Pharmacokinetics of Florfenicol and Its Metabolite Florfenicol Amine in Atlantic Salmon

The plasma pharmacokinetic profile of the antibacterial agent florfenicol was studied in Atlantic salmon Salmo salar in seawater. In a single‐dose study, each fish was given 10 mg/kg body weight by intravenous (iv) injection or by oral gavage of feed coated with the drug. In a multidose study, pelleted feed coated with 2 g florfenicol/kg was hand‐fed daily for 10 d at a dosage of 10 mg florfenicol/kg body weight. The pellets contained small, X‐ray‐dense glass beads (hallotini). The feed intake was assessed by counting the number of ballotini on X‐ray pictures of sampled fish. In a depletion study, the concentration of the marker residue, florfenicol amine, was determined in muscle and liver after a field trial in which the fish were medicated with feed coated with florfenicol for 10 d at a dosage of 10 mg/kg body weight. In the single‐dose study, the iv data were best described by a two‐compartment open model; the elimination half time. t1/2β, was estimated at 14.7 h. The maximum plasma concentration was 9.l μg/mL, observed at 6 h. The bioavailability was 99%. The maximum plasma concentration observed in the multidose study and the maximum concentration predicted from the single‐dose study corresponded very well, which demonstrated the value of single‐dose studies. In the depletion study, the upper 95% confidence band for individual observations intercepted the limit of detection for the analytical method after 21 d in muscle and after 27 d in the liver.