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The pharmacokinetics of isoniazid in man are described. Pronounced interindividual variation in circulating isoniazid concentration and clearance which occur after dosing with the drug are associated with hereditary differences in the acetylator status. The variations in rate of isoniazid inactivation and elimination in different (rapid and slow) acetylator phenotypes are primarily due to differences in the rate of acetylation of isoniazid by a genetically controlled polymorphic N-acetyltransferase in liver and small intestine. An appreciable 'first-pass' effect is observed following oral isoniazid administration, particularly in the rapid acetylator phenotype. Liver disease can cause a significant prolongation in the clearance of isoniazid; in the acutely ill patient, the prolongation correlates most closely with serum bilirubin elevation, although the degree of prolongation is less important than the intrinsic genetic difference between acetylator phenotypes. The effect of renal impairment on isoniazid excretion is relatively unimportant, even in slow acetylators. Methods for monitoring blood and urine concentrations of isoniazid and for acetylator phenotype determination which are convenient for the patient and clinician are available. Implications of phenotype differences in acetylator status for the optimal management of tuberculosis with isoniazid are considered. Attempts to devise new isoniazid formulations for this purpose are being evaluated.

Classification of humans as rapid or slow acetylators is based on hereditary differences in rates of N-acetylation of therapeutic and carcinogenic agents, but N-acetylation of certain arylamine drugs displays no genetic variation. Two highly homologous human genes for N-acetyltransferase (NAT; arylamine acetyltransferase, acetylCoA:arylamine N-acetyltransferase, EC 2.3.1.5), NAT] and NAT2, presumably code for the genetically invariant and variant NAT proteins, respectively. In the present investigation, 1.9-kilobase human genomic EcoRI fragments encoding NAT2 were generated by the polymerase chain reaction with liver and leukocyte DNA from seven subjects phenotyped as homozygous and heterozygous acetylators. Hepatic cytosolic N-acetyltransferase (NAT; arylamine acetyltransferase, acetyl CoA:arylamine N-acetyltransferase, EC 2.3.1.5) participates in the detoxification of a plethora of hydrazine and arylamine drugs, as well as in activation pathways of occupational carcinogens (1, 2). A reactive cysteinyl thiolate is part of the active site of NAT (3), which displays a ping-pong reaction mechanism with a covalent acetyl-cysteinyl-NAT species as the catalytic intermediate (3, 4). Hereditary differences in N-acetylation activity among individuals and in populations of diverse raciogeographic origin have led to a phenotypic classification of humans as rapid or slow acetylators. This genetic heterogeneity in NAT activity, widely referred to as the N-acetylation polymorphism, is determined in people and several animal species by a single autosomal gene with two major alleles expressed codominantly (2, 5). On the other hand, rates of elimination in vivo and acetylation in vitro of certain drugs (e.g., p-aminosalicylic acid) do not differ appreciably among rapid and slow acetylators, indicating that metabolism of these substrates takes place without genetic variation. The term monomorphic has been coined for genetically invariant N-acetylation activity (ref. 2, pp. 137-138). The molecular mechanisms for monomorphic and polymorphic N-acetylation in humans are not yet fully understood.The amino acid composition reported for several tryptic peptides of electrophoretically homogeneous liver NAT from homozygous rapid acetylator rabbits (4) proved pivotal in the design of oligonucleotide screening probes and isolation of rabbit NAT cDNAs (6, 7), which eventually enabled identification of human NAT cDNA and genomic clones (7,8). The libraries screened for human NAT cDNAs were made with mRNA from two livers of undetermined acetylator phenotype (7). It was inferred from the substrate selectivity profiles of the cDNA products expressed in Chinese hamster ovary cells that two of the isolated cDNA clones coded for the genetically variant NAT and one coded for the monomorphic protein (7). Three NAT gene loci, corresponding to EcoRI fragments of 1.3 kilobases (kb) (NATJ), 1.9 kb (NAT2), and 4.7 kb (NA TP), were identified upon screening of a library constructed with leukocyte DNA from a heterozygous person. Indirect evidence f...

The human N-acetylation polymorphism is a genetic trait phenotypically reflected by differences in N-acetyltransferase (NAT) activity with therapeutic agents (rapid and slow acetylation), but a genetic invariability in N-acetylation of some arylamine drugs is also known. There are two highly similar human NAT genes: NAT1 is thought to encode a genetically invariant protein, whereas NAT2 has conclusively been shown to represent a polymorphic locus. This study demonstrates the presence of discrete NAT1 structural variants among Caucasians. These were detected by direct sequencing of 1.6-kilobase NAT1 fragments generated by the polymerase chain reaction with liver and leukocyte DNA from 13 subjects of established acetylator phenotype and NAT2 genotype. A prominent alteration in one of the variants was obliteration of the consensus polyadenylation signal (AATAAA-->AAAAAA). Several mutations were discernible in all regions of the second variant allele, including silent (codon 153) and nonsilent (Ser-214-->Ala) substitutions in the coding region and deletion of nine bases from an AT-rich segment in the 3' untranslated region. One-half of the unrelated subjects were either homozygous or heterozygous for the mutant NAT1 alleles, both of which obeyed a Mendelian inheritance pattern. These novel results unambiguously show that human NAT1, like NAT2, is a polymorphic locus.

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