Polymerase chain reaction and its potential as a pharmacokinetic tool.

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Interindividual variability of pharmacokinetic processes such as drug distribution, rates of drug metabolism and elimination results from genetic differences and nongenetic variables including age, sex, liver size, liver function, circadian rhythms, body temperature, as well as nutritional and environmental factors, such as exposure to inducers or inhibitors of drug metabolism. Genetic influence on pharmacokinetics can be either polygenic or of the single-gene type. The latter category can be subdivided into common polymorphisms such as the debrisoquine/sparteine polymorphism or the N-acetyltransferase polymorphism (Meyer et al. 1990) and rare pharmacogenetic traits such as impaired phenytoin metabolism (Inaba 1990) or serum cholinesterase defects (La Du 1989). In the last few years a number of single-gene type pharmacogenetic defects have been investigated on the molecular level and, for some of them, genotyping is already possible (table I). Identification of the 2 alleles of an individual at a specific gene locus is achieved by analysis of restriction fragment length polymorphisms (RFLPs) and with the help of the polymerase chain reaction. Restriction endonucleases recognise specific nucleotide sequences of variable length, known as restriction sites. An average of I in 500 nucleotides differs between 2 randomly selected alleles and, if restriction sites are changed, this variability can be recognised by one of the known restriction endonucleases (Antonarakis 1989). Alleles may then be identified by characteristic fragments after digestion of genomic DNA with a specific endonuclease. These RFLPs may be generated by a primary mutation causing the altered phenotype or by allelic restriction sites in the flanking DNA that are linked to the primary mutation. RFLPs and their application in DNA diagnostics have been reviewed by Gusella (1986). In the past few years a new method has revolutionised DNA analysis: the polymerase chain reaction (PeR).