Abstract 1534: Bioassay-guided identification of novel P-glycoprotein inhibitors from traditional Chinese medicines

Abstract

Abstract Multi-drug resistance (MDR) is an obstacle for therapeutic success of anticancer agents. Overexpression of P-glycoprotein (P-gp), encoded by ABCB1 gene, plays a key role in conferring MDR via active efflux of anticancer drugs from cells. Previous efforts with several generations of P-gp inhibitors had not been successful in modulating clinical MDR. Recently, an increasing number of cancer patients are using complementary and alternative medicines, in particular, traditional Chinese medicines (TCMs), in combination with conventional anticancer agents. The purpose of this study is to examine the effects of TCM extracts on modulating P-gp function. We initiated our study from four single herbal products (Glycyrrhiza uralensis, Hibiscus syriacus, Tripterygium wilfordii, and Cephalotaxus sinensis) and one formula of herbal mixture (Si-Wu-Tang), using a bio-assay guided fractionation method. Crude extracts were firstly prepared with ethanol. The most active extracts identified were further extracted with hexane, chloroform, acetone and ethanol sequentially to obtain fractions of increasing polarity. The MDR modulating effects of the crude extracts and individual fractions were evaluated based on their capability to enhance daunorubicin (DNR) accumulation in P-gp over-expressing human leukemia cell line K562/DOX. The intracellular fluorescence intensity in the presence or absence of test drugs or known P-gp inhibitor cyclosporine A (10 µM) was measured by flow cytometer. The crude extracts were also tested for cytotoxicity against K562/DOX cells by the MTS cell proliferation assay. This study revealed that among the five TCMs, the crude extracts of Tripterygium wilfordii demonstrated a dose-dependent effect on increasing the DNR accumulation, indicating the presence of novel P-gp inhibitor(s). Upon further separation, the chloroform fractions of this herb showed the highest P-gp inhibitory activity. However, known compounds present in this herb, celastrol, triptolide and myristic acid did not show effects on DNR accumulation. Most of the plant extracts tested were able to inhibit the growth of K562/DOX cells with Cephalotaxus sinensis being the most effective, consistent with literature reports that this herb contains highly cytotoxic compound homoharringtonine and congeners. The potency of P-gp inhibition for the five TCMs was not correlated with their cytotoxicity. In conclusion, our results demonstrated that the chloroform fractions of Tripterygium wilfordii contain unknown components for P-gp inhibition. Our current efforts are focused on isolation and identification of active compounds from it. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 1534.