Yeast cells expressing differential levels of human or yeast DNA topoisomerase II: a potent tool for identification and characterization of topoisomerase II-targeting antitumour agents

Purpose: To identify and characterize the specificity and potency of topoisomerase II-interacting antitumour drugs in an in vivo model utilizing the yeast Saccharomyces cerevisiae. Methods: Four yeast transformants were selected for the expression of either human or yeast DNA topoisomerase II at different, biologically relevant, levels under the tight control of promoters of various strengths. Results: Analyses of 24 drugs permitted their classification into three distinct groups, depending on whether they induced topoisomerase II-related cytotoxicity (etoposide), showed nonspecific cytotoxicity (camptothecin), or exerted no cytotoxicity at all (vinorelbine). Within the first group different patterns of action were distinguishable: (1) classical topoisomerase II expression-dependent cytotoxicity for both species of enzyme (e.g. etoposide, amsacrine, doxorubicin, actinomycin D), although amsacrine and TOP 53 were more active, respectively, on human and yeast topoisomerase II; and (2) compounds that appeared to poison only one species of topoisomerase II with, for example, genistein and the bisdioxopiperazine ICRF-193 lethally targeting only the human type, and mitoxantrone only the yeast isozyme. Three of the 16 known topoisomerase II inhibitors tested were not correctly identified with this assay, possibly owing to restricted cell wall permeability or to the absence of correct processing pathways such as, for example, in the case of the prodrug etopophos. Conclusion: This methodology, in vivo in yeast, selected for a large range of potent topoisomerase II-targeting anticancer agents. Of particular interest in this yeast model, and in contrast to yeast topoisomerase II, human topoisomerase II was shown to confer dominant sensitivity in the presence of the series of bisdioxopiperazine derivatives tested. This assay therefore has the potential easily to identify and characterize the potency and specificity of synthesized anticancer drugs with modified original chemical structures or those present, for example, in natural plant extracts or marine organisms.