Abstract

Scutellarin [scutellarein-7-O-glucuronide (S-7-G)] displayed a unique pharmacokinetic profile in humans after oral administration: the original compound was hardly detected, whereas its isomeric metabolite isoscutellarin [scutellarein-6-O-glucuronide (S-6-G)] had a markedly high exposure. Previous rat study revealed that S-7-G and S-6-G in the blood mainly originated from their aglycone in enterocytes, and that the S-7-G/S-6-G ratio declined dramatically because of a higher hepatic elimination of S-7-G. In the present study, metabolite profiling in human excreta demonstrated that the major metabolic pathway for S-6-G and S-7-G was through further glucuronidation. To further understand the cause for the exposure difference between S-7-G and S-6-G in humans, studies were conducted to uncover mechanisms underlying their formation and elimination. In vitro metabolism study suggested that S-7-G was formed more easily but metabolized more slowly in human intestinal and hepatic microsomes. Efflux transporter study showed that S-6-G and S-7-G were good substrates of breast cancer resistance protein and multidrug resistance-associated protein (MRP) 2 and possible substrates of MRP3; however, there was no preference great enough to alter the S-7-G/S-6-G ratio in the blood. Among the major hepatic anion uptake transporters, organic anion-transporting polypeptide (OATP) 2B1 played a predominant role in the hepatic uptake of S-6-G and S-7-G and showed greater preference for S-7-G with higher affinity than S-6-G (K_m values were 1.77 and 43.9 μM, respectively). Considering the low intrinsic permeability of S-6-G and S-7-G and the role of OATP2B1 in the hepatic clearance of such compounds, the selective hepatic uptake of S-7-G mediated by OATP2B1 is likely a key determinant for the much lower systemic exposure of S-7-G than S-6-G in humans.