The cytochrome P450 (CYP) enzyme system is involved in the metabolism and elimination of numerous widely used drugs. The capacity of this system varies from one person to another, leading to variable drug excretion rates and intersubject differences in the final serum drug concentrations. For this reason, therapeutic response and side-effects vary widely between patients treated with the same dose of drug. The intersubject variability in metabolic rate is largely determined by genetic factors. Some CYP enzymes, including CYP2D6 and CYP2C19, are genetically polymorphic. Several mutant alleles have been described. Environmental factors such as smoking, diet and co-administration of medications might also influence the CYP enzyme activity. By the use of genotyping or phenotyping methods every individual can be classified as either a poor, an intermediate, an extensive or an ultrarapid metabolizer. If this could be performed prior to drug therapy, the knowledge could be applied to drug selection and dose adjustment in order to reach therapeutic serum drug levels.
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