In vitro means "in glass": experimentation performed outside a living organism. The category covers cell-free work such as receptor binding, enzyme kinetics and surface plasmon resonance, and cell-based work such as monolayer culture, primary cells, organoids and tissue explants.
What in vitro data can and cannot support
Its strengths are control and resolution. Concentration at the target is known rather than inferred, confounders are removed, and mechanism can be isolated — which is why binding affinities, EC50 values and selectivity profiles are generated this way. That precision is exactly what makes over-extrapolation tempting.
The standard limitations are worth naming. Concentrations used in culture frequently exceed anything achievable in a whole organism, so a micromolar cell result may be irrelevant at attainable exposures. Nothing in a dish reproduces absorption, plasma protein binding, hepatic metabolism, renal clearance or immune response — a peptide with a two-minute plasma half-life can look indefinitely stable in serum-free medium. Immortalised lines drift genetically and are frequently misidentified. And peptide stability in culture medium is itself a variable most protocols under-report.
An in vitro finding therefore supports a mechanistic hypothesis. Progression to in vivo work is what tests whether it survives contact with an organism.
Related terms and material
preclinical · ELISA · research use only. Reference material: assay kits, peptide libraries. Further reading: pH and solubility of peptides.