N-acetylation genotype and risk of severe reactions to sulphonamides in AIDS patients

N-acetyltransferase 2 enzyme genotype-phenotype discordances in both HIV-negative and HIV-positive Nigerians

BACKGROUND

The N-acetyltransferase 2 (NAT2) enzyme has been understudied in Nigerians including genotype-phenotype association studies.

OBJECTIVE

The aim of this study was NAT2 haplotype identification and genotype-phenotype investigations in HIV-positive and HIV-negative Nigerians.

PATIENTS AND METHODS

Phenotypes included self-reported sulphonamide hypersensitivity survey, experimental and computational NAT2 phenotyping. The NAT2 gene was amplified by PCR. Gene sequencing used ABI 3730 and Haploview 4.2 for haplotype reconstruction. Genotype-phenotype analyses used the χ P-value and odds ratio with a 95% confidence interval.

RESULTS

Self-reported sulphonamide hypersensitivity showed a prevalence of 3.1 and 12.4% in HIV-positive and HIV-negative Nigerians, respectively. NAT2 genetic variants 191G>A, 282C>T, 341T>C, 481C>T, 590G>A, 803A>G and 857G>A were not significantly different between both groups (odds ratio=0.87; 95% confidence interval: 0.54-1.38, P=0.55). Nine haplotypes: NAT2*4, NAT2*12A, NAT2*13A, NAT2*5B, NAT2*6A, NAT2*7B, NAT2*5C, NAT2*14B and NAT2*14A had frequencies more than 1%, whereas NAT2*12B had 1.1% in the HIV-positive and 0.4% in the HIV-negative group. Overall, slow acetylator haplotypes made up 68%. The NAT2*12 signature single-nucleotide polymorphism was in high linkage disequilibrium with signature single-nucleotide polymorphism for NAT2*13 (D'=0.97, r=0.61) and NAT2*5 (D'=0.98, r=0.64). Genotype-phenotype association analysis showed haplotypes NAT2*13A, NAT2*5C, NAT2*7B and NAT2*14A to be associated strongly with the slow metabolic phenotype (P=0.002, 0.029, 0.032 and 0.050, respectively). Computational phenotypes were similar, with 30.9, 66 and 3.1% for slow, intermediate and rapid acetylators, respectively, among HIV-positive Nigerians and 31.2, 66.3 and 2.5% among the HIV-negative group. Overall, slow phenotypes made up 31%.

CONCLUSION

NAT2 haplotype frequencies are similar in Nigerians, irrespective of HIV status, but genotype-phenotype discordances exist.

Role of Oxidative Stress in Hypersensitivity Reactions to Sulfonamides

Antimicrobial sulfonamides are important medications. However, their use is associated with major immune-mediated drug hypersensitivity reactions with a rate that ranges from 3% to 4% in the general population. The pathophysiology of sulfa-induced drug hypersensitivity reactions is not well understood, but accumulation of reactive metabolites (sulfamethoxazole [SMX] hydroxylamine [SMX-HA] and SMX N-nitrosamine [SMX-NO]) is thought to be a major factor. These reactive metabolites contribute to the formation of reactive oxygen species (ROS) known to cause cellular damage and induce cell death through apoptosis and necroptosis. ROS can also serve as "danger signals," priming immune cells to mount an immunological reaction. We recruited 26 sulfa-hypersensitive (HS) patients, 19 healthy control subjects, and 6 sulfa-tolerant patients to this study. Peripheral blood monocytes and platelets were isolated from blood samples and analyzed for in vitro cytotoxicity, ROS and carbonyl protein formation, lipid peroxidation, and GSH (glutathione) content after challenge with SMX-HA. When challenged with SMX-HA, cells isolated from sulfa-HS patients exhibited significantly (P ≤ .05) higher cell death, ROS and carbonyl protein formation, and lipid peroxidation. In addition, there was a high correlation between cell death in PBMCs and ROS levels. There was also depletion of GSH and lower GSH/GSSG ratios in peripheral blood mononuclear cells from sulfa-HS patients. The amount of ROS formed was negatively correlated with intracellular GSH content. The data demonstrate a major role for oxidative stress in in vitro cytotoxicity of SMX reactive metabolites and indicate increased vulnerability of cells from sulfa-HS patients to the in vitro challenge.

Hepatic expression profiles in retroviral infection: relevance to drug hypersensitivity risk

HIV‐infected patients show a markedly increased risk of delayed hypersensitivity (HS) reactions to potentiated sulfonamide antibiotics (trimethoprim/sulfamethoxazole or TMP/SMX). Some studies have suggested altered SMX biotransformation in HIV infection, but hepatic biotransformation pathways have not been evaluated directly. Systemic lupus erythematosus (SLE) is another chronic inflammatory disease with a higher incidence of sulfonamide HS, but it is unclear whether retroviral infection and SLE share risk factors for drug HS. We hypothesized that retroviral infection would lead to dysregulation of hepatic pathways of SMX biotransformation, as well as pathway alterations in common with SLE that could contribute to drug HS risk. We characterized hepatic expression profiles and enzymatic activities in an SIV‐infected macaque model of retroviral infection, and found no evidence for dysregulation of sulfonamide drug biotransformation pathways. Specifically, NAT1,NAT2,CYP2C8,CYP2C9,CYB5R3,MARC1/2, and glutathione‐related genes (GCLC,GCLM,GSS,GSTM1, and GSTP1) were not differentially expressed in drug naïve SIVmac239‐infected male macaques compared to age‐matched controls, and activities for SMX N‐acetylation and SMX hydroxylamine reduction were not different. However, multiple genes that are reportedly over‐expressed in SLE patients were also up‐regulated in retroviral infection, to include enhanced immunoproteasomal processing and presentation of antigens as well as up‐regulation of gene clusters that may be permissive to autoimmunity. These findings support the hypothesis that pathways downstream from drug biotransformation may be primarily important in drug HS risk in HIV infection.

Genome-Wide Association Study in Immunocompetent Patients with Delayed Hypersensitivity to Sulfonamide Antimicrobials

Background

Hypersensitivity (HS) reactions to sulfonamide antibiotics occur uncommonly, but with potentially severe clinical manifestations. A familial predisposition to sulfonamide HS is suspected, but robust predictive genetic risk factors have yet to be identified. Strongly linked genetic polymorphisms have been used clinically as screening tests for other HS reactions prior to administration of high-risk drugs.

Objective

The purpose of this study was to evaluate for genetic risk of sulfonamide HS in the immunocompetent population using genome-wide association.

Methods

Ninety-one patients with symptoms after trimethoprim-sulfamethoxazole (TMP-SMX) attributable to “probable” drug HS based on medical record review and the Naranjo Adverse Drug Reaction Probability Scale, and 184 age- and sex-matched patients who tolerated a therapeutic course of TMP-SMX, were included in a genome-wide association study using both common and rare variant techniques. Additionally, two subgroups of HS patients with a more refined clinical phenotype (fever and rash; or fever, rash and eosinophilia) were evaluated separately.

Results

For the full dataset, no single nucleotide polymorphisms were suggestive of or reached genome-wide significance in the common variant analysis, nor was any genetic locus significant in the rare variant analysis. A single, possible gene locus association (COL12A1) was identified in the rare variant analysis for patients with both fever and rash, but the sample size was very small in this subgroup (n = 16), and this may be a false positive finding. No other significant associations were found for the subgroups.

Conclusions

No convincing genetic risk factors for sulfonamide HS were identified in this population. These negative findings may be due to challenges in accurately confirming the phenotype in exanthematous drug eruptions, or to unidentified gene-environment interactions influencing sulfonamide HS.

Variability in the population pharmacokinetics of isoniazid in South African tuberculosis patients

AIM

This study was designed to characterize the population pharmacokinetics of isoniazid in South African pulmonary tuberculosis patients.

METHODS

Concentration-time measurements obtained from 235 patients receiving oral doses of isoniazid as part of routine tuberculosis chemotherapy in two clinical studies were pooled and subjected to nonlinear mixed-effects analysis.

RESULTS

A two-compartmental model, including first-order absorption and elimination with allometric scaling, was found to describe the observed dose-exposure relationship for oral isoniazid adequately. A mixture model was used to characterize dual rates of isoniazid elimination. Estimates of apparent clearance in slow and fast eliminators were 9.70 and 21.6 l h(-1) , respectively. The proportion of fast eliminators in the population was estimated to be 13.2%. Central volume of distribution was estimated to be 10% smaller in female patients and clearance was found to be 17% lower in patients with HIV. Variability in absorption rate (90%) was completely interoccasional in nature, whereas in relative bioavailability, interoccasional variability (8.4%) was lower than interindividual variability (26%). Oral doses, given once daily according to dosing policies at the time, were sufficient to reach therapeutic concentrations in the majority of the studied population, regardless of eliminator phenotype. Simulations suggested that current treatment guidelines (5 mg kg(-1) ) may be suboptimal in fast eliminators with low body weight.

CONCLUSIONS

A population pharmacokinetic model was developed to characterize the highly variable pharmacokinetics of isoniazid in a South African pulmonary tuberculosis patient population. Current treatment guidelines may lead to underexposure in rapid isoniazid eliminators.

Variability in drug metabolizing enzyme activity in HIV-infected patients

AIMS

To evaluate variability in cytochrome P450 (CYP) 1A2, CYP2D6, CYP3A, N-acetyltransferase 2 (NAT2), and xanthine oxidase (XO) activity in HIV-infected patients and compare this with data from uninfected, healthy volunteers.

METHODS

Ten HIV-infected men and seven women on medication affecting CYP enzyme activity were phenotyped four times over 2 months using caffeine, dextromethorphan, and midazolam. Urinary caffeine and dextromethorphan metabolite ratios were used to phenotype CYP1A2, NAT2, XO, and CYP2D6 activity and midazolam plasma clearance was used to phenotype CYP3A activity. Plasma and urine samples were analyzed by validated LC/UV or LC/MS methods for midazolam, caffeine, and dextromethorphan. Noncompartmental pharmacokinetics and nonparametric statistical analyses were performed, and the data compared with those of healthy volunteer historic controls.

RESULTS

Compared with age and sex-matched healthy volunteers, HIV-infected subjects had 18% lower hepatic CYP3A4 activity, 90% lower CYP2D6 activity, 53% lower NAT2 activity, and 22% higher XO activity. No significant difference was found in CYP1A2 activity. Additionally, 25% genotype-phenotype discordance in CYP2D6 activity was noted in HIV-infected subjects. Intraindividual variability in enzyme activity increased by 42-62% in HIV-infected patients for CYP1A2, NAT2, and XO, and decreased by 33% for CYP2D6. Interindividual variability in enzyme activity increased by 27-63% in HIV-infected subjects for CYP2D6, CYP1A2, and XO, and decreased by 38% for NAT2. Higher plasma TNFalpha concentrations correlated with lower CYP2D6 and CYP3A4 activity.

CONCLUSIONS

Infection with HIV or stage of HIV infection may alter Phase I and II drug metabolizing enzyme activity. HIV infection was related to an increase in variability of these drug-metabolizing enzymes. Altered metabolism may be a consequence of immune activation and cytokine exposure.

Pharmacogenetics, drug-metabolizing enzymes, and clinical practice

The application of pharmacogenetics holds great promise for individualized therapy. However, it has little clinical reality at present, despite many claims. The main problem is that the evidence base supporting genetic testing before therapy is weak. The pharmacology of the drugs subject to inherited variability in metabolism is often complex. Few have simple or single pathways of elimination. Some have active metabolites or enantiomers with different activities and pathways of elimination. Drug dosing is likely to be influenced only if the aggregate molar activity of all active moieties at the site of action is predictably affected by genotype or phenotype. Variation in drug concentration must be significant enough to provide "signal" over and above normal variation, and there must be a genuine concentration-effect relationship. The therapeutic index of the drug will also influence test utility. After considering all of these factors, the benefits of prospective testing need to be weighed against the costs and against other endpoints of effect. It is not surprising that few drugs satisfy these requirements. Drugs (and enzymes) for which there is a reasonable evidence base supporting genotyping or phenotyping include suxamethonium/mivacurium (butyrylcholinesterase), and azathioprine/6-mercaptopurine (thiopurine methyltransferase). Drugs for which there is a potential case for prospective testing include warfarin (CYP2C9), perhexiline (CYP2D6), and perhaps the proton pump inhibitors (CYP2C19). No other drugs have an evidence base that is sufficient to justify prospective testing at present, although some warrant further evaluation. In this review we summarize the current evidence base for pharmacogenetics in relation to drug-metabolizing enzymes.

Immune response to xenobiotics in the skin: from contact sensitivity to drug allergy

Skin is the most frequent target of adverse drug reactions. These cutaneous drug reactions (CDRs) show varied clinical manifestations ranging from mildly discomforting rashes to life-threatening Stevens-Johnson syndrome or toxic epidermal necrolysis. Most CDRs appear to be immune mediated, although the mechanism by which they are initiated remains unclear. In this review, current knowledge of the mechanisms by which xenobiotics provoke immune responses in the skin after epicutaneous administration and how similar reactions may occur after systemic routes are summarised. This review also discusses a variety of genetic or environmental factors that may determine the susceptibility of individuals towards immune responses in skin following drug exposure.

Slow acetylator phenotype and genotype in HIV-positive patients with sulphamethoxazole hypersensitivity

AIMS

To test the role of acetylator status, and to investigate the reported discrepancy between acetylator phenotype and genotype in HIV-positive patients with sulphamethoxazole (SMX) hypersensitivity.

METHODS

Forty HIV-positive patients (32 of whom were SMX-hypersensitive), and 26 healthy volunteers, were genotyped by polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) analysis, and phenotyped using dapsone (50 mg) as a probe, for acetylator status. Sequencing of the NAT2 exon was performed where discrepancy between phenotyping and genotyping was detected. Our results were also pooled with published studies addressing slow acetylator status in HIV-positive SMX-hypersensitive patients.

RESULTS

Slow acetylator genotype and phenotype frequencies did not differ between HIV-positive SMX-hypersensitive and nonhypersensitive patients, and healthy controls, which was further confirmed in a meta-analysis of published studies (pooled odds ratio 2.25, 95% confidence interval 0.45, 11.17). Discordance between phenotype and genotype was resolved in four of the subjects by sequencing of the whole NAT2 exon, which revealed rare mutations, leaving three (9%) HIV-positive SMX-hypersensitive patients and one (4%) healthy volunteer who continued to demonstrate the discordance.

CONCLUSIONS

Slow acetylator phenotype or genotype is unlikely to predispose to SMX hypersensitivity in HIV-positive patients, although a minor role cannot be excluded. Phenotype-genotype discrepancies are partly due to nondetection of all rare alleles by PCR methodology, and can be circumvented by sequencing of the gene in patients showing a discrepancy.

Prospective evaluation of risk factors of cutaneous drug reactions to sulfonamides in patients with AIDS

BACKGROUND

Persons with HIV infection have increased rates of drug eruptions.

OBJECTIVE

Our aim was to evaluate the risk factors of drug eruptions in response to sulfonamides in patients with AIDS, using a case-control analysis.

METHODS

One hundred thirty-six patients who were hospitalized for pneumocystosis or toxoplasmosis were evaluated at the onset of treatment for various risk factors, which were then compared among patients with (48, 36%) and without (88, 64%) a drug eruption.

RESULTS

In multivariate analysis, high CD8(+) cell count and age less than 36 years indicated a risk of drug eruption (respective odds ratios: 3.5 [95% CI 1.6-7.8], P =.002, and 2.1 [95% CI 1-4.6], P =.06). Markers of viral replication for HIV, Epstein-Barr virus, cytomegalovirus, human herpesvirus 6, and parvovirus B19, slow acetylation phenotype or genotype, and glutathione level were not associated with a risk. Administration of corticosteroids had no preventive effect.

CONCLUSIONS

Our results challenge several current concepts regarding drug eruptions by discarding a strong association with glutathione deficiency, slow acetylation, or active viral infections and by showing no preventive effect of corticosteroids.

Acetylator phenotype and genotype in HIV-infected patients with and without sulfonamide hypersensitivity

Adverse reactions to sulfonamides occur at a higher frequency in patients infected with the human immunodeficiency virus (HIV) than noninfected patients. Some studies have suggested that patients with the slow acetylator phenotype are predisposed to these reactions, whereas other studies suggest that the slow acetylator genotype is not a predisposing factor. To rationalize these seemingly contradictory observations, the authors determined the N-acetyltransferase 2 (NAT2) genotype and phenotype in patients with and without a history of hypersensitivity reactions to sulfonamides. HIV-infected patients with a history of a delayed-type hypersensitivity reaction to trimethoprim-sulfamethoxazole were enrolled, along with a group of AIDS patients with no history of hypersensitivity (delayed or immediate). NAT2 phenotype was determined in both groups using dapsone, while the genotype was determined using a polymerase chain reaction-restriction fragment length polymorphism assay. Ten of 14 patients (71%) with a history of hypersensitivity exhibited the slow acetylator phenotype, while 8 of 14 patients (57%) without such a history exhibited this same phenotype (odds ratio [OR] = 1.9, 95% confidence interval [CI] = 0.4-9.0; p = 0.69, Fisher's Exact Test). While 9 of 14 patients (64%) with a history of hypersensitivity exhibited a slow acetylator genotype, only 4 of 14 patients (29%) without such a history exhibited this genotype (ns). There were more instances of discordance between deduced and actual phenotype in the nonhypersensitive patients (n = 4) than in the hypersensitive patients (n = 1). The reported higher frequency of the slow acetylator phenotype among patients with a history of hypersensitivity to sulfonamides does not appear to be explained by metabolic changes that would cause discordance between acetylator genotype and phenotype.

Arylamine N-acetyltransferases and drug response

Arylamine N-acetyltransferases (NATs) play an important role in the interaction of competing metabolic pathways determining the fate of and response to xenobiotics as therapeutic drugs, occupational chemicals and carcinogenic substances. Individual susceptibility for drug response and possible adverse drug reactions are modulated by the genetic predisposition (manifested for example, by polymorphisms) and the phenotype of these enzymes. For all drugs metabolized by NATs, the impact of different in vivo enzyme activities is reviewed with regard to therapeutic use, prevention of side effects and possible indications for risk assessment by phenotyping and/or genotyping. As genes of NATs are susceptibility genes for multifactorial adverse effects and xenobiotic-related diseases, risk prediction can only be made possible by taking the complexity of events into consideration.

Prospective evaluation of detoxification pathways as markers of cutaneous adverse reactions to sulphonamides in AIDS

The use of sulphonamides is complicated by a high rate of cutaneous reactions in AIDS. Metabolic risk factors have been suspected for these reactions. We conducted a prospective study to evaluate whether glutathione S-transferase M1 null genotype, glutathione deficiency and acetylator status as risk factors. To explain the high frequency of slow acetylator phenotype in AIDS patients, we compared N-acetyltransferase-2 phenotype and genotype in this population. AIDS patients treated with sulphonamides for Pneumocystis carinii pneumonia or toxoplasmosis were followed up for cutaneous reactions. Glutathione S-transferase genotyping, glutathione level determination, N-acetyltransferase-2 genotyping and phenotyping were performed. One hundred and thirty-six AIDS patients were studied. Glutathione S-transferase M1 and T1 null genotypes, intracellular glutathione level, slow acetylator genotype and phenotype were not risk factors for cutaneous sulphonamides reactions. The association of glutathione S-transferase M1 null genotype and the slow acetylator one was a risk factor [Fisher's exact test, odds ratio (OR) = 2.6, 95% confidence interval (CI) = 1.2-5.9; P = 0.02]. A discordance between acetylator genotype and phenotype was found in 35% of patients. This frequency was significantly higher than the 6-7% expected (Fisher's exact test: OR = 7.5, 95% CI = 4.2-13.4; P < 0.0001). Suspected metabolic risk factors for sulphonamides cutaneous reactions were not confirmed prospectively. However, the association of glutathione S-transferase M1 null genotype and the slow acetylator one appeared to increase the risk of reactions. We clearly showed that the acetylation phenotype measured by caffeine probe could be modified by the disease.

Association analysis of drug metabolizing enzyme gene polymorphisms in HIV-positive patients with co-trimoxazole hypersensitivity

The use of co-trimoxazole in HIV-positive patients has been associated with a high frequency (40-80%) of hypersensitivity reactions. This has been attributed to the bioactivation of the sulphonamide component, sulphamethoxazole (SMX), to its toxic hydroxylamine and nitroso metabolites. The aim of this study was to determine whether functionally significant polymorphisms in the genes coding for enzymes involved in SMX metabolism influence susceptibility to SMX hypersensitivity. HIV-positive patients with (n = 56) and without (n = 89) SMX hypersensitivity were genotyped for allelic variants in CYP2C9, GSTM1, GSTT1, GSTP1 and NAT2 using polymerase chain reaction (PCR) and/or PCR-restriction fragment length polymorphism analysis. The CYP2C9*2/*3 genotype and CYP2C9*3 allele frequencies were nine- and 2.5-fold higher in the hypersensitive group compared to non-sensitive patients, respectively, although they were not statistically significant when corrected for multiple testing. There were no differences in the frequencies of the GSTM1 and GSTT1 null genotypes, and the slow acetylator genotype, between hypersensitive and non-sensitive patients, while GSTP1 frequency was lower (although non-significant) in the hypersensitive group [21% versus 32%, odds ratio (OR) = 0.5, Pc = 0.24]. Comparison of the genotype frequencies in HIV-positive and -negative patients showed that the NAT2 slow acetylator genotype frequency in the HIV-positive patients (74%) was significantly (Pc = 0.0003, OR = 2.3) higher than in control subjects (56%). Our results show that genetic polymorphisms in drug metabolizing enzymes are unlikely to be major predisposing factors in determining individual susceptibility to co-trimoxazole hypersensitivity in HIV-positive patients.

Acetylator phenotype and genotype in patients infected with HIV: discordance between methods for phenotype determination and genotype

The acetylator phenotype and genotype of AIDS patients, with and without an acute illness, was compared with that of healthy control subjects (30 per group). Two probe drugs, caffeine and dapsone, were used to determine the phenotype in the acutely ill cohort. Polymerase chain reaction amplification and restriction fragment length polymorphism analysis served to distinguish between the 26 known NAT2 alleles and the 21 most common NAT1 alleles. The distribution (%) of slow:rapid acetylator phenotype seen among acutely ill AIDS patients differed with the probe substrate used: 70:30 with caffeine versus 53:47 with dapsone. Phenotype assignment differed considerably between the two methods and there were numerous discrepancies between phenotype and genotype. The NAT2 genotype distribution was 45:55 slow:rapid. Control subjects, phenotyped only with caffeine, were 67:33 slow:rapid versus 60:40 genotypically. Stable AIDS patients, phenotyped only with dapsone, were 55:45 slow:rapid versus 46:54 genotypically. Following resolution of their acute infections, 12 of the acutely ill subjects were rephenotyped with dapsone. Phenotype assignment remained unchanged in all cases. The distribution of NAT1 alleles was similar in all three groups. It is evident from the amount of discordance between caffeine phenotype and dapsone phenotype or genotype that caution should be exercised in the use of caffeine as a probe for NAT2 in acutely ill patients. It is also clear that meaningful study of the acetylation polymorphism requires both phenotypic and genotypic data.

Glucuronidation and sulphation of paracetamol in HIV-positive patients and patients with AIDS

AIMS

To gauge the effect of disease state and disease progression on the glucuronidation and sulphation of paracetamol (APAP) among HIV-positive patients and patients with AIDS.

METHODS

The extent of APAP glucuronidation and APAP sulphation was assessed using a spot urine sample collected 4 h after the oral administration of 500 mg of APAP to 108 patients with AIDS or HIV infection. The molar concentrations of APAP and its glucuronide and sulphate metabolites were determined using a validated h.p.l.c. method and glucuronidation and sulphation indices were constructed using APAP metabolite/APAP molar concentration ratios.

RESULTS

No effect of disease state, AIDS vs asymptomatic HIV positive vs control, on APAP glucuronidation or sulphation was observed. The patient population was studied over time and disease progression also did not significantly alter the calculated glucuronidation and sulphation indices. The effect of the concomitant administration of other therapeutic agents was assessed and in the cross sectional portion of the study dapsone appeared to significantly decrease APAP sulphation as did lamivudine. In the longitudinal portion of the study the latter effect was not observed but zidovudine was seen to increase APAP glucuronidation. The data also indicates that APAP glucuronidation may be reduced in patients who are >10% below their ideal body weight.

A validated method for the determination of paracetamol and its glucuronide and sulphate metabolites in the urine of HIV+/AIDS patients using wavelength-switching UV detection

Paracetamol is a safe drug which has been used as an in-vivo probe to determine phase II metabolism in a HIV+/AIDS population. Due to the biohazard nature of HIV-infected samples, a high performance liquid chromatography (HPLC) assay which offers minimal sample manipulation and maximal specificity was developed. This reverse-phase HPLC method uses wavelength-switching UV detection for the simultaneous determination of paracetamol and its glucuronide and sulfate metabolites in HIV-infected urine samples. The solvent systems involves a simple isocratic elution with a composition of 50 mM sodium acetate buffer, pH adjusted to 3.5; acetonitrile (96:4 v/v) modified with 0.35% trifluroacetic acid. The validated method is highly reproducible with an inter-assay variation of < 7%. This method also shows good precision and sensitivity, making it an ideal assay for phenotyping studies to determine the extent of glucurondiation and sulfation activities.

Genotyping of the polymorphic N-acetyltransferase (NAT2) and loss of heterozygosity in bladder cancer patients

Acetylation is one of the major routes in metabolism and detoxification of a large number of drugs, chemicals and carcinogens. Slow acetylators are said to be more susceptible to developing bladder cancer and because of investigations about tumor risk based on phenotyping procedures, it was our aim to study the distribution of allelic constellations of the N-acetyltransferase (NAT2) by genotyping patients with bladder cancer. We analysed NAT2 gene of blood and tumor DNA from 60 patients with primary bladder cancer and DNA of blood samples from 154 healthy individuals. Using ASO-PCR/RFLP techniques we identified 70% of patients with bladder cancer (n = 42) to be slow acetylators while genotyping of controls resulted in 61% with slow acetylators (n = 94). In addition, dividing bladder cancer patients in males and females the genotype NAT2*5B/NAT2*6A occured with much higher frequencies in males (OR = 4, 95%); CI = 1.8-8.9). Furthermore, investigating bladder cancer tissues we could detect loss of heterozygosity (LOH) in slow and rapid acetylator genotypes. In eleven out of 60 tumor samples (18.3%) we observed allelic loss at the NAT2 locus while in control DNA of blood from the same patients both alleles were still detectable.

N-acetylation among HIV-positive patients and patients with AIDS: when is fast, fast and slow, slow?

BACKGROUND

The discrepancy between genotype and expressed phenotype of the polymorphic N-acetyltransferase (NAT2) has been suggested by separate genotypic and phenotypic studies in populations with human immunodeficiency virus (HIV). Only one study has examined both genotype and phenotype in the same population, and no discrepancies were observed.

METHODS

In a cross-sectional study, 105 HIV-positive patients and patients with acquired immunodeficiency syndrome (AIDS) were phenotyped for NAT2 activity with use of caffeine as an in vivo probe; 50 of these patients were also genotyped by restriction mapping and allele-specific amplification. In a longitudinal study, 23 patients were phenotyped at least twice during the 2-year study.

RESULTS

The distribution of the NAT2 phenotype among the 105 patients was unimodal and skewed toward slow acetylators as opposed to the bimodal distribution observed in healthy white populations. The genotype distribution was 26:24 slow:fast. There were 18 discrepancies between genotype and phenotype: 12 slow acetylators with fast genotypes and six fast acetylators with slow genotypes. No drug-related effects on NAT2 activity were apparent, but the role of disease progression was evident. Among the slow acetylators whose genotype was fast, the incidence of AIDS was higher (six of 12) than that among the fast acetylators whose genotype was fast (two of 14). Among patients phenotyped more than once (mean time between samples, 10.4 months) changes in phenotype from fast to slow were associated with progression of HIV infection.

CONCLUSIONS

Disease progression in HIV infection and AIDS may alter expression of the NAT2 gene. The genotype and the phenotype are not interchangeable measurements. In the HIV population, to know the genotype is useful only if the phenotype is also known and vice versa.

Impairment of the metabolism of dipyrone in asymptomatic carriers of the hepatitis B virus

BACKGROUND

The pharmacokinetics of a number of drugs has been shown to be impaired in patients with acute or chronic viral liver disease.

OBJECTIVE

To examine the effect of the asymptomatic hepatitis B virus carrier state on the metabolism of dipyrone (INN, metamizole) as a model drug.

METHODS

The pharmacokinetics of the metabolites of dipyrone-4-methylaminoantipyrine, 4-aminoantipyrine, 4-formylaminoantipyrine, and 4-acetylaminoantipyrine-after a 1.0 gm oral dose of dipyrone were evaluated in nine asymptomatic carriers of hepatitis B virus with normal liver function tests and nine healthy subjects. All subjects displayed the slow acetylator phenotype.

RESULTS

The nonrenal (metabolic) clearance of 4-methylaminoantipyrine was significantly reduced (mean +/- SEM) (123.3 +/- 15.8 versus 182.9 +/- 15.1 ml.min-1, respectively; p < 0.02) in the carriers of hepatitis B virus compared with the healthy subjects, and the elimination half-life of this metabolite was significantly longer (3.69 +/- 0.35 versus 2.64 +/- 0.28 hours, respectively; p < 0.03). The formation clearances of 4-aminoantipyrine and 4-formylaminoantipyrine were significantly smaller in the carriers of hepatitis B virus compared with healthy subjects (33.8 +/- 6.2 versus 55.2 +/- 6.4 ml.min-1; p < 0.03, and 16.7 +/- 2.2 versus 34.2 +/- 4.2 ml.min-1; p < 0.002; respectively). However, the elimination half-life of 4-formylaminoantipyrine was found to be slightly shorter in the carriers of hepatitis B virus. No significant differences were noted between the groups in the pharmacokinetics of 4-acetylaminoantipyrine.

CONCLUSION

The metabolism of dipyrone is impaired in asymptomatic carriers of hepatitis B virus. Clinically latent infection with hepatitis B virus seems to exert a differential effect on metabolism of the drug. Oxidative pathways to produce 4-aminoantipyrine and 4-formylaminoantipyrine were significantly affected, whereas acetylation remained intact. This study provided an additional example of the effect of a virus on the disposition of a drug.

Human acetyltransferase polymorphisms

Conjugation of primary amino and hydroxylamino groups with acetate, catalyzed by acetyl CoA-dependent arylamine acetyltransferase (NAT) enzymes, may play an important role in the intricate series of metabolic pathways that produce or prevent toxicity following exposure to homo- and heterocyclic arylamine and hydrazine xenobiotics. Two independently regulated and kinetically distinct human acetyltransferases are now known to exist, namely NAT1 and NAT2. Interindividual variation in NAT2 function is associated with the classical isoniazid acetylation polymorphism which was discovered over forty years ago. At last count, fifteen variant alleles at the NAT2 gene locus have been linked to the isoniazid 'acetylator phenotype', and each of these can be identified in population studies using specific PCR-based genotyping tests. On the other hand, NAT1 shows kinetic selectivity for compounds whose disposition is unrelated to the classical isoniazid acetylation polymorphism. NAT1 expression is also phenotypically variable in human populations, at least in part due to allelic differences at the NAT1 gene locus. Nine NAT1 variant alleles have been described to date, of which NAT1* 14 and NAT1* 15 clearly produce defective NAT1 proteins and lead to functional impairment in the metabolism of NAT1-selective substrates both in vivo and in vitro. On the other hand, it has been reported that the NAT1* 10 variant associates with elevated NAT1 activity and increased risk for cancers of the bladder and colon. Because of the important toxicologic consequences of allelic variation in NAT1 and NAT2 function for the metabolic activation of arylamine and heterocyclic amine procarcinogens, further studies are needed to improve our understanding of the extent of NAT allelic variation, to determine the functional capacity of each variant gene product, and to develop accurate methods of detecting them in population and epidemiological studies.

Role of viral infections in the induction of adverse drug reactions

A spectrum of adverse drug reactions that are caused by the combined action of drugs and viruses has been described: ampicillin rash in acute infectious mononucleosis; Reye's syndrome; hypersensitivity reactions to sulphonamides in patients with HIV infection; drug-induced agranulocytosis; paracetamol (acetaminophen) hepatotoxicity; aspirin (acetylsalicyclic acid)-induced asthma; Epstein-Barr virus-associated lymphoma and methotrexate; and AIDS-related Kaposi's sarcoma and nitrite use. Changes in pharmacokinetics have been reported for: caffeine, sulfamethoxazole and fluconazole in patients with HIV infection; theophylline, following influenza and influenza vaccination; and recently, dipyrone metabolites in carriers of the hepatitis B virus. In addition increased drug- and drug metabolite-related toxicity has been observed in virally infected cells. Pathogenetic mechanisms for the interaction between drugs and viruses are varied, and include biological mechanisms (often immunological) and changes in drug metabolism. The combined effects of chemical and biological exposure provide a unique model for the study of disease induction.

Dextromethorphan as an in vivo probe for the simultaneous determination of CYP2D6 and CYP3A activity

Dextromethorphan (DM) is O-demethylated into dextrorphan (DEX) in humans by the cytochrome P450 designated as CYP2D6 and N-demethylated into 3-methoxymorphinan (3MM) via CYP3As. Clinically, DM has been successfully used as an index of CYP2D6 and this paper describes analytical and clinical data that will help evaluate the use of DM hydrobromide as a probe of CYP3A activity. DM and its three demethylated metabolites were measured in a 4-h spot urine sample using a HPLC method employing solid-phase extraction (C(18)), analysis on a phenyl column [mobile phase, methanol-acetonitrile-phosphate buffer (10 mM, pH 3.5, 20:25:55, v/v)] and fluorescence detection (excitation at lambda=228 nm, no emission cut-off filter). The urinary molar ratio DM-DEX was used to assess CYP2D6 activity while DM-3MM was used for CYP3As. The DM-3MM ratios were sensitive to the co-administration of selective CYP3A inhibitors grapefruit juice and erythromycin. In addition, in healthy volunteers and cancer patients, the N-demethylation of DM correlated with the CYP3A-mediated metabolism of verapamil and tamoxifen. DM appears to be a promising way to simultaneously phenotype patients for CYP2D6 and CYP3As.