NADH-dependent reduction of sulphamethoxazole hydroxylamine in dog and human liver microsomes

A single-nucleotide polymorphism in the canine cytochrome b5 reductase (CYB5R3) gene is associated with sulfonamide hypersensitivity and is overrepresented in Doberman Pinschers

Canine sulfonamide hypersensitivity (HS) has been associated with a variant in the cytochrome b₅ reductase gene (CYB5R3 729A>G), which encodes a drug-detoxifying enzyme. Study objectives were to determine variant allele frequency in Doberman Pinschers (DOBE), a breed which may be predisposed to sulfonamide HS, and to characterize the effects of CYB5R3 729G on gene expression and function. CYB5R3 729A>G allele frequencies were compared between DOBE (n = 24) vs. non-Doberman (non-DOBE; n = 60) dogs. CYB5R3mRNA expression, protein expression, and reduction of sulfamethoxazole hydroxylamine were compared between banked canine liver samples of 729AA vs. GG genotype. The 729G allele was overrepresented in DOBE (1.00) vs. non-DOBE dogs (0.567, p < .0001). mRNA and protein expressions as well as cyt b₅ reductase activity were similar between livers of AA and GG genotype. All Doberman Pinschers in this study were homozygous for CYB5R3 729G, which could contribute to this breed's apparent predisposition to sulfonamide HS. However, CYB5R3 729G does not alter sulfamethoxazole detoxification capacity, so a direct role could not be demonstrated. It is possible that this marker is linked to another contributing variant.

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.

Immunogenicity of trimethoprim/sulfamethoxazole in a macaque model of HIV infection

BACKGROUND

Sulfonamide hypersensitivity has a high incidence in HIV infection and correlates with low CD4+ counts, but the mechanisms are not understood. The aims of this study were to determine whether trimethoprim/sulfamethoxazole (TMP/SMX) led to SMX adduct formation, immunogenicity, or signs of drug hypersensitivity in SIV-infected rhesus macaques, and whether differences in antioxidants, pro-inflammatory mediators, or SMX disposition were predictive of drug immunogenicity.

METHODS

Nine macaques chronically infected with SIVmac239 and 7 non-infected controls were studied. Baseline blood ascorbate, glutathione, IFN-γ, LPS, sCD14, and cytochrome b₅ reductase measurements were obtained, macaques were dosed with TMP/SMX (120mg/kg/day p.o. for 14days), and SMX metabolites, lymph node drug adducts, drug-responsive T cells, and anti-SMX antibodies were measured.

RESULTS

Four of 9 of SIV-positive (44%), and 3 of 7 SIV negative (43%) macaques had drug-responsive T cells or antibodies to SMX. Two macaques developed facial or truncal rash; these animals had the highest levels of lymph node drug adducts. Antioxidants, pro-inflammatory mediators, and SMX metabolites were not predictive of drug immunogenicity; however, the Mamu DRB1*0401/0406/0411 genotype was significantly over-represented in immune responders.

CONCLUSIONS

Unlike other animal models, macaques develop an immune response, and possible rash, in response to therapeutic dosages of TMP/SMX. Studying more animals with CD4+ counts <200cells/μl, along with moderately restricted ascorbate intake to match deficiencies seen in humans, may better model the risk of SMX hypersensitivity in HIV-infection. In addition, the role of Mamu-DRB1 genotype in modeling drug hypersensitivity in retroviral infection deserves further study.

Evaluation of polymorphisms in the sulfonamide detoxification genes CYB5A and CYB5R3 in dogs with sulfonamide hypersensitivity

BACKGROUND

Delayed hypersensitivity (HS) reactions to potentiated sulfonamide antimicrobials occur in both dogs and humans, and involve an intermediate hydroxylamine metabolite that is detoxified by cytochrome b(5) and NADH cytochrome b(5) reductase.

HYPOTHESIS/OBJECTIVES

We hypothesized that polymorphisms in the genes (CYB5A and CYB5R3) encoding these 2 enzymes would be associated with risk of sulfonamide HS in dogs.

ANIMALS

A total of 18 dogs with delayed HS to potentiated sulfonamide antimicrobials and 16 dogs that tolerated (TOL) a therapeutic course of these drugs without adverse effect.

METHODS

CYB5A and CYB5R3 were sequenced from canine liver, and the promoter, exons, and 3' untranslated regions of both genes were resequenced from genomic DNA obtained from all dogs.

RESULTS

Multiple polymorphisms were found in both genes. When controlled for multiple comparisons, the 729GG variant in CYB5R3 was significantly overrepresented in dogs with sulfonamide HS (78% of dogs), compared to TOL dogs (31%; P = .003).

CONCLUSIONS AND CLINICAL IMPORTANCE

The CYB5R3 729GG variant may contribute to the risk of sulfonamide HS in dogs. Functional characterization of this polymorphism, as well as genotyping in a larger number of HS and TOL dogs, is warranted.

Drug antigenicity, immunogenicity, and costimulatory signaling: evidence for formation of a functional antigen through immune cell metabolism

Recognition of drugs by immune cells is usually explained by the hapten model, which states that endogenous metabolites bind irreversibly to protein to stimulate immune cells. Synthetic metabolites interact directly with protein-generating antigenic determinants for T cells; however, experimental evidence relating intracellular metabolism in immune cells and the generation of physiologically relevant Ags to functional immune responses is lacking. The aim of this study was to develop an integrated approach using animal and human experimental systems to characterize sulfamethoxazole (SMX) metabolism-derived antigenic protein adduct formation in immune cells and define the relationship among adduct formation, cell death, costimulatory signaling, and stimulation of a T cell response. Formation of SMX-derived adducts in APCs was dose and time dependent, detectable at nontoxic concentrations, and dependent on drug-metabolizing enzyme activity. Adduct formation above a threshold induced necrotic cell death, dendritic cell costimulatory molecule expression, and cytokine secretion. APCs cultured with SMX for 16 h, the time needed for drug metabolism, stimulated T cells from sensitized mice and lymphocytes and T cell clones from allergic patients. Enzyme inhibition decreased SMX-derived protein adduct formation and the T cell response. Dendritic cells cultured with SMX and adoptively transferred to recipient mice initiated an immune response; however, T cells were stimulated with adducts derived from SMX metabolism in APCs, not the parent drug. This study shows that APCs metabolize SMX; subsequent protein binding generates a functional T cell Ag. Adduct formation above a threshold stimulates cell death, which provides a maturation signal for dendritic cells.

Discovery and characterization of a cytochrome b5 variant in humans with impaired hydroxylamine reduction capacity

OBJECTIVES

We have shown that cytochrome b5 (cyt b5), along with its reductase, NADH cytochrome b5 reductase (b5R), is capable of direct xenobiotic biotransformation. We hypothesized that functionally significant genetic variability in cyt b5 could be found in healthy individuals.

BASIC METHODS

Cyt b5 cDNAs were prepared from peripheral blood mononuclear cells from 63 individuals.

MAIN RESULTS

One individual was heterozygous for a sequence variant in cyt b5 (A178G), with a predicted amino acid substitution of T60A. This variant, when expressed in Escherichia. coli, maintained a similar Vmax for the hydroxylamines of sulfamethoxazole, 4-aminobiphenyl, and 2-amino-l-methyl-6-phenylimidazo[4,5-b] pyridine (PhIP), compared with wild type cyt b5, with a modestly increased Km (2 to 3.5-fold) for each substrate. When expressed in a mammalian system (HeLa cells), however, T60A was associated with a 70% reduction in cyt b5 protein expression compared with wild type. mRNA expression for both isoforms were comparable in HeLa cells, and translation of these mRNAs in a rabbit reticulocyte lysate system with inhibited proteasomal machinery were also similar. Incubation of these translated enzymes with uninhibited rabbit reticulocyte lysate, however, indicated greater susceptibility of T60A to proteasomal degradation.

CONCLUSIONS

These data indicate that a naturally occurring variant in cyt b5, T60A, leads to modestly altered affinity for hydroxylamine substrates and dramatically reduced cyt b5 expression. Work is underway to determine the prevalence of this and other variants in cyt b5 or b5R in a larger population, and to determine the association of such variants with differences in hydroxylamine reduction in vivo.

Reductive detoxification of arylhydroxylamine carcinogens by human NADH cytochrome b5 reductase and cytochrome b5

Heterocyclic and aromatic amine carcinogens are thought to lead to tumor initiation via the formation of DNA adducts, and bioactivation to arylhydroxylamine metabolites is necessary for reactivity with DNA. Carcinogenic arylhydroxylamine metabolites are cleared by a microsomal, NADH-dependent, oxygen-insensitive reduction pathway in humans, which may be a source of interindividual variability in response to aromatic amine carcinogens. The purpose of this study was to characterize the identity of this reduction pathway in human liver. On the basis of our findings with structurally similar arylhydroxylamine metabolites of therapeutic drugs, we hypothesized that the reductive detoxification of arylhydroxylamine carcinogens was catalyzed by NADH cytochrome b5 reductase (b5R) and cytochrome b5 (cyt b5). We found that reduction of the carcinogenic hydroxylamines of the aromatic amine 4-aminobiphenyl (4-ABP; found in cigarette smoke) and the heterocyclic amine 2-amino-1-methyl-6-phenylimidazo [4,5-b] pyridine (PhIP; found in grilled meats) was indeed catalyzed by a purified system containing only human b5R and cyt b5. Specific activities were 56-346-fold higher in the purified system as compared to human liver microsomes (HLM), with similar Michaelis-Menten constants (K(m) values) in both systems. The stoichiometry for b5R and cyt b5 that yielded the highest activity in the purified system was also similar to that found in native HLM ( approximately 1:8 to 1:10). Polyclonal antisera to either b5R or cyt b5 significantly inhibited N-hydroxy-4-aminobiphenyl (NHOH-4-ABP) reduction by 95 and 89%, respectively, and immunoreactive cyt b5 protein content in individual HLM was significantly correlated with individual reduction of both NHOH-4-ABP and N-hydroxy-PhIP (NHOH-PhIP). Finally, titration of HLM into the purified b5R/cyt b5 system did not enhance the efficiency of reduction activity. We conclude that b5R and cyt b5 are together solely capable of the reduction of arylhydroxylamine carcinogens, and we further hypothesize that this pathway may be a source of individual variability with respect to cancer susceptibility following 4-ABP or PhIP exposure.

Association of drug-serum protein adducts and anti-drug antibodies in dogs with sulphonamide hypersensitivity: A naturally occurring model of idiosyncratic drug toxicity

BACKGROUND

Sulphonamide antimicrobials, such as sulphamethoxazole (SMX), provide effective infection prophylaxis in immunocompromised patients, but can lead to drug hypersensitivity (HS) reactions. These reactions also occur in dogs, with a similar time course and clinical presentation as seen in humans.

OBJECTIVES

Drug-serum adducts and anti-drug antibodies have been identified in sulphonamide HS humans. The aim of this study was to determine whether similar markers were present in dogs with sulphonamide HS.

METHODS

Thirty-four privately owned sulphonamide HS dogs, 10 sulphonamide-'tolerant' dogs, 18 sulphonamide-naïve dogs, and four dogs experimentally dosed with SMX and the oxidative metabolite SMX-nitroso, were tested for drug-serum adducts by immunoblotting, and anti-drug antibodies by ELISA.

RESULTS

Sulphonamide-serum adducts were found in 10/20 HS dogs tested (50%), but in no tolerant dogs. Anti-sulphonamide IgG antibodies were detected in 17/34 HS dogs (50%), but in only one tolerant dog; antibody absorbance values were significantly higher in HS dogs. There was a significant association between the presence of sulphonamide-serum adducts and anti-sulphonamide antibodies (P = 0.009). Anti-drug antibodies were also found in dogs experimentally dosed with SMX-nitroso followed by SMX, but not in a dog dosed with drug vehicle, followed by SMX.

CONCLUSION

Similar humoral markers are present in dogs and humans with sulphonamide HS, supporting the use of dogs as a naturally occurring model for this syndrome in humans. These data suggest the potential use of drug-serum adducts and anti-drug antibodies as markers for sulphonamide HS. Preliminary data indicate that anti-sulphonamide antibodies may be triggered by the SMX-nitroso metabolite, not by the parent drug, in dogs.

Roles of endogenous ascorbate and glutathione in the cellular reduction and cytotoxicity of sulfamethoxazole-nitroso

Sulfamethoxazole (SMX) is an effective drug for the management of opportunistic infections, but its use is limited by hypersensitivity reactions, particularly in HIV-infected patients. The oxidative metabolite SMX-nitroso (SMX-NO), is thought to be a proximate mediator of SMX hypersensitivity, and can be reduced in vitro by ascorbate or glutathione. Leukocytes from patients with SMX hypersensitivity show enhanced cytotoxicity from SMX metabolites in vitro; this finding has been attributed to a possible "detoxification defect" in some individuals. The purpose of this study was to determine whether variability in endogenous ascorbate or glutathione could be associated with individual differences in SMX-NO cytotoxicity. Thirty HIV-positive patients and 23 healthy control subjects were studied. Both antioxidants were significantly correlated with the reduction of SMX-NO to its hydroxylamine, SMX-HA, by mononuclear leukocytes, and both were linearly depleted during reduction. Controlled ascorbate supplementation in three healthy subjects increased leukocyte ascorbate with no change in glutathione, and significantly enhanced SMX-NO reduction. Ascorbate supplementation also decreased SMX-NO cytotoxicity compared to pre-supplementation values. Rapid reduction of SMX-NO to SMX-HA was associated with enhanced direct cytotoxicity from SMX-NO. When forward oxidation of SMX-HA back to SMX-NO was driven by the superoxide dismutase mimetic, Tempol, SMX-NO cytotoxicity was increased, without enhancement of adduct formation. This suggests that SMX-NO cytotoxicity may be mediated, at least in part, by redox cycling between SMX-HA and SMX-NO. Overall, these data indicate that endogenous ascorbate and glutathione are important for the intracellular reduction of SMX-NO, a proposed mediator of SMX hypersensitivity, and that redox cycling of SMX-HA to SMX-NO may contribute to the cytotoxicity of these metabolites in vitro.

Oxygen-insensitive enzymatic reduction of oximes to imines

The reduction of oximes to imines under anaerobic and aerobic conditions was studied using (E)- and (Z)-2,4,6-trimethylacetophenone oxime, benzaldoxime and (E)-2,4,6-trimethylbenzaldoxime. Pig and human liver microsomes, pig liver mitochondria and cytosol to a minor extent catalyzed the conversion of both isomeric ketoximes to the corresponding stable imine, the (E)-isomer being the better substrate. All reactions were oxygen-insensitive and required active protein and NADH or NADPH; however, NADH was preferred as cofactor. The reconstituted liver microsomal system of a pig liver CYP2D enzyme (NADH-benzamidoxime reductase), which is known to reduce N-hydroxylated derivatives of strongly basic functional groups, such as amidoximes, is also capable of reducing oximes. As expected, the corresponding imine was detected in relevant amounts when incubating 2,4,6-trimethyl-acetophenone oxime using the reconstituted enzyme system, but reduction rates were significantly lower compared to rates obtained when incubating benzamidoxime. Steric hindrance due to the methyl groups in ortho position to the oxime functionality could be excluded as being responsible for the lower conversion rates according to results obtained in incubations of 2,4,6-trimethylbenzamidoxime. When incubating benzaldoxime, only benzoic acid could be detected as metabolite, since the aldehyde is easily oxidized during incubation procedures, whereas incubations of (E)-2,4,6-trimethylbenzaldoxime also showed the formation of the corresponding aldehyde. These results allow us to postulate that the metabolism of aldoximes like 2,4,6-trimethylbenzaldoxime most likely proceeds through enzymatic reduction of the oxime to yield the intermediate imine, which is subsequently hydrolyzed to the aldehyde and then oxidized to the corresponding benzoic acid.

Reduction of sulfamethoxazole and dapsone hydroxylamines by a microsomal enzyme system purified from pig liver and pig and human liver microsomes

Biotransformation involving nitrogen are of pharmacological and toxicological relevance. In principle, nitrogen containing functional groups can undergo all the known biotransformation processes such as oxidation, reduction, hydrolysis and formation of conjugates. For the N-reduction of benzamidoxime an oxygen-insensitive liver microsomal enzyme system that required cytochrome b5, NADH-cytochrome b5 reductase and a cytochrome P450 isoenzyme of the subfamily 2D has been described. In previous studies it was demonstrated that N-hydroxylated derivates of strongly basic functional groups are easily reduced by this enzyme system. The N-hydroxylation of sulfonamides such sulfamethoxazole (SMX) and dapsone (DDS) to sulfamethoxazole-hydroxylamine (SMX-HA) and dapsone-hydroxylamine (DDS-N-OH), respectively is the first step in the formation of reactive metabolites. Therefore it seemed reasonable to study the potential of cytochrome b5, NADH-cytochrome b5 reductase and CYP2D to detoxify these N-hydroxylated metabolites by N-reduction. Metabolites were analysed by HPLC analysis. SMX-HA and DDS-N-OH are reduced by cytochrome b5, NADH-cytochrome b5 reductase and CYP2D but also only by cytochrome b5 and NADH-cytochrome b5 reductase without addition of CYP2D. The reduction rate for SMX-HA by cytochrome b5, NADH-cytochrome b5 reductase and CYP2D was 0,65 +/- 0,1 nmol SMX/min/mg protein. The reduction rate by b5 and b5 reductase was 0,37 +/- 0,15 nmol SMX/min/mg protein. For DDS-N-OH the reduction rate by cytochrome b5, NADH-cytochrome b5 reductase and CYP2D was 1.79 +/- 0.85 nmol DDS/min/mg protein and by cytochrome b5 and NADH-cytochrome b5 reductase 1.25 +/- 0.15 nmol DDS/min/mg protein. Cytochrome b5, NADH-cytochrome b5 reductase are therefore involved in the detoxification of these reactive hydroxylamines and CYP2D increased the N-reduction.

CHARACTERIZATION AND PARTIAL PURIFICATION OF THE RAT AND HUMAN ENZYME SYSTEMS ACTIVE IN THE REDUCTION OFN-HYDROXYMELAGATRAN AND BENZAMIDOXIME

The enzymic basis for intracellular reduction of N-hydroxylated amidines to their corresponding amidines, and hydroxylamines to their corresponding amines, is unknown. The hydroxylated amidines can be used as prodrug moieties, and an understanding of the enzyme system active in the reduction can contribute to more efficient drug development. In this study, we examined the properties of this enzyme system using benzamidoxime and N-hydroxymelagatran as substrates. In rats and humans, the hepatic enzyme system was localized in mitochondria as well as in microsomes, using preferably NADH as cofactor. Potassium cyanide, N-methylhydroxylamine, p-hydroxymercuribenzoate, and desferrioxamine were efficient inhibitors, whereas typical cytochrome P450 (P450) inhibitors were ineffective. In rats, the highest specific activity was found in liver, adipose tissue, and kidneys, whereas in humans, the specific activity in the preparations of adipose tissue examined was lower. A sex difference was observed in rat liver, where 4-fold higher activity was seen in microsomes from female rats. No gender differences were present in any other tissue investigated. Partial purification of the hepatic system was achieved using polyethylene glycol fractionation followed by Octyl Sepharose chromatography at low detergent concentrations, whereas the enzyme was denatured after complete solubilization. The unique appearance of the enzyme activity in adipose tissue, together with the cyanide sensitivity and the failure of typical P450 inhibitors to impede the reaction, indicates that the enzyme system active in reduction of benzamidoxime and N-hydroxymelagatran formation is not of cytochrome P450 origin, but likely consists of an NADH-dependent electron transfer chain with a cyanide-sensitive protein as the terminal component.

NADH cytochrome b5 reductase and cytochrome b5 catalyze the microsomal reduction of xenobiotic hydroxylamines and amidoximes in humans

Hydroxylamine metabolites, implicated in dose-dependent and idiosyncratic toxicity from arylamine drugs, and amidoximes, used as pro-drugs, are metabolized by an as yet incompletely characterized NADH-dependent microsomal reductase system. We hypothesized that NADH cytochrome b5 reductase and cytochrome b5 were responsible for this enzymatic activity in humans. Purified human soluble NADH cytochrome b5 reductase and cytochrome b5, expressed in Escherichia coli, efficiently catalyzed the reduction of sulfamethoxazole hydroxylamine, dapsone hydroxylamine, and benzamidoxime, with apparent Km values similar to those found in human liver microsomes and specific activities (Vmax) 74 to 235 times higher than in microsomes. Minimal activity was seen with either protein alone, and microsomal protein did not enhance activity other than additively. All three reduction activities were significantly correlated with immunoreactivity for cytochrome b5 in individual human liver microsomes. In addition, polyclonal antibodies to both NADH cytochrome b5 reductase and cytochrome b5 significantly inhibited reduction activity for sulfamethoxazole hydroxylamine. Finally, fibroblasts from a patient with type II hereditary methemoglobinemia (deficient in NADH cytochrome b5 reductase) showed virtually no activity for hydroxylamine reduction, compared with normal fibroblasts. These results indicate a novel direct role for NADH cytochrome b5 reductase and cytochrome b5 in xenobiotic metabolism and suggest that pharmacogenetic variability in either of these proteins may effect drug reduction capacity.

Plasma Ascorbate Deficiency Is Associated With Impaired Reduction of Sulfamethoxazole-Nitroso in HIV Infection

OBJECTIVE

The objective of these studies was to determine the role of ascorbate deficiency in HIV infection in the defective detoxification of sulfamethoxazole-nitroso, the metabolite thought to mediate sulfonamide hypersensitivity reactions.

METHODS

Fifty-one HIV-infected patients and 26 healthy volunteers were evaluated. Vitamin supplementation histories were obtained, and blood samples were collected for determination of plasma ascorbate, dehydroascorbate, and cysteine concentrations, erythrocyte glutathione concentrations, and plasma reduction of sulfamethoxazole-nitroso in vitro.

RESULTS

Plasma ascorbate concentrations were significantly lower in HIV-positive patients not taking vitamin supplements (29.5 +/- 22.3 microM) than in healthy subjects (54.8 +/- 22.3 microM; P = 0.0005) and patients taking 500-1000 mg of ascorbate daily (82.5 +/- 26.3 microM; P < 0.0001). Plasma ascorbate deficiency was strongly correlated with impaired reduction of sulfamethoxazole-nitroso to its hydroxylamine (r = 0.60, P < 0.0001), and during in vitro reduction, the loss of plasma ascorbate was strongly associated with the amount of nitroso reduced (r = 0.70, P < 0.0001). Ascorbate added ex vivo normalized this reduction pathway. Erythrocyte glutathione concentrations were significantly lower in HIV-positive patients (0.98+/-0.32 mM) than in healthy subjects (1.45+/-0.49 mM; P = 0.001), but this finding was unrelated to ascorbate supplementation. There was trend toward lower plasma cysteine concentrations in patients (8.4+/-3.9 microM) than in controls (10.3+/-4.3 microM), but this trend was similarly unrelated to ascorbate supplementation. Dehydroascorbate concentrations were not significantly higher in HIV-positive patients (7.4+/-10.5%) than in healthy controls (4.0+/-6.2%), even in the subset of patients taking ascorbate (8.4+/-9.4%).

CONCLUSIONS

Ascorbate deficiency is common in HIV-positive patients and is associated with impaired detoxification of sulfamethoxazole-nitroso, the suspected proximate toxin in sulfonamide hypersensitivity. Patients taking daily ascorbate supplements (500-1000 mg) achieved high plasma ascorbate concentrations and did not show this detoxification defect. Ascorbate deficiency (or supplementation) was not associated with changes in glutathione or cysteine concentrations. These data suggest that ascorbate deficiency, independent of thiol status, may be an important determinant of impaired drug detoxification in HIV infection.

Idiosyncratic toxicity associated with potentiated sulfonamides in the dog

Idiosyncratic toxicity to potentiated sulfonamides occurs in both humans and dogs, with considerable clinical similarities. The syndrome in dogs can consist of fever, arthropathy, blood dyscrasias (neutropenia, thrombocytopenia, or hemolytic anemia), hepatopathy consisting of cholestasis or necrosis, skin eruptions, uveitis, or keratoconjunctivitis sicca. Other manifestations seen less commonly include protein-losing nephropathy, meningitis, pancreatitis, pneumonitis, or facial nerve palsy. The pathogenesis of these reactions is not completely understood, but may be due to a T-cell-mediated response to proteins haptenated by oxidative sulfonamide metabolites. Our laboratory is working on tests to characterize dogs with possible idiosyncratic sulfonamide reactions, to include ELISA for anti-drug antibodies, immunoblotting for antibodies directed against liver proteins, flow cytometry for drug-dependent anti-platelet antibodies, and in vitro cytotoxicity assays. The management of idiosyncratic sulfonamide toxicity involves client education to identify clinical signs early and allow rapid drug discontinuation, supportive care to include possibly ascorbate and glutathione precursors, and avoidance of subsequent re-exposure. It is important to realize that only antimicrobial sulfonamides, such as sulfamethoxazole, sulfadiazine, and sulfadimethoxine, share this clinical syndrome. There is no evidence for cross-reactivity with drugs that have different underlying structures but share a sulfonamide moiety, such as acetazolamide, furosemide, glipizide, or hydrochlorthiazide.

Mechanisms of NSAID-induced hepatotoxicity: focus on nimesulide

Nonsteroidal anti-inflammatory drugs (NSAIDs) have been associated with idiosyncratic hepatotoxicity in susceptible patients. The molecular mechanisms underlying this toxicity have not yet been fully elucidated. However, experimental evidence suggests that they include increased concentration of the drugs in the hepatobiliary compartment, formation of reactive metabolites that covalently modify proteins and produce oxidative stress, and mitochondrial injury. Genetic and/or acquired patient factors can either augment the pathways leading to hepatic toxicity or impede the protective and detoxifying pathways. An example is nimesulide, a selective cyclo-oxygenase-2 inhibitor widely used for the treatment of inflammatory and pain conditions, which has been recently associated with rare but serious and unpredictable adverse reactions in the liver (increases in serum aminotransferase activities, hepatocellular necrosis, and/or intrahepatic cholestasis). Similar to other drugs causing idiosyncratic hepatotoxicity, both the molecule and the patient contribute to the hazard. Here, the weakly acidic sulfonanilide drug undergoes bioreductive metabolism of the nitroarene group to reactive intermediates that have been implicated in oxidative stress, covalent binding, and mitochondrial injury. It is only in a small number of susceptible patients, however, that genetic or nongenetic factors will cause this potential toxicity to become clinically manifest. In view of the very large recipient population, the incidence of nimesulide-induced liver injury has been low (approximately 0.1 per 100,000 patients treated). Although this estimation is based on spontaneous reporting data versus sales units and needs correction due to the classical bias of this system, the type and incidence of these rare but severe hepatic adverse reactions are comparable to that of other NSAIDs.

Reduction of N-hydroxylated compounds: amidoximes (N-hydroxyamidines) as pro-drugs of amidines

In order to examine the importance of metabolic cycles and in particular of reductions of N-hydroxylated compounds, the reversible metabolism at the amidine, guanidine, and amidinohydrazone nitrogen atoms of various drugs and model compounds was investigated. Many of these N-oxygenated metabolites are very easily reduced back into the starting materials. A comparison of the kinetic data for the N-hydroxylation and reduction suggests that the reduction should predominate in vivo. This could be verified by in vivo studies. Thus, N-hydroxylated amidines (amidoximes) can be used as pro-drugs of amidines. Because of their strong basicity, amidines, guanidines, and amidinohydrazones are protonated under physiological conditions, are very hydrophilic, and are usually not absorbed from the gastrointestinal tract. The N-hydroxylated derivatives of amidines (amidoximes), guanidines (N-hydroxyamidines), and amidinohydrazones (N-hydroxyamidinohydrazones) are less basic because of the introduction of the oxygen atom. They are absorbed from the gastrointestinal tract and then reduced to the active amidines, guanidines, and amidinohydrazones. The pro-drug principle was originally developed in our laboratory for pentamidine and then applied to other amidines such as sibrafiban and melagatran (ximelagatran). The enzymatic basis of N-oxidative processes is very well understood, whereas reductions have been less extensively investigated. We purified an enzyme system from pig and human liver consisting of cytochrome b5, its reductase, and a P450 enzyme, which is involved in the reduction of the N-hydroxylated compounds. Similar activities were found in all species studied so far. Furthermore, comparable reductive reactions could also be demonstrated with microsomal fractions from organs other than liver. In addition, mitochondria are highly capable of performing the reductions of these N-hydroxylated compounds. Thus, several organs and cell organelles are involved in the reduction explaining the extensive reduction of the pro-drugs in vivo underlying the suitability of the concept for drug development.