[Pharmacogenetics and pharmacogenomics].

Genetic polymorphisms can lead to drug adverse effects because certain allelic variants of genes that encode enzymes, targets or carriers involved in drug metabolism, are associated with an increase or a loss of function. Drug metabolism takes place essentially in the liver and is regulated by phase I enzymes (including several cytochrome P450 isoenzymes), the role of which is to make drug metabolites more polar by hydroxylation, and by phase II enzymes that catalyse conjugation reactions. Cytochromes P450 isoenzymes control 80% of oxidative reactions, owing to their low substrate specificity. The most extensively studied polymorphisms are those of CYP2D6 and CYP2C9, which are frequent and affect the metabolism of many drugs. For example, several CYP2C9 gene variants are associated with lower activity of the corresponding enzyme, potentially leading to drug overdose. Thiopurine methyl transferase and UDP-glucuronyl transferase are phase II enzymes that conjugate respectively 6-mercaptopurine metabolites with a methyl radical and metabolites of irinotecan (an anti-tumour drug) with a glucuronyl radical. Mutations in the corresponding genes can, through a loss of function, lead to excessively high levels of active metabolites, with a risk of bone marrow aplasia. The action of vitamin K antagonists is influenced by polymorphisms of vitamine K epoxyde reductase, the target molecule of vitamin K. A mutation in the methylene tetrahydrofolate reductase gene diminishes the folate pool and thereby increases the effects of methotrexate, a folic acid antagonist. Resistance to the anti-platelet effect of aspirin can be due to polymorphisms that affect other platelet aggregation pathways. Genotyping results must be confirmed by phenotyping, which examines the rate of transformation of a drug into its metabolites and shows whether the increase or decrease in this rate is linked to a specific polymorphism. Somatic mutations in malignant tumours can modify the response to anticancer drugs, and should therefore be taken into account. The National Academy of Medicine recommends:--development of genomic analyses for frequent polymorphisms in patients warranting treatment with drugs that have severe adverse effects;--collection of DNA samples from patients participating in drug trials in order to examine the possible relation between adverse effects and gene polymorphisms;--creation of biological resource centers in hospitals for the storage and analysis of tissue specimens.