Effect of bleomycin on murine tumor cells at elevated temperatures and two different pH values

The cytotoxic effect of bleomycin, an antibiotic chemotherapeutic agent, at elevated temperatures was investigated. Single cell suspensions of FSa-II tumor cells were treated at elevated temperatures with or without bleomycin either at pH 7.4 or 6.7. Immediately after treatment, cells were cooled and diluted for lung colony assay. Cyclophosphamide of 200 mg/kg was injected i.p. into the recipient mice 48 h before i.v. injection of tumor cells. Lungs were removed 13 days thereafter and fixed in Bouin's solution. Colonies formed on the surface of each lobe were counted, and the surviving fractions were calculated. Cell survival was determined as a function of treatment time at various temperatures. Survival curves following bleomycin treatment at various temperatures were biphasic. Initial steep portion was followed by a resistant tail. The surviving fractions were reduced to 0.1 within 20 min of treatment at pH 7.4 in the temperature range from 39.0 degrees-43.5 degrees C, and 10 min at pH 6.7. The slope of the resistant tail becomes steeper with increasing temperature, indicating that the cytotoxic effect of bleomycin was enhanced at elevated temperatures. The reciprocal of D0 (treatment time to reduce surviving fraction from 1.0 to 0.37 on the exponential portion of survival curve) of the resistant tail was plotted as a function of the treatment temperature, and activation energy was calculated. This analysis indicates that the enhancement of the cytotoxic effect increases with increasing temperature. However, above 42.5 degrees C, this enhancement appears to be dominated by lethal thermal damage.