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Abstract: Ethnopharmacological relevance: Angelicae Dahurica (Hoffm.) Benth. & Hook. f. ex Franch. & Sav combined with Radix Scutellariae baicalensis Georgi has been widely used in traditional Chinese medicine (TCM) as an antipyretic analgesic and anti-inflammatory drug. Modern pharmacological studies have demonstrated that the compatible application of these two drugs is an effective treatment for hepatitis. A previous study indicated that a Radix Angelicae Dahuricae extract enhanced the intestinal absorption of the baicalin found in Radix Scutellariae; however, the underlying compatibility mechanism of these two herbs remains unknown. In this study, we further examined the effect of a Radix Angelicae Dahuricae extract on the absorption and transport properties of baicalin in a Caco-2 cell model to determine the compatibility mechanism of these two herbs. Aim of the study: The aim of this work was to study the transport properties of baicalin in Radix Scutellariae across cell membranes and the effects of a Radix Angelicae Dahuricae extract on baicalin absorption using the well-characterized, human-based intestinal Caco-2 cell model. Materials and methods: We assessed the absorption, bidirectional transport and toxicity of baicalin using a range of parameters, including drug concentration, pH, a P-glycoprotein (P-gp) inhibitor (Verapamil), an MRP inhibitor (MK-571) and EDTA-Na2 (tight junction modulator). Next, we studied the influence of a Radix Angelicae Dahuricae extract on the transport of baicalin under the same conditions. Drug concentration was measured by HPLC, and the apparent permeability coefficient (Papp) and apparent permeability ratio (PDR) were subsequently calculated. Results: The results showed that baicalin is non-toxic within a concentration range of 800µg/mL to 4800µg/mL. The transport of baicalin showed time and concentration dependence. The absorption of baicalin was optimal at pH 7.4 in 37°C; however, the absorption decreased at 4°C. The Papp of baicalin transport through the Caco-2 cell monolayer model was altered when specific inhibitors of P-gp or MRP were added to the cells. However, there was no significant difference in the PDR value. The Papp of baicalin improved when it was combined with the Radix Angelicae Dahuricae extract. The influence of EDTA-Na2 on the transport of baicalin showed that the permeability of baicalin significantly increased. The result further indicated that the mechanism of baicalin intestinal absorption in the Caco-2 cell monolayer involves passive transcellular diffusion. Conclusions: Passive diffusion is the main mode of intestinal absorption of bacalin and it involved in the efflux of proteins. The enhanced intestinal absorption of baicalin by Radix Angelicae Dahuricae can be due to opening of the tight junctions between cells and inhibition of MRP efflux protein expression or function. [Copyright &y& Elsevier]