Oxidation of anthracycline anticancer agents by the peroxidase mimic microperoxidase 11 and hydrogen peroxide

The interaction of two clinically important anticancer agents doxorubicin (DXR) and daunorubicin (DNR) and the DNR analog 5-iminodaunorubicin (5IDNR) with the model mammalian peroxidase microperoxidase 11 (MP11) and H2O2 has been investigated using spectrophotometric and EPR techniques. We demonstrate that DNR, DXR, and 5IDNR undergo irreversible oxidation by MP11/H2O2, forming colorless products in both phosphate buffer pH 7.0 and in phosphate buffer pH 7.0/MeOH mixture (1:1 vol/vol), suggesting an extensive modification of the compounds’ chromophores. The initial rate of the anthracyclines’ oxidation is independent of anthracycline concentrations, but is linearly dependent on [H2O2]i at constant [MP11]i (and vice versa), indicating that the reaction is zero order in [anthracycline], first order with respect to [H2O2] and [MP11], and second order overall. Based on data obtained using DNR, DXR, 5IDNR, and p-hydroquinone k2app, the apparent second order rate constant for the formation of a reactive intermediate from MP11 and H2O2 (an analog of peroxidase compound I) has been determined to be in the range of (2.51–5.11) × 103 M−1 s−1 in both solvent systems. EPR studies show that oxidation of DNR, DXR, or 5IDNR with MP11/H2O2 generates free radicals, suggesting that the reaction may be a one-electron process. This study also shows that 5IDNR, but not DNR or DXR, efficiently protects MP11 heme against degradation by H2O2. Our overall conclusion is that MP11 is an effective catalyst of oxidation of anthracyclines by H2O2. Given that, at sites of inflammation or cancer, the anthracyclines can colocalize with peroxidases, protein degradation products, and with H2O2, peroxidation could be one possible fate of these anticancer agents in vivo.