David B. Lovejoy

Cancer cell iron metabolism and the development of potent iron chelators as anti-tumour agents.

Cancer contributes to 50% of deaths worldwide and new anti-tumour therapeutics with novel mechanisms of actions are essential to develop. Metabolic inhibitors represent an important class of anti-tumour agents and for many years, agents targeting the nutrient folate were developed for the treatment of cancer. This is because of the critical need of this factor for DNA synthesis. Similarly to folate, Fe is an essential cellular nutrient that is critical for DNA synthesis. However, in contrast to folate, there has been limited effort applied to specifically design and develop Fe chelators for the treatment of cancer. Recently, investigations have led to the generation of novel di-2-pyridylketone thiosemicarbazone (DpT) and 2-benzoylpyridine thiosemicarbazone (BpT) group of ligands that demonstrate marked and selective anti-tumour activity in vitro and also in vivo against a wide spectrum of tumours. Indeed, administration of these compounds to mice did not induce whole body Fe-depletion or disturbances in haematological or biochemical indices due to the very low doses required. The mechanism of action of these ligands includes alterations in expression of molecules involved in cell cycle control and metastasis suppression, as well as the generation of redox-active Fe complexes. This review examines the alterations in Fe metabolism in tumour cells and the systematic development of novel aroylhydrazone and thiosemicarbazone Fe chelators for cancer treatment.

Antitumor Activity of Metal-Chelating Compound Dp44mT Is Mediated by Formation of a Redox-Active Copper Complex That Accumulates in Lysosomes

The metal-chelating compound Dp44mT is a di-2-pyridylketone thiosemicarbazone (DpT) which displays potent and selective antitumor activity. This compound is receiving translational attention, but its mechanism is poorly understood. Here, we report that Dp44mT targets lysosome integrity through copper binding. Studies using the lysosomotropic fluorochrome acridine orange established that the copper-Dp44mT complex (Cu[Dp44mT]) disrupted lysosomes. This targeting was confirmed with pepstatin A-BODIPY FL, which showed redistribution of cathepsin D to the cytosol with ensuing cleavage of the proapoptotic BH3 protein Bid. Redox activity of Cu[Dp44mT] caused cellular depletion of glutathione, and lysosomal damage was prevented by cotreatment with the glutathione precursor N-acetylcysteine. Copper binding was essential for the potent antitumor activity of Dp44mT, as coincubation with nontoxic copper chelators markedly attenuated its cytotoxicity. Taken together, our studies show how the lysosomal apoptotic pathway can be selectively activated in cancer cells by sequestration of redox-active copper. Our findings define a novel generalized strategy to selectively target lysosome function for chemotherapeutic intervention against cancer.

Novel aroylhydrazone and thiosemicarbazone iron chelators with anti-malarial activity against chloroquine-resistant and -sensitive parasites

Iron (Fe) is crucial for cellular proliferation, and Fe chelators have shown activity at preventing the growth of the malarial parasite in cell culture and in animal and human studies. We investigated the anti-malarial activity of novel aroylhydrazone and thiosemicarbazone Fe chelators that show high activity at inhibiting the growth of tumour cells in cell culture [Blood 100 (2002) 666]. Experiments with the chelators were performed using the chloroquine-sensitive, 3D7, and chloroquine-resistant, 7G8, strains of Plasmodium falciparum in vitro. The new ligands were significantly more active in both strains than the Fe chelator in widespread clinical use, desferrioxamine (DFO). The most effective chelators examined were 2-hydroxy-1-naphthylaldehyde isonicotinoyl hydrazone and 2-hydroxy-1-naphthylaldehyde-4-phenyl-3-thiosemicarbazone. The anti-malarial activity correlates with anti-proliferative activity against neoplastic cells demonstrated in a previous study. Our studies suggest that this class of lipophilic chelators may have potential as useful agents for the treatment of malaria.

Chelators at the Cancer Coalface: Desferrioxamine to Triapine and Beyond

The importance of iron and copper in cancer biology has been well established. Iron plays a fundamental role in cellular proliferation and copper has been shown to be a significant cofactor for angiogenesis. Early observations with the chelator used for the treatment of iron overload, desferrioxamine, showed that it had promise as an anticancer agent. These results sparked great interest in the possibility of developing more effective iron chelators for cancer therapy. The recent entry into clinical trials of the iron-binding drug, Triapine, provides evidence of the potential of this antitumor strategy. Likewise, chelators originally designed to treat disorders of copper overload, such as penicillamine, trientine, and tetrathiomolybdate, have also emerged as potential anticancer drugs, as they are able to target the key angiogenic cofactor, copper. In this review, we will discuss the development of these and other chelators that show potential as anticancer agents.

2-Acetylpyridine thiosemicarbazones are potent iron chelators and antiproliferative agents: redox activity, iron complexation and characterization of their antitumor activity.

Through systematic structure-activity studies of the 2-benzoylpyridine thiosemicarbazone (HBpT), 2-(3-nitrobenzoyl)pyridine thiosemicarbazone (HNBpT) and dipyridylketone thiosemicarbazone (HDpT) series of iron (Fe) chelators, we identified structural features necessary to form Fe complexes with potent anticancer activity (J. Med. Chem. 2007, 50, 3716-3729). In this investigation, we generated the related 2-acetylpyridine thiosemicarbazone (HApT) analogues to examine the influence of the methyl group at the imine carbon. Four of the six HApT chelators had potent antitumor activity (IC(50): 0.001-0.002 microM) and Fe chelation efficacy that was similar to the most effective HBpT and HDpT ligands. The HApT Fe complexes had the lowest Fe(III/II) redox potentials of any thiosemicarbazone series we have generated. This property, in combination with their ability to effectively chelate cellular Fe, make the HApT series one of the most potent antiproliferative agents developed by our group.

Iron chelators of the dipyridylketone thiosemicarbazone class: Precomplexation and transmetalation effects on anticancer activity

We previously reported a series of di-2-pyridylketone thiosemicarbazone (HDpT) chelators that showed marked and selective antitumor activity (Whitnall, M.; et al. Proc. Natl. Acad. Sci. U.S.A. 2006, 103, 14901-14906). To further understand their biological efficacy, we report the characterization and activity of their Mn(II), Co(III), Ni(II), Cu(II), and Zn(II) complexes. The X-ray crystal structures of four divalent (Mn, Ni, Cu, and Zn) and one trivalent (Fe) complexes are reported. Electrochemistry shows the Fe(III/II) and Cu(II/I) potentials of the complexes may be redox-active within cells. Stability constants were also determined for the Mn(II), Ni(II), Cu(II), and Zn(II) complexes. All divalent complexes underwent transmetalation upon encountering Fe(II), to form low spin ferrous complexes. Importantly, the divalent Mn(II), Ni(II), Cu(II), and Zn(II) complexes of the HDpT analogues are equally active in preventing proliferation as their ligands, suggesting the complexes act as lipophilic vehicles facilitating intracellular delivery of the free ligand upon metal dissociation.

Novel Second-Generation Di-2-Pyridylketone Thiosemicarbazones Show Synergism with Standard Chemotherapeutics and Demonstrate Potent Activity against Lung Cancer Xenografts after Oral and Intravenous Administration in Vivo

We developed a series of second-generation di-2-pyridyl ketone thiosemicarbazone (DpT) and 2-benzoylpyridine thiosemicarbazone (BpT) ligands to improve the efficacy and safety profile of these potential antitumor agents. Two novel DpT analogues, Dp4e4mT and DpC, exhibited pronounced and selective activity against human lung cancer xenografts in vivo via the intravenous and oral routes. Importantly, these analogues did not induce the cardiotoxicity observed at high nonoptimal doses of the first-generation DpT analogue, Dp44mT. The Cu(II) complexes of these ligands exhibited potent antiproliferative activity having redox potentials in a range accessible to biological reductants. The activity of the copper complexes of Dp4e4mT and DpC against lung cancer cells was synergistic in combination with gemcitabine or cisplatin. It was demonstrated by EPR spectroscopy that dimeric copper compounds of the type [CuLCl](2), identified crystallographically, dissociate in solution to give monomeric 1:1 Cu:ligand complexes. These monomers represent the biologically active form of the complex.

Novel Thiosemicarbazone Iron Chelators Induce Up-Regulation and Phosphorylation of the Metastasis Suppressor N-myc Down-Stream Regulated Gene 1: A New Strategy for the Treatment of Pancreatic Cancer

Pancreatic cancer is an aggressive neoplasm, with a mortality rate close to 100%. The most successful agent for pancreatic cancer treatment is gemcitabine, although the overall effect in terms of patient survival remains very poor. This study was initiated to evaluate a novel class of anticancer agents against pancreatic cancer. This group of compounds belongs to the dipyridyl thiosemicarbazone class that have been shown to have potent and selective activity against a range of different neoplasms in vitro and in vivo. We demonstrate for the first time in pancreatic cancer that these agents increase the expression of the growth and metastasis suppressor N-myc downstream-regulated gene 1 and its phosphorylation at Ser330 and Thr346 that is important for its activity against this tumor. In addition, these agents increased expression of the cyclin-dependent kinase inhibitor p21CIP1/WAF1, whereas decreasing cyclin D1 in pancreatic cancer cells. Together, these molecular alterations account, in part, for the pronounced antitumor activity observed. Indeed, these agents had significantly higher antiproliferative activity in vitro than the established treatments for pancreatic cancer, namely gemcitabine and 5-fluorouracil. Studies in vivo demonstrated that a novel thiosemicarbazone, namely di-2-pyridylketone 4-cyclohexyl-4-methyl-3-thiosemicarbazone hydrochloride, completely inhibited the growth of pancreatic cancer xenografts with no evidence of marked alterations in normal tissue histology. Together, our studies have identified molecular effectors of a novel and potent antitumor agent that could be useful for pancreatic cancer treatment.

Identification of the di-pyridyl ketone isonicotinoyl hydrazone (PKIH) analogues as potent iron chelators and anti-tumour agents.

In an attempt to develop chelators as potent anti‐tumour agents, we synthesized two series of novel ligands based on the very active 2‐pyridylcarboxaldehyde isonicotinoyl hydrazone (PCIH) group. Since lipophilicity and membrane permeability play a critical role in Fe chelation efficacy, the aldehyde moiety of the PCIH series, namely 2‐pyridylcarboxaldehyde, was replaced with the more lipophilic 2‐quinolinecarboxaldehyde or di‐2‐pyridylketone moieties. These compounds were then systematically condensed with the same group of acid hydrazides to yield ligands based on 2‐quinolinecarboxaldehyde isonicotinoyl hydrazone (QCIH) and di‐2‐pyridylketone isonicotinoyl hydrazone (PKIH). To examine chelator efficacy, we assessed their effects on proliferation, Fe uptake, Fe efflux, the expression of cell cycle control molecules, iron‐regulatory protein‐RNA‐binding activity, and 3H‐thymidine, 3H‐uridine and 3H‐leucine incorporation. Despite the high lipophilicity of the QCIH ligands and the fact that they have the same Fe‐binding site as the PCIH series, surprisingly none of these compounds were effective. In contrast, the PKIH analogues showed marked anti‐proliferative activity and Fe chelation efficacy. Indeed, the ability of these ligands to inhibit proliferation and DNA synthesis was similar or exceeded that found for the highly cytotoxic chelator, 311. In contrast to the PCIH and QCIH analogues, most of the PKIH group markedly increased the mRNA levels of molecules vital for cell cycle arrest. In conclusion, our studies identify structural features useful in the design of chelators with high anti‐proliferative activity. We have identified a novel class of ligands that are potent Fe chelators and inhibitors of DNA synthesis, and which deserve further investigation.

The Medicinal Chemistry of Novel Iron Chelators for the Treatment of Cancer

Cancer is one of the leading causes of death worldwide and there is an increasing need for novel anti-tumor therapeutics with greater selectivity and potency. A new strategy in the treatment of cancer has focused on targeting an essential cell metabolite, iron (Fe). Iron is vital for cell growth and metabolism, forming a crucial component of the active site of ribonucleotide reductase (RR), the rate-limiting enzyme in DNA synthesis. Cancer cells in particular require large amounts of Fe to proliferate, making them more susceptible to the Fe deficiency caused by Fe chelators. Beginning with primordial siderophores, Fe chelators have since evolved to a new generation of potent and efficient anti-cancer agents. Recently, investigations have led to the generation of novel di-2-pyridylketone thiosemicarbazone (DpT) and 2-benzoylpyridine thiosemicarbazone (BpT) ligands that demonstrate marked and selective anti-tumor activity both in vitro and in vivo against a wide spectrum of tumors. The mechanism of action of these novel ligands includes alterations in the expression of key regulatory molecules as well as the generation of redox active Fe complexes. Interestingly, non-synthetic Fe chelators including silybin and curcumin, both of which are derived from plants, also have vast potential in the treatment of cancer. This review explores the development of novel Fe chelators for the treatment of cancer and their mechanisms of action.