FENBENDAZOLE-RELATED DRUG RESIDUES IN MILK FROM TREATED DAIRY COWS

Oral administration of [C-14] fenbendazole (FBZ) at a dose of 5.0 mg/k g leads to the presence of radiolabel in the milk of lactating dairy c ows. However, the maximum mean concentration of total FBZ equivalents quantitated to one-third of the recommended safe concentrations in mil k (1.67 mu g/mL). The label is equally distributed to the fat and aque ous portions of the milk. The maximum level, in general, is attained a pproximately 24-36 h after drug administration, with the highest level s ranging from 24 to 48 h after drug administration. The residues rapi dly deplete, attaining levels of 10-20 ng/mL by day 5, and are essenti ally undetectable by radiolabel monitoring by day 6. Extraction of the milk by matrix solid phase dispersion indicated that the label was di stributed between traces of the parent drug, FBZ, and predominantly, t he FBZ sulphoxide (SO) and sulphone (SO2) metabolites. No other radiol abelled peaks were observed. Based on these data the metabolites of FB Z, FBZ-sulphone and FBZ-sulphoxide, could be used as market residues f or monitoring the administration of FBZ to lactating dairy cows.