In vitro flow cytometry method to quantitatively assess inhibitors of P-glycoprotein

P-glycoprotein (Pgp)-mediated drug efflux is a major factor contributing to the variance of absorption and distribution of many drugs (Hall et al., 19 99). A simple and reliable in vitro method to identify inhibitors of Pgp he lps to prevent the potential of drug interactions. Using daunorubicin as a fluorescent marker and vanadate as a positive control compound, a functiona l flow cytometry method for assessing the ability of a drug to inhibit Pgp- mediated drug efflux from CR1R12 multidrug-resistant cells has been evaluat ed. Quantitation of the relative fluorescence was used to compare potency o f individual inhibitors. Known Pgp inhibitors, such as cyclosporin A, nicar dipine, verapamil, quinidine, terfenadine, tamoxifen, and vinblastine were demonstrated to inhibit the Pgp-mediated efflux of daunorubicin. Cyclospori n A and terfenadine were the most potent inhibitors among the compounds tes ted. Tetraphenylphosphonium and alpha-tocopherol had little inhibitory effe ct. Progesterone produced significant inhibition at relatively high concent rations. This study demonstrated that this in vitro flow cytometry method i s a simple, sensitive, and quantitative tool to assess the capacity of a dr ug to inhibit Pgp transporters, and is useful for screening or identifying inhibitors of Pgp as well as evaluation of potential for drug interactions.