Conversion of the
carbamazepine metabolite
3-hydroxycarbamazepine (3-OHCBZ) to the
catechol 2,3-dihydroxycarbamazepine (2,3-diOHCBZ) followed by subsequent oxidation to a reactive o-
quinone species has been proposed as a possible bioactivation pathway in the pathogenesis of
carbamazepine-induced
hypersensitivity. Initial in vitro phenotyping studies implicated
CYP3A4 as a primary catalyst of 2,3-diOHCBZ formation: 2-hydroxylation of 3-OHCBZ correlated significantly (r(2) > or = 0.929, P < 0.001) with
CYP3A4/5 activities in a panel of human liver microsomes (n = 14) and was markedly impaired by
CYP3A inhibitors (>80%) but not by inhibitors of other
cytochrome P450 enzymes (< or = 20%). However, in the presence of
troleandomycin, the rate of 2,3-diOHCBZ formation correlated significantly with
CYP2C19 activity (r(2) = 0.893, P < 0.001) in the panel of human liver microsomes. Studies with a panel of
cDNA-expressed
enzymes revealed that
CYP2C19 and
CYP3A4 were high (S50 = 30 microM) and low (S50 = 203 microM) affinity
enzymes, respectively, for 2,3-diOHCBZ formation and suggested that
CYP3A4, but not
CYP2C19, might be inactivated by a metabolite formed from 3-OHCBZ. Subsequent experiments demonstrated that preincubation of 3-OHCBZ with human liver microsomes or recombinant
CYP3A4 led to decreased
CYP3A4 activity, which was both preincubation time- and concentration-dependent, but not inhibited by inclusion of
glutathione or
N-acetylcysteine.
CYP3A4,
CYP3A5, CYP3A7,
CYP2C19, and
CYP1A2 converted [14C]3-OHCBZ into
protein-reactive metabolites, but
CYP3A4 was the most catalytically active
enzyme. The results of this study suggest that CYP3A4-dependent secondary oxidation of 3-OHCBZ represents a potential
carbamazepine bioactivation pathway via formation of reactive metabolites capable of inactivating
CYP3A4, potentially generating a neoantigen that may play a role in the etiology of
carbamazepine-induced idiosyncratic toxicity.