The current state of GPCR‐based drug discovery to treat metabolic disease

Abstract

One approach of modern drug discovery is to identify agents that enhance or diminish signal transduction cascades in various cell types and tissues by modulating the activity of GPCRs. This strategy has resulted in the development of new medicines to treat many conditions, including cardiovascular disease, psychiatric disorders, HIV/AIDS, certain forms of cancer and Type 2 diabetes mellitus (T2DM). These successes justify further pursuit of GPCRs as disease targets and provide key learning that should help guide identifying future therapeutic agents. This report reviews the current landscape of GPCR drug discovery with emphasis on efforts aimed at developing new molecules for treating T2DM and obesity. We analyse historical efforts to generate GPCR‐based drugs to treat metabolic disease in terms of causal factors leading to success and failure in this endeavour.

Linked Articles

This article is part of a themed section on Molecular Pharmacology of GPCRs. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v175.21/issuetoc

Abbreviations

DPP4

dipeptidyl peptidase‐4

α‐MSH

α melanocyte stimulating hormone

PAM

positive allosteric modulator

POMC

pro‐opiomelanocortin peptide

SSTR5

somatostatin receptor 5

T2DM

Type 2 diabetes mellitus.

Introduction

The http://www.guidetopharmacology.org/GRAC/FamilyDisplayForward?familyId=694 are a large family of heptahelical plasma membrane‐spanning proteins that have evolved to couple extracellular signals into intracellular responses (Lefkowitz, 2007). It is often claimed that 20–50% of approved drugs act in some way via GPCRs (Santos et al., 2017). GPCRs typically couple to two main families of intracellular effector proteins to transduce signals: heterotrimeric G‐proteins and β‐arrestins (Lefkowitz, 2007). Heterotrimeric G‐proteins regulate well‐described biochemical pathways such as cAMP synthesis, phospholipase C activation, ion channel activity and small GTPase signalling (Lefkowitz, 2007). In contrast, β‐arrestins desensitize GPCRs and activate alternative signal transduction pathways such as kinase cascades (Lefkowitz, 2007). A detailed coverage of the intricacies of canonical GPCR biology and signal transduction can be found in the literature and as such is outside the scope of this review. The GPCR family has members that selectively respond to an exceptionally diverse palette of ligands, including photons, protons, small organic molecules, carbohydrates, lipids, peptides and large proteins (Alexander et al., 2017a). Moreover, many GPCRs exhibit selective distribution in organs, tissues and cell types. Not surprisingly, many individual GPCRs have been investigated as potential therapeutic targets for Type 2 diabetes mellitus (T2DM), obesity and cardiovascular disease. In this review, we retrospectively evaluate the outcomes of targeting single GPCRs for T2DM and metabolic disease, with the thought that this might provide insights for future efforts. This review is intended to sample areas we feel are worthy of illumination and is not meant to be exhaustive.

Overview

It is generally asserted that GPCRs are highly amenable to drug discovery and represent the best target‐class for identifying novel therapeutic agents (Roth and Kroeze, 2015; Santos et al., 2017). The GPCR phylogenetic tree represents five families of transmembrane receptors as designated by the GRAFS (or Class) nomenclature Glutamate (Class C), Rhodopsin (Class A), Adhesion (Class B2), Frizzled (Class F), Secretin (Class B1) (Alexander et al., 2017a). The GPCR‐ome consists of more than 400 olfactory receptors and approximately 300 potential drug‐target receptors (Roth and Kroeze, 2015). Within this selection of 300 drug‐target GPCRs, approximately 150 are orphan receptors (no endogenous ligand has been described for them). Among the GPCR drug‐target set, there is a diversity of natural ligand, protein structure, biological function and consequently druggability of the individual GPCRs.

Many pharmacological modalities exist by which to target GPCRs. Although these are well‐elucidated concepts, they are worth listing (see Table 1). Briefly, agents to target GPCR biology can act indirectly, such as an injectable antibody to sequester the natural agonist ligand (such as anti‐CGRP antibodies for migraine) (Dodick et al., 2014) or a small molecule enzyme inhibitor to prevent degradation of the endogenous agonist, such as dipeptidyl peptidase‐4 (http://www.guidetopharmacology.org/GRAC/ObjectDisplayForward?objectId=1612) inhibitors that block the degradation of glucagon‐like peptide‐1 (http://www.guidetopharmacology.org/GRAC/ObjectDisplayForward?objectId=1612) and glucose‐dependent insulinotropic peptide (http://www.guidetopharmacology.org/GRAC/LigandDisplayForward?ligandId=3542) and thereby enhance the effects of http://www.guidetopharmacology.org/GRAC/ObjectDisplayForward?objectId=249 receptor and http://www.guidetopharmacology.org/GRAC/ObjectDisplayForward?objectId=248 activation respectively (Ahren, 2007). Canonical gene therapy approaches such as siRNA and antisense as well as emerging approaches such as CRISPR have shown some promise, but drug delivery has been a limiting factor with these methods.

Table 1.

Different pharmacological approaches to target GPCRs

Pharmacology	Mechanisms of action	Representative targets	Examples: drug, disease
Indirect orthosteric agonism or antagonism by changing natural ligand concentration	Enzyme inhibitors, antibodies, transporter inhibitors	DPP4 (GLP‐1, GIP receptors)	http://www.guidetopharmacology.org/GRAC/DatabaseSearchForward?searchString=Sitagliptin&searchCategories=all&species=none&type=all&comments=includeComments&order=rank&submit=Search+Database, T2DM
		http://www.guidetopharmacology.org/GRAC/ObjectDisplayForward?objectId=1613&familyId=741&familyType=ENZYME (http://www.guidetopharmacology.org/GRAC/ObjectDisplayForward?objectId=34, http://www.guidetopharmacology.org/GRAC/ObjectDisplayForward?objectId=359 receptors)	http://www.guidetopharmacology.org/GRAC/LigandDisplayForward?ligandId=5158, hypertension
		Cycloxygenase (many PG, lipoxin and Tx receptors)	Aspirin, pain and inflammation
Indirect antagonism by reducing receptor expression	siRNA, CRISPR, ASO	Glucagon receptor	ISIS 325568 (Phase II), T2DM
Negative allosteric modulation	Small molecules, peptides, antibodies.	CCR5	http://www.guidetopharmacology.org/GRAC/LigandDisplayForward?ligandId=806, HIV/AIDS
	Alter the affinity and/or efficacy of the natural ligand
Positive allosteric modulation	Small molecules, peptides, antibodies.	CaS receptors	Cinacalcet, secondary hyperparathyroidism
	Alter the affinity and/or efficacy of the natural ligand
Orthosteric antagonist	Small molecules, peptides, antibodies.	β1‐ and β2‐adrenoceptors	http://www.guidetopharmacology.org/GRAC/LigandDisplayForward?ligandId=564, cardiac arrythmia and heart failure
	Block the binding of the natural ligand in a competitive manner	H2 receptors	http://www.guidetopharmacology.org/GRAC/LigandDisplayForward?ligandId=1234, dyspepsia
		GnRH receptors	Degarlix, prostate cancer
Orthosteric agonist	Small molecules, peptides,antibodies.	β2‐adrenoceptors	http://www.guidetopharmacology.org/GRAC/LigandDisplayForward?ligandId=558, asthma
	Mimic the binding of the natural ligand to activate the receptor.	GLP‐1 receptors	Exenatide, T2DM
		SST2 and SST5 receptors	http://www.guidetopharmacology.org/GRAC/LigandDisplayForward?ligandId=2055, GH secreting tumours and acromegaly

Examples of the key pharmacological methods to drug GPCRs and some examples from approved medicines.

Molecules that act by directly binding GPCRs are classified as either orthosteric (bind in the same pocket as the natural ligand) or allosteric (bind in a different pocket to the natural ligand). The pharmacology of orthosteric GPCR ligands is well understood. Blocking the action of the natural ligand to stimulate a GPCR is the effect of an antagonist (e.g. histamine H₁ receptor antagonists for allergy) (Sadek and Stark, 2016). Agonists, such as the http://www.guidetopharmacology.org/GRAC/ObjectDisplayForward?objectId=29 agonists for asthma, mimic the cellular pharmacology of the endogenous agonist (adrenaline for the β₂‐adrenoceptor) and typically have an enhanced duration of action. Recently, attention has focused on the discovery and characterization of allosteric modulators of GPCRs. Such molecules cooperatively interact with the natural orthosteric ligand to potentiate receptor signalling (positive allosteric modulator, PAM) or attenuate receptor signalling (negative allosteric modulator). Typically, these molecules work by stabilizing unique conformations of the receptor and modulating either the affinity or efficacy of an orthosteric ligand. For example, http://www.guidetopharmacology.org/GRAC/LigandDisplayForward?ligandId=3308 is a PAM of the http://www.guidetopharmacology.org/GRAC/ObjectDisplayForward?objectId=54 used to treat hypoparathyroidism (Jensen and Brauner‐Osborne, 2007). Allosteric modulators provide several potential advantages over orthosteric molecules with respect to selectivity, mechanism and kinetics of action and the ability to modulate receptors for which the orthosteric site appears undruggable with conventional approaches. To date, unfortunately, limited success has been demonstrated with GPCR allosteric modulators in the clinic. Other pharmacological ligand categorizations exist including partial agonists, inverse agonists, bitopic ligands, kinetically selective and biased ligands. Discussion of these is beyond the scope of this article but they are well reviewed in the literature.

The medical need in metabolic disease: an opportunity for GPCRs

One of the major medical needs of the 21st century is the prevention and treatment of the epidemic of T2DM, obesity and related co‐morbidities (Zimmet et al., 2014). It is projected that by 2035, over half a billion individuals will have T2DM. A sizeable component of this epidemic is likely resultant from dietary and societal changes over the last century. Given the magnitude and seriousness of the oncoming problem, it is probable that in concert with preventative strategies, an enhanced armamentarium for pharmacological treatment of these conditions will be needed.

Design of new therapeutic approaches must take into account that glucose homeostasis and energy metabolism are regulated by highly coordinated signalling networks linking the enteric nervous system and the CNS. Normal interplay among the processes controlling glucose disposal and energy expenditure can, however, become dysregulated when excessive caloric intake leads to profound lipid accumulation and development of obesity. This condition is characterized by an increase in intra‐abdominal visceral fat and ectopic deposition of lipid in non‐adipose tissues such as liver and skeletal muscle. Adiposity raises intracellular lipid content and intermediate metabolites such as DAG, acetyl‐CoA, and various ceramides, which are associated with the development of insulin resistance (Samuel and Shulman, 2016). Reduced insulin sensitivity in muscle is particularly detrimental to overall glucose control because this tissue accounts for the majority of whole‐body glucose utilization. Furthermore, impaired insulin action in adipocytes promotes lipolysis, which increases the availability of fatty acids for uptake by the liver. This heightened flux of pre‐formed fatty acids increases hepatic triglyceride synthesis via re‐esterification and promotes gluconeogenesis by elevating acetyl‐CoA from fatty acid oxidation, thereby contributing to overall hyperglycaemia (Samuel and Shulman, 2016). If an increase in insulin secretion by pancreatic beta cells fails to sufficiently compensate, impaired glucose tolerance and T2DM eventually ensue (DeFronzo and Tripathy, 2009).

The underlying genetic basis that predisposes susceptibility and/or influences the onset of T2DM is still not well understood. However, consistent with the critical physiological impairments that characterize progression of the disease, most genetic loci linked to T2DM risk are associated with insulin secretion and pathways controlling pancreatic islet development (Shungin et al., 2015). It is also becoming clear that genetic variance points to a role of the brain in obesity susceptibility as adiposity is associated with genes involved in hypothalamic pathways that control food intake and energy expenditure (Shungin et al., 2015). These genetic findings support other emerging evidence of a ‘brain‐centred glucoregulatory system’ that works in concert with islets to integrate the complex control of glucose homeostasis (Schwartz et al., 2013). Defining the neuronal circuity that helps control glucose homeostasis should provide new therapeutic targets, many of which may be GPCRs.

GPCR‐targeted drug discovery for T2DM and obesity: historical approaches

Academia and the pharmaceutical industry have invested substantial resources into basic science, target validation, drug discovery and drug development supporting GPCR‐targeted medications. The genomic era of GPCR discovery was presaged by the molecular cloning of the β‐adrenoceptor in 1986 (Lefkowitz, 2007). This led to the identification of the genes encoding numerous GPCRs for hormones, neurotransmitters and regulatory peptides many of which had been hypothesized decades earlier. The current molecular biology toolkit for GPCRs has powered drug discovery research toward the generation of several investigational molecules for treating metabolic diseases, and there is a wealth of publications, patents and clinical trials that exemplify this. To simplify our analysis, we have created a top 20 list of what we regard as the most extensively studied and historically promising GPCRs for T2DM and obesity that have been tested in humans (Table 2). Of these, we further highlight some of the most intensely investigated GPCRs in short vignettes on the http://www.guidetopharmacology.org/GRAC/ObjectDisplayForward?objectId=251, the http://www.guidetopharmacology.org/GRAC/ObjectDisplayForward?objectId=30, the melanocortin http://www.guidetopharmacology.org/GRAC/ObjectDisplayForward?objectId=285, the cannabinoid http://www.guidetopharmacology.org/GRAC/ObjectDisplayForward?objectId=56, the orphan receptor http://www.guidetopharmacology.org/GRAC/ObjectDisplayForward?objectId=126, the http://www.guidetopharmacology.org/GRAC/ObjectDisplayForward?objectId=225/(GPR40) and the glucagon‐like peptide‐1 (GLP‐1) receptor.

Table 2.

Top 20 GPCRs for Type 2 diabetes mellitus, metabolic syndrome, and obesity that have been tested in humans

		Receptor	Representative ligands	Pharmacology	Proven or proposed site of action	Therapeutic approach	Route of administration	Preclinical Validation	Outcome of the most advanced phase of testing	Reference
Approved Drugs	1	GLP‐1 receptor	Exenatide	Agonist	CNS, GI Tract, Islet, PNS	Peptide, protein	Parenteral	Glucose lowering in rodent and primate models	Approved drug/Byetta and others	Graaf et al. (2016)
	2	5HT2C receptor	http://www.guidetopharmacology.org/GRAC/LigandDisplayForward?ligandId=2941, http://www.guidetopharmacology.org/GRAC/LigandDisplayForward?ligandId=4613	Agonist, combination (fenfluramine + phentermine)	CNS	Small molecule	oral	Fen‐Phen	Approved drug/Belviq; approved drug (Fen‐Phen) subsequently withdrawn	Weintraub et al. (1992)
	3	http://www.guidetopharmacology.org/GRAC/FamilyIntroductionForward?familyId=11	Pramlintide	Agonist	CNS/GI tract/PNS/Islet	Peptide	Parenteral	Inhibition of gastric emptying and glucagon secretion in rodents	Approved drug/Symlin	Mietlicki‐Baase (2016)
	4	μ‐opioid receptor	http://www.guidetopharmacology.org/GRAC/LigandDisplayForward?ligandId=1639	Antagonist (combination with http://www.guidetopharmacology.org/GRAC/LigandDisplayForward?ligandId=7135)	CNS	Small molecule	oral	Inhibition of food intake in rodents	Approved drug/Contrave	Greenway et al. (2009)
	5	CB1 receptor	Rimonabant	Antagonist/inverse agonist	CNS	Small molecule	oral	Appetite suppression and weight loss in rodent and primate models	Approved drug subsequently withdrawn/Accomplia	Colombo et al. (1998)
Phase III	6	FFA1 receptor (GPR40)	TAK875	Agonist	Islet, L‐cell	Small molecule	oral	Glucose lowering in rodent and primate models	Glucose lowering in Phase III	Kaku et al. (2015)
Phase I and Phase II, other human studies	7	β3‐adrenoceptor	CL‐316243, L‐796568, TAK‐677	Agonist	Adipose	Small molecule	oral	Lipolytic, hypophagic and insulin sensitizing effects in rodents	Phase II	Larsen et al. (2002)
	8	http://www.guidetopharmacology.org/GRAC/ObjectDisplayForward?objectId=76	Gl181771X	Agonist	PNS	Small molecule	oral	Hypophagic effects in rodents	Not efficacious, Phase II	Jordan et al. (2008)
	9	Glucagon receptor	TT‐401	Agonist (oxyntomodulin, triagonists)	Liver, islet	Peptide	Parenteral	Glucose lowering in rodents	Phase II	Muller et al. (2017)
			LY2409241	Antagonist	Liver	Small molecule, antibody, antisense oligonucleotides	oral, parenteral	Glucose lowering in rodents	Glucose lowering in Phase II	Pearson et al. (2016)
	10	http://www.guidetopharmacology.org/GRAC/FamilyIntroductionForward?familyId=28	http://www.guidetopharmacology.org/GRAC/LigandDisplayForward?tab=biology&ligandId=9060	Antagonist, inverse agonist	CNS	Small molecule	oral	Hypophagic and weight loss effects in rodents	Phase II diabetes – discontinued for strategic reasons	Asakawa et al. (2003)
	11	GIP receptor	SAR438335/RG7697	Agonist (twincretins, triagonists)	Adipose, islet	Peptide	Parenteral	Glucose lowering, reduced insulin resistance in rodents and primates	Phase II, efficacy in Phase I	Finan et al. (2013)
	12	GPBA receptor	SB‐756050	Agonist	L‐cell	Small molecule	oral	Glucose lowering in rodents	Phase I	Hodge et al. (2013)
	13	GPR119	PSN821	Agonist	L‐Cell, islet	Small molecule	oral	Glucose lowering and incretin elevation in rodents	Limited efficacy in Phase II	Ritter et al. (2016)
	14	H3 receptor	Betahistine (H3 antagonist, H1 agonist)	Antagonist/inverse agonist	CNS	Small molecule	oral	Prevention of weight gain in rodents	Not efficacious, Phase II	Provensi et al. (2016)
	15	http://www.guidetopharmacology.org/GRAC/ObjectDisplayForward?objectId=312	http://www.guidetopharmacology.org/GRAC/LigandDisplayForward?tab=summary&ligandId=8470, http://www.guidetopharmacology.org/GRAC/LigandDisplayForward?tab=summary&ligandId=1596	Agonist	Adipose	Small molecule	oral	Reduction of lipolysis and plasma NEFA, glucose lowering in rodents	Glucose lowering and reduced NEFA in Phase II. Response not durable (GSK256073)	Dobbins et al. (2015)
	16	MC4 receptor	http://www.guidetopharmacology.org/GRAC/LigandDisplayForward?ligandId=9272	Agonist	CNS	Small molecule and peptide	oral, parenteral	Weight loss, increased energy expenditure and improved glucose tolerance in rodents and primates	Phase II, increased energy expenditure in obese individuals	Chen et al. (2015)
	17	http://www.guidetopharmacology.org/GRAC/ObjectDisplayForward?objectId=280 receptor	AMG076	Small molecule	CNS	Small molecule	oral	Weight loss, increased energy expenditure and improved glucohomeostasis in rodents.	Phase I	Motani et al. (2013)
	18, 19	http://www.guidetopharmacology.org/GRAC/ObjectDisplayForward?objectId=306 /http://www.guidetopharmacology.org/GRAC/ObjectDisplayForward?objectId=307	http://www.guidetopharmacology.org/GRAC/LigandDisplayForward?ligandId=1554, TM30388	Agonist	CNS	Peptide	Parenteral	Inhibition of food intake in rodents	Short‐term efficacy in humans, inhibition of food intake	Batterham et al. (2002)
	20	http://www.guidetopharmacology.org/GRAC/ObjectDisplayForward?objectId=308	MK‐0557, velneperit	Antagonist	CNS	Small molecule	oral	Numerous genetic and pharmacological rodent models of obesity	Phase II, limited efficacy for weight loss	Erondu et al. (2006)

This table summarizes GPCRs that have been investigated for the treatment of diabetes and obesity in humans. Data are obtained from publications and http://www.clinicaltrials.gov. It represents a summary of information known to the authors and is not intended to be exhaustive.

Why so challenging: lost in translation from mice to men?

The receptor for glucagon, the principal counter‐regulatory hormone to insulin, was pharmacologically characterized in the early 1970s (Rodbell, 1973), but it was not until the cloning of the glucagon receptor in 1993 (Jelinek et al., 1993) that heterologous expression of this receptor became possible. This enabled structure–function studies to be undertaken and initiated the identification of small molecule and antibody antagonists (Pearson et al., 2016). Glucagon receptor‐null mice and studies using glucagon receptor antisense oligonucleotides in diabetic rodent models confirmed the glucose‐lowering potential of antagonizing glucagon receptors but also demonstrated that strong interference in this pathway results in compensatory alpha‐cell proliferation in the pancreas (Sloop et al., 2004). Ultimately, several antibody and small molecule glucagon receptor antagonists were identified as putative anti‐diabetic agents, with some showing indications of efficacy in humans (Pearson et al., 2016). However, based on the overall benefit–risk analysis, it remains unclear whether glucagon receptor antagonists will ultimately be submitted to, and approved by, regulatory agencies.

A presumed drawback of GCGR antagonists is that these agents may only affect hyperglycaemia and have little effect on body weight. In this light, one of the first GPCR targets investigated to reduce adiposity was the β₃‐adrenoceptor. These receptors are expressed in brown (thermogenic) adipocytes in rodents and humans (Lowell and Bachman, 2003), and receptor activation was proposed to stimulate energy expenditure and promote weight loss and improve insulin action. Agonists at the β₃‐adrenoceptor, such as http://www.guidetopharmacology.org/GRAC/LigandDisplayForward?ligandId=3462, reduced food intake, and stimulated lipolysis and thermogenesis consequently inducing weight loss and improved insulin sensitivity in rodents (Susulic et al., 1995). Genetic evidence from β₃‐adrenoceptor knockout mice (Susulic et al., 1995) and obese T2DM humans (Walston et al., 1995) supported the in vitro and preclinical pharmacological studies bringing much excitement to the development of novel orally available β₃‐adrenoceptor agonists. However, clinical studies with several β₃‐adrenoceptor agonists demonstrated little effect on lipolysis, energy expenditure and body weight in humans (Larsen et al., 2002). An explanation for this lack of efficacy is that β₃‐adrenoceptors in humans are expressed only in brown adipocytes, which are relatively sparse in humans, while in rodents, β₃‐adrenoceptors are found in both white and brown adipocytes (Ito et al., 1998). Moreover, activation of β₃‐adrenoceptors in rodent white adipose tissue stimulates the lipolysis necessary to fuel β‐oxidation of fatty acids in brown adipose tissue, an effect lacking in humans (Lowell and Bachman, 2003).

Derived from the proopiomelanocortin (POMC) gene, http://www.guidetopharmacology.org/GRAC/LigandDisplayForward?ligandId=1320) analogues offered similar promise in preclinical studies, inhibiting food intake and stimulating energy expenditure in both dietary (DIO) and genetic, leptin‐deficient rodent obesity/diabetes models (such as the ob/ob, agouti and db/db mouse models) (Fisher et al., 1999). Indeed, the adipokine http://www.guidetopharmacology.org/GRAC/LigandDisplayForward?ligandId=5015 stimulates POMC signalling in a specific population of hypothalamic arcuate nucleus‐containing and brainstem POMC‐containing neurons (Berglund et al., 2012). The anti‐obesity effects of http://www.guidetopharmacology.org/GRAC/LigandDisplayForward?ligandId=1320 analogues are attributed to activation of MC₄ receptors in the CNS. Consistent with the effects of administration of α‐MSH analogues, genetic null mutations of either POMC or MC₄ receptors result in severe obesity in both mice and humans (Tao, 2009). Of particular interest is the weight loss promoting effects of α‐MSH administration in human POMC‐deficient patients (Kuhnen et al., 2016). However, the effect of α‐MSH analogues on body weight in the general obese patient population is not clear (Krishna et al., 2009). It may be that cardiovascular side effects have been dose‐limiting in human studies. Future work will need to examine if cardiovascular side effects exhibit tachyphylaxis following repeated dosing, whether α‐MSH analogues with differing pharmacokinetic properties, partial agonist pharmacology or possibly PAMs can safely and efficaciously be used to modulate MC₄ receptors in obese and T2DM patients.

The endocannabinoid system is involved in the regulation of food intake, body weight and peripheral metabolism. Genetic and pharmacological evidence in rodents and humans support that blockade of CB₁ receptors produces reductions in food intake and body weight and improves insulin sensitivity (Kang, 2013). The prototypical CB₁ receptor antagonist SR141716 (rimonabant) was the first to advance to the clinic and new drug registration in Europe for the treatment of obesity and metabolic disease (Lafontan et al., 2007). However, the excitement over this new obesity medication was short‐lived as significant neuropsychiatric side effects were produced in clinical settings by rimonabant, causing withdrawal of the drug from the market. Thus, although the genetic and pharmacological validation of antagonists of CB₁ receptors in the CNS for obesity was successful, rimonabant failed clinically because of safety concerns that were not evident from preclinical studies. Recently, peripherally‐restricted, non‐brain penetrant, CB₁ receptor antagonists have demonstrated efficacy in obese diabetic preclinical models (Hsiao et al., 2015). Thus, there may yet be a path forward for clinical development of a CB₁ receptor antagonist for metabolic disease.

The lysophospholipid receptor GPR119 received much attention as a novel anti‐diabetic target because it is expressed both in pancreatic beta cells and in L and K cells of the gut. The therapeutic potential of this receptor was supported by key data showing GPR119 agonists stimulate insulin and incretin secretion and also reduce body weight in rodents (Ritter et al., 2016). These findings are in line with studies of Gpr119 null mice that show a reduction in post‐prandial GLP‐1 secretion (Lan et al., 2009). Numerous companies advanced GPR119 agonists into clinical trials. However, to date, none have progressed beyond Phase II development (Kang, 2013). It is generally understood that these compounds are well tolerated, but development has been discontinued primarily due to a lack of efficacy with respect to glucose control (Katz et al., 2012). These results represent another example of the difficulty in translating efficacy signals from rodent models to glucose lowering in man.

Small molecule agonists of a related GPCR, the long‐chain fatty acid FFA1 receptor (formerly GPR40), were shown to stimulate insulin secretion and lower blood glucose in diabetic rodents (Yashiro et al., 2012) and humans with T2DM (Mancini and Poitout, 2015). Consistent with these results, the stimulatory effects of fatty acids on insulin secretion are lost in FFA1 receptor knockout mice (Lan et al., 2008). Interestingly, a new report shows the ability of FFA1 receptor agonists or potentiators to increase GLP‐1 to levels sufficient for reducing food intake and body weight in obese rodents (Gorski et al., 2017). Several FFA1 receptor agonists have entered clinical testing with mixed results. An early molecule http://www.guidetopharmacology.org/GRAC/LigandDisplayForward?ligandId=6485 did not lower glucose in Phase I trials (Luo et al., 2012). However, another agonist, http://www.guidetopharmacology.org/GRAC/LigandDisplayForward?tab=summary&ligandId=6484 demonstrated sustained glucose lowering as a monotherapy in Phase III trials (Mancini and Poitout, 2015). Unfortunately, due to a possible hepatotoxicity risk, TAK‐875 is no longer being advanced in the clinic. As with the glucagon receptor antagonists, despite the extensive efforts of the academic and industrial scientific community, it is unclear at this time if FFA1 receptor agonists will be submitted for regulatory review.

In summary, there are many different factors that can prevent the translation of preclinical projects into fully fledged drugs that could be administered to patients. These can range from non‐translatable animal models, safety signals and commercial concerns to unexplainable lack of efficacy in the clinic. As a counter to this list of failures, it is useful to consider the GLP‐1 receptor agonists as an example of successful GPCR drug discovery.

A key success: GLP‐1 receptor agonist therapy

Discovery and characterization of the physiological actions of the incretin hormones, GIP and GLP‐1, and their respective receptors (both secretin family members) during the late 1980s and mid‐1990s provided a fundamental understanding of the glucose‐regulatory roles of these peptides (Drucker, 2006). This work set the stage for clinical investigations of several GLP‐1 receptor agonists during the 2000s. It is noteworthy that the main impetus for pursuing these agents came from preclinical and clinical studies characterizing the ability of GLP‐1 to enhance glucose‐dependent insulin secretion, regulate gastric transit and reduce food intake, not from genetic studies. Further, analyses of the reported Glp‐1r null mouse lines have not revealed profound physiological effects resulting from loss of the receptor (Hansotia et al., 2004). In spite of a lack of ‘genetic validation’ that is often demanded as critical evidence, GLP‐1 receptor agonists were successfully developed and are now entrenched in the modern algorithm for treating T2DM because these agents improve glucose homeostasis and reduce body weight. In light of the aetiology and progressive nature of T2DM, the ability to both enhance peripheral glucose disposal and decrease energy intake is clearly a desirable attribute. A key component of the efficacy of GLP‐1 receptor agonists is the pleiotropic effects they induce by acting on many different cell types and target tissues (Drucker, 2016). Similarly, there are a wide range of receptors and signal transduction systems that can regulate endogenous GLP‐1 secretion making it a critical nexus for pharmaceutical intervention. Small molecule inhibitors of DPP4, the enzyme that inactivates incretins, are also a successful class of drugs. They are extensively reviewed in the literature; thus, we will not discuss them in detail here.

Pharmaceutical development of agonists targeting the GLP‐1 receptor has largely focused on engineering strategies to prolong the pharmacokinetic properties of native GLP‐1 or http://www.guidetopharmacology.org/GRAC/LigandDisplayForward?ligandId=1135, a potent GLP‐1 receptor agonist isolated from the salivary gland of the Gila monster lizard (39 amino acids, 53% similarity with GLP‐1) (Eng et al., 1992). Analogues of GLP‐1 include http://www.guidetopharmacology.org/GRAC/LigandDisplayForward?ligandId=1133, http://www.guidetopharmacology.org/GRAC/LigandDisplayForward?ligandId=7638 and http://www.guidetopharmacology.org/GRAC/LigandDisplayForward?ligandId=7386. Although there are subtle differences in the efficacy responses for these compounds reported from various clinical trials, each of these agents improves glycaemic control and leads to some degree of weight loss. Overall, this therapeutic class is well tolerated, but the incidence of nausea often limits aggressive dose escalation (Harris and McCarty, 2015). The benefit of GLP‐1 receptor agonist therapy beyond improving glycaemic control and body weight has been investigated in large cardiovascular outcome trials for liraglutide. Consistent with improving overall cardiometabolic health, these studies showed that liraglutide therapy reduced the rate of myocardial infarction, stroke and death from cardiovascular causes (Marso et al., 2016). Furthermore, high‐risk patients had lower rates of cardiovascular events if they were undergoing treatment with liraglutide (Marso et al., 2016). In other trials, liraglutide 3.0 mg (a higher dose than the standard 1.8 mg) has been shown to be an effective adjunctive therapy to diet and exercise for weight management itself and was approved for this by the FDA. Clearly, the demonstration that GLP‐1 receptor agonism improves cardiovascular outcomes and has anti‐obesity effects in humans is a major breakthrough in the field of metabolic disease. While it has been determined that activation of GLP‐1 receptors lowers body weight by decreasing energy intake, as distinct from increasing fat oxidation, the mechanism(s) by which this occurs continues to be a significant area of investigation.

Exploring GPCR‐based mechanisms to increase circulating GLP‐1

Based on the success of direct acting GLP‐1 receptor agonists and DPP4 inhibitors that stabilize endogenously released GLP‐1, more recent efforts have focused on enhancing the secretion of GLP‐1 from intestinal L cells as well as other incretins, for example, GIP from K cells. Following bariatric surgery, GLP‐1 levels are significantly increased, and these enhanced levels are thought to be partly responsible for the observed improvement in metabolic disease (Jorgensen et al., 2013). Can compounds acting directly on the L cell raise GLP‐1 levels to those seen in bariatric surgery or to levels that exceed those observed with currently available therapeutics?

Initial efforts targeting GPCRs expressed on the L cell yielded candidate molecules for GPR119 and the bile acid http://www.guidetopharmacology.org/GRAC/ObjectDisplayForward?objectId=37 (TGR5). The GPBA receptor agonist SB‐756050 and the dual http://www.guidetopharmacology.org/GRAC/ObjectDisplayForward?objectId=603 and GPBA receptor agonist INT‐767 have progressed to the clinic (Hodge et al., 2013); however, positive data have not been forthcoming. Although GPBA receptor agonists have shown robust GLP‐1 secretagogue activity and can improve glucose control in rodent models, these compounds appear to also induce gallbladder filling, which may limit their use due to on‐target adverse pharmacology (Li et al., 2011). Intestinally restricted GPBA receptor agonists have been suggested as a means of mitigating unwanted on‐target pharmacology (Duan et al., 2015). Efficacy of the GPR119 agonist GSK1292263 has been examined in humans with T2DM (Nunez et al., 2014) but this compound had no effect on either GLP‐1 or GIP levels nor did it improve glucose control in patients.

As mentioned previously, some agonists targeting the FFA1 receptor stimulate release of GLP‐1 and thus present a novel way of targeting the incretin system. First generation FFA1 receptor agonists (e.g. TAK‐875) were found to selectively act on islet beta cells and increase insulin secretion with little effect on GLP‐1 secretion possibly through selective coupling to Gα_q signal transduction pathways (Hauge et al., 2016). However, newer classes of FFA1 receptor agonists molecules increase GLP‐1 secretion by apparently coupling to both G_s and G_q pathways (Hauge et al., 2016), making these molecules significantly more attractive as candidate therapeutic agents. The potential for FFA1 receptor agonists to stimulate sufficient GLP‐1 secretion for reducing body weight may enhance the attraction of this type of molecule. A related receptor, http://www.guidetopharmacology.org/GRAC/ObjectDisplayForward?objectId=127/GPR120, is also activated by long‐chain fatty acids and ω‐3 fatty acids. Activation of FFA4 receptors induces GLP‐1 secretion from the L‐cell in rodents in addition to inducing anti‐lipolytic, insulin sensitizing and anti‐inflammatory effects by action on adipose tissue and macrophages (Oh et al., 2014). Much effort has been put into the generation of FFA4 receptor agonists, however, it appears that none have yet reached clinical testing. An interesting concept has been advanced from demonstrations that simultaneous activation of FFA1 and FFA4 receptors has synergistic anti‐diabetic effects in animal models by virtue of dual glucose‐lowering and insulin‐sensitizing effects (Oh et al., 2014; Satapati et al., 2017). It has been proposed that a dual FFA1/FFA4 receptor agonist could be a highly efficacious anti‐diabetic agent as it is acting on several key tissues (i.e. L‐cell, beta cell, adipocyte and macrophage) and physiological systems thought to be necessary for an effective antidiabetic agent.

Another potential mechanism to increase endogenous GLP‐1 concentrations is to inhibit signalling pathways that negatively regulate GLP‐1 release. The hormone http://www.guidetopharmacology.org/GRAC/LigandDisplayForward?ligandId=2020 plays such a role in the gut as it exerts an inhibitory tone on GLP‐1 secretion. In support of this concept, blockade of the somatostatin http://www.guidetopharmacology.org/GRAC/ObjectDisplayForward?objectId=359 has been shown to enhance GLP‐1 release (Farb et al., 2017). Although different groups have reported SST₅ receptor antagonists, data from clinical trials evaluating this mechanism in humans have not been disclosed. Further efforts aimed at deep mining of GPCRs expressed in most entero‐endocrine cell types have been reported; this may lead to a host of potential new targets to explore either alone or in combination, ultimately with the goal of eliciting a therapeutically relevant increase in GLP‐1 and other incretins that can mimic those seen in bariatric surgery.

A look to the future of GPCR drug discovery in metabolic disease

Drug discovery is beset by failure at all stages. This is exemplified by the estimate of >99% of projects initiated in preclinical industrial labs failing to deliver new medicines (Paul et al., 2010). The success rate of developing T2DM therapies for all target classes (not just GPCRs) typifies this. Within the last 30 years, there have only been four classes of medications launched for T2DM that engage novel targets: the thiazolidinediones (oral http://www.guidetopharmacology.org/GRAC/ObjectDisplayForward?objectId=595 agonists), GLP‐1‐based drugs (injectable GLP‐1 analogues and oral DPP4 inhibitors), http://www.guidetopharmacology.org/GRAC/LigandDisplayForward?ligandId=7482 (the injectable amylin receptor agonist) and glucose reabsorption inhibitors (oral http://www.guidetopharmacology.org/GRAC/ObjectDisplayForward?objectId=916 inhibitors). Our opinion is that there is no obvious panacea or magic solution that can be applied to revolutionize GPCR‐based drug discovery for metabolic disease.

We conclude this review article by offering 10 questions that frame the modern drug discovery paradigm and summarise some of the key factors to consider when initiating and progressing a GPCR drug discovery campaign.

• 1 What is the target validation? Will engaging the GPCR be efficacious for the pathophysiology being targeted? This can encompass a wide variety of information including human genetic data, an efficacious compound in the clinic, compelling data from preclinical models using tool compounds and knockout mice phenotypes. A good example is the GLP‐1 receptor, as it does not show particularly strong validation attributes in mouse knockout models or from human genetics – yet this receptor is the pre‐eminent T2DM GPCR drug target.
• 2 Are preclinical models predictive of clinical efficacy? It will be difficult to discover and develop a drug if they are not. Clearly, some GPCRs, such as the β₃‐adrenoceptor and the GPR119, display robust efficacy in rodents, but this does not translate to treating human disease.
• 3 Will the approach be safe? There is enhanced scrutiny on the safety of metabolic medications because they are typically used chronically. Natural biological processes do not usually have target engagement to the extent that synthetic molecules do. This supra‐pharmacological action of synthetic molecules can lead to undesirable side‐effects or toxicology in some cases. Ligands of both CB₁ and GPBA receptors demonstrated robust clinical and preclinical‐efficacy respectively. However, safety concerns led to termination of those efforts.
• 4 Is it technically feasible? What is the probability of technical success? Are there basic science problems such as selectivity, potency, adsorption, metabolism, excretion and toxicology that cannot be solved in a reasonable timeframe?
• 5 Are there preclinical and clinical biomarkers for target engagement and translational pharmacology? If the compound progresses into clinical development, it is important to have a method to show the compound is actually engaging the target.

These first five questions are predominantly science rooted and occur at the front end of projects. Often, groups may focus on these first questions and conduct experiments to obtain answers.

• 6 How will efficacy of the medicine compare to the existing standard of care (that may be a generic or biosimilar molecule)?
• 7 Where will the medicine fit in the treatment paradigm and what will the therapeutic landscape look like in >10 years time?
• 8 Can a solid intellectual property position be established?
• 9 Is the new medicine commercially viable? What is the overall probability of success given cost of goods, patent life, projected market, etc.
• 10 Can efficacy data be generated to warrant reimbursement of the product by payers? Statistical significance is the historical benchmark to get a medicine approved. In the modern era, drugs need to show substantial benefit to the healthcare system to engender payment.

These remaining questions have more of a commercial and medical bent and are often out of the control of a GPCR‐focused scientist, but awareness of these issues is important in resource‐constrained environments.

Nomenclature of targets and ligands

Key protein targets and ligands in this article are hyperlinked to corresponding entries in http://www.guidetopharmacology.org, the common portal for data from the IUPHAR/BPS Guide to PHARMACOLOGY (Harding et al., 2018) and are permanently archived in the Concise Guide to PHARMACOLOGY 2017/2018 (Alexander et al., 2017a,b,c,d).

Conflict of interest

All authors are employees of Eli Lilly and Company and may own company stock or possess stock options.

Sloop, K. W. , Emmerson, P. J. , Statnick, M. A. , and Willard, F. S. (2018) The current state of GPCR‐based drug discovery to treat metabolic disease. British Journal of Pharmacology, 175: 4060–4071. 10.1111/bph.14157.