Cross-reactivity in drug hypersensitivity reactions to sulfasalazine and sulfamethoxazole.

BACKGROUND: Sulfonamides are generally classified into 2 groups: antibiotics and non-antibiotics. Recent studies showed that patients allergic to sulfonamide antibiotics do not have a specific risk for an allergy to sulfonamide non-antibiotic. However, the anti-inflammatory drug sulfasalazine represents an important exception. Used in rheumatic diseases, it is classified as a non-antibiotic sulfonamide, but is structurally related to antibiotic sulfonamides. Therefore, we aimed to analyze in vitro the cross-reactivity between the antimicrobial sulfamethoxazole and the anti-inflammatory drug sulfasalazine. METHODS: PBMC from 2 patients with severe hypersensitivity syndrome to sulfasalazine, 3 patients with sulfamethoxazole allergy and 5 healthy donors were isolated and incubated with medium only (negative control), 2 concentrations (10, 100 µg/ml) of sulfapyridine, 2 concentrations (100, 200 µg/ml) of sulfamethoxazole, and tetanus toxoid (10 µg/ml) as a positive control. After 6 days of culture, (3)H-thymidine was added and cell proliferation was measured. RESULTS: In all patients tested, the lymphocyte transformation tests were positive for both sulfapyridine and sulfamethoxazole, suggesting a strong cross-reactivity to these drugs. None of the healthy donors reacted to any of the drugs tested. We refrained from provoking our patients with either sulfasalazine or sulfamethoxazole, as they had a clear, typical history, severe symptoms and were positive on in vitro tests to both compounds. CONCLUSIONS: We demonstrate that in the case of sulfamethoxazole and sulfasalazine, cross-reactivity is dependent on chemical features rather than the indication of the drugs. Therefore, patients with hypersensitivity to sulfasalazine or sulfamethoxazole should be specifically advised to avoid both drugs.

In vitro tests in drug hypersensitivity diagnosis.

The diagnosis of a drug hypersensitivity reaction (DHR) is a challenging task because multiple and complex mechanisms are involved. Better understanding of immunologic pathomechanisms in DHRs and rapid progress in cellular-based in-vitro tests can help to adjust the correct diagnostic strategy to individual patients with different clinical manifestations of drug allergy. Thus, drug hypersensitivity diagnosis needs to rely on a combination of medical history and different in vivo and in vitro tests. In this article, the authors discuss current in vitro techniques, most recent findings, and new promising tools in the diagnosis of T-cell-mediated drug hypersensitivity.

Pharmacological interaction of drugs with immune receptors: the p-i concept.

Drug-induced hypersensitivity reactions have been explained by the hapten concept, according to which a small chemical compound is too small to be recognized by the immune system. Only after covalently binding to an endogenous protein the immune system reacts to this so called hapten-carrier complex, as the larger molecule (protein) is modified, and thus immunogenic for B and T cells. Consequently, a B and T cell immune response might develop to the drug with very heterogeneous clinical manifestations. In recent years, however, evidence has become stronger that not all drugs need to bind covalently to the MHC-peptide complex in order to trigger an immune response. Rather, some drugs may bind directly and reversibly to immune receptors like the major histocompatibility complex (MHC) or the T cell receptor (TCR), thereby stimulating the cells similar to a pharmacological activation of other receptors. This concept has been termed pharmacological interaction with immune receptors the (p-i) concept. While the exact mechanism is still a matter of debate, non-covalent drug presentation clearly leads to the activation of drug-specific T cells as documented for various drugs (lidocaine, sulfamethoxazole (SMX), lamotrigine, carbamazepine, p-phenylendiamine, etc.). In some patients with drug hypersensitivity, such a response may occur within hours even upon the first exposure to the drug. Thus, the reaction to the drug may not be due to a classical, primary response, but rather be mediated by stimulating existing, pre-activated, peptide-specific T cells that are cross specific for the drug. In this way, certain drugs may circumvent the checkpoints for immune activation imposed by the classical antigen processing and presentation mechanisms, which may help to explain the peculiar nature of many drug hypersensitivity reactions.