THE MULTIDRUG-RESISTANCE-REVERSER VERAPAMIL INTERFERES WITH CELLULAR P-GLYCOPROTEIN-MEDIATED PUMPING OF DAUNORUBICIN AS A NON-COMPETING SUBSTRATE

We examined P-glycoprotein-mediated verapamil transport, using two dru g-sensitive and multidrug resistant cell-line couples, i.e. A2780, 278 0(AD) and SW-1573, SW-1573/1R500. The interaction of H-3-labeled verap amil with cells was measured using a flow-through system. The verapami l-containing medium was pumped over the cells and monitored on-line fo r radioactivity. In the P-glycoprotein-expressing cells, verapamil acc umulation was increased by vinblastine and some known multidrug resist ant (MDR) modifiers. Subsequent removal of these modifiers caused rele ase of verapamil into the medium against a verapamil concentration gra dient. In this manner, we obtained evidence that verapamil is actively transported by the MDR-related P-glycoprotein. Using the flow-through system, we also exposed the cells to flowing culture medium containin g daunorubicin, and measured the inhibition of daunorubicin efflux by verapamil. We found that, although the active efflux of daunorubicin w as maximally blocked by verapamil short-term, longer-term active efflu x of daunorubicin resumed. At a daunorubicin concentration in the flow ing medium of 5 mu M, increasing the verapamil concentration resulted in the same short-term effects, but in a significantly longer period o f a maximal inhibition of daunorubicin efflux from the cells. At a dau norubicin concentration of 20 mu M, increasing the verapamil concentra tion affected neither the short-term nor the long-term effects. These and other observations are in agreement with a model in which daunorub icin and verapamil are non-competing substrates for P-glycoprotein. In conclusion, we obtained evidence that verapamil is actively transport ed by the MDR-related P-glycoprotein and that verapamil and daunorubic in are non-competing substrates for P-glycoprotein. Consequently, the effectiveness of verapamil as an MDR antagonist may be compromised bec ause it is extruded by P-glycoprotein.