Polymorphic acetylation and aminopyrine demethylation in Gilbert's syndrome

N-acetyltransferase: the practical consequences of polymorphic activity in man

Over the years, numerous studies have supported the premise that individuals possessing the "slow acetylator" phenotype are more at risk from developing drug side-effects. Most prominent amongst these reports are those concerned with hepatotoxicity and peripheral neuropathy following treatment with isoniazid, lupus-like symptoms during procainamide therapy and experiencing hypersensitivity reactions to the various sulphonamide derivatives. Similarly, "slow acetylators" undergoing heavy exposure to arylamines and related carcinogens are more likely to develop bladder cancer. Contrariwise, there appears a slight risk of "rapid acetylators" developing pancreatic tumours.Other therapeutic agents for which polymorphic N-acetylation plays a minor role in their metabolism have been investigated but any impact of this metabolic difference on clinical efficacy or associated toxicity is still under question. In the search for clues as to the underlying aetiology, patient groups with many disease states have been examined for association with differences in N-acetylation and the majority have provided data that could be interpreted as equivocal. Studies have given contradictory, often opposing, results, calculated risk factors that are (perhaps) just significant but certainly not high, and patients within the cohorts who are always exceptions. Undoubtedly, other as yet unappreciated factors are at play.

[Occupational exposure to isocyanates and individual susceptibility]

Patients with organic diisocyanate-induced pulmonary disease may be specially susceptible to the toxic effects of agent. Among 11 cases diagnosed in one year, the majority (10/11) were slow acetylators. The same patients were different from the control population in terms of alpha-1-antitrypsin phenotypes. Heterozygous combinations were more frequent than among controls. It seems that the combination of low N-acetylation capacity and a heterozygous alpha-1-antitrypsin predisposes to the disease.

N-acetylation and debrisoquine hydroxylation polymorphisms in patients with Gilbert's syndrome

1. N-acetylation and debrisoquine hydroxylation phenotypes were determined in 54 patients with Gilbert's syndrome and in 247 (sulphamethazine) and 76 (debrisoquine) non-related healthy volunteers. 2. Forty (74.1%) of the patients and 135 (54.7%) of the healthy volunteers were slow acetylators (chi 2 = 6.87). In the patients, the cumulative urinary excretion of sulphamethazine up to 6 h (Ae(0,6)) was significantly lower. No differences in the frequency of debrisoquine poor metabolizers were observed: Gilbert's syndrome 5/54 (9.3%), healthy volunteers 5/76 (6.6%). The metabolic ratios were similar in both groups as well as the urinary recoveries of debrisoquine and its 4-hydroxy metabolite. 3. Gilbert's syndrome seems to be related in some way to N-acetylation but not to the debrisoquine hydroxylation polymorphism.

N-acetylation in healthy and diseased children

Acetylation capacity was examined in three groups of Czech children by measuring the plasma and urine concentrations of sulphamethazine and its acetylated metabolite 6 h after an oral test dose of 20 mg/kg sulphamethazine. Amongst 82 healthy children aged 4-15 y there were 32 (39%) fast acetylators; there was no significant difference between the number of boys and girls, or between children over or less than 6 years of age. In 41 patients aged 3-15 y with phenylketonuria, the acetylation indices showed a significantly higher proportion of fast acetylators - 24 (58.5%) using plasma measurements and 29 (70.7%) using urine data. In them the ratio between slow and fast acetylators was inverted compared to normal children. The preponderance of fast acetylators was greater in boys than in girls and in children over 6 years of age. An increased acetylation capacity in patients with phenylketonuria was apparent even in individuals classified as slow acetylators, because in them the plasma concentration of the acetylated metabolite was higher than in control acetylators. Amongst 48 young patients (5-15 y) with insulin-dependent diabetes there were 19 (39.6%) fast and 29 (60.4%) slow acetylators, which corresponded well to the phenotype distribution in control children. This did not support the suggested association between the fast acetylator phenotype and Type I diabetes.

Human arylamine N-acetyltransferase genes: isolation, chromosomal localization, and functional expression

N-Acetylation by hepatic arylamine N-acetyltransferase (NAT, EC 2.3.1.5) is a major route in the metabolism and detoxification of numerous drugs and foreign chemicals. NAT is the target of a common genetic polymorphism of clinical relevance in human populations. We have used our recently isolated rabbit cDNA rnat to clone three human NAT genes from human leukocyte DNA. None of the three genomic coding sequences was interrupted by introns. Two genes, designated NAT1 and NAT2, each possessed open reading frames of 870 bp. Both genes have been assigned to human chromosome 8, pter-q11. Following transfection they were transiently expressed in monkey kidney COS-1 cells. NAT1 and NAT2 gave rise to functional NAT proteins, as judged by their NAT enzyme activity with the arylamine substrate sulfamethazine. Western blots with NAT-specific antisera detected proteins of apparent molecular weight of 33 and 31 kD in NAT1- and NAT2-transfected cultures, respectively. The product of NAT2 had an identical apparent molecular weight as that of NAT detected in human liver cytosol. The deduced amino acid sequence of NAT2 also contained 6 peptide sequences which had previously been determined from tryptic peptides of the polymorphic NAT purified from human liver. These data suggest that NAT2 encodes the polymorphic NAT protein. The third gene, NATP, had multiple deleterious mutations and did not encode a functional NAT protein; it most likely represents a pseudogene.

N-acetylation pharmacogenetics: a gene deletion causes absence of arylamine N-acetyltransferase in liver of slow acetylator rabbits

The New Zealand White rabbit provides a widely used animal model for the human acetylation polymorphism, which confers marked interindividual variation in the effect and toxicity of numerous drugs, chemicals, and potential carcinogens. The relationship of a recently isolated cDNA clone, designated rnat, to genetically polymorphic arylamine N-acetyltransferase (NAT; acetyl-CoA:arylamine N-acetyltransferase, EC 2.3.1.5) of rabbit liver was established by its expression in monkey kidney COS-1 cells: (i) cytosols from transfected cultures contained high levels of an Ac-CoA-dependent NAT activity, which was kinetically indistinguishable from that observed in cytosols from livers of genetically rapid-acetylator rabbits; (ii) transfected cells also contained an immunoreactive protein, recognized by NAT-specific antibodies, with identical electrophoretic mobility to NAT from rabbit liver. The rnat clone and anti-NAT antibodies were then used to study the relationship between NAT activity, liver enzyme protein, and the level of mRNA in livers from in vivo phenotyped rapid- and slow-acetylator rabbits. Livers from slow acetylators were devoid of both immunodetectable NAT protein and its corresponding mRNA. Analysis of genomic DNA with a panel of restriction enzymes revealed the loss of specific hybridizing bands in the DNA of slow-acetylator rabbits. These data strongly suggest that defective arylamine N-acetylation in the rabbit model is caused by a gene deletion resulting in an absence of specific mRNA and NAT enzyme protein.

Gilbert's syndrome

While Gilbert's syndrome is extremely common and benign, its pathogenesis may not be as straightforward as once believed. It has been used as a model to examine aberrations of virtually every step in bilirubin metabolism. The clinical hallmarks are of a hereditary, chronic, mild unconjugated hyperbilirubinaemia. Not infrequently subclinical haemolysis may coexist. Liver histology is normal although some minor ultrastructural abnormalities may be evident. The universal defect appears to be a reduction in hepatic bilirubin-GT activity. However, other associated abnormalities in bilirubin metabolism, which occur less consistently, suggest that this may not be the sole defect in all patients. The syndrome is almost certainly part of a spectrum which includes the Crigler-Najjar syndromes; molecular biology data suggests that there is an absence of one (or even more) GT isoenzymes in these disorders. Whether one or more genes is consistently culpable remains open to speculation. Despite the complicated pathogenesis of Gilbert's syndrome, management remains simply reassurance alone.

Normal pathways for glucuronidation, sulphation and oxidation of paracetamol in Gilbert's syndrome

A group of eleven subjects with Gilbert's syndrome was characterized by conventional tests and determination of bilirubin and its conjugates in plasma by alkaline methanolysis and thin layer chromatography. After a 1 g dose of paracetamol h.s. the drug and its metabolites were measured by high performance liquid chromatography (HPLC) in the overnight 8-h urine sample. The amounts of paracetamol and of its metabolites recovered in urine were almost identical with those found in the control group (n = 10). The glucuronide:paracetamol ratio, which is considered to be an index of glucuronidation, was not correlated with the fraction of bilirubin present in plasma as glucuronides. These data do not suggest that in subjects with Gilbert's syndrome therapeutic doses of paracetamol are associated with an increased risk for hepatic or systemic toxicity.

Thalidomide-induced peripheral neuropathy. A prospective clinical, neurophysiological and pharmacogenetic evaluation

Prospective clinical and electrophysiological follow-up was performed on nine patients under thalidomide treatment in order to detect the very beginning of possible drug-induced peripheral neuropathy. For neurophysiological assessment, nerve conduction measurements of the median, peroneal and sural nerves (7 conduction parameters) and needle EMG examination of the anterior tibial muscle were performed. The results of a first control after about 3 months of treatment were compared with the starting point examination, and the patients were then classified as "affected" or "not affected" according to clinical and neurophysiological criteria. At this point, three patients showed clinical and electrophysiological, and another two only electrophysiological alterations suggesting early neuropathy. This classification did not change after further clinical and electrophysiological controls. Without starting-point values, the early detection of neuropathy would not have been possible in all patients. No single reliable neurophysiological parameter for detection of thalidomide-induced neuropathy could be found. Pharmacogenetic classification with regard to hydroxylation and acetylation phenotypes was then performed in some patients and interpreted with relation to thalidomide neurotoxicity. A possible relationship between slow acetylators and development of thalidomide-induced neuropathy was found.

Effect of concomitant isoniazid administration on determination of acetylator phenotype by sulphadimidine

The influence of concomitant administration of isoniazid (INH) on the acetylation of sulphadimidine has been studied in 6 healthy volunteers, previously identified as having the fast acetylator phenotype. INH was administered in a slow release form (500 mg tablet) 1 hour before the sulphadimidine. Acetylation of sulphadimidine was measured in plasma 6 h after its intake and in urine collected between 5 and 6 hours. INH significantly decreased the acetylated fraction of sulphadimidine in plasma from 69.0 to 54.0 and in urine from 85.9 to 81.2%. This was reflected in a significantly higher plasma concentration of unconjugated sulphadimidine and reduced urinary excretion of acetylated sulphadimidine. It is concluded that concomitant administration of INH inhibits acetylation of sulphadimidine. Fast acetylators at the border line of discrimination, may be misclassified if phenotyped with sulphadimidine during concomitant administration of INH.

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.

Antipyrine clearance in patients with Gilbert's syndrome

The pharmacokinetic parameters of antipyrine (AP) were examined in 45 normal healthy subjects (18 heavy smokers, 5 mild smokers, and 22 nonsmokers) and in 12 patients with Gilbert's syndrome (GS), amongst whom 2 mild and 1 heavy smokers were included. Heavy smokers were defined as persons smoking more than 20 cigarettes/day and mild smokers as those smoking less than 10 cigarettes/day. Significant differences (unpaired Student's t-test) in the elimination t1/2 of AP among the study groups and in its total plasma clearance (CL) were observed without any change in the apparent volume of distribution. The individual CL values varied within the same study groups, but the mean +/- SD (0.026 +/- 0.004 l/h/kg) in the GS patients did not significantly differ from that in normal nonsmokers (0.025 +/- 0.006 l/h/kg) or in normal mild smokers (0.028 +/- 0.001 l/h/kg). When the 3 patients with GS who smoked were excluded, the mean CL of the group (0.025 l/h/kg) was again comparable to that of the normal nonsmokers and mild smokers. The mean (+/- SD) CL in normal heavy smokers (0.040 +/- 0.012 l/h/kg) was significantly greater than in normal mild smokers (p less than 0.05), in normal nonsmokers (p less than 0.001) and in patients with GS (p less than 0.001). The results suggest that drug oxidation capacity estimated from the total plasma CL of AP appears unimpaired in GS.

Polymorphic acetylation of sulphamethazine in rural bedouin and urban-dwellers in Saudi Arabia

1. The polymorphic acetylation of sulphamethazine (sulphadimidine, sulphamezathine) has been investigated in a population of 109 Saudi male arabs of rural bedouin origin and in 126 Saudi female arabs from urban cosmopolitan areas of Jeddah. 2. Rural males excreted 5-79% of the administered dose (1 X 5 g/m2 body surface area) in the 0-12 h urine and the urban females excreted 5-97%. 3. The frequency distribution of the ratio acetyl sulphamethazine/sulphamethazine was bimodal in rural, urban and the combined populations with a clear antimode at 70% acetylation of the recovered dose. 4. The incidence of slow acetylators was 67 X 9, 59 X 5 and 63 X 4% in the rural, urban and combined populations. The incidence of the As allele in Saudi arabs was thus 0 X 80+/-0 X 03 S.E.M., which is similar to that found in the neighbouring countries, of Egypt and Sudan. Since no significant difference in As frequency was apparent between the rural (pure) and urban (cosmopolitan) arabs, it is concluded that immigrants to Saudi Arabia from other muslim countries have not affected the gene frequencies with respect to acetylation. 5. Methodology of assessing acetylation phenotype is discussed. It would appear that urine analysis alone gives satisfactory discrimination between phenotypes.