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AND THE ANSWER IS . . .
Discussion
Allergic type reactions to S-ABx are not unusual although it is unclear how many of such reactions (generally cutaneous) are truly immune based (1). It has often been recommended that non-antibiotic agents that also contain the sulfonamide moiety (N-ABx) such as thiazides, furosemide, some oral anti-diabetes drugs and (more recently) celecoxib should be avoided by individuals with histories of allergic-type reactions to S-ABx (2). This recommendation has been based on the concern that those allergic to S-ABx will exhibit similar adverse reactions to other non-antibiotic compounds which contain the sulfonamide moiety 0N-ABx. However, more recent reports have concluded that no interaction of the human immune system with the sulfonamide component of S-ABx has been demonstrated (reviewed in 3). The antigenic determinant in S-ABx responsible for inducing immediate hypersensitivity reactions appears to be a heterocyclic ring side chain at the N1 position. Non-immediate hypersensitivity reactions are largely directed against reactive metabolites formed at the N4. Neither the N1 or N4 determinants are present in the sulfonamide containing N-ABx (2,3). There does not appear to be convincing epidemiologic evidence that individuals "allergic" to S-ABx are at increased risk for similar reactions to N-ABx (2,3).
However, T cell recognition of unmetabolized S-ABx may be responsible for a small percentage of adverse reactions to S-ABx. It is not yet clear whether such sensitized T cells will react against the sulfonamide in N-ABx (3).
Studies by Gruchalla et al have shown that skin tests with a N4-SMX hapten (not the whole sulfamethoxazole compound) elicited a positive response in 27% of a group of individuals with convincing histories pf allergic-type reactions to S-ABx (4). Therefore, skin testing with a S-ABX component is currently considered insufficiently sensitive to detect such S-ABx allergy.
References
1. Eng J Med 2004;350:302-303
2. Drug safety 2001;24:39-47
3. Pharmacotherapy 2004;24:856-70
4. J Allergy Clin Immunol. 1991;88:784-92
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