Fenbendazole-related drug residues in milk from treated dairy cows

Oral administration of [14C] fenbendazole (FBZ) at a dose of 5.0 mg/kg leads to the presence of radiolabel in the milk of lactating dairy cows. However, the maximum mean concentration of total FBZ equivalents quantitated to one-third of the recommended safe concentration in milk (1.67 micrograms/mL). The label is equally distributed to the fat and aqueous portions of the milk. The maximum level, in general, is attained approximately 24-36 h after drug administration, with the highest levels ranging from 24 to 48 h after administration. The residues rapidly deplete, attaining levels of 10-20 ng/mL by day 5, and are essentially undetectable by radiolabel monitoring by day 6. Extraction of the milk by matrix solid phase dispersion indicated that the label was distributed between traces of the parent drug, FBZ, and predominantly, the FBZ sulphoxide (SO) and sulphone (SO2) metabolites. No other radiolabelled peaks were observed. Based on these data the metabolites of FBZ, FBZ-sulphone and FBZ-sulphoxide, could be used as marker residues for monitoring the administration of FBZ to lactating dairy cows.