Elsevier

Neuropharmacology

Volume 60, Issue 1, January 2011, Pages 24-35
Neuropharmacology

Review
Allosteric modulation of G protein-coupled receptors: A pharmacological perspective

https://doi.org/10.1016/j.neuropharm.2010.07.010Get rights and content

Abstract

G protein-coupled receptor (GPCR)-based drug discovery has traditionally focused on targeting the orthosteric site for the endogenous agonist. However, many GPCRs possess allosteric sites that offer enormous potential for greater selectivity in drug action. The complex behaviors ascribed to allosteric ligands also present challenges to those interested in preclinical lead discovery. These challenges include the need to detect and quantify various phenomena when screening for allosteric ligands, such as saturability of effect, probe dependence, differential effects on orthosteric ligand affinity vs. efficacy, system-dependent allosteric agonism, stimulus-bias (functional selectivity), and the potential existence of bitopic (hybrid orthosteric/allosteric) ligands. These issues are also critical when interpreting structure–function studies of allosteric GPCR modulators because mutations in receptor structure, either engineered or naturally occurring, can differentially affect not only modulator affinity, but also the nature, magnitude and direction of the allosteric effect on orthosteric ligand function. The ever-expanding array of allosteric modulators arising from both academic and industrial research also highlights the need for the development of a uniform approach to nomenclature of such compounds.

Introduction

G protein-coupled receptors (GPCRs) comprise the largest superfamily of cell-surface receptors, accounting for 1–2% of the human genome (Foord, 2002). Despite sharing a structural similarity, namely a characteristic seven-transmembrane-spanning architecture linked by an N-terminal extracellular domain, C-terminal intracellular domain, three extracellular loops and three intracellular loops, GPCRs recognize a vast array of diverse extracellular stimuli (e.g., photons, ions, biogenic amines, peptides, lipids and proteins) and transduce their signals to the intracellular environment via a conformational rearrangement linked to an increasing number of cytosolic interactants that extends well beyond the classic G protein paradigm (Kristiansen, 2004). For decades, the development of ligands in traditional GPCR drug discovery has focused on targeting the orthosteric binding site of the receptor, which is defined as the site where the endogenous ligand binds to elicit signal transduction. This approach has guided the development of classical orthosteric ligands that directly activate the target receptor (agonists) or block the actions of the endogenous ligand (antagonists/inverse agonists). However, it is now well known that the rate of traditional GPCR-based novel drug discovery, in common with drugs for other target classes, is rapidly in decline (Booth and Zemmel, 2004). Part of this decline may relate to the relative intractability of many GPCRs (e.g., peptide receptors) to orthosteric small molecule discovery, as well as the difficulty in selectively targeting orthosteric sites at receptors that display high homology in this region between subtypes.

Accordingly, the last decade and a half has seen a pronounced shift in the acceptance and pursuit of small molecules that target topographically distinct allosteric sites on GPCRs (Christopoulos, 2002, Christopoulos and Kenakin, 2002, May et al., 2007b). Ligands that bind to these allosteric sites are called “allosteric modulators”, because they elicit a conformational change in the receptor while still allowing, in many instances, the concomitant binding of orthosteric ligands – thus modulating the pharmacological characteristics of the latter agent. Allosteric drugs offer enormous potential with regards to greater selectivity of action and novel modes of sculpting GPCR responsiveness in a spatially and temporally advantageous manner. However, despite the (now) broad acceptance of the phenomenon within academia and the pharmaceutical industry, substantial challenges remain in the detection, validation, classification and translation of putative allosteric GPCR drug candidates. This review will briefly consider some of the issues associated with the pharmacological exploitation of allosteric GPCR ligands.

Section snippets

Development of the concept

The allosteric concept, first coined and formalized by Monod, Wyman, Changeux and colleagues (Monod et al., 1963, Monod et al., 1965, Monod and Jacob, 1961), was built on the synthesis of observations related to cooperativity in enzyme behavior and the study of mechanisms governing end-product inhibition by substances that were structurally distinct from compounds that recognized classic enzyme substrate-binding sites. The first model that was proposed to account for allosteric behavior, the

Operational approaches to quantifying GPCR allosterism

Ideally, it is desirable if the parameters describing allosteric modulator effects on GPCR function could be readily determined from experimental observations in order to facilitate studies of modulator structure–activity relationships (SAR). Unfortunately, such parameters are not as readily attainable as in an orthosteric setting, where most potency/selectivity optimization is facilitated by the relatively straightforward determination of compound affinity values. At a minimum, allosteric

Pharmacological characteristics of GPCR allosterism

Whether viewed from a mechanistic or an operational perspective, GPCR allosterism is characterized by a number of properties that reflect the underlying reciprocity of effect between two different ligands, be they small molecules or large proteins, that interact concomitantly via topographically distinct sites on a GPCR. These key properties are: saturability of effect, probe dependence, differential effects on orthosteric ligand affinity vs. efficacy, and stimulus-bias (functional

The best of both worlds: bitopic orthosteric/allosteric ligands

In recent years, studies have focused on an interesting mode of ligand–GPCR engagement that combines the principles of allostery and functional selectivity with a novel twist. Specifically, compounds are being discovered, or designed, that behave as hybrid molecules that engage both orthosteric and allosteric sites on a GPCR to achieve functionally selective signaling (Mohr et al., 2010, Valant et al., 2009). These compounds have been dubbed “bitopic”, “dualsteric” or “multivalent” (Antony

Structure–function analyses of GPCR allosteric modulation

The enormous diversity of orthosteric ligands that act at the GPCR superfamily indicates that, despite sharing a common architecture, these receptors can display multiple orthosteric binding modes. Thus, for biogenic amine GPCRs, the orthosteric binding site comprises a cavity near the top third of the transmembrane helical bundle, whereas for many peptide-recognizing GPCRs, the primary orthosteric site contacts are composed of regions in the extracellular N terminus and/or loops; for the

A word on terminology

Since its introduction, the word “allosteric” has come to be associated with an increasing array of protein behaviors and phenomena, extending well beyond the description of protein structural changes elicited by the transmission of conformational changes between two topographically distinct ligand-binding sites. It has been argued recently that the uses of the term should be restricted to those instances where the free energy of coupling between a protein and a ligand is altered, in a

Conclusions

The study of allosteric modulation of GPCRs has now progressed from the realm of pharmacological rarity to an established paradigm. The potential therapeutic advantages of allosteric ligands as drugs are well accepted, but have yet to be widely realized in practice because the clinical translation of such compounds is still in its early days. The ever-expanding portfolio of complex behaviors ascribed to allosteric ligands also presents both challenges and opportunities to preclinical lead

Acknowledgments

Work in the authors’ laboratory is funded by Program Grant no. 519461 of the National Health and Medical Research Council (NHMRC) of Australia. Arthur Christopoulos is a Senior, and Patrick Sexton a Principal, Research Fellow of the NHMRC. Peter Keov is a recipient of an Australian Postgraduate Award. The authors would like to thank Rick Neubig, Terry Kenakin, Fred Ehlert and Andrew Tobin for helpful discussions on allosteric ligand classification.

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