PT  - JOURNAL ARTICLE
AU  - Kilicarslan, Tansel
AU  - Haining, Robert L.
AU  - Rettie, Allan E.
AU  - Busto, Usanda
AU  - Tyndale, Rachel F.
AU  - Sellers, Edward M.
TI  - Flunitrazepam Metabolism by Cytochrome P450s 2C19 and 3A4
DP  - 2001 Apr 01
TA  - Drug Metabolism and Disposition
PG  - 460--465
VI  - 29
IP  - 4
4099  - http://dmd.aspetjournals.org/content/29/4/460.short
4100  - http://dmd.aspetjournals.org/content/29/4/460.full
SO  - Drug Metab Dispos2001 Apr 01; 29
AB  - We have identified CYP2C19 and CYP3A4 as the principal cytochrome P450s involved in the metabolism of flunitrazepam to its major metabolites desmethylflunitrazepam and 3-hydroxyflunitrazepam. Human CYP2C19 and CYP3A4 mediated the formation of desmethylflunitrazepam with Km values of 11.1 and 108 μM, respectively, and 3-hydroxyflunitrazepam withKm values of 642 and 34.0 μM, respectively. In human liver microsomes (n = 4) formation of both metabolites followed biphasic kinetics. Desmethylflunitrazepam formation was inhibited 31% byS-mephenytoin and 78% by ketoconazole, suggesting involvement of both CYP2C19 and CYP3A4. Formation of 3-hydroxyflunitrazepam was also significantly inhibited by ketoconazole (94%) and S-mephenytoin (18%). In support of these chemical inhibition data, antibodies directed against CYP2C19 and CYP3A4 selectively inhibited formation of desmethylflunitrazepam by 26 and 45%, respectively, while anti-CYP3A4 antibodies reduced 3-hydroxyflunitrazepam formation by 80%. Our data also suggest that CYP1A2, -2B6, -2C8, -2C9, -2D6, and -2E1 are not involved in either of these metabolic pathways. We estimate that the relative contributions of CYP2C19 and CYP3A4 to the formation of desmethylflunitrazepam in vivo are 63 and 37%, respectively, at therapeutic flunitrazepam concentrations (0.03 μM). We conclude that the polymorphic enzyme CYP2C19 importantly mediates flunitrazepam demethylation, which may alter the efficacy and safety of the drug, while CYP3A4 catalyzes the formation of 3-hydroxyflunitrazepam. The American Society for Pharmacology and Experimental Therapeutics