A survey of small molecule glucagon receptor antagonists from recent patents (2006 – 2010)

In silico screening-based discovery of benzamide derivatives as inhibitors of Rho-associated kinase-1 (ROCK1)

As a pivotal node in modulating various cell behaviors, Rho-associated kinase-1 (ROCK1) has attracted significant attention as a promising therapeutic target in a variety of diseases. Benzamide has been widely reported as a ROCK1 inhibitors in recent years. To better understand its pharmacological properties and to explore its potential inhibitors, a series of ROCK1 inhibitors derived from N-methyl-4-(4-pyrazolidinyl) benzamides (MPBs) were investigated by using three-dimensional quantitative structure-activity relationship (3D-QSAR) models, pharmacophore models, molecular docking, and molecular dynamics (MD) simulation. The comparative Molecular Field Analysis (CoMFA) model (q2 = 0.616, R2 = 0.972, ONC = 4, and r2pred = 0.983) and the best Comparative Molecular Similarity Indices Analysis (CoMSIA) model (q2= 0.740, R2 = 0.982, ONC = 6, and r2pred = 0.824) exhibited reliable predictability with satisfactory validation parameters. In the subsequent virtual screening, VS03 and VS05 were identified to have superior predicted activities and higher docking scores, meanwhile they demonstrated to be reasonably stable in the binding pocket through MD simulations. These results provide a significant theoretical direction for the rational design and development of novel ROCK1 inhibitors.Communicated by Ramaswamy H. Sarma.

The relationship between glucose and the liver-alpha cell axis - A systematic review

Until recently, glucagon was considered a mere antagonist to insulin, protecting the body from hypoglycemia. This notion changed with the discovery of the liver-alpha cell axis (LACA) as a feedback loop. The LACA describes how glucagon secretion and pancreatic alpha cell proliferation are stimulated by circulating amino acids. Glucagon in turn leads to an upregulation of amino acid metabolism and ureagenesis in the liver. Several increasingly common diseases (e.g., non-alcoholic fatty liver disease, type 2 diabetes, obesity) disrupt this feedback loop. It is important for clinicians and researchers alike to understand the liver-alpha cell axis and the metabolic sequelae of these diseases. While most of previous studies have focused on fasting concentrations of glucagon and amino acids, there is limited knowledge of their dynamics after glucose administration. The authors of this systematic review applied PRISMA guidelines and conducted PubMed searches to provide results of 8078 articles (screened and if relevant, studied in full). This systematic review aims to provide better insight into the LACA and its mediators (amino acids and glucagon), focusing on the relationship between glucose and the LACA in adult and pediatric subjects.

Newly discovered endocrine functions of the liver

The liver, the largest solid visceral organ of the body, has numerous endocrine functions, such as direct hormone and hepatokine production, hormone metabolism, synthesis of binding proteins, and processing and redistribution of metabolic fuels. In the last 10 years, many new endocrine functions of the liver have been discovered. Advances in the classical endocrine functions include delineation of mechanisms of liver production of endocrine hormones [including 25-hydroxyvitamin D, insulin-like growth factor 1 (IGF-1), and angiotensinogen], hepatic metabolism of hormones (including thyroid hormones, glucagon-like peptide-1, and steroid hormones), and actions of specific binding proteins to glucocorticoids, sex steroids, and thyroid hormones. These studies have furthered insight into cirrhosis-associated endocrinopathies, such as hypogonadism, osteoporosis, IGF-1 deficiency, vitamin D deficiency, alterations in glucose and lipid homeostasis, and controversially relative adrenal insufficiency. Several novel endocrine functions of the liver have also been unraveled, elucidating the liver’s key negative feedback regulatory role in the pancreatic α cell-liver axis, which regulates pancreatic α cell mass, glucagon secretion, and circulating amino acid levels. Betatrophin and other hepatokines, such as fetuin-A and fibroblast growth factor 21, have also been discovered to play important endocrine roles in modulating insulin sensitivity, lipid metabolism, and body weight. It is expected that more endocrine functions of the liver will be revealed in the near future.

Effect of hydrophobic and hydrogen bonding interactions on the potency of ß-alanine analogs of G-protein coupled glucagon receptor inhibitors

G-protein coupled glucagon receptors (GCGRs) play an important role in glucose homeostasis and pathophysiology of Type-II Diabetes Mellitus (T2DM). The allosteric pocket located at the trans-membrane domain of GCGR consists of hydrophobic (TM5) and hydrophilic (TM7) units. Hydrophobic interactions with the amino acid residues present at TM5, found to facilitate the favorable orientation of antagonist at GCGR allosteric pocket. A statistically robust and highly predictive 3D-QSAR model was developed using 58 β-alanine based GCGR antagonists with significant variation in structure and potency profile. The correlation coefficient (R² ) and cross-validation coefficient (Q² ) of the developed model were found to be 0.9981 and 0.8253, respectively at the PLS factor of 8. The analysis of the favorable and unfavorable contribution of different structural features on the glucagon receptor antagonists was done by 3D-QSAR contour plots. Hydrophobic and hydrogen bonding interactions are found to be main dominating non-bonding interactions in docking studies. Presence of highest occupied molecular orbital (HOMO) in the polar part and lowest unoccupied molecular orbital (LUMO) in the hydrophobic part of antagonists leads to favorable protein-ligand interactions. Molecular mechanics/generalized born surface area (MM/GBSA) calculations showed that van der Waals and nonpolar solvation energy terms are crucial components for thermodynamically stable binding of the inhibitors. The binding free energy of highly potent compound was found to be -63.475 kcal/mol; whereas the least active compound exhibited binding energy of -41.097 kcal/mol. Further, five 100 ns molecular dynamics simulation (MD) simulations were done to confirm the stability of the inhibitor-receptor complex. Outcomes of the present study can serve as the basis for designing improved GCGR antagonists.

Glucagon revisited: Coordinated actions on the liver and kidney

Glucagon secretion is stimulated by a low plasma glucose concentration. By activating glycogenolysis and gluconeogenesis in the liver, glucagon contributes to maintain a normal glycemia. Glucagon secretion is also stimulated by the intake of proteins, and glucagon contributes to amino acid metabolism and nitrogen excretion. Amino acids are used for gluconeogenesis and ureagenesis, two metabolic pathways that are closely associated. Intriguingly, cyclic AMP, the second messenger of glucagon action in the liver, is released into the bloodstream becoming an extracellular messenger. These effects depend not only on glucagon itself but on the actual glucagon/insulin ratio because insulin counteracts glucagon action on the liver. This review revisits the role of glucagon in nitrogen metabolism and in disposal of nitrogen wastes. This role involves coordinated actions of glucagon on the liver and kidney. Glucagon influences the transport of fluid and solutes in the distal tubule and collecting duct, and extracellular cAMP influences proximal tubule reabsorption. These combined effects increase the fractional excretion of urea, sodium, potassium and phosphates. Moreover, the simultaneous actions of glucagon and extracellular cAMP are responsible, at least in part, for the protein-induced rise in glomerular filtration rate that contributes to a more efficient excretion of protein-derived end products.

Renal potassium handling in carriers of the Gly40Ser mutation of the glucagon receptor suggests a role for glucagon in potassium homeostasis

Plasma potassium concentration (PK ) is tightly regulated. Insulin is known to favor potassium entry into cells. But how potassium leaves the cells later on is not often considered. Previous studies in rats showed that glucagon infusion increased urinary potassium excretion dose-dependently and reversibly. This prompted us to investigate the possible influence of glucagon on potassium handling in humans. We took advantage of the Gly40Ser mutation of the glucagon receptor (GR) that results in a partial loss of function of the GR. In the Olivetti cohort (male workers), 25 subjects who carried this mutation were matched 1:4 to 100 noncarriers for age and weight. Estimated osmolarity of serum and 24-h urine (Sosm and Uosm, respectively) was calculated from the concentrations of the main solutes: [(Na+K)*2 + urea (+glucose for serum)]. Transtubular potassium gradient (TTKG), reflecting the intensity of K secretion in the distal nephron, was calculated as [(urine K/serum K)(Uosm /Sosm )]. There was no significant difference in serum K, or 24-h urine urea, Na and K excretion rates. But urine K concentration was significantly lower in carriers than in noncarriers. Means (interquartile range): 38 (34-43) versus 47 (43-51) mmol/L, P = 0.030. TTKG was also significantly lower in carriers: 4.2 (3.9-4.6) versus 5.0 (4.7-5.2), P = 0.015. This difference remained statistically significant after adjustments for serum insulin and 24-h Na and urea excretions. These results in humans suggest that glucagon stimulates K secretion in the distal nephron. Thus, in conjunction with insulin, glucagon may also participate in K homeostasis by promoting renal K excretion.

Investigational glucagon receptor antagonists in Phase I and II clinical trials for diabetes

INTRODUCTION

Despite type 2 diabetes (T2D) being recognized as a bihormonal pancreatic disease, current therapies are mainly focusing on insulin, while targeting glucagon has been long dismissed. However, glucagon receptor (GCGr) antagonists are currently investigated in clinical trials. Area covered: Following a brief description of the rationale for antagonizing GCGr in T2D, lessons from GCGr knock-out mice and pharmacological means to antagonize GCGr, a detailed description of the main results obtained with GCGr antagonists in Phase I-II clinical trials is provided. The development of several small molecules has been discontinued, while new ones are currently considered as well as innovative approaches such as monoclonal antibodies or antisense oligonucleotides inhibiting GCGr gene expression. Their potential benefits but also limitations are discussed. Expert opinion: The proof-of-concept that antagonizing GCGr improves glucose control in T2D has been confirmed in humans. Nevertheless, some adverse events led to stopping the development of some of these GCGr antagonists. New approaches seem to have a better benefit/risk balance, although none has progressed to Phase III clinical trials so far. Pharmacotherapy of T2D is becoming a highly competitive field so that GCGr antagonists should provide clear advantages over numerous existing glucose-lowering medications before eventually reaching clinical practice.

Structure and Function of Peptide-Binding G Protein-Coupled Receptors

G protein-coupled receptors (GPCRs) are the largest family of cell surface receptors and are important human drug targets. Of the 826 human GPCRs, 118 of them recognize endogenous peptide or protein ligands, and 30 of the 118 are targeted by approved drug molecules, including the very high-profile class B glucagon-like peptide 1 receptor. In this review, we analyze the 21 experimentally determined three-dimensional structures of the known peptide-binding GPCRs in relation to the endogenous peptides and drug molecules that modulate their cell signaling processes. Our integrated analyses reveal that half of the marketed drugs and most of the drugs in clinical trials that interact with peptide GPCRs are small molecules with a wide range of binding modes distinct from those of large peptide ligands. As we continue to collect additional data on these receptors from orthogonal approaches, including nuclear magnetic resonance and electron microscopy, we are beginning to understand how these receptors interact with their ligands at the molecular level and how improving the pharmacology of GPCR signal transduction requires us to study these receptors using multiple biophysical techniques.

Glucagon actions on the kidney revisited: possible role in potassium homeostasis

It is now recognized that the metabolic disorders observed in diabetes are not, or not only due to the lack of insulin or insulin resistance, but also to elevated glucagon secretion. Accordingly, selective glucagon receptor antagonists are now proposed as a novel strategy for the treatment of diabetes. However, besides its metabolic actions, glucagon also influences kidney function. The glucagon receptor is expressed in the thick ascending limb, distal tubule, and collecting duct, and glucagon regulates the transepithelial transport of several solutes in these nephron segments. Moreover, it also influences solute transport in the proximal tubule, possibly by an indirect mechanism. This review summarizes the knowledge accumulated over the last 30 years about the influence of glucagon on the renal handling of electrolytes and urea. It also describes a possible novel role of glucagon in the short-term regulation of potassium homeostasis. Several original findings suggest that pancreatic α-cells may express a “potassium sensor” sensitive to changes in plasma K concentration and could respond by adapting glucagon secretion that, in turn, would regulate urinary K excretion. By their combined actions, glucagon and insulin, working in a combinatory mode, could ensure an independent regulation of both plasma glucose and plasma K concentrations. The results and hypotheses reviewed here suggest that the use of glucagon receptor antagonists for the treatment of diabetes should take into account their potential consequences on electrolyte handling by the kidney.

A novel series of indazole-/indole-based glucagon receptor antagonists

A novel, potent series of glucagon receptor antagonists (GRAs) was discovered. These indazole- and indole-based compounds were designed on an earlier pyrazole-based GRA lead MK-0893. Structure-activity relationship (SAR) studies were focused on the C3 and C6 positions of the indazole core, as well as the benzylic position on the N-1 of indazole. Multiple potent GRAs were identified with excellent in vitro profiles and good pharmacokinetics in rat. Among them, GRA 16d was found to be orally active in blunting glucagon induced glucose excursion in an acute glucagon challenge model in glucagon receptor humanized (hGCGR) mice at 1, 3 and 10mg/kg (mpk), and significantly lowered acute glucose levels in hGCGR ob/ob mice at 3 mpk dose.

Recent progress in the development of small-molecule glucagon receptor antagonists

The endocrine hormone glucagon stimulates hepatic glucose output via its action at the glucagon receptor (GCGr) in the liver. In the diabetic state, dysregulation of glucagon secretion contributes to abnormally elevated hepatic glucose output. The inhibition of glucagon-induced hepatic glucose output via antagonism of the GCGr using small-molecule ligands is a promising mechanism for improving glycemic control in the diabetic state. Clinical data evaluating the therapeutic potential of small-molecule GCGr antagonists is currently emerging. Recently disclosed clinical data demonstrates the potential efficacy and possible therapeutic limitations of small-molecule GCGr antagonists. Recent pre-clinical work on the development of GCGr antagonists is also summarized.

Modulation of Glucagon Receptor Pharmacology by Receptor Activity-modifying Protein-2 (RAMP2)

The glucagon and glucagon-like peptide-1 (GLP-1) receptors play important, opposing roles in regulating blood glucose levels. Consequently, these receptors have been identified as targets for novel diabetes treatments. However, drugs acting at the GLP-1 receptor, although having clinical efficacy, have been associated with severe adverse side-effects, and targeting of the glucagon receptor has yet to be successful. Here we use a combination of yeast reporter assays and mammalian systems to provide a more complete understanding of glucagon receptor signaling, considering the effect of multiple ligands, association with the receptor-interacting protein receptor activity-modifying protein-2 (RAMP2), and the role of individual G protein α-subunits. We demonstrate that RAMP2 alters both ligand selectivity and G protein preference of the glucagon receptor. Importantly, we also uncover novel cross-reactivity of therapeutically used GLP-1 receptor ligands at the glucagon receptor that is abolished by RAMP2 interaction. This study reveals the glucagon receptor as a previously unidentified target for GLP-1 receptor agonists and highlights a role for RAMP2 in regulating its pharmacology. Such previously unrecognized functions of RAMPs highlight the need to consider all receptor-interacting proteins in future drug development.

Pancreatic α-cell hyperplasia: facts and myths

CONTEXT

Pancreatic α-cell hyperplasia (ACH) was once an esoteric pathological entity, but it has become an important differential diagnosis of hyperglucagonemia after inactivating glucagon receptor (GCGR) genomic mutations were found in patients with ACH. Recently, the controversy over the pancreatic effects of incretins has stimulated much discussion of ACH that often includes inaccurate statements not supported by the literature.

DATA ACQUISITION

Literature related to ACH was reviewed.

EVIDENCE SYNTHESIS

ACH is defined as a diffuse and specific increase in the number of α-cells. A dozen cases have been reported and fall into three clinical types: reactive, functional, and nonfunctional. Characterized by remarkable hyperglucagonemia without glucagonoma syndrome, reactive ACH is caused by inactivating GCGR mutations, and its main clinical significance is pancreatic neuroendocrine tumors diagnosed at middle age. The Gcgr(-/-) mice, a model of reactive ACH, exhibit a multistage tumorigenesis in their pancreata. Pharmacological agents that inhibit glucagon signaling also cause reactive ACH in animals and possibly in humans as well. The pancreata of incretin-treated humans and those of reactive ACH murine models share similarities. Functional ACH features hyperglucagonemia with glucagonoma syndrome. Nonfunctional ACH is associated with normal glucagon levels. The causes of functional and nonfunctional ACH are unknown as yet.

CONCLUSIONS

ACH is a histological diagnosis and clinically heterogeneous. Caused by GCGR mutations, reactive ACH is a preneoplastic lesion giving rise to slow-developing pancreatic neuroendocrine tumors. The effects of treatments targeting glucagon signaling in this regard remain controversial. The strong negative feedback control of glucagon signaling conserved in all mammals studied, including humans, makes long-term pancreatic tumor surveillance advisable for the glucagon signaling-targeting therapies.

Anti-diabetic efficacy and impact on amino acid metabolism of GRA1, a novel small-molecule glucagon receptor antagonist

Hyperglucagonemia is implicated in the pathophysiology of hyperglycemia. Antagonism of the glucagon receptor (GCGR) thus represents a potential approach to diabetes treatment. Herein we report the characterization of GRA1, a novel small-molecule GCGR antagonist that blocks glucagon binding to the human GCGR (hGCGR) and antagonizes glucagon-induced intracellular accumulation of cAMP with nanomolar potency. GRA1 inhibited glycogenolysis dose-dependently in primary human hepatocytes and in perfused liver from hGCGR mice, a transgenic line of mouse that expresses the hGCGR instead of the murine GCGR. When administered orally to hGCGR mice and rhesus monkeys, GRA1 blocked hyperglycemic responses to exogenous glucagon. In several murine models of diabetes, acute and chronic dosing with GRA1 significantly reduced blood glucose concentrations and moderately increased plasma glucagon and glucagon-like peptide-1. Combination of GRA1 with a dipeptidyl peptidase-4 inhibitor had an additive antihyperglycemic effect in diabetic mice. Hepatic gene-expression profiling in monkeys treated with GRA1 revealed down-regulation of numerous genes involved in amino acid catabolism, an effect that was paralleled by increased amino acid levels in the circulation. In summary, GRA1 is a potent glucagon receptor antagonist with strong antihyperglycemic efficacy in preclinical models and prominent effects on hepatic gene-expression related to amino acid metabolism.

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

A potent, selective glucagon receptor antagonist 9m, N-[(4-{(1S)-1-[3-(3,5-dichlorophenyl)-5-(6-methoxynaphthalen-2-yl)-1H-pyrazol-1-yl]ethyl}phenyl)carbonyl]-β-alanine, was discovered by optimization of a previously identified lead. Compound 9m is a reversible and competitive antagonist with high binding affinity (IC(50) of 6.6 nM) and functional cAMP activity (IC(50) of 15.7 nM). It is selective for glucagon receptor relative to other family B GPCRs, showing IC(50) values of 1020 nM for GIPR, 9200 nM for PAC1, and >10000 nM for GLP-1R, VPAC1, and VPAC2. Compound 9m blunted glucagon-induced glucose elevation in hGCGR mice and rhesus monkeys. It also lowered ambient glucose levels in both acute and chronic mouse models: in hGCGR ob/ob mice it reduced glucose (AUC 0-6 h) by 32% and 39% at 3 and 10 mpk single doses, respectively. In hGCGR mice on a high fat diet, compound 9m at 3, and 10 mpk po in feed lowered blood glucose levels by 89% and 94% at day 10, respectively, relative to the difference between the vehicle control and lean hGCGR mice. On the basis of its favorable biological and DMPK properties, compound 9m (MK-0893) was selected for further preclinical and clinical evaluations.