DOI: 10.1002/cmdc.201100102 Identification of Xanthones as Selective Killers of Cancer Cells Overexpressing the ABC Transporter MRP1 Estelle Genoux-Bastide, [a] Doriane Lorendeau, [b] Edwige Nicolle, [a] Samir Yahiaoui, [a] Sandrine Magnard, [b] Attilio Di Pietro, [b] HØlne Baubichon-Cortay, [b] and Ahcne Boumendjel* [a] Introduction The development of multidrug resistance (MDR) in cancer cells is a major hurdle in the treatment of cancer by chemotherapy. MDR is the phenomenon in which exposure of tumor cells to a single cytotoxic agent results in cross-resistance to other struc- turally unrelated classes of cytotoxics. Notably, MDR is fre- quently restricted to anticancer drugs, although it covers other classes of drugs such as antibiotics, antifungal, antiviral, and antiparasitic agents. The decreased accumulation of anticancer drugs in cancer cells may be mediated by several membrane proteins which belong to the ABC (ATP-binding cassette) family of proteins. [1] The best-known and well-studied ABC pro- teins are P-glycoprotein (P-gp) encoded by the mdr1 gene and multidrug-resistance protein 1 (MRP1) encoded by the mrp1 gene. [2–4] More recently, breast cancer resistance protein (BCRP/ ABCG2) has been described. [5] Discovered in 1992, [4] MRP1 (also known as ABCC1) was found to be ubiquitously expressed in normal tissues, with high levels in the lung, testis, kidney, and peripheral blood mononuclear cells. [6] It was established that MRP1 mediates MDR by catalyzing drug efflux, either by conjugation with glu- tathione (GSH) or by co-transport with free GSH (without cova- lent bonding between the drug and GSH). [7, 8] It was observed that the levels of free GSH are decreased in MRP1-overexpress- ing cells and increased in tissues from ABCC null mice (target- ed abcc1 À/À mice). [9–12] Furthermore, the ability of MRP1 to transport both GSH and GSSG (oxidized glutathione) raises the possibility that the protein contributes to maintenance of the cell redox state. [13–15] Several studies have revealed that intra- cellular glutathione depletion is correlated to progression of the execution phase of apoptosis. [16, 17] We recently reported [18, 19] that the calcium channel blocker verapamil, a well-known inhibitor of P-gp which was not clear- ly described as an MRP1 inhibitor, [20–22] is able to stimulate glu- tathione transport out of the cell by MRP1 after binding to the transporter. The fast and massive glutathione extrusion led MRP1-overexpressing cells to apoptosis, suggesting that vera- pamil could be considered as a strong anticancer agent specifi- cally targeting resistant tumors. However, a major drawback of using verapamil relates to its cardiotoxicity. Accordingly, the discovery of compounds able to stimulate efflux of GSH among resistant cells without inducing side effects can be con- sidered as new and promising antitumor agents acting through an original mechanism. Xanthones are a class of dibenzo-g-pyrone heterocyclic com- pounds commonly found in plants which have been shown to [a] Dr. E. Genoux-Bastide, + Dr. E. Nicolle, Dr. S. Yahiaoui, Prof. A. Boumendjel ++ UniversitØ de Grenoble/CNRS, UMR 5063 DØpartement de Pharmacochimie MolØculaire Bâtiment E, AndrØ Rassat, Pôle Chimie BP 53, 38041 Grenoble Cedex 9 (France) Fax: (33) 4-76-63-52-98 E-mail : Ahcene.Boumendjel@ujf-grenoble.fr [b] Dr. D. Lorendeau, + S. Magnard, Dr. A. Di Pietro, Dr. H. Baubichon-Cortay ++ Equipe LabellisØe Ligue 2009 Institut de Biologie et Chimie des ProtØines FR 3302, BM2SI UMR 5086 CNRS/UniversitØ Lyon 1 7 passage du Vercors, 69367 Lyon Cedex 07 (France) [ + ] These authors contributed equally to this work. [ ++ ] Both senior investigators for this study. Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/cmdc.201100102. Multidrug-resistance protein 1 (MRP1) belongs to the ATP-bind- ing cassette (ABC) transporter family. MRP1 mediates MDR (multidrug resistance) by causing drug efflux either by conju- gation to glutathione (GSH) or by co-transport with free GSH (without covalent bonding between the drug and GSH). We re- cently reported that the calcium channel blocker verapamil can activate massive GSH efflux in MRP1-overexpressing cells, leading to cell death through apoptosis. However, clinical use of verapamil is hampered by its cardiotoxicity. Then, in the search for compounds that act similarly to verapamil, but with- out major side effects, we investigated xanthones. Herein we show that xanthones induce apoptosis among resistant cells overexpressing MRP1 similarly to the verapamil effect. Among the xanthones studied, 1,3-dihydroxy-6-methoxyxanthone was identified as the most active derivative, able to specifically kill cells transfected with human MRP1 with even greater potency than verapamil. Under the same conditions, the active xan- thones have no toxic effect on control (sensitive) cells. Xan- thones could therefore be considered as new potential anti- cancer agents for the selective treatment of MRP1-positive tumors. 1478  2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim ChemMedChem 2011, 6, 1478 – 1484 MED