Terry P. Kenakin

Agonist-receptor efficacy II: agonist trafficking of receptor signals

There is evidence to suggest that receptors with seven transmembrane domains can exist in G protein-activating conformations. It is not known how many activated receptor forms exist for each receptor. Furthermore, if there are multiple forms, does the chemical structure of the agonist determine which form dominates, and therefore, which response pathway is activated? This latter scheme is referred to as agonist-receptor trafficking, and is discussed in this, the second of two articles by Terry Kenakin. One way to approach these questions is to study receptors that couple to more than one G protein and, in essence, to try to allow the G protein to indicate the receptor state.

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.

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.

Inverse, protean, and ligand-selective agonism: matters of receptor conformation

Concepts regarding the mechanisms by which drugs activate receptors to produce physiological response have progressed beyond considering the re¬ceptor as a simple on‐off switch. Current evidence suggests that the idea that agonists produce only vary¬ing degrees of receptor activation is obsolete and must be reconciled with data to show that agonist efficacy has texture as well as magnitude. Thus, agonists can block system constitutive response (inverse agonists), behave as positive and inverse agonists on the same receptor (protean agonists), and differ in the stimulus pattern they produce in physiological systems (ligand‐selective agonists). The molecular mechanism for this seemingly diverse array of activities is the same, namely, the selective microaffinity of ligands for different conformational states of the receptor. This paper reviews evidence for the existence of the various types of agonism and the potential therapeutic utility of differ¬ent agonist types.—Kenakin, T. Inverse, protean, and ligand‐selective agonism: matters of receptor confor¬mation. FASEB J. 15, 598‐611 (2001)

International Union of Pharmacology Committee on Receptor Nomenclature and Drug Classification. XXXVIII. Update on Terms and Symbols in Quantitative Pharmacology

The recommendations that follow have been updated from the proposals of a Technical Subcommittee set up by the International Union of Pharmacology Committee on Receptor Nomenclature and Drug Classification (Jenkinson DH, Barnard EA, Hoyer D, Humphrey PPA, Leff P, and Shankley NP (1995) International Union of Pharmacology Committee on Receptor Nomenclature and Drug Classification. IX. Recommendations on terms and symbols in quantitative pharmacology. Pharmacol Rev 47:255–266).

Functional Selectivity and Biased Receptor Signaling

With the emergence of information describing functional selectivity and biased agonists and antagonists has come a lack of confidence in “one size fits all” assays for detection of agonism. Seven-transmembrane receptors are pleiotropic with respect to the signaling protein to which they couple in a cell, and many conformations of the receptor can be formed; this leads to systems where ligands can stabilize unique conformations that go on to selectively activate signaling pathways. Thus, such “biased” ligands can produce cell-specific agonism that may require targeted assays to detect and quantify. It also predicts that ligands can have many different efficacies for the many behaviors that the receptor can exhibit (referred to as “pluridimensional efficacy”), leading to a breakdown in the common classifications of agonist and antagonist. This all poses unique challenges to the pharmacologic nomenclature of drugs, the detection and optimization of new drugs, and the association of phenotypic clinical profiles with pharmacological properties of drugs.

Pharmacological regulation of insulin secretion in MIN6 cells through the fatty acid receptor GPR40: identification of agonist and antagonist small molecules

1 Long chain fatty acids have recently been identified as agonists for the G protein‐coupled receptors GPR40 and GPR120. Here, we present the first description of GW9508, a small‐molecule agonist of the fatty acid receptors GPR40 and GPR120. In addition, we also describe the pharmacology of GW1100, a selective GPR40 antagonist. These molecules were used to further investigate the role of GPR40 in glucose‐stimulated insulin secretion in the MIN6 mouse pancreatic β‐cell line. 2 GW9508 and linoleic acid both stimulated intracellular Ca2+ mobilization in human embryonic kidney (HEK)293 cells expressing GPR40 (pEC50 values of 7.32±0.03 and 5.65±0.06, respectively) or GPR120 (pEC50 values of 5.46±0.09 and 5.89±0.04, respectively), but not in the parent HEK‐293 cell line. 3 GW1100 dose dependently inhibited GPR40‐mediated Ca2+ elevations stimulated by GW9508 and linoleic acid (pIC50 values of 5.99±0.03 and 5.99±0.06, respectively). GW1100 had no effect on the GPR120‐mediated stimulation of intracellular Ca2+ release produced by either GW9508 or linoleic acid. 4 GW9508 dose dependently potentiated glucose‐stimulated insulin secretion in MIN6 cells, but not in primary rat or mouse islets. Furthermore, GW9508 was able to potentiate the KCl‐mediated increase in insulin secretion in MIN6 cells. The effects of GW9508 on insulin secretion were reversed by GW1100, while linoleic acid‐stimulated insulin secretion was partially attenuated by GW1100. 5 These results add further evidence to a link between GPR40 and the ability of fatty acids to acutely potentiate insulin secretion and demonstrate that small‐molecule GPR40 agonists are glucose‐sensitive insulin secretagogues.

New Concepts in Drug Discovery: Collateral Efficacy and Permissive Antagonism

New perspectives on the complexity of G-protein-coupled receptor (GPCR) signalling and the increased resolution of existing tools for studying GPCR behaviour has led to the conception of new hypotheses that affect the discovery of drugs acting at GPCRs. Taking into consideration the novel concepts of collateral efficacy and permissive antagonism in the search for synthetic agonists and antagonists, respectively, will be essential in the search for drugs with unique therapeutic profiles. Here, the design of drugs against HIV is used as an example of how these concepts might be taken into consideration for GPCR-targeted drugs in general.

Functional Selectivity through Protean and Biased Agonism: Who Steers the Ship?

This article describes functional selectivity of agonists and antagonists and distinguishes conventional cell-based functional selectivity, where the strength of signal produces selective signaling in various organs, from true receptor active-state based selectivity, also alternatively referred to in the literature as “stimulus trafficking,” “biased agonism,” and “collateral efficacy.” This latter mechanism of selectivity depends on the ligand-related conformation of the receptor and is not compatible with the parsimonious view that agonists produce a single receptor active state. In addition, protean agonism is described, whereby a ligand produces positive agonism in quiescent systems and inverse agonism in constitutively active systems. This is a special case of active state-based selectivity in which the ligand produces an active state that is of lower efficacy than the natural constitutively active state. It is postulated that receptor active-state based selectivity, unlike cell-based functional selectivity, is controllable through the chemical structure of the ligand and is therefore more likely to be a viable avenue for therapeutic selectivity in the clinic. Reasons are given for differentiating receptor active-state based selectivity from conventional functional organ selectivity.

Differences between natural and recombinant G protein-coupled receptor systems with varying receptor/G protein stoichiometry

The increasing accessibility of genetically engineered receptor systems for the study of drug-receptor interaction has led to a corresponding increase in the testing of new drug entities in recombinant receptor systems. In this article Terry Kenakin illustrates some possible conditions within these recombinant systems where the relative stoichiometry of the receptors to other cellular components may differ from that found in natural systems and where this difference may lead to anomalies in drug testing.