Receptor theory
Receptor theory is the application of receptor models to explain drug behavior.
History
The receptor concept
In 1901, Langley challenged the
Nature of receptor–drug interactions
Receptor occupancy model
The receptor occupancy model, which describes agonist and competitive antagonists, was built on the work of Langley, Hill, and Clark. The occupancy model was the first model put forward by Clark to explain the activity of drugs at receptors and quantified the relationship between drug concentration and observed effect. It is based on mass-action kinetics and attempts to link the action of a drug to the proportion of receptors occupied by that drug at equilibrium.[9][10] In particular, the magnitude of the response is directly proportional to the amount of drug bound, and the maximum response would be elicited once all receptors were occupied at equilibrium. He applied mathematical approaches used in enzyme kinetics systematically to the effects of chemicals on tissues.[2]
He showed that for many drugs, the relationship between drug concentration and biological effect corresponded to a hyperbolic curve, similar to that representing the adsorption of a gas onto a metal surface
Competitive inhibition models
The development of the classic theory of drug antagonism by Gaddum, Schild and Arunlakshana built on the work of Langley,
Agonist models
The flaw in Clark's receptor-occupancy model was that it was insufficient to explain the concept of a partial agonist. This led to the development of agonist models of drug action by Ariens in 1954 and by Stephenson in 1956 to account for the intrinsic activity (efficacy) of a drug (that is, its ability to induce an effect after binding).[12][13]
Two-state receptor theory
The two-state model is a simple linear model to describe the interaction between a ligand and its receptor, but also the active receptor (R*).
Ternary complex model
The original Ternary complex model was used to describe ligand, receptor, and G-protein interactions. It uses equilibrium dissociation constants for the interactions between the receptor and each ligand (Ka for ligand A; Kb for ligand B), as well as a cooperativity factor (α) that denotes the mutual effect of the two ligands on each other's affinity for the receptor. An α > 1.0 refers to positive allosteric modulation, an α < 1.0 refers to negative allosteric modulation, and an α = 1.0 means that binding of either ligand to the receptor does not alter the affinity of the other ligand for the receptor (i.e., a neutral modulator).[15] Further, the α parameter can be added as a subtle but highly useful extension to the ATCM in order to include effects of an allosteric modulator on the efficacy (as distinct from the affinity) of another ligand that binds the receptor, such as the orthosteric agonist. Some ligands can reduce the efficacy but increase the affinity of the orthosteric agonist for the receptor.[15]
Although it is a simple assumption that the proportional amount of an active receptor state should correlate with the biological response, the experimental evidence for receptor overexpression and spare receptors suggests that the calculation of the net change in the active receptor state is a much better measure for response than is the fractional or proportional change. This is demonstrated by the effects of agonist/ antagonist combinations on the desensitization of receptors. This is also demonstrated by receptors that are activated by overexpression, since this requires a change between R and R* that is difficult to understand in terms of a proportional rather than a net change, and for the molecular model that fits with the mathematical model.[21][22][23]
Postulates of receptor theory
- Receptors must possess structural and steric specificity.
- Receptors are saturableand finite (limited number of binding sites)
- Receptors must possess high affinity for its endogenous ligand at physiological concentrations
- Once the endogenous ligand binds to the receptor, some early recognizable chemical event must occur
References
- PMID 18204481.
- ^ PMID 15063082.
- ^ PMID 16402126.
- ^ Langley J. On the stimulation and paralysis of nerve cells and of nerve-endings. Part 1. J Physiol 1901 October 16; 27(3): 224–236.
- ^ J. N. Langley. On the reaction of cells and of nerve-endings to certain poisons, chiefly as regards the reaction of striated muscle to nicotine and to curare. J Physiol 1905; 33: 374–413.
- ^ PMID 15616162.
- S2CID 1518772.
- ISBN 0-387-23069-6
- ^ PMID 10449182.
- ^ E.M Ross, and T.P. Kenakin. (2001) Pharmacodynamics. Mechanisms of drug action and the relationship between drug concentration and effect. In Goodman & Gilman’s The Pharmacological Basis of Therapeutics, Vol. Tenth. J.G. Hardman & L.E. Limbird, Eds. McGraw-Hill. New York.
- PMID 16502872.
- ^ a b c d D. Colquhoun, The relation between classical and cooperative models for drug action. In: H.P. Rang, Editor, Drug Receptors, Macmillan Press (1973), pp. 149–182. http://www.ucl.ac.uk/Pharmacology/dc-bits/colquhoun-1973.pdf
- ^ S2CID 205479063.
- PMID 16483674.
- ^ PMID 18652471.
- ^ J.W. Black and P. Leff. (1983) Operational Models of Pharmacological Agonism. In: Proc. R. Soc. London Ser. B 220, pp. 141–162.
- PMID 7540781.
- S2CID 2580606.
- PMID 15660959.
- ^ PMID 9846630.
- ^ Optimal Agonist/Antagonist Combinations Maintain Receptor Response by Preventing Rapid β-adrenergic Receptor Desensitization | BIO BALANCE
- ^ Molecular dynamics of a biophysical model for b2-adrenergic and G protein-coupled receptor activation | BIO BALANCE
- ^ The Biophysical Basis for the Graphical Representations | BIO BALANCE