Oxidative degradation of cardiotoxic anticancer anthracyclines to phthalic acids: novel function for ferrylmyoglobin.

We show that the pseudoperoxidase activity of ferrylmyoglobin(MbIV) promotes oxidative degradation ofdoxorubicin (DOX), an anticancer anthracycline knownto induce severe cardiotoxicity. MbIV, formed in vitro byreacting horse heart MbIII with H2O2, caused disappearanceof the spectrum of DOX at 477 nm and appearanceof UV-absorbing chromophores that indicated openingand degradation of its tetracyclic ring. Electron sprayionization mass spectrometry analyses of DOX/MbIV ultrafiltratesshowed that DOX degradation resulted information of 3-methoxyphthalic acid, the product of oxidativemodifications of its methoxy-substituted ring D.Other methoxy-substituted anthracyclines similarly released3-methoxyphthalic acid after oxidation by MbIV,whereas demethoxy analogs released simple phthalicacid. Kinetic and stoichiometric analyses of reactionsbetween DOX and MbIII/H2O2 or hemin/H2O2 showedthat the porphyrin radical of MbIV-compound I and theiron-oxo moiety of MbIV-compound II were sequentiallyinvolved in oxidizing DOX; however, oxidation by compoundI formed more 3-methoxyphthalic acid than oxidationby compound II. Sizeable amounts of 3-methoxyphthalicacid were formed in the heart of micetreated with DOX, in human myocardial biopsies exposedto DOX in vitro, and in human cardiac cytosol thatoxidized DOX after activation of its endogenous myoglobinby H2O2. Importantly, H9c2 cardiomyocytes weredamaged by low concentrations of DOX but could tolerateconcentrations of 3-methoxyphthalic acid higherthan those measured in murine or human myocardium.These results unravel a novel function for MbIV in theoxidative degradation of anthracyclines to phthalic acidsand suggest that this may serve a salvage pathwayagainst cardiotoxicity.