Use In Food-producing Animals Research Articles

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.

Human safety data collection and evaluation for the approval of new animal drugs.

Before a new drug is approved for use in food-producing animals, data are required which demonstrate that food derived from the animal does not contain unsafe residues. Also, an analytical method for residues must be provided which is practicable for government surveillance and enforcement activities. Residue information is derived by using radiolabeled drugs to study metabolism in the animal for which the drug is intended. Residues in the edible tissues are characterized, and the amount of residues and their rates of depletion from the different tissues after cessation of drug treatment are determined. Bioassays with laboratory animals are used to study the toxicity of the drug and its important metabolites and to establish tolerance limitations. From this information, the conditions for the analytical method are established--that is, the compound(s) measured (the marker), the tissue (target tissue) monitored to ensure control of "total residue", and the required sensitivity. The method is subjected to validation in government laboratories and must meet the Food and Drug Administration's standards of precision, accuracy, specificity, and practicability. Finally, the method is used in simulated field trials to establish the required withdrawal time after drug treatment before the animals can be marketed for their milk or for slaughter for food.