Use of Somatic Cell Hybrids to Analyze Role of Specific Enzymes in Daunorubicin Cytotoxicity1

Through the use of drug-adapted tissue culture cells, correlations have been observed between the level of specific enzymes and drug resistance. Drug resistance, however, may be due to multiple factors. To test whether the activity of daunorubicin reducÃ-aseor NADPH diaphorase independ ently influences in vitro daunorubicin-induced Cytotoxicity, we developed somatic cell hybrid clones to partially isolate these factors. This was accomplished by fusing daunorubicin-resistant myeloblast cells obtained from a patient with monosomy 7 leukemia to a daunorubicin-sensitive Chinese hamster cell line. The in vitro Cytotoxicity of daunorubicin was compared in hybrid clones having variable enzyme activities; the concen trations of daunorubicin that inhibited the growth of clones by 50% did not differ by more than 2-fold, whereas daunorubicin reducÃ-aseactivities and NADPH diaphorase isozynte activities differed by more than Killami 15-fold, respectively. These large differences in enzymatic activity were obtained in part by the suppression of specific hamster genes, indicating a regulatory control mechanism for xenobiotic enzymes. Our findings suggest that in this system substantial intercellular variation in the activity of these xenobiotic enzymes does not independently influence cellular resistance to daunorubicin.