MECHANISM OF ACTION OF BISIMIDAZOACRIDONES, NEW DRUGS WITH POTENT, SELECTIVE ACTIVITY AGAINST COLON-CANCER

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 clinica l development. We have studied the mechanism of action of one bisimida zoacridone, WMC26, which is 1,000-10,000 times more toxic to human col on 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 bi nding. These results led us to investigate whether WMC26 was selective ly toxic due to a specific recognition of DNA bends by repair excinucl eases, as has been demonstrated with the DNA bisintercalator, ditercal inium. Both prokaryotic and eukaryotic cells with intact repair capaci ty were shown to be selectively sensitive to WMC26, strongly indicatin g that excision repair plays a role in its toxicity. Confocal microsco py studies utilizing fluorescence of the WMC26 chromophore showed comp ound localization in the perinuclear cytoplasmic area, as had been pre viously noted for ditercalinium, indicating that cytoplasmic DNA could be the target, This irreversible accumulation of compound was gradual ly followed by vacuolization of the cytoplasm and cell death. Cell cyc le analysis of both lines treated with WMC26 or with ditercalinium sho wed that, while the latter induced HCT116 growth arrest at G(1)-G(0), WMC26 also blocked the cell cycle at G(2)-M; SKMEL2 cells did not unde rgo any changes in cell cycle as a result of either treatment. Our dat a show that WMC26 is 10-100 times more cytotoxic than ditercalinium il l vitro. Like ditercalinium, WMC26 appears to exert its toxicity via c ytoplasmic elements, through a mechanism involving excision repair pro cesses. However, its highly selective cytotoxicity may stem from addit ional undefined targets in sensitive colon cancer cells.