Opinion
What is pharmacological ‘affinity’? Relevance to biased agonism and antagonism

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Highlights

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    Because receptors are allosteric proteins, affinity estimates are always conditional.

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    At present, all known methods to estimate bias utilize functional affinity.

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    Binding estimates of affinity can be very different from functional affinity.

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    Affinity must be considered when measuring bias.

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    Concepts around signaling bias can be extended to development of biased antagonists.

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.

Section snippets

Binding: ‘Langmuirian’ affinity

Pharmacological affinity is a measure of the attraction a ligand has for a biological target. It can be quantified with an equilibrium dissociation constant, defined as the ratio of the rate that the ligand approaches the protein binding site (denoted k1 in temporal units such as s−1M−1) and the rate that the bound ligand diffuses away from the protein binding site (denoted k2 with units of s−1). Thus the characteristic number used to quantify affinity is defined as k2/k1 and denoted KA. The

7TMRs as ensembles of allosteric proteins

7TMRs are the major means by which chemical signals are transmitted from the extracellular space to the cell cytosol. Their main function is to change their shape (conformation) in response to interactions with extracellular ligands and intracellular signaling proteins. There is a great deal of evidence to suggest that proteins such as these do not stay in static conformations but instead exist in ensembles of different conformations that interchange with the available free energy of the system

Functional affinity

This discussion will define the term ‘functional’ affinity as being the KA value – defining the strength of binding of a ligand as it forms a complex with the receptor to both induce a cellular response and also to interfere with the effects of another ligand as it attempts to co-bind to the receptor to induce a physiological response. In the latter case, several published pharmacological procedures are available to measure this value for different agonists, including orthosteric simple

Measuring agonist bias

The term ‘bias’ will be used to denote the preferential ability of a ligand to cause the receptor to interact with a distinct signaling pathway in the cell as opposed to other pathways. It will be seen that the molecular determinants of bias are both the efficacy and affinity of the ligand for the receptor as it interacts with these pathways. The current model for biased signaling is based on the notion that different agonist molecules stabilize different receptor conformations interacting with

Biased antagonism

Biased antagonism is an established phenomenon for negative allosteric modulators. For example, the allosteric antagonist LP1805 [N,N-(2-methylnaphyl-benzyl)-2-aminoacetonitrile] converts normal signaling by NK2 receptors for the natural agonist neurokinin A (activation of Gαq and Gαs) to a biased signal of enhanced Gαq but diminished Gαs [50]. Similarly, the allosteric antagonist indole1 (Nα-tosyltryptophan) changes normal prostaglandin D2 (PDG2) signaling of the chemoattractant

Concluding remarks

There are theoretical and practical reasons for not utilizing binding affinities as descriptors of functional effects. In addition, functional experiments can provide more accurate estimates of functional affinity which, in turn, are more relevant to functional receptor activity. Specifically, the functional KA value for partial agonists can be identified through fitting of the Black/Leff operational model, and τ/KA ratios are identified for full agonists with the same model. Given this,

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