AZATOXIN DERIVATIVES WITH POTENT AND SELECTIVE ACTION ON TOPOISOMERASE-II

Azatoxin was rationally designed as a DNA topoisomerase II (top2) inhi bitor [Leteurtre et al., Cancer Res 52: 4478-4483, 1992] and was also found to inhibit tubulin polymerization. Its cytotoxicity is due to ac tion on tubulin at lower concentrations and on top2 at higher concentr ations. At intermediate concentrations, the combination of the two mec hanisms appears antagonistic [Solary et al., Biochem Pharmacol 45: 244 9-2456, 1993]. The aim of this study was to design azatoxin derivative s that would act only on tubulin or on top2. Selective targeting of to p2 or tubulin was tested using top2-mediated DNA cleavage assays, and tubulin polymerization and tubulin proteolysis assays, as well as COMP ARE analyses of cytotoxicity assays in the National Cancer Institute i n vitro Drug Screening Program. Selective inhibitors of top2 and tubul in polymerization have been obtained. Top2 inhibition, abolished by me thylation at position 4', was enhanced by the addition of a bulky grou p at position 11. Bulky substitution at position 11 determined differe nt patterns of top2 cleavage sites and suppressed the action on tubuli n. Selective inhibition of tubulin was obtained with 4'-methylazatoxin that was found to bind to the colchicine site. These results are cons istent with those obtained in the podophyllotoxin family to which azat oxin is structurally related. Some azatoxin derivatives are under cons ideration for further preclinical development.