐pass metabolism of glucagon in the liver.

Pharmacokinetics of Intraperitoneally Delivered Glucagon in Pigs: A Hypothesis of First Pass Metabolism

Background and Objective

Artificial pancreases administering low-dose glucagon in addition to insulin have the scope to improve glucose control in patients with diabetes mellitus type 1. If such a device were to deliver both hormones intraperitoneally, it would mimic normal physiology, which may be beneficial. However, the pharmacokinetic properties of glucagon after intraperitoneal administration are not well known. Hence, the current study aims to evaluate the relationship between the amount of intraperitoneally delivered glucagon and pharmacokinetic variables in a pig model.

Methods

Pharmacokinetic data was retrieved from experiments on 19 anaesthetised pigs and analysed post hoc. The animals received a single intraperitoneal bolus of glucagon ranging from 0.30 to 4.46 µg/kg. Plasma glucagon was measured every 2–10 min for 50 min.

Results

Peak plasma concentration and area under the time–plasma concentration curve of glucagon correlated positively with the administered dose, and larger boluses provided a relatively greater increase. The mean (standard deviation) time to maximum glucagon concentration in plasma was 11 (5) min, and the mean elimination half-life of glucagon in plasma was 19 (7) min.

Conclusions

Maximum plasma concentration and area under the time–plasma concentration curve of glucagon increase nonlinearly in relation to the intraperitoneally administered glucagon dose. We hypothesise that the results are compatible with a satiable first-pass metabolism in the liver. Time to maximum glucagon concentration in plasma and the elimination half-life of glucagon in plasma seem independent of the drug dose.

Intraperitoneal and subcutaneous glucagon delivery in anaesthetized pigs: effects on circulating glucagon and glucose levels

Glucagon is a pancreatic hormone and increases the blood glucose levels. It may be incorporated in a dual hormone artificial pancreas, a device to automatically and continuously control blood glucose levels of individuals with diabetes. Artificial pancreas systems have been developed for use in the subcutaneous tissue; however, the systems are not fully automated due to slow dynamics. The intraperitoneal space is therefore investigated as an alternative location for an artificial pancreas. Glucose dynamics after subcutaneous and intraperitoneal glucagon delivery in ten anaesthetized pigs were investigated. The pigs received intraperitoneal boluses of 0.3 µg/kg and 0.6 µg/kg and a subcutaneous bolus of 0.6 µg/kg in randomized order. They also received an intraperitoneal bolus of 1 mg given at the end of the experiments to test the remaining capacity of rapid glucose release. Six pigs were included in the statistical analysis. The intraperitoneal glucagon bolus of 0.6 µg/kg gave a significantly higher glucose response from 14 to 30 min compared with the subcutaneous bolus. The results indicate that glucagon induces a larger glucose response after intraperitoneal delivery compared with subcutaneous delivery and is encouraging for the incorporation of glucagon in an intraperitoneal artificial pancreas.

Effect of crude Moringa oleifera Lam. seed extract on the blood markers of metabolic syndrome in high-fructose diet-fed growing Sprague-Dawley rats

Background Moringa oleifera seed has anti-diabetic and anti-obesogenic properties. This study interrogated the effect of crude hydroethanolic M. oleifera seed extract on the blood markers of metabolic syndrome (MetS) in high-fructose diet fed growing Sprague-Dawley rats. Methods Sixty 21-day old female and male Sprague-Dawley rat pups were randomly allocated to and administered one of the following treatment regimens daily for twelve weeks: group I - plain drinking water (PW)+plain gelatine cube (PC), group II - 20% (w/v) fructose solution (FS)+PC, group III - FS+100 mg/kg body mass fenofibrate in gelatine cube (FN), group IV - FS+low dose (50 mg/kg body mass) of M. oleifera in gelatine cube (LMol) and group V - FS+high dose (500 mg/kg body mass) of M. oleifera in gelatine cube (HMol). The rats in each treatment regimen had ad libitum access to a standard rat chow. After the 12-week trial, the rats were subjected to an oral glucose tolerance test and then euthanised 48 h later. Blood was collected. Plasma triglyceride, cholesterol and insulin concentration were determined. HOMA-IR was then computed. Results The high-fructose diet increased (p<0.05) plasma insulin concentration and HOMA-IR in female rats only. It increased plasma triglyceride concentration in both female and male rats and plasma cholesterol concentration in male rats only. The crude hydroethanolic M. oleifera seed extract prevented the high-fructose diet-induced metabolic derangements in male and female rats. Conclusion Crude hydroethanolic M. oleifera seed extract can potentially be used as a prophylactic intervention for diet-induced MetS in children.

Intraperitoneal, subcutaneous and intravenous glucagon delivery and subsequent glucose response in rats: a randomized controlled crossover trial

OBJECTIVE

Hypoglycemia is a frequent and potentially dangerous event among patients with diabetes mellitus type 1. Subcutaneous glucagon is an emergency treatment to counteract severe hypoglycemia. The effect of intraperitoneal glucagon delivery is sparsely studied. We performed a direct comparison of the blood glucose response following intraperitoneally, subcutaneously and intravenously administered glucagon.

RESEARCH DESIGN AND METHODS

This is a prospective, randomized, controlled, open-label, crossover trial in 20 octreotide-treated rats. Three interventions, 1  week apart, in a randomized order, were done in each rat. All 20 rats were given intraperitoneal and subcutaneous glucagon injections, from which 5 rats were given intravenous glucagon injections and 15 rats received placebo (intraperitoneal isotonic saline) injection. The dose of glucagon was 5 µg/kg body weight for all routes of administration. Blood glucose levels were measured before and until 60 min after the glucagon/placebo injections.

RESULTS

Compared with placebo-treated rats, a significant increase in blood glucose was observed 4 min after intraperitoneal glucagon administration (p=0.009), whereas after subcutaneous and intravenous glucagon administration significant increases were seen after 8 min (p=0.002  and p<0.001, respectively). In intraperitoneally treated compared with subcutaneously treated rats, the increase in blood glucose was higher after 4 min (p=0.019) and lower after 40 min (p=0.005) and 50 min (p=0.011). The maximum glucose response occurred earlier after intraperitoneal compared with subcutaneous glucagon injection (25 min vs 35 min; p=0.003).

CONCLUSIONS

Glucagon administered intraperitoneally gives a faster glucose response compared with subcutaneously administered glucagon in rats. If repeatable in humans, the more rapid glucose response may be of importance in a dual-hormone artificial pancreas using the intraperitoneal route for administration of insulin and glucagon.

Effect of neonatal orally administered S-allyl cysteine in high-fructose diet fed Wistar rats

S-allyl cysteine (SAC) has antioxidant, antidiabetic and antiobesity properties. We hypothesized that neonatal oral administration of SAC would protect rats against neonatal and adulthood high-fructose diet-induced adverse metabolic outcomes in adulthood. In total, 112 (males=56; females=56), 4-day-old Wistar rat pups were randomly allocated to groups and administered the following treatment regimens daily for 15 days from postnatal day (PND) 6-20: group I - 10 ml/kg distilled water, group II - 10 ml/kg 20% fructose solution (FS), group III - 150 mg/kg SAC and group IV - SAC+FS. On PND 21, the pups were weaned and allowed to grow on a standard rat chow (SRC) until PND 56. The rats from each treatment regimen were then randomly split into two subgroups: one on a SRC and plain drinking water and another on SRC and 20% FS as drinking fluid and then subjected to these treatment regimens for 8 weeks after which they were euthanized and tissues collected for analyzes. Neonatal oral administration of SAC attenuated the neonatal high-fructose diet-induced programming for hepatic lipid accretion in adulthood but not against adulthood high-fructose diet-induced visceral obesity. Neonatal oral administration of SAC programmes for protection against neonatal fructose-induced programming for hepatic lipid accumulation thus could potentially protect against fat-mediated liver derangements in adult life.

Effect of dietary Ximenia caffra kernel meal on blood and liver metabolic substrate content and the general clinical biochemistry of Sprague Dawley rats

We investigated (at the University of the Witwatersrand: GPS coordinates 26°10' 52.96″S; 28°2' 33.61″E) the effects of substituting soya bean meal (SBM) with Ximenia caffra kernel meal (XCKM) as a dietary protein source on blood and liver metabolic substrates content, serum markers of liver and kidney function and the general clinical biochemistry of Sprague Dawley (SD) rats. Five diets with similar energy and protein content were formulated (D1-D5) where XCKM replaced SBM on a crude protein basis at 0, 25, 50, 75 and 100%. Forty weanling male SD rats were randomly assigned to diets D1-D5, fed for 37 days and weighed twice weekly. The rats were then fasted overnight, and fasting blood glucose and triglyceride concentrations were determined from tail-vein-drawn blood. Immediately thereafter, the rats were euthanised and blood was collected via cardiac puncture. Serum was used to assay for markers of the general health profile. Livers were removed and weighed, and samples were used to determine lipid and glycogen content. Rats fed D4 (75% substitution level) had significantly lower (p < 0.05) blood triglyceride content compared with rats fed D2 (25% level of substitution). The substitution of SBM with XCKM did not affect (p > 0.05) fasting blood glucose and cholesterol concentrations, liver glycogen and lipid content. Additionally, it had no effect (p > 0.05) on serum activity/concentration of surrogate markers of liver (alanine aminotransferase and alkaline phosphatase activity and urea, total bilirubin, globulin and albumin concentrations) and kidney (phosphorus, calcium and creatinine concentrations) function and the general clinical biochemistry of the rats. Defatted XCKM could substitute SBM in rat diets without compromising blood glucose and cholesterol homeostasis, liver and kidney function and the general clinical biochemistry of growing male Sprague Dawley rats.

Glycogen phosphorylase inhibitors: a patent review (2008 - 2012)

INTRODUCTION

Glycogen phosphorylase (GP) is the enzyme responsible for the synthesis of glucose-1-phosphate, the source of energy for muscles and the rest of the body. The binding of different ligands in catalytic or allosteric sites assures activation and deactivation of the enzyme. A description of the regulation mechanism and the implications in glycogen metabolism are given.

AREAS COVERED

Deregulation of GP has been observed in diseases such as diabetes mellitus or cancers. Therefore, it appears as an attractive therapeutic target for the treatment of such pathologies. Numbers of inhibitors have been published in academic literature or patented in the last two decades. This review presents the main patent claims published between 2008 and 2012.

EXPERT OPINION

Good inhibitors with interesting IC50 and in vivo results are presented. However, such therapeutic strategy raises questions and some answers are proposed to bring new insights in the field.

Glucose metabolism in fish: a review

Teleost fishes represent a highly diverse group consisting of more than 20,000 species living across all aquatic environments. This group has significant economical, societal and environmental impacts, yet research efforts have concentrated primarily on salmonid and cyprinid species. This review examines carbohydrate/glucose metabolism and its regulation in these model species including the role of hormones and diet. Over the past decade, molecular tools have been used to address some of the downstream components of these processes and these are incorporated to better understand the roles played by carbohydrates and their regulatory paths. Glucose metabolism remains a contentious area as many fish species are traditionally considered glucose intolerant and, therefore, one might expect that the use and storage of glucose would be considered of minor importance. However, the actual picture is not so clear since the apparent intolerance of fish to carbohydrates is not evident in herbivorous and omnivorous species and even in carnivorous species, glucose is important for specific tissues and/or for specific activities. Thus, our aim is to up-date carbohydrate metabolism in fish, placing it to the context of these new experimental tools and its relationship to dietary intake. Finally, we suggest that new research directions ultimately will lead to a better understanding of these processes.

An assessment of the in vivo efficacy of the glycogen phosphorylase inhibitor GPi688 in rat models of hyperglycaemia

BACKGROUND AND PURPOSE

Studies in cultured hepatocytes demonstrate glycogen synthase (GS) activation with glycogen phosphorylase (GP) inhibitors. The current study investigated whether these phenomena occurred in vivo using a novel GP inhibitor.

EXPERIMENTAL APPROACH

An allosteric GP inhibitor, GPi688, was evaluated against both glucagon-mediated hyperglycaemia and oral glucose challenge-mediated hyperglycaemia to determine the relative effects against GP and GS in vivo.

KEY RESULTS

In rat primary hepatocytes, GPi688 inhibited glucagons-mediated glucose output in a concentration dependent manner. Additionally GP activity was reduced and GS activity increased seven-fold. GPi688 inhibited glucagon-mediated hyperglycaemia in both Wistar (65%) & obese Zucker (100%) rats and demonstrated a long duration of action in the Zucker rat. The in vivo efficacy in the glucagon challenge model could be predicted by the equation; % glucagon inhibition=56.9+34.3[log ([free plasma]/rat IC50)], r=0.921). GPi688 also reduced the blood glucose of obese Zucker rats after a 7 h fast by 23%. In an oral glucose tolerance test in Zucker rats, however, GPi688 was less efficacious (7% reduction) than a glycogen synthase kinase-3 (GSK-3) inhibitor (22% reduction), despite also observing activation (by 45%) of GS in vivo.

CONCLUSIONS AND IMPLICATIONS

Although GP inhibition can inhibit hyperglycaemia mediated by increased glucose production, the degree of GS activation induced by allosteric GP inhibitors in vivo, although discernible, is insufficient to increase glucose disposal. The data suggests that GP inhibitors might be more effective clinically against fasting rather than prandial hyperglycaemic control.

Development of potent, orally active 1-substituted-3,4-dihydro-2-quinolone glycogen phosphorylase inhibitors

A series of substituted 3,4-dihydro-2-quinolone glycogen phosphorylase inhibitors, which have potential as antidiabetic agents, is described. Initial members of the series showed good enzyme inhibitory potency but poor physical properties. Optimisation of the 1-substituent led to 2,3-dihydroxypropyl compounds which showed good in vitro potency and improved physical properties, together with good DMPK profiles and acute in vivo efficacy in a rat model. X-ray crystallographic data are presented, showing an unexpected variety of binding orientations at the dimer interface site.