ABSTRACT
BACKGROUND: Delayed hypersensitivity (HS) reactions to potentiated sulfonamide antimicrobials occur in both dogs and humans, and involve an intermediate hydroxylamine metabolite that is detoxified by cytochrome b(5) and NADH cytochrome b(5) reductase. HYPOTHESIS/OBJECTIVES: We hypothesized that polymorphisms in the genes (CYB5A and CYB5R3) encoding these 2 enzymes would be associated with risk of sulfonamide HS in dogs. ANIMALS: A total of 18 dogs with delayed HS to potentiated sulfonamide antimicrobials and 16 dogs that tolerated (TOL) a therapeutic course of these drugs without adverse effect. METHODS: CYB5A and CYB5R3 were sequenced from canine liver, and the promoter, exons, and 3' untranslated regions of both genes were resequenced from genomic DNA obtained from all dogs. RESULTS: Multiple polymorphisms were found in both genes. When controlled for multiple comparisons, the 729GG variant in CYB5R3 was significantly overrepresented in dogs with sulfonamide HS (78% of dogs), compared to TOL dogs (31%; P = .003). CONCLUSIONS AND CLINICAL IMPORTANCE: The CYB5R3 729GG variant may contribute to the risk of sulfonamide HS in dogs. Functional characterization of this polymorphism, as well as genotyping in a larger number of HS and TOL dogs, is warranted.