Free radical formation and DNA damage in the cytotoxicity of alkylaminoanthraquinone antitumour agents
The role of NAD(P)H dependent reductase mediated redox cycling in DNA strand breakage and cytotoxicity by doxorubicin (DOX), mitozantrone (MIT), CI941 and a series of alkylaminoanthraquinones (AQ) based on MIT has been investigated in MCF-7 human breast cancer cells. lAQ, 1,8AQ and DOX were found to redox cycle in MCF-7 cells as evidenced by stimulation of NADPH oxidation, superoxide anion formation and hydroxyl radical formation associated with generation of a drug free radical in MCF-7 S9 cell fraction. Furthermore, DOX formed a free radical, stimulated oxygen consumption and produced hydroxyl radicals in intact viable MCF-7 cells. 1,5AQ formed a free radical species and produced hydroxyl radicals but did not stimulate NADPH oxidation or superoxide anion formation in MCF-7 S9 fraction. Reactive oxygen formation correlated with the DNA strand breakage and cytotoxicity produced by IAQ, I,SAQ, 1,8AQ and DOX in MCF-7 cells. DOX also produced plasmid DNA strand breakage in the presence of purified cytochrome P4S0 reductase or xanthine oxidase. lAQ, 1,SAQ and 1,8AQ mediated plasmid DNA strand breakage only in the presence of purified cytochrome P4S0 reductase. MIT, CI941 and 1,4AQ did not redox cycle in MCF-7 cell S9 fraction yet produced equivalent cellular DNA strand breakage to DOX and AQ's at LDSO concentrations. The order of cytotoxicity of these compounds after a 1 hour exposure ~nd at least 6 d~ys further cell growth was;- (LDSO);- CI941(I.SxlO-lUM) >MIT(S.2xlO- YM) >lAQ(6.0xlO- 8M) >1,8AQ(O.5xlO- 6M) >1,4AQ(I.2xIO- 6M) >DOX(3.0xlO- 6M) >1,SAQ(12.3xIO- 6M). Variation in cellular uptake between the agents did not account for differences in cytotoxicity and DNA strand breakage observed. The 1,4 substitution pattern of MIT, CI941 and 1,4AQ appears to prevent metabolic activation. In further studies;- i) MIT and CI941 were more potent inhibitors of 3 MCF-7 topoisomerase I activity than other quinones, ii) MIT and DOX-Fe + complexes mediated plasmid DNA strand breakage in the presence of reduced glutathione iii) Only MIT was oxidatively activated by horseradish peroxidase!H202' producing plasmid DNA damage. The results indicate that future development of quinone anti tumour agents should concentrate on compounds which do not redox cycle but have functional groups that assist iron binding, activation by peroxidases and/or topoisomerase inhibition. In this respect, these agents should possess chromophore hydroxyl groups as these appear to be responsible for the increased cytotoxicity of MIT and CI941 compared to the other AQ's.