Permeability Assays Background The Caco-2 cell line, derived from a human colorectal carcinoma, has become an established in vitro model for the prediction of drug absorption across the human intestine. When cultured on semipermeable membranes, Caco-2 cells differentiate into a highly functionalized epithelial barrier with remarkable morphological and biochemical similarity to the small intestinal columnar epithelium. The membrane transport properties of novel compounds can thereby be assessed using these differentiated cell monolayers. The apparent permeability coefficients (Papp) obtained from Caco-2 cell transport studies have been shown to correlate to human intestinal absorption.1,2 Mandin Darby Canine Kidney (MDCK) cells are a common model for studying drug transport mechanisms in distal renal epithelia. Like Caco-2 cells, MDCK cells differentiate into columnar epithelium and form tight junctions when cultured on semi-permeable membranes. Primarily for passively absorbed compounds, drug permeability data from MDCK cell assays have been shown to be similar to permeability data from Caco-2 cell assays.1 As a consequence, MDCK cells are gaining acceptance within the pharmaceutical industry as an alternate model to Caco-2 cells for rapidly screening compounds for absorption potential, as part of the pre-clinical drug selection process. In addition, preliminary data from some laboratories show a good correlation between MDCK cell and Bovine Brain Endothelial Cell (BBEC) culture permeabilities for several compounds. BBEC cultures are used as an in vitro model to predict compound permeability across the blood brain barrier (BBB). Key Features of the Assay • model of drug absorption from the intestine to blood: mimicking the intestinal environment • model of drug uptake from the blood into the brain: mimicking the blood brain barrier Assay Applications • mechanistic studies of drug absorption • screening assay for pre-clinical drug selection Diagram of assay insert FIG. 1 Cell monolayers are cultured on a semipermeable membrane to form tight cell-cell junctions. The inner well containing the cells is called the apical compartment while the outer well is called the basolateral compartment. Assay Principle Permeability across the Caco-2 or MDCK cell layer is determined by growing the cells on a membrane placed between two (donor and acceptor) chambers. Drug candidates are typically added to the apical side of the cell layer and their