We have studied the formation of hydroxyl radical (OH•) induced by doxorubicin in a series of doxorubicin- or vincristine-selected variants of C6 rat glioblastoma cells in culture by electron-spin resonance spectroscopy using 5,5′-dimethyl-1-pyrroline-1-oxide as a spin trap. Wild-type cells, sensitive to doxorubicin, exhibited in the presence of this drug a concentration-dependent OH• formation which could be inhibited by preincubation with superoxide dismutase, catalase or an antibody against cytochrome P450-reductase. In highly doxorubicin-resistant cells, OH• formation was reduced to about 20% of the level obtained in sensitive cells. In cells presenting a very low level of resistance to doxorubicin or in cells selected with vincristine, both presenting a pure multidrug-resistant phenotype, OH• formation was identical to that obtained in sensitive cells. In cells of intermediate resistance or in revertant cells, intermediate levels of OH• formation were obtained. Protection against OH• formation and action can be identified at the levels of superoxide dismutase and glutathione peroxidase activities, which are both enhanced in the resistant cells.
