ABSTRACT:Whereas ketoconazole is often used to study the worst-case scenario for clinical pharmacokinetic drug-drug interactions (DDIs) for drugs that are primarily metabolized by CYP3A4, fluconazole is considered to be a moderate inhibitor of CYP3A4, providing assessment of the moderate-case scenario of CYP3A-based DDIs. Fluconazole is also a moderate inhibitor of CYP2C9 and CYP2C19. For predicting clinical DDIs using conventional approaches, determining the in vivo inhibitor concentration at the enzymatic site [I], a critical parameter, is still not practical. In our previous study, a novel method involving hepatocyte suspension in plasma was used to circumvent the need to determine the elusive [I] value. In this study, the CYP1A2, 2C9, 2C19, 2D6, and 3A4 activities remaining in the presence of fluconazole were determined in human hepatocytes suspended in human plasma, covering a range of fluconazole clinical plasma concentrations (C avg and C max ). Because the protein-binding effect of fluconazole is expected to be close to that in vivo, the inhibition observed in vitro will be similar to that in vivo. This inhibition information was then applied to the cytochrome P450 (P450) phenotypic data to predict DDIs. Using the available P450 phenotypic information on theophylline, tolbutamide, omeprazole, S-warfarin, phenytoin, cyclosporine, and midazolam and that determined in this study for sirolimus and tacrolimus, we found that the predictions for area under the curve increases for most of these drugs in the presence of fluconazole were remarkably similar (within 35%) to the observed clinical values. This study proves the general applicability of our approach using human hepatocyte incubation in human plasma to predict DDIs.Fluconazole is an antifungal agent being widely used for oropharyngeal candidiasis and coccidioidal meningitis (Hardman et al., 2001). Fluconazole is also a moderate inhibitor of multiple cytochromes P450 (P450s) in humans, such as CYP2C9, CYP2C19, and CYP3A4 (Niwa et al., 2005a). Fluconazole shows linear pharmacokinetics (Balant, 1981;Balani et al., 2006), high gastrointestinal absorption, low plasma protein binding, and low metabolic clearance. It is cleared largely via renal excretion (Ͼ90%) (Debruyne, 1997). In addition, fluconazole is found to inhibit UDP glucuronosyltransferases (Trapnell et al., 1998;Uchaipichat et al., 2006), as well as transporters such as Pgp (Kodawara et al., 2001). One advantage of fluconazole over other antifungal drugs such as ketoconazole and itraconazole is that it can "freely" penetrate into tissues including cerebrospinal fluid (Lazar and Wilner, 1990). In the clinic, for the purpose of overcoming possible drug resistance, the usage of fluconazole is often very high. At a 200 mg b.i.d. dose, the maximum plasma concentration (C max ) could reach as high as 34.6 M (Hardman et al., 2001). Whereas ketoconazole is often used to study the worst-case scenario of CYP3A4-mediated drug-drug interaction (DDI) potential for a drug candidate, fluconazole can be considere...

ABSTRACT:Traditional cytochrome P450 (P450) based drug-drug interaction (DDI) predictions are based on the ratio of an inhibitor's physiological concentration [I] and its inhibition constant K i . Determining [I] at the enzymatic site, although critical for predicting clinical DDIs, remains a technical challenge. In our previous study, a novel approach using cryopreserved human hepatocytes suspended in human plasma was investigated to mimic the in vivo concentration of ketoconazole at the enzymatic site (Lu et al., 2007), effectively eliminating the estimation of the elusive [I] value. P450 inhibition in this system appears to model that in vivo. Using the ketoconazole inhibition information in a human hepatocyte-plasma suspension together with quantitative P450 phenotypic information, we successfully predicted the pharmacokinetic DDIs for a small set of drugs, such as theophylline, tolbutamide, omeprazole, desipramine, midazolam, loratadine, cyclosporine, and alprazolam, as well as an investigational compound. For the applicability of this model on a wider scale the in vitro-in vivo correlation data set needed to be expanded. However, for most drugs in the literature there is not enough quantitative information on the involvement of individual P450s to predict DDIs retrospectively. To facilitate that, in this study we determined quantitative P450 phenotyping for seven marketed drugs: budesonide, buprenorphine, loratadine, sirolimus, tacrolimus, docetaxel, and methylprednisolone. Augmentation of the new data set with the one generated previously produced broader a database that provided further support for the wider applicability of this approach using ketoconazole as a potent CYP3A inhibitor. This application is predicted to be equally effective with other P450 inhibitors that are not substrates of efflux pumps.The prediction of pharmacokinetic drug-drug interactions (DDIs) for humans in the recent past has relied heavily on the ratio [I]/K i , use of which has shown some success. However, to date its broader applicability has not been demonstrated, mainly because of the elusive parameter [I], which signifies the free inhibitor concentration at the enzyme site. Because the value cannot be determined directly, scientists have resorted to finding the next best parameter to estimate that concentration, as described in the previous communication (Lu et al., 2007). But the applicability of one method over another generally leads to not-so-useful predictions. Thus, use of the [I]/K i ratio to predict drug-drug interaction remains a challenge.We recently proposed a new method for DDI prediction that enabled us to avoid the dependence on [I]/K i ratio (Lu et al., 2007). In our method, various concentrations of the inhibitor ketoconazole were incubated in human hepatocytes that were suspended in human plasma to construct an inhibition titration curve. After equilibrium, the major P450 activities remaining were measured using prototypical substrates. It was assumed that if an extracellular (plasma) concentration of that...

The great emphasis on ethical and humane treatment of animals in biomedical research has culminated in the promulgation of the rule of 3Rs - Replacement, Reduction, and Refinement. We have proposed an addition to the 3Rs - a fourth R for Recycling the animal. In drug discovery single-dose pharmacokinetic studies in rats, each animal is generally used only once and then euthanized. A reduction in the number of rats used in this high-throughput era can be readily implemented by reusing animals, just as larger animals are reused in multiple pharmacokinetic studies, consequently reducing the overall number of animal lives sacrificed in research. We provide evidence here for the reproducibility of pharmaco-kinetic studies of tolbutamide and fluconazole, used as test compounds, in rats receiving once-weekly oral and intravenous doses for 4 weeks, proving that recycling rats for multiple single low-dose pharmacokinetic studies is a viable option.