Peroxidase-Mediated Bioactivation of Hydroxylated Metabolites of Carbamazepine and Phenytoin

Abstract

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/H 2 O 2 , 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/H 2 O 2 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.Carbamazepine (CBZ) and phenytoin (PHN) are effective and widely used anticonvulsants. However, they are associated with a relatively high incidence of idiosyncratic drug reactions (IDR) including a syndrome called anticonvulsant hypersensitivity syndrome (Haruda, 1979;Askmark and Wiholm, 1990). The syndrome is similar for both drugs (Vittorio and Muglia, 1995) with fever, skin rash, and lymphadenopathy being most common, and other organs such as the liver are also often affected. The rash can range from a mild maculopapular exanthem to Stevens-Johnson syndrome or toxic epidermal necrolysis. Neutropenia and agranulocytosis can also occur in CBZ-and PHN-treated patients (Turner, 1960;Hart and Easton, 1982;Joffe et al., 1985). In addition to the similarity in the syndromes associated with these two anticonvulsants, there is cross-sensitivity among these anticonvulsants, that is, up to 80% of patients who have an IDR to one of these drugs will also have an adverse reaction to the other, which suggests that a common pathogenic mechanism is involved (Shear and Spielberg, 1988). Such cross-sensitivity has not been observed between these agents and other anticonvulsants such as valproate and benzodiazepines (Hyson and Sadler, 1997).There is a large amount of circumstantial evidence to suggest that most such IDRs are due to reactive metabolites rather than the parent drug . When a reactive metabolite is found for a drug that causes IDRs it is often assumed that this r...

Pathways of Carbamazepine Bioactivation in Vitro. III. The Role of Human Cytochrome P450 Enzymes in the Formation of 2,3-Dihydroxycarbamazepine

“…It is possible that CYP3A4 converts 3-OHCBZ into metabolites other than (or in addition to) CBZ-quinone that are sufficiently reactive to inactivate CYP3A4 directly. Evidence is presented in Lu and Uetrecht (2008) for myeloperoxidase-and H 2 O 2 -mediated formation of one or more free radical intermediates from 3-OHCBZ (depicted in Fig. 1).…”

Phenytoin induced severe agranulocytosis and hepatitis

“…This generation of ROS, initiated within secondsafter stimulation of neutrophils and lasting for hours, leadsto the production ofhypochlorous acid, HOCl.This acid represents a major system for oxidizing a susceptible drug to a reactive product. That may be covalentbound to cellular molecules and, subsequently, the complex can serve as a hapten, inducing antibodiesmainly directed against neutrophils andneutrophil precursors in the bone marrow 2,3 .…”

Normales Knochenmark und häufige reaktive Veränderungen

“…4c). Die besondere Empfindlichkeit der Myelopoiese gegenüber Medikamenten, speziell Arylamine, wird mit der aktiven intrazellulä-ren Metabolisierung durch die Myeloperoxidase und der In-situ-Generation von freien Radikalen erklärt [23]. Paraneoplastische Neutropenien bei T-LGL sind oft von einer deutlichen Linksverschiebung der Myelopoiese begleitet [12].…”