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TAS-103 is a novel antineoplastic agent that is active against in vivo tumor 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 it enhances both topoisomerase I- and topoisomerase II-mediated DNA cleavage in treated cells. However, the relative importance of these two enzymes for the cytotoxic actions of TAS-103 is not known. Therefore, the primary cellular target of the drug and its mode of action were determined. TAS-103 stimulated DNA cleavage mediated by mammalian topoisomerase I and human topoisomerase IIalpha and beta in vitro. The drug was less active than camptothecin against the type I enzyme but was equipotent to etoposide against topoisomerase IIalpha. A yeast genetic system that allowed manipulation of topoisomerase activity and drug sensitivity was used to determine the contributions 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 IIalpha in the absence of DNA, suggesting that enzyme-drug interactions play a role in formation of the ternary topoisomerase II.drug.DNA complex. TAS-103 induced topoisomerase II-mediated DNA cleavage at sites similar to those observed in the presence of etoposide. Like etoposide, it enhanced cleavage primarily by inhibiting the religation reaction of the enzyme. Based on these findings, it is suggested that TAS-103 be classified as a topoisomerase II-targeted drug.