Slow release bolus for small ruminants: in vitro release of tetracycline compared with serum concentrations of the antibiotic in sheep.

Two formulations of slow release boluses for small ruminants containing tetracycline hydrochloride in a compressed form were investigated in vitro and in vivo in adult sheep. An in vitro dissolution test was used for the preliminary selection of the 2 boluses. It was shown that the non-cumulative kinetics of tetracycline release in vitro were predictive of the tetracycline serum levels in sheep treated orally with the 2 boluses. The maximum concentrations revealed by the in vitro and in vivo curves were obtained at almost the same Tmax and, in vivo, the therapeutic serum levels were maintained for about 5 days. It is concluded that an in vitro approach is of value in predicting the kinetic profiles of a long-acting tetracycline bolus in sheep.