Pharmacokinetics, therapeutic efficacy, and prolonged residue depletion of flumequine in Japanese eel (Anguilla japonica) and its implications for withdrawal period determination.

The Japanese eel (Anguilla japonica) is a major aquaculture species in South Korea. However, a limited number of officially approved antimicrobial agents have led to frequent off-label drug use, raising concerns about therapeutic efficacy, food safety, and regulatory compliance. Flumequine (FQ), a first-generation quinolone effective against gram-negative pathogens, has been used in eels without species-specific pharmacokinetic or residue data. This study evaluated the pharmacokinetics, therapeutic efficacy, and residue depletion of FQ in the Japanese eel. The oral administration of 20 mg/kg significantly protected against Edwardsiella piscicida, with a relative survival rate exceeding 80%. Pharmacokinetic analysis revealed slow elimination (Cmax 1.09 µg/mL; t1/2λz 53.9 h; are under the curve (AUC)0–∞ 84.8 µg·h/mL). Residual depletion in the muscle was markedly prolonged, requiring 128 days at 28 °C and 166 days at 20 °C to reach the maximum residue limit (0.5 mg/kg). These findings confirm the efficacy of FQ, but demonstrate an unusually extended withdrawal period in the Japanese eel, which is attributable to species-specific drug disposition and high muscle lipid content. These results have critical implications for aquaculture practice, food safety, and regulatory policy, underscoring the urgent need for evidence-based guidelines and optimized dosage regimens tailored for eel aquaculture.