Permeability is determined commonly using cell culture based models utilizing Caco-2 cells (a continuous line of heterogeneous human epithelial colorectal adenocarcinoma cells) or MDCK cells (Madin-Darby Canine Kidney Cells). In studies where the contribution of specific transporters is examined, MDCK cells over expressing a particular transporter of interest are often utilized. Examples of such MDCK cell lines include MDR1-MDCK (P-gp over expressing cells) and MRP2-MDCKII cells (MRP2 over expressing cells).
In brief, cells are grown on permeable culture inserts in transwells until they reach confluence. Compound is applied to either the apical or basolateral side of the cell monolayer depending on whether permeability is being measured from apical to basolateral or vice versa. Permeability is determined as follows:
Papp is the apparent permeability dR/dt is the rate of appearance of the compound in the receiver side (i.e., if compound is applied to the apical side, the basolateral side is the receiver).
A is area of the transwell insert
CDonor imtiai is the initial concentration on the donor side at time 0.
The efflux ratio (ER) can also be determined using an in vitro system and is defined as:
where Papp (BA) is the apparent permeability from the basolateral to apical side in an in vitro permeability assay and Papp (AB) is the apparent permeability from the apical to basolateral side in an in vitro permeability assay.
An ER that is >1 suggests that Papp (BA) > Papp (AB). Practically speaking, ER >3 indicate the presence of efflux. ERs can also vary with the cell system used. For example, a compound can have an ER of approximately 1 in an assay using Caco-2 cells but can have an ER >3 in a MDR1-MDCK assay. In this case, the higher ER observed in MDR1-MDCK cells may be related to the degree of over expression of MDR1 (P-gp) in the MDR1-MDCK cells.
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