New concepts in pharmacological efficacy at 7TM receptors: IUPHAR review 2

Abstract

The present-day concept of drug efficacy has changed completely from its original description as the property of agonists that causes tissue activation. The ability to visualize the multiple behaviours of seven transmembrane receptors has shown that drugs can have many efficacies and also that the transduction of drug stimulus to various cellular stimulus-response cascades can be biased towards some but not all pathways. This latter effect leads to agonist 'functional selectivity', which can be favourable for the improvement of agonist therapeutics. However, in addition, biased agonist potency becomes cell type dependent with the loss of the monotonic behaviour of stimulus-response mechanisms, leading to potential problems in agonist quantification. This has an extremely important effect on the discovery process for new agonists since it now cannot be assumed that a given screening or lead optimization assay will correctly predict therapeutic behaviour. This review discusses these ideas and how new approaches to quantifying agonist effect may be used to circumvent the cell type dependence of agonism. This article, written by a corresponding member of the International Union of Basic and Clinical Pharmacology Committee on Receptor Nomenclature and Drug Classification (NC-IUPHAR), reviews our current understanding of the interaction of ligands with seven transmembrane receptors. Further information on these pharmacological concepts is being incorporated into the IUPHAR/BPS database GuideToPharmacology.org.

Figure 1

Scheme for binding of a ligand (A) to a receptor (R) that can exist in two states R and R*. It is assumed that the ligand-directed endpoint is production of the R* state through binding to form AR*. A mechanism of selection is through a selective binding of A to the R* state to cause an enrichment of AR*. A mechanism of induction has A binding to the R state followed by a conformational change to the R* state.

Figure 2

7TMR agonism as an allosteric system. (A) Classic guest allosterism whereby the effects of a ligand A are modified by the binding of a modulator B to a separate site on the receptor. The affinity of A is altered by a factor α upon binding of B; similarly, the efficacy of A is altered by a factor β. The effects of A are reciprocated on the affinity and efficacy of B. The equation yields the response to the ligand A in terms of the Black/Leff operational model with A as an agonist of affinity K_A⁻¹ and efficacy τ_A. (B) 7TMR agonism with the agonist as a modulator M and cytosolic signalling protein φ as the allosteric guest. The equation yields response as the Black/Leff fucntion with the receptor/signal protein complex ([φR]) as the agonist species.

Figure 3

Keeping the conformation of a region of the 7TMR relatively fixed (through binding of a ligand) will lead to a biased ensemble of conformations throughout the protein. Insofar as some of those are associated with coupling to cellular signalling proteins, this will define the pharmacologic action of the ligand when the receptor is in the cell membrane. These various signalling regions of the receptor will adopt an ‘active state’ conformation with a range of probabilities depending on the nature of the ligand (histogram in panel B). Specifically, the frequency of occurrence of any given conformation will be influenced by the nature of the ligand binding to the receptor leading to a unique frequency distribution of conformations associated with that ligand.

Figure 4

Receptor concormational ensembles as frequency distributions. Top panel shows distributions of the unliganded receptor for a defined instant in time (lines identify relative quantity of a distribution). Grey distributions define the arrays of conformations that have an identifiable cellular function (i.e. activation of G-proteins, phosphorylation of receptor protein, etc.). Presence of a conformation coincident with a conformation in the pharmacologically functional ensemble indicates the basal activity of the system. For the unliganded receptor, few spontaneously formed conformations produce cellular efect. Middle panels show the effects of the binding of three separate ligands (A, B and C). With ligand binding comes clustering of conformations coincident with pharmacologically active ensembles. The presence of these pharmacologically active conformations define the observed activity of the ligand. Lower panels show how differences in ligand-stabilized clusters of conformations result in varying ratios of pharmacological activity for different pathways.

Figure 5

Effects of varying receptor/coupling protein stoichiometry on relative agonist activity. The receptor mediates cellular response through activation of two effectors 1 and 2. Agonist A produces a relatively uniform activation of both response effectors, whereas agonist B is biased towards effector-2. In cells with an increased stoichiometric ratio of receptor to effector 2, the response to agonist B will be disporportionateloy augmented to yield a different relative potency ratio for the two agonists in the two systems.

Figure 6

General view of three mechanisms for agonist selectivity in vivo. Thus, stabilization of unique receptor conformations can lead to bias of interaction of the receptor with cellular signalling molecules (stimulus trafficking) and presentation of conformations to GRKs for selective barcoding through phosphorylation. Binding of the agonist to the receptor interferes with the interaction of the receptor with the endogenous agonist.

Figure 7

Bias plot for two agonists: A (relatively equal efficacy for pathways 1 and 2) and agonist B (higher affinity but lower efficacy for response 2). Panels on the right show the dose–response curves for both agonists for the two signalling pathways. The bias plot shows the response of each agonist for pathway 2 as a function of the response for pathway 1. For this particular tissue, agonist A produces relatively uniform activation of both pathways, while agonist B produces a selective effect for pathway 2 at low levels of stimulation followed by a saturated and lower level of response 2 at higher stimulation levels. Thus, bias is concentration and system dependent in that low concentrations of agonist B are biased 2 > 1, while higher concentrations the bias is reversed.

Figure 8

Relative agonist response (calculated as the arithmetic difference of pathway stimulations indicated by dotted and solid dose–response curves in separate panels) for a wide range of tissues of differing receptor density. At low receptor density, the agonist produces primarily response denoted by the dotted dose–response curve, while at high receptor densities, a more balanced stimulation is obtained. The surface shows the complexity of overall stimulation to the range of systems with varying receptor densities.