Influence of a pig respiratory disease on the pharmacokinetic behaviour of amoxicillin after oral ad libitum administration in medicated feed

The pharmacokinetic properties of amoxicillin in healthy and respiratory-diseased pigs were studied, after ad libitum administration of medicated feed. In addition, amoxicillin dose linearity and drug penetration into respiratory tract tissues were evaluated in diseased animals. The respiratory disease involves porcine reproductive and respiratory syndrome virus and bacterial agents such as Pasteurella multocida, Bordetella bronchiseptica and Streptococcus suis. Typical clinical signs and gross lesions of respiratory disease were observed. The plasma pharmacokinetic analysis was performed by means of a noncompartmental approach. After single intravenous bolus administration of amoxicillin to healthy pigs, the steady-state volume of distribution was 0.61 L/kg, the total plasma clearance was 0.83 L/h/kg and the mean residence time was 0.81 h. After oral bolus administration, the mean absorption time was 1.6 h and the peak plasma concentration (3.09 mu g/mL) reached at 1.1 h postadministration. The oral bioavailability was 34%. For oral ad libitum administration, plasma concentration-time profiles were related to the feeding behaviour. Plasma concentrations at steady-state were established between 12 and 120 h. The pharmacokinetic parameters calculated (C-maxss, C-minss, C-avss and AUC(24ss)) showed significantly lower values in healthy pigs compared to diseased animals. This was in accordance with the significantly higher amoxicillin bioavailability (44.7% vs. 14.1%) and longer absorption period observed in diseased pigs. Amoxicillin dose linearity in diseased animals was established in a dose range of 4-18 mg/kg. On the other hand, tissue distribution ratio in diseased animals was 0.65 for bronchial mucosa, 0.48 for lung tissue and 0.38 for lymph nodes. Our results suggest that the pharmacokinetic properties and disposition of amoxicillin can be influenced by the disease state or by related factors such as changes in the gastrointestinal transit.