Biliary excretion of metabolites of baicalin and baicalein in rats

Biliary excretion of metabolites of baicalin, present in Scutellariae Radix, was investigated using rats. The bile of rats administered baicalin orally was shown to contain five major metabolites, which were identified as baicalein 6-O-beta-glucopyranuronoside (M1), 6-O-methyl-baicalein 7-O-beta-glucopyranuronoside (oroxylin A 7-O-beta-glucuronide (M2], baicalein 7-O-beta-glucopyranuronoside (M3), 6-O-beta-glucopyranuronosyl- baicalein 7-O-sulfate (M4), and baicalein 6,7-di-O-beta-glucopyranuronoside (M5) on the basis of chemical and spectroscopic evidence. The bile of rats treated with baicalein also contained the above metabolites. Slower biliary excretion of the metabolites after baicalin administration suggested that it was absorbed as baicalein after hydrolysis in the gastrointestinal tract. The total cumulative amounts of the five metabolites excreted in the bile during 30 h after oral administration of baicalin and that of baicalein were approximately 54% and 40% of the doses, respectively. In addition the bilary metabolites of both drugs were shown to be mainly composed of M5 and M4, which have high polarity and large molecular weight.