The relative importance of passive and P-glycoprotein mediated anthracycline efflux from multidrug resistant cells.

For the four anthracyclines idarubicin, daunorubicin, epirubicin and doxorubicin the passive and active efflux rates in intact multidrug resistant cells were compared. Although highly similar structurally, these anti-tumor agents differ in lipophilicity and membrane permeability (k). The method we used was based on the continuous measurement of the cellular efflux and determination of the ratio (R(Vp)) of transport rates just before and just after inhibition of the active transport with verapamil (Vp). Hence, R(Vp) - 1 should reflect the active transport rate relative to the passive transport rate. If cells were single, well-stirred compartments, R(Vp) - 1 should equal V(max)/(k·K(m)), where V(max) is the maximal pumping rate and K(m) is the Michaelis constant. However, using the plasma membrane permeabilizing agent digitonin, we found an effective intracellular anthracycline store. Particularly, when the efflux was fast, e.g. with idarubicin or in intensively pumping cells, the intracellular transport began to control the cellular efflux. Under these conditions, k underestimated the true plasma membrane permeability (k