Bioavailability of oxytetracycline, tetracycline and chlortetracycline after oral administration to fed and fasted pigs

The disposition of oxytetracycline (OTC), tetracycline (TC) and chlortetracycline (CTC) was measured after intravenous and oral administration to pigs. Eighteen healthy pigs (six for each compound) weighing 22-43 kg received a dose of 10 mg/kg intravenously, and 45 mg/kg (OTC and TC) or 40 mg/kg (CTC) orally in both a fasted and a fed condition in a three-way crossover design. The three tetracyclines were present in plasma up to 30 hours after intravenous and after oral administration to fasted as well as fed pigs. The volume of distribution was 1.4, 1.2 and 0.7 L/kg body weight for OTC, TC and CTC respectively. The bioavailability was in general low for all the three tetracyclines. The presence of food did not affect the bioavailability of OTC, which was only 3% in both fasted and fed pigs. For TC there was a significantly higher bioavailability in fasted (18%) than in fed (5%) pigs, whereas for CTC the difference was not significant, being 11% in fasted vs. 6% in fed pigs. Even though the presence of food affected the bioavailability only for TC, it prolonged the absorption phase for all three tetracyclines. Based on the bioavailability and the resulting plasma concentrations, it is concluded that it is not possible to obtain a therapeutically active concentration in plasma or tissues after oral administration of any of the three tetracyclines to fed or fasted pigs.