Absorption and disposition of scutellarin in rats: a pharmacokinetic explanation for the high exposure of its isomeric metabolite.

Scutellarin or scutellarein-7-O-glucuronide (S-7-G) is a flavonoid used in the treatment of cardiovascular diseases. After oral administration to humans, S-7-G can hardly be detected, whereas its isomeric metabolite [scutellarein-6-O-glucuronide (S-6-G)] dominates in plasma. A preliminary study in rats also revealed a low bioavailability of S-7-G, as well as a high plasma concentration of S-6-G. Therefore, the present study tried to explore the possible causes of the unusual pharmacokinetics of scutellarin in humans through investigating the absorption and disposition of S-7-G in rats. After oral administration to rats, S-7-G was largely hydrolyzed in the intestinal tract and was absorbed as aglycone. While passing through the intestinal wall, aglycone was extensively glucuronidated into S-7-G and S-6-G (approximately 20:1), which subsequently entered the mesenteric blood (approximately 15:1). However, because S-7-G exhibited more rapid uptake in hepatocytes, was glucuronidated at a 2.7-fold higher rate in the liver, and was excreted in greater amounts through bile and urine than S-6-G, the S-7-G/S-6-G ratio eventually declined to approximately 1.5:1 in the systemic circulation. Findings revealed that S-7-G cannot be absorbed directly; S-7-G and S-6-G in the body were mostly generated from aglycone in the intestinal wall; a larger amount of S-7-G than S-6-G entered the mesenteric blood at the absorption stage, but the gap between them shrank quickly mainly because of the higher hepatic first-pass elimination of S-7-G. These findings in rats are of great value as reference for further study to accurately interpret the pharmacokinetics of S-7-G in humans.