Pharmacokinetics of ivermectin in sheep following intravenous, intra-abomasal or intraruminal administration

The pharmacokinetics of ivermectin in plasma following intravenous, intra-abomasal, and intraruminal administration to sheep was determined. When given intravenously, ivermectin was very slowly eliminated with a terminal half-life of 178 h and a volume of distribution at steady state of 5.3 l/kg indicating sequestration in a temporary depot. Intra-abomasal administration resulted in rapid absorption, a peak plasma concentration of 60.6 ng/ml at 4.4 h, and 100% bioavailability. However, intraruminal administration produced a much lower peak concentration (17.6 ng/ml at 23.5 h) and bioavailability (25.1%). A subsequent in vitro study indicated that ivermectin may be rapidly metabolized in the rumen.