Doxorubicin-Dependent Reduction of Ferrylmyoglobin and Inhibition of Lipid Peroxidation:  Implications for Cardiotoxicity of Anticancer Anthracyclines

Lipid peroxidation has been proposed to mediate cardiotoxicity induced by doxorubicin (DOX) and other anticancer anthracyclines; however, there have been reports showing that DOX can also inhibit lipid peroxidation. Here we characterized the effects of DOX on the oxo-ferryl moiety [FeIVO, MbIV] of H2O2-activated myoglobin, a lipid oxidant likely formed in the heart during treatment with DOX. MbIV was formed in vitro by reacting 100 μM H2O2 with 50 μM horse heart metmyoglobin (MbIII). Spectral studies showed that DOX reduced MbIV to MbIII, half-maximal regeneration of MbIII occurring at ∼18 μM DOX. Comparisons between DOX, its aglycone doxorubicinone, and other approved or investigational anthracyclines or model compounds (daunorubicin, idarubicin, aclarubicin, and naphthazarin), showed that DOX reduced MbIV through the hydroquinone moiety of its tetracyclic ring. DOX inhibited MbIV-dependent peroxidation of arachidonic acid, suppressing the formation of thiobarbituric acid-reactive substances with an IC50 of ∼18 μM. Lipid peroxidation was inhibited also by the hydroquinone-containing daunorubicin and idarubicin but not by the hydroquinone-deficient aclarubicin; moreover, neither simple hydroquinone nor other known MbIV reductants (ascorbate, glutathione, and ergothioneine) reached measurable IC50s in a micromolar range. DOX-dependent inhibition of lipid peroxidation correlated with its ability to reduce MbIV to MbIII in competition with arachidonic acid (r = 0.83, P = 0.029); it did not correlate with its ability to scavenge other free radical species [like e.g., peroxyl radicals generated through the thermal decomposition of 2,2‘-azo-bis(2-amidinopropane)]. DOX reduced MbIV and inhibited lipid peroxidation also when H2O2, MbIII and arachidonic acid were reacted in cytosol of human myocardial biopsies, a model developed to predict the cardiotoxic mode of action of DOX in patients. These results illustrate “antioxidant” properties of DOX, mediated by reduction of MbIV to MbIII, and cast doubts on lipid peroxidation as a causative mechanism of anthracycline-induced cardiotoxicity.