Receptor affinity describes the tendency of a ligand to occupy and remain bound to a specific receptor site. The relationship is quantified by the equilibrium dissociation constant (Kd): a lower Kd value indicates higher affinity, because a smaller concentration of ligand is required to occupy half the available receptor population at equilibrium. Conversely, a high Kd reflects weaker binding and a greater tendency for the ligand–receptor complex to dissociate.
Affinity is distinct from efficacy. A ligand may bind a receptor with high affinity yet produce little or no functional response (a property associated with competitive antagonists and some partial agonists), whilst a lower-affinity ligand that does bind may still produce a full functional response. Both properties together determine the pharmacological profile of a compound.
In cannabinoid pharmacology, receptor affinity measurements , typically derived from radioligand displacement assays , are used to characterise how strongly a given phytocannabinoid or synthetic cannabinoid binds to CB1 and CB2 receptors. These figures appear throughout the preclinical literature and underpin comparative classification of cannabinoid compounds, but Kd values obtained in isolated-tissue or cell-line systems do not automatically predict behaviour in a living organism, where factors such as bioavailability, metabolism, and receptor density all influence observed effects.
Australian scheduling decisions under the Poisons Standard (administered by the Therapeutic Goods Administration) are informed by pharmacological data including receptor affinity profiles, particularly when classifying novel psychoactive substances and synthetic cannabinoids under Schedules 8 or 9.
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