Cytotoxic Mechanism of XK469: Resistance of Topoisomerase IIβ Knockout Cells and Inhibition of Topoisomerase I

Topoisomerase IIβ knockout mouse cells (β−/−) were found to have only slight resistance to m-AMSA, a dual topoisomerase IIα-IIβ poison, as compared to wild-type cells (β+/+) during 1 h or 3 day exposures to the drug. In contrast, the β−/− cells were greater than threefold resistant to XK469, a selective topoisomerase IIβ poison during three day drug exposures (β+/+ IC50 = 175 μM, β−/− IC50 = 581 μM). Short term (1 h) exposure to XK469 was not cytotoxic to either β−/− or β+/+ cells, suggesting that anticancer therapy with XK469 may be more efficacious if systemic levels can be prolonged. During studies on topoisomerase activity in nuclear extracts of the β+/+ and β−/− cells, we found evidence that XK469 is a weak topoisomerase I catalytic inhibitor. The high IC50 for topoisomerase I inhibition (2 mM) suggests that topoisomerase I is not a significant target for XK469 cytotoxicity.