Intestinal Absorption of Isoalantolactone and Alantolactone, Two Sesquiterpene Lactones from Radix Inulae, Using Caco-2 Cells. 
Isoalantolactone and alantolactone are the main sesquiterpene lactones in Radix Inulae (dried root of Inula helenium L. or I. racemosa Hook.  F.), which is a frequently utilized herbal medicine.  They also occur in several plants and have various pharmacologic effects.  However, they have been found to have poor oral bioavailability in rats.  To understand the intestinal absorptive characteristics of isoalantolactone and alantolactone as well specific influx and efflux transporters in their absorption.  Bidirectional permeabilities of isoalantolactone and alantolactone were investigated across Caco-2 cell monolayers.  Transport assays were performed using different concentrations of two lactones and specific inhibitors of ATP-binding cassette transporters and influx transporters.  The absorption permeability of isoalantolactone and alantolactone was high at the tested concentrations (5, 20 and 80 μmol/l), and the major permeation mechanism of both lactones was found to be passive diffusion with active efflux mediated by multidrug resistance-associated proteins (MRPs) and breast cancer resistance protein (BCRP).  Our results demonstrated that the absorption permeability of isoalantolactone and alantolactone was good in the Caco-2 cell model.  The isoalantolactone and alantolactone absorption elucidated in this study provides useful information for further pharmacokinetics studies.  Since low intestinal absorption can now be ruled out as a cause, further studies are needed to explain the low oral bioavailability of the two sesquiterpene lactones.