A series of 13 disubstituted chromones was synthesized. Two types of substituents, on each side of the scaffold, contributed to both the potency of ABCG2 inhibition and the cytotoxicity. The best compound, 5-(4-bromobenzyloxy)-2-(2-(5-methoxyindolyl)ethyl-1-carbonyl)-4H-chromen-4-one (6g), displayed high-affinity inhibition and low cytotoxicity, giving a markedly high therapeutic index. The chromone derivative specifically inhibited ABCG2 versus other multidrug ABC transporters and was not transported. It constitutes a highly promising candidate for in vivo chemosensitization of ABCG2-expressing tumors.

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Multidrug-resistance protein 1 (MRP1) belongs to the ATP-binding cassette (ABC) transporter family. MRP1 mediates MDR (multidrug resistance) by causing drug efflux either by conjugation to glutathione (GSH) or by co-transport with free GSH (without covalent bonding between the drug and GSH). We recently 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 without 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 xanthones have no toxic effect on control (sensitive) cells. Xanthones could therefore be considered as new potential anticancer agents for the selective treatment of MRP1-positive tumors.

Chalcones continue to attract considerable interest due to their anti-inflammatory and antiangiogenic properties. We recently reported the ability of 2′,5′-dihydroxychalcone (2′,5′-DHC) to induce both breast cancer resistance protein-mediated export of glutathione (GSH) and c-Jun N-terminal kinase-mediated increased intracellular GSH levels. Herein, we report a structure–activity relationship study of a series of 30 synthetic chalcone derivatives with hydroxyl, methoxyl, and halogen (F and Cl) substituents and their ability to increase intracellular GSH levels. This effect was drastically improved with one or two electrowithdrawing groups on phenyl ring B and up to three methoxyl and/or hydroxyl groups on phenyl ring A. The optimal structure, 2-chloro-4′,6′-dimethoxy-2′-hydroxychalcone, induced both a potent NF-E2-related factor 2-mediated transcriptional response and an increased formation of glutamate cysteine ligase holoenzyme, as shown using a human breast cancer cell line stably expressing a luciferase reporter gene driven by antioxidant response elements.

Most anticancer drugs are rendered less efficacious due to cell resistance to chemotherapy related to various mechanisms. A major mechanism is associated with the overexpression of ATP binding cassette (ABC) transporters, especially P-glycoprotein (Pgp/ABCB1), multidrug resistance-associated protein 1 (MRP1/ ABCC1) and breast cancer resistance protein (BCRP/ABCG2), [1] which traffic chemotherapeutic agents out of cancer cells.ABCG2 was simultaneously discovered by three research groups and named ABCP for its abundance in placenta, [2] BCRP for its identification in breast cancer cell lines, [3] and MXR for its resistance to mitoxantrone. [4] ABCG2 constitutes an important target for the design of efflux inhibitors that would, when co-administered with an anticancer agent, give increased intracellular drug concentrations and hence greater cytotoxicity. While several types of ABCG2 inhibitors have been evaluated in vitro, very few have entered preclinical trials. [5][6][7][8][9] We recently discovered that some substituted chromones are selective and potent ABCG2 inhibitors. [10] These compounds were synthesized in five steps, and the overall yields were quite low. In pursuing our efforts toward structurally simple and easily accessible specific inhibitors of BCRP, we investigated 6-halogenochromones linked to a tryptamine unit, obtained in only one step, as new potent inhibitors (Scheme 1).The choice of C-6 as the site of halogenation was motivated by a number of considerations: 1) the presence of a hydrophobic halogen at the C-6 position fulfills the previously identified need for a hydrophobic substituent in this part of the mole-cule; [10] 2) halogens, especially bromine and iodine, have a positive contribution to inhibitory activity; [11,12] 3) halogens could open interesting opportunities for the generation of further potential inhibitors, as they can be easily replaced by a number of chemical entities.Access to target compounds 3-7 was achieved in one step by coupling 6-substituted-4-oxo-4H-chromene-2-carboxylic acid (1) with tryptamine (2) in the presence of bis(2-oxo-3-oxazolidinyl)phosphonic chloride (BOP-Cl) as the coupling agent (Scheme 2; full details are given in the Supporting Information). 6-Iodo-4-oxo-4H-chromene-2-carboxylic acid (R = I) was not commercially available, but was easily obtained by hydrolysis of the commercially available corresponding ethyl ester with sodium hydrogen carbonate (20 % in water) at 80 8C.The test compounds were first screened by flow cytometry for their effects on the inhibition of mitoxantrone efflux in Scheme 1. Retrosynthetic rationale for the synthesis of targeted BCRP inhibitors, and the structures of the commercially available starting materials 1 and 2.Scheme 2. Synthesis of targeted inhibitors 3-7. Reagents and conditions: a) BOP-Cl, Et 3 N, DMF, RT, 24 h, 41-55 %.