For elucidation of the mechanism behind the differences in cellular accumulation of daunorubicin and doxorubicin, the uptake and subcellular localization of these drugs were studied in cultured fibroblasts by cell fractionation techniques. Daunorubicin and doxorubicin accumulated to the same saturation level in nuclei, whereas the lysosomal concentration of daunorubicin always exceeded that of doxorubicin. No saturation of lysosomes could be achieved under the experimental conditions used. In both cellular storage pools, more of the drug was found when the cells had been incubated in a medium at pH 7.8 than after incubation at pH 6.9. Metabolic inhibitors enhanced the accumulation of both drugs under conditions under which most of the drug was stored in nuclei. On the other hand, low incubation temperature inhibited drug accumulation under conditions under which most of the drug was stored in lysosomes. The accumulation of N-acetyldaunorubicin markedly exceeded that of N-acetyldoxorubicin, although these derivatives were not stored in significant amounts in either nuclei or lysosomes. Our results support the hypothesis that transport of daunorubicin and doxorubicin across the plasma membrane occurs by a “leak and pump” system, the leak being inward diffusion of non-ionized drug molecules and the pump being an active efflux. Since daunorubicin is less polar than is doxorubicin, it can be assumed to diffuse faster across biological membranes, which should lead to a higher cytoplasmic steady-state level. On this basis a hypothesis is presented to explain the observed differences in cellular accumulation and subcellular distribution between daunorubicin and doxorubicin.
Supported in part by grants from Caisse Général d'Epargne et de Retraite, Bruxelles, Belgium, and Rhone-Poulenc S. A., Paris, France.