Chemical inhibitors that are selective for specific human cytochrome P450 (P4501) enzymes are essential tools in the determination of the contribution of these enzymes in the metabolism of new chemical entities (Ring and Wrighton, 2000). While potent, specific inhibitors are available for many of the human P450 enzymes (e.g., quinidine for CYP2D6, furafylline for CYP1A2, etc.), such a compound has not been available for CYP2C19. S-Mephenytoin has been used as an inhibitor for CYP2C19, however this compound is a substrate for this enzyme, and high concentrations are required for its use as an inhibitor (Yoshimoto et al., 1995; Mankowski, 1999). Omeprazole has been used as a CYP2C19 inhibitor, but it also inhibits CYP2C9 (Ko et al., 1997). Ticlopidine has also been recently used as an inhibitor of CYP2C19, however this compound is not selective for this enzyme as it also is a potent inhibitor of CYP2D6 (Mankowski, 1999; Ko et al., 2000). Although these agents can be used for inhibition of CYP2C19, they are not optimal.
In a recent report in this journal, the synthesis and testing ofN-3-benzylnirvanol and N-3-benzylphenobarbital enantiomers as inhibitors of human cytochrome P450 enzymes were described (Suzuki et al., 2002). (+)N-3-Benzylnirvanol was shown to be a potent inhibitor of CYP2C19 catalyzedS-mephenytoin 4′-hydroxylase activity in human liver microsomes with a Ki value of 0.25 μM, while not appreciably inhibiting CYP1A2, 2A6, 2C8, 2C9, 2D6, 2E1, and 3A4 activities. Thus, (+)N-3-benzylnirvanol appeared to be a promising tool for selective inhibition of CYP2C19.
However, the potential for (+)N-benzylnirvanol to inhibit human CYP2B6 was not tested in the aforementioned report. SinceN-benzylnirvanol is an analog of nirvanol, the metabolite arising from CYP2B6 catalyzed mephenytoin metabolism (Heyn et al., 1996; Ekins et al., 1998), we had concerns that this promising new inhibitor may lack true selectivity for CYP2C19 and cause appreciable inhibition of CYP2B6. To this end, we prepared (+)N-benzylnirvanol according to the described procedure (Suzuki et al., 2002), and tested it as an inhibitor of CYP2B6-catalyzed bupropion hydroxylase activity, a marker activity for this P450 enzyme (Faucette et al., 2000; Hesse et al., 2000), as well as other human P450 enzymes using standard enzyme-specific marker activities.
We confirmed the results reported by Suzuki et al. (2002). (+)N-Benzylnirvanol potently inhibited CYP2C19 activity (IC50 = 0.41 μM) and did not inhibit CYP1A2, 2A6, 2C9, 2D6, and 3A activities in human liver microsomes at concentrations of up to 30 μM. (+)N-Benzylnirvanol mildly inhibited CYP2B6 (IC50 = 58 μM), yielding an approximate 140X selectivity between CYP2C19 and CYP2B6. Our conclusion is that (+)N-benzylnirvanol possesses appropriate potency and selectivity as an inhibitor for CYP2C19 and should be the agent of choice when determining whether CYP2C19 is involved in the metabolism of a new drug or chemical.
Footnotes
- Abbreviations used are::
- P450
- cytochrome P450
- Received November 19, 2002.
- Accepted November 20, 2002.
- The American Society for Pharmacology and Experimental Therapeutics