Cytochrome p450 pharmacogenetic testing

Abstract

The cytochrome P450 (CYP450) family of enzymes catalyse the metabolism of many drugs and xenobiotics. The genes that code for these enzymes are polymorphic which can significantly affect drug metabolism in certain individuals. CYP2D6, CYP2C19 and CYP2C9 are responsible for the metabolism of a large number of commonly prescribed drugs, including warfarin, clopidogrel, codeine, tamoxifen and some antidepressants. Over 70 allelic variants have been described for the gene that codes CYP2D6. These variants result in four distinct phenotypes: poor metabolisers, intermediate metabolisers, extensive metabolisers and ultrarapid metabolisers. Both the CYP2C9 and the CYP2C19 genes have two major variant alleles that result in enzyme deficiency. Differences in drug metabolism due to CYP450 gene variants influence plasma levels of both the active drug and its metabolites. Poor metabolisers treated with drugs that are metabolised by these enzymes are at increased risk for prolonged therapeutic effect or toxicity, while ultrarapid metabolisers may not achieve therapeutic plasma levels. Laboratories providing pharmacogenetic testing for the CYP450 genes are required to screen for 20 or more clinically relevant variants in a cost-effective manner, with expected turn-around times that make the test clinically useful. Although direct DNA sequencing remains the gold standard for DNA variant detection, it is often unsuitable for routine use, as the number of variants screened lie across multiple genes and involve many exons. Strategies to genotype common allelic variants in CYP2D6 will be presented, including single base extension, real-time PCR and microarray analysis. The cytochrome P450 (CYP450) family of enzymes catalyse the metabolism of many drugs and xenobiotics. The genes that code for these enzymes are polymorphic which can significantly affect drug metabolism in certain individuals. CYP2D6, CYP2C19 and CYP2C9 are responsible for the metabolism of a large number of commonly prescribed drugs, including warfarin, clopidogrel, codeine, tamoxifen and some antidepressants. Over 70 allelic variants have been described for the gene that codes CYP2D6. These variants result in four distinct phenotypes: poor metabolisers, intermediate metabolisers, extensive metabolisers and ultrarapid metabolisers. Both the CYP2C9 and the CYP2C19 genes have two major variant alleles that result in enzyme deficiency. Differences in drug metabolism due to CYP450 gene variants influence plasma levels of both the active drug and its metabolites. Poor metabolisers treated with drugs that are metabolised by these enzymes are at increased risk for prolonged therapeutic effect or toxicity, while ultrarapid metabolisers may not achieve therapeutic plasma levels. Laboratories providing pharmacogenetic testing for the CYP450 genes are required to screen for 20 or more clinically relevant variants in a cost-effective manner, with expected turn-around times that make the test clinically useful. Although direct DNA sequencing remains the gold standard for DNA variant detection, it is often unsuitable for routine use, as the number of variants screened lie across multiple genes and involve many exons. Strategies to genotype common allelic variants in CYP2D6 will be presented, including single base extension, real-time PCR and microarray analysis.