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This article is available online at http://dmd.aspetjournals.org ABSTRACT:Selegiline was used as a model compound in a project aimed at comparing, evaluating, and integrating different in vitro approaches for the prediction of cytochrome P450 (P450)-catalyzed hepatic drug metabolism in humans (EUROCYP). Metabolic predictions were generated using homology modeling, cDNA-expressed P450 enzymes, human liver microsomes, primary cultured human hepatocytes, and precision-cut human liver slices. All of the in vitro systems correctly indicated the formation of two dealkylated metabolites, desmethylselegiline and methamphetamine. The metabolic instability of selegiline was demonstrated by all of the in vitro systems studied. Estimates of clearance varied from 16 l/h to 223 l/h. With the exception of one approach, all systems underpredicted the in vivo clearance in humans (236 l/h). Despite this, all approaches successfully classified selegiline as a high clearance compound. Homology modeling suggested the participation of CYP2B6 in the demethylation of selegiline and of CYP2D6 in the depropargylation of the drug. Studies with recombinant expressed enzymes and with human hepatic microsomal fraction supported the involvement of CYP2B6 but not of CYP2D6. These techniques also suggested the involvement of CYP1A2, CYP2C8, and CYP2C19 in the biotransformation of selegiline. In vitro, CYP2B6 was the most active form of P450 involved in selegiline metabolism. Metabolism by several enzymes operating in parallel implies a low interaction potential for the drug. None of the techniques alone was able to predict all aspects of the metabolic and kinetic behavior of selegiline in vivo. However, when used as an integrated package, all significant characteristics were predictable.For more than a decade, in vitro techniques have had a well established role in the early phases of drug development in predicting the pharmacokinetic and metabolic behavior of new chemical entities. When properly used, predictions based on results from in vitro studies will save both development costs and time. In addition, they greatly reduce the number of experimental animals used by the industry in absorption, distribution, metabolism, and excretion studies. Support for the increased utilization of in vitro techniques in drug metabolism studies has come from regulatory authorities (CDER/FDA, 1997). Most of these techniques are easily applicable to high throughput screening, which has further promoted their use recently (Rodrigues, 1997).Several in vitro methods for metabolic predictions are in common use, and even commercially available, but little attention has been paid to the validation of the systems in terms of quality of the predictions obtained and their usefulness in the process of drug development. In particular, comparisons of the data produced by different systems have been scarce. The present study was part of the EUROCYP project within the Biomed2 (Framework IV) program of the European Union. The goals of the project were to evaluate, compare, a...

Rifampin markedly reduces the plasma concentrations of tamoxifen and toremifene by inducing their CYP3A4-mediated metabolism. Concomitant use of rifampin or other potent inducers of CYP3A4 with tamoxifen and toremifene may reduce the efficacy of these antiestrogens.