Depletion of semicarbazide and 5-nitrofuraldehyde in rainbow trout after nitrofurazone single oral administration*

Nitrofurazone is a prohibited antimicrobial agent in veterinary medicine in many countries due to its carcinogenicity. Like all nitrofurans, it is rapidly metabolised in animals, and its detection is based on the identification of its side chain metabolite, the semicarbazide. However, the latter lacks specificity as it can be formed during food processing that has not been treated with nitrofurazone. This study investigates the fate of nitrofurazone and a potentially relevant metabolite, the 5-nitrofuraldehyde, in rainbow trout. Fish were orally administered either with nitrofurazone or 5-nitrofuraldehyde at 2 mg.kg^-1 bw, and plasma, muscle, and liver samples were analysed using LC-MS/MS. Our results showed that 5-nitrofuraldehyde is not a persistent metabolite for nitrofurazone abuse. Although it was detected in plasma and edible tissues at day 1 after withdrawal, it was rapidly eliminated in these samples by day 7. In contrast, semicarbazide persisted in trout tissues for several weeks. Tissue-bound semicarbazide concentrations were initially 30.5 ± 6.9 µg.kg^-1 in muscle and 45.8 ± 5.9 µg.kg^-1 in liver, then decreased to 17.3 ± 1.96 µg.kg^-1 and 24.1 ± 3.61 µg.kg^-1, respectively, after 14 days. The estimated half-lives ranged between 16 days in muscle and 14 days in liver. These findings suggest that semicarbazide remains a reliable marker for nitrofurazone abuse in trout compared to 5-nitrofuraldehyde.