Radioimmunoassay of oxfendazole in sheep fat

Oxfendazole (methyl 5-(phenylsulfinyl)-2-benzimidazole-carbamate) is a broad spectrum anthelmintic agent designed for use in food-producing animals. A simple radioimmunoassay (RIA) for determination of oxfendazole in plasma was modified for determining oxfendazole in sheep fat. Fat tissue was enzymatically hydrolyzed to an oily residue with collagenase-hyaluronidase, and oxfendazole was then extracted into an acidified aqueous phase. An aliquot of this phase was used directly for RIA. Bound radioactivity was separated from free by using polyethylene glycol-bovine gamma globulin because oils and other components in the aqueous aliquot preclude the use of charcoal for the separation. The lower limit of sensitivity of the assay is 0.003 ppm. Accuracy experiments carried out in the range 0.01-0.5 ppm gave a regression line of y (ng/g) = 0.91 x (ng/g) + 2.89, with r = 0.99. Fat tissue derived from sheep given an oral dose of 6.0 mg/kg was analyzed by this method and by a high pressure liquid chromatographic (HPLC) method. Values obtained by the 2 methods agreed well.