Antitumor bisimidazoacridones are bifunctional
DNA binders which have recently been shown to selectively target human colon
carcinoma cells in vitro and in vivo and appear to be excellent candidates for clinical development. We have studied the mechanism of action of one bisimidazoacridone,
WMC26, which is 1,000-10,000 times more toxic to human colon
carcinoma cells (HCT116) than to
melanoma cells (SKMEL2) in vitro. Plasmid
DNA exposed to
WMC26 showed enhanced digestion by
DNase I at A-T-rich sites, suggesting alterations in DNA conformation upon
drug binding. These results led us to investigate whether
WMC26 was selectively toxic due to a specific recognition of
DNA bends by repair excinucleases, as has been demonstrated with the
DNA bisintercalator,
ditercalinium. Both prokaryotic and eukaryotic cells with intact repair capacity were shown to be selectively sensitive to
WMC26, strongly indicating that excision repair plays a role in its toxicity. Confocal microscopy studies utilizing fluorescence of the
WMC26 chromophore showed compound localization in the perinuclear cytoplasmic area, as had been previously noted for
ditercalinium, indicating that cytoplasmic
DNA could be the target. This irreversible accumulation of compound was gradually followed by vacuolization of the cytoplasm and cell death. Cell cycle analysis of both lines treated with
WMC26 or with
ditercalinium showed that, while the latter induced HCT116 growth arrest at G1-G0,
WMC26 also blocked the cell cycle at G2-M; SKMEL2 cells did not undergo any changes in cell cycle as a result of either treatment. Our data show that
WMC26 is 10-100 times more cytotoxic than
ditercalinium in vitro. Like
ditercalinium,
WMC26 appears to exert its toxicity via cytoplasmic elements, through a mechanism involving excision repair processes. However, its highly selective cytotoxicity may stem from additional undefined targets in sensitive
colon cancer cells.