Drug recovery from medicated animal feeds
Investigations into the poor recovery of sulphadimidine from medicated animal feeds have shown that irreversible drug-feed binding, and not drug degradation, is responsible for the poor recoveries. The experimental work involved the novel use of C-sulphadimidine in analytical studies and in autoradiography of C-sulphadimidine-bound feed. The latter showed that the drug was not bound preferentially to specific feed constituents but was widely distributed on nearly all the feed particles. Further work on sulphadimidine recovery from feeds demonstrated an inverse relationship between drug recovery and feed moisture content. The role of moisture in the binding mechanism was then considered, and experiments conducted on the adsorption of moisture by feeds showed that the rate controlling mechanism was diffusion. A hypothesis is presented in which sulphadimidine is partially dissolved by the moisture in the feed and the resulting solution then diffuses into the internal regions of the feed particles via pores and cracks in the constituent particles. The deep penetration of the drug into the feed prevents the drug from being recovered by the extraction solution. Experimental evidence was found to support this hypothesis. Experimental work also investigated the causes of poor recoveries of sulphadiazine, trimethoprim and dinitolmide.