Pharmacokinetics of eprinomectin in plasma and milk following subcutaneous administration to lactating dairy cattle

Eprinomectin is only available as a topically applied anthelmintic for dairy cattle. To determine whether eprinomectin can be applied as an injectable formulation in dairy cattle, a novel injectable formulation was developed and was subcutaneously delivered to four lactating dairy cattle at a dose rate of 0.2 mg/ kg. Plasma and milk samples were collected. The concentrations of eprinomectin in all samples were determined by HPLC. The peak plasma concentration (C-max)of 44.0 +/- 24.2 ng/ml occurred 39 +/- 19.3 h after subcutaneous administration, equivalent to the C-max (43.76 +/- 18.23 ng/ml) previously reported for dairy cattle after a pour-on administration of 0.5 mg/kg eprinomectin. The area under the plasma concentration-time curve (AUC) after subcutaneous administration was 7354 +/- 1861 (ng h)/ml, higher than that obtained after pour-on delivery (5737.68 +/- 412.80 (ng h)/ml). The mean residence time (MRT) of the drug in plasma was 211 +/- 55.2 h. Eprinomectin was detected in the milk at the second sampling time. The concentration of drug in milk was parallel to that in plasma, with a milk to plasma ratio of 0.16 +/- 0.01. The highest detected concentration of eprinomectin in milk was 9.0 ng/ml, below the maximum residue limit (MRL) of eprinomectin in milk established by the Joint FAO/WHO Expert Committee on Food Additives in 2000. The amount of eprinomectin recovered in the milk during this trial was 0.39%+/- 0.08% of the total administered dose. This study demonstrates that subcutaneous administration of eprinomectin led to higher bioavailability and a lower dose than a pour-on application, and that an injectable formulation of eprinomectin may be applied in dairy cattle with a zero withdrawal period.