Azatoxin was rationally designed as
a DNA topoisomerase II (top2) inhibitor [Leteurtre et al.,
Cancer Res 52: 4478-4483, 1992] and was also found to inhibit
tubulin polymerization. Its cytotoxicity is due to action on
tubulin at lower concentrations and on top2 at higher concentrations. At intermediate concentrations, the combination of the two mechanisms appears antagonistic [Solary et al., Biochem Pharmacol 45: 2449-2456, 1993]. The aim of this study was to design
azatoxin derivatives that would act only on
tubulin or on top2. Selective targeting of top2 or
tubulin was tested using top2-mediated DNA cleavage assays, and
tubulin polymerization and
tubulin proteolysis assays, as well as COMPARE analyses of cytotoxicity assays in the National Cancer Institute in vitro
Drug Screening Program. Selective inhibitors of top2 and
tubulin polymerization have been obtained. Top2 inhibition, abolished by methylation at position 4', was enhanced by the addition of a bulky group at position 11. Bulky substitution at position 11 determined different patterns of top2 cleavage sites and suppressed the action on
tubulin. Selective inhibition of
tubulin was obtained with
4'-methylazatoxin that was found to bind to the
colchicine site. These results are consistent with those obtained in the
podophyllotoxin family to which
azatoxin is structurally related. Some
azatoxin derivatives are under consideration for further preclinical development.