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The inhibition of HIV-1 integrase by flavones and related compounds was investigated biochemically and by means of structure-activity relationships. Purified enzyme and synthetic oligonucleotides were used to assay for three reactions catalysed by integrase: (1) processing of 3' termini by cleavage of the terminal dinucleotide; (2) strand transfer, which models the integration step; and (3) "disintegration," which models the reversal of the strand transfer reaction. Inhibitions of all three reactions by flavones generally occurred in parallel, but caffeic acid phenethyl ester (CAPE) appeared to inhibit reaction 2 selectively. CAPE, however, inhibited reactions 1 and 3 effectively when preincubated with the enzyme, suggesting that this compound differs from the flavones primarily in requiring more time to block the enzyme. The core integrase fragment consisting of amino acids 50-212 retained the ability to catalyse reaction 3, and flavones and CAPE retained the ability to inhibit. Hence, the putative zinc-finger region that is deleted in this fragment is probably not the target of inhibition. Inhibition by flavones usually required the presence of at least one ortho pair of phenolic hydroxyl groups and at least one or two additional hydroxyl groups. Potency was enhanced by the presence of additional hydroxyl groups, especially when present in ortho pairs or in adjacent groups of three. Inhibitory activity was reduced or eliminated by methoxy or glycosidic substitutions or by saturation of the 2,3 double bond. These structure-activity findings for flavones were generally concordant with those previously reported for reverse transcriptase and topoisomerase II. These findings are discussed in the context of a review of the effects of flavones on various enzymes, the possible mechanisms of inhibition, and the potential for building upon a general pharmacophore to generate target specificity.

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.

Azatoxin (NSC 640737) is a synthetic molecule that was rationally designed as a topoisomerase II inhibitor (Leteurtre et al., Cancer Res 52: 4478-4483, 1992). The present study was undertaken in order to investigate the molecular pharmacology and the cytotoxic activity of azatoxin in human tumor cells. Alkaline elution experiments performed in HL-60 cells demonstrated that: (1) azatoxin induces DNA-protein cross-links and protein-linked DNA single- and double-strand breaks characteristics of topoisomerase II inhibition in HL-60 cells; and (2) the potency of azatoxin is comparable to that of etoposide (VP-16). Testing of azatoxin in 45 human cell lines in the National Cancer Institute (NCI) in vitro Drug Screening Program indicated that azatoxin was potent (mean IC50 = 0.13 microM), but that its cell line sensitivity profile was correlated with that of tubule inhibitors rather than that of topoisomerase II inhibitors. These data led us to investigate the anti-tubulin activity of azatoxin. We found that azatoxin inhibited tubulin polymerization in vitro and was a mitotic inhibitor at 1 microM and above in the human colon cancer cell line KM20L2. In these cells topoisomerase II inhibition, as detected by the induction of protein-linked DNA strand breaks, required azatoxin concentrations of at least 10 microM. In summary, azatoxin is a potent cytotoxic agent that inhibited both tubulin and topoisomerase II. At lower azatoxin concentrations the former activity prevailed whereas at higher concentrations topoisomerase II inhibition became prominent.