Schisandrin B: A dual inhibitor of P-glycoprotein and multidrug resistance-associated protein 1

Abstract

We recently reported that schisandrin B (Sch B) was a novel P-glycoprotein (P-gp) inhibitor. In this study, we revealed that Sch B was also an effective inhibitor of multidrug resistance-associated protein 1 (MRP1). The activities of Sch B to reverse MRP1-mediated drug resistance was tested using HL60/ADR and HL60/MRP, the human promyelocytic leukemia cell lines with the overexpression of MRP1 but not P-gp. Sch B resumed daunorubicin and carboxyfluorescein diacetate (CFDA, a specific substrate for MRP1) accumulation and retention in HL60/ADR cells in a time and concentration dependent manner. At the equimolar concentration, Sch B demonstrated significantly stronger potency than probenecid, a MRP1 inhibitor. This study, together with the previous findings, demonstrated that Sch B was a dual inhibitor of P-gp and MRP1, a type suggested to be preferable to the use of combination of two specific modulators to prevent drug–drug interaction and cumulative toxicities.

Introduction

Cancer multidrug resistance (MDR) is one of the major causes for the failure of clinical chemotherapy. While there are many mechanisms underlying the MDR, the ATP binding cassette members, particularly P-glycoprotein (P-gp, ABCB1) and multidrug resistance-associated protein 1 (MRP1, ABCC1), are responsible for most of the clinical cancer MDR [1], [2].

Several lines of evidence have proved that MRP1 conferred cancer with multiple drug resistance. MRP1 is a transmembrane protein, functioning mainly as a (co-)transporter of amphipathic organic anions [2]. While it transports hydrophobic drugs conjugated to the anionic tripeptide glutathione, the efficient extrusion of non-conjugated drugs by this protein depends on a normal cellular supply of glutathione. MRP1 has a broad substrate specificities, including anthracyclines, vinca alkaloids, epipodophylotoxins, mitoxantrones, methotrexates, and to a less extent, taxanes [2]. The expression of MRP1 was found in almost every tumor type, including the solid tumors (lung, gastrointestinal and urothelial carcinomas, neuroblastoma, glioma, retinoblastoma, melanoma, cancers of the breast endometrium, ovary, prostate, and thyroid) and hematological malignancies [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21]. It has been implicated that the expression of MRP1 is negatively correlated with the prognosis in certain types of cancers [3], [22], [23].

One of the ways to overcome MRP1 mediated MDR is to use an inhibitor to block the function of MRP1. Although the roles of MRP1 in MDR were known a decade ago, the discovery and development of the inhibitors to MRP1 with high efficacies and appreciable safety apparently have been much more difficult than to P-gp, most probably because MRP1 is an anionic transporter. Therefore, in principle, the MPR1 inhibitor should be anionic in nature. The anionic molecule, however, enters intact cells poorly, so that the effective intracellular concentration was difficult to be attained. Another potential problem related to the anionic transporter inhibitors is that these molecules may cause a widespread inhibition of the anionic transporter system in human body and hence a series consequences [2]. Probenecid is anionic in nature and a MRP1 inhibitor. The development of this compound as a clinical MDR modulator appears to be not optimistic due to its dose-limiting toxicities [2]. On the other hand, several P-gp inhibitors were found to cross-react with MRP1, among which, VX-710 (a pipecolinate derivative) and MS-209 (a quinoline derivative) are of particular interest [24], [25], [26]. Since the clinical MDR appears to be multifactorial, the MDR inhibitors with broad specificity are apparently preferable to the use of combination of several specific modulators to reduce the drug–drug interaction and cumulative toxicities [24].

Sch B (Fig. 1), the most abundant dibenzocyclooctadiene lignan present in Schisandra chinensis (Turcz.) Baill, is of multiple biological functions to protect against carbon tetrachloride-induced hapatotoxicity [27], myocardical ischemia/reperfusion injury [28], and brain oxidative damage [29]. We recently discovered that this compound functioned as a P-gp inhibitor [30], [31]. In addition, we proved that this compound could enhance doxorubicin-induced apoptosis in cancer cells but not in normal cells [32]. In this study, we further demonstrated that Sch B could effectively reverse MRP1-mediated cancer MDR.