STUDIES ON TISSUE DISTRIBUTION AND THE ELIMINATION REGULARITY OF DOXYCYCLINE RESIDUES IN ICTALURUS PUNCTATUS

Doxycycline (DOTC), a tetracycline broad-spectrum antibiotic, is widely used for the treatment of systemic bacterial infections. The broad spectrum antibiotic doxycycline has commonly been used in channel catfish (Ictalurus punctatus) aquaculture to prevent and treat bacterial diseases. In order to study the tissue distribution and elimination of doxycycline in channel catfish, single and continuous for 5 days oral administration (20 mg/kg) body weight of DOTC has been investigated in healthy channel catfish at (28±1) . This is the first report about tissue distribution and residue℃ s of DOTC in channel catfish. The results indicated that: 1) The concentration-time course of DOTC in the tissues could be described by a two-compartment open model after a single oral administration at a dose of 20 mg/kg, which showed an obvious double-peak phenomenon. The main pharmacokinetic parameters were as follows: Tmax (1) was about 30min in kidney, blood and muscle, which was 1h in liver. Cmax(1) was 16.641 ± 0.335, 1.139 ± 0.204, 0.58 ± 0.394, 0.881 ± 0.020 mg/kg in liver, kidney, blood and muscle respectively. Cmax(2) was exceeded the corresponding Cmax(1). Tmax (2) was 8h in kidney, blood and muscle, which was 12h in liver. Cmax(2) was 35.907 ± 0.378, 1.173 ± 0.213, 4.26 ± 0.714 and 2.027 ± 0.051 μg/kg in liver, kidney, blood and muscle respectively. The pharmocokinetics of DOTC displayed various favorable properties, including large distribution volume and prolonged elimination half-life. The area under concentration-time curve (AUC) was 63.242, 1282.076, 142.379 and 62.348 μg/mL·h in kidney, liver, blood and muscle respectively. The elimination half-life (T1/2b) was 31.091—126.527h, and the retention time (MRT) was 40.85—56.585h. 2) At the temperature of (28±1)℃, channel catfish were treated with DOTC by oral administration at a dose of 20 mg/kg body weight body for five days. The result indicated that the concentration of DOTC in liver was higher than other tissues, and the concentration of DOTC in skin was the lowest after 5 days withdrawal. The speed from fast to slow was muscle and skin, kidney and liver. It showed that liver was the main absorbing tissue, and muscle had a poor absorbability. These observations indicated that the drug might constitute a good option for the treatment of certain infectious diseases in channel catfish. If take liver as target tissue in this experiment, according to the maximum residue limit (MRL) of 300 μg/kg in liver, the withdrawal period should not be less than 30d under this experiment condition. And if take an edible tissue of muscle and skin as target tissue in this experiment, according to the maximum residue limit (MRL) of 300 μg/kg in muscle and skin,the withdrawal period should not be less than 19d. In conclusion, the reliable theory was the basis to make dosage regimen and withdrawal time based on the study.