• Jan B Kuhlmann, Georg Wensing, and Jochen Kuhlmann.
• Int J Clin Pharm Th. 2014 Feb 1; 52 (2): 143-50.

IntroductionThe genetic polymorphism of drug metabolizing enzymes of the cytochrome P450 (CYP) families, especially CYP2D6 and CYP2C19, is the most important cause of variable responses of many drugs. Enzyme activity ranges from complete deficiency, so called poor metabolizers (PMs), to an ultrafast metabolism. While PMs and extensive metabolizers (EMs) can be well distinguished by genotyping, phenotyping is necessary to subdivide EMs from intermediate metabolizers (IMs). The aim of the study was to evaluate if messenger RNA (mRNA) concentration for CYP-enzymes in peripheral blood leukocytes (PBLs) will be predictive of systemic enzyme activity, allowing an easy and safe determination of metabolic activity.MethodsThe genotype, phenotype, and mRNA-expression in PBLs were evaluated in 124 healthy Caucasian volunteers (males and females, age range 23 - 59 years) on three occasions (every 4 weeks). Genotyping was performed by Taqman allelic discrimination on the most common null alleles for CYP2D6 (*3, *4, *6, *7, and *8) and CYP2C19 (*2 and *3). For phenotyping CYP2D6, dextromethorphan/dextrorphan metabolic ratios were determined in collected urine (8 hours) after administration of 30 mg dextromethorphan. For phenotyping CYP2C19, we used the plasma concentration ratio of omeprazole/hydroxyomeprazole 4 hours after ingestion of 40 mg omeprazole. mRNA-expression in PBLs for CYP2D6 and CYP2C19 was measured by Taqman real-time PCR before medication and 4 hours afterwards.ResultsGenotyping for CYP2D6 and CYP2C19 showed a regular distribution of EMs and PMs compared to studies of a comparable population. The median dextromethorphan/dextrorphan metabolic ratio was 0.47 in EMs/IMs and 2.29 in PMs. The median omeprazole/hydroxyomeprazole metabolic ratio was 3.06 in EMs/IMs and 35.29 in PMs. CYP2D6 and CYP2C19 mRNA expression was detected without evidence of correlation to the respective metabolic ratio.ConclusionThe results do not support the concept of using mRNA expression profiles for CYP2D6 and CYP2C19 enzymes in PBLs for prediction of systemic enzyme activity.

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