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.

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.

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.

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.