Serum and tissue concentrations of doxycycline in broilers after the sub-cutaneous injection of a long-acting formulation.

1. The antibacterial agent doxycycline hyclate (Dox) is usually administered to broilers in drinking water or as a feed supplement. Parenteral injection is not the usual route for administration, so a long-acting formulation (Dox-LA) was tested to evaluate if serum concentrations can achieve the pharmacokinetic/pharmacodynamic (PK/PD) ratios regarded as adequate for the drug. 2. A poloxamer-based matrix was used to provide Dox-LA. Serum and tissue concentrations of Dox vs time were determined in two day-old broilers after subcutaneous (SC) injection of Dox-LA or oral administration of a single bolus of aqueous Dox (Dox-PO), at a dose of 20 mg/kg. Weight gain, feed conversion rate, haematological variables, aspartate aminotransferase and alanine aminotransferase activities, blood urea and creatinine were determined and compared for Dox-LA with Dox-PO and non-medicated controls. 3. Dox-LA had a high relative bioavailability (1200%). Maximum serum concentrations were not statistically different (5·1 ± 1·1 µg/ml for Dox-LA and 6·1 ± 1.4 µg/ml for Dox-PO), but half-life of Dox-LA was much greater than the value obtained for Dox-PO (73·0 ± 0·9 h and 2·0 ± 0·02 h, respectively). Tissue concentrations were higher, and stayed higher for longer periods in the Dox-LA group. 4. In conclusion, considering the minimum effective serum concentration against Mycoplasma spp is 0·5 µg/ml, a dose-interval of 180 h can be achieved with Dox-LA, but only for 24 h after Dox-PO. Better PK/PD ratios for Dox-LA should result in improved clinical outcomes compared with Dox-PO.