Small molecule glucagon receptor antagonists: an updated patent review (2015–2019)

Hepatic glucagon action: beyond glucose mobilization

Glucagon's ability to promote hepatic glucose production has been known for over a century, with initial observations touting this hormone as a diabetogenic agent. However, glucagon receptor agonism [when balanced with an incretin, including glucagon-like peptide 1 (GLP-1) to dampen glucose excursions] is now being developed as a promising therapeutic target in the treatment of metabolic diseases, like metabolic dysfunction-associated steatotic disease/metabolic dysfunction-associated steatohepatitis (MASLD/MASH), and may also have benefit for obesity and chronic kidney disease. Conventionally regarded as the opposing tag-team partner of the anabolic mediator insulin, glucagon is gradually emerging as more than just a "catabolic hormone." Glucagon action on glucose homeostasis within the liver has been well characterized. However, growing evidence, in part thanks to new and sensitive "omics" technologies, has implicated glucagon as more than just a "glucose liberator." Elucidation of glucagon's capacity to increase fatty acid oxidation while attenuating endogenous lipid synthesis speaks to the dichotomous nature of the hormone. Furthermore, glucagon action is not limited to just glucose homeostasis and lipid metabolism, as traditionally reported. Glucagon plays key regulatory roles in hepatic amino acid and ketone body metabolism, as well as mitochondrial turnover and function, indicating broader glucagon signaling consequences for metabolic homeostasis mediated by the liver. Here we examine the broadening role of glucagon signaling within the hepatocyte and question the current dogma, to appreciate glucagon as more than just that "catabolic hormone."

The Molecular Pharmacology of Glucagon Agonists in Diabetes and Obesity

Within the past couple decades glucagon receptor agonism has drawn attention as a therapeutic tool for the treatment of type 2 diabetes and obesity. In both mice and humans glucagon-induced enhancements in energy expenditure and suppression of food intake suggest promising utility, therefore interest has advanced in the synthetic optimization of glucagon-based pharmacology to further resolve the physiological and cellular underpinnings. Modifications within the glucagon peptide sequence have allowed for greater solubility, stability, circulating half-life, and understanding of the structure-function potential behind partial and "super"-agonists. This knowledge gained from such modifications has provided a basis for the development of long-acting therapeutically useful glucagon analogues, chimeric unimolecular dual- and tri-agonists, and novel strategies for the targeting of nuclear hormones into glucagon receptor-expressing tissues. In this review, we summarize the peptide path leading to these glucagon-based developments in the field of anti-diabetes and anti-obesity pharmacology, while highlighting the associated biological and therapeutic effects.

Alpha-cells and therapy of diabetes: Inhibition, antagonism or death?

Absolute or relative hyperglucagonaemia is a characteristic of both Type 1 and Type 2 diabetes, resulting in fasting hyperglycaemia due in part to increased hepatic glucose production and lack of postprandial suppression of circulating glucagon concentrations. Consequently, therapeutics that target glucagon secretion or biological action may be effective antidiabetic agents. In this regard, specific glucagon receptor (GCGR) antagonists have been developed that exhibit impressive glucose-lowering actions, but unfortunately may cause off-target adverse effects in humans. Further to this, several currently approved antidiabetic agents, including GLP-1 mimetics, DPP-4 inhibitors, metformin, sulphonylureas and pramlintide likely exert part of their glucose homeostatic actions through direct or indirect inhibition of GCGR signalling. In addition to agents that inhibit the release of glucagon, compounds that enhance the transdifferentiation of glucagon secreting alpha-cells towards an insulin positive beta-cell phenotype could also help curb excess glucagon secretion in diabetes. Use of alpha-cell toxins represents another possible strategy to address hyperglucagonaemia in diabetes. In that respect, research from the 1920 s with diguanides such as synthalin A demonstrated effective glucose-lowering with alpha-cell ablation in both animal models and humans with diabetes. However, further clinical use of synthalin A was curtailed due its adverse effects and the increased availability of insulin. Overall, these observations with therapeutics that directly target alpha-cells, or GCGR signaling, highlight a largely untapped potential for diabetes therapy that merits further detailed consideration.

Hepatocyte cholesterol content modulates glucagon receptor signalling

OBJECTIVE

To determine whether glucagon receptor (GCGR) actions are modulated by cellular cholesterol levels.

METHODS

We determined the effects of experimental cholesterol depletion and loading on glucagon-mediated cAMP production, ligand internalisation and glucose production in human hepatoma cells, mouse and human hepatocytes. GCGR interactions with lipid bilayers were explored using coarse-grained molecular dynamic simulations. Glucagon responsiveness was measured in mice fed a high cholesterol diet with or without simvastatin to modulate hepatocyte cholesterol content.

RESULTS

GCGR cAMP signalling was reduced by higher cholesterol levels across different cellular models. Ex vivo glucagon-induced glucose output from mouse hepatocytes was enhanced by simvastatin treatment. Mice fed a high cholesterol diet had increased hepatic cholesterol and a blunted hyperglycaemic response to glucagon, both of which were partially reversed by simvastatin. Simulations identified likely membrane-exposed cholesterol binding sites on the GCGR, including a site where cholesterol is a putative negative allosteric modulator.

CONCLUSIONS

Our results indicate that cellular cholesterol content influences glucagon sensitivity and indicate a potential molecular basis for this phenomenon. This could be relevant to the pathogenesis of non-alcoholic fatty liver disease, which is associated with both hepatic cholesterol accumulation and glucagon resistance.

Regulation of Hepatic Lipid and Glucose Metabolism by INSP3R1

With the rising epidemics of obesity and nonalcoholic fatty liver disease (NAFLD) and its downstream consequences including steatohepatitis, cirrhosis, and type 2 diabetes in the U.S. and worldwide, new therapeutic approaches are urgently needed to treat these devastating conditions. Glucagon, known for a century to be a glucose-raising hormone and clearly demonstrated to contribute to fasting and postprandial hyperglycemia in both type 1 and type 2 diabetes, represents an unlikely target to improve health in those with metabolic syndrome. However, recent work from our group and others' identifies an unexpected role for glucagon as a potential means of treating NAFLD, improving insulin sensitivity, and improving the lipid profile. We propose a unifying, calcium-dependent mechanism for glucagon's effects both to stimulate hepatic gluconeogenesis and to enhance hepatic mitochondrial oxidation: signaling through the inositol 1,4,5-trisphosphate receptor type 1 (INSP3R1), glucagon activates phospholipase C (PKC)/protein kinase A (PKA) signaling to enhance adipose triglyceride lipase (ATGL)-dependent intrahepatic lipolysis and, in turn, increase cytosolic gluconeogenesis by allosteric activation of pyruvate carboxylase. Simultaneously in the mitochondria, calcium transferred through mitochondria-associated membranes activates several dehydrogenases in the tricarboxylic acid cycle, correlated with an increase in mitochondrial energy expenditure and reduction in ectopic lipid. This model suggests that short-term, cyclic treatment with glucagon or other INSP3R1 antagonists could hold promise as a means to reset lipid homeostasis in patients with NAFLD.

Pancreatic alpha cells and glucagon secretion: Novel functions and targets in glucose homeostasis

Diabetes is the result of dysregulation of both insulin and glucagon. Still, insulin has attracted much more attention than glucagon. Glucagon is released from alpha cells in the islets of Langerhans in response to low glucose and certain amino acids. Drugs with the primary aim of targeting glucagon signalling are scarce. However, glucagon is often administered to counteract severe hypoglycaemia, and commonly used diabetes medications such as GLP-1 analogues, sulfonylureas and SGLT2-inhibitors also affect alpha cells. Indeed, there are physiological and developmental similarities between the alpha cell and the insulin-secreting beta cell and new data confirm that alpha cells can be converted into insulin-secreting cells. These aspects and attributes, the need to find novel therapies targeting the alpha cell and more are considered in this review.

Research progress of coumarins and their derivatives in the treatment of diabetes

Diabetes is a group of metabolic diseases characterised by chronic hyperglycaemia caused by multiple causes, which is caused by insulin secretion and/or utilisation defects. It is characterised by increased fasting and postprandial blood glucose levels due to insulin deficiency or insulin resistance. It is reported that the harm of diabetes mainly comes from its complications, and the cardiovascular disease caused by diabetes is the primary cause of its harm. China has the largest number of diabetic patients in the world, and the prevention and control of diabetes are facing great challenges. In recent years, many kinds of literature have been published abroad, which have proved that coumarin and its derivatives are effective in the treatment of diabetic complications such as nephropathy and cardiovascular disease. In this paper, the types of antidiabetic drugs and the anti-diabetic mechanism of coumarins were reviewed.

Role of Glucagon and Its Receptor in the Pathogenesis of Diabetes

Various theories for the hormonal basis of diabetes have been proposed and debated over the past few decades. Insulin insufficiency was previously regarded as the only hormone deficiency directly leading to metabolic disorders associated with diabetes. Although glucagon and its receptor are ignored in this framework, an increasing number of studies have shown that they play essential roles in the development and progression of diabetes. However, the molecular mechanisms underlying the effects of glucagon are still not clear. In this review, recent research on the mechanisms by which glucagon and its receptor contribute to the pathogenesis of diabetes as well as correlations between GCGR mutation rates in populations and the occurrence of diabetes are summarized. Furthermore, we summarize how recent research clearly establishes glucagon as a potential therapeutic target for diabetes.

Molecular and in vivo phenotyping of missense variants of the human glucagon receptor

Naturally occurring missense variants of G protein–coupled receptors with loss of function have been linked to metabolic disease in case studies and in animal experiments. The glucagon receptor, one such G protein–coupled receptor, is involved in maintaining blood glucose and amino acid homeostasis; however, loss-of-function mutations of this receptor have not been systematically characterized. Here, we observed fewer glucagon receptor missense variants than expected, as well as lower allele diversity and fewer variants with trait associations as compared with other class B1 receptors. We performed molecular pharmacological phenotyping of 38 missense variants located in the receptor extracellular domain, at the glucagon interface, or with previously suggested clinical implications. These variants were characterized in terms of cAMP accumulation to assess glucagon-induced Gα_s coupling, and of recruitment of β-arrestin-1/2. Fifteen variants were impaired in at least one of these downstream functions, with six variants affected in both cAMP accumulation and β-arrestin-1/2 recruitment. For the eight variants with decreased Gα_s signaling (D63^ECDN, P86^ECDS, V96^ECDE, G125^ECDC, R225^3.30H, R308^5.40W, V368^6.59M, and R378^7.35C) binding experiments revealed preserved glucagon affinity, although with significantly reduced binding capacity. Finally, using the UK Biobank, we found that variants with wildtype-like Gα_s signaling did not associate with metabolic phenotypes, whereas carriers of cAMP accumulation-impairing variants displayed a tendency toward increased risk of obesity and increased body mass and blood pressure. These observations are in line with the essential role of the glucagon system in metabolism and support that Gα_s is the main signaling pathway effecting the physiological roles of the glucagon receptor.

Striking the Balance: GLP-1/Glucagon Co-Agonism as a Treatment Strategy for Obesity

Obesity and Type 2 diabetes represent global health challenges, and there is an unmet need for long-lasting and effective pharmacotherapies. Although long-acting glucagon-like peptide-1 (GLP-1) analogues are now in routine use for diabetes and are now being utilised for obesity per se, the need for ever better treatments has driven the development of co-agonists, with the theoretical advantages of improved efficacy by targeting multiple pathways and reduced adverse effects. In this review, we highlight the past and present progress in our understanding and development of treatments based on GLP-1/glucagon co-agonism. We also reflect on the divergent effects of varying the GLP-1:glucagon activity and ratio in the context of pre-clinical and human clinical trial findings. In particular, the multiple metabolic actions of glucagon highlight the importance of understanding the contributions of individual hormone action to inform the safe, effective and tailored use of GLP-1/glucagon co-agonists to target weight loss and metabolic disease in the future.

A Patent Review on the Current Developments of Benzoxazoles in Drug Discovery

The benzoxazole moiety is widely found in various natural compounds, which are often found to be biologically active. Due to its versatile biological properties, benzoxazole has been incorporated as an essential pharmacophore and substructure in many medicinal compounds. In the past years, numerous benzoxazole derivatives have been synthesised and evaluated for their biological potential. The wide range in therapeutic potential of benzoxazole derivatives is related to the favourable interactions of the benzoxazole moiety with different protein targets. Herein we review the biological activities of benzoxazole derivatives patented within the past six years. Using the Lens database, granted patents issued from 2015 to 2020 were retrieved. The patented benzoxazole derivatives demonstrated excellent activity against various protein targets and diseases, with some reaching clinical trial stage. Pharmacological and medicinal aspects of patented benzoxazole derivatives are discussed. The recent development and drawbacks are also reviewed.

Optimization of Truncated Glucagon Peptides to Achieve Selective, High Potency, Full Antagonists

Antagonism of glucagon's biological action is a proven strategy for decreasing glucose in diabetic animals and patients. To achieve full, potent, and selective suppression, we chemically optimized N-terminally truncated glucagon fragments for the identification and establishment of the minimum sequence peptide, [Glu9]glucagon(6-29) amide (11) as a full antagonist in cellular signaling and receptor binding (IC₅₀ = 36 nM). Substitution of Phe6 with l-3-phenyllactic acid (Pla) produced [Pla6, Glu9]glucagon(6-29) amide (21), resulting in a 3-fold improvement in receptor binding (IC₅₀ = 12 nM) and enhanced antagonist potency. Further substitution of Glu9 and Asn28 with aspartic acid yielded [Pla6, Asp28]glucagon amide (26), which demonstrated a further increase in inhibitory potency (IC₅₀ = 9 nM), and improved aqueous solubility. Peptide 26 and a palmitoylated analogue, [Pla6, Lys10(γGluγGlu-C16), Asp28]glucagon(6-29) amide (31), displayed sustained duration in vivo action that successfully reversed glucagon-induced glucose elevation in mice.

Integrated Metabolomics and Lipidomics Analysis Reveal Remodeling of Lipid Metabolism and Amino Acid Metabolism in Glucagon Receptor-Deficient Zebrafish

The glucagon receptor (GCGR) is activated by glucagon and is essential for glucose, amino acid, and lipid metabolism of animals. GCGR blockade has been demonstrated to induce hypoglycemia, hyperaminoacidemia, hyperglucagonemia, decreased adiposity, hepatosteatosis, and pancreatic α cells hyperplasia in organisms. However, the mechanism of how GCGR regulates these physiological functions is not yet very clear. In our previous study, we revealed that GCGR regulated metabolic network at transcriptional level by RNA-seq using GCGR mutant zebrafish (gcgr ^-/-). Here, we further performed whole-organism metabolomics and lipidomics profiling on wild-type and gcgr ^-/- zebrafish to study the changes of metabolites. We found 107 significantly different metabolites from metabolomics analysis and 87 significantly different lipids from lipidomics analysis. Chemical substance classification and pathway analysis integrated with transcriptomics data both revealed that amino acid metabolism and lipid metabolism were remodeled in gcgr-deficient zebrafish. Similar to other studies, our study showed that gcgr ^-/- zebrafish exhibited decreased ureagenesis and impaired cholesterol metabolism. More interestingly, we found that the glycerophospholipid metabolism was disrupted, the arachidonic acid metabolism was up-regulated, and the tryptophan metabolism pathway was down-regulated in gcgr ^-/- zebrafish. Based on the omics data, we further validated our findings by revealing that gcgr ^-/- zebrafish exhibited dampened melatonin diel rhythmicity and increased locomotor activity. These global omics data provide us a better understanding about the role of GCGR in regulating metabolic network and new insight into GCGR physiological functions.