title: Pathways of carbamazepine bioactivation in vitro I. Characterization of human cytochromes P450 responsible for the formation of 2- and 3-hydroxylated metabolites.
abstract: In vitro studies were conducted to identify the cytochromes P450 (P450s) involved in the formation of 2- and 3-hydroxycarbamazepine, metabolites that may serve as precursors in the formation of protein-reactive metabolites. Human liver microsomes (HLMs) converted carbamazepine (30-300 microM) to 3-hydroxycarbamazepine at rates >25 times those of 2-hydroxycarbamazepine. Both the 2- and 3-hydroxylation of carbamazepine appeared to conform to monophasic Michaelis-Menten kinetics in HLMs (apparent K(m) values, approximately 1640 and approximately 217 microM; apparent V(max) values, approximately 5.71 and approximately 46.9 pmol/mg of protein/min, respectively). Rates of carbamazepine 2- and 3-hydroxylation correlated strongly with CYP2B6 activity (r >or= 0.757) in a panel of HLMs (n = 8). Carbamazepine 3-hydroxylation also correlated significantly with CYP2C8 activity at a carbamazepine concentration of 30 microM. Formation of 2- and 3-hydroxycarbamazepine did not correlate significantly with any other P450 activities. The chemical inhibitors ketoconazole (CYP3A) and 7-EFC (CYP2B6) inhibited both 2- and 3-hydroxycarbamazepine formation whereas 4-methylpyrazole (CYP2E1) markedly decreased 2-hydroxycarbamazepine formation. Several recombinant P450s catalyzed carbamazepine 2- and 3-hydroxylation, but after adjustment for relative hepatic abundance, CYP3A4 and CYP2B6 appeared to be the major catalysts of carbamazepine 3-hydroxylase activity, and at least five P450s were significant contributors to 2-hydroxycarbamazepine formation; CYP2E1 made the greatest contribution to the Cl(int) of carbamazepine 2-hydroxylation (approximately 30%), but P450s CYP1A2, 2A6, 2B6, and 3A4 also made significant contributions (approximately 13-18%). These results suggest that CYP2B6 and CYP3A4 are largely responsible for the formation of 3-hyrdoxycarbamazepine, whereas multiple P450s (CYP1A2, 2A6, 2B6, 2E1, and 3A4) contributed to 2-hydroxycarbamazepine formation.
authors.affiliation: Section of Developmental Pharmacology and Experimental Therapeutics, Division of Pediatric Clinical Pharmacology and Medical Toxicology, Children's Mercy Hospitals and Clinics, Kansas City, Missouri 64108, USA. rpearce@cmh.edu
authors.forename: Robin E
authors.initials: RE
authors.lastname: Pearce
authors.forename: Gangadhara R
authors.initials: GR
authors.lastname: Vakkalagadda
authors.forename: J Steven
authors.initials: JS
authors.lastname: Leeder
mesh_terms: D000927:Anticonvulsants / Q000493:pharmacokinetics*; D001711:Biotransformation; D002220:Carbamazepine / Q000493:pharmacokinetics*; D002851:Chromatography, High Pressure Liquid; D003577:Cytochrome P-450 Enzyme System / Q000096:biosynthesis / Q000235:genetics / Q000378:metabolism*; D018076:DNA, Complementary / Q000378:metabolism; D006801:Humans; D006900:Hydroxylation; D066298:In Vitro Techniques; D008862:Microsomes, Liver / Q000201:enzymology; D012044:Regression Analysis
chemical_list: D000927:Anticonvulsants; D018076:DNA, Complementary; D002220:Carbamazepine; D003577:Cytochrome P-450 Enzyme System