To cite this article: Neuroendocrinol Lett 2009; 30(Suppl 1):92–95 ORIGINAL ARTICLE Neuroendocrinology Letters Volume 30 Suppl. 1 2009 Evaluation of the activity of P450 enzymes in rats: use of the single marker or combined drug administration Jan Jurica 1 , Michal Kyr 3 , Eva McCaskey Hadasova 1 , Josef Tomandl 2 1 Department of Pharmacology, Faculty of Medicine, Masaryk University, Brno, Czech Republic 2 Department of Biochemistry, Faculty of Medicine, Masaryk University, Brno, Czech Republic 3 Department of Children’s Oncology, Faculty Hospital Brno, Czech Republic Correspondence to: Josef Tomandl, MSc, Ph.D., Dept. of Biochemistry, Kamenice 5, Brno, 625 00, Czech Republic. tel: +420-549 497 930, e-mail: tomandl@med.muni.cz Submitted: 2009-08-28 Accepted: 2009-09-08 Published online: 2009-10-05 Key words: P450; cocktail; marker; phenacetin; tolbutamide; rat; liver perfusion Neuroendocrinol Lett 2009; 30(Suppl.1):92–95 PMID: 20027151 NEL300709A14 © 2009 Neuroendocrinology Letters • www.nel.edu Abstract OBJECTIVES: A “cocktail” of several substrates is frequently used to assess meta- bolic activity of multiple cytochrome P450 enzymes in one session. Some interac- tions among substrates can appear and may influence the rate of biotransforma- tion of other ones. Our current work was aimed on the influence of tolbutamide on cytochrome P450-mediated metabolism of phenacetin and vice versa. DESIGN: In the presented work, the biotransformation rates of phenacetin and tolbutamide (markers of rat CYP1A2 and CYP2C6/11 metabolic activities, respec- tively) administered either separately or both simultaneously were compared. The model of isolated perfused rat liver was used. RESULTS: Phenacetin had no significant effect on tolbutamide hydroxylation. Tolbutamide addition to the perfusion medium significantly increased the rate of O-deethylation of phenacetin. CONCLUSION: Some differences in the rate of P450-mediated metabolism can be observed when comparing assessment using combination of two model substrates with the common way (single marker administration). Due to these differences, results obtained by the mentioned methodologies might not be fully comparable. INTRODUCTION Cytochrome P450 enzyme (CYP) metabolic activ- ity may be influenced by many endogenous and exogenous factors, especially by co-administered drugs (Krizkova et al. 2008). Metabolic activity of various CYP’s is most often measured using selective substrate of distinct P450 enzyme. In early phase of drug discovery, there are utilized so called “high-throughput” methods increasing the efficiency and effectiveness of assay to assess metabolic activity of many P450 enzymes in short time (Smith et al. 2007; Testino & Patonay, 2003; Zlokarnik et al. 2005). There are often applied several substrates simultaneously, so as the activ- ity of multiple P450 enzymes could be measured (so called “cocktail approach”) (Petsalo et al. 2008; Scott et al. 1999; Sharma et al. 2004; Smith et al. 2007; Tanaka et al. 2003; Testino & Patonay, 2003; Yao et al. 2007; Zhang et al. 2008; Zlokarnik et al. 2005). Some pharmacodynamic-based drug-drug interactions and adverse effects can appear in combination of the substrates in vivo. A “cocktail” approach can open a further question - can one marker influence the biotransformation rate of the other one? The aim of the present work was to compare biotransformation rate of phenacetin (PHE) and tolbutamide (TB) (markers of CYP1A2