Residue levels of nalidixic acid and its metabolites in cultured rainbow trout

Residue levels of nalidixic acid (1-ethyl-1, 4-dihydro-7-methyl-4-oxo-1, 8-naphthyridine-3-carboxylic acid, NA) and its metabolites, 6-OHNA (1-ethyl-1, 4-dihydro-6-hydroxy-7-methyl-4-oxo-1, 8-naphthyridine-3-carboxylic acid) and 7-OHNA (1-ethyl-1, 4-dihydro-7-hydroxy-methyl-4-oxo-1, 8-naphthyridine-3-carboxylic acid) in cultured rainbow trout (Salmo gairdnerii) after oral administration of 50mg/kg of NA were determined. The following results were obtained. The unconjugated metabolites, 6-OHNA and 7-OHNA, which had antibacterial activity, were detected in bile and liver, but their tissue levels were much lower than those of unchanged NA and their residence times were not so long as that of unchanged NA. The amount of the unconjugated form in bile 24 hours after dosing about one-eighth that of the conjugated form. These results indicate that the metabolic pattern of NA differs from that of PMA, and if the existing withholding period prior to consumption of rainbow trout is observed, 6-OHNA and 7-OHNA would be undetectable in tissues except for bile. The disappearance rate of unchanged NA 96 hours after dosing was so slow that a suitable with-holding period prior to consumption of rainbow trout could not be evaluated.