Pharmacokinetics and residue elimination of oxytetracycline in grass carp, Ctenopharyngodon idellus

Pharmacokinetics and residue elimination of oxytetracycline (OTC) are important in the determination of optimal dosage regimens and in establishing safe withdrawal periods in farmed fishes. In this study the grass carp (Ctenopharyngodon idellus L.) were divided at random into two groups: one was treated orally via a stomach tube at a single dose of 100 mg/kg body weight and the other group was treated orally with medicated feed at a dosage of 100 mg/kg per day for 7 days at water temperature of 21 ±1 °C. OTC concentrations were determined by means of high performance liquid chromatography (HPLC) with 0.05 μg/ml detection limit. The data of pharmacokinetics conformed to a two-compartment model. The absorption half-life (t1/2 kα) was 1.34 h and the distribution half-life (t1/2 α) was 5.45 h and the elimination half-life (t1/2 β) of OTC was 83.66 h in serum. The maximum OTC concentration (Cmax) was 4.99 μg/ml and the time to peak concentration (Tmax) was 5.69 h. Residue elimination in grass carp treated orally with medicated feed at a dosage of 100 mg/kg per day for 7 days showed that the post-dosing OTC residue concentrations were highest in kidney and lowest in muscle in the detected tissues during an OTC-elimination period. OTC residues in the muscle of grass carp fell below the detection limit of 0.05 μg/ml (less than the maximum residue limit) on the 25th day at 21 ± 1 °C. The results suggest that a withdrawal time should be at least 25 days prior to the use of dosed fish as human food following oral administrations at a dosage of 100 mg/kg body weight per day for 7 days at water temperature of 21 ± 1 °C.