Decreased Nucleotide Excision Repair Activity and Alterations of Topoisomerase IIα Are Associated with the in Vivo Resistance of a P388 Leukemia Subline to F11782, a Novel Catalytic Inhibitor of Topoisomerases I and II

Purpose: The purpose of the study was to investigate the mechanisms associated with antitumor activity and resistance to F11782, a novel dual catalytic inhibitor of topoisomerases with DNA repair-inhibitory properties. Experimental Design: For that purpose, an F11782-resistant P388 leukemia subline (P388/F11782) has been developed in vivo and characterized. Results: Weekly subtherapeutic doses of F11782 (10 mg/kg) induced complete resistance to F11782 after 8 weekly passages. This resistant P388/F11782 subline retained some in vivo sensitivity to several DNA-topoisomerase II and/or I complex-stabilizing poisons and showed marked collateral sensitivity to cisplatin, topotecan, colchicine, and Vinca alkaloids, while proving completely cross-resistant only to merbarone and doxorubicin. Therefore, resistance to F11782 did not appear to be associated with a classic multidrug resistance profile, as further reflected by unaltered drug uptake and no overexpression of resistance-related proteins or modification of the glutathione-mediated detoxification process. In vivo resistance to F11782 was, however, associated with a marked reduction in topoisomerase IIα protein (87%) and mRNA (50%) levels, as well as a diminution of the catalytic activity of topoisomerase IIα. In contrast, only minor reductions in topoisomerases IIβ and I levels were recorded. However, of major interest, nucleotide excision repair activity was decreased 3-fold in these P388/F11782 cells and was more specifically associated with a decreased (67%) level of XPG (human xeroderma pigmentosum group G complementing protein), an endonuclease involved in this DNA repair system. Conclusions: These findings suggest that both topoisomerase IIα and XPG are major targets of F11782 in vivo and further demonstrate the original mechanism of action of this novel compound.