Matters of Receptor Conformation

Inverse, protean, and ligand-selective agonism: matters of receptor conformation TERRY KENAKIN Department of Receptor Biochemistry, Glaxo SmithKline Research and Development, Research Triangle Park, North Carolina 27709, USA ABSTRACT Concepts regarding the mechanisms by allosteric interaction resulted in an alteration of the which drugs activate receptors to produce physiological affinity of the receptor for the radiolabel. With the response have progressed beyond considering the re- technological advances enabling high-throughput func- ceptor as a simple on-off switch. Current evidence tional receptor screens should come an increase in the suggests that the idea that agonists produce only vary- types of GPCR ligands in the new millennium. This will ing degrees of receptor activation is obsolete and must result in an increase in the number of allosteric ligands be reconciled with data to show that agonist efficacy has (modulators, agonists, enhancers) that modify receptor texture as well as magnitude. Thus, agonists can block function without necessarily modifying steric access of system constitutive response (inverse agonists), behave the endogenous ligands to the receptor. Therefore, the as positive and inverse agonists on the same receptor changing mode of high-throughput screening can be (protean agonists), and differ in the stimulus pattern predicted to lead to an increase in the texture of drug they produce in physiological systems (ligand-selective types for GPCRs. Another reason for the increasing agonists). The molecular mechanism for this seemingly number of drug targets is increased knowledge of diverse array of activities is the same, namely, the GPCR behavior in cellular systems. This has led to the selective microaffinity of ligands for different confor- discovery of inverse agonism. This review will concen- mational states of the receptor. This paper reviews trate on a subset of these new chemical targets, namely, evidence for the existence of the various types of those that possess efficacy, either positive or negative, agonism and the potential therapeutic utility of differ- and these will be discussed in terms of their mecha- ent agonist types.—Kenakin, T. Inverse, protean, and nisms of action and possible relevance to therapy of ligand-selective agonism: matters of receptor confor- disease. mation. FASEB J. 15, 598–611 (2001) Drugs with ‘efficacy’ Key Words: G-protein-coupled receptors ⅐ receptor activation ⅐ inverse agonism ⅐ receptor theory ⅐ constitutive receptor activity Drugs can be thought of as having two properties with respect to biological systems: affinity for the receptor and intrinsic efficacy. A common usage of the word NEW MOLECULAR TARGETS FOR DRUG efficacy in clinical pharmacology is ‘therapeutically DISCOVERY useful activity’. Thus, a drug is considered ‘efficacious’ if it alleviates the symptoms of a disease in a patient. The past decade has brought an explosion of new in- Within this context, even a competitive antagonist formation about G-protein-coupled receptors (GPCRs), would have ‘efficacy’. This review will discuss efficacy in their nature, and how they behave. With the sequenc- terms of its formal definition in pharmacological recep- ing of the human genome will come a plethora of new tor theory, that is, the property of a molecule that GPCR targets. This alliance of information is leading to causes it to produce some observable physiological a revolution in the drug discovery process. Similarly, response. In terms of GPCRs, a useful working defini- there is a burgeoning number of chemical targets for tion of receptor is the property of a molecule that therapeutic advantage; the list of drug targets for causes the receptor to change its behavior toward the receptors has grown considerably (see Fig. 1). Before host system (1). 1995, the major targets for drug development were full Three types of efficacious drugs will be discussed. and partial agonists and antagonists. Since the princi- Inverse agonists are an established drug class and pal mode of high-throughput screening has been ra- possess what is termed ‘negative efficacy’. Protean dioligand binding, orthosteric ligands (those that steri- agonists are a theoretical class that produce receptor cally hinder the access of the radiolabel to the receptor binding site) primarily were discovered. Allosteric li- 1 Correspondence: Department of Receptor Biochemistry, gands (those that affect receptor function through GlaxoWellcome Research and Development, 5 Moore Dr., binding to their own binding site separate from that of Research Triangle Park, NC 27709, USA. E-mail: the endogenous ligand) were detected only if the [email protected] 598 0892-6638/01/0015-0598 © FASEB Downloaded from www.fasebj.org by Univ of North Carolina Health Sciences Library (152.2.71.222) on June 07, 2018. The FASEB Journal Vol. ${article.issue.getVolume()}, No. ${article.issue.getIssueNumber()}, pp. 598-611. GPCR systems in order to understand how ligands can function as inverse, protean, and structure-specific agonists. The first is that, like all proteins, receptors can exist in various conformations. However, in the case of GPCRs, some of these conformations reveal sequences in their cytosolic loops, which can then activate G- proteins to initiate response. These conformations are referred to as the ‘active state’ (Ra) of the receptor; correspondingly, the conformation(s) that do not activate G-proteins are referred to as the ‘inactive state’ (Ri). In the simplest case, one single conformation of each will be assumed with the two conformations existing in an equilibrium defined by an ‘allosteric constant’ (denoted L and defined as [Ra]/[Ri]). Figure 1. Chemical targets for GPCRs. Prior to 1995, the A second property of GPCR systems is that they are principal targets were full and partial agonists and antago- synoptic and interactive. Therefore, it is incorrect to nists. It is predicted that the types of ligands discovered will increase with increased screening technology. Full agonist: describe GPCR function simply in terms of the receptor produces full receptor activation leading to production of the (two-state theory). Rather, the G-protein is an interac- system maximal response. Partial agonist: produces submaxi- tive and essential part of the system. The G-protein mal receptor activation leading to production of submaximal influences the receptor in ways that modify the behav- system response and possible blockade of full agonist activa- ior of the receptor and vice versa. Of particular rele- tion. Antagonist: produces no physiological response but vance is the fact that a receptor can spontaneously rather blocks the response to endogenous or exogenous interact with G-proteins in the absence of agonist agonists. Inverse agonist: functions as an antagonist in non- constitutively active systems, but has the added property of ligands. Thus, if the affinity of Ri for a G-protein is actively reducing receptor-mediated constitutive activity of denoted Kg (equilibrium association constant), the GPCR systems (response not resulting from agonist activation affinity of the active state Ra for the same protein is but rather spontaneously emanating from the system itself). denoted ␤Kg where ␤Ͼ1. Response emanates from the Allosteric agonist: functions as an agonist but activates the hydrolysis of GTP by the G-protein resulting from receptor through interaction at a site distinct from that of the activation by Ra. From these elements the simplest endogenous agonist (usually a nonpeptide ligand for a pep- version of a GPCR system can be constructed: tide receptor). Allosteric modulator (antagonist): blocks receptor function but does not necessarily interfere with ligand L ␤Kg receptor interaction (receptor occupancy). Allosteric en- Ri ϭ Ra ϩ G ϭ RaG (1) hancer: potentiates the effects of agonists on the receptor. It can be seen that such a system defines the possibility of constitutive activity whereby a response can be activation of lower magnitude than that emanating produced by the GPCR system in the absence of an from spontaneous receptor constitutive activity. The agonist. The system can be made to produce a response predicted behavior for this class would be the observa- through stoichiometry of the reactants, namely, Ri and tion of positive agonism in some GPCR systems and G. Thus, the constitutive activity (as defined by elevated inverse agonism in others. Although this has been levels of [RaG]) can be increased by raising the recep- observed experimentally, an explanation of the effect tor concentration: in terms of receptor conformation is still theoretical. Finally, ‘ligand-specific’ agonism, which considers that ␤L͓Ri͔/KG some agonists have a different quality as well as quantity Constitutive Activity ϭ (2) 1 ϩ ␤L͓Ri͔/KG of efficacy for a given GPCR, will be considered. All of these classes will be discussed in terms of the evidence where KG is the equilibrium dissociation constant of the for their classification and their possible therapeutic receptor/G-protein complex (KGϭ1/Kg), or by in- relevance. As a preface to discussion of these drug creasing the concentration of G-protein: entities, it is useful to discuss the dynamics of the GPCR systems with which they interact. ␤L͓G͔/KG Constitutive Activity ϭ (3) 1 ϩ L͑1 ϩ ␤͓G͔/KG͒ GPCR systems Another way in which constitutive activity can be produced is through alteration of L, the allosteric constant. G-protein-coupled receptors are allosteric proteins de- Under normal circumstances, L is a unique molecular signed

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