Assay

An assay is a controlled laboratory procedure designed to measure a specific property of a compound — typically how strongly it binds a receptor, how it activates or blocks a cellular pathway, or how quickly an enzyme breaks it down. Assay results appear throughout this encyclopedia as receptor affinity values, potency figures, and selectivity profiles.

Understanding what those numbers mean — and what they do not mean — is important context for reading any pharmacological profile. An assay captures one aspect of a molecule's behaviour under controlled conditions, which may or may not predict how it behaves inside a living body.

How it works · its role

Two broad classes of assay are used in psychopharmacology. Binding assays measure how well a compound competes with a known molecule — often a radiolabelled probe — for a receptor site. They produce a value, typically reported as Ki or IC₅₀, representing the concentration at which competition is half-maximal. Lower values indicate tighter binding.

Functional assays go a step further, measuring what happens after a compound binds: does it activate the receptor, block it, or produce a partial response? These yield measures like Emax (the maximum possible effect) and EC₅₀ (the concentration producing half that maximum). A compound can bind tightly in a binding assay while behaving as a weak activator — or even an antagonist — in a functional one.

Compounds are often tested across panels covering dozens or hundreds of receptor types. The resulting selectivity profile maps which targets a compound hits at what concentrations, and how those compare across the receptor landscape.

Relevance to substances & effects

The receptor binding tables and pharmacology sections on substance pages are built directly from assay data. When a substance is listed as a potent 5-HT₂A agonist or a serotonin reuptake inhibitor, that characterisation traces back to assay results — typically collected from cell lines or isolated membrane preparations in a dish.

Assay data has real limits in explaining subjective effects. A compound's in vitro binding profile does not automatically predict its in vivo pharmacology, because bioavailability, metabolism, blood–brain barrier penetration, and interactions across multiple receptor populations all modify the outcome.

Two substances with nearly identical receptor binding profiles in an assay may produce quite different experiences. Conversely, drugs with apparently different profiles sometimes converge on similar effects because they act on overlapping downstream pathways. Assay results are a starting point for understanding a drug, not a complete account of what it will do.