DNA topoisomerases as targets for the anticancer drug TAS-103: DNA interactions and topoisomerase catalytic inhibition

TAS-103 is a novel anticancer drug that kills cells by increasing levels of DNA cleavage mediated by topoisomerase II. While most drugs that stimulate topoisomerase II-mediated DNA scission (i.e., topoisomerase II poisons) al so inhibit the catalytic activity of the enzyme, they typically do so only at concentrations above the clinical range. TAS-103 is unusual in that it r eportedly inhibits the catalytic activity of both topoisomerase I and II an d does so at physiologically relevant concentrations [Utsugi, T., et al. (1 997) Jpn. J. Cancer Res. 88, 992-1002]. Without a topoisomerase activity to relieve accumulating torsional stress, the DNA tracking systems that promo te the action of TAS-103 as a topoisomerase II poison would be undermined. Therefore, the effects of TAS-103 on the catalytic activity of topoisomeras e I and II were characterized. DNA binding and unwinding assays indicate th at the drug intercalates into DNA with an apparent dissociation constant of similar to 2.2 mu M. Furthermore, DNA strand passage assays with mammalian topoisomerase I indicate that TAS-103 does not inhibit the catalytic activ ity of the type I enzyme. Rather, the previously reported inhibition of top oisomerase I-catalyzed DNA relaxation results from a drug-induced alteratio n in the apparent topology of the nucleic acid substrate. TAS-103 does inhi bit the catalytic activity of human topoisomerase II alpha, apparently by b locking the DNA religation reaction of the enzyme. The lack of inhibition o f topoisomerase I catalytic activity by TAS-103 explains how the drug is ab le to function as a topoisomerase II poison in treated cells.