Caco-2 permeability

The Caco-2 assay is a laboratory stand-in for the lining of the human intestine. Caco-2 cells, derived from a colon cancer, are grown until they form a tight single-layer sheet that behaves much like real gut wall. A drug is placed on one side and chemists measure how fast it crosses to the other, giving an apparent permeability value.

Because the cell layer also carries the same transporter and efflux proteins found in the gut, the assay captures more than passive diffusion. By comparing movement in both directions, it can reveal whether a compound is being actively pumped back out (notably by P-glycoprotein), which a simple membrane test would miss. This makes it a workhorse predictor of oral absorption.

It is more laborious, variable, and expensive than a purely artificial-membrane assay, and results depend on the particular cell batch and protocol, so absolute numbers transfer poorly between labs. Chemists therefore read it as a ranking and a flag for efflux liabilities rather than an exact forecast of human absorption.