Dimethoxyaurones: Potent inhibitors of ABCG2 (breast cancer resistance protein).

A series of 4,6-dimethoxyaurones were synthesized by reacting 4,6-dimethoxybenzofuran-3(2H)-one with various benzaldehydes in a base-catalyzed aldol reaction. A Z configuration was assigned to the aurones based on spectroscopic and crystallographic data. The aurones were tested for their ability to modulate ABCG2 (breast cancer resistance protein)-mediated multidrug resistance in vitro. Several members (0.5 microM) increased the accumulation of mitoxantrone (MX) in human breast cancer cells (MDA-MB-231) transfected with ABCG2 and re-sensitized these cells to the cytotoxic effects of MX. In the re-sensitization assay, aurones at 0.5 microM reduced the resistance of the transfected cells to MX to just twice that of the parental cells, exceeding fumitremorgin C (FTC) tested at the same concentration. The aurones (10 microM) also increased calcein-AM accumulation in MDCKII/MDR1 cells that were transfected with ABCB1 (P-glycoprotein), at levels comparable to verapamil tested at the same concentration. Structure-activity analysis showed that substitution of the benzylidene ring B of the aurone template was less important for ABCG2 inhibition, with little variation in activity noted for compounds with an unsubstituted ring B or one that was substituted. In contrast, substitution of ring B gave rise to better inhibitors of ABCB1. A preference for the 3' position of ring B was noted. There was also some indication from the data that aurones with good ABCG2 inhibitory activity were poor ABCB1 inhibitors and vice versa, but further confirmation would be required. Limited antiproliferative activity (>70% cell survival) was observed for many aurones on four different cell lines. Thus, functionalized 4,6-dimethoxyaurones are promising ABCG2 inhibitors that combine good activity at submicromolar concentrations with limited antiproliferative activity.