In vitro

In vitro (Latin for "in glass") describes any experiment conducted outside a living organism — in isolated cells, tissue preparations, purified enzymes, or receptor assays performed in a dish or tube.

It occupies the foundational tier of pharmacological research. Most knowledge about how psychoactive compounds bind to receptors, inhibit enzymes, or disrupt cellular processes originates here, which is why mechanism claims on these pages are frequently labelled in vitro only. That label is not a dismissal — it is a statement about what the evidence can and cannot carry.

In vitro methods span a range of complexity: a simple enzyme-inhibition assay uses purified protein; a receptor-binding study uses membrane fragments; a hepatocyte assay uses live liver cells in culture. Each narrows the biological context further than a whole organism provides, which is both their utility and their limit.

What this design can establish

In vitro work reliably establishes that a specific molecular interaction is chemically possible. When a compound is shown to bind a receptor, inhibit an enzyme, or trigger a cellular response under these conditions, that finding reflects something real about the compound's chemical properties.

It is the primary source of binding-affinity data — figures such as Ki and IC₅₀ values that appear in pharmacology tables — and the standard method for identifying which enzymes (particularly the Cytochrome P450 family) are involved in a substance's metabolism. These are foundational facts that inform every tier of research above them.

In vitro studies also produce concentration-response relationships within the experimental system: they can show that an effect grows with concentration, and identify the point at which it first appears. This makes them useful for ranking compounds by potency at a given target.

What it cannot

In vitro work cannot establish that any observed interaction occurs at concentrations a living body ever reaches. Absorption, protein binding, first-pass metabolism, and tissue distribution all shape actual exposure at the target — none of these are present in a dish.

The inference readers are most likely to draw — that an in-vitro interaction predicts a real-world effect or risk — is not supported by the design alone. A compound may inhibit an enzyme at ten times the peak plasma concentration produced by any realistic dose; a receptor binding may be blocked by plasma proteins before the compound reaches that site in tissue.

This gap between in-vitro and in-vivo findings is large and well documented. Many interaction signals identified in cell studies do not replicate in animals or humans. Many that do require concentrations far outside those produced by ordinary use.

When this encyclopedia marks a mechanism or interaction claim as in vitro only, that phrase means exactly this: the chemistry is demonstrated; the clinical relevance is not.