What is In vitro?

In vitro ("in glass") describes experiments carried out outside a living organism, in tubes, plates and culture vessels. It covers a purified receptor in a binding assay, an enzyme reaction, an HPLC stability study and a dish of cultured cells alike; what they share is that the experimenter sets the environment rather than a living body. Research peptides labelled "for in-vitro research use only" are supplied for this kind of laboratory work and not for administration to people or animals. The label states intended use; regulatory status in Australia is a separate question.

Where it sits among related terms

Nominal is not actual

The concentration on a plate map is what was added, not necessarily what the cells experience.

• Protein binding. Semaglutide was designed with a fatty acid moiety to give it high affinity for serum albumin ^[1]. In medium containing serum or albumin, part of the added semaglutide is bound rather than free, so the nominal concentration overstates the free one.
• Proteolysis depends on the matrix. In samples from mice, peptides from three families degraded faster in serum than in plasma, all were more stable in fresh blood, and their rank order of stability changed across six incubation conditions ^[2]. A "serum half-life" belongs to that serum, not to the peptide in general.
• Chemistry at 37 °C. Culture conditions are also the conditions under which deamidation and oxidation proceed.
• Contaminants. Endotoxin and solvents can produce responses of their own.

Worked example: how much peptide a plate needs

A 96-well assay uses 100 µL per well at a final 10 µM. Each well needs 10 µM × 100 µL = 1 nmol of peptide.

The vehicle follows from the stock. Diluting a 1 mM stock in neat DMSO to 10 µM is a 1:100 step and leaves 1% v/v DMSO in the well; in a retinal neuronal cell line, DMSO was toxic above 1% v/v ^[3]. A 10 mM stock would cut that to 0.1%, but for tesamorelin it means dissolving about 51 mg/mL. The same study recommended calculating the final solvent concentration and running an untreated control alongside the vehicle control ^[3]. The molarity calculator and dilution calculator handle the arithmetic; the dose–response design guide covers concentration series and controls.

Common misunderstandings

• "An in-vitro EC50 transfers to an animal." Potency in a well depends on assay medium, cell type and receptor density. Exposure in an organism also depends on half-life and distribution, which a plate does not model.
• "A response proves the receptor mechanism." Without vehicle, untreated and receptor-blocked controls, solvent effects or contaminants can look like agonism.
• "Going serum-free removes the binding problem." It removes albumin binding but also the carrier protein that limits losses to plastic; adding 1% bovine serum albumin improved recovery of radiolabelled peptides from tube surfaces ^[4]. At low concentrations, labware choice matters, as the aliquot entry explains.

The research peptides overview explains how these reagents fit into laboratory research more broadly.