Many undesired side-effects or therapeutic failures of drugs are the results of differences or changes in drug-metabolism, primarily depending on the levels and activities of cytochrome P450 (CYP) enzymes. In order to assess whether CYP-expression profiles can reflect the hepatic drug-metabolism, we compared CYP mRNA levels in the liver or in peripheral leukocytes with the corresponding hepatic CYP-activities. A preliminary CYP-genotyping for the most frequent polymorphisms in Caucasian populations (CYP2C9*2, CYP2C9*3, CYP2C19*2, CYP2C19*3, CYP2D6*3, CYP2D6*4, CYP2D6*6 and CYP3A5*3) was carried out before CYP-phenotyping, excluding the donors with non-functional alleles of CYP2C9, CYP2C19 and CYP2D6, and those with functional CYP3A5*1 allele from a correlation analysis. The hepatic mRNA levels of CYP1A2, CYP2B6, CYP2C9, CYP2C19, CYP2D6 and CYP3A4 displayed a strong association with CYP-activities in the liver, whereas the expression of CYP1A2, CYP2C9, CYP2C19 and CYP3A4 in leukocytes was proven to reflect the hepatic activities of these CYP species. The leukocytes were found to be inappropriate cells for the assessment of hepatic CYP2B6 and CYP2D6 activities. Combining the results of CYP-genotyping and CYP-phenotyping analyses, patients' drug-metabolizing capacities can be estimated by the CYP-expression in the liver and also in leukocytes, with some limitations. Patients' genetic and non-genetic variations in CYP-status can guide the appropriate selection of drugs and the optimal dose, minimizing the risk of harmful side-effects and ensuring a successful outcome of drug therapy.
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