Abstract

The pharmacokinetics of oxytetracycline in two-year old loggerhead sea turtles (Caretta caretta) following single intravenous (i.v.) and intramuscular (i.m.) injections were studied for biological marking and antibiotic applications. Twenty juvenile turtles, weighing a mean of 7.98 kg and housed individually in two 5775 L raceways at 28.5-30.0°C, were divided into two treatment groups. Ten animals received 25 mg/kg of oxytetracycline i.v., and 10 received the same dose i.m. Blood was collected from the dorsal cervical sinus (external jugular vein) at -24 (predose sample), 0.5, 1.5, 3, 6, 12, 24, 48, 96, 120 and 240 hours after injection. Plasma oxytetracycline concentrations were analyzed by high performance liquid chromatography (HPLC). Data from the i.v. route best fit a three-compartment model, while non-compartmental analysis was used for data from the i.m route. For the i.v. route, means for maximum plasma concentration, terminal phase half-life, systemic clearance, and apparent volume of distribution at steady state were 6.6 µg/mL, 66.1 hr, 290.7 mL/hr/kg and 18.4 L, respectively. Means for i.m. systemic availability, maximum plasma concentration, and elimination half-life were 91.8 percent, 1.6 µg/mL, and 61.9 hr, respectively. The remarkably high apparent volume of distribution represents a very deep compartment, likely bone deposition, reflecting the large skeletal mass of turtles and the fact that these were well-nourished growing juveniles. Bone deposition is one of the characteristics that makes oxytetracycline a useful tool for aging studies by skeletochronology. Though maximum plasma concentration by i.m. administration was low, the long elimination time indicates that an infrequent dosing interval may be effective for sensitive bacteria.