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Examination of the Antiproliferative Activity of Iron Chelators

Multiple Cellular Targets and the Different Mechanism of Action of Triapine Compared with Desferrioxamine and the Potent Pyridoxal Isonicotinoyl Hydrazone Analogue 311¹

The Iron Metabolism and Chelation Group, The Heart Research Institute, Camperdown, Sydney, New South Wales, 2050 Australia

Purpose: Tumors are sensitive to iron (Fe) chelation therapy with the clinically used chelator desferrioxamine (DFO). Recently, the potent inhibitor of ribonucleotide reductase, Triapine, has entered clinical trials as an anticancer agent. This compound is a potential Fe chelator, but despite this, no investigations have examined its effect on cellular Fe metabolism. This is essential for understanding its mechanism of action and clinical effects.

Experimental Design: We compared the effect of Triapine with DFO, and also with the novel Fe chelator, 311, which shows marked antiproliferative activity. This latter ligand was relevant to compare, because it is tridentate like Triapine and shares structural similarity. We assessed the effects of chelators on proliferation, Fe uptake, Fe efflux, the expression of cell cycle control molecules, and iron-regulatory protein-RNA-binding activity. Redox activity was determined by ascorbate oxidation, benzoate hydroxylation, plasmid DNA degradation, and the precipitation of cellular DNA. These studies have been performed using several neuroepithelioma and neuroblastoma cell lines and a variety of normal cell types including fibroblasts, umbilical vein endothelial cells, skeletal muscle cells, monocyte-derived macrophages, and bone marrow stem cells.

Results: Triapine was twice as effective as DFO at mobilizing ⁵⁹Fe from prelabeled cells but was much less efficient than 311. In terms of preventing ⁵⁹Fe uptake from Tf, Triapine and DFO had similar activity, having far less efficacy than 311. All three of the chelators showed greater activity against the proliferation of neoplastic than of normal cells, the effect of 311 and Triapine being similar and these two chelators being significantly (P < 0.0001) more active than DFO. Complexation of Triapine with Fe had no appreciable effect on its antiproliferative activity, whereas addition of Fe totally inhibited the effects of DFO and 311. Furthermore, the Triapine Fe complex was shown to be redox active.

Conclusion: The cytotoxic mechanism of action of Triapine was different from that of DFO and 311, with the combined action of Fe chelation and free radical generation being involved.

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