An investigation was undertaken of the mechanism of a previously reported cross-resistance between vincristine (VCR) and daunorubicin (DNR) in Ehrlich ascites tumor cells. No significant difference was demonstrated for the time course of [3H]VCR uptake in cells resistant to VCR (EHR 2/VCR+) and in cells resistant to DNR (EHR 2/DNR+), whereas wild-type cells accumulated nearly 6-fold more drug at steady state. The energy dependence of [3H]VCR and of DNR transport was investigated by the metabolic inhibitors sodium azide and iodoacetic acid. These studies revealed that uptake of [3H]VCR and of DNR was depressed in both resistant sublines by an energy-dependent process that mostly requires energy from glycolysis. If glucose was omitted from the medium together with addition of sodium azide, the uptake of [3H]VCR and of DNR in EHR 2/VCR+ reached a level nearly equal to that of wild-type cells. If glycolysis was restored by addition of glucose to the resistant cells loaded with drug in this way, a pronounced extrusion of [3H]VCR and of DNR was induced. In a similar experiment with wild-type cells, a slight but significant extrusion of [3H]VCR could be induced.
The studies showed that, for nearly unidirectional influx, the cells must be incubated in the medium without glucose but with sodium azide. In this medium the influx of [3H]VCR and of DNR was significantly higher in wild-type cells than in cells from the resistant sublines. The flux of DNR was not competitively inhibited by VCR either in wild-type cells or in resistant cells. The data indicate that the mechanism of cross-resistance between VCR and DNR in Ehrlich ascites tumor cells is a result of at least two different mechanisms: (a) an energy-dependent drug extrusion common to VCR and DNR; and (b) unspecific changes in the membrane, which reduce the influx of both compounds.
Supported by grants from the Danish Cancer Society and the Anders Hasselbalch Foundation.