KINETIC-ANALYSIS IN LIVING CELLS OF THE INHIBITION OF THE P-GLYCOPROTEIN-MEDIATED EFFLUX OF ANTHRACYCLINES BY VINCA ALKALOIDS

Cells that overexpress the mdr I gene have decreased steady-state accu mulation and increased efflux of many anticancer drugs including anthr acyclines and vinca alkaloids. The mechanism(s) of P-glycoprotein-medi ated efflux of drugs is (are) still poorly understood. In an attempt t o identify mechanism(s) by which multidrug resistance can be circumven ted, the cellular accumulation has been examined of pirarubicin, doxor ubicin and idarubicin alone and in conjunction with four vinca alkaloi d derivatives-vinblastine, navelbine, vindesine and vincristine. The p resent study was performed using a spectrofluorometric method with whi ch it is possible to follow continuously the uptake and release of flu orescent molecules by living cells, as the incubation of the cells wit h the drug proceeds. Erythroleukemia K562 cell lines were used. It has been shown that the P-glycoprotein-mediated efflux of these three ant hracyclines can be inhibited by vinca alkaloids derivatives. At pH 7.2 , 50% of the P-glycoprotein-mediated efflux of daunorubicin and idarub icin was inhibited by about 40 +/- 10 mu M vinblastine and that of pir arubicin by 10 +/- 2 mu M vinblastine. The vinblastine concentration r equired to inhibit 50% of the active efflux of these anthracyclines di d not depend on the anthracycline concentrations used, indicating that the inhibition was non competitive. The ability of navelbine, vincris tine and vindesine to inhibit the active efflux of pirarubicin was als o checked; 15 +/- 3 mu M navelbine are required to inhibit 50% of the active efflux but at concentrations lower than 100 mu M, neither vincr istine nor vindesine were able to inhibit this efflux, indicating that the vinca alkaloids compounds which are the most efficient are the mo st lipophilic. For the four vinca alkaloids, the concentration require d to inhibit 50% of the efflux was lower as the pH was higher. A detai led kinetics analysis of the P-glycoprotein-mediated efflux of pirarub icin in the presence of vinblastine indicates a non competitive inhibi tion with K-1 = 12 +/- 2 mu M. (C) 1998 Elsevier Science Ireland Ltd. All rights reserved.