Adjuvant therapies that enhance the anti-tumor effects of cis-diammineplatinum(II) dichloride (cisplatin, CDDP) are actively being pursued. Growing evidence supports the involvement of mitochondrial dysfunction in the anti-cancer effect of cisplatin. We examined the potential of using selective flavonoids that are effective in depleting tumor cells of glutathione (GSH) to potentiate cisplatin-mediated cytotoxicity in human lung adenocarcinoma (A549) cells. We found that cisplatin (40 µM, 48-h treatment) disrupts the steady-state levels of mitochondrial respiratory complex I, which correlates with elevated mitochondrial reactive oxygen species (ROS) production and cytochrome c release. The flavonoids, 2',5'-dihydroxychalcone (2',5'-DHC, 20 µM) and chrysin (20 µM) potentiated the cytotoxicity of cisplatin (20 µM), which could be blocked by supplementation of the media with exogenous GSH (500 µM). Both 2',5'-DHC and chrysin were more effective than the specific inhibitor of GSH synthesis, L-buthionine sulfoximine (BSO, 20 µM), in inducing GSH depletion and potentiating the cytotoxic effect of cisplatin. These data suggest that the flavonoid-induced potentiation of cisplatin's toxicity is due, in part, to synergetic pro-oxidant effects of cisplatin by inducing mitochondrial dysfunction, and the flavonoids by depleting cellular GSH, an important antioxidant defense.

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