Metabolism and disposition of ractopamine hydrochloride by turkey poults

Ractopamine HCl, ((1R*,3R*),(1R*,3S*)-4-hydroxy-alpha-[[[3-(4- hydroxyphenyl)-1-methylpropyl]-amino]methyl]benzenemethanol hydrochloride), is a beta-adrenergic agonist that is under evaluation as a nutrient repartitioning agent in livestock. Because ractopamine metabolism has not been evaluated in turkeys, the objectives of this study were to synthesize and identify products of ractopamine metabolism and to determine the stereoselective metabolism and tissue distribution of [14C]ractopamine HCl in orally dosed turkey poults. Glucuronides of diastereoisomeric [14C]ractopamine, and the (1R,3R) and (1R,3S) stereoisomers of ractopamine were synthesized by use of microsomal proteins immobilized on Sepharose beads. Monoglucuronides conjugated to the phenols at C-10 or C-10' were isolated and identified by 1H-NMR and negative ion FAB/MS. Urine and feces were collected from colostomized turkey poults after oral dosing with 20 mg of [14C]ractopamine HCl (9.28 microCi). Radioactive residues in tissues were highest in the gallbladder and liver. Radioactivity was not detectable in blood 48 hr after dosing and was slightly above background (< 100 dpm/g tissue) in skeletal and cardiac muscle. Urine contained 47.5% of the administered radioactivity by 16 hr after dosing, and by 48 hr 52.0% of the radioactivity was excreted in the urine. Feces contained 36.6% and 41.5% of the dose 16 and 48 hr after dosing, respectively. Unmetabolized ractopamine represented only 8% of the urinary radioactivity; ractopamine glucuronides represented 72% of the urinary radioactivity. Glucuronides conjugated to the C-10 phenol of ractopamine represented 59.8% of the urinary metabolites and were composed of all four ractopamine stereoisomers. Glucuronides conjugated to the C-10' phenol of ractopamine represented 12.7% of the urinary metabolites and were composed of the (1R,3R) and (1R,3S) stereoisomers. Ractopamine was rapidly eliminated from turkeys after oral dosing, and glucuronidation was the major pathway of metabolism. Regioselective glucuronidation occurred favoring the C-10 phenol of ractopamine; glucuronidation of the C-10' phenol of ractopamine was specific for the (1R,3R) and (1R,3S) stereoisomers.