Pathways of Carbamazepine Bioactivation in Vitro I. Characterization of Human Cytochromes P450 Responsible for the Formation of 2- and 3-Hydroxylated Metabolites

• Published in: Drug metabolism and disposition Vol. 30; no. 11; pp. 1170 - 1179
• Main Authors: PEARCE, Robin E, VAKKALAGADDA, Gangadhara R, LEEDER, J. Steven
• Format: Journal Article
• Language: English
• Published: Bethesda, MD American Society for Pharmacology and Experimental Therapeutics 01-11-2002
• Subjects: Anticonvulsants - pharmacokinetics; Anticonvulsants. Antiepileptics. Antiparkinson agents; Biological and medical sciences; Biotransformation; Carbamazepine - pharmacokinetics; Chromatography, High Pressure Liquid; Cytochrome P-450 Enzyme System - biosynthesis; Cytochrome P-450 Enzyme System - genetics; Cytochrome P-450 Enzyme System - metabolism; DNA, Complementary - metabolism; Humans; Hydroxylation; In Vitro Techniques; Medical sciences; Microsomes, Liver - enzymology; Neuropharmacology; Pharmacology. Drug treatments; Regression Analysis; Human; Hydroxylation; Digestive system; Isozyme; Metabolite; Cytochrome P450; Liver; Anticonvulsant; Tricyclic compound; Microsome; Metabolism; In vitro; Carbamazepine; Dibenzazepine derivatives
• ISSN: 0090-9556; 1521-009X
• DOI: 10.1124/dmd.30.11.1170

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 μM) 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, ∼1640 and ∼217 μM; apparent V max values, ∼5.71 and ∼46.9 pmol/mg of protein/min, respectively). Rates of carbamazepine 2- and 3-hydroxylation correlated strongly
with CYP2B6 activity ( r ≥ 0.757) in a panel of HLMs ( n = 8). Carbamazepine 3-hydroxylation also correlated significantly with CYP2C8 activity at a carbamazepine concentration of
30 μM. 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 (∼30%), but P450s CYP1A2, 2A6, 2B6, and 3A4 also made significant contributions (∼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.