Glucagon antagonism

Glucagon is secreted by pancreatic α-cells and is the main counter-regulatory hormone to insulin, opposing its actions in the liver. Glucagon acts primarily on liver and increases blood glucose by increasing hepatic glucose production via stimulation of glycogen breakdown and gluconeogenesis. Glucagon plays a key role in maintaining euglycaemia and avoiding fasting hypoglycaemia. In diabetes, where insulin action is defective, there is an imbalance in the insulin-glucagon axis and inadequately opposed glucagon induces inappropriate over-production of glucose. This is a major causative feature of hyperglycaemia in type 2 diabetes and led to the concept that glucagon antagonism could be a viable anti-diabetic approach. In diabetic animal models, strong anti-hyperglycaemic effects were achieved by antagonism of glucagon using either anti-glucagon neutralising antibodies, anti-sense oligonucleotides to decrease expression of the glucagon receptor, or peptide and small molecule (both competitive and non-competitive) antagonists of glucagon binding to its receptor. Glucagon antagonism approaches are undergoing trials in type 2 diabetic patients in Phase 1 or Phase 2 clinical development (June 2014).

Glucagon

Glucagon is a 29 amino acid polypeptide hormone synthesised and stored in pancreatic islet α-cells, and then secreted in response to reductions of blood glucose [1]. Glucagon half-life in plasma is only a few minutes and its dynamic range is around 2 to 40 pmol/l.

In α-cells glucagon is produced from the same proglucagon precursor as GLP-1, GLP-2 and oxyntomodulin (all biologically active) and glicentin are produced from in gut L-cells and in some brain cells. In α-cells processing is catalysed by prohormone convertase-2 (PC2) as opposed to PC1/3 in the gut cells. Processing in α-cells gives glucagon and the co-released biologically inactive glicentin-related polypeptide and major proglucagon fragment.

The primary stimulus for glucagon secretion is reduction in blood glucose below the normo-glycaemic range, although how much of the effects of glucose are directly on α-cells or are indirect and involve intra-islet paracrine effects and glucose-sensing cells in the CNS is a matter of debate [2]. β-cell products (insulin, amylin, zinc ions and gamma-aminobutyric acid), somatostatin from delta cells, autonomic innervations, and glucagon itself may all be involved, and plasma concentrations of amino acids and gut hormones GLP-1, GLP-2 and GIP may also play a role, so there are potentially many different positive and negative regulators of glucagon secretion.

Within α-cells low glucose allows moderate KATP channel activity which sets the membrane potential in a range where voltage dependent Na+ and Ca2+ channels open and trigger action potentials and Ca2+ influx and secretion of glucagon into the portal vein in a pulsatile manner [2]. Elevated glucose depolarises α-cells and inactivates the voltage-gated cation channels to suppress electrical activity and secretion.

Glucagon action and mechanism

Glucagon is secreted into the portal vein to its main site of action the liver, the tissue with highest expression of the glucagon receptor (Gcgr), a class II seven-trans-membrane G-protein coupled receptor (GPCR) coupled to G proteins of GSα (primarily) or Gq types. Gcgr shares some homology with GLP-1 and GIP receptors.

There is some Gcgr expression in other tissues including heart, kidney, intestinal smooth muscle, brain (hypothalamus and brainstem), adipose tissue and pancreatic α-cells; its action and function in these other tissues is less well described.

Glucagon binding to liver Gcgr activates GSα which activates adenylyl cyclase to increase cytosolic cyclic AMP and activate Protein Kinase A (PKA) (Figure 1) as its main mechanism; additionally Gq activation can activate phospholipase C to increase cytosolic Ca2+ [1].

Figure 1.  Schematic of the actions of glucagon to stimulate hepatic glucose production, and the different current approaches (dashed lines) by which these can be antagonised.
Figure 1. Schematic of the actions of glucagon to stimulate hepatic glucose production, and the different current approaches (dashed lines) by which these can be antagonised.
PKA activation leads to activation of glycogen phosphorylase and increased expression of phosphoenol pyruvate carboxykinase (PEPCK) which then stimulate glycogenolysis and gluconeogenesis, respectively. PKA also phosphorylates and inactivates glycogen synthase and pyruvate kinase to inhibit glucose flow to glycogen and glycolysis (Figure 1). These combined actions lead to increases in hepatic glucose production to ensure euglycaemia.

Glucagon and diabetes

In diabetes, where insulin action is defective, one of the key factors driving hyperglycaemia is that glucagon continues to act and promote hepatic glucose production even when blood glucose is high. Type 2 diabetics exhibit elevated fasting and post-prandial glucagon levels, especially when considered in terms of prevailing plasma glucose and insulin levels, and their insulin resistance means that glucagon secretion and action is not sufficiently suppressed [1]. It has been estimated that > 50% of pathological increments in plasma glucose are due to inappropriate glucagon action. This led to the concept that antagonism of glucagon secretion or action may be a viable anti-diabetic approach.

Antagonism of glucagon action

There have been no direct drug discovery attempts to inhibit α-cell glucagon secretion, though somatostatin is often used in preclinical and clinical clamp studies to inhibit endogenous glucagon (and insulin) secretion in order to be able to control and set plasma glucose and insulin levels as desired. However, it is worth noting that an induced glucagon lowering effect may be a major component in the treatment of type 2 diabetes with GLP-1 analogues and dipeptidyl peptidase 4 (DPP4) inhibitors [3].

Drug discovery approaches have targeted glucagon antagonism. Studies on Gcgr knock-out mice indicated the potential benefits of this but also indicated some potential risks. These mice exhibited lowered fasting and post-prandial blood glucose compared to wild type though did not exhibit overt hypoglycaemia. However, plasma glucagon was markedly raised and accompanied by α-cell hyperplasia. Plasma GLP-1 levels were also raised and phenotypic traits associated with this evident. Mice with knock-outs of α-cell transcription factor Arx which have no α-cells also display a good anti-hyperglycaemic phenotype.

Some descriptions of rare deactivating human mutations in the Gcgr gene have indicated similar findings as in knock-out mice with an overall relatively mild phenotype, although presentation with epigastric tumours appears to have been a common feature.

Proofs of concept

The anti-diabetic potential of glucagon antagonism (Figure 1) was demonstrated in several different animal models using (i) intravenous injections of peptide antagonists of glucagon binding to Gcgr [4] (ii) intravenous injections of monoclonal antibodies against glucagon [5] (iii) intra-peritoneal injections of antisense oligonucleotides to reduce Gcgr expression [6]. Longer term treatments with (i) – (iii) have shown powerful anti-diabetic effects and in these various diabetic models there was no overt hypoglycaemia.

A common means of showing efficacy of glucagon antagonism in vivo, used pre-clinically and clinically, is a “glucagon challenge” where effective agents will suppress increases in blood glucose elicited by the injected specific agonist.

As with other class II GPCRs (secretin-like, such as that for GLP-1), and in contrast to class I GPCRs (rhodopsin-like), it has proven much more challenging to find selective small molecule antagonists of glucagon binding to Gcgr that could be developed into orally available pharmaceuticals. Nevertheless, there has been significant effort directed against this and with some degree of success. A “three pharmacophoric groups” general structure emanating from a central core appears to be a common feature of such molecules. Both reversible and non-reversible antagonists have been described. A concern with the former is that this could lead to enhanced glucagon secretion (as in Gcgr-/- mice) which could then overcome antagonism, whereas a concern with the latter is that hypoglycaemia might then be more difficult to reverse. Gcgr antisense has also been explored from a drug discovery perspective.

There are some species differences in Gcgr responses to antagonists and humanised Gcgr mice are often used (and crossed with diabetic genetic models) for drug discovery.

Glucagon antagonism has also been postulated as a potential therapeutic approach for hypo-aminoacidemia and muscle wasting.

Clinical studies

The first orally available antagonist trialled clinically was BAY 27-9955 (reported 2001) [7] and proof-of-concept in (healthy) man was shown using a glucagon challenge; however, BAY 27-9955 was discontinued.

Merck’s MK-0893 [8] reached Phase 2; 12 weeks of once daily oral dosing led to significant and dose-dependent reductions in fasting and post-prandial plasma glucose and in HbA1c, with similar low incidences of hypoglycaemia as a metformin cohort. Combinations with metformin and sitagliptin were also trialled, and risk for hypoglycaemia was assessed in healthy males which showed the drug caused lengthening of recovery times. However, plasma levels of LDL cholesterol and liver transaminases were increased in some studies, as was body weight and blood pressure, all of which were not evident pre-clinically, and MK-0893 was discontinued. Whether these potential side-effects were compound or mechanism-related is unknown.

Eli Lilly’s LY-2409021 is currently in Phase 2 (June 2014). Its structure and mechanism have not been disclosed, however (as with MK-0893) some clinical data has been presented at EASD and ADA meetings. LY-2409021 has shown good anti-hyperglycaemic efficacy, although some reversible elevations of liver enzymes was seen. Ligand recently reported (June 2014) a Phase I trial with its small molecule antagonist LGD-6972 [9].

The other approach currently in Phase 2 is antisense oligonucleotide ISIS-GCGRRx which antagonises glucagon by reducing Gcgr expression (Figure 1). When injected once weekly for 13 weeks (after three week 1 loading doses) to type 2 diabetics who were not well controlled by (and continued on) metformin, dose-dependent and significant reductions in HbA1c and fructosamine were evident [10]. At the higher dose, a >2% HbA1c reduction was observed. Moreover, treated patients had significantly increased GLP-1 levels, as was also observed pre-clinically, but no clinically significant changes in hypoglycaemia, body weight, LDL cholesterol or blood pressure. However, as with small molecule antagonists, some elevation of plasma liver enzymes was seen but without any elevation of bilirubin or other indicators of liver damage; these liver enzymes elevations were ascribed to the expected pharmacology of glucagon antagonism. There would be no drug-drug interaction concerns with such an agent which could be given in combination with other anti-diabetic agents.

Glucagon antagonism currently (June 2014) remains a potential novel therapeutic approach for diabetes and future clinical trials will determine whether this potential can be realised.

See also the website: www.glucagon.com

Dr. Jim McCormack held stock or options in (OSI)Pharmaceuticals Inc. until 2009 and in Novo Nordisk until 2010, however, he has no conflicting interests relevant to this Diapedia page written in June 2014.

References

  1. ^ Bagger JI et al. Glucagon antagonism as a potential therapeutic target in type 2 diabetes. Diab Ob Metab 2011;13:965-71

  2. ^ Quesada I et al. Physiology of the pancreatic a-cell and glucagon secretion: role in

  3. ^ Christensen M et al. The alpha-cell as target for type 2 diabetes therapy Diab Ob Metab 2011;8:369-81

  4. ^ Ahn JM et al. Development of potent truncated glucagon antagonists. J Med Chem 2001;44:1372-9

  5. ^ Brand CL et al. Evidence for a major role of glucagon in regulation of plasma glucose in conscious, nondiabetic and alloxan-induced diabetic rabbits. Diabetes 1996;45:1076-83

  6. ^ Sloop KW et al. Hepatic and glucagon-like peptide-1-mediated reversal of diabetes by glucagon receptor antisense oligonucleotide inhibitors. J Clin Invest 2004;113:1571-81

  7. ^ Petersen KF, Sullivan JT. Effects of a novel glucagon receptor antagonist (Bay 27-9955) on glucagon-stimulated glucose production in humans. Diabetologia 2001;44:2018-24

  8. ^ Xiong Y et al. Discovery of a Novel Glucagon Receptor Antagonist N-[(4-{(1S)-1-[3-(3, 5-Dichlorophenyl)-5-(6-methoxynaphthalen-2-yl)-1H-pyrazol-1-yl]ethyl}phenyl)carbonyl]-β-alanine (MK-0893) for the Treatment of Type II Diabetes J Med Chem 2012;55:6137-48

  9. ^ Vajda EG et al. Pharmacokinetics of the glucagon receptor antagonist LGD-6972. American Diabetes Association (ADA) Scientific meeting 2014, abstract number 1116-P; www.diabetes.org

  10. ^ ISIS Pharmaceuticals Press Releases of 14th May and 16th June 2014; www.isispharm.com

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