Conversion of the
carbamazepine metabolite,
2-hydroxycarbamazepine, to the potentially reactive species,
carbamazepine iminoquinone (CBZ-IQ), has been proposed as a possible bioactivation pathway in the pathogenesis of
carbamazepine-induced
hypersensitivity. Generation of CBZ-IQ has been proposed to proceed through the intermediate,
2-hydroxyiminostilbene (2-OHIS); however, data suggested that
2-hydroxycarbamazepine is oxidized by
cytochromes P450 (P450s) directly to CBZ-IQ, followed by
NADPH-mediated reduction to 2-OHIS. In vitro studies were conducted to identify the P450s responsible for converting
2-hydroxycarbamazepine to 2-OHIS and to determine functional consequences of this bioactivation pathway. Formation of 2-OHIS in human liver microsomes (HLMs) was consistent with monophasic, Michaelis-Menten kinetics. The sample-to-sample variation in the rate of 2-OHIS formation correlated significantly (r2 > or = 0.706) with
CYP3A4/5 and
CYP2B6 activities in a panel of HLMs (n = 10). Studies with a panel of
cDNA-expressed
enzymes revealed that
CYP3A4 preferentially catalyzed 2-OHIS formation;
CYP3A4 formed 2-OHIS at a rate >10 times that of other
enzymes capable of forming 2-OHIS (
CYP1A1,
CYP2C19, and CYP3A7). Inhibitors of
CYP3A enzymes markedly impaired 2-OHIS formation in HLMs, whereas inhibitors of other P450s resulted in < or = 20% inhibition. Although
CYP3A4 was primarily responsible for converting
2-hydroxycarbamazepine to 2-OHIS, neither
2-hydroxycarbamazepine, 2-OHIS, nor CBZ-IQ caused time-dependent inactivation of
CYP3A activity. No
thiol adducts were formed directly from
2-hydroxycarbamazepine. However,
glutathione- and
N-acetylcysteine-conjugates were formed with 2-OHIS or CBZ-IQ as substrates. Thus, CYP3A4-dependent secondary oxidation of
2-hydroxycarbamazepine represents a potential
carbamazepine bioactivation pathway leading to the formation of
thiol-reactive metabolites, intermediates that may play a role in the etiology of idiosyncratic toxicity attributed to
carbamazepine.