Modulation of bleomycin cytotoxicity.

Lethal effects of a 75-microgram/ml concentration (approximately 5 X 10-5 M) of bleomycin on stationary-phase haploid or diploid cells of the eucaryote Saccharomyces cerevisiae were negated in the presence of 0.05 M phosphate buffer (pH 7). High cell densities (2 X 10(8) cells per ml) further inhibited killing. Multiphasic survival curves resulting after treatments in deionized water (pH 6.7) suggested the presence of cells with differing susceptibilities either at the start of treatment periods or as a result of resistance which developed during exposure to antibiotic. To identify a delayed effect, prolonged lethal consequences of the action of bleomycin were investigated under liquid-holding conditions. Survival of untreated early-stationary-phase yeast cells was not significantly affected by incubation without antibiotic for 6 or 36 h in non-nutrient buffer or water. However, increased killing resulted after bleomycin-treated cells were incubated in the absence of bleomycin or buffer. Moreover, cells which had never been exposed to the antibiotic lost considerable colony-forming ability as a result of incubation with bleomycin-treated cells, indicating the efflux of bleomycin or a reaction product. The findings have implications for both experimental cell studies and cancer therapy, as well as for the chemical mechanisms by which a metal bleomycin complex could cause killing.