This article was published on June 23, 2022.
To be able to understand the effects of drugs at various receptors, clinicians do need to know just a little bit about what the term “receptor binding affinity” means. So, here’s a brief, simple explanation for busy clinicians.
One commonly used measure of the affinity (strength) with which a drug binds to a particular type (or subtype) of receptor is its Ki or inhibitory constant (also called the inhibition constant).
In plain English, the Ki represents the concentration of the drug (in nanomoles or nM) required to occupy 50% of those receptors. For example, the Ki of a particular antipsychotic at postsynaptic D2 receptors is the concentration of that drug that would result in the drug occupying 50% of those receptors.
We need to remember the following, which is obvious from the above:
The LOWER the Ki for a particular drug at a particular receptor, the STRONGER its binding affinity for that receptor.
This is because the lower Ki means that the drug can occupy 50% of those receptors even when the drug is present in a lower concentration.
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Preskorn SH. Clinical relevance of relative receptor binding affinity: quetiapine and ziprasidone as examples. J Psychiatr Pract. 2007 Nov;13(6):393-8. doi: 10.1097/01.pra.0000300125.22064.a9. PMID: 18032984.
Preskorn SH. Drug-Drug Interactions (DDIs) in Psychiatric Practice, Part 7: Relative Receptor Binding Affinity as a Way of Understanding the Differential Pharmacology of Currently Available Antipsychotics. J Psychiatr Pract. 2019 Nov;25(6):461-465. doi: 10.1097/PRA.0000000000000431. PMID: 31821222.
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