The genetic origin of responses to drugs.

Individual variation in drug metabolism has been extensively investigated. Population studies have shown that there is a wide range in metabolising ability for all detoxification pathways; the distributions may be unimodal (Gaussian) or polymodal, with subsets of individuals who differ from the majority. These may be poor metabolisers (PM) or extensive metabolisers (EM). In many cases, these phenotypes can be linked with the genotype. Frequently the PM phenotype is more susceptible to drug toxicity, while the EM phenotype requires increased dosage for therapeutic benefit. In some cases, phenotypes or genotypes appear to have increased susceptibility to clinical disease. These ideas are discussed for the cytochrome P-450 isozymes, FMO system, cysteine dioxygenase-linked oxidations, glucuronidation, sulphation, acetylation, glutathione conjugation and methylation pathways.