Polymorphic drug metabolism.
The three best-described genetic polymorphisms of drug metabolism--the debrisoquin/sparteine type of oxidative polymorphism (hereafter referred to as the debrisoquin polymorphism), the polymorphism of N-acetylation, and the mephenytoin type of oxidative polymorphism--are reviewed. For all three polymorphisms, the poor-metabolizer phenotype is inherited as an autosomal recessive trait. The debrisoquin and mephenytoin oxidative polymorphisms involve defects in two separate cytochrome P450 enzymes. The prevalence of the poor-metabolizer phenotype for debrisoquin ranges between 2% and 10% for groups of various ethnic origins. The poor-metabolizer phenotype for mephenytoin comprises about 5% of the Caucasian population and about 20% of the Japanese population. N-acetyltransferase is a cytosolic enzyme whose clinical polymorphism was discovered using isoniazid as the substrate probe. The prevalence of the slow-acetylator phenotype among American and European Caucasian and American black groups is about 50%; among the Japanese it is about 10%. More than 20 agents are substrates for debrisoquin hydroxylase, about 15 for N-acetyltransferase, and 3-5 for mephenytoin. In poor metabolizers, debrisoquin can cause hypotension, and sparteine can cause blurred vision, headache, and dizziness. Clinical consequences of the slow-acetylator phenotype include increased susceptibility to systemic lupus erythematosus induced by procainamide and hydralazine, peripheral neuropathy induced by isoniazid, hydralazine, and dapsone, and sulfasalazine-induced dose-related leukopenia, nausea, vomiting, headache, and vertigo. After administration of mephenytoin, poor metabolizers have increased somnolence and intellectual impairment. Awareness of genetic polymorphisms of drug metabolism should improve understanding of interindividual variability in drug disposition and response.