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

Cationic aromatic tricyclic compounds including triphenylmethane dyes, phenazines, phenoxazines, acridines, phenothiazines, phenanthridinium compounds, anthracenes and xanthene dyes, which amplify cell killing in phleomycin-treated Escherichia coli B cells also modified phleomycin-induced breakdown of DNA to acid-soluble fragments. A plot of DNA breakdown as a function of concentration was bell-shaped for each of the active compounds, i.e. as the concentration increased, DNA breakdown was enhanced initially, but above a certain concentration, the proportion of DNA degraded declined, often to zero. One of the compounds, acriflavine, when tested also inhibited DNA breakdown following ultraviolet irradiation. A study, by sedimentation methods, of DNA single-strand breakage in phleomycin-treated E. coli cells, using 3 representative compounds, Crystal Violet, 3,6-diaminoacridine and Methylene Blue, revealed a consistent increase in DNA strand breaks as concentration of compound increased. In similar experiments with ethidium bromide the breakage yield/concentration curve exhibited a maximum. In general, however, it seems that the inhibition of DNA-breakdown observed at higher concentrations of these amplifying compounds is not explicable by an effect on the primary breakage event, but is due to suppression of exonucleolytic activity in the cells.

Amplifications of phleomycin and bleomycin-induced antibiotic activity in Escherichia coli by aromatic cationic compounds.

A wide range of aromatic compounds has been shown to amplify phleomycin-induced cell killing in Escherichia coli. They include acridines, acridinium chlorides, dihydroanthracenes, anthracenes, dianthracenes, phenanthridinium salts, phenazinium chlorides, phenoxazones, triphenyl methane dyes, benzoquinolizinium chloride, diphenylmethane derivatives, stilbene and diphenyl derivatives. Low concentrations of these amplifiers also amplified the DNA breakage and degradation effects of phleomycin. The minimum structural specification for activity as an amplifying agent is suggested. A representative sample of compounds effective as amplifiers of phleomycin also amplified the antibiotic effects of bleomycins B4 and B6. The amplifiers described are known to vary in their ability to penetrate and accumulate in different organisms or tissues. This suggests the possibility of developing a series of antibiotic regimes using these amplifiers (or the large number of derivative compounds also likely to be active) where the therapeutic index is determined by the properties of the amplifier chosen rather than of the phleomycin or the bleomycin.