In Vitro

In vitro is Latin for "in glass" and refers to experiments carried out outside a living organism. It includes cell-free methods — receptor binding, enzyme kinetics, surface plasmon resonance — as well as cell-based systems such as monolayer cultures, primary cells, organoids and tissue explants.

Strengths and limits of in vitro evidence

The approach excels at control and resolution. The concentration reaching the target is known instead of estimated, confounding factors are stripped away, and a mechanism can be studied in isolation — the reason binding affinities, EC50 values and selectivity profiles come from such systems. Yet that very precision invites over-extrapolation.

The usual caveats deserve spelling out. Culture concentrations often exceed anything a whole organism could reach, so a micromolar effect in cells may mean nothing at achievable exposures. A dish cannot mimic absorption, plasma protein binding, liver metabolism, kidney clearance or immune reactions — a peptide whose plasma half-life is two minutes may appear stable indefinitely in serum-free medium. Immortalised cell lines drift genetically and are often misidentified. And many protocols fail to report how stable the peptide actually is in the culture medium.

An in vitro result, then, supports a hypothesis about mechanism. Moving on to in vivo experiments is what shows whether it holds up inside an organism.

Related terms and material

preclinical · ELISA. Reference material: assay kits, peptide libraries. Further reading: pH and solubility of peptides.