Immune response to sulfamethoxazole in patients with AIDS

Presumed Sulfonamide-Associated Uveitis With Stevens-Johnson Syndrome in a Quarter Horse Mare

We describe the case of a four-year-old Quarter Horse mare that presented with fever, respiratory infection with productive cough, disorientation, and bilateral anterior uveitis with discharge that had been previously treated with trimethoprim-sulfadiazine (TMPS). Acinetobacter johnsonii was cultured from an endoscopic tracheal wash. Treatment was initiated with cefquinome, systemic flunixin-meglumine, local ocular atropine, and corticosteroids. On subsequent days, the mare exhibited bilateral edematous, painful swelling of the face, primarily affecting the eyelids and lips. There were neither swellings nor pulsations of the metatarsal arteries. On day five of treatment, the facial swelling disappeared, the uveitis improved markedly, and the mare's periorbital skin, muzzle, and vulva began to slough, revealing underlying, nonpigmented skin. Although systemic use of sulfonamides has been associated with bilateral anterior uveitis and Stevens-Johnson syndrome (SJS or erythema multiforme major) in humans, these conditions are rare in horses. Stevens-Johnson syndrome has been associated most commonly with sulfonamide use but also with a range of other medications, including anti-infectives, anti-inflammatories, anticonvulsants, analgesics, and infections. A possible pathway for sulfonamide-induced SJS is discussed. To our knowledge, this is the first reported case of sulfonamide-associated uveitis and SJS in the horse.

Drug-induced oral erythema multiforme: A diagnostic challenge

Trimethoprim-sulfamethoxazole (cotrimoxazole or TMP/SMX) is commonly used for the treatment of urinary or lower respiratory tract infections. A comprehensive review is presented of the world literature on adverse reactions to TMP/SMX since its first use in 1968, when concern was expressed about possible hematologic toxicity. The adverse reactions to TMP/SMX occur in approximately 1%–3% of persons in general population. Here, we report a case of oral erythema multiforme (EM) secondary to TMP/SMX that presented with oral and lip ulcerations typical of EM without any skin lesions and highlights the importance of distinguishing them from other ulcerative disorders involving oral cavity. The patient was treated symptomatically and given tablet prednisolone. The condition improved with stoppage of TMP/SMX therapy. It has been reported that primary attacks of oral EM is confined to the oral mucosa, but the subsequent attacks can produce more severe forms of EM involving the skin. Hence, it is important to identify for early diagnosis and management. Although EM is quite common with TMP/SMX, there are no reports of oral EM appearing after intake of this drug.

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.

Trimethoprim-sulfamethoxazole-induced Steven Johnson syndrome in an HIV-infected patient

Trimethoprim-sulfamethoxazole (TMP/SMX) is a widely prescribed antimicrobial for the management of several uncomplicated infections. It is commonly used for the treatment and prophylaxis of Pneumocystis jirovecii pneumonia (PCP) in the HIV-infected population. The adverse reaction to TMP/SMX is more frequent and severe in HIV-infected patients as compared to the general population. Here, we report a case of Stevens-Johnson syndrome (SJS) secondary to TMP/SMX. The patient had a generalized cutaneous reaction with involvement of the eyes, oral cavity, and genitals. He had elevated hepatic alanine aminotransferase and aspartate aminotransferase enzyme. TMP/SMX therapy was stopped and supportive treatment was started. His condition improved after eight days of stopping TMP/SMX therapy.

Idiosyncratic adverse drug reactions: current concepts

Idiosyncratic drug reactions are a significant cause of morbidity and mortality for patients; they also markedly increase the uncertainty of drug development. The major targets are skin, liver, and bone marrow. Clinical characteristics suggest that IDRs are immune mediated, and there is substantive evidence that most, but not all, IDRs are caused by chemically reactive species. However, rigorous mechanistic studies are very difficult to perform, especially in the absence of valid animal models. Models to explain how drugs or reactive metabolites interact with the MHC/T-cell receptor complex include the hapten and P-I models, and most recently it was found that abacavir can interact reversibly with MHC to alter the endogenous peptides that are presented to T cells. The discovery of HLA molecules as important risk factors for some IDRs has also significantly contributed to our understanding of these adverse reactions, but it is not yet clear what fraction of IDRs have a strong HLA dependence. In addition, with the exception of abacavir, most patients who have the HLA that confers a higher IDR risk with a specific drug will not have an IDR when treated with that drug. Interindividual differences in T-cell receptors and other factors also presumably play a role in determining which patients will have an IDR. The immune response represents a delicate balance, and immune tolerance may be the dominant response to a drug that can cause IDRs.

Drug hypersensitivity reactions targeting the skin in dogs and cats

Adverse drug reactions (ADRs) can be dose dependent or idiosyncratic. Most idiosyncratic reactions are believed to be immune-mediated; such drug hypersensitivities and allergies are unpredictable. Cutaneous reactions are the most common presentation of drug allergies. In veterinary medicine it can be difficult to assess the true prevalence of adverse drug reactions, although reports available suggest that they occur quite commonly. There are multiple theories that attempt to explain how drug allergies occur, because the pathogenesis is not yet well understood. These include the (pro)-hapten hypothesis, the Danger Theory, the pi concept, and the viral reactivation theory. Cutaneous drug allergies in veterinary medicine can have a variety of clinical manifestations, ranging from pruritus to often fatal toxic epidermal necrolysis. Diagnosis can be challenging, as the reactions are highly pleomorphic and may be mistaken for other dermatologic diseases. One must rely heavily on history and physical examination to rule out other possibilities. Dechallenge of the drug, histopathology, and other diagnostic tests can help to confirm the diagnosis. New diagnostic tools are beginning to be used, such as antibody or cellular testing, and may be used more in the future. There is much yet to learn about drug allergies, which makes future research vitally important. Treatment of drug allergies involves supportive care, and additional treatments, such as immunosuppressive medications, depend on the manifestation of the disease. Of utmost importance is to avoid the use of the incriminating drug in future treatment of the patient, as subsequent reactions can be worse, and ultimately can prove fatal.

Drug allergy: an updated practice parameter

Adverse drug reactions (ADRs) result in major health problems in the United States in both the inpatient and outpatient setting. ADRs are broadly categorized into predictable (type A and unpredictable (type B) reactions. Predictable reactions are usually dose dependent, are related to the known pharmacologic actions of the drug, and occur in otherwise healthy individuals, They are estimated to comprise approximately 80% of all ADRs. Unpredictable are generally dose independent, are unrelated to the pharmacologic actions of the drug, and occur only in susceptible individuals. Unpredictable reactions are subdivided into drug intolerance, drug idiosyncrasy, drug allergy, and pseudoallergic reactions. Both type A and B reactions may be influenced by genetic predisposition of the patient

Anti-myeloperoxidase and anti-cathepsin G antibodies in sulphonamide hypersensitivity

BACKGROUND

Anti-neutrophil cytoplasmic antibodies (ANCA) are associated with vasculitis in humans. Sulphonamide antimicrobials cause drug hypersensitivity (HS) reactions with some clinical signs that are suggestive of vasculitis.

OBJECTIVE

The purpose of this study was to determine whether sulphonamide HS is associated with anti-neutrophil antibodies, using the dog as a spontaneous clinical model.

METHODS

Thirty-four sulphonamide-HS dogs, 11 sulphonamide-'tolerant' dogs, and nine healthy sulphonamide-naïve dogs were evaluated for anti-neutrophil antibodies using a commercial ELISA against human myeloperoxidase (MPO), a commercial human ANCA Western blot protocol, and immunoblotting against whole canine neutrophils.

RESULTS

Using ELISA, anti-MPO antibodies were found with an apparent higher frequency in HS dogs (50%), compared with 'tolerant' dogs (18%), which also showed significantly lower absorbances. Among HS dogs, anti-MPO antibodies were significantly more common, with significantly higher absorbances, in dogs that did not survive the HS reaction (78%) compared with survivors (35%). Using immunoblotting, ANCA were detected with similar overall frequencies in HS and 'tolerant' dogs. However, one protein targeted by several HS dogs, but no 'tolerant' dogs, was identified as cathepsin G.

CONCLUSION

These data indicate that anti-MPO antibodies and anti-cathepsin G antibodies are associated with sulphonamide HS. Anti-MPO antibodies have been shown to be pathogenic both in vitro and in vivo, leading to vasculitis lesions and vasculitis-like syndromes. The present study therefore suggests that vasculitis might be one mechanism of tissue damage in this sulphonamide HS. Furthermore, the evaluation of ANCA, and its relationship to disease severity and clinical outcome, should be considered in human patients with sulphonamide drug HS.

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.

Evaluation of the clinical, immunologic, and biochemical effects of nitroso sulfamethoxazole administration to dogs: a pilot study

Sulfonamide antimicrobials such as sulfamethoxazole (SMX) have been associated in humans with hypersensitivity reactions, to include fever, skin eruptions, hepatotoxicity, and blood dyscrasias. These reactions also occur in dogs, the only non-human species known to develop a similar spectrum of sulfonamide hypersensitivity. Sulfonamide hypersensitivity is not well understood, but has been hypothesized to be due to the generation of the reactive oxidative metabolite, nitroso sulfamethoxazole (SMX-NO). SMX-NO, unlike the parent sulfonamide, is cytotoxic in vitro, haptenizes tissue proteins, and is immunogenic in rodents. The purpose of this pilot study was to determine whether SMX-NO, when administered to dogs, would lead to drug-tissue adducts, anti-drug antibodies, antioxidant depletion, or clinical evidence of drug hypersensitivity. Four dogs were randomized to one of four treatments: SMX-NO 1 mg/kg; SMX-NO 3 mg/kg; SMX-NO 10 mg/kg; or vehicle control. Dosing was by the intraperitoneal route, once daily for four consecutive days per week, for 2 weeks total, followed by a third week of observation. Following this, all dogs were challenged with trimethoprim-sulfamethoxazole, 25 mg/kg for 12 h for 2 weeks. No dog developed clinical or biochemical evidence of drug hypersensitivity. Plasma cysteine and leukocyte reduced glutathione were not depleted during dosing; however, ascorbate was significantly depleted by week 2 following SMX-NO at 10 mg/kg. Anti-SMX antibodies (IgG or IgM by ELISA) were not detected in any dogs at any time points. SMX-hemoglobin adducts were detected in the spleen in SMX-NO dosed dogs; however, these adducts were not accompanied by an immunologic or systemic response. The results of this pilot study indicate that SMX-NO dosing in dogs, using a dosing protocol shown to be immunogenic in other species, produces modest ascorbate depletion and hemoglobin adduct formation, but is insufficient to produce an immunologic response or a clinical syndrome of sulfonamide hypersensitivity in this susceptible species.

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.

Clinical Findings in 40 Dogs with Hypersensitivity Associated with Administration of Potentiated Sulfonamides

The purpose of this study was to summarize the clinical findings in 40 dogs with systemic hypersensitivity reactions associated with the administration of potentiated sulfonamides. Dogs ranged from 6 months to 14 years of age, with a mean of 5.7 +/- 3.2 years. Spayed female dogs were overrepresented (24 of 40, or 60% of the dogs), as were Samoyeds (3 of 40; 8%) and Miniature Schnauzers (5 of 40; 13%). Mean dosages of potentiated sulfonamides were 47.0 +/- 14.9 mg/kg/d (range, 23.4-81.4 mg/kg/d). The time from the 1st administration of the drug to the onset of the clinical signs of hypersensitivity ranged from 5 to 36 days, with a mean of 12.1 +/- 5.9 days. There was no relationship between either the dosage or type of sulfonamide given and the time to the onset of the clinical signs. Fever was the most common clinical sign observed (55% of the dogs); thrombocytopenia was 2nd (54%), and hepatopathy (28%) was 3rd. Neutropenia, keratoconjunctivitis sicca (KCS), hemolytic anemia. arthropathy, uveitis, skin and mucocutaneous lesions, proteinuria, facial palsy, suspected meningitis, hypothyroidism, pancreatitis, facial edema, and pneumonitis were also observed in some patients. Of 39 dogs with adequate follow-up, 30 (77%) recovered, whereas 8 (21%) either died or were euthanized, and 1 recovered clinically but had persistent increases in alanine aminotransferase (ALT) activity. Dogs with hepatopathy generally had a poorer prognosis (46% recovery) than dogs without hepatopathy (89% recovery; P = .0035). Sixty-three percent of the dogs with thrombocytopenia recovered, compared to 90% of the dogs without thrombocytopenia (P = .042). Recovery was not associated with sex, age, breed, or type of sulfonamide administered.

Haptenation of sulfonamide reactive metabolites to cellular proteins

Adverse drug reactions are a major problem complicating medical therapy. The pathogenesis of many severe adverse drug reactions, notably hypersensitivity reactions, is poorly understood. The sulfonamides are associated with severe hypersensitivity reactions. The initial pathogenesis seems to be caused by bioactivation of the parent drug to a reactive intermediate and subsequent propagation by the immune system. The determinants of the immune response are not known. We explored the formation of sulfonamide haptens in Molt-3 and HEPA 1C1C7 cells after incubation with sulfamethoxazole (SMX), the hydroxylamine of sulfamethoxazole (SMX-HA), or the nitroso of sulfamethoxazole (SMX-NO). Haptenation was demonstrated with SMX-HA and SMX-NO but not SMX; this occurred at concentrations below that associated with toxicity (significant haptenation was seen at 25 to 50 microM). Thus, haptenation occurred presumably onto viable cells. Haptenation occurred rapidly; haptenation of cell surface proteins was demonstrated within 5 min. This did not occur indiscriminately; confocal microscopy demonstrated haptenation onto specific sites on the cell membrane. We found that haptenation was significantly inhibited by thiols and other antioxidants (p < 0.05). Sulfonamide-specific haptens were rapidly internalized by what seemed to be a caveolae-dependent process. It seems that sulfonamide reactive metabolites haptenated specific cell surface proteins that are rapidly internalized. Understanding the specific protein target(s) for haptenation and how these haptens are processed will be important in understanding the immune mediation of sulfonamide hypersensitivity adverse drug reactions.

Mechanistic perspectives on sulfonamide-induced cutaneous drug reactions

PURPOSE OF REVIEW

Idiosyncratic drug reactions continue to limit the therapeutic utility of sulfonamide drugs because of their associated morbidity and mortality. Cutaneous reactions are the predominant reasons for withdrawal of such drugs from use in patients. As a consequence of the recognized metabolic and immunologic capability of the skin, an understanding of the pathogenic role of this tissue in the development of sulfonamide-induced cutaneous drug reactions may provide insight into the mechanisms and risk factors for these and other adverse drug events.

RECENT FINDINGS

In the present review we discuss currently available mechanistic information, including issues related to drug bioactivation and adduct formation, immunoresponsiveness, and immune dysregulation, for the development of sulfonamide-induced (delayed-type) cutaneous drug reactions. The potential application of findings from several related areas of research are also discussed within the context of the pathogenesis of these cutaneous reactions.

SUMMARY

Despite progress, numerous unresolved issues support the testing of novel hypotheses, the search for additional risk factors, and the need for a global approach, including links between laboratory and clinical paradigms. These issues must be addressed if we are to gain an understanding of the mechanistic bases for these cutaneous drug reactions.

Antigenicity and immunogenicity of sulphamethoxazole: demonstration of metabolism-dependent haptenation and T-cell proliferation in vivo

Sulphamethoxazole has been associated with the occurrence of hypersensitivity reactions. There is controversy as to whether the immune response is metabolism-dependent or -independent. We have therefore investigated the site of antigen formation and the nature of the drug signal presented to the immune system in vivo. Male Wistar rats were dosed with sulphamethoxazole, sulphamethoxazole hydroxylamine or nitroso sulphamethoxazole. Antigen formation on cell surfaces was determined by flow cytometry using a specific anti-sulphamethoxazole antibody. Immunogenicity was determined by assessment of ex vivo T-cell proliferation. Administration of nitroso sulphamethoxazole, but not sulphamethoxazole or sulphamethoxazole hydroxylamine, resulted in antigen formation on the surface of lymphocytes, splenocytes and epidermal keratinocytes, and a strong proliferative response of splenocytes on re-stimulation with nitroso sulphamethoxazole. Rats dosed with sulphamethoxazole or sulphamethoxazole hydroxylamine did not respond to any of the test compounds. CD4+ or CD8+ depleted cells responded equally to nitroso sulphamethoxazole. The proliferative response to nitroso sulphamethoxazole was seen even after pulsing for only 5 min, and was not inhibited by glutathione. Responding cells produced IFN-gamma, but not IL-4. Haptenation of cells by sulphamethoxazole hydroxylamine was seen after depletion of glutathione by pre-treating the rats with diethyl maleate. Splenocytes from the glutathione-depleted sulphamethoxazole hydroxylamine-treated rats responded weakly to nitroso sulphamethoxazole, but not to sulphamethoxazole or sulphamethoxazole hydroxylamine. Dosing of rats with sulphamethoxazole produced a cellular response to nitroso sulphamethoxazole (but not to sulphamethoxazole or its hydroxylamine) when the animals were primed with complete Freund's adjuvant. These studies demonstrate the antigenicity of nitroso sulphamethoxazole in vivo and provide evidence for the role of drug metabolism and cell surface haptenation in the induction of a cellular immune response in the rat.

Plasma cysteine deficiency and decreased reduction of nitrososulfamethoxazole with HIV infection

The aim of these studies was to determine whether HIV-infected patients have a plasma thiol deficiency and whether this is associated with decreased detoxification of the toxic metabolites of sulfamethoxazole. Reduced, oxidized, protein-bound, and total thiol levels were measured in 33 HIV-positive patients and 33 control subjects by an HPLC method utilizing the fluorescent probe bromobimane. The reduction of sulfamethoxazole hydroxylamine and nitrososulfamethoxazole by plasma and the plasma redox balance in the presence of nitrososulphamethoxazole were also determined by HPLC. Reduced plasma cysteine was significantly (p<0.0001) lower in HIV-positive patients (13.0+/-3.0 microM) when compared with control subjects (16.9+/-3.0 microM). Although there was no difference in oxidized, protein-bound, and total cysteine, the thiol/disulfide ratios were lower in HIV-positive patients. Reduced homocysteine was elevated in patients. Plasma from HIV-positive patients was less able to detoxify nitrososulfamethoxazole than control plasma. These findings show that the disturbance in redox balance in HIV-positive patients may alter metabolic detoxification capacity, and thereby predispose to sulfamethoxazole hypersensitivity.

Immunological principles of adverse drug reactions: the initiation and propagation of immune responses elicited by drug treatment

Adverse drug reactions account for between 2 to 5% of all hospital admissions and can prevent the administration of an otherwise effective therapeutic agent. Hypersensitivity or immune-mediated reactions, although less common, tend to be proportionately more serious. There is convincing evidence to implicate the immune system in the pathogenesis of hypersensitivity reactions. Our understanding of the way in which the immune system recognises drugs is based on the hapten hypothesis; the onset of hypersensitivity involves drug bioactivation, covalent binding to proteins, followed by uptake, antigen processing and T cell proliferation. Central to this hypothesis is the critical role of drug metabolism, with the balance between metabolic bioactivation and detoxification being one important component of individual susceptibility. The purpose of this review is to classify drug hypersensitivity reactions in terms of their clinical presentation, and also to consider recent advances in our understanding of the chemical, biochemical and, in particular, cellular immunological mechanisms of hypersensitivity. The following topics are reviewed: (i) drug disposition and cellular metabolism; (ii) mechanisms of antigen processing and presentation; (iii) the role of cytokines and co-stimulatory molecules in the induction and maintenance of a polarised immune response; and (iv) the application of the hapten hypothesis, danger hypothesis and serial triggering model to drug hypersensitivity. A greater understanding of the mechanism(s) of hypersensitivity may identify novel therapeutic strategies and help to combat one of the more severe forms of adverse reactions to drugs.

Cellular disposition of sulphamethoxazole and its metabolites: implications for hypersensitivity

1. Bioactivation of sulphamethoxazole (SMX) to chemically-reactive metabolites and subsequent protein conjugation is thought to be involved in SMX hypersensitivity. We have therefore examined the cellular metabolism, disposition and conjugation of SMX and its metabolites in vitro. 2. Flow cytometry revealed binding of N-hydroxy (SMX-NHOH) and nitroso (SMX-NO) metabolites of SMX, but not of SMX itself, to the surface of viable white blood cells. Cellular haptenation by SMX-NO was reduced by exogenous glutathione (GSH). 3. SMX-NHOH and SMX-NO were rapidly reduced back to the parent compound by cysteine (CYS), GSH, human peripheral blood cells and plasma, suggesting that this is an important and ubiquitous bioinactivation mechanism. 4. Fluorescence HPLC showed that SMX-NHOH and SMX-NO depleted CYS and GSH in buffer, and to a lesser extent, in cells and plasma. 5. Neutrophil apoptosis and inhibition of neutrophil function were induced at lower concentrations of SMX-NHOH and SMX-NO than those inducing loss of membrane viability, with SMX having no effect. Lymphocytes were significantly (P<0.05) more sensitive to the direct cytotoxic effects of SMX-NO than neutrophils. 6. Partitioning of SMX-NHOH into red blood cells was significantly (P<0.05) lower than with the hydroxylamine of dapsone. 7. Our results suggest that the balance between oxidation of SMX to its toxic metabolites and their reduction is an important protective cellular mechanism. If an imbalance exists, haptenation of the toxic metabolites to bodily proteins including the surface of viable cells can occur, and may result in drug hypersensitivity.