Pharmacokinetics and muscle residue depletion of amoxicillin in cage cultured hybrid red tilapia (Oreochromis mossambicus x Oreochromis niloticus)

Amoxicillin (AMX) is commonly used to treat Streptococcus spp. infection in hybrid red tilapia (Oreochromis mossambicus x Oreochromis niloticus), despite the lack of plasma pharmacokinetic data, and suitable edible tissue residue withdrawal time estimations. The aim of this study was therefore to investigate plasma pharmacokinetics and muscle tissue residues depletion of AMX in hybrid red tilapia raised in cages. Cage raised hybrid red tilapia (223 +/- 11.5 g) from Southern Vietnam were treated with medicated feed containing AMX at a dose of 5000 mg/kg feed (equal to 50 mg/kg body weight fish) for five consecutive days. During administration of the medicated feed, fish muscle (including skin) samples were taken at days 1 and 5 (each time at 2 h, 12 h and 24 h after feeding) and at 3, 7 and 14 days after the cessation of medicated feed administration. Plasma was collected at 30 min, and at 1, 2, 6, 12 and 24 h after the first medication and at 2, 12 and 24 h after administration of medicated feed on day 2, 3, 4 and 5. AMX levels were determined by liquid chromatography coupled to tandem mass spectrometry. Results showed that AMX depleted rapidly in fish muscle and skin and was below the limit of quantification (5 mu g/kg) within 24 h after five consecutive days of medication. Noncompartmental pharmacokinetic analysis of AMX revealed that maximum plasma concentration (C-max) on day 1 was 2.75 +/- 0.98 mu g/mL (T-max,1h) whereas it ranged from 0.81 to 0.97 mu g/mL (day 2 to 4) and decreased to 0.27 +/- 0.07 mu g/mL (T-max to 2-12 h) on day five of medication. Also, the highest area under the plasma concentration-time curve from time 0 to 24 h (AUC(0-)(24h) ; 28.06 +/- 3.32 h.mu g/mL) was found on the first day of treatment and decreased on the following days. The elimination rate constant (k(el)) was 0.17 h(-1) and elimination half-life (T-1/2el) was 4.1 h. Our results show that AMX is depleted from edible muscle tissue in a short time period, which suggests a low, if any, food safety hazard associated with common treatment regimes. However, further research is needed to define a well-suited therapeutic dose.