Elucidation of the metabolic pathway of S-equol in rat, monkey and man

► 14C-S-equol metabolites were studied in plasma, urine and feces of rats and monkey. ► The 4′-gluconronide, the 7-sulfate, and the diconjugate were major metabolites. ► Similar metabolites were seen in hepatocytes from the rat, monkey and man. S-equol is a selective estrogen receptor β (ERβ) agonist which is produced in certain individuals after ingestion of its precursor daidzein, an isoflavone present in soy. S-equol is thought to provide certain health benefits, including reduced menopausal symptoms. The metabolic profile of S-equol was determined in vivo in Sprague–Dawley rats and cynomolgus monkeys, and in vitro using hepatocytes from rat, monkey, and human. High resolution MS fragmentation patterns indicated that the major metabolite of S-equol in rat plasma and urine was the 4′-glucuronide conjugate, with lesser amounts of unconjugated S-equol, the 7-sulfate conjugate, and the 4′-glucuronide-7-sulfate diconjugate. Monkeys also showed extensive metabolism, with the major species in plasma being the 4′-glucuronide and the 7-sulfate-4′-glucuronide diconjugate; urine contained primarily the 4′-glucuronide, as seen in the rat. In vitro metabolism by hepatocytes was extensive and similar in all species, with fragmentation patterns also indicating that the 4′-glucuronide was the major metabolite. No oxidative metabolites of [14C] S-equol were detected in either in vivo or in vitro studies. These findings show that glucuronidation is the primary pathway for the metabolism of S-equol in rat, monkey and man, and that all metabolic routes of S-equol observed in vitro were also observed in vivo.