RHODAMINE EFFLUX PATTERNS PREDICT P-GLYCOPROTEIN SUBSTRATES IN THE NATIONAL-CANCER-INSTITUTE DRUG SCREEN

Fifty-eight cell lines in the National Cancer Institute drug screen we re analyzed for their ability to efflux the fluorescent dye rhodamine 123 as a functional assay for P-glycoprotein (Pgp). Using flow cytomet ry, the rhodamine fluorescence was measured for each cell line under f our incubation conditions, i.e., after accumulation in the presence or absence of the Pgp antagonist cyclosporin A and after efflux in rhoda mine-free medium in the presence or absence of cyclosporin A. The resu lts in some cell lines were compatible with Pgp-mediated efflux. There was a significant correlation between mdr-1 expression and rhodamine efflux in the 58 cell lines (r = 0.788, p = 0.0001). Using the rhodami ne efflux data as a seed for COMPARE analysis with the cytotoxicity da ta on >30,000 compounds in the National Cancer Institute drug screen d atabase, hundreds of compounds with high correlation coefficients were identified. Selected compounds were tested for reversal of cross-resi stance in a multidrug-resistant cell line. A high degree of reversibil ity, up to 10,000-fold, for some of the compounds was noted in the pre sence of the Pgp antagonist PSC 833. This finding suggested that compo unds with predominately Pgp-mediated resistance were being identified. Using these compounds as seeds for COMPARE analysis against a more re stricted database of 187 standard agents, a series of standard compoun ds were repeatedly identified as having high correlation coefficients with the newly identified Pgp substrates. These standard agents, inclu ding phyllanthoside, bisantrene, and homoharringtonine, constitute an mdr-1 profile. New agents identified as being highly correlated with t hese compounds may benefit from clinical trials with Pgp antagonists.