FLIP-FLOP OF DOXORUBICIN ACROSS ERYTHROCYTE AND LIPID-MEMBRANES

Doxorubicin, an anticancer drug, is extruded from multidrug resistant (MDR) cells and from the brain by P-glycoprotein located in the plasma membrane and the blood-brain barrier, respectively. MDR-type drugs ar e hydrophobic and, as such, enter cells by diffusion through the membr ane without the requirement for a specific transporter. The apparent: contradiction between the presumably free influx of MDR-type drugs int o MDR cells and the efficient removal of the drugs by P-glycoprotein, an enzyme with a limited ATPase activity, prompted us to examine the m echanism of passive transport within the membrane. The kinetics of dox orubicin transport demonstrated the presence of two similar sized drug pools located in the two leaflets of the membrane. The transbilayer m ovement of doxorubicin occurred by a flip-flop mechanism of the drug b etween the two membrane leaflets. At 37 degrees, the flip-flop exhibit ed a half-life of 0.7 min,in both erythrocyte membranes and cholestero l-containing lipid membranes. The flip-flop was inhibited by cholester ol and accelerated by high temperatures and the fluidizer benzyl alcoh ol. The rate of doxorubicin flux across membranes is determined by bot h the massive binding to the membranes and the slow flip-flop across t he membrane. The long residence-time of the drug in the inner leaflet of the plasma membrane allows P-glycoprotein a better opportunity to r emove it from the cell. (C) 1997 Elsevier Science Inc.