Non-drug-related residues in tracer studies

Factors involved in residue studies with radioactive tracers are discussed. Three types of metabolic management of 14C-labeled compounds are described. Intact incorporation into protein of chick muscle is demonstrated with L-O-ethyl-[14C]threonine. Retention of unchanged and quantitatively recoverable parent compound occurred with [14C]thiabendazole sprayed on oranges. The latter drug fed to animals was retained in tissue partly intact, but mostly metabolized to recognizable detoxification products, and ultimately to endogenous tissue components. The latter type of residue was termed a "nonmetabolite" residue. The extreme case of extensive metabolism to products structurally unrelated to the parent compound was illustrated with [14C]ronidazole administered to turkeys.