Acetylator phenotype and serum levels of sulfapyridine in patients with inflammatory bowel disease

N-acetyltransferase 1 and 2 genotypes do not predict response or toxicity to treatment with mesalamine and sulfasalazine in patients with ulcerative colitis

OBJECTIVES

5-Aminosalicylate is metabolized in colonic mucosa by N-acetyltransferase 1 (NAT1), and sulfapyridine is metabolized in the liver by N-acetyltransferase 2 (NAT2). Common genetic polymorphisms in these enzymes result in rapid and slow acetylation. We determined the association between NAT1 genotype and response to mesalamine and sulfasalazine, as well as between NAT2 genotype and toxicity to sulfasalazine, in a population-based cohort of patients with ulcerative colitis.

METHODS

DNA was obtained from 77 white patients with ulcerative colitis from Olmsted County, MN. NAT1 and NAT2 genotyping was performed using microelectronic array devices. Phenotypes were deduced from previously published genotype/phenotype correlations. Clinical response to mesalamine and sulfasalazine, and toxicity to sulfasalazine, were determined by medical record review and associated with NAT1 and NAT2 genotypes.

RESULTS

The clinical response rates among 52 patients treated with mesalamine were 67% (31 of 46) for rapid acetylators and 83% (five of six) for slow acetylators (odds ratio = 0.4, 95% CI = < 0.1-3.9, p = 0.65). Similarly, the clinical response rates among 64 patients treated with sulfasalazine were 74% (43 of 58) for rapid acetylators and 67% (four of six) for slow acetylators (odds ratio = 1.4, 95% CI = 0.2-8.6, p = 0.65). The toxicity rates among the 64 patients treated with sulfasalazine were 34% (12 of 35) for slow acetylators and 45% (13 of 29) for rapid acetylators (odds ratio = 0.6, 95% CI = 0.2-1.8, p = 0.65).

CONCLUSIONS

NAT1 and NAT2 genotypes did not predict response to mesalamine or sulfasalazine, or toxicity to sulfasalazine.

Genetic polymorphisms of human N-acetyltransferase, cytochrome P450, glutathione-S-transferase, and epoxide hydrolase enzymes: relevance to xenobiotic metabolism and toxicity

In this review, an overview is presented of the current knowledge of genetic polymorphisms of four of the most important enzyme families involved in the metabolism of xenobiotics, that is, the N-acetyltransferase (NAT), cytochrome P450 (P450), glutathione-S-transferase (GST), and microsomal epoxide hydrolase (mEH) enzymes. The emphasis is on two main topics, the molecular genetics of the polymorphisms and the consequences for xenobiotic metabolism and toxicity. Studies are described in which wild-type and mutant alleles of biotransformation enzymes have been expressed in heterologous systems to study the molecular genetics and the metabolism and pharmacological or toxicological effects of xenobiotics. Furthermore, studies are described that have investigated the effects of genetic polymorphisms of biotransformation enzymes on the metabolism of drugs in humans and on the metabolism of genotoxic compounds in vivo as well. The effects of the polymorphisms are highly dependent on the enzyme systems involved and the compounds being metabolized. Several polymorphisms are described that also clearly influence the metabolism and effects of drugs and toxic compounds, in vivo in humans. Future perspectives in studies on genetic polymorphisms of biotransformation enzymes are also discussed. It is concluded that genetic polymorphisms of biotransformation enzymes are in a number of cases a major factor involved in the interindividual variability in xenobiotic metabolism and toxicity. This may lead to interindividual variability in efficacy of drugs and disease susceptibility.

Predominance of slow acetylators of N-acetyltransferase in a Hmong population residing in the United States

Pharmacogenetics can be an important determinant of pharmacologic response. To learn more about interpopulation differences in drug metabolism between ethnically diverse populations of subjects cared for by an International Clinic, a study was conducted to describe the prevalence of fast or slow acetylators of N-acetyltransferase (NAT2) in a population of Hmong residing in Minnesota. Ninety-eight healthy Hmong refugees from Laos volunteered to take caffeine as an oral probe drug to establish acetylator phenotype. Participants were classified as either rapid or slow acetylators based on the urinary molar ratio of select metabolites of caffeine. Assignment of phenotype was based on results from analysis of urine collected subsequent to ingestion of caffeine. The ratio of 5-acetylamino-6-formylamino-3-methyluracil (AFMU) to the combined products of the 7-demethylation pathway of paraxanthine (AFMU, 1-methylxanthine (1X), and 1-methylurate (1U)] formed the basis for this determination. A probit plot of the data collected in our subjects qualified a metabolic ratio of 0.34 as an acceptable cut point for phenotype assignment. Participants with an AFMU/(AFMU + 1X + 1U) ratio of < 0.34 were classified as slow acetylators and all others as rapid acetylators. Analysis of the data suggested a bimodal distribution with an excess (74.5%) of slow acetylators in the population. The predominance of slow acetylators found in the Hmong contrast with the prevalence of slow acetylators seen in other ethnic groups. These findings may have important clinical implications given the large number of Hmong treated each year in our International Clinic and the increasing use of medications metabolized by NAT2 in this population.

Genetically Determined Polymorphisms in Drug Metabolism

Many factors can influence the metabolism and disposition of drugs. Genetically determined differences in an individual's capacity to metabolize drugs are known causes of interindividual and interethnic variabilities in drug disposition and response. In general, a poor metabolizer for a specific metabolic pathway would likely develop adverse effects, and an extensive metabolizer for the same metabolic pathway might have less than optimal response. Although there are different types of polymorphism in drug metabolism, polymorphisms in debrisoquine-type oxidation, S-mephenytoin oxidation, and N-acetylation have been the most extensively studied. This article will present the basic concepts of pharmacogenetics, review the major types of metabolic polymorphisms, outline ways to determine phenotyping and genotyping differences in metabolizing enzyme activities, and discuss how these differences relate to drug metabolism, response, and toxicity. When evaluating drug response and adverse reactions in individual patients, an awareness of genetic differences in metabolic capacities would help contribute to optimization in drug therapy.

Genetically determined variability in acetylation and oxidation. Therapeutic implications

The clinical significance of two separate genetic polymorphisms which alter drug metabolism, acetylation and oxidation is discussed, and methods of phenotyping for both acetylator and polymorphic oxidation status are reviewed. Particular reference is made to the dapsone method, which provides a simple means of distinguishing fast and slow - and possibly intermediate - acetylators, and to the sparteine method which allows a clear separation of oxidation phenotypes. Although acetylation polymorphism has been known for some time, definite indications for phenotyping are few. It is doubtful whether acetylator phenotype makes a significant difference to the outcome in most isoniazid treatment regimens, and peripheral neuropathy from isoniazid in slow acetylators is easily overcome by pyridoxine administration. However, in comparison with rapid acetylators, slow acetylators receiving isoniazid have an increased susceptibility to phenytoin toxicity, and perhaps also to carbamazepine toxicity. It is also possible that rapid acetylators receiving isoniazid attain higher serum fluoride concentrations from enflurane and similar anaesthetics than do similarly treated slow acetylators. Thus, when drug interactions of these types are suspected, phenotyping for acetylator status may be advisable. If routine monitoring of serum procainamide and N-acetylprocainamide concentrations is practised, phenotyping of subjects prior to therapy with these agents should not be necessary. Although acetylator phenotype influences serum concentrations of hydralazine, when this drug is given in combination with other drugs acetylator phenotype has not been shown to influence the therapeutic response. Slow acetylator phenotype along with female gender and the presence of HLA-DR antigens appear to be risk factors in the development of hydralazine-induced systemic lupus erythematosus (SLE). Determination of acetylator phenotype may therefore help determine susceptibility to this adverse reaction. In the case of sulphasalazine, adult slow acetylators require a lower daily dose of the drug than fast acetylators in order to maintain ulcerative colitis in remission without significant side effects. It is therefore advisable to determine acetylator phenotype prior to sulphasalazine therapy. Work on the association of acetylation polymorphism with various disease states is also reviewed. It is possible that a higher incidence of bladder cancer is associated with slow acetylation phenotype - especially in individuals exposed to high levels of arylamines. The question as to whether idiopathic SLE is more common in slow acetylators remains unresolved. There appears to be no difference between fa

Survey of the human acetylator polymorphism in spontaneous disorders

There is ample evidence that the human acetylator phenotypes are associated with drug induced phenomena. It is principally the slow acetylators who exhibit toxic adverse effects because of their relative inability to detoxify the original drug compounds. In rare instances, however, it is the rapid acetylators who are at a disadvantage. In the matter of association of spontaneous disease with either acetylator phenotype, there are two groups of disorders to consider. First, disorders in which carcinogenic amines are known to be an aetiological factor. This is because these amines are substrates for the polymorphic N-acetyltransferase activity and hence there is a possible rational basis for searching for an association. Secondly, other disorders where searches for associations are based more on hunches. In the first group there is a definite statistical association between cancer of the bladder and the slow acetylator phenotype. In prevalence studies the slow phenotype is 39% more associated with bladder cancer than is the rapid phenotype. On the basis of the evidence now available it is not possible to say whether this association is because slow acetylators develop the disease more frequently or whether they survive longer. In the second group the relevant studies show (1) a greatly increased prevalence of slow acetylators in Gilbert's disease; (2) a confirmed association between the rapid acetylator phenotype and diabetes; (3) a possible association between the rapid acetylator phenotype and breast cancer; (4) a possible association between the slow acetylator phenotype and leprosy in Chinese patients; (5) an earlier age of onset of thyrotoxicosis (Graves' disease) in slow acetylators than in rapid acetylators; (6) no evidence of an association between either phenotype and spontaneous systemic lupus erythematosus.