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Vol. 304, Issue 3, 1228-1235, March 2003
Department of Pharmaceutical Sciences, College of Pharmacy,
Washington State University, Pullman, Washington
The purpose of this study was to determine the importance of intestinal
disposition in the first-pass metabolism of flavonoids. A four-site
perfused rat intestinal model, rat liver and intestinal microsomes,
Caco-2 cell microsomes, and the Caco-2 cell culture model were used. In
the four-site model,
28% of perfused aglycones are absorbed (
450
nmol/30 min). Both absorption and subsequent excretion of metabolites
were rapid and site-dependent (p < 0.05). Maximal
amounts of intestinal conjugates excreted per 30 min were 61 and 150 nmol for genistein and apigenin, respectively. Maximal amounts of
biliary conjugates excreted per 30 min were 50 and 30 nmol for
genistein and apigenin, respectively. Microsomes, prepared from Caco-2
cells, rat intestine, and rat liver, always glucuronidated apigenin
faster than genistein (p < 0.05). In addition, rat
jejunal microsomes glucuronidated both flavonoids faster
(p < 0.05) than rat intestinal microsomes prepared
from other regions. When comparing glucuronidation in different organs,
jejunal microsomes often but not always glucuronidated both flavonoids
faster than liver microsomes. In the Caco-2 model, both flavonoids were
rapidly absorbed and rapidly conjugated, and the conjugates were
excreted apically and basolaterally. Similar to the four-site perfusion model, apigenin conjugates were excreted much faster than genistein conjugates (>2.5 times for glucuronic acid, >4.5 times for sulfate; p < 0.05). In conclusion, intestinal disposition
may be more important than hepatic disposition in the first-pass
metabolism of flavonoids such as apigenin. In conjunction with
enterohepatic recycling, enteric recycling may be used to explain why
flavonoids have poor systemic bioavailabilities.
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