Ki

Ki (the inhibition constant) is the equilibrium concentration of a competing molecule at which it occupies exactly half of a target receptor's binding site, measured against a known reference ligand in a competition assay. It is the standard unit in receptor binding tables and the most widely reported measure of binding affinity in psychopharmacology.

The value is expressed in molar concentration — nanomolar (nM) for most psychoactive compounds, picomolar for exceptionally potent binders. Lower means tighter: a compound with a Ki of 0.5 nM outcompetes one with a Ki of 500 nM at concentrations a thousand times smaller. That difference in affinity often corresponds to differences in the dose range at which a substance produces effects.

Ki figures are not universal constants. The same compound tested at the same receptor can yield different values depending on the radioligand used, the species of tissue, and the assay conditions. Values from human and rodent preparations frequently diverge. Reading binding tables with that variability in mind prevents false precision.

How it works · its role

The measurement starts with a competition binding assay. A radioligand — a reference molecule tagged with a radioactive atom — is applied to receptor-expressing cells or membrane preparations. The test compound is then added at increasing concentrations and competes for the same binding site; as its concentration rises, it displaces radioligand. The concentration that displaces half is the IC50.

Ki is derived from IC50 through the Cheng-Prusoff equation, which corrects for the radioligand's own concentration and affinity. That correction is why Ki travels better across laboratories than raw IC50: it removes the dependency on experimental conditions that IC50 carries.

Affinity is only part of what binding reveals. A nanomolar Ki at a receptor confirms the molecule gets there efficiently; it says nothing about what happens once it arrives. Whether the molecule activates, blocks, or partially activates that receptor — its intrinsic activity — is a separate question, answered by functional assays, not binding assays.

Relevance to substances & effects

Binding affinity tables appear throughout substance pharmacology because selectivity at specific receptors and transporters explains much of what a substance does. Serotonergic psychedelics carry sub-nanomolar Ki values at the 5-HT₂A receptor, a profile that anchors their perceptual effects to that target at low doses.

Opioids are characterised by their relative affinity at mu, kappa, and delta opioid receptors — a fingerprint that predicts broad patterns of pain suppression, sedation, and dependence risk in general terms. When a compound's Ki values cluster tightly around one receptor subtype, effects at low concentrations are more predictable; when affinities overlap across several targets, responses become more complex and harder to attribute to a single mechanism.

Where Ki reaches its limit is in predicting subjective or clinical outcomes. Two compounds can share an identical Ki at the same receptor and produce very different experiences if one is a full agonist and the other a partial agonist or antagonist. The binding profile is a useful fingerprint for understanding a substance's pharmacology — not a conclusion about what using it feels like.