Pharmacokinetics of doxycycline in tilapia ( O reochromis aureus × O reochromis niloticus ) after intravenous and oral administration

The pharmacokinetics of doxycycline was studied in plasma after a single dose (20 mg/kg) of intravenous or oral administration to tilapia ( O reochromis aureus  ×  O reochromis niloticus ) reared in fresh water at 24 °C. Plasma samples were collected from six fish per sampling point. Doxycycline concentrations were determined by high‐performance liquid chromatography with a 0.005  μ g/mL limit of detection, then were subjected to noncompartmental analysis. Following oral administration, the double‐peak phenomenon was observed, and the first ( C max1 ) and second ( C max2 ) peaks were 1.99 ± 0.43  μ g/mL at 2.0 h and 2.27 ± 0.38  μ g/mL at 24.0 h, respectively. After the intravenous injection, a C max2 (12.12 ± 1.97  μ g/mL) was also observed, and initial concentration of 45.76  μ g/mL, apparent elimination rate constant ( λ z ) of 0.018 per h, apparent elimination half‐life ( t 1/2 λ z ) of 39.0 h, systemic total body clearance (Cl) of 41.28 mL/h/kg, volume of distribution (Vz) of 2323.21 mL/kg, and volume of distribution at steady‐state (Vss) of 1356.69 mL/kg were determined, respectively. While after oral administration, the λ z , t 1/2 λ z , and bioavailability of doxycycline were 0.009 per h, 77.2 h, and 23.41%, respectively. It was shown that doxycycline was relatively slowly and incompletely absorbed, extensively distributed, and slowly eliminated in tilapia, in addition, doxycycline might undergo enterohepatic recycling in tilapia.