METABOLIC INTERACTIONS OF METHOXSALEN AND COUMARIN IN HUMANS AND MICE

Methoxsalen (8-methoxypsoralen) is a very potent inhibitor of human cy tochrome P450 2A6 (CYP2A6) and mouse Cyp2a-5-mediated coumarin 7-hydro xylation in vitro. To determine the effect of methoxsalen on coumarin 7-hydroxylation in humans in vivo, five subjects were given 45 mg of m ethoxsalen and 5 mg of coumarin. Methoxsalen inhibited in vivo coumari n metabolism by 47 +/- 9.2% (mean +/- SEM). Methoxsalen was metabolize d in human liver microsomes at the rate of 50-100 pmol/ mg protein/min (approx. 30% of the activity in mouse liver microsomes). Metabolism w as not inhibited by the anti-Cyp2a-5 antibody in human liver microsome s. 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 microsome s, methoxsalen metabolism was inhibited by the anti-Cyp2a-5 antibody. Cyp2a-5 protein expressed in the yeast Saccharomyces cerevisiae was ca pable of metabolizing methoxsalen, indicating that methoxsalen is a su bstrate of Cyp2a-5. Although kinetic studies indicated that the inhibi tion of coumarin 7-hydroxylation by methoxsalen is competitive in huma n liver microsomes, methoxsalen does not appear to be a substrate for CYP2A6. Methoxsalen and coumarin have the potential of strong metaboli c interactions in man.