Inhibitory effects of herbal constituents on P-glycoprotein in vitro and in vivo: Herb-drug interactions mediated via P-gp

Modulation of drug transporters via herbal medicines which have been widely used in combination with conventional prescription drugs may result in herb-drug interactions in clinical practice. The present study was designed to investigate the inhibitory effects of 50 major herbal constituents on P-glycoprotein (P-gp) in vitro and in vivo as well as related inhibitory mechanisms. Among these herbal medicines, four constituents, including emodin, 18 beta-glycyrrhetic acid (18 beta-GA), dehydroandrographolide (DAG), and 20(S)-ginsenoside F-1 [20(S)-GF(1)] exhibited significant inhibition (>50%) on P-gp in MDR1-MDCKII and Caco-2 cells. Emodin was the strongest inhibitor of P-gp (IC50 = 9.42 mu M), followed by 18 beta-GA (IC50 = 21.78 mu M), 20(S)-GF(1) (IC50 = 76.08 mu M) and DAG (IC50 = 77.80 mu M). P-gp ATPase activity, which was used to evaluate the affinity of substrates to P-gp, was stimulated by emodin and DAG with K-m and V-max values of 48.61, 29.09 mu M and 7129, 38.45 nmol/min/mg protein, respectively. However, 18 beta-GA and 20(S)-GF(1) exhibited significant inhibition on both basal and verapamil-stimulated P-gp ATPase activities at high concentration. Molecular docking analysis (CDOCKER) further elucidated the mechanism for structure-inhibition relationships of herbal constituents with P-gp. When digoxin was co-administered to male SD rats with emodin or 18 beta-GA, the AUC(0 - t) and Cmax of digoxin were increased by approximately 51% and 58%, respectively. Furthermore, 18 beta-GA, DAG, 20(S)-GF(1) and Rh-1 at 10 mu M significantly inhibited CYP3A4/5 activity, while emodin activated the metabolism of midazolam in human liver microsomes. In conclusion, four herbal constituents demonstrated inhibition of P-gp to specific extents in vitro and in vivo. Taken together, our findings provided the basis for the reliable assessment of the potential risks of herb-drug interactions in humans. (C) 2014 Elsevier Inc. All rights reserved.