title: Peroxidase-mediated bioactivation of hydroxylated metabolites of carbamazepine and phenytoin.
abstract: Carbamazepine (CBZ) and phenytoin (PHN) are associated with a relatively high incidence of idiosyncratic drug reactions. Most such reactions are believed to be due to reactive metabolites. The reactions associated with these two drugs are similar, and if a patient has a reaction to one, he or she is at increased risk of having a reaction to the other, suggesting that a similar reactive metabolite may be involved. CBZ causes neutropenia in approximately 10% of patients; this suggests that reactive metabolites are formed by myeloperoxidase (MPO), the major oxidative enzyme in neutrophils. Major metabolites of CBZ are the 2- and 3-OH metabolites, and that of PHN is the 4-OH metabolite. We found that both 2-OH-CBZ and 3-OH-CBZ were further oxidized by MPO/H2O2, and the oxidation of 3-OH-CBZ was much faster than the oxidation of 2-OH-CBZ or CBZ itself. Oxidation by MPO formed dimers of 3-OH-CBZ and 4-OH-PHN and, in the presence of N-acetyltyrosine, cross dimers were formed. This strongly suggests free radical intermediates. Bioactivation of 3-OH-CBZ and 4-OH-PHN by MPO/H2O2 led to covalent binding to the tyrosine of a model protein. Free radicals usually generate reactive oxygen species (ROS). We also tested the ability of these metabolites to generate ROS and found that 3-OH-CBZ generated more ROS than 2-OH-CBZ, which was, in turn, greater than that generated by CBZ. These results suggest that bioactivation of 3-OH-CBZ and 4-OH-PHN to free radicals by peroxidases may play a role in the ability of these drugs to cause idiosyncratic drug reactions.
authors.affiliation: Leslie Dan Faculty of Pharmacy, University of Toronto, 144 College Street, Toronto, ON, Canada.
authors.forename: Wei
authors.initials: W
authors.lastname: Lu
authors.forename: Jack P
authors.initials: JP
authors.lastname: Uetrecht
mesh_terms: D000818:Animals; D000927:Anticonvulsants / Q000493:pharmacokinetics*; D001711:Biotransformation; D002220:Carbamazepine / Q000493:pharmacokinetics*; D002460:Cell Line; D002851:Chromatography, High Pressure Liquid; D019281:Dimerization; D006735:Horseradish Peroxidase / Q000378:metabolism; D006900:Hydroxylation; D009682:Magnetic Resonance Spectroscopy; D051379:Mice; D018384:Oxidative Stress; D009195:Peroxidase / Q000037:antagonists & inhibitors / Q000378:metabolism*; D010672:Phenytoin / Q000493:pharmacokinetics*; D021241:Spectrometry, Mass, Electrospray Ionization; D053719:Tandem Mass Spectrometry
chemical_list: D000927:Anticonvulsants; D002220:Carbamazepine; D010672:Phenytoin; D006735:Horseradish Peroxidase; D009195:Peroxidase