Scientific Publications by FDA Staff

J Vet Pharmacol Ther 1978 Jun;1(2):119-28

Drug residues in food animals II. Plasma and tissue kinetics of oxytetracycline in young cross-bred swine

Mercer HD, Teske RH, Long PE, Showalter DH

Tissue distribution and elimination kinetics of oxytetracycline in sixteen organs and body fluids were determined in young pigs following intravenous and oral administration. Seventeen non-fasted pigs, 8¿10 weeks of age, weight range 16.4¿34.5 kg were dosed intravenously at a dose rate of 11 mg/kg bodyweight. An additional seventeen weaning pigs, 12¿14 weeks of age, weight range 27.2¿36.3 kg were dosed orally at a dose rate of 48¿65 mg/kg bodyweight. Oxytetracycline was rapidly distributed (half-life, 6.71 ± 1.13 min) in swine. The mean volume of distribution was 1.26 ± 0.18 l/kg and overall body clearance was 3.82 ± 0.59 ml/kg/min. The elimination half-life of oxytetracycline in pigs was 3.87 ± 0.62 h, which is shorter than has been observed in other domestic animal species. Oxytetracycline became rapidly and efficiently involved in enterohepatic cycling, with as much as 70% of a total intravenous dose being available for reabsorption from the gastrointestinal tract within 1 h after administration. This high degree of enterohepatic recycling prolonged the half-life, and the large amount of drug that entered the enteric tract contributed to the high volumes of distribution and high k12/k21 ratios. The excellent tissue penetration of this drug further contributed to the high volume of distribution and high k12/k21 ratios obtained. Relationships between plasma and tissue depletion for several major edible organs were found to be statistically significant. Blood plasma is proposed as a body fluid for monitoring oxytetracycline tissue residues.