Received for publication April 1, 2005.
Revised August 30, 2005.
Accepted for publication August 31, 2005.

Pathways of Carbamazepine Bioactivation In Vitro II. The Role of Human Cytochrome P450 Enzymes in the Formation of 2-Hydroxyiminostilbene

Abstract

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 (CYPs) directly to CBZ-IQ, followed by NADPH-mediated reduction to 2-OHIS. In vitro studies were conducted to identify the CYPs 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 (HLM) was consistent with monophasic, Michaelis-Menten kinetics. The sample-to-sample variation in the rate of 2-OHIS formation correlated significantly (r²0.706) with CYP3A4/5 and CYP2B6 activities in a panel of HLM (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 HLM, whereas inhibitors of other CYPs resulted in 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.

Key words: adverse drug reactions, bioactivation, CYP3A, cytochrome P450, human CYP enzymes, hypersensitivity

This article has been cited by other articles:

				W. Lu and J. P. Uetrecht Peroxidase-Mediated Bioactivation of Hydroxylated Metabolites of Carbamazepine and Phenytoin Drug Metab. Dispos., August 1, 2008; 36(8): 1624 - 1636. [Abstract] [Full Text] [PDF]

				R. E. Pearce, W. Lu, Y. Wang, J. P. Uetrecht, M. A. Correia, and J. S. Leeder Pathways of Carbamazepine Bioactivation in Vitro. III. The Role of Human Cytochrome P450 Enzymes in the Formation of 2,3-Dihydroxycarbamazepine Drug Metab. Dispos., August 1, 2008; 36(8): 1637 - 1649. [Abstract] [Full Text] [PDF]

				P. Kang, M. Liao, M. R. Wester, J. S. Leeder, R. E. Pearce, and M. A. Correia CYP3A4-Mediated Carbamazepine (CBZ) Metabolism: Formation of a Covalent CBZ-CYP3A4 Adduct and Alteration of the Enzyme Kinetic Profile Drug Metab. Dispos., March 1, 2008; 36(3): 490 - 499. [Abstract] [Full Text] [PDF]