ABSTRACT:Understanding the potential for cytochrome P450 (P450)-mediated drug-drug interactions is a critical part of the drug discovery process. Factors such as nonspecific binding, atypical kinetics, poor effector solubility, and varying ratios of accessory proteins may alter the kinetic behavior of an enzyme and subsequently confound the extrapolation of in vitro data to the human situation. The architecture of the P450 active site and the presence of multiple binding regions within the active site may also confound in vitro-in vivo extrapolation, as inhibition profiles may be dependent on a specific inhibitor-substrate interaction. In these studies, the inhibition profiles of a set of 24 inhibitors were paneled against the CYP2C19 substrate probes ( The cytochrome P450 superfamily of drug-metabolizing enzymes is involved in the metabolism of the majority of currently prescribed drugs and new chemical entities. Within the P450 superfamily, members of the human CYP2C family (CYP2C8, CYP2C9, CYP2C18, and CYP2C19) are responsible for the metabolism of approximately 20% of marketed drugs (Goldstein, 2001). Of the four clinically relevant CYP2C isoforms noted above, CYP2C9 and CYP2C19 are the most highly conserved, with approximately 91% structural similarity between the two enzymes (Romkes et al., 1991). There is roughly 80% sequence identity among all four CYP2C isoforms (Ridderström et al., 2001). Crystal structures have been published for CYP2C8 and CYP2C9 (Williams et al., 2003;Schoch et al., 2004), although currently only homology models exist for CYP2C18 and CYP2C19 (Ridderström et al., 2001;Suzuki et al., 2004).CYP2C19 is a polymorphic enzyme that accounts for less than 5% of hepatic P450 (Ring et al., 2001) and 2 to 3% of intestinal P450 content (Paine et al., 2006). The CYP2C19 poor metabolizer phenotype is found in approximately 25% of Asians and only 3 to 5% of Caucasians (Rodrigues and Rushmore, 2002). Commonly used drugs that are metabolized by CYP2C19 include proton pump inhibitors such as omeprazole and lansoprazole, psychotropic drugs including diazepam and imipramine, and anticonvulsants such as phenobarbital and mephenytoin. Unlike CYP2D6, another polymorphic drug-metabolizing enzyme, CYP2C19, has also been shown to be inducible by both rifampicin and dexamethasone.In drug discovery, phenotyping new chemical entities for CYP2C19-mediated metabolism as well as for their potential to inhibit CYP2C19 is a common practice because of the potential liabilities of the CYP2C19 polymorphism. In vitro assays to examine drug-drug interactions (DDIs) commonly use (S)-mephenytoin as a probe substrate for CYP2C19-catalyzed reactions, although difficulties can arise from the relatively low rate of turnover to its 4Ј-hydroxy metabolite. According to recent U.S. Food and Drug Administration guidance (http://www.fda.gov/cder/drug/ druginteractions/default.htm), alternative metabolic pathways that can be used as probes of CYP2C19 activity in vitro include omeprazole metabolism to 5-hydroxyomeprazole and the format...