Individual differences in metabolism of the sulfonamides may predispose patients to idiosyncratic reactions. Sulfonamides are metabolized by N-acetylation (mediated by a genetically polymorphic enzyme) and oxidation to potentially toxic metabolites. We examined 6 patients who had severe reactions to sulfonamides and compared them with 20 controls. Acetylator phenotype was determined with caffeine, a safe in-vivo probe of enzyme activity. All 6 patients were slow acetylators (expected, 55%; p less than 0.05). Detoxification of oxidative metabolites was studied in vitro with a lymphocyte assay evaluating cell death from metabolites generated by a murine hepatic microsomal system. Cells from each patient showed increased toxicity from sulfonamide metabolites but not from the drugs themselves. Cells from parents of 3 patients had intermediate toxicity from sulfonamide metabolites, whereas cells from a sibling of 1 patient had a normal response. Susceptibility to sulfonamide reactions may be due to interaction of metabolic pathways, possibly under genetic control, regulating N-acetylation and specific detoxification of toxic metabolites of the drugs.

Differences in metabolism of sulfonamides predisposing to idiosyncratic toxicity.

Study Objective:To determine whether differences in in-vitro detoxification of sulfonamide-reactive metabolites can be detected among the lymphocytes from controls, patients with sulfonami...

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

Delayed hypersensitivity reactions are among the most severe adverse effects of the sulfonamides in current clinical use. These reactions appear to occur because of differences in the metabolism and detoxification of reactive metabolites of the sulfonamides. N-Acetylation is a major metabolic pathway for the sulfonamides. Slow acetylation phenotype might be a risk factor for the development of these reactions. We determined the acetylation phenotype of 21 patients who had suffered hypersensitivity reactions to the sulfonamides. There were 11 females and 10 males in the group, with a mean age of 15 years (age range, 1.8 to 50 years). Their acetylator phenotype was determined by determining the ratio of urinary caffeine metabolites (1-methylxanthine to 5-amino-6-formylmethyluracil after an oral dose of 50 mg caffeine). Nineteen (90%) of the patients were slow acetylators compared to a 55% incidence of slow acetylators in a race-matched control population (p < 0.008). This suggests that a slow acetylator phenotype is a risk factor for the development of sulfonamide hypersensitivity reactions and provides further support for the role of imbalances in genetically determined pathways of metabolism and detoxification of the sulfonamides in the pathogenesis of these reactions.



Clinical Pharmacology and Therapeutics (1991) 49, 13–17; doi:10.1038/clpt.1991.3

Prominence of slow acetylator phenotype among patients with sulfonamide hypersensitivity reactions.

Drug disposition and drug hypersensitivity.

Evidence strongly suggests that many adverse drug reactions, including idiosyncratic drug reactions, involve reactive metabolites. Furthermore, certain functional groups, which are readily oxidized to reactive metabolites, are associated with a high incidence of adverse reactions. Most drugs can probably form reactive metabolites, but a simple comparison of covalent binding in vitro is unlikely to provide an accurate indication of the relative risk of a drug causing an idiosyncratic reaction because it does not provide an indication of how efficiently the metabolite is detoxified in vivo. In addition, the incidence and nature of adverse reactions associated with a given drug is probably determined in large measure by the location of reactive metabolite formation, as well as the chemical reactivity of the reactive metabolite. Such factors will determine which macromolecules the metabolites will bind to, and it is known that covalent binding to some proteins, such as those in the leukocyte membrane, is much more likely to lead to an immune-mediated reaction or other type of toxicity. Some reactive metabolites, such as acyl glucuronides, circulate freely and could lead to adverse reactions in almost any organ; however, most reactive metabolites have a short biological half-life, and although small amounts may escape the organ where they are formed, these metabolites are unlikely to reach sufficient concentrations to cause toxicity in other organs. Many idiosyncratic drug reactions involve leukocytes, especially agranulocytosis and drug-induced lupus. We and others have demonstrated that drugs can be metabolized by activated neutrophils and monocytes to reactive metabolites. The major reaction appears to be reaction with leukocyte-generated hypochlorous acid. Hypochlorous acid is quite reactive, and therefore it is likely that many other drugs will be found that are metabolized by activated leukocytes. Some neutrophil precursors contain myeloperoxidase and the NADPH oxidase system, and it is likely that these cells can also oxidize drugs. Therefore, although there is no direct evidence, it is reasonable to speculate that reactive metabolites generated by activated leukocytes, or neutrophil precursors in the bone marrow, could be responsible for drug-induced agranulocytosis and aplastic anemia. This could involve direct toxicity or an immune-mediated reaction. These mechanisms are not mutually exclusive, and it may be that both mechanisms contribute to the toxicity, even in the same patient. In the case of drug-induced lupus, a prevalent hypothesis for lupus involves modification of class II MHC antigens.(ABSTRACT TRUNCATED AT 400 WORDS)

The role of leukocyte-generated reactive metabolites in the pathogenesis of idiosyncratic drug reactions

We have demonstrated the in vitro production of a potentially toxic metabolite of sulfadiazine Human lymphocytes were incubated with sulfadiazine and a murine hepatic microsomal drug metabolizing system. Toxicity to cells was assessed by trypan blue dye exclusion. Covalent binding of labelled sulfadiazine to microsomes also was studied. Sulfadiazine toxicity to cells was dependent on microsomes and NADPH. Binding and toxicity were decreased when microsomes were boiled or cytochrome P-450 inhibited, and by the addition of N-acetylcysteine or glutathione. The data suggest the production of a toxic intermediate of oxidative metabolism of sulfadiazine which is detoxified by conjugation with glutathione. Covalent binding of such metabolites to cell macromolecules could lead to cell death and, by acting as haptens, to secondary hypersensitivity reactions.

S P Spielberg

Papers

In vitro evaluation of a toxic metabolite of sulfadiazine.