Pharmacogenetics and blood dyscrasias

Pharmacogenetic differences in the handling of and response to drugs can markedly alter the risk of severe idiosyncratic adverse drug reactions, including neutropenia, agranulocytosis and aplastic anaemia. Inherited deficiencies of drug metabolizing enzymes can shunt the metabolism of drugs to metabolites which are directly toxic (e.g. 6-mercaptopurine metabolism to 6-thioguanine nucleotides) or towards electrophilic metabolites which can kill cells and/or lead to a host immune response (e.g. sulphonamide metabolism to hydroxylamine metabolites). Defects in detoxification pathways (e.g. glutathione conjugation) similarly can predispose patients to adverse outcomes. The advent of molecular screening tools to define individual (rather than population) risk may lead to the use of clinical laboratory tests to identify/predict idiosyncratic adverse drug reactions.

Concordance between tramadol and dextromethorphan parent/metabolite ratios: the influence of CYP2D6 and non-CYP2D6 pathways on biotransformation

Cytochrome P4502D6 (CYP2D6) activity has been shown to be a determinant of both the pharmacokinetics and pharmacodynamics of tramadol in adults. This study evaluated the association between CYP2D6 activity, as determined by dextromethorphan (DM) urinary metabolite ratio, and tramadol biotransformation in 13 children (7-16 years). CYP2D6 genotype was determined by XL-PCR and PCR/RFLP. Phenotype was assessed by HPLC quantitation of DM and its metabolites from a 12- to 24-hour urine collection following a single oral dose of DM. There was only a modest correlation between tramadol/M1 (metabolite 1) plasma concentration or AUC and the DM/dextrorphan (DX) urinary molar ratio in the study cohort; however, when subjects were segregated based on the number of functional CYP2D6 alleles, a much stronger relationship was observed for subjects with two functional alleles, with essentially no relationship evident in those individuals with one functional allele. Further evaluation of these data suggested that the CYP2D6-mediated metabolite (M1) is formed to a lesser extent, and the formation of the non-CYP2D6 product (M2) is more pronounced in subjects with one versus two functional alleles. Thus, the number of functional CYP2D6 alleles and the availability of alternative cytochromes P450 capable of metabolizing tramadol may explain the poor association between DM, a well-characterized CYP2D6 probe, and tramadol in a population of CYP2D6 extensive metabolizers.

Deficiency of cytosolic arylamine N-acetylation in the domestic cat and wild felids caused by the presence of a single NAT1-like gene

The purpose of this study was to determine the molecular basis for a relative deficiency in the cat of cytosolic arylamine N-acetyltransferase (NAT), an enzyme family that is important in the metabolism of xenobiotics and that normally consists of at least two related enzymes, NAT1 and NAT2. N-acetyltransferase in feline liver showed high affinity (mean Km = 2.1 microM) for p-aminobenzoic acid, an NAT1 selective substrate in humans and rabbits, but showed a very poor affinity (mean Km > 10 mM) for sulfamethazine, an NAT2 selective substrate in humans and rabbits. Immunoreactive N-acetyltransferase was detected in feline liver, bladder and colon using an NAT1-specific antipeptide antibody, but was not detected in any tissues using an NAT2-specific antibody. Southern blot analysis of genomic DNA demonstrated a single band in domestic cats using each of six restriction digests; single bands were also found on Southern blot analysis of six wild felids. The deduced amino acid sequence of the central portion of feline N-acetyltransferase, obtained by polymerase chain reaction amplification in both domestic cats and seven wild felids (lion, tiger, lynx, snow leopard, bobcat, Asian leopard cat and cheetah), contained three residues, Phe125, Arg127, and Tyr129, which determine NAT1-like substrate specificity in humans. These results support the conclusion that cytosolic arylamine N-acetylation activity is low in the cat because of the presence of a single N-acetyltransferase that has substrate specificity, immunogenicity and sequence characteristics similar to human NAT1, and that the unusual presence of only a single N-acetyltransferase gene appears to be a family wide trait shared by other felids.

Patients with delayed-onset sulfonamide hypersensitivity reactions have antibodies recognizing endoplasmic reticulum luminal proteins

Sulfonamide antimicrobials cause a delayed-onset, hypersensitivity-type syndrome characterized by fever, skin rash and multiorgan toxicity occurring 7 to 14 days after initiation of therapy. The pathogenesis is believed to be immune-mediated. We investigated whether patients with delayed-onset sulfonamide hypersensitivity reactions had antibodies recognizing hapten-microsomal protein conjugates and/or native microsomal proteins. By immunoblotting using rat liver as a source of microsomal protein, 17 of 21 patients had antibodies recognizing one or more of three native endoplasmic reticulum proteins of 55 kDa (14 of 21 patients), 80 kDa (4 of 21 patients) or 96 kDa (3 of 21 patients) in size on sodium dodecyl sulfate-polyacrylamide gel electrophoresis. No control subjects (n = 11) and only 1 of 18 patients with adverse events not consistent with sulfonamide hypersensitivity reactions had antibodies against these microsomal proteins under the conditions used. Only 1 patient had antibodies that recognized the sulfonamide hapten, sulfamethoxazole. The 55-kDa protein was identified as protein disulfide isomerase. The 80-kDa protein was identified as grp78. The 96-kDa protein was not identified. Delayed-onset sulfonamide hypersensitivity reactions are therefore primarily associated with antibodies recognizing specific protein epitopes and not anti-drug antibodies.

Cytosolic arylamine N-acetyltransferase (NAT) deficiency in the dog and other canids due to an absence of NAT genes

The purpose of this study was to determine the molecular basis in the dog for an unusual and absolute deficiency in the activity of cytosolic N-acetyltransferase (NAT), an enzyme important for the metabolism of arylamine and hydrazine compounds. NAT activity towards two NAT substrates, p-aminobenzoic acid and sulfamethazine, was undetectable in dog liver cytosol, despite substrate concentrations ranging from 10 microM to 4 mM and a wide range of incubation times. Similarly, no protein immunoreactive to NAT antibody was evident on western blot analysis of canine liver cytosol. Southern blot analysis of genomic DNA from a total of twenty-five purebred and mixed bred dogs, and eight wild canids, probed with a full-length human NAT2 cDNA, suggested an absence of NAT sequences in all canids. Polymerase chain reaction amplification of genomic DNA using degenerate primers designed to mammalian NAT1 and NAT2 consensus sequences generated products of the expected size in human, mouse, rabbit, and cat DNA, but no NAT products in any dog or wild canids. These results support the conclusion that cytosolic NAT deficiency in the domestic dog is due to a complete absence of NAT genes, and that this defect is shared by other canids.

N-acetyltransferases: pharmacogenetics and clinical consequences of polymorphic drug metabolism

Since the discovery of polymorphic N-acetylation of drugs nearly 40 years ago, great progress has been made in understanding the molecular genetics of acetylation as well as the clinical consequences of being a rapid or slow acetylator. Inborn errors (several different alleles) at the NAT2 locus are responsible for the traditional acetylator polymorphism. Studies have revealed variant alleles at the NAT1 locus as well. The consequences of pharmacogenetic variation in these enzymes include (i) altered kinetics of specific drug substrates; (ii) drug-drug interactions resulting from altered kinetics; (iii) idiosyncratic adverse drug reactions. The latter have been extensively investigated for the arylamine-containing sulfonamide antimicrobial drugs. Individual differences in multiple metabolic pathways can increase the likelihood of covalent binding of reactive metabolites of the drugs to cell macromolecules with resultant cytotoxicity and immune response to neoantigens. This can result clinically in an idiosyncratic hypersensitivity reaction, manifested by fever, skin rash, and variable toxicity to organs including liver, bone marrow, kidney, lung, heart, and thyroid. Slow acetylation by NAT2 is a risk factor for such reactions to sulfonamides. Given the incidence of these severe adverse drug reactions (much less than 1/1000), slow acetylation cannot be the sole mechanism of predisposition in the population. Differences in rates of production of hydroxylamine metabolites of the drugs by cytochrome P450 (CYP2C9), myeloperoxidase, and thyroid, roxidase, along with an inherited abnormality in detoxification of the hydroxylamines are critically important in determining individual differences in adverse reaction risk. Both NATs, particularly NAT1, also can further metabolize hydroxylamine metabolites to N-acetoxy derivatives. Intensive investigation of patients with these rare adverse reactions using a variety of tools from in vitro cell toxicity assays through molecular genetic analysis will help elucidate mechanisms of predisposition and ultimately lead to diagnostic tools to characterize individual risk and prevent idiosyncratic drug toxicity.

Covalent binding of sulfamethoxazole reactive metabolites to human and rat liver subcellular fractions assessed by immunochemical detection

Potentially serious idiosyncratic reactions associated with sulfamethoxazole (SMX) include systemic hypersensitivity reactions and hepatotoxicity. Covalent binding of SMX to proteins subsequent to its N-hydroxylation to form N4-hydroxysulfamethoxazole (SMX-HA) is thought to be involved in the pathogenesis of these reactions. A polyclonal antibody was elicited in rabbits against a SMX--keyhole limpet hemocyanin conjugate that recognized covalent protein adducts of SMX in microsomal protein and was used to characterize the covalent binding of SMX and its putative reactive metabolites to hepatic protein in vivo and in vitro. In vitro covalent binding of SMX to rat and human liver microsomal protein was NADPH-dependent, while binding of SMX-HA was not dependent on NADPH. SMX and SMX-HA produced similar patterns of covalent binding, with major protein targets in the region of 150, 100 (two bands), 70 (two bands), and 45-55 kDa. The pattern of covalent binding to human and rat liver microsomal protein was similar. Binding of SMX-HA was completely eliminated by GSH or by addition of cytosolic fractions and acetylcoenzyme A. The acetoxy metabolite of SMX also led to covalent binding, but it was primarily attributable to the formation of SMX-HA from acetoxySMX. In vivo exposure of rats to SMX did not result in detectable covalent binding by the methods employed. When rat liver slices were incubated with 2 mM SMX or 500 microM SMX-HA, no toxicity was observed and yet covalent binding of SMX-HA to 130, 100, 70, and 55 kDa proteins could be detected. These results confirm that covalent binding of SMX occurs via the formation of SMX-HA and that covalent binding of SMX-HA in vitro results from its conversion to the more reactive nitroso metabolite. Acetylation of SMX-HA protected against its covalent binding. Further studies are required to determine how this in vitro covalent binding relates to in vivo covalent binding in humans and to either direct or immune-mediated cytotoxicity in SMX idiosyncratic drug reactions.

In vitro formation, disposition and toxicity of N-acetoxy-sulfamethoxazole, a potential mediator of sulfamethoxazole toxicity

Variation in the formation and disposition of the hydroxylamine of (SMX-HA) is thought to play an important role in the pathogenesis of sulfamethoxazole (SMX)-induced idiosyncratic adverse drug reactions. We hypothesized that, in analogy to carcinogenic arylamines, SMX-HA might be further converted to an electrophilic N-acetoxy metabolite which could play a role in mediating SMX toxicity. Accordingly, we chemically synthesized N-acetoxy-SMX, and examined the characteristics of its formation, metabolism, cytotoxicity and mutagenicity in human and bacterial test systems. The human arylamine N-acetyl-transferases, (NAT)1 and NAT2, were capable of converting SMX-HA to N-acetoxy-SMX. NAT1 and NAT2 possessed similar affinities for SMX-HA (apparent Km values of 650 and 520 microM, respectively), but the apparent maximal velocity of the NAT1-mediated acetylation was higher than that of NAT2. (1332 vs. 37 nmol/min/U of immunoreactive NAT protein). Human peripheral blood mononuclear cells 12,000 x g supernatant fractions converted N-acetoxy-SMX mainly back to SMX-HA, and also to a lesser extent to SMX, at clinically relevant concentrations. Similar pathways were observed in human hepatic cytosolic fractions. In a cytotoxicity assay, N-acetoxy-SMX was significantly more toxic to human peripheral blood mononuclear cells than SMX-HA (16.6 vs. 11.5% dead cells at a concentration of 300 microM). N-acetoxy-SMX was weakly mutagenic to the Salmonella typhimurium TA100 strain in the Ames test. These data suggest that the N-acetoxy metabolites of sulfonamides could potentially play a role in mediating sulfonamide idiosyncratic adverse drug reactions.

N4-hydroxylation of sulfamethoxazole by cytochrome P450 of the cytochrome P4502C subfamily and reduction of sulfamethoxazole hydroxylamine in human and rat hepatic microsomes

The N4-hydroxylation of sulfamethoxazole (SMX) to its hydroxylamine (SMX-HA) metabolite is the first step in the formation of reactive metabolites responsible for mediating hypersensitivity reactions associated with this compound. In rat hepatic microsomes, the NADPH-dependent oxidation of SMX to SMX-HA was increased 3-fold by pretreatment of rats with phenobarbital. Other cytochrome P450 (CYP) inducers were ineffective. The constitutive and induced SMX N-hydroxylation activities were inhibited by tolbutamide, and induction of SMX-HA activity paralleled the induction of progesterone 21-hydroxylase activity, a marker for CYP2C6. SMX N-hydroxylation in phenobarbital-treated rat hepatic microsomes was inhibited 70% by anti-CYP2C6 antisera. Thus, the N4-hydroxylation of SMX by rat hepatic microsomes was mediated by members of the CYP2C subfamily, probably CYP2C6. In a panel of human microsomes, SMX-HA formation correlated with tolbutamide hydroxylase activity (r = 0.75; p = 0.01); CYP2C9 content (r = 0.79; p < 0.01) and was inhibited 70% by 500 microM tolbutamide and 90% by 100 microM sulfaphenazole. Recombinant CYP2C9 catalyzed the N-hydroxylation of SMX. SMX-HA formation in human hepatic microsomes was therefore mediated predominantly by CYP2C9. CYP-mediated reduction of SMX-HA to SMX was markedly induced in dexamethasone and phenobarbital-treated rat hepatic microsomes, and was attributed to CYP3A and CYP2B forms. In uninduced rat and human hepatic microsomes, SMX-HA reduction was mediated predominantly by an NADH-dependent microsomal hydroxylamine reductase under aerobic conditions. Under anaerobic conditions, troleandomycin at > or = 1 microM inhibited the reduction of SMX-HA in human hepatic microsomes by 45%, whereas sulfaphenazole had no effect.(ABSTRACT TRUNCATED AT 250 WORDS)

Enhanced differential diagnosis of anticonvulsant hypersensitivity reactions by an integrated Bayesian and biochemical approach

OBJECTIVE

The differential diagnosis of hypersensitivity reactions associated with anticonvulsants requires accuracy because of the many implications for patient management. We tested an integrated Bayesian and biochemical diagnostic approach.

METHODS

The patients were analyzed clinically by two tests. One test, the Bayesian Adverse Reaction Diagnostic Instrument (BARDI), calculates the posterior probability of a drug being the cause based on epidemiologic and case data. The other, the lymphocyte toxicity assay, is an in vitro rechallenge that determines the percentage of cell death attributable to a drug's toxic metabolites. The setting for the study was an adverse drug reaction clinic at Sunnybrook Health Science Centre and the Hospital for Sick Children, Toronto, Ontario, Canada. Fifty-one patients who had hypersensitivity reactions after receiving aromatic anticonvulsants were tested. Four of these patients had more than one reaction reported, with different anticonvulsants generating 56 distinct events.

RESULTS

Compared to the lymphocyte toxicity assay, BARDI had 94% sensitivity, 93% accuracy, and 50% specificity. When lymphocyte toxicity assay data were incorporated into BARDI, agreement rose from 93% to 100%. BARDI also identified which drug was a more likely cause for 11 patients receiving multiple anticonvulsants.

CONCLUSION

These findings show that BARDI and the lymphocyte toxicity assay have high concordance and, when used in an integrated approach, these tests can improve the diagnostic accuracy and enhance the management of patients with hypersensitivity reactions.

The efficacy and safety of granisetron in pediatric cancer patients who had failed standard antiemetic therapy during anticancer chemotherapy

PURPOSE

This study was undertaken to evaluate the safety and efficacy of granisetron (a 5-hydroxytryptamine. antagonist) in children with malignant disease who had previously experienced unacceptable nausea and vomiting and/or adverse effects associated with standard antiemetic therapy.

PATIENTS AND METHODS

Thirty children 3-18 years of age who were receiving anticancer chemotherapy were enrolled in the study. Patients received a prophylactic dose of granisetron before chemotherapy and two subsequent doses as needed. If further antiemetics were required, standard therapy was given and those patients were classified as treatment failures. Patients received granisetron during one to three cycles of chemotherapy; a total of 66 courses were given.

RESULTS

Eighty-seven percent of patients had good control of nausea and vomiting with granisetron alone; 90% of patients elected to receive granisetron with subsequent chemotherapy. No loss of efficacy was noted with repeated cycles in 21 patients. No serious adverse events occurred.

CONCLUSIONS

Intravenous granisetron (20 micrograms/kg/dose) appears to be a safe and effective drug for pediatric patients receiving emetogenic chemotherapy.

Characterization of the microsomal epoxide hydrolase gene in patients with anticonvulsant adverse drug reactions

Therapy with the aromatic anticonvulsants phenytoin, phenobarbital and carbamazepine has been associated with the occurrence of rare idiosyncratic hypersensitivity reactions. These drugs are thought to be activated to potentially reactive arene oxide (epoxide) metabolites by cytochrome P450-dependent monooxygenation, while liver microsomal epoxide hydrolase (mEH) plays a detoxifying role by converting such reactive intermediates to non-toxic dihydrodiols. Evidence from in vitro lymphocyte toxicity tests and enzyme inhibitor studies has suggested that an inherited defect in mEH function may be responsible for the enhanced drug toxicity observed in affected individuals. To test this hypothesis we designed methods to directly compare mEH gene structure in patients presenting with anticonvulsant adverse reactions and in control subjects in which no in vivo or in vitro toxicity to anticonvulsants could be demonstrated. Southern analysis of peripheral lymphocyte DNA using a full-length mEH cDNA as hybridization probe revealed no gross differences in mEH gene structure in selected patients when compared with DNA samples from unaffected control subjects. The human mEH gene was then cloned and characterized from a control individual. Nine exons were identified within a 22 kb region and sequences of selected regions, including all exons, were determined. Single strand conformation polymorphism (SSCP) analysis was performed on all exonic regions in genomic DNA from each of 26 subjects, including six unrelated patients with previous toxicity to anticonvulsant therapy and seven siblings (three of whom had displayed toxicity). Several distinct SSCP patterns could be observed among the subjects tested, each corresponding to a specific point mutation within one of the amplified fragments of the mEH gene. However, none of the SSCP patterns reflecting point mutations was correlated with the occurrence of anticonvulsant toxicity. From these observations we conclude that a genetic defect altering the structure and function of the mEH protein is unlikely to be responsible for predisposing patients to anticonvulsant adverse reactions.

A comparative study of the toxicity of chemically reactive xenobiotics towards adherent cell cultures: selective attenuation of menadione toxicity by buthionine sulphoximine pretreatment

Metabolic activation to reactive intermediates is a prerequisite for many forms of chemically-induced toxicity. Hepa 1c1c-9 cells were exposed to varying concentrations of several reactive metabolites implicated in adverse drug reactions and the toxicity of the compounds assessed using applied fluorescence technology. Cytotoxicity was assessed using the fluorescence of 2', 7'-bis-(2-carboxyethyl)-5-(6)-carboxy-fluorescein as an index of cell viability. The role of glutathione in cellular defence against these chemicals was investigated by pretreating the target cells overnight with buthionine sulphoximine, a specific inhibitor of glutathione synthesis. Depletion of intracellular glutathione augmented the toxicity of N-acetyl-p-benzoquinone imine (1.5-3-fold at 100 and 10 microM). Toxicity produced by the hydroxylamine of sulphamethoxazole (500 microM) was dependent entirely on pretreatment of the cells with buthionine sulphoximine (% cell death = 33 +/- 16 compared with 0 +/- 4 in untreated cells, P < 0.05). By contrast, the lethal effects of the model quinone, menadione, were attenuated markedly following glutathione depletion. The data obtained suggest that this assay, previously used with suspension cultures, may be useful in the rapid in-vitro screening of putative reactive intermediates. Moreover, the application of such methodology should prove beneficial for the elucidation of cellular mechanisms of defence and detoxification.

Role of polymorphic and monomorphic human arylamine N-acetyltransferases in determining sulfamethoxazole metabolism

Sulfonamides are associated with a variety of adverse reactions, some of which have been linked with the classical acetylator phenotypes. Although the slow acetylator phenotype has been identified as a risk factor for hypersensitivity reactions to sulfamethoxazole (SMX), the disposition of this compound appears not to be affected by the acetylation polymorphism in vivo in humans. We therefore investigated the acetylation of SMX by monomorphic (NAT1) and polymorphic (NAT2) arylamine N-acetyltransferases in humans with the objective of determining their role in the metabolism of SMX. SMX was acetylated by both NAT1 and NAT2. Km values determined in hepatic cytosol for NAT1- and NAT2-mediated acetylation of SMX were 1.2 mM and approximately 5 mM, respectively, at an acetyl coenzyme A concentration of 100 microM. Mononuclear leukocytes, which contain only NAT1, had a Km value of 1.2 mM. Km values determined with recombinant NAT1 and NAT2 proteins expressed in Escherichia coli were 1.5 mM and approximately 15 mM, respectively. The higher affinity of NAT1 for SMX indicates that acetylation by this enzyme will predominate at therapeutic plasma concentrations, in agreement with the observed in vivo monomorphic acetylation of SMX. NAT1 may be the primary determinant of SMX systemic metabolic clearance. However, in the hepatocyte NAT2 variation may be an important competitive pathway which influences the extent of oxidative metabolism of SMX to its reactive hydroxylamine metabolite. Therefore, variation in both monomorphic and polymorphic N-acetyltransferases may play a role in determining susceptibility to sulfamethoxazole toxicity.

Human anti-cytochrome P450 antibodies in aromatic anticonvulsant-induced hypersensitivity reactions

Aromatic anticonvulsants such as phenytoin, phenobarbital and carbamazepine are associated with a hypersensitivity syndrome (fever, rash lymphadenopathy, hepatitis) suggestive of an immune component. We have identified immunoglobulin G antibodies in the sera of nine affected patients which recognize a 53-kD protein which is constitutively expressed and PB inducible in rat liver microsomes. No such reactivity was observed in sera from healthy controls, patients on chronic phenytoin therapy without toxicity or patients with hepatic failure not receiving anticonvulsants. Using highly purified rat hepatic cytochrome P450, P450 3A1 was identified as the major antigenic species, whereas less intense reactivity was noted with P450 2C11. P450 2C6 and 3A2 were minor antigens in some patients. In all patients, the apparent constitutive and phenobarbital-inducible expression of the antigen was a composite effect of antibodies reacting with at least two isozymes, one of which was constitutively expressed and the other PB inducible. In human liver, a 53-kD antigen was expressed to a greater extent in microsomes from a patient with a fatal hepatotoxic reaction to phenytoin compared to microsomes from normal liver or from a sulfonamide hepatitis patient. Western blotting with microsomes prepared from lymphoblastoid cell lines transfected with different human hepatic cytochromes P450 failed to identify P450s 1A1, 1A2, 2A3, 2B6, 2C9, 2D6, 2E1, 3A4 or epoxide hydrolase as the target antigen. Identification of the antigen will be important in understanding the relationship between drug metabolism and the subsequent immune response in the pathogenesis of these rare but severe forms of drug toxicity.

Sulfamethoxazole is metabolized to the hydroxylamine in humans

The oxidation of sulfamethoxazole to its hydroxylamine metabolite was investigated in vitro with human liver microsomes and in vivo by detection in the urine. Sulfamethoxazole was oxidized to the hydroxylamine in an NADPH-dependent process by liver microsomes prepared from two human livers. Three healthy volunteers ingested 1000 mg sulfamethoxazole, and urine was collected for 24 hours. Sulfamethoxazole hydroxylamine constituted 3.1% +/- 0.7% of the drug excreted in the urine in 24 hours. Fifty-four percent of the ingested dose was excreted during this same time period. We conclude that sulfamethoxazole hydroxylamine is an authentic in vivo metabolite in humans, probably formed predominantly by cytochrome P450 in the liver. It could be responsible for mediation of sulfonamide adverse reactions, particularly hypersensitivity reactions.

Idiosyncratic drug reactions: interaction of development and genetics

There is a major need for biologic markers closely linked to the mechanisms of susceptibility and toxicity of xenobiotics that can aid in elucidation of populations at risk. Such markers, genetic or developmental, need to be interpreted in light of the mechanism of toxicity of the specific compound. It is likely that maximum progress in this area will be made by integration of biologic markers of susceptibility (closely linked to the basic toxicologic mechanisms of the compound in question) with human epidemiologic studies. "New" pharmacogenetic variants in drug metabolism are being discovered at an ever-increasing rate. Molecular biology approaches to variant genes, as well as in vivo and cellular probes of toxicologically important metabolic pathways, are likely to serve as significant tools in predicting who is at risk for an ADR, and in confirming clinical impressions of ADR causality. Linked with epidemiologic studies of ADR incidence, it will be increasingly possible to discover the relative roles of development and genetics in ADRs.

Drug-induced hypothyroidism: the thyroid as a target organ in hypersensitivity reactions to anticonvulsants and sulfonamides

Inherited defects in detoxification of reactive metabolites of drugs predispose patients to "hypersensitivity" reactions. Covalent interaction of metabolites with cell macromolecules leads to cytotoxic and immunologic outcomes, manifested clinically by multisystem syndromes with variable organ involvement. Hypothyroidism developed in 5 of 202 patients (age range, 1 to 81 years) we investigated for hypersensitivity reactions to anticonvulsants or sulfonamides shortly after their reaction. None had previous personal or family histories of autoimmune disease. All had low thyroxine levels, elevated levels of thyroid stimulating hormone, and autoantibodies including antimicrosomal antibodies. Patients were 2 to 18 years of age at presentation, and two were male. All returned to a euthyroid state within a year of presentation, and all remain well. The demographics, clinical presentation, and course of the patients is atypical of idiopathic lymphocytic thyroiditis. We investigated the pathogenesis of thyroid toxicity using the hydroxylamine metabolite of sulfamethoxazole as a model. The hydroxyalmine was toxic to thyroid cells in vitro, which did or did not express thyroid peroxidase activity, whereas the parent sulfonamide was toxic only to cells with active thyroid peroxidase. The purified enzyme converted sulfamethoxazole to the hydroxylamine. Formation of reactive drug metabolites by thyroid peroxidase in a host who is genetically unable to detoxify the metabolites may lead directly to cytotoxicity. Covalent binding to macromolecules, including thyroid peroxidase, also may lead to expression of neoantigens and formation of autoantibodies. Patients who have sustained hypersensitivity reactions to drugs should be investigated for possible involvement of the thyroid.

Expression of monomorphic arylamine N-acetyltransferase (NAT1) in human leukocytes

The expression of arylamine N-acetyltransferase (NAT) in leukocytes was investigated using p-aminobenzoic acid (PABA) and sulfamethazine (SMZ), substrates which are preferentially acetylated by the monomorphic NAT1 and polymorphic NAT2 enzymes, respectively. Activity towards both substrates was detected in mononuclear leukocytes (MNL; preparation containing approximately 80% lymphocytes), monocytes and neutrophils. PABA and SMZ acetylation rates were highly correlated in each of the isolated cell types. The NAT in leukocytes displayed a much higher affinity and turnover rate for the acetylation of PABA than for SMZ. These kinetic characteristics suggest that the acetylating activity in human leukocytes is predominantly attributable to the monomorphic enzyme NAT1. Neutrophils showed evidence of biphasic kinetics for SMZ which would indicate the coexpression of NAT1 and low levels of the polymorphic enzyme, NAT2. NAT activity in MNL was not influenced by the acetylator phenotype of the individual. There was, however, a significant correlation between NAT activity in MNL and the in vivo acetylation (urinary metabolite ratio) of p-aminosalicylic acid, which is monomorphically acetylated in humans. The expression of NAT1 in leukocytes and the virtuall absence of NAT2 may have important toxicological implications. The in vitro/in vivo correlation suggests that leukocytes may be a useful marker of systemic NAT1 activity.

Familial occurrence of hypersensitivity to phenytoin

PURPOSE

Therapy with anticonvulsants such as phenytoin, phenobarbital, and carbamazepine can be complicated by severe hypersensitivity reactions. Previous work has suggested that the predisposition to such reactions is based on an inherited abnormality in the detoxification of reactive metabolites of the drugs. However, there are no reports of familial occurrence of the reactions in the literature. In the current study, we examined a family in which three siblings developed hypersensitivity reactions to phenytoin, confirming the inheritance of a predisposition to the reactions. Detoxification of reactive metabolites of the anticonvulsants was studied in cells from the patients and their siblings.

PATIENTS AND METHODS

Three siblings from a family of 12 siblings developed hypersensitivity reactions to phenytoin characterized by fever, rash, lymphadenopathy, and anicteric hepatitis. All recovered completely after discontinuation of treatment. One sibling tolerated phenobarbital without toxic sequelae. Peripheral blood mononuclear cells from the three patients and five additional siblings who had never taken anticonvulsants were exposed to oxidative metabolites of phenytoin, phenobarbital, and carbamazepine generated by a hepatic microsomal drug-metabolizing system in vitro. The toxicity of metabolites in the cells from the siblings was compared with that in cells from control subjects.

RESULTS

Cells from each of the patients who had experienced a hypersensitivity reaction exhibited increased toxicity from metabolites of phenytoin and carbamazepine, while the cellular response to metabolites of phenobarbital was within normal limits. Cells from four of the other siblings showed an abnormal response to phenytoin metabolites, while cells from the final sibling detoxified phenytoin metabolites normally.

CONCLUSION

Our observations on the patients confirm the inherited nature of phenytoin hypersensitivity reactions in vivo. In vitro studies demonstrated abnormal metabolite detoxification in the patients and several of their siblings. The detoxification defect included metabolites of phenytoin and carbamazepine but not of phenobarbital. A family history of a drug hypersensitivity reaction should alert physicians to the probability of a markedly increased risk of an adverse reaction in family members. In vitro assays to confirm adverse reaction risks may ultimately be able to provide individualized risk assessment for patients who must take anticonvulsants.

Reactions of the nitroso and hydroxylamine metabolites of sulfamethoxazole with reduced glutathione. Implications for idiosyncratic toxicity

N4-oxidation of sulfonamides has been implicated in the pathogenesis of idiosyncratic reactions to these antimicrobials. In vitro toxicity assays employing mononuclear leukocytes as target cells have shown that the toxicity of sulfamethoxazole hydroxylamine (SMX-HA) is inhibited by exogenous glutathione, suggesting that conjugation with glutathione is an important detoxification pathway. However, in these experiments, significant depletion of cellular glutathione only occurred at concentrations of SMX-HA greater than or equal to 300 microM. At concentrations of SMX-HA which produce 50% toxicity in mononuclear leukocytes (approximately 100 microM), there was not a significant loss of glutathione. SMX-HA also caused a small but significant increase in oxidized glutathione concentrations. In cell-free experiments, reduced glutathione (GSH) prevented the autooxidation of SMX-HA to nitrososulfamethoxazole (nitroso-SMX). During this process, oxidized glutathione was formed. GSH rapidly reacted with nitroso-SMX to form a labile semimercaptal conjugate. Physiologically relevant concentrations of GSH (i.e. 1 mM) favored thiolytic cleavage of the semimercaptal to form SMX-HA. Isomerization of the semimercaptal to the more stable sulfinamide occurred at low GSH concentrations. Purified glutathione transferases had no effect on the reaction of SMX-HA with GSH. Therefore, glutathione is important in protecting cells from the toxicity of SMX-HA largely by preventing its further oxidation to nitroso-SMX. Stable glutathione conjugates are likely to be formed only in small quantities under physiological conditions. Conjugation with glutathione would not be expected to be a major pathway for clearance of the hydroxylamine and nitroso metabolites of sulfonamides.

Glutathione transferase mu deficiency is not a marker for predisposition to sulphonamide toxicity

Glutathione transferase mu activity, a marker for susceptibility to lung cancer and chemically induced cytogenetic damage, is not a predictive index for the predisposition to sulphonamide hypersensitivity reactions. However, considering the functional diversity and broad, overlapping substrate specificity of GSH-dependent enzymes, it is conceivable that an as yet unidentified deficiency in another GST isozyme or GSH-related enzyme may be a marker for sulphonamide toxicity. In addition, heterogeneity in cellular repair mechanisms and the diversity of the human immune response [22] may also contribute to the manifestation of the toxic effects of sulphonamides. Experiments are currently in progress to determine which of this myriad of variables is predominantly responsible for inter-individual susceptibility to the idiosyncratic reactions produced by these antibacterial agents.

In-vitro assessment of a hypersensitivity syndrome associated with sorbinil

Sorbinil is a hydantoin aldose reductase inhibitor that has shown promise as therapy for patients with diabetic complications such as neuropathy and retinopathy. However, as many as 10% of patients receiving sorbinil have had adverse reactions characterized by fever, skin rash, and myalgia. Our previous studies of phenytoin suggested that susceptibility to reactions might result from an inherited detoxification defect. We did the current study to determine if sorbinil is metabolized to reactive intermediates and if cells from patients with a history of a reaction to sorbinil are appropriate for the in-vitro investigation of susceptibility. Microsome-generated metabolites of sorbinil (50 microM) were toxic to normal peripheral blood lymphocytes (7.9% +/- 0.3% dead cells [mean +/- SE]). Toxicity was increased in the presence of an epoxide hydrolase inhibitor (17.5% +/- 0.3% dead cells) and abolished by an inhibitor of cytochrome P-450. In contrast to cells from healthy controls and diabetics who tolerated sorbinil (7.9% +/- 0.7% and 7.8% +/- 0.4% dead cells, respectively), cells from the six patients who had sorbinil reactions showed significantly increased toxicity from metabolites of sorbinil and phenytoin (19.7% +/- 2.3% dead cells, P less than 0.001). Cells from three patients who had reactions to phenytoin were similarly sensitive to sorbinil metabolites (23.4% +/- 0.3% dead cells). We conclude that sorbinil is oxidatively metabolized to a potentially toxic intermediate. Certain patients may be at increased risk for developing hypersensitivity reactions. Development of this important new drug has been hampered by uncommon but potentially severe reactions. An increased understanding of the steps involved in the development of adverse reactions could lead to screening tests or to the development of safer compounds.

Cellular toxicity of sulfamethoxazole reactive metabolites--II. Inhibition of natural killer activity in human peripheral blood mononuclear cells

Based on the identification of intracellular esterase activity as one early target of sulfamethoxazole hydroxylamine (SMX-HA), we wished to determine if the metabolite affected immune functions which involve esterases. The natural killer (NK) activity of human peripheral blood mononuclear cells (PBMC) was assessed with a cell concentration fluorescence technique following exposure to SMX-HA. When K562 target cells were incubated (4 hr/37 degrees) with various ratios of untreated PBMC effector to K562 target cells (E:T), NK activity increased from 17.8 +/- 3.1% (mean +/- SEM; N = 12) at an E:T ratio of 5:1 to 46.2 +/- 2.0% at an E:T ratio of 40:1. Pretreatment of fresh PBMC with 0.1 to 1.0 mM SMX-HA produced a concentration-dependent inhibition of NK activity (E:T ratio 40:1) reaching approximately 80% at 1 mM SMX-HA. Maximum suppression of NK activity was completed within a 60-min pretreatment period with measurable inhibition detected within 30 min. The viability of effector cells was not affected by the metabolite during the pretreatment period. Therefore, the SMX-HA effects could not be directly attributed to decreased viability of the effector cells; they were irreversible and could be prevented by the inclusion of exogenous reduced glutathione (GSH) in a concentration-dependent manner. Given the important roles of NK cells in immune responsiveness and host resistance, our findings of rapid functional inactivation of the cytolytic effector function provide a possible link between idiosyncratic drug toxicity and drug effects directly on components of the immune system.

Cellular toxicity of sulfamethoxazole reactive metabolites--I. Inhibition of intracellular esterase activity prior to cell death

Reactive metabolites produced by oxidative metabolism of the parent compound are considered responsible for the toxicity of a number of drugs, including idiosyncratic reactions to sulfonamide antibiotics. Using sulfamethoxazole hydroxylamine (SMX-HA) as a model compound, we report the use of a pH-sensitive fluorescent probe, 2',7'-biscarboxyethyl-5(6)-carboxyfluorescein (BCECF), to identify early subcellular targets of chemically synthesized, toxic drug metabolites in peripheral blood mononuclear cells. When toxicity was assessed with this probe immediately after a 2-hr drug challenge, SMX-HA produced a concentration-dependent decrease in cellular fluorescence which was not accompanied by the development of compromised cell membrane integrity until 18 hr later. Dissipation of pH gradients across the cell membrane with nigericin and monensin demonstrated that decreased intracellular pH was only a small component of SMX-HA-induced toxicity. Loading cells with BCECF 30 min prior to SMX-HA challenge produced only a 3% decrease in cellular fluorescence at an SMX-HA concentration of 1 mM, whereas addition of BCECF after drug challenge resulted in a 71% decrease in fluorescence, consistent with a direct drug effect on cellular esterase activity. This was confirmed by monitoring BCECF cleavage in cell lysates in the presence and absence of SMX-HA. These studies demonstrate that inhibition of cellular esterase activity accounted for the observed loss of cellular fluorescence after drug exposure. Since changes in cellular fluorescence at 2 hr correlated well with cell death at 18 hr, we conclude that SMX-HA inhibition of intracellular esterase activity is an early event in the process that terminates in metabolite-induced cell death.

Interactions between N-acetyl-p-benzoquinone imine and fluorescent calcium probes: implications for mechanistic toxicology

Intracellular free calcium ([Ca2+]i) homeostasis has been implicated as an early target in both cellular necrosis and apoptosis. In this study, we have used peripheral blood mononuclear cells (PBMC) as target cells to investigate the effects of several reactive metabolites associated with drug toxicity on [Ca2+]i in order to delineate further early events in cytotoxicity. Compounds implicated in both drug-induced necrosis (N-acetyl-p-benzoquinone imine; NAPQI) and drug hypersensitivity (sulfamethoxazole hydroxylamine; SMX-HA) were examined and their effects on [Ca2+]i compared with those of the T cell mitogen phytohemagglutinin (PHA; 1.5 micrograms/ml) and the calcium ionophore ionomycin (2.5 microM). PHA and ionomycin produced characteristic elevations in [Ca2+]i as monitored by an increase in the fluorescence of fluo-3-loaded cells. SMX-HA did not significantly affect [Ca2+]i at concentrations previously shown to be cytotoxic to PBMC (100 and 500 microM), suggesting that Ca2+ homeostasis is not an early target for SMX-HA toxicity. Addition of NAPQI (250 microM) to fluo-3-loaded cells produced a marked decrease in fluorescence which was not reversed by ionomycin. Conversely, addition of NAPQI to cells loaded with indo-1 resulted in a rapid increase in fluorescence. This effect, however, was found to be attributable to NAPQI addition per se rather than to an increase in [Ca2+]i. HPLC and fluorescence analysis of samples generated from the decomposition of NAPQI revealed the presence of several products which fluoresced intensely at the excitation/emission wavelength pairs of a number of fluorescent probes commonly used to monitor [Ca2+]i.(ABSTRACT TRUNCATED AT 250 WORDS)

Peroxidase-dependent oxidation of sulfonamides by monocytes and neutrophils from humans and dogs

The hydroxylamine and nitroso metabolites formed by N4-oxidation of sulfonamides are thought to be involved in the pathogenesis of idiosyncratic reactions to this class of drugs. Idiosyncratic reactions to sulfonamides are characterized by multisystemic toxicity, including hepatitis, nephritis, dermatitis, and blood dyscrasias (aplastic anemia, agranulocytosis). We have previously shown that cytochrome P-450 in the liver metabolizes sulfamethoxazole to its hydroxylamine metabolite. In this paper we report the N4-oxidation of sulfamethoxazole by activated monocytes and neutrophils (human and canine) to form sulfamethoxazole hydroxylamine and nitrosulfamethoxazole. The presumed nitroso intermediate was not detected. Purified myeloperoxidase and prostaglandin H synthase were also capable of mediating the oxidation of sulfamethoxazole. The present studies suggest that myeloperoxidase is responsible for the observed oxidation by phagocytic cells. Oxidation by neutrophils may play a role in agranulocytosis, and oxidation by monocytes may facilitate antigen presentation. Extrahepatic bioactivation of sulfonamides by peroxidases in phagocytic cells and other tissues may be important in determining the range of adverse reactions to sulfonamides that occur.

The caffeine breath test and caffeine urinary metabolite ratios in the Michigan cohort exposed to polybrominated biphenyls: a preliminary study

A field biochemical epidemiology study was conducted using the Michigan cohort consisting of 51 rural residents exposed to polybrominated biphenyls (PBB). The study had three major objectives: a) to determine the serum half-life of the major PBB congener, hexabromobiphenyl (HBB), in the human, b) to determine if the PBB-exposed subjects had elevated cytochrome P-450I function as determined by the caffeine breath test (CBT) and the caffeine urinary metabolite ratio (CMR), and c) to determine the applicability of the CBT and CMR in field studies. PBB serum levels were detected in 36 of the 51 PBB-exposed subjects. The serum half-life of HBB was determined by comparing the current serum HBB values to the subject's previous serum values obtained 5 to 8 years earlier. The median HBB half-life was 12 years (range 4-97 years). The CBT and CMR were elevated in the subjects exposed to PBBs as compared to the values obtained from urban nonsmokers and were similar to those found in adults who smoke. A gender effect was seen in the PBB-exposed subjects, the median CBT and CMR values of the females being lower than the values of males. There was a correlation between the CBT and the HBB serum values (r2 = 0.2, p = 0.01) but not between CMR and HBB serum values. The CBT and CMR were easily conducted in the field and appear to be useful metabolic probes of cytochrome P-450I activity in human environmental toxicology.

Hepatic microsomal metabolism of sulfamethoxazole to the hydroxylamine

Sulfonamides are oxidized to protein reactive cytotoxic metabolites by murine hepatic microsomes. Mononuclear leukocytes from patients with idiosyncratic reactions to sulfonamides were more susceptible to toxicity from these metabolites than were leukocytes from a control population, suggesting that these metabolites play a role in the pathogenesis of such reactions. Here we have shown that murine hepatic microsomes oxidize sulfamethoxazole at the N4-position to form the hydroxylamine. Formation of the hydroxylamine was dependent on the presence of microsomes, NADPH, and oxygen. The addition of SKF 525-A, cimetidine, or gassing with carbon monoxide inhibited formation. The enzymic activity was stable at 37 degrees C in the absence of NADPH. Ascorbic acid, N-acetylcysteine, and reduced glutathione significantly increased the yield of hydroxylamine, presumably by decreasing further oxidation and covalent binding. Microsomes prepared from mice treated with phenobarbital or beta-naphthoflavone catalyzed the formation of the hydroxylamine more readily than did microsomes from untreated mice. These results demonstrate that cytochrome P-450-mediated oxidation of sulfamethoxazole results in the formation of hydroxylamines, which can be further oxidized to more reactive intermediates. These metabolites are likely involved in the pathogenesis of idiosyncratic reactions.

Phenobarbital hepatotoxicity in an 8-month-old infant

Severe hepatotoxicity from phenobarbital occurred in an infant boy who had a complicated illness with chronic bilateral subdural hematomas and sepsis. Skin rash began after 2 weeks of treatment, and signs of hepatocellular failure developed 3 weeks after phenobarbital had been started. Signs of severe liver disease included elevated aminotransferases, conjugated hyperbilirubinemia, significant coagulopathy, hepatosplenomegaly and ascites. Other features of this adverse drug reaction were unremitting fever, leukocytosis with eosinophilia and atypical lymphocytosis, and proteinuria. Sepsis, viral hepatitis, and metabolic liver disease were excluded. The child was on no other medication and had been previously well. In-vitro rechallenge of the patient's lymphocytes with cytochrome P-450 generated metabolites of phenobarbital showed extensive cytotoxicity compared to control. These data support the hypothesis that a defect in drug detoxification was responsible for the child's susceptibility to this drug hepatotoxicity.

An in vitro investigation of predisposition to sulphonamide idiosyncratic toxicity in dogs

Sulphonamide idiosyncratic toxicosis has been reported in 28 dogs. Non-septic polyarthritis and fever occurring after 8 to 21 days therapy was the most common manifestation. Of 22 dogs with this syndrome, 7 were Doberman Pinschers. In humans, inherited decreased ability to detoxify sulphonamide hydroxylamine metabolites (as reflected in an in vitro mononuclear leukocyte (MNL) toxicity assay) has been associated with susceptibility to sulphonamide idiosyncratic toxicity. We have demonstrated that microsomes obtained from the liver of a dog were capable of metabolizing sulphamethoxazole to sulphamethoxazole hydroxylamine (SMX-HA). Production of SMX-HA was an NADPH dependent process and the yield was increased by the presence of 1 mmol/L ascorbic acid. SMX-HA was toxic to isolated MNL from mixed breed dogs (MBD) and Doberman Pinschers. The toxicity of SMX-HA to MNL from Dobermans was significantly different from that to MNL from MDB. MNL from 7 out of 15 Dobermans (including a dog with a history of an idiosyncratic reaction to a sulphonamide) had an LD-50 (concentration of SMX-HA required to produce 50% cytotoxicity in MNL) less than 100 mumols/L, while MNL from 0 out of 10 MBD had an LD-50 less than 100 mumols/L. These results suggest that the basis for the observed predisposition of Dobermans to sulphonamide idiosyncratic toxicity may be a limited capacity to detoxify the hydroxylamine metabolites of sulphonamides.

Use of a microplate reader in an assay of glutathione reductase using 5,5'-dithiobis(2-nitrobenzoic acid)

The use of 96-well microtiter plates and a programmable microplate reader to measure glutathione reductase in an assay based on reduction of 5,5'-dithiobis(2-nitrobenzoic acid) by GSH generated from an excess of GSSG is described. Samples are prepared in 96-well plates and absorbance at 415 nm with a reference wavelength of 595 is determined every 30 s for 3 min. The rate of increase in absorbance is directly proportional to the amount of glutathione reductase in the sample. Activity in an unknown sample is determined from a standard curve. The assay is rapid and allows many small samples to be analyzed in replicates of two or more at the same time.

Acute changes in renal function associated with deferoxamine therapy

In three patients who received intravenous deferoxamine there was a twofold to eightfold increase in plasma creatinine level and a parallel decrease in creatinine clearance that resolved when treatment with the drug was discontinued. In two thalassemic patients, diuresis was evident by urine output exceeding fluid intake. The mechanism was studied in dogs that exhibited an acute and significant decrease in inulin and para-aminohippuric acid clearances induced by intravenous deferoxamine. Saline diuresis could prevent the decrease in the glomerular filtration rate but not the decrease in renal blood flow caused by deferoxamine. Deferoxamine induced an acute increase in the fractional excretion of sodium, potassium, chloride, phosphate, and urate, which may explain the relative diuresis observed in two of the patients. In a subsequent experiment, ferrioxamine induced an increase in the fractional excretion of sodium and chloride but did not affect the glomerular filtration rate and renal blood flow. Our studies suggest that adequate hydration may be needed to preserve renal hemodynamics during intravenous deferoxamine therapy. Repeated measurements of renal function should accompany treatment with this agent.

Neuroblastoma after prenatal exposure to phenytoin: cause and effect?

We evaluated the causality of the association between intrauterine exposure to phenytoin and postnatal neuroblastoma using an in vitro lymphocyte toxicity assay for phenytoin-induced reactions in an unusual sibship. In addition, we investigated intrauterine phenytoin exposure in a case series of infants and children with neuroblastoma diagnosed over 17 years at our center. The response of lymphocytes from our index case with neuroblastoma exposed in utero to phenytoin was within the normal range, whereas the mother and a sibling with fetal hydantoin syndrome (FHS) exhibited an intermediate toxicity. None of the 188 cases of childhood neuroblastoma diagnosed between 1969 and 1988 had been exposed in utero to phenytoin, indicating that, statistically, the drug cannot be associated with neuroblastoma in more than two cases with this malignancy in our cohort, or in 1.5% of all cases of neuroblastoma. Although our data do not suggest an association between phenytoin in pregnancy and postnatal neuroblastoma, it is still possible that there is an increased risk for neuroblastoma in children with FHS.

Fluorescence-based viability assay for studies of reactive drug intermediates

Studies of drug toxicity, toxicologic structure-function relationships, screening of idiosyncratic drug reactions, and a variety of cytotoxic events and cellular functions in immunology and cell biology require the sensitive and rapid processing of often large numbers of cell samples. This report describes the development of a high-sensitivity, high-throughput viability assay based on (a) the carboxyfluorescein derivative 2'-7'-biscarboxyethyl-5(6)-carboxyfluorescein (BCECF) as a vital dye, (b) instrumentation capable of processing multiple small (less than 100 cells) samples, and (c) a 96-well unidirectional vacuum filtration plate. Double staining of cultured peripheral blood mononuclear cells with BCECF and propidium iodide (PI) showed no overlap between PI+ (nonviable) and BCECF+ (viable) cells by flow cytometric analysis. Optimal conditions were developed for dye loading and minimizing physical cell damage and fluorescence quench during the assay procedure. The ratio of BCECF fluorescence to internal standard fluorescent particles was linear from 40 to greater than 20,000 cells with a signal:noise ratio of approximately 3 at 40 cells/well. Sulfamethoxazole hydroxylamine (SMX-HA) was used as a model toxic drug metabolite to explore the validity of the BCECF procedure. SMX-HA, but not its parent compound sulfamethoxazole, resulted in a dose dependent loss of cellular fluorescence and the parallel accumulation of PI+ nonviable cells. When compared to the currently used tetrazolium dye reduction viability assay, the BCECF method was 3-fold more sensitive, greater than 10-fold faster, and required 1/10-1/100 the cell numbers.

Diagnosis of sulfonamide hypersensitivity reactions by in-vitro "rechallenge" with hydroxylamine metabolites

STUDY OBJECTIVE

To determine whether differences in in-vitro detoxification of sulfonamide-reactive metabolites can be detected among the lymphocytes from controls, patients with sulfonamide hypersensitivity reactions, and patients with nonhypersensitivity reactions to the sulfonamide agents.

DESIGN

In-vitro toxicity assay on lymphocytes.

SETTING

Clinics for adverse drug reactions in an adult and pediatric tertiary care center.

PATIENTS

Peripheral blood lymphocytes were obtained from 46 normal volunteers and 76 patients referred to the clinic for assessment of adverse drug reactions to sulfonamide agents. Thirty-one patients had clinical histories consistent with a diagnosis of hypersensitivity reaction, whereas 45 patients had clinical histories felt to be inconsistent with a diagnosis of hypersensitivity reaction.

INTERVENTIONS

Lymphocytes were assayed with tetrazolium to determine toxicity from the hydroxylamine of sulfamethoxazole.

MEASUREMENTS AND MAIN RESULTS

The lymphocytes from patients with a history of hypersensitivity reactions showed markedly increased toxicity across a tenfold-concentration toxicity-concentration curve compared with those from controls and patients with a history of nonhypersensitivity reactions. These differences were highly significant (P less than 0.01). No difference was found between the toxicity shown by the lymphocytes from controls and that shown by the lymphocytes from patients with a history of nonhypersensitivity reactions.

CONCLUSIONS

Metabolic differences in the production and detoxification of reactive metabolites of sulfonamide agents are important determinants of hypersensitivity reactions to these agents. These results suggest that the hydroxylamine derivative of sulfamethoxazole may be a reactive metabolite mediating these reactions. Sulfonamide hydroxylamines are useful in the diagnosis and study of the pathogenesis of hypersensitivity reactions to sulfonamide agents.

Anticonvulsant hypersensitivity syndrome. In vitro assessment of risk

Arene oxide metabolites of aromatic anticonvulsants (phenytoin, phenobarbital, and carbamazepine) may be involved in the pathogenesis of hypersensitivity reactions. We investigated 53 patients with clinical sensitivity to anticonvulsants by exposing their lymphocytes in vitro to drug metabolites generated by a murine hepatic microsomal system. The diagnosis of a hypersensitivity reaction was corroborated by in vitro rechallenge for each drug (phenytoin, n = 34; phenobarbital, n = 22; carbamazepine, n = 25) when cytotoxicity (% dead cells) exceeded 3 SD above the mean result for controls. Cross-reactivity among the drugs was noted. 7 out of 10 patients who had received all three anticonvulsants had adverse reactions to each. 40 out of 50 patients tested to all three drugs in vitro were positive to each. Adverse reactions were indistinguishable among anti-convulsants. Skin rash (87%), fever (94%), hepatitis (51%), and hematologic abnormalities (51%) were common clinical features of each drug. 62% of reactions involved more than two organs. Cells from patients' parents exhibited in vitro toxicity that was intermediate between values for controls and patients. In vitro testing can help diagnose hypersensitivity to anticonvulsants. Cells from patients may also be used for prospective individualization of therapy to decrease risk of adverse reaction. Cross-reactivity among the major anticonvulsants is common and should be considered before deciding on alternative therapy.

An unusual skin exposure to copper; clinical and pharmacokinetic evaluation

Skin exposure to copper is rare and has been described only with copper sulfate. A case of skin exposure to copper after an explosion of copper azide is presented. The amount of copper absorbed by this route was estimated to be 7.7 mg. Calculated distribution volume was 2.02 I/kg, half-life was 167.4 days and clearance was 0.0058 ml/min/kg. The authors also demonstrated metallic copper to be radiopaque, in contrast to copper salts. It is suggested that copper may be absorbed from the skin even if it is in the metal form. Careful clinical follow up as well as serial determinations of serum copper should guide the need for chelation therapy.

Drug metabolite toxicity assessed in human lymphocytes with a purified, reconstituted cytochrome P-450 system

Evaluation of idiosyncratic drug reactions in predisposed individuals is limited by ethical concerns arising from rechallenge with the suspected offending agent. A previously developed in vitro method using human lymphocytes and a murine microsomal drug metabolizing system has been used to examine toxicity due to acetaminophen (APAP), sulfonamide antibiotics and aromatic anticonvulsants. An improved method is described in which toxic APAP metabolites are generated by a purified and reconstituted cytochrome P-450 system, minimizing the amount of exogenous detoxification enzymes in the assay. Toxicity is assessed by an objective, automated method based on the reduction of 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide to an insoluble purple formazan by the mitochondria of viable cells and correlates with that based on trypan blue exclusion. Toxicity required cytochrome P-450 and NADPH, and was inhibited by SKF 525A. Exogenous glutathione also decreased toxicity in a concentration-dependent manner. Lymphocytes from a glutathione synthetase-deficient patient exhibited markedly enhanced toxicity to APAP exceeding the 95% CL of 10 control subjects over a concentration range of 10 to 1000 micrograms/ml. The data are consistent with the generation of cytochrome P-450-dependent reactive metabolites which subsequently can be detoxified by glutathione. This method allows one to address specifically individual differences in detoxification pathways. The use of an automated assessment of cell viability may prove useful in preclinical screening of new compounds for their propensity to cause "idiosyncratic" drug reactions in a predisposed population.

Synthesis and in vitro toxicity of hydroxylamine metabolites of sulfonamides

Among the most serious side effects of sulfonamides are hypersensitivity reactions, the pathogenesis of which has been suggested to be mediated by reactive metabolites. We have previously demonstrated dose-related covalent binding and toxicity of reactive intermediates of sulfonamides generated by a murine hepatic microsomal activating system. We hypothesized that hydroxylamine (H/A) metabolites might be likely candidates for mediating such toxicity; accordingly, we synthesized chemically the H/As of sulfadiazine and sulfamethoxazole. Synthesis was performed using 4-nitrobenzenesulfonyl chloride and either 2-aminopyrimidine or 3-amino-5-methylisoxazole, respectively, as starting materials. The resulting nitro derivatives were reduced to the corresponding H/A with hydrogen in the presence of a poisoned platinum catalyst. After synthesis and purification, toxicity of the H/As to lymphocytes of normal volunteers was evaluated using three cytotoxicity assays: 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide dye conversion, trypan blue dye exclusion and propidium iodide dye exclusion. The H/As of sulfadiazine and sulfamethoxazole displayed dose-related toxicity. 1.6 mM sulfadiazine H/A produced 82% cell death, whereas 400 microM sulfamethoxazole H/A produced 62% cell death; the parent sulfonamides were not toxic to cells. The toxicity of sulfamethoxazole H/A was decreased by coincubation with glutathione or N-acetylcysteine; there was a 47% decrease in toxicity when coincubated with 100 microM glutathione, whereas there was a 55% decrease displayed when coincubation was done with 500 microM N-acetylcysteine. H/A metabolites of the sulfonamides or their nitroso derivatives, normally detoxified by conjugation to glutathione, may be the proximate toxins mediating sulfonamide hypersensitivity.