PT - JOURNAL ARTICLE AU - Jae-Wook Ko AU - Zeruesenay Desta AU - David A. Flockhart TI - Human <em>N</em>-Demethylation of (<em>S</em>)-Mephenytoin by Cytochrome P450s 2C9 and 2B6 DP - 1998 Aug 01 TA - Drug Metabolism and Disposition PG - 775--778 VI - 26 IP - 8 4099 - http://dmd.aspetjournals.org/content/26/8/775.short 4100 - http://dmd.aspetjournals.org/content/26/8/775.full SO - Drug Metab Dispos1998 Aug 01; 26 AB - 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)-MPN-demethylation suggested two contributing activities. A high-affinity/low-capacity component exhibited aKM of 174.1 μM and aVmax of 170.5 pmol/mg protein/min, whereas a low-affinity/high-capacity component exhibited aKM of 1911 μM and aVmax 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 &gt;1000 μM, we estimate that &gt;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 &gt;1000 μM are used, but it is unlikely to be a suitable phenotyping tool for this isoform in vivo, where concentrations of &gt;1000 μM are rarely encountered. The American Society for Pharmacology and Experimental Therapeutics