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Vol. 26, Issue 8, 775-778, August 1998

Human N-Demethylation of (S)-Mephenytoin by Cytochrome P450s 2C9 and 2B6

Jae-Wook Ko, Zeruesenay Desta, and David A. Flockhart

Division of Clinical Pharmacology, Departments of Medicine and Pharmacology, Georgetown University Medical Center

We tested the ability of human liver microsomes (HLMs) and recombinant human cytochrome P450 (CYP or P450) isoforms to catalyze the N-demethylation of nirvanol-free (S)-mephenytoin [(S)-MP] in vitro. In mixed HLMs, the kinetics of (S)-MP N-demethylation suggested two contributing activities. A high-affinity/low-capacity component exhibited a KM of 174.1 µM and a Vmax of 170.5 pmol/mg protein/min, whereas a low-affinity/high-capacity component exhibited a KM of 1911 µM and a Vmax of 3984 pmol/mg protein/min. The activity of the high-affinity component was completely abolished by sulfaphenazole, with little effect on the low-affinity component. Of the recombinant P450 isoforms tested, only CYP2B6 and CYP2C9 formed nirvanol from (S)-MP. The KM value (150 ± 42 µM) derived for recombinant CYP2C9 was close to that obtained for the high-affinity/low-capacity component in mixed HLMs (KM = 174.1 µM). The predicted contribution of this activity at concentrations (1-25 µM) achieved after a single 100-mg dose of racemic MP is approximately 30% of the rate of nirvanol formation. At concentrations of >1000 µM, we estimate that >90% of the rate can be explained by the low-affinity activity (CYP2B6). Therefore, the N-demethylation of (S)-MP to nirvanol may be a useful means of probing the activity of CYP2B6 in vitro when concentrations of >1000 µM are used, but it is unlikely to be a suitable phenotyping tool for this isoform in vivo, where concentrations of >1000 µM are rarely encountered.

Copyright © 1998 by The American Society for Pharmacology and Experimental Therapeutics


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