Glabridin is a major constituent of the root of Glycyrrhiza glabra, which is commonly used in the treatment of cardiovascular and central nervous system diseases. This study aimed to investigate the role of P-glycoprotein (PgP/MDR1) in the intestinal absorption of glabridin. The systemic bioavailability of glabridin was approximately 7.5% in rats, but increased when combined with verapamil. In single-pass perfused rat ileum with mesenteric vein cannulation, the permeability coefficient of glabridin based on drug disappearance in luminal perfusates (P(lumen)) was approximately 7-fold higher than that based on drug appearance in the blood (P(blood)). Glabridin was mainly metabolized by glucuronidation, and the metabolic capacity of intestine microsomes was 1/15 to 1/20 of that in liver microsomes. Polarized transport of glabridin was found in Caco-2 and MDCKII monolayers. Addition of verapamil in both apical (AP) and basolateral (BL) sides abolished the polarized transport of glabridin across Caco-2 cells. Incubation of verapamil significantly altered the intracellular accumulation and efflux of glabridin in Caco-2 cells. The transport of glabridin in the BL-AP direction was significantly higher in MDCKII cells overexpressing PgP/MDR1 than in the control cells. Glabridin inhibited PgP-mediated transport of digoxin with an IC(50) value of 2.56 microM, but stimulated PgP/MDR1 ATPase activity with a K(m) of 25.1 microM. The plasma AUC(0-24h) of glabridin in mdr1a(-/-) mice was 3.8-fold higher than that in wild-type mice. These findings indicate that glabridin is a substrate for PgP and that both PgP/MDR1-mediated efflux and first-pass metabolism contribute to the low oral bioavailability of glabridin.