The partition and fate of soluble and digesta particulate associated oxfendazole and its metabolites in the gastrointestinal tract of sheep.

The disposition of oxfendazole (OFZ) containing a trace of [14C]OFZ was examined in the gastrointestinal tract and bloodstream of sheep fitted with rumen and abomasal cannulae. Within 2 h of intraruminal (IR) administration, OFZ and its metabolites were almost completely associated with rumen particulate digesta. The proportion of metabolites in digesta fluid increased with their passage from the rumen into the abomasum. To determine the fate of 14C-labelled metabolites after distribution throughout rumen digesta, the rumen particulate and fluid digesta phases from a donor sheep were separated and each transferred to the rumen of an untreated recipient sheep. The 14C-labelled metabolites which derived from the donor rumen fluid quickly associated with recipient rumen particulate material. The metabolites were then progressively desorbed, as were metabolites which were transferred already associated with rumen particulate digesta. Desorption occurred faster in the abomasum than in the rumen. There was no difference in uptake kinetics between administration routes, indicating rapid equilibrium. Consequently the disposition of [14C] OFZ and its metabolites in the bloodstream was similar in each group. It is suggested that the progressive desorption of particulate associated metabolites is a principal determinant of the duration of OFZ availability.