Production of hydroxyl radical by iron(III)-anthraquinone complexes through self-reduction and through reductive activation by the xanthine oxidase/hypoxanthine system.

The iron(III) complexes of doxorubicin and epirubicin were observed to undergo a self-reduction (autoxidation) reaction in the absence of added reductants under aerobic conditions that resulted in the formation of ferrous anthracycline complexes. These self-reduction reactions resulted in significant hydrogen peroxide-mediated hydroxyl radical formation, as determined by electron paramagnetic resonance spin trapping. In contrast, the iron(III) complexes of daunorubicin, idarubicin, and mitoxantrone produced no significant amount of hydroxyl radical formation. Only the anthraquinones with an alpha-ketol side chain were observed to undergo both self-reduction and hydroxyl radical formation. Thus, the alpha-ketol side chain must be undergoing concomitant oxidation. The rate of self-reduction of the iron(III)-doxorubicin complex is consistent with a mechanism in which unbound doxorubicin binds to an iron(III)-doxorubicin complex of decreased coordination and after binding undergoes an intramolecular electron transfer. Molecular modeling was used to identify iron(III)-doxorubicin complexes that could result in electron transfer from the doxorubicin side chain hydroxyl group to the iron(III). All of the iron(III)-anthracycline complexes were able to produce hydroxyl radicals at significantly increased rates in the presence of the xanthine oxidase/hypoxanthine superoxide-generating system. In this system the iron(III)-epirubicin complex gave the greatest rate of hydroxyl radical production, with iron(III)-idarubicin giving the least.