Comparative pharmacokinetics of oxytetracycline in tilapia (Oreochromis spp.) maintained at three different salinities

Environmental factors, such as temperature, pH, and salinity of water may affect the pharmacokinetics (PK) of a drug in aquatic animals and in most instances water salinity is ignored in PK studies. This study compared PK profiles of oxytetracycline (OTC) following a single oral dosage of 50 mg/kg in tilapia (Oreochromis spp.) maintained in three aquatic environments: freshwater (FW: 0 ppt salinity), brackish water (BW: 15 ppt salinity) and salt water (SW: 30 ppt salinity). Water quality parameters between the three systems were similar except salinity. Following OTC administration, blood samples were collected at 23 time points: 0.25, 0.5, 1, 2, 4, 6, 9, 12, 24 h, and 2, 4, 6, 8, 10, 12, 14, 18, 22, 26, 30, 34, 38 and 42 days. At each sampling time, six fish from each group were netted, sedated with buffered MS-222, bled and then euthanized. The OTC was extracted from plasma by Solid Phase Extraction (SPE) and analyzed by Ultra High Pressure Liquid Chromatography coupled by as tandem quadrupole mass spectrometer. The plasma concentration versus time data of OTC for the FW, BW and SW tilapia were subjected to PK analysis using non-compartment methods. Pharmacokinetics of OTC was characterized by rapid absorption and slow excretion in the FW and BW tilapia. Compared to the FW and BW groups, absorption and elimination of OTC was faster in the SW tilapia. The AUC0-∞ of OTC was in order of FW (165 h.μg/mL) > BW (145 h.μg/mL) > SW (55.5 h.μg/mL) group. In SW tilapia, terminal half-life (69 h) of OTC was >2 times shorter than FW (177 h) and BW (155 h) groups. However, AUCs and terminal half-lives of the FW and BW groups were not significantly different. The study indicated that rise in water salinity level increases clearance of OTC in tilapia. It is suggested that OTC residues in tissues will not be the same in tilapia maintained at different water salinity levels. The results confirmed that infectious diseases associated with bacteria having a MIC of 0.5-1.0 μg/mL can be treated with the 50 mg/kg dosage of OTC in the FW and BW group, but the same dosage in the SW tilapia may lead to therapeutic failure and increased risk of resistance emergence.