title: Metabolic interactions of methoxsalen and coumarin in humans and mice.
abstract: Methoxsalen (8-methoxypsoralen) is a very potent inhibitor of human cytochrome P450 2A6 (CYP2A6) and mouse Cyp2a-5-mediated coumarin 7-hydroxylation in vitro. To determine the effect of methoxsalen on coumarin 7-hydroxylation in humans in vivo, five subjects were given 45 mg of methoxsalen and 5 mg of coumarin. Methoxsalen inhibited in vivo coumarin metabolism by 47 +/- 9.2% (mean +/- SEM). Methoxsalen was metabolized in human liver microsomes at the rate of 50-100 pmol/mg protein/min (approx. 30% of the activity in mouse liver microsomes). Metabolism was not inhibited by the anti-Cyp2a-5 antibody in human liver microsomes. NIH 3T3 cells stably expressing catalytically active CYP2A6 enzyme did not metabolize methoxsalen, indicating that CYP2A6 does not accept methoxsalen as a substrate. In pyrazole-induced mouse liver microsomes, methoxsalen metabolism was inhibited by the anti-Cyp2a-5 antibody. Cyp2a-5 protein expressed in the yeast Saccharomyces cerevisiae was capable of metabolizing methoxsalen, indicating that methoxsalen is a substrate of Cyp2a-5. Although kinetic studies indicated that the inhibition of coumarin 7-hydroxylation by methoxsalen is competitive in human liver microsomes, methoxsalen does not appear to be a substrate for CYP2A6. Methoxsalen and coumarin have the potential of strong metabolic interactions in man.
authors.affiliation: Department of Pharmacology and Toxicology, University of Oulu, Finland.
authors.forename: J
authors.initials: J
authors.lastname: Mäenpää
authors.forename: R
authors.initials: R
authors.lastname: Juvonen
authors.forename: H
authors.initials: H
authors.lastname: Raunio
authors.forename: A
authors.initials: A
authors.lastname: Rautio
authors.forename: O
authors.initials: O
authors.lastname: Pelkonen
mesh_terms: D000818:Animals; D001189:Aryl Hydrocarbon Hydroxylases; D003374:Coumarins / Q000378:metabolism* / Q000493:pharmacokinetics; D065637:Cytochrome P-450 CYP2A6; D065607:Cytochrome P-450 Enzyme Inhibitors; D003577:Cytochrome P-450 Enzyme System / Q000378:metabolism / Q000652:urine; D004347:Drug Interactions; D006801:Humans; D007700:Kinetics; D008297:Male; D008730:Methoxsalen / Q000378:metabolism*; D051379:Mice; D008811:Mice, Inbred DBA; D008862:Microsomes, Liver / Q000378:metabolism; D006899:Mixed Function Oxygenases / Q000037:antagonists & inhibitors / Q000378:metabolism / Q000652:urine
chemical_list: D003374:Coumarins; D065607:Cytochrome P-450 Enzyme Inhibitors; D003577:Cytochrome P-450 Enzyme System; C030123:coumarin; D006899:Mixed Function Oxygenases; D001189:Aryl Hydrocarbon Hydroxylases; C585710:CYP2A6 protein, human; D065637:Cytochrome P-450 CYP2A6; D008730:Methoxsalen