A Novel Model for the Prediction of Drug-Drug Interactions in Humans Based on in Vitro Cytochrome P450 Phenotypic Data

Abstract

ABSTRACT:Ketoconazole has generally been used as a standard inhibitor for studying clinical pharmacokinetic drug-drug interactions (DDIs) of drugs that are primarily metabolized by CYP3A4/5. However, ketoconazole at therapeutic, high concentrations also inhibits cytochromes P450 (P450) other than CYP3A4/5, which has made the predictions of DDIs less accurate. Determining the in vivo inhibitor concentration at the enzymatic site is critical for predicting the clinical DDI, but it remains a technical challenge. Various approaches have been used in the literature to estimate the human hepatic free concentrations of this inhibitor, and application of those to predict DDIs has shown some success. In the present study, a novel approach using cryopreserved human hepatocytes suspended in human plasma was applied to mimic the in vivo concentration of ketoconazole at the enzymatic site. Ketoconazole is an antifungal medicine and is also a potent CYP3A4/5 (CYP3A4) inhibitor in humans. Ketoconazole is widely used in vitro as a selective CYP3A4 inhibitor at low concentrations (Pelkonen et al., 1998;Li et al., 1999). However, at high concentrations it also inhibits cytochrome P450 (P450) isozymes other than CYP3A4 (Venkatakrishnan et al., 2001;Stresser et al., 2004) and UDP glucuronosyltransferase (Yong et al., 2005), as well as transporters such as sodium taurocholate cotransporting polypeptide, Pglycoprotein (Pgp), breast cancer resistance protein, and multidrug resistance-associated protein 2 (Azer et al., 1995;Salphati and Benet 1998, Achira et al., 1999;Xia et al., 2005b). For the purpose of overcoming possible drug resistance, clinical usage of ketoconazole involves much higher concentrations (ϳ25-fold) than its in vitro efficacious concentration (Schaefer-Korting et al., 1984). For studying the worst-case scenario of CYP3A4-mediated drug-drug interaction (DDI) potential on a compound, dosing of ketoconazole at 200 mg b.i.d. is recommended by the Pharmaceutical Research and Manufacturers of America (Bjornsson et al., 2003). At such a high dose, the maximum plasma concentration (C max ) could reach as high as 20 M (Rochlitz et al., 1988;Hsu and Chen, 1997). Other dose regimens, such as 200 mg/day, have also been reported. These doses showed plasma C max ranging from 3.2 to 10 M with most of the values about 10 M (Schaefer- Korting et al., 1984;Ito et al., 1998;Pelkonen et al., 1998;Hardman et al., 2001;Blanchard et al., 2004). Ketoconazole treatment has been found to completely inhibit CYP3A4 activity in vivo (Obach et al., 2006), but the cross-inhibition to P450s other than CYP3A4 has not yet been thoroughly shown in vivo as it has been in vitro. The cross-P450 isozyme inhibition by ketoconazole in vivo is usually overlooked when predicting DDIs between ketoconazole and CYP3A4 substrates, and ketoconazole is still treated as a selective CYP3A4 inhibitor in vivo.P450-reactive phenotyping is a quantitative measurement of the relative contributions of each P450 to the overall metabolism of a drug, when this drug is primar...

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