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The study of iron chelators as anti-tumor agents is still in its infancy. Iron is important for cellular proliferation and this is demonstrated by observations that iron-depletion results in cell cycle arrest and also apoptosis. In addition, many iron chelators are known to inhibit ribonucleotide reductase, the iron-containing enzyme that is the rate-limiting step for DNA synthesis. Desferrioxamine is a well known chelator used for the treatment of iron-overload disease, but it has also been shown to possess anti-cancer activity. Another class of chelators, namely the thiosemicarbazones, have been shown to possess anti-cancer activity since the 1950's, although their mechanism(s) of action have only recently been more comprehensively elucidated. In fact, the redox activity of thiosemicarbazone iron complexes is thought to be important in mediating their potent cytotoxicity. Moreover, unlike typical iron chelators which simply act to deplete tumors of iron, several thiosemicarbazones (i.e., Bp44mT and Dp44mT) do not induce this effect, their anti-cancer efficacy being due to other mechanisms e.g., redox activity. Other reports have also shown that some thiosemicarbazones inhibit topoisomerase IIα, demonstrating that this class of agents have multiple molecular targets and act by various mechanisms. The most well characterized thiosemicarbazone iron chelator in terms of its assessment in humans is 3-aminopyridine-2-carboxaldehyde thiosemicarbazone (3-AP). Observations from these clinical trials highlight the less than optimal activity of this ligand and several side effects related to its use, including myelo-suppression, hypoxia and methemoglobinemia. The mechanisms responsible for these latter effects must be elucidated and the design of the ligand altered to minimize these problems and increase efficacy. This review discusses the development of chelators as unique agents for cancer treatment.

Thiosemicarbazones are a group of compounds that have received comprehensive investigation as anticancer agents. The antitumor activity of the thiosemicarbazone, 3-amino-2-pyridinecarboxaldehyde thiosemicarbazone (3-AP; triapine), has been extensively assessed in more than 20 phase I and II clinical trials. These studies have demonstrated that 3-AP induces methemoglobin (metHb) formation and hypoxia in patients, limiting its usefulness. Considering this problem, we assessed the mechanism of metHb formation by 3-AP compared with that of more recently developed thiosemicarbazones, including di-2-pyridylketone-4,4-dimethyl-3-thiosemicarbazone (Dp44mT). This was investigated using intact red blood cells (RBCs), RBC lysates, purified oxyhemoglobin, and a mouse model. The chelation of cellular labile iron with the formation of a redox-active thiosemicarbazone-iron complex was found to be crucial for oxyhemoglobin oxidation. This observation was substantiated using a thiosemicarbazone that cannot ligate iron and also by using the chelator, desferrioxamine, that forms a redox-inactive iron complex. Of significance, cellular copper chelation was not important for metHb generation in contrast to its role in preventing tumor cell proliferation. Administration of Dp44mT to mice catalyzed metHb and cardiac metmyoglobin formation. However, ascorbic acid administered together with the drug in vivo significantly decreased metHb levels, providing a potential therapeutic intervention. Moreover, we demonstrated that the structure of the thiosemicarbazone is of importance in terms of metHb generation, because the DpT analog, di-2-pyridylketone-4-cyclohexyl-4-methyl-3-thiosemicarbazone (DpC), does not induce metHb generation in vivo. Hence, DpC represents a next-generation thiosemicarbazone that possesses markedly superior properties. This investigation is important for developing more effective thiosemicarbazone treatment regimens.

Background: Autophagy is a prosurvival mechanism contributing to resistance against anticancer agents. Results: We demonstrate that the selective antitumor thiosemicarbazone di-2-pyridylketone 4,4-dimethyl-3-thiosemicarbazone (Dp44mT) increases autophagosome synthesis but induces cell death by reducing autophagosome degradation. Conclusion: Dp44mT overcomes autophagy and utilizes the autophagic machinery to provoke cell death. Significance: These results indicate a new mechanism by which Dp44mT induces tumor cell death.