Increased Oral AUC of Baicalin in Streptozotocin-Induced Diabetic Rats due to the Increased Activity of Intestinal β-Glucuronidase

The purpose of the study was to investigate the pharmacokinetics of baicalin, a major bioactive component of Scutellariae radix, in diabetic conditions. The 4-week diabetic rats were induced by intraperitoneal administration of streptozotocin. Plasma concentrations of baicalin were measured following oral (200 mg/kg) or intravenous (12 mg/kg) administration. Everted intestinal transport, intestinal mucosal metabolism of baicalin and intestinal β-glucuronidase activity were also investigated. It was found that the diabetic condition significantly increased the exposure of baicalin following oral doses (AUC 100.77 ± 4.16 µg · h/mL in diabetic rats vs. 48.48 ± 7.94 µg · h/mL in normal rats). In contrast, the diabetic condition significantly decreased the exposure of baicalin following intravenous doses (AUC 11.20 ± 2.28 µg · h/mL in diabetic rats vs. 18.02 ± 3.45 µg · h/mL in normal rats). We also found lower apparent permeability coefficients of baicalin in the ileum of diabetic rats (8.43 × 10−6 ± 2.40 × 10−6 cm/s in diabetic rats vs. 5.21 × 10−5 ± 1.55 × 10−5 cm/s in normal rats). Further studies showed that the diabetic condition enhanced the hydrolysis of baicalin to baicalein in intestinal mucosal, accompanied by an increase of β-glucuronidase activity. All these results suggested that the higher oral exposure of baicalin in diabetic rats did not result from the decreased hepatic metabolism or increased intestinal absorption of baicalin. The enhancement of intestinal β-glucuronidase activity may partly account for the higher exposure of baicalin in diabetic rats after oral administration.