PT - JOURNAL ARTICLE AU - Annie Albin Lumen AU - Poulomi Acharya AU - Joseph W. Polli AU - Andrew Ayrton AU - Harma Ellens AU - Joe Bentz TI - If the <em>K</em><sub>I</sub> Is Defined by the Free Energy of Binding to P-Glycoprotein, Which Kinetic Parameters Define the IC<sub>50</sub> for the Madin-Darby Canine Kidney II Cell Line Overexpressing Human Multidrug Resistance 1 Confluent Cell Monolayer? AID - 10.1124/dmd.109.029843 DP - 2010 Feb 01 TA - Drug Metabolism and Disposition PG - 260--269 VI - 38 IP - 2 4099 - http://dmd.aspetjournals.org/content/38/2/260.short 4100 - http://dmd.aspetjournals.org/content/38/2/260.full SO - Drug Metab Dispos2010 Feb 01; 38 AB - From previous fits of drug transport kinetics across confluent Madin-Darby canine kidney II cell line overexpressing human multidrug resistance 1 cell monolayers, we found that a drug's binding constant to P-glycoprotein (P-gp) was significantly smaller than its IC50 when that drug was used as an inhibitor against another P-gp substrate. We tested several IC50 candidate functions, including the standard function, the Kalvass-Pollack function, and the efflux ratio, to determine whether any of them yielded an IC50 = KI, as would be expected for water-soluble enzymes. For the confluent cell monolayer, the IC50/KI ratio is greater than 1 for all candidate functions tested. From the mass action kinetic model, we have derived a simple approximate equation that shows how the IC50/KI ratio depends on the elementary rate constants from our mass action model. Thus, the IC50 will differ between cell lines and tissues, for the same probe substrate and inhibitor, if there are different membrane concentrations of P-gp, or the probe substrate's elementary rate constants, partition coefficient, binding constant to P-gp, passive permeability, and ability to access the other transporters (if any) in the two cell lines. The mass action model and the approximate equation for the IC50/KI ratio derived here can be used to estimate the elementary rate constants needed to extrapolate in vitro drug-drug interactions for compounds to the in vivo environment.Copyright © 2010 by The American Society for Pharmacology and Experimental Therapeutics