The effect of combinations of the anthracyclines aclarubicin and daunorubicin 
was investigated in a clonogenic assay using the human small cell lung cancer 
cell line OC-NYH and a multidrug-resistant (MDR) murine subline of Ehrlich 
ascites tumor (EHR2/DNR+). It was found that the cytotoxicity of daunorubicin in 
OC-NYH cells was antagonized by simultaneous exposure to nontoxic concentrations 
of aclarubicin. Coordinately, aclarubicin inhibited the formation of 
daunorubicin-induced protein-concealed DNA single-strand breaks and DNA-protein 
cross-links in OC-NYH cells when assayed by the alkaline elution technique. 
Aclarubicin had no influence on the accumulation of daunorubicin in these cells. 
In contrast, the accumulation of daunorubicin in EHR2/DNR+ cells was enhanced by 
more than 300% when the cells were simultaneously incubated with the MDR 
modulator verapamil, aclarubicin, or the two agents combined. Yet the 
cytotoxicity of daunorubicin was potentiated significantly only by verapamil. 
The increased cytotoxicity of daunorubicin in the presence of verapamil was 
completely antagonized when aclarubicin was used together with the MDR 
modulator. Finally, the effect of daunorubicin on the DNA cleavage activity of 
purified topoisomerase II in the presence and absence of aclarubicin was 
examined. It was found that daunorubicin stimulated DNA cleavage by 
topoisomerase II at specific DNA sites. The addition of aclarubicin completely 
inhibited the daunorubicin-induced stimulation of DNA cleavage. Taken together, 
these data indicate that aclarubicin-mediated inhibition of daunorubicin-induced 
cytotoxicity is due mainly to a drug interaction with the nuclear enzyme 
topoisomerase II. This antagonism at the nuclear level explains why aclarubicin 
is a poor modulator of daunorubicin resistance even though aclarubicin is able 
to increase the intracellular accumulation of daunorubicin in a MDR cell line.