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.

ABCG2 plays a major role in anticancer-drug efflux and related tumor multidrug resistance. Potent and selective ABCG2 inhibitors with low cytotoxicity were investigated among a series of 44 chalcones and analogues (1,3-diarylpropenones), by evaluating their inhibitory effect on the transport of mitoxantrone, a known ABCG2 substrate. Six compounds producing complete inhibition with IC50 values below 0.5 µM and high selectivity for ABCG2 were identified. The number and position of methoxy substituents appeared to be critical for both inhibition and cytotoxicity. The best compounds, with potent inhibition and low toxicity, contained an N-methyl-1-indolyl (compound 38) or a 6′-hydroxyl-2′,4′-dimethoxy-1-phenyl (compound 27) moiety (A-ring) and two methoxy groups at positions 2 and 6 of the 3-phenyl moiety (B-ring). Methoxy substitution contributed to inhibition at positions 3 and 5, but had a negative effect at position 4. Finally, methoxy groups at positions 3, 4, and 5 of the B-ring markedly increased cytotoxicity and, therefore, should be avoided.

Apigenin has been reported to inhibit proliferation of cancer cells; however, the mechanism underlying its action is not completely understood. Here, we evaluated the effects of apigenin on the levels of expression and activity of antioxidant enzymes, and the involvement of ROS in the mechanism of cell death induced by apigenin in HepG2 human hepatoma cells. Upon treatment with apigenin, HepG2 cells displayed a reduction in cell viability in a dose- and time-dependent manner, and some morphological changes. In addition, apigenin treatment induced ROS generation and significantly decreased the mRNA levels and activity of catalase and levels of intracellular GSH. On the other hand, apigenin treatment did not alter the expression or activity levels of other antioxidant enzymes. Addition of exogenous catalase significantly reduced the effects of apigenin on HepG2 cell death. We also demonstrated that HepG2 cells are more sensitive to apigenin-mediated cell death than are primary cultures of mouse hepatocytes, suggesting a differential toxic effect of this agent in tumor cells. Our results suggest that apigenin-induced apoptosis in HepG2 cells may be mediated by a H(2)O(2)-dependent pathway via reduction of the antioxidant defenses.

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The ABCG2 multidrug transporter is known to confer cancer cell multidrug resistance by causing the efflux of anticancer drugs; therefore, selective inhibitors have the potential to improve chemotherapeutic treatments. Here, various methoxy derivatives of resveratrol are shown to be potent inhibitors of mitoxantrone efflux by ABCG2: among a series of 11 derivatives, compound 9 (3,5,3',4'-tetramethoxy trans-stilbene) had an IC(50) of 0.16 μM and showed a maximal inhibition of 75%, as measured by flow cytometry. It was not transported, as shown by HPLC fractionation and mass spectrometry titration and the lack of any cross-resistance in cell survival experiments. Compound 9 had a very low intrinsic cytotoxicity and was able to chemosensitize the growth of resistant ABCG2-transfected HEK293 cells at submicromolar concentrations. Drug-efflux inhibition was specific for ABCG2 since very low effects were observed with ABCB1 and ABCC1. The action mechanism of compound 9 was different from that of GF120918, which produced a complete and partly competitive but not ABCG2-specific inhibition, since ABCB1 was even more strongly inhibited. The two inhibitors also displayed different effects on the ABCG2 vanadate-sensitive ATPase activity, suggesting that they either bound to distinct sites or induced different conformational changes. Mitoxantrone efflux was fully inhibited by combining low concentrations of compound 9 with either GF120918 or a transport substrate such as prazosin or nilotinib. We conclude that methoxy derivatives of stilbene are good candidates for investigating future in vivo modulation of ABCG2 drug-efflux activity.

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Melanins have been associated with the development of melanoma and its resistance to photodynamic therapy (PDT). Singlet molecular oxygen ((1)O(2)), which is produced by ultraviolet A solar radiation and the PDT system, is also involved. Here, we investigated the effects that these factors have on DNA damage and repair. Our results show that both types of melanin (eumelanin and pheomelanin) lead to DNA breakage in the absence of light irradiation and that eumelanin is more harmful than pheomelanin. Interestingly, melanins were found to bind to the minor grooves of DNA, guaranteeing close proximity to DNA and potentially causing the observed high levels of strand breaks. We also show that the interaction of melanins with DNA can impair the access of repair enzymes to lesions, contributing to the perpetuation of DNA damage. Moreover, we found that after melanins interact with (1)O(2), they exhibit a lower ability to induce DNA breakage; we propose that these effects are due to modifications of their structure. Together, our data highlight the different modes of action of the two types of melanin. Our results may have profound implications for cellular redox homeostasis, under conditions of induced melanin synthesis and irradiation with solar light. These results may also be applied to the development of protocols to sensitize melanoma cells to PDT.

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