GPR119 is a Gαs protein-coupled receptor with some possible endogenous agonists such as fatty acid amides. Agonists of GPR119 increase intracellular cAMP leading to increased glucose-dependent insulin secretion from pancreatic β-cells and to increased incretin secretion from gut entero-endocrine cells. Synthetic GPR119 agonists, selective, potent and orally available, showed significant anti-diabetic and anti-obesic effects in various animal models of type 2 diabetes and obesity by: (1) lowering blood glucose without hypoglycaemia, (2) slowing diabetes progression, (3) reducing food intake and body weight. Thus the discovery and development of orally active small-molecule GPR119 agonists may have the potential to lead to a novel treatment that could control blood glucose and decrease body weight in type 2 diabetic patients, an outcome only achievable currently with injectable GLP-1R agonists. This chapter also illustrates some of the key steps from basic research to drug discovery and development that have been undertaken to characterize and de-orphanise GPR119, then to identify synthetic selective and potent GPR119 agonists and to explore and understand their mechanism of action, leading to the identification of novel clinical candidates for the treatment of type 2 diabetes and associated obesity.
Characterisation and de-orphanisation of GPR119
GPR119[a] was identified initially by several research groups and described as a class A (rhodopsin-type) orphan GPCR having no close primary sequence relative in the human genome.
Several molecules (such as trans-retinoic acids and some phospholipids) have been described as potential endogenous ligands, acting as agonists of GPR119. The fatty acid amide oleoylethanolamide (OEA[b]]) and the endovanilloid N-oleoyldopamine (OLDA) have been reported so far as the most potent natural GPR119 agonists in vitro, although they appear less potent and selective than natural ligands identified for other GPCRs. However, these endogenous compounds may be synthesized and/or accumulate in vivo at specific local sites.
GPR119 is a Gαs-coupled receptor predominantly expressed in pancreas and gut
Several endogenous ligands and synthetic small-molecule agonists of GPR119 have been shown to increase intracellular cAMP levels in mammalian cells expressing endogenous or (transiently or stably) transfected GPR119, indicating that GPR119 should function via coupling to Gαs[c], leading to the stimulation of adenylate cyclase
GPR119 displays a restricted expression pattern in human tissues, with the pancreas, foetal liver and gastrointestinal tract being consistently identified as the major sites of expression (at the mRNA level). In rodents, GPR119 mRNA was also highly expressed in the pancreas and gastrointestinal tract (in particular the colon and small intestine). In addition, significant levels of mRNA expression were reported in many areas of the rat and mouse brain[d], in contrast to human brain.
Consistent with its expression in pancreas, GPR119 has been detected in isolated rodent pancreatic islets (at the mRNA level), and more specifically in insulin secreting pancreatic β-cells[e] (at the protein level). Strong GPR119 expression has been observed (by immunocytochemistry using polyclonal GPR119 antibody) in subsets of human pancreatic islet cells. Receptor expression in other islet cell types than β-cells, such as the glucagon-secreting islet α-cells, cannot be excluded at present.
Consistent with its expression in intestinal tissues, GPR119 is strongly expressed (at the mRNA level) in the mouse L-cell-derived entero-endocrine GLUTag cell line, and in microdissected murine intestinal L-cells and GIP-secreting K cells. Strong GPR119 expression has been also observed (by immunohistochemical staining) in endocrine cells within villi of human small intestine.
Discovery of synthetic small-molecule agonists of GPR119
The class A (rhodopsin-like) GPCRs, family to which GPR119 belongs, have been shown to be more tractable drug targets than the class B (secretin-like) GPCRs, which include the receptors for glucagon-like peptide-1 (GLP-1) and for glucose-dependent insulinotropic peptide (GIP).
A large number of orally active synthetic small-molecule GPR119 agonists have now been identified by many pharmaceutical companies, and many patents have been published. One of the first GPR119 agonists on which data have been disclosed is Arena Pharmaceuticals’ compound AR231453. This compound, identified through chemical optimization of an inverse agonist found during a high through-put screening (HTS) campaign, has been reported to be highly selective for GPR119, having no measurable affinity for over 200 other GPCRs or enzymes, including dipeptidyl peptidase-IV (DPP-IV). Around the same time, a separate HTS campaign by (OSI)Prosidion[f] using a yeast-based functional assay led to the discovery of novel small-molecule GPR119 agonists, with initial chemical optimization yielding PSN632408, a compound similar in efficacy and potency to OEA at both human and mouse GPR119 in vitro. Moreover, PSN632408, with physicochemical properties associated with good oral absorption, showed efficacy in vivo after oral administration. Further chemical optimization led to the identification of more potent, selective, orally available, small-molecule agonists of GPR119, which has then enabled extensive exploration of mechanism of action and efficacy studies to be conducted in relevant functional cellular systems, such as pancreatic β-cells and entero-endocrine intestinal cell models, and in a variety of type 2 diabetes and obesity rodent models.
GPR119 agonists modulate glucose homeostasis by stimulating pancreatic glucose-dependent insulin secretion and gut incretin secretion
GPR119 expression in pancreatic islet β-cells led to the hypothesis that this receptor could play a role in modulating insulin secretion. Therefore, GPR119 agonists, which have been shown also to raise intracellular cAMP levels in vitro in different pancreatic β-cell sytems expressing endogenous GPR119, would be expected to potentiate glucose-stimulated insulin secretion (GSIS) in a similar manner to the incretins GLP-1 and GIP, hormones which also act via Gαs coupled receptors in β-cells.
The insulinotropic actions of GPR119 agonists have been demonstrated in several models of pancreatic β-cells[g]. Moreover, several studies have indicated that small-molecule GPR119 agonists can suppress glucose excursions when administered orally before the oral glucose load in oral glucose tolerance tests (OGTTs) in normoglycaemic or hyperglycaemic rodent models[h]. On the other hand, following either intra-peritoneal or intravenous administration of glucose, the anti-hyperglycaemic effects of GPR119 were reduced, implying that not all of the effects of GPR119 agonists seen in the OGTTs were achieved only by direct induction of GSIS in the pancreatic β-cell, and was strongly suggestive of some additional incretin mediation of the response.
The presence of GPR119 in gut entero-endocrine L-cells suggested that intestinally-expressed GPR119 may also be involved in glucose homeostasis via the modulation of incretin hormone release. Small-molecule GPR119 agonists have been shown to elevate cAMP levels and stimulate GLP-1 secretion from GLUTag cells which respond to the signals controlling GLP-1 release in a manner similar to that of primary intestinal L-cells. Furthermore, these results translated to an in vivo setting where GPR119 agonists increased plasma GLP-1 levels acutely when administered to rodents[i].
Therefore, GPR119 agonists appear to lower blood glucose concentrations via a dual approach, acting directly at the pancreatic β-cell to promote GSIS, and indirectly, via the gut entero-endocrine cells, by stimulating the release of the incretin hormones GLP-1 and GIP, themselves powerful anti-hyperglycaemic agents. It should be noted that, while GLP-1-dependent blood glucose lowering may depend partly on a direct insulinotropic effect on the β-cell, GLP-1 also exerts its actions by suppressing meal-associated glucagon secretion and slowing delivery and absorption of food to and from the gut by reducing gastric motility.
GPR119 agonists may slow the progression of diabetes
GPR119 agonists, like GLP-1 receptor agonists, act at Gαs-coupled receptors and raise cAMP levels in the pancreatic islet β-cell. And it has been reported that elevated intracellular cAMP levels resulting from agonists binding to Gαs-coupled receptors directly protected β-cells from 2-deoxy-D-ribose-induced oxidative damage and lipid-induced apoptosis. Taken together, these data strongly suggested that GPR119 agonists could potentially exert beneficial effects on disease progression beyond those which could be achieved by improving glucose homeostasis alone, and that the concomitant GRP119 agonists action of increasing circulating GLP-1, itself capable of raising β-cell cAMP levels, might also contribute to such a protective effect.
The potential of GPR119 agonists to exert β-cell protective effects by raising cAMP levels has been shown in MIN6 pancreatic β-cells with AR231453 inducing Akt phosphorylation and IRS-2 expression, key measures of islet and β-cell mass protection. In addition, chronic oral administration of PSN119-1 (once daily for 21 days) to young pre-diabetic db/db mice was able to maintain normal fed blood glucose levels and glucose tolerance and therefore GPR119 agonist treatment appeared to slow the progression of diabetes in this model.
GPR119 agonists modulate food intake and body weight
The idea that GPR119 activation might also provide a basis for anti-obesity therapy was first suggested by the effects reported on one of its most active endogenous agonists, OEA[j], on food intake and body weight in rodent studies. In addition, the demonstration that GPR119 agonists stimulate GLP-1 release supported the idea of these agents having a possible effect on body weight, since GLP-1 causes gastric deceleration and increases satiety, leading to reduced food intake and weight loss in both rodent models and human subjects.
GPR119 agonists have been shown to inhibit gastric emptying and to suppress food intake in rats when dosed acutely, and without any evidence of drug-induced malaise or conditioned taste aversion. These hypophagic actions of GPR119 agonists have led to reductions in weight gain, fat pad masses and plasma leptin and triglyceride levels in several rodent models of obesity when administered sub-chronically. These actions are probably a result of their effects on GLP-1 secretion and/or perhaps directly through GPR119 activation itself.
GPR119 agonists are potential novel agents for the treatment of type 2 diabetes
Current publicly-available pre-clinical data on the effects of GPR119 agonists in animal models indicate that potent and selective, orally available, synthetic GPR119 agonists could be potential therapies for diabetes and related metabolic disorders by: (1) stimulating glucose-dependent insulin secretion, (2) inducing incretin release, (3) protecting pancreatic β-cells through raised cAMP levels,
Figure 1: Schematic diagram illustrating the possible actions of GPR119 agonistsand (4) reducing food intake and body weight[k]. Although the details of their mechanisms of action (see Figure 1) are not yet fully understood, GPR119 agonists could represent a rare opportunity to achieve, with an oral therapy, blood glucose control without hypoglycemia and with concomitant body weight loss and attenuation of disease progression, an outcome only achievable currently with injectable GLP-1R agonists.
Future perspectives for GPR119 agonists
The value of GPR119 agonists as a new class of therapeutics for type 2 diabetes and associated obesity is likely to be determined within the next few years, following the recent entry of many of these compounds into clinical development.
Currently, only a few results of some of the clinical studies have been reported. Encouraging results have been reported from Phase 1 clinical trials assessing the safety, tolerability and proof of pharmacology of orally available GPR119 agonists in human subjects. Moreover, several oral GPR119 agonists have been advanced into Phase 2 clinical trials for the treatment of type 2 diabetes. Among the few data reported, several GPR119 agonists have shown some beneficial pharmacological effects in patients with type 2 diabetes, consistent with predicted GPR119 agonism effects based upon preclinical diabetic models: JNJ-38431055 showed glucose lowering and incretin increases after single dose administration; PSN821 showed glucose lowering and decreased energy intake after 14 days treatment; MBX-2982 lowered weighted mean glucose (WMG[l]) and postprandial glucose during an extended mixed-meal tolerance test in a 4-week study. These agonists have been reported to be safe and well tolerated, with no associated hypoglycemia and no serious adverse effects.
So far, the very limited set of published data, which provide clinical validation of GPR119 agonism, could eventually indicate that a GPR119 agonist alone may not provide sufficient glucose-lowering efficacy to translate into a clinically useful and competitive therapeutic agent. It is probable that more conclusive evaluations of clinical studies will appear as data continue to accumulate and/or be published. However, more preclinical studies and clinical studies in type 2 diabetic patients associated with obesity, exploring several relevant end-points, may be required to fully explore the potential of GPR119 agonism and its mechanisms of action.
GPR119 agonism, stimulating secretion of endogenous GLP-1 at its appropriate sites of action, may still provide at present a relevant third approach to the two current approaches based around GLP-1 action, i.e. (1) injection of an “incretin mimetic”, i.e., a peptidic GLP-1 receptor agonist resistant to degradation by DPP-IV, and (2) inhibition of DPP-IV, to preserve endogenous GLP-1 in its active form. In view of recently published data, it is also worth pointing out again the obvious potential advantages that could theoretically be obtained by the oral co-administration of a GPR119 agonist (acting as double GLP-1 and insulin secretagogue) and a DPP-IV inhibitor (protecting secreted GLP-1). This attractive oral combined approach could provide a greater augmentation of endogenous active GLP-1 that could potentially elicit similar beneficial effects that are currently induced by injected GLP-1 mimetics.
[Dr C. Reynet worked at (OSI)Prosidion until December 2007 and held stock in (OSI)Pharmaceuticals, Inc. until 2008. She no longer has any conflicting interest relevant to this review article written in April 2014.]
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^ GPR119 has been studied and described in the literature under various synonyms including SNORF25, RUP3, GPCR2, 19AJ, OSGPR116 and Glucose-Dependent Insulinotropic Receptor (GDIR). GPR119 is encoded by a single exon gene, localised on chromosome X (in both humans and rodents - GenBank NM_178471). Isoforms of GPR119 have been identified in a number of mammalian species, including rats, mice, hamsters, chimpanzees, rhesus monkeys, cattle and dogs, with some differences reported between rodent and human isoforms. Human GPR119 spans 335 amino acids (NP_848566). Phylogenetic analysis assigned GPR119 to a new subgroup within the bioamine receptors, whilst homology clustering analysis indicated that the closest relatives of GPR119 are the cannabinoid receptors.
^ The identification of OEA as a potential endogenous GPR119 agonist was of particular interest as OEA has been reported to produce pharmacological effects in rodents, including: (1) reducing food intake and body weight gain, (2) altering feeding behaviour and motor activity, and (3) increasing fatty acid uptake by adipocytes and enterocytes. Mechanistically, it has been proposed that these effects may be brought about by (1) activation of peroxisome proliferator-activated receptor-α, (2) activation of the transient receptor potential vanilloid type 1 receptor, and (3) through raised fatty acid translocase expression, respectively. However, it has also been proposed that the effects of OEA on feeding may be mediated at least in part by GPR119 because OEA achieves high concentrations in the small intestine, a region of high GPR119 expression where this fatty acid ethanolamide is known to be synthesized.
^ In transfected cells, GPR119 also exhibited poor coupling efficiency to Gαi and Gαq, implying a selective interaction with Gαs.
^ GPR119 has been shown to be expressed in several rodent brain areas including cerebellum, cerebral cortex, choroid plexus, dorsal root ganglion, hippocampus and hypothalamus
^ Contrary evidence has also been presented, where immunofluorescence studies with an anti-peptide antiserum revealed the pancreatic polypeptide-secreting cells as the only site of GPR119 expression in mouse and rat islets. However, evidence of GPR119 expression in rodent β-cells derived insulinoma cell lines, such as HIT-T15, NIT-1, MIN6 and RIN5, strongly supports the β-cells being a site of expression within pancreatic islets.
^ (OSI)Prosidion was a subsidiary of OSI Pharmaceuticals, Inc. which was acquired by Astellas Pharmaceuticals in 2010.
^ The selective GSIS effects of GPR119 agonists in pancreatic cells have been demonstrated in several control experiments performed in different laboratories in several in vitro systems such as HIT-T15 hamster insulinoma cells, NIT-1 mouse β-cells, isolated mouse and rat pancreatic islets, and perfused rat pancreas. For example, the insulinotropic effects of the GPR119 ligand oleoyl LPC were attenuated in the presence of either an adenylate cyclase inhibitor or a GPR119-selective siRNA in the NIT-1 mouse β-cell line, and were absent in islets from GPR119-deficient mice. It has also been shown that the synthetic GPR119 agonist AR231453 compound had no insulinotropic actions in islets isolated from GPR119-null mice.
^ The observation that the in vivo anti-hyperglycaemic actions of synthetic selective GPR119 agonists were lost in GPR119-deficient mice confirmed that these observed pharmacological effects were indeed mediated through GPR119.
^ In addition, a small-molecule GPR119 agonist has been reported to stimulate the release of not just GLP-1, but also GIP, in wild type, but not GPR119-deficient mice.
^ It is relevant to note that OEA is a relatively non-selective and low-potency GPR119 agonist, which could explain in part why a study has observed that the hypophagic effect of OEA was maintained in GPR119-null mice. The testing of potent, selective GPR119 agonists for food intake and body weight effects in GPR119-deficient mouse models has not been reported thus far.
^ Reducing food intake and body weight may also indirectly contribute to the protection of pancreatic β-cells because of the consequent reductions in the glucose demand on these cells to produce insulin.
^ WMG: 24-h Weighted Mean Glucose is a clinical biomarker often used as a primary end-point in clinical studies for assessing glucose-lowering efficacy of compounds. Based upon collection of multiple blood samples, WMG is calculated as AUC (area under concentration-time curve)0-24h/24 and is expressed in mg/dl.