Prediction of Metabolic Activity From Genotype: The Gene–Dose Effect of N-Acetyltransferase Peter Meisel,* Dorit Arndt,* Eberhard Scheuch,* Klaus-Jürgen Klebingat,† and Werner Siegmund* Departments of *Pharmacology and †Urology, Ernst Moritz Arndt University Greifswald, Greifswald, Germany Summary: Metabolic activity of the polymorphic N-acetyltransferase (NAT2) is de- termined by the mutation pattern of the NAT2 gene. This results in interindividual differences in the metabolic capacity (the phenotype), with continuous distribution of the activities rather than qualitative distinction between rapid and slow acetylators. To determine whether the phenotype might be predicted solely from the mutation pattern of NAT2, quantitative relationships were calculated between mutation patterns of the NAT2 gene and the phenotype of NAT2 assessed either in vitro or in vivo. Healthy volunteers were examined for the velocity at which they metabolized sulfamethazine, and human liver cytosols were measured for NAT2 enzymatic activity, obtaining in vivo and in vitro metabolic phenotype, respectively. Typing of the NAT2 gene was performed by polymerase chain reaction, restriction fragment length analysis, or allele- specific polymerase chain reaction. Multiple linear regression analysis provided quan- titative relationships between allelic pattern and the NAT2 activities measured in vivo and in vitro. Estimates showed the influence of particular allelic configurations on enzyme activity in vitro and the extent of acetylation of the probe drug in vivo, resulting in a strict gene–dose effect. Comparison of in vitro results with in vivo phenotyping figures showed a high degree of correspondence, indicating that the one is the reflection of the other. Key Words: Arylamine N-acetyltransferase— Polymorphism—Genotype—Phenotype—Human liver. Polymorphisms of drug-metabolizing enzymes have a major impact on the fate of xenobiotic substances, whether used as medical drugs or taken up from the environment. Thus, studies on these polymorphisms are undertaken to individualize drug therapy or to evaluate genetic predisposition for exposure-related risks in dis- ease development. Polymorphisms of drug-metabolizing enzymes deter- mine interindividual differences in their function in vitro as well as in the elimination rate of their substrates in vivo. Traditionally, such differences are viewed qualita- tively, leading to classification of subjects as rapid or slow, extensive or poor metabolizers (sometimes also as intermediate). This approach was justified at a time when only procedures of phenotyping were available to distin- guish between different metabolic rates of drug disap- pearance. With the advent of genotyping methods, it be- came clear that many drug-metabolizing enzymes are coded by genes bearing multiple mutations or allelic combinations. Slow acetylator phenotype is a common genetic trait in humans. It results from the presence of several muta- tions in one of the genes encoding for arylamine N- acetyltransferases (1). N-acetyltransferase-2 (NAT2) is capable of metabolizing drugs and carcinogenic sub- stances containing an arylamine or hydrazine moiety. Expression of this isozyme is accomplished mainly in the liver and is coded by 27 different alleles occurring in different frequency distributions. In white subjects, only seven of these alleles have an epidemiologic impact. The dominant wild-type allele NAT2*4 expresses a Received May 23, 2000; accepted September 7, 2000. Address correspondence and reprint requests to Peter Meisel, De- partment of Pharmacology, Ernst Moritz Arndt University Greifswald, F.-Loeffler.Str. 23d, D-17487 Greifswald, Germany; e-mail: meiselp@uni-greifswald.de. Therapeutic Drug Monitoring 23:9–14 © 2001 Lippincott Williams & Wilkins, Inc., Philadelphia 9