Binding affinity

Binding affinity describes how strongly a drug molecule attaches to its target — typically a receptor protein — measured under controlled laboratory conditions. It is usually expressed as a Ki (inhibition constant) or Kd (dissociation constant): the concentration of the molecule needed to occupy half of the available binding sites. The lower the value, the tighter the bind.

Crucially, affinity is a measure of attraction, not action. A molecule can grip a receptor tightly and produce a strong effect, produce no effect at all, or block other molecules from getting in — that depends on a separate property called efficacy. Affinity describes how well a molecule fits the receptor; efficacy describes what the receptor does as a result.

How it works · its role

Binding affinity is typically measured in a radioligand displacement assay. A known molecule, tagged with a radioactive marker, is allowed to bind its target receptor. The compound being tested is then introduced; as it competes for the same site, it displaces the tracer. A concentration-response curve from this competition yields the Ki — a constant that reflects how tightly the test compound holds on.

Affinity emerges from physical complementarity between a molecule and its binding pocket: the fit of shape, charge, and polarity. A molecule that fits well makes stable contact across many points; one that fits poorly detaches quickly.

Because these measurements are made in isolated systems — purified proteins, cell membranes, or tissue homogenates — binding affinity describes a single interaction stripped of biological context. What actually happens in a living organism depends on many additional factors: how much of the drug reaches the receptor, how long it stays, how the surrounding system responds, and what other receptors the molecule also binds.

Relevance to substances & effects

Binding affinity shapes which effects a substance produces and at what doses. High-affinity ligands occupy receptors at low concentrations; substances with very high affinity at a given receptor tend to dominate it even when competing molecules are present.

Classic psychedelics such as LSD have exceptionally high affinity at the 5-HT₂A receptor — a key reason small doses produce pronounced and prolonged effects. Opioids vary considerably in their affinity at the μ-opioid, which underlies potency differences across compounds in that class.

Affinity also explains competitive displacement. High-affinity antagonists such as naloxone can push opioids off their binding sites, which is the pharmacological basis of overdose reversal: a molecule with greater affinity for a receptor will displace one already occupying it, regardless of which arrived first.

On substance pages, binding affinity data reflects in-vitro assay values. These indicate which receptors a substance is likely to engage at physiologically relevant concentrations, and provide a mechanistic foundation for understanding the effects and interactions described elsewhere in the record.