Interaction of hyperthermia with bleomycin and liblomycin: effects on CHO cells in vitro

The relative ability of hyperthermia to potentiate the cytotoxicity of bleomycin A2 (BLM) and the lipophilic bleomycin analog liblomycin (LBM) was examined in chinese hamster ovary (CHO) cells. A 60-min exposure to either BLM or LBM inhibited CHO clonogenic survival in a concentration-dependent manner (IC50 values: 11 microM and 2 microM, respectively). Hyperthermia alone also inhibited CHO cell survival in a temperature- and time-dependent manner. In combination with hyperthermia, the cytotoxicity of both drugs was enhanced, however, combination of LBM with hyperthermia exhibited a greater synergistic effect than that observed with the BLM + heat regimen. The greatest inhibitory effects of combined drug + heat treatments occurred when hyperthermia was applied simultaneously with drug exposure. Hyperthermic potentiation of drug cytotoxicity could not be explained by increased uptake of radiolabeled BLM or LBM by CHO cells, or heat-mediated potentiation of the ability of these drugs to cleave plasmid pBR322 DNA in vitro. Our data demonstrate the greater efficacy of LBM, as well as the greater synergy between LBM and hyperthermia against the BLM-sensitive CHO cell line.