Pharmacokinetics and Withdrawal Time of Doxycycline in Snakehead ( <i>Channa striata</i> ) Following Oral Medication

The study consisted of two experiments. First, a PK experiment was set up with a single oral dose at 20 mg/kg body weight, and second, a withdrawal time determination study, where the fish were fed once a day for 5 consecutive days at the same dosage. The DOX concentration in medicated feed, fish muscle, plasma, liver, and kidney was analysed by LC-MS/MS. Phoenix 8.1 and WT 1.4 software were used in the calculation for PK parameters and withdrawal time, respectively. DOX was absorbed quickly into plasma after oral medication with an absorption half-life T_1/2abs of 0.12 h, reaching the C_max (116 μg/L) at 1 h (T_max). DOX distribution was highest in the kidney, followed by the liver and plasma. The high DOX concentration in the kidney and liver supports the possibility of using DOX to treat relevant bacterial infections in snakehead. In conclusion, DOX is rapidly absorbed in snakehead after oral medication, has a significant tissue distribution, followed by slow elimination. Finally, after feeding medicated feed for 5 consecutive days, the withdrawal times of DOX in composite muscle and skin samples of snakehead were 4 days and 6 days (at 28.5°C) according to the maximum residue limits set by the European Commission (100 μg/kg for bovine, porcine and poultry) and Japanese authority (50 μg/kg), respectively.