Evaluation of the clinical, immunologic, and biochemical effects of nitroso sulfamethoxazole administration to dogs: a pilot study

Sulfonamide antimicrobials such as sulfamethoxazole (SMX) have been associated in humans with hypersensitivity reactions, to include fever, skin eruptions, hepatotoxicity, and blood dyscrasias. These reactions also occur in dogs, the only non-human species known to develop a similar spectrum of sulfonamide hypersensitivity. Sulfonamide hypersensitivity is not well understood, but has been hypothesized to be due to the generation of the reactive oxidative metabolite, nitroso sulfamethoxazole (SMX-NO). SMX-NO, unlike the parent sulfonamide, is cytotoxic in vitro, haptenizes tissue proteins, and is immunogenic in rodents. The purpose of this pilot study was to determine whether SMX-NO, when administered to dogs, would lead to drug-tissue adducts, anti-drug Antibodies, antioxidant depletion, or clinical evidence of drug hypersensitivity. Four dogs were randomized to one of four treatments: SMX-NO 1 mg/kg; SMX-NO 3 mg/kg; SMX-NO 10 mg/kg; or vehicle control. Dosing was by the intraperitoneal route, once daily for four consecutive days per week, for 2 weeks total, followed by a third week of observation. Following this, all dogs were challenged with trimethoprim-sulfamethoxazole, 25 mg/kg for 12 h for 2 weeks. No dog developed clinical or biochemical evidence of drug hypersensitivity. Plasma cysteine and leukocyte reduced glutathione were not depleted during dosing; however, ascorbate was significantly depleted by week 2 following SMX-NO at 10 mg/kg. Anti-SMX Antibodies (IgG or IgM by ELISA) were not detected in any dogs at any time points. SMX-hemoglobin adducts were detected in the spleen in SMX-NO dosed dogs; however, these adducts were not accompanied by an immunologic or systemic response. The results of this pilot study indicate that SMX-NO dosing in dogs, using a dosing protocol shown to be immunogenic in other species, produces modest ascorbate depletion and hemoglobin adduct formation, but is insufficient to produce an immunologic response or a clinical syndrome of sulfonamide hypersensitivity in this susceptible species.