Simultaneous treatment with 1-beta-D-arabinofuranosylcytosine and daunorubicin induces cross-resistance to both drugs due to a combination-specific mechanism in HL60 cells.

We have established a human myelogenous leukemia cell line (HL60/AD) that is 10-fold cross-resistant to both 1-beta-D-arabinofuranosylcytosine (ara-C) and daunorubicin; the cell line was isolated from HL60 by simultaneous treatment with these two agents at low drug concentrations attainable in clinical trials. HL60/AD was found to have multiple resistance mechanisms. With regard to ara-C, HL60/AD cells showed decreased deoxycytidine kinase activity but did not show elevation of cytidine deaminase activity or a decrease in ara-C influx. With regard to daunorubicin, a decrease in topoisomerase II activity was found. A decrease in intracellular accumulation of daunorubicin was also found. P-glycoprotein was not detected, but the multidrug resistance-associated protein was expressed. Furthermore, an increase of total cellular glutathione (GSH) content was found. Interestingly, the resistance of HL60/AD cells not only to daunorubicin but also to ara-C was markedly reversed by treatment with L-buthionine-(S,R)-sulfoximine (BSO), a potent inhibitor of GSH synthesis. After exposure of HL60/AD to ara-C, mitochondrial membrane potential and reactive oxygen intermediates showed no significant change, but a considerable loss of mitochondrial membrane potential and an increase in reactive oxygen intermediate generation were caused by pre-incubation with BSO. Neither elevation of GSH nor reversal of resistance by BSO was found in ara-C-resistant HL60 cells that were selected only with ara-C. These findings suggest that in addition to the summation of the mechanisms of resistance to each agent reported previously, an increased level of GSH plays an important role in the cross-resistance induced in HL60/AD cells by simultaneous exposure to both drugs.