Knowledge of the passive permeability coefficient for new drugs is useful for estimating the fraction absorbed across the gastrointestinal tract. The commonly used approximate formula for the passive permeability coefficient is based on the initial rate of permeation across cell monolayers, requires measurement during the linear phase of permeation, and is not applicable when there is significant back flux of compound or mass balance problem. To develop a rigorous equation that can be used at any time point, i.e., that is valid outside of the linear phase, the mass action equations were integrated for a standard single barrier model of passive permeability. The simple analytical solution found also allows correction for both loss of drug (e.g., due to binding and/or hydrolysis) and sampling volume loss for multiple time point experiments. To test this equation, we measured the passive permeation of three well characterized drugs (amprenavir, quinidine, and loperamide) across confluent monolayers of MDCKII-hMDR1 cells. The potent P-glycoprotein inhibitor GF120918 was used to inhibit P-glycoprotein activity, so only passive permeability was determined. Dramatically different time-dependent behavior was observed for the three compounds, with loperamide showing significant loss of compound, and loperamide and quinidine causing plasma membrane modifications over time. The simple and exact equation for the permeability coefficient developed here works from start of transport to equilibrium, being valid when the commonly used approximate equation may not be. Thus, the exact equation is safer to use in any context, even for single time point estimates in high-throughput permeability assays.
Copyright 2004 Wiley-Liss, Inc. and the American Pharmacists Association J Pharm Sci 93:2108-2123, 2004