Inhibition and enhancement of phleomycin-induced DNA breakdown by aromatic tricyclic compounds

Enhancement of bleomycin-induced micronucleus formation in V79 cells as a rapid and sensitive screen for non-covalent DNA-binding compounds

Non-covalent drug/DNA interactions are difficult to study and because of this, the significance of such interactions from a safety standpoint and their contribution to positive genetic toxicology test findings is poorly understood. It is shown in the present study that such interactions may be detected and quantified in Chinese hamster V79 cells by an adaptation of the bleomycin amplification assay. This assay measures the ability of a test compound to enhance the DNA damaging activity of the antibiotic bleomycin using micronucleus formation as an endpoint. Results are presented examining the bleomycin amplification activity of known intercalating agents, groove-binding agents and other structurally diverse classes of compounds for which intercalative status has not been reported. The assay reveals a strong and predictable SAR for amplification activity based on number and orientation of aromatic rings. Moreover, excellent correlations are observed between DNA binding (viscometric analyses) and DNA amplification in V79 cells for a series of seven experimental compounds. The assay is shown to be useful in understanding the genotoxicity of marketed antihistamines and to help explain genetic toxicology findings observed in a series of novel pharmaceutical entities. It is proposed that assessment of bleomycin amplification activity of novel compounds in early genotoxicity prescreening may provide important information upon which to base synthesis of compounds with minimal or no genotoxic liability.

Molecular basis for potentiation of bleomycin-mediated degradation of DNA by polyamines. Experimental and molecular mechanical studies

The bleomycin-mediated degradation of DNA is stimulated (amplified) by certain DNA binding compounds, such as polyamines, that distort the double helix. Computer modelling studies suggest that putrescine (1), spermidine (2), and spermine (3) bind preferentially on the floor of the major groove of (dGdC)5.(dGdC)5. This interaction results in a bend of the oligomer helix toward the major groove and enlargement of the minor groove, both effects being in the order 1 less than 2 less than 3. These polyamine-induced distortions, as obtained from theoretical studies, parallel the experimental values of the amplification activities of 1-3 in the bleomycin-mediated degradation of poly(dGdC).poly(dGdC). The amplification mechanism of non-competitive binding of amplifier molecules in the major groove, and bleomycin in the minor groove, is proposed. It is suggested that the amplifier-induced conformational changes of the DNA helix increase affinity of the activated bleomycin complex toward the DNA minor groove and, consequently, result in an increased efficiency of the bleomycin-mediated degradation of the helix.