[Show abstract][Hide abstract] ABSTRACT: We have previously described a mitoxantrone-resistant human breast carcinoma cell line, MCF7/MX, in which resistance was associated with a defect in the energy-dependent accumulation of mitoxantrone in the absence of P-glycoprotein overexpression (M. Nakagawa et al., Cancer Res. 52: 6175-6181, 1992). We now report that this cell line is highly cross-resistant to the camptothecin analogues topotecan (180-fold), 9-aminocamptothecin (120-fold), CPT-11 (56-fold), and SN38 (101-fold), but is only mildly cross-resistant to the parent compound camptothecin (3.2-fold) and 10,11-methylenedioxy-camptothecin (2.9-fold). Topotecan accumulation was decreased in MCF7/MX cells compared to parental MCF7/WT cells, and there was a corresponding reduction in topotecan-mediated stimulation of the enzyme/DNA complex formation in MCF7/MX cells compared to MCF7/WT cells. No overexpression of the multidrug resistance-associated protein was detected compared to parental MCF7/WT cells. Furthermore, both sensitive MCF7/WT and mitoxantrone-resistant MCF7/MX cells contain equal amounts of DNA topoisomerase I protein, and DNA relaxation activities were equal in both cell lines and inhibited to the same extent by topotecan and camptothecin. Thus, these results suggest a novel mechanism of resistance to topoisomerase I inhibitors in cancer cells.