TRANSCELLULAR PERMEABILITY OF CHLORPROMAZINE DEMONSTRATING THE ROLES OF PROTEIN-BINDING AND MEMBRANE PARTITIONING

Transcellular permeability of the neuroleptic-anesthetic chlorpromazin e (CPZ) was examined using a cell type (MDCK) that forms a confluent m onolayer of polarized cells resulting in distinct apical (AP) and baso lateral (BL) membrane domains separated by tight junctions. Because CP Z is membrane interactive, transmonolayer flux was analyzed as two kin etic events: cell uptake from the AP donor solution and efflux into th e BL side receiver. Using the rate of cell uptake in the presence of d ifferent concentrations of BSA, an intrinsic cell partition coefficien t of 3700 +/- 130 and an operational dissociation binding constant of 0.4 +/- 0.05 mM were calculated. In contrast to uptake, efflux of CPZ from either the AP or the BL side of the cell monolayer was similar to 10(4)-fold slower and was dependent upon the avidity of CPZ for the p rotein acceptor in the receiver solution. These results emphasized the importance of simultaneously measuring disappearance of a lipophilic molecule from the donor solution and its appearance in the receiver an d demonstrated how interactions with proteins on either side of the ce llular barrier influence permeability. Appearance kinetics showed that the composition of the receiving environment is critical to model a p articular in vivo situation and implied that the intrinsic permeabilit y of membrane-interactive molecules in vitro does not necessarily pred ict penetration beyond the initial cellular barrier in vivo.