DNA topoisomerases as targets for the anticancer drug TAS-103: Primary cellular target and DNA cleavage enhancement

TAS-103 is a novel antineoplastic agent that is active against in vivo tumo r models [Utsugi, T., et al. (1997) Jpn, J, Cancer Res. 88, 992-1002]. This drug is believed to be a dual topoisomerase I/II-targeted agent, because i t enhances both topoisomerase I- and topoisomerase II-mediated DNA cleavage in treated cells. However, the relative importance of these two enzymes fo r the cytotoxic actions of TAS-103 is not known. Therefore, the primary cel lular target of the drug and its mode of action were determined. TAS-103 st imulated DNA cleavage mediated by mammalian topoisomerase I and human topoi somerase II alpha and beta in vitro. The drug was less active than camptoth ecin against the type I enzyme but was equipotent to etoposide against topo isomerase II alpha. A yeast genetic system that allowed manipulation of top oisomerase activity and drug sensitivity was used to determine the contribu tions of topoisomerase I and II to drug cytotoxicity. Results indicate that topoisomerase II: is the primary cellular target of TAS-103. In addition, TAS-103 binds to human topoisomerase II alpha in the absence of DNA, sugges ting that enzyme-drug interactions play a role in formation of the ternary topoisomerase II.drug.DNA complex. TAS-103 induced topoisomerase II-mediate d DNA cleavage at sites similar to those observed in the presence of etopos ide. Like etoposide, it enhanced cleavage primarily by inhibiting the relig ation reaction of the enzyme. Based on these findings, it is suggested that TAS-103 be classified as a topoisomerase II-targeted drug.