A novel, conformation-specific allosteric inhibitor of the tachykinin NK2 receptor (NK2R) with functionally selective properties

Allostery: The Good, the Bad, and the Ugly

With the advent of functional screening, more allosteric molecules are being discovered and developed as possible therapeutic entities. Allosteric proteins are unique because of two specific properties: 1) separate binding sites for allosteric modulators and guests and 2) mandatory alteration of receptor conformation upon binding of allosteric modulators. For G protein-coupled receptors, these properties produce many beneficial effects on pharmacologic systems that are described here. Allosteric discovery campaigns also bring with them added considerations that must be addressed for the endeavor to be successful, and these are described herein as well. SIGNIFICANCE STATEMENT: Recent years have seen the increasing presence of allosteric molecules as possible therapeutic drug candidates. The scientific procedures to characterize these are unique and require special techniques, so it is imperative that scientists understand the new concepts involved in allosteric function. This review examines the reasons why allosteric molecules should be considered as new drug entities and the techniques required to optimize the discovery process for allosteric molecules.

The wonderful and masterful G protein-coupled receptor (GPCR): A focus on signaling mechanisms and the neuroendocrine control of fertility

Human GnRH deficiency, both clinically and genetically, is a heterogeneous disorder comprising of congenital GnRH deficiency with anosmia (Kallmann syndrome), or with normal olfaction [normosmic idiopathic hypogonadotropic hypogonadism (IHH)], and adult-onset hypogonadotropic hypogonadism. Our understanding of the neural mechanisms underlying GnRH secretion and GnRH signaling continues to increase at a rapid rate and strikingly, the heterotrimeric guanine nucleotide-binding protein (G protein)-coupled receptors (GPCRs) continue to emerge as essential players in these processes. GPCRs were once viewed as binary on-off switches, where in the "on" state they are bound to their Gα protein, but now we understand that view is overly simplistic and does not adequately characterize GPCRs. Instead, GPCRs have emerged as masterful signaling molecules exploiting different physical conformational states of itself to elicit an array of downstream signaling events via their G proteins and the β-arrestins. The "one receptor-multiple signaling conformations" model is likely an evolved strategy that can be used to our advantage as researchers have shown that targeting specific receptor conformations via biased ligands is proving to be a powerful tool in the effective treatment of human diseases. Can biased ligands be used to selectively modulate signaling by GPCR regulators of the neuroendocrine axis in the treatment of IHH? As discussed in this review, the grand possibility exists. However, while we are still very far from developing these treatments, this exciting likelihood can happen through a much greater mechanistic understanding of how GPCRs signal within the cell.

Prescient Indices of Activity: The Application of Functional System Sensitivity to Measurement of Drug Effect

Through pharmacological procedures, indices of drug activity can be obtained that transcend the systems in which they are measured. If (i) affinity, (ii) efficacies, (iii) orthosteric versus allosteric interaction, and (iv) rate of receptor offset can be determined, activity can be predicted in all systems. This can yield more detailed profiles (fingerprints) of efficacy to better define the required activities of follow-up molecules should the original candidates fail in the clinic. The use of functional assays of varying sensitivity is a major tool in the lead optimization process and the observation of candidate molecule profiles in multiple functional assays can reveal all properties of candidate molecules. In this review, the different indices for agonists, antagonists, and allosteric modulators are defined while highlighting the application of functional assays in deriving these indices.

Biased signaling of G protein coupled receptors (GPCRs): Molecular determinants of GPCR/transducer selectivity and therapeutic potential

G protein coupled receptors (GPCRs) convey signals across membranes via interaction with G proteins. Originally, an individual GPCR was thought to signal through one G protein family, comprising cognate G proteins that mediate canonical receptor signaling. However, several deviations from canonical signaling pathways for GPCRs have been described. It is now clear that GPCRs can engage with multiple G proteins and the line between cognate and non-cognate signaling is increasingly blurred. Furthermore, GPCRs couple to non-G protein transducers, including β-arrestins or other scaffold proteins, to initiate additional signaling cascades. Receptor/transducer selectivity is dictated by agonist-induced receptor conformations as well as by collateral factors. In particular, ligands stabilize distinct receptor conformations to preferentially activate certain pathways, designated 'biased signaling'. In this regard, receptor sequence alignment and mutagenesis have helped to identify key receptor domains for receptor/transducer specificity. Furthermore, molecular structures of GPCRs bound to different ligands or transducers have provided detailed insights into mechanisms of coupling selectivity. However, receptor dimerization, compartmentalization, and trafficking, receptor-transducer-effector stoichiometry, and ligand residence and exposure times can each affect GPCR coupling. Extrinsic factors including cell type or assay conditions can also influence receptor signaling. Understanding these factors may lead to the development of improved biased ligands with the potential to enhance therapeutic benefit, while minimizing adverse effects. In this review, evidence for ligand-specific GPCR signaling toward different transducers or pathways is elaborated. Furthermore, molecular determinants of biased signaling toward these pathways and relevant examples of the potential clinical benefits and pitfalls of biased ligands are discussed.

Allosteric Modulation of Class A GPCRs: Targets, Agents, and Emerging Concepts

G-protein-coupled receptors (GPCRs) have been tractable drug targets for decades with over one-third of currently marketed drugs targeting GPCRs. Of these, the class A GPCR superfamily is highly represented, and continued drug discovery for this family of receptors may provide novel therapeutics for a vast range of diseases. GPCR allosteric modulation is an innovative targeting approach that broadens the available small molecule toolbox and is proving to be a viable drug discovery strategy, as evidenced by recent FDA approvals and clinical trials. Numerous class A GPCR allosteric modulators have been discovered recently, and emerging trends such as the availability of GPCR crystal structures, diverse functional assays, and structure-based computational approaches are improving optimization and development. This Perspective provides an update on allosterically targeted class A GPCRs and their disease indications and the medicinal chemistry approaches toward novel allosteric modulators and highlights emerging trends and opportunities in the field.

Discovery of a Locally and Orally Active CXCL12 Neutraligand (LIT-927) with Anti-inflammatory Effect in a Murine Model of Allergic Airway Hypereosinophilia

We previously reported Chalcone-4 (1) that binds the chemokine CXCL12, not its cognate receptors CXCR4 or CXCR7, and neutralizes its biological activity. However, this neutraligand suffers from limitations such as poor chemical stability, solubility, and oral activity. Herein, we report on the discovery of pyrimidinone 57 (LIT-927), a novel neutraligand of CXCL12 which displays a higher solubility than 1 and is no longer a Michael acceptor. While both 1 and 57 reduce eosinophil recruitment in a murine model of allergic airway hypereosinophilia, 57 is the only one to display inhibitory activity following oral administration. Thereby, we here describe 57 as the first orally active CXCL12 neutraligand with anti-inflammatory properties. Combined with a high binding selectivity for CXCL12 over other chemokines, 57 represents a powerful pharmacological tool to investigate CXCL12 physiology in vivo and to explore the activity of chemokine neutralization in inflammatory and related diseases.

PAM-Antagonists: A Better Way to Block Pathological Receptor Signaling?

Seven transmembrane receptor (7TMR) responses are modulated by orthosteric and allosteric ligands to great therapeutic advantage. Here we introduce a unique class of negative allosteric modulator (NAM) - the positive allosteric modulator (PAM)-antagonist - that increases the affinity of the receptor for the agonist but concomitantly decreases agonist efficacy when cobound. Notably, the reciprocation of allosteric energy causes the orthosteric agonist to increase the affinity of the receptor for the PAM-antagonist; thus, this modulator seeks out and destroys agonist-bound receptor complexes. When contrasted with standard orthosteric and allosteric antagonists it is clear that PAM-antagonists are uniquely well suited to reversing ongoing persistent agonism and provide favorable target coverage in vivo. Specifically, the therapeutic application of PAM-antagonists to reverse pathological overactivation (e.g., endothelin vasoconstriction) is emphasized.

Allosteric Modulation of GPCRs: New Insights and Potential Utility for Treatment of Schizophrenia and Other CNS Disorders

G-protein-coupled receptors (GPCRs) play critical roles in regulating brain function. Recent advances have greatly expanded our understanding of these receptors as complex signaling machines that can adopt numerous conformations and modulate multiple downstream signaling pathways. While agonists and antagonists have traditionally been pursued to target GPCRs, allosteric modulators provide several mechanistic advantages, including the ability to distinguish between closely related receptor subtypes. Recently, the discovery of allosteric ligands that confer bias and modulate some, but not all, of a given receptor's downstream signaling pathways can provide pharmacological modulation of brain circuitry with remarkable precision. In addition, allosteric modulators with unprecedented specificity have been developed that can differentiate between subpopulations of a given receptor subtype based on the receptor's dimerization state. These advances are not only providing insight into the biological roles of specific receptor populations, but hold great promise for treating numerous CNS disorders.

Signaling bias in drug discovery

INTRODUCTION

The availability of different functional pharmacological assays has revealed that agonists for receptors that are pleiotropically coupled to multiple signaling pathways in the cell can emphasize signals to some pathways over others, i.e. can be biased toward certain signals. This, in turn, opens the possibility that molecules can be made to emphasize favorable signals, de-emphasize harmful signals or selectively block the ability of the natural agonist to produce unfavorable signals. Areas covered: This paper discusses the mechanism of biased signaling, the possible therapeutic implications of this effect, methods to quantify and measure bias and the current literature describing the translation of biased measure in vitro to in vivo systems. In addition, the challenges of exploiting this mechanism for therapy are outlined. Expert opinion: While this mechanism is well established and ubiquitous in pharmacology and easily measured in vitro, there are theoretical and practical hurdles to overcome to the fruitful utilization of signaling bias in therapeutic systems. There will be failures in the translation of biased molecules in vivo because of these challenges but hopefully also success and these latter translations hopefully will provide guidance in exploiting this effect further for therapy.

G Protein-coupled Receptor Biased Agonism

G protein-coupled receptors are the largest family of targets for current therapeutics. The classic model of their activation was binary, where agonist binding induced an active conformation and subsequent downstream signaling. Subsequently, the revised concept of biased agonism emerged, where different ligands at the same G protein-coupled receptor selectively activate one downstream pathway versus another. Advances in understanding the mechanism of biased agonism have led to the development of novel ligands, which have the potential for improved therapeutic and safety profiles. In this review, we summarize the theory and most recent breakthroughs in understanding biased signaling, examine recent laboratory investigations concerning biased ligands across different organ systems, and discuss the promising clinical applications of biased agonism.

Kinetic binding and activation profiles of endogenous tachykinins targeting the NK1 receptor

Ligand-receptor binding kinetics (i.e. association and dissociation rates) are emerging as important parameters for drug efficacy in vivo. Awareness of the kinetic behavior of endogenous ligands is pivotal, as drugs often have to compete with those. The binding kinetics of neurokinin 1 (NK1) receptor antagonists have been widely investigated while binding kinetics of endogenous tachykinins have hardly been reported, if at all. Therefore, the aim of this research was to investigate the binding kinetics of endogenous tachykinins and derivatives thereof and their role in the activation of the NK1 receptor. We determined the binding kinetics of seven tachykinins targeting the NK1 receptor. Dissociation rate constants (k_off) ranged from 0.026±0.0029min^-1 (Sar⁹,Met(O₂)¹¹-SP) to 0.21±0.015min^-1 (septide). Association rate constants (k_on) were more diverse: substance P (SP) associated the fastest with a k_on value of 0.24±0.046nM^-1min^-1 while neurokinin A (NKA) had the slowest association rate constant of 0.001±0.0002nM^-1min^-1. Kinetic binding parameters were highly correlated with potency and maximal response values determined in label-free impedance-based experiments on U-251 MG cells. Our research demonstrates large variations in binding kinetics of tachykinins which correlate to receptor activation. These findings provide new insights into the ligand-receptor interactions of tachykinins and underline the importance of measuring binding kinetics of both drug candidates and competing endogenous ligands.

Theoretical Aspects of GPCR-Ligand Complex Pharmacology

Over the past 50 years in pharmacology, an understanding of seven transmembrane (7TMR) function has been gained from the comparison of experimental data to receptor models. These models have been constructed from building blocks composed of systems consisting of series and parallel mass action binding reactions. Basic functions such as the the isomerization of receptors upon ligand binding, the sequential binding of receptors to membrane coupling proteins, and the selection of multiple receptor conformations have been combined in various ways to build receptor systems such as the ternary complex, extended ternary complex, and cubic ternary complex models for 7TMR function. Separately, the Black/Leff operational model has furnished an extremely valuable method of quantifying drug agonism. In the past few years, incorporation of the basic allosteric nature of 7TMRs has led to additional useful models of functional receptor allosteric mechanisms; these models yield valuable methods for quantifying allosteric effects. Finally, molecular dynamics has provided yet another new set of models describing the probability of formation of multiple receptor states; these radically new models are extremely useful in the prediction of functionally selective drug effects.

Noribogaine is a G-protein biased κ-opioid receptor agonist

Noribogaine is the long-lived human metabolite of the anti-addictive substance ibogaine. Noribogaine efficaciously reaches the brain with concentrations up to 20 μM after acute therapeutic dose of 40 mg/kg ibogaine in animals. Noribogaine displays atypical opioid-like components in vivo, anti-addictive effects and potent modulatory properties of the tolerance to opiates for which the mode of action remained uncharacterized thus far. Our binding experiments and computational simulations indicate that noribogaine may bind to the orthosteric morphinan binding site of the opioid receptors. Functional activities of noribogaine at G-protein and non G-protein pathways of the mu and kappa opioid receptors were characterized. Noribogaine was a weak mu antagonist with a functional inhibition constants (Ke) of 20 μM at the G-protein and β-arrestin signaling pathways. Conversely, noribogaine was a G-protein biased kappa agonist 75% as efficacious as dynorphin A at stimulating GDP-GTP exchange (EC50=9 μM) but only 12% as efficacious at recruiting β-arrestin, which could contribute to the lack of dysphoric effects of noribogaine. In turn, noribogaine functionally inhibited dynorphin-induced kappa β-arrestin recruitment and was more potent than its G-protein agonistic activity with an IC50 of 1 μM. This biased agonist/antagonist pharmacology is unique to noribogaine in comparison to various other ligands including ibogaine, 18-MC, nalmefene, and 6'-GNTI. We predict noribogaine to promote certain analgesic effects as well as anti-addictive effects at effective concentrations>1 μM in the brain. Because elevated levels of dynorphins are commonly observed and correlated with anxiety, dysphoric effects, and decreased dopaminergic tone, a therapeutically relevant functional inhibition bias to endogenously released dynorphins by noribogaine might be worthy of consideration for treating anxiety and substance related disorders.

Gaddum Memorial Lecture 2014: receptors as an evolving concept: from switches to biased microprocessors

This review is based on the JR Vane Medal Lecture presented at the BPS Winter Meeting in December 2014 by T. Kenakin. A recording of the lecture is included as supporting information and can also be viewed online here: https://www.youtube.com/watch?v=xrP81AQ8l-8. Pharmacological models used to describe drug agonism and antagonism have evolved over the past 20 years from a parsimonious model describing single active and inactive receptor states to models of multiconformational receptor systems modified by ligand conformational selection. These latter models describe the observed, presently underexploited, pharmacological mechanism of ligand-directed biased signalling. Biased signals can be quantified with transduction coefficients (ΔΔLog(τ/KA) values), a scale grounded in the Black/Leff operational model; this enables the optimization of biased profiles through medicinal chemistry. The past decades have also brought the availability of new technologies to measure multiple functional effects mediated by seven transmembrane receptors. These have confirmed that drugs can have many efficacies, which may be collaterally linked, that is there is no linear sequence of activities required. In addition, new functional screening assays have introduced increasing numbers of allosteric ligands into drug discovery. These molecules are permissive (they do not necessarily preclude endogenous signalling in vivo); therefore, they may allow better fine tuning of pathological physiology. The permissive quality of allosteric ligands can also change the quality of endogenous signalling efficacy ('induced bias') as well as the quantity of signal; in this regard, indices related to ΔΔLog(τ/KA) values (namely ΔLog(αβ) values) can be used to quantify these effects for optimization in the drug discovery process. All of these added scales of drug activity will, hopefully, allow better targeting of candidate molecules towards therapies.

The Effective Application of Biased Signaling to New Drug Discovery

The ability of agonists to selectively activate some but not all signaling pathways linked to pleiotropically signaling receptors has opened the possibility of obtaining molecules that emphasize beneficial signals, de-emphasize harmful signals, and concomitantly deemphasize harmful signals while blocking the harmful signals produced by endogenous agonists. The detection and quantification of biased effects is straightforward, but two important factors should be considered in the evaluation of biased effects in drug discovery. The first is that efficacy, and not bias, determines whether a given agonist signal will be observed; bias only dictates the relative concentrations at which agonist signals will appear when they do appear. Therefore, a Cartesian coordinate system plotting relative efficacy (on a scale of Log relative Intrinsic Activities) as the ordinates and Log(bias) as the abscissae is proposed as a useful tool in evaluating possible biased molecules for progression in discovery programs. Second, it should be considered that the current scales quantifying bias limit this property to the allosteric vector (ligand/receptor/coupling protein complex) and that whole-cell processing of this signal can completely change measured bias from in vitro predictions.

Towards tissue-specific pharmacology: insights from the calcium-sensing receptor as a paradigm for GPCR (patho)physiological bias

The calcium-sensing receptor (CaSR) is a widely expressed G protein-coupled receptor (GPCR) that mediates numerous tissue-specific functions. Its multiple ligands and diverse roles attest to the need for exquisite control over the signaling pathways that mediate its effects. 'Biased signaling' is the phenomenon by which distinct ligands stabilize preferred receptor signaling states. The CaSR is subject to biased signaling in response to its endogenous ligands. Interestingly, the 'natural' bias of the CaSR is altered in disease states, and small molecule drugs engender biased allosteric modulation of downstream signaling pathways. Thus, biased signaling from the CaSR also has important implications pathophysiologically and therapeutically. As outlined in this review, this novel paradigm extends to other GPCRs, making the CaSR a model for studies of ligand-biased signaling and for understanding how it may be used to foster selective drug activity in different tissues.

Specific targeting of the IL-23 receptor, using a novel small peptide noncompetitive antagonist, decreases the inflammatory response

IL-23 is part of the IL-12 family of cytokines and is composed of the p19 subunit specific to IL-23 and the p40 subunit shared with IL-12. IL-23 specifically contributes to the inflammatory process of multiple chronic inflammatory autoimmune disorders, including psoriasis, multiple sclerosis, inflammatory bowel disease, and rheumatoid arthritis. So far, one antibody targeting the shared p40 subunit of IL-12 and IL-23, Ustekinumab, is approved clinically to treat psoriasis. However, there are no treatments inhibiting specifically the IL-23 proinflammatory response. We have developed small IL-23R-specific antagonists by designing all D-peptides arising from flexible regions of IL-23R. Of these peptides, we selected 2305 (teeeqqly), since in addition to its soluble properties, it inhibited IL-23-induced STAT3 phosphorylation in spleen cells. Peptide 2305 specifically binds to IL-23R/IL-12Rβ1-expressing HEK-293 cells and not to cells devoid of the receptor. Peptide 2305 showed functional selectivity by modulating IL-23-induced gene expression in IL-23R/IL-12Rβ1-expressing cells and in Jurkat cells; 2305 does not inhibit IL-12-induced cytokine expression in IL-12Rβ-IL-12Rβ2-HEK-293 cells. Finally, compared with anti-p40 treatment, 2305 effectively and selectively inhibits IL-23-induced inflammation in three in vivo mouse models: IL-23-induced ear inflammation, anti-CD40-induced systemic inflammatory response, and collagen-induced arthritis. We, hereby, describe the discovery and characterization of a potent IL-23R small-peptide modulator, 2305 (teeeqqly), that is effective in vivo. 2305 may be more convenient, less cumbersome, less costly, and most importantly, more specific than current biologics for the treatment of inflammatory conditions, and conceivably complement the actual therapies for these chronic and debilitating inflammatory diseases.

Opportunities and challenges in the discovery of allosteric modulators of GPCRs for treating CNS disorders

Novel allosteric modulators of G protein-coupled receptors (GPCRs) are providing fundamental advances in the development of GPCR ligands with high subtype selectivity and novel modes of efficacy that have not been possible with traditional approaches. As new allosteric modulators are advancing as drug candidates, we are developing an increased understanding of the major advantages and broad range of activities that can be achieved with these agents through selective modulation of specific signalling pathways, differential effects on GPCR homodimers versus heterodimers, and other properties. This understanding creates exciting opportunities, as well as unique challenges, in the optimization of novel therapeutic agents for disorders of the central nervous system.

Advances in G protein-coupled receptor allostery: from function to structure

It is now widely accepted that G protein-coupled receptors (GPCRs) are highly dynamic proteins that adopt multiple active states linked to distinct functional outcomes. Furthermore, these states can be differentially stabilized not only by orthosteric ligands but also by allosteric ligands acting at spatially distinct binding sites. The key pharmacologic characteristics of GPCR allostery include improved selectivity due to either greater sequence divergence between receptor subtypes and/or subtype-selective cooperativity, a ceiling level to the effect, probe dependence (whereby the magnitude and direction of the allosteric effect change with the nature of the interacting ligands), and the potential for biased signaling. Recent chemical biology developments are beginning to demonstrate how the incorporation of analytical pharmacology and operational modeling into the experimental workflow can enrich structure-activity studies of allostery and bias, and have also led to the discovery of a new class of hybrid orthosteric/allosteric (bitopic) molecules. The potential for endogenous allosteric modulators to play a role in physiology and disease remains to be fully appreciated but will likely represent an important area for future studies. Finally, breakthroughs in structural and computational biology are beginning to unravel the mechanistic basis of GPCR allosteric modulation at the molecular level.

What is pharmacological 'affinity'? Relevance to biased agonism and antagonism

The differences between affinity measurements made in binding studies and those relevant to receptor function are described. There are theoretical and practical reasons for not utilizing binding data and, in terms of the quantification of signaling bias, it is unnecessary to do so. Finally, the allosteric control of ligand affinity through receptor-signaling protein interaction is discussed within the context of biased antagonism. In this regard, it is shown that both the bias and relative efficacy of a ligand are essential data for fully predicting biased effects in vivo.

A simple method to detect allostery in GPCR dimers

G protein-coupled receptors (GPCRs) represent one of the largest families of cell surface receptors as key targets for pharmacological manipulation. G proteins have long been recognized as allosteric modulators of GPCR ligand binding. More recently, small molecule allosteric modulators have now been widely characterized for a number of GPCRs, and some are now used clinically. Many studies have also underscored the importance of GPCR dimerization or higher-order oligomerization in the control of the physiological responses they modulate. Thus, allosterism can also, between monomer equivalents in the context of a dimer, oligomer, or receptor mosaic, influence signaling pathways downstream. It therefore becomes essential to characterize both small molecule allosteric ligands and allosteric interactions between receptors modulated by canonical orthosteric ligands, in a pathway-specific manner. Here, we describe a simple, radioligand-binding method, which is designed to probe for allosteric modulation mediated by any GPCR interactor, from small molecules to interacting proteins. It can also detect allosteric asymmetries within a GPCR heterodimer, via orthosteric or allosteric ligands. This assay measures time-dependent ligand occupancy of radiolabeled orthosteric or (with adaptations) allosteric ligands as modulated by either small molecules or receptor dimer partners bound or unbound with their own ligands.

Biased ligands: pathway validation for novel GPCR therapeutics

G protein-coupled receptors (GPCRs), in recent years, have been shown to signal via multiple distinct pathways. Furthermore, biased ligands for some receptors can differentially stimulate or inhibit these pathways versus unbiased endogenous ligands or drugs. Biased ligands can be used to gain a deeper understanding of the molecular targets and cellular responses associated with a GPCR, and may be developed into therapeutics with improved efficacy, safety and/or tolerability. Here we review examples and approaches to pathway validation that establish the relevance and therapeutic potential of distinct pathways that can be selectively activated or blocked by biased ligands.

Biased agonism: a novel paradigm in G protein-coupled receptor signaling observed in acquired hypocalciuric hypercalcemia

The classical model of G protein-coupled receptor (GPCR) activation is the two-state model, in which the GPCR exists in equilibrium between an active and inactive state. Based on this model, GPCR ligands have been classified as agonists, inverse agonists, or antagonists depending on their actions in shifting this equilibrium. Recently, however, accumulating evidence has indicated that GPCRs may exist in multiple active and inactive conformational states. In this situation, each ligand recognizes and stabilizes a specific conformation of the GPCR, leading to a set of specific biological effects. Based on this new model, a unique agonist or a combination of the usual agonist and an allosteric modulator may enable activation of a specific signaling pathway via a GPCR that activates multiple signals (biased agonism, functional selectivity). The calcium-sensing receptor autoantibody that we have identified in the serum of a patient with acquired hypocalciuric hypercalcemia (AHH) is the first example of a biased allosteric modulator of a GPCR working in a pathophysiological context. Our findings may indicate the presence of physiological allosteric modulators and provide new directions for the future drug development.

Minireview: Signal bias, allosterism, and polymorphic variation at the GLP-1R: implications for drug discovery

The glucagon-like peptide-1 receptor (GLP-1R) controls the physiological responses to the incretin hormone glucagon-like peptide-1 and is a major therapeutic target for the treatment of type 2 diabetes, owing to the broad range of effects that are mediated upon its activation. These include the promotion of glucose-dependent insulin secretion, increased insulin biosynthesis, preservation of β-cell mass, improved peripheral insulin action, and promotion of weight loss. Regulation of GLP-1R function is complex, with multiple endogenous and exogenous peptides that interact with the receptor that result in the activation of numerous downstream signaling cascades. The current understanding of GLP-1R signaling and regulation is limited, with the desired spectrum of signaling required for the ideal therapeutic outcome still to be determined. In addition, there are several single-nucleotide polymorphisms (used in this review as defining a natural change of single nucleotide in the receptor sequence; clinically, this is viewed as a single-nucleotide polymorphism only if the frequency of the mutation occurs in 1% or more of the population) distributed within the coding sequence of the receptor protein that have the potential to produce differential responses for distinct ligands. In this review, we discuss the current understanding of GLP-1R function, in particular highlighting recent advances in the field on ligand-directed signal bias, allosteric modulation, and probe dependence and the implications of these behaviors for drug discovery and development.

An antedrug of the CXCL12 neutraligand blocks experimental allergic asthma without systemic effect in mice

The chemokine receptor CXCR4 and its chemokine CXCL12 are involved in normal tissue patterning but also in tumor cell growth and survival as well as in the recruitment of immune and inflammatory cells, as successfully demonstrated using agents that block either CXCL12 or CXCR4. In order to achieve selectivity in drug action on the CXCR4/CXCL12 pair, in particular in the airways, drugs should be delivered as selectively as possible in the treated tissue and should not diffuse in the systemic circulation, where it may reach undesired organs. To this end, we used a previously unexploited Knoevenagel reaction to create a short lived drug, or soft drug, based on the CXCL12-neutralizing small molecule, chalcone 4, which blocks binding of CXCL12 to CXCR4. We show that the compound, carbonitrile-chalcone 4, blocks the recruitment of eosinophils to the airways in ovalbumin-sensitized and challenged mice in vivo when administered directly to the airways by the intranasal route, but not when administered systemically by the intraperitoneal route. We show that the lack of effect at a distant site is due to the rapid degradation of the molecule to inactive fragments. This approach allows selective action of the CXCL12 neutraligands although the target protein is widely distributed in the organism.

Troubleshooting and deconvoluting label-free cell phenotypic assays in drug discovery

INTRODUCTION

Central to drug discovery and development is to comprehend the target(s), potency, efficacy and safety of drug molecules using pharmacological assays. Owing to their ability to provide a holistic view of drug actions in native cells, label-free biosensor-enabled cell phenotypic assays have been emerging as new generation phenotypic assays for drug discovery. Despite the benefits associated with wide pathway coverage, high sensitivity, high information content, non-invasiveness and real-time kinetics, label-free cell phenotypic assays are often viewed to be a blackbox in the era of target-centric drug discovery.

METHODS

This article first reviews the biochemical and biological complexity of drug-target interactions, and then discusses the key characteristics of label-free cell phenotypic assays and presents a five-step strategy to troubleshooting and deconvoluting the label-free cell phenotypic profiles of drugs.

RESULTS

Drug-target interactions are intrinsically complicated. Label-free cell phenotypic signatures of drugs mirror the innate complexity of drug-target interactions, and can be effectively deconvoluted using the five-step strategy.

DISCUSSION

The past decades have witnessed dramatic expansion of pharmacological assays ranging from molecular to phenotypic assays, which is coincident with the realization of the innate complexity of drug-target interactions. The clinical features of a drug are defined by how it operates at the system level and by its distinct polypharmacology, ontarget, phenotypic and network pharmacology. Approaches to examine the biochemical, cellular and molecular mechanisms of action of drugs are essential to increase the efficiency of drug discovery and development. Label-free cell phenotypic assays and the troubleshooting and deconvoluting approach presented here may hold great promise in drug discovery and development.

Allosteric modulation of a chemogenetically modified G protein-coupled receptor

Designer receptors exclusively activated by designer drugs (DREADDs) are chemogenetically modified muscarinic acetylcholine receptors (mAChRs) that have minimal responsiveness to acetylcholine (ACh) but are potently and efficaciously activated by an otherwise inert synthetic ligand, clozapine-N-oxide (CNO). DREADDs have been used as tools for selectively modulating signal transduction pathways in vitro and in vivo. Recent comprehensive studies have validated how the pharmacology of a CNO-bound DREADD mirrors that of an ACh-bound wild-type (WT) mAChR. However, nothing is known about whether this equivalence extends to the allosteric modulation of DREADDs by small molecules. To address this, we investigated the actions at an M(1) DREADD of benzyl quinolone carboxylic acid (BQCA), a positive allosteric modulator of ACh binding and function that is known to behave according to a simple two-state mechanism at the WT receptor. We found that allosteric modulation of the CNO-bound DREADD receptor is not equivalent to the corresponding modulation of the ACh-bound WT receptor. We also found that BQCA engenders stimulus bias at the M(1) DREADD, having differential types of cooperativity depending on the signaling pathway. Furthermore, the modulation of ACh itself by BQCA at the DREADD is not compatible with the two-state model that we previously applied to the M(1) WT receptor.

Allosteric modulators of rhodopsin-like G protein-coupled receptors: opportunities in drug development

Rhodopsin-like (class A) G protein-coupled receptors (GPCRs) are one of the most important classes of drug targets. The discovery that these GPCRs can be allosterically modulated by small drug molecules has opened up new opportunities in drug development. It will allow the drugability of "difficult targets", such as GPCRs activated by large (glyco)proteins, or by very polar or highly lipophilic physiological agonists. Receptor subtype selectivity should be more easily achievable with allosteric than with orthosteric ligands. Allosteric modulation will allow a broad spectrum of pharmacological effects largely expanding that of orthosteric ligands. Furthermore, allosteric modulators may show an improved safety profile as compared to orthosteric ligands. Only recently, the explicit search for allosteric modulators has been started for only a few rhodopsin-like GPCRs. The first negative allosteric modulators (allosteric antagonists) of chemokine receptors, maraviroc (CCR5 receptor), used in HIV therapy, and plerixafor (CXCR4 receptor) for stem cell mobilization, have been approved as drugs. The development of allosteric modulators for rhodopsin-like GPCRs as novel drugs is still at an early stage; it appears highly promising.

A biased ligand for OXE-R uncouples Gα and Gβγ signaling within a heterotrimer

Differential targeting of heterotrimeric G protein versus β-arrestin signaling are emerging concepts in G protein-coupled receptor (GPCR) research and drug discovery, and biased engagement by GPCR ligands of either β-arrestin or G protein pathways has been disclosed. Herein we report on a new mechanism of ligand bias to titrate the signaling specificity of a cell-surface GPCR. Using a combination of biomolecular and virtual screening, we identified the small-molecule modulator Gue1654, which inhibits Gβγ but not Gα signaling triggered upon activation of Gα(i)-βγ by the chemoattractant receptor OXE-R in both recombinant and human primary cells. Gue1654 does not interfere nonspecifically with signaling directly at or downstream of Gβγ. This hitherto unappreciated mechanism of ligand bias at a GPCR highlights both a new paradigm for functional selectivity and a potentially new strategy to develop pathway-specific therapeutics.

Quantification of allosteric interactions at G protein-coupled receptors using radioligand binding assays

Allosteric interactions involve the simultaneous binding of two ligands to the same receptor. An allosteric modulator causes a conformational change in the receptor protein that yields a change in the binding or signaling of an orthosteric agent, i.e., an agonist or competitive antagonist that binds to the endogenous agonist binding site. Because of the complex nature of allosteric phenomena, the detection and quantification of their effects on orthosteric ligand binding relies on the use of both equilibrium and non-equilibrium assays to ensure proper interpretation of the findings. Outlined in this unit are the most common experimental approaches for measuring allosteric effects on orthosteric ligand affinity at G protein-coupled receptors. There is also a discussion of the analysis of experimental data derived from such assays.

Fluorescent derivatives of AC-42 to probe bitopic orthosteric/allosteric binding mechanisms on muscarinic M1 receptors

Two fluorescent derivatives of the M1 muscarinic selective agonist AC-42 were synthesized by coupling the lissamine rhodamine B fluorophore (in ortho and para positions) to AC42-NH(2). This precursor, prepared according to an original seven-step procedure, was included in the study together with the LRB fluorophore (alone or linked to an alkyl chain). All these compounds are antagonists, but examination of their ability to inhibit or modulate orthosteric [(3)H]NMS binding revealed that para-LRB-AC42 shared several properties with AC-42. Carefully designed experiments allowed para-LRB-AC42 to be used as a FRET tracer on EGFP-fused M1 receptors. Under equilibrium binding conditions, orthosteric ligands, AC-42, and the allosteric modulator gallamine behaved as competitors of para-LRB-AC42 binding whereas other allosteric compounds such as WIN 51,708 and N-desmethylclozapine were noncompetitive inhibitors. Finally, molecular modeling studies focused on putative orthosteric/allosteric bitopic poses for AC-42 and para-LRB-AC42 in a 3D model of the human M1 receptor.

Signalling bias in new drug discovery: detection, quantification and therapeutic impact

Agonists of seven-transmembrane receptors, also known as G protein-coupled receptors (GPCRs), do not uniformly activate all cellular signalling pathways linked to a given seven-transmembrane receptor (a phenomenon termed ligand or agonist bias); this discovery has changed how high-throughput screens are designed and how lead compounds are optimized for therapeutic activity. The ability to experimentally detect ligand bias has necessitated the development of methods for quantifying agonist bias in a way that can be used to guide structure-activity studies and the selection of drug candidates. Here, we provide a viewpoint on which methods are appropriate for quantifying bias, based on knowledge of how cellular and intracellular signalling proteins control the conformation of seven-transmembrane receptors. We also discuss possible predictions of how biased molecules may perform in vivo, and what potential therapeutic advantages they may provide.

Therapeutic potential of CCR1 antagonists for multiple myeloma

The chemokine receptor CCR1 has been the target of intensive research for nearly two decades. Small-molecule antagonists were first reported in 1998 and, since then, many inhibitors for CCR1 have been brought forth. Yet, with all the money and time spent, to date, no small-molecule antagonists have successfully moved past Phase II clinical trials. With the current advancement of CCR1 antagonists by Bristol-Myers Squibb and Chemocentrix, there has been renewed interest. In this review, we present an overview of CCR1, its activating ligands, methods of signaling, and downstream response. We discuss studies that indicate CCR1 plays an important role in multiple myeloma and the underlying molecular mechanisms. Finally, we present an overview of the clinical and preclinical compounds for CCR1. We address individual structures, discuss their pharmacological précis, and summarize the published evidence to assess their value for use in multiple myeloma.

Allostery in GPCRs: 'MWC' revisited

G protein-coupled receptors (GPCRs) constitute the largest family of receptors in the genome and are the targets for at least 30% of current medicines. In recent years, there has been a dramatic increase in the discovery of allosteric modulators of GPCR activity and a growing appreciation of the diverse modes by which GPCRs can be regulated by both orthosteric and allosteric ligands. Interestingly, some of the contemporary views of GPCR function reflect characteristics that are shared by prototypical allosteric proteins, as encompassed in the classic Monod-Wyman-Changeux (MWC) model initially proposed for enzymes and subsequently extended to other protein families. In this review, we revisit the MWC model in the context of emerging structural, functional and operational data on GPCR allostery.

The best of both worlds? Bitopic orthosteric/allosteric ligands of g protein-coupled receptors

It is now acknowledged that G protein-coupled receptors, the largest class of drug targets, adopt multiple active states that can be preferentially stabilized by orthosteric ligands or allosteric modulators, thus giving rise to the phenomenon of pathway-biased signaling. In the past few years, researchers have begun to explore the potential of linking orthosteric and allosteric pharmacophores to yield bitopic hybrid ligands. This approach is an extension of the more traditional bivalent ligand concept and shares some of the same challenges, including the choice and role of the linker between the two pharmacophores and the validation of mechanism of action. Nonetheless, the promise of bitopic ligands is the generation of novel chemical tools that have improved affinity and/or selectivity profiles. Previously identified functionally selective compounds (and medicines) also may act via a bitopic mechanism, suggesting that the phenomenon is more widespread than currently appreciated.

Biased Ligands for Better Cardiovascular Drugs

Drug discovery efforts targeting G-protein-coupled receptors (GPCR) have been immensely successful in creating new cardiovascular medicines. Currently marketed GPCR drugs are broadly classified as either agonists that activate receptors or antagonists that prevent receptor activation by endogenous stimuli. However, GPCR couple to a multitude of intracellular signaling pathways beyond classical G-protein signals, and these signals can be independently activated by biased ligands to vastly expand the potential for new drugs at these classic targets. By selectively engaging only a subset of a receptor's potential intracellular partners, biased ligands may deliver more precise therapeutic benefit with fewer side effects than current GPCR-targeted drugs. In this review, we discuss the history of biased ligand research, the current understanding of how biased ligands exert their unique pharmacology, and how research into GPCR signaling has uncovered previously unappreciated capabilities of receptor pharmacology. We focus on several receptors to illustrate the approaches taken and discoveries made, and how these are steadily illuminating the intricacies of GPCR pharmacology. Discoveries of biased ligands targeting the angiotensin II type 1 receptor and of separable pharmacology suggesting the potential value of biased ligands targeting the β-adrenergic receptors and nicotinic acid receptor GPR109a highlight the powerful clinical promise of this new category of potential therapeutics.

Refining efficacy: allosterism and bias in G protein-coupled receptor signaling

Receptors on the surface of cells function as conduits for information flowing between the external environment and the cell interior. Since signal transduction is based on the physical interaction of receptors with both extracellular ligands and intracellular effectors, ligand binding must produce conformational changes in the receptor that can be transmitted to the intracellular domains accessible to G proteins and other effectors. Classical models of G protein-coupled receptor (GPCR) signaling envision receptor conformations as highly constrained, wherein receptors exist in equilibrium between single "off" and "on" states distinguished by their ability to activate effectors, and ligands act by perturbing this equilibrium. In such models, ligands can be classified based upon two simple parameters; affinity and efficacy, and ligand activity is independent of the assay used to detect the response. However, it is clear that GPCRs assume multiple conformations, any number of which may be capable of interacting with a discrete subset of possible effectors. Both orthosteric ligands, molecules that occupy the natural ligand-binding pocket, and allosteric modulators, small molecules or proteins that contact receptors distant from the site of ligand binding, have the ability to alter the conformational equilibrium of a receptor in ways that affect its signaling output both qualitatively and quantitatively. In this context, efficacy becomes pluridimensional and ligand classification becomes assay dependent. A more complete description of ligand-receptor interaction requires the use of multiplexed assays of receptor activation and screening assays may need to be tailored to detect specific efficacy profiles.

Label-Free Receptor Assays

Label-free biosensors offer integrated, kinetic and multi-parametric measures of receptor biology and ligand pharmacology in whole cells. Being highly sensitive and pathway-unbiased, label-free receptor assays can be used to probe the systems cell biology including pleiotropic signaling of receptors, and to characterize the functional selectivity and phenotypic pharmacology of ligand molecules. These assays provide a new dimension for elucidating receptor biology and for facilitating drug discovery.

The N-terminal domain of human hemokinin-1 influences functional selectivity property for tachykinin receptor neurokinin-1

Human hemokinin-1 (hHK-1) is a substance P-like tachykinin peptide preferentially expressed in non-neuronal tissues. It is involved in multiple physiological functions such as inflammation, hematopoietic cells development and vasodilatation via the interaction with tachykinin receptor neurokinin-1 (NK1). To further understand the intracellular signal transduction mechanism under such functional multiplicity, current study was focused on the differential activation of Gs and Gq pathways by hHK-1 and its C-terminal fragments, which is termed as functional selectivity. We demonstrated these hHK-1 and related peptide fragments can independently activate Gs and Gq pathways, showing a relative bias toward Gq over Gs pathway. The T1, K3 and Q6 of hHK-1 might play roles in the activation of adenylate cyclase mediated by Gs, while having negligible effect on Gq mediated intracellular calcium release. The stepwise truncation of N-terminal amino acid of hHK-1 caused gradual decrease in ERK1/2 phosphorylation level and NF-κB activity. However, it had little influence on the induction of NK1 receptor desensitization and internalization. Taken together these data support that hHK-1 and its C-terminal fragments are human NK1 receptor agonists with different functional selectivity properties and that such functional selectivity leads to differential activation of downstream signaling and receptor trafficking.

Allosteric ligands for G protein-coupled receptors: a novel strategy with attractive therapeutic opportunities

Allosteric receptor ligands bind to a recognition site that is distinct from the binding site of the endogenous messenger molecule. As a consequence, allosteric agents may attach to receptors that are already transmitter-bound. Ternary complex formation opens an avenue to qualitatively new drug actions at G protein-coupled receptors (GPCRs), in particular receptor subtype selective potentiation of endogenous transmitter action. Consequently, suitable exploitation of allosteric recognition sites as alternative molecular targets could pave the way to a drug discovery paradigm different from those aimed at mimicking or blocking the effects of endogenous (orthosteric) receptor activators. The number of allosteric ligands reported to modulate GPCR function is steadily increasing and some have already reached routine clinical use. This review aims at introducing into this fascinating field of drug discovery and at providing an overview about the achievements that have already been made. Various case examples will be discussed in the framework of GPCR classification (family A, B, and C receptors). In addition, the behavior at muscarinic receptors of hybrid derivatives incorporating both an allosteric and an orthosteric fragment in a common molecular skeleton will be illustrated.

A novel biased allosteric compound inhibitor of parturition selectively impedes the prostaglandin F2alpha-mediated Rho/ROCK signaling pathway

The prostaglandin F2alpha (PGF2alpha) receptor (FP) is a key regulator of parturition and a target for pharmacological management of preterm labor. However, an incomplete understanding of signaling pathways regulating myometrial contraction hinders the development of improved therapeutics. Here we used a peptidomimetic inhibitor of parturition in mice, PDC113.824, whose structure was based on the NH(2)-terminal region of the second extracellular loop of FP receptor, to gain mechanistic insight underlying FP receptor-mediated cell responses in the context of parturition. We show that PDC113.824 not only delayed normal parturition in mice but also that it inhibited both PGF2alpha- and lipopolysaccharide-induced preterm labor. PDC113.824 inhibited PGF2alpha-mediated, G(alpha)(12)-dependent activation of the Rho/ROCK signaling pathways, actin remodeling, and contraction of human myometrial cells likely by acting as a non-competitive, allosteric modulator of PGF2alpha binding. In contrast to its negative allosteric modulating effects on Rho/ROCK signaling, PDC113.824 acted as a positive allosteric modulator on PGF2alpha-mediated protein kinase C and ERK1/2 signaling. This bias in receptor-dependent signaling was explained by an increase in FP receptor coupling to G(alpha)(q), at the expense of coupling to G(alpha)(12). Our findings regarding the allosteric and biased nature of PDC113.824 offer new mechanistic insights into FP receptor signaling relevant to parturition and suggest novel therapeutic opportunities for the development of new tocolytic drugs.

Prediction of functionally selective allosteric interactions at an M3 muscarinic acetylcholine receptor mutant using Saccharomyces cerevisiae

Saccharomyces cerevisiae is a tractable yeast species for expression and coupling of heterologous G protein-coupled receptors with the endogenous pheromone response pathway. Although this platform has been used for ligand screening, no studies have probed its ability to predict novel pharmacology and functional selectivity of allosteric ligands. As a proof of concept, we expressed a rat M(3) muscarinic acetylcholine receptor (mAChR) bearing a mutation (K(7.32)E) recently identified to confer positive cooperativity between acetylcholine and the allosteric modulator brucine in various strains of S. cerevisiae, each expressing a different human Galpha/yeast Gpa1 protein chimera, and probed for G protein-biased allosteric modulation. Subsequent assays performed in this system revealed that brucine was a partial allosteric agonist and positive modulator of carbachol when coupled to Gpa1/G(q) proteins, a positive modulator (no agonism) when coupled to Gpa1/G(12) proteins, and a neutral modulator when coupled to Gpa1/G(i) proteins. It is noteworthy that these results were validated at the human M(3)K(7.32)E mAChR expressed in a mammalian (Chinese hamster ovary) cell background by determination of calcium mobilization and membrane ruffling as surrogate measures of G(q) and G(12) protein activation, respectively. Furthermore, the combination of this functionally selective allosteric modulator with G protein-biased yeast screens allowed us to ascribe a potential G protein candidate (G(12)) as a key mediator for allosteric modulation of M(3)K(7.32)E mAChR-mediated ERK1/2 phosphorylation, which was confirmed by small interfering RNA knockdown experiments. These results highlight how the yeast platform can be used to identify functional selectivity of allosteric ligands and to facilitate dissection of convergent signaling pathways.

Seven transmembrane receptors as shapeshifting proteins: the impact of allosteric modulation and functional selectivity on new drug discovery

It is useful to consider seven transmembrane receptors (7TMRs) as disordered proteins able to allosterically respond to a number of binding partners. Considering 7TMRs as allosteric systems, affinity and efficacy can be thought of in terms of energy flow between a modulator, conduit (the receptor protein), and a number of guests. These guests can be other molecules, receptors, membrane-bound proteins, or signaling proteins in the cytosol. These vectorial flows of energy can yield standard canonical guest allostery (allosteric modification of drug effect), effects along the plane of the cell membrane (receptor oligomerization), or effects directed into the cytosol (differential signaling as functional selectivity). This review discusses these apparently diverse pharmacological effects in terms of molecular dynamics and protein ensemble theory, which tends to unify 7TMR behavior toward cells. Special consideration will be given to functional selectivity (biased agonism and biased antagonism) in terms of mechanism of action and potential therapeutic application. The explosion of technology that has enabled observation of diverse 7TMR behavior has also shown how drugs can have multiple (pluridimensional) efficacies and how this can cause paradoxical drug classification and nomenclatures.