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.     

Methyl and ethyl ketone analogs of salicylaldehyde isonicotinoyl hydrazone: novel iron chelators with selective antiproliferative action

Salicylaldehyde isonicotinoyl hydrazone (SIH) is a lipophilic, orally-active tridentate iron chelator providing both effective protection against various types of oxidative stress-induced cellular injury and anticancer action. However, the major limitation of SIH is represented by its labile hydrazone bond that makes it prone to plasma hydrolysis. Recently, nine new SIH analogues derived from aromatic ketones with improved hydrolytic stability were developed. Here we analyzed their antiproliferative potential in MCF-7 breast adenocarcinoma and HL-60 promyelocytic leukemia cell lines. Seven of the tested substances showed greater selectivity than the parent agent SIH towards the latter cancer cell lines compared to non-cancerous H9c2 cardiomyoblast-derived cells. The tested chelators induced a dose-dependent dissipation of the inner mitochondrial membrane potential, an induction of apoptosis as evidenced by Annexin V positivity or significant increases of activities of caspases 3, 7, 8 and 9 and cell cycle arrest. With the exception of nitro group-bearing NHAPI, the studies of iron complexes of the chelators confirmed the crucial role of iron in the mechanism of their antiproliferative action. Finally, all the assayed chelators inhibited the oxidation of ascorbate by iron ions indicating lack of redox activity of the chelator-iron complexes. In conclusion, this study identified several important design criteria for improvement of the antiproliferative selectivity of the aroylhydrazone iron chelators. Several of the novel compounds--in particular the ethylketone-derived HPPI, NHAPI and acetyl-substituted A2,4DHAPI--merit deeper investigation as promising potent and selective anticancer agents.     

Desferrithiocin analogue iron chelators: iron clearing efficiency,tissue distribution,and renal toxicity

The current solution to iron-mediated damage in transfusional iron overload disorders is decorporation of excess unmanaged metal, chelation therapy. The clinical development of the tridentate chelator deferitrin (1, Table 1) was halted due to nephrotoxicity. It was then shown by replacing the 4′-(HO) of 1 with a 3,6,9-trioxadecyloxy group, the nephrotoxicity could be ameliorated. Further structure–activity relationship studies have established that the length and the position of the polyether backbone controlled: (1) the ligand’s iron clearing efficiency (ICE), (2) chelator tissue distribution, (3) biliary ferrokinetics, and (4) tissue iron reduction. The current investigation compares the ICE and tissue distribution of a series of (S)-4,5-dihydro-2-[2-hydroxy-4-(polyether)phenyl]-4-methyl-4-thiazolecarboxylic acids (Table 1, 3–5) and the (S)-4,5-dihydro-2-[2-hydroxy-3-(polyether)phenyl]-4-methyl-4-thiazolecarboxylic acids (Table 1, 8–10). The three most effective polyether analogues, in terms of performance ratio (PR), defined as mean ICE_primate/ICE_rodent, are 3 (PR 1.1), 8, (PR 1.5), and 9, now in human trials, (PR 2.2). At the onset of the clinical trial on 9, no data were available for ligand 3 or 8. This is unfortunate, as 3 has many advantages over 9, e.g., the ICE of 3 in rats is 2.5-fold greater than that of 9 and analogue 3 achieves very high levels in the liver, pancreas, and heart, the organs most affected by iron overload. Finally, the impact of 3 on the urinary excretion of kidney injury molecule-1 (Kim-1), an early diagnostic biomarker for monitoring acute kidney toxicity, has been carried out in rats; no evidence of nephrotoxicity was found. Overall, the results suggest that 3 would be a far superior clinical candidate to 9.     

Ischemia-induced brain iron delocalization: Effect of iron chelators

Tissue damage in cerebral ischemia may be produced by acidosis-induced delocalization of intracellular iron which acts as a catalyst in oxidative reactions. Acidosis was induced either by homogenization and incubation of rat cortical homogenates in acidified buffers or by submitting hyperglycemic rats to complete ischemia, a procedure that leads to intracellular lactic acidosis. The level of low molecular weight species (LMWS) iron was measured after filtration of tissue homogenates through a 10,000 Mr ultrafiltration membrane. When cortical tissue was homogenized in buffer pH 7, the level of LMWS iron was equal to 0.21 μg/g. It was significantly enhanced by acidification of the homogenization medium, reaching 0.34 μg/g at pH 6 and 0.75 μg/g at pH 5. When the tissue was homogenized in water, the LMWS iron level reached 0.17 μg/g in normoglycemic rats and 0.38 μg/g (p < 0.5) in hyperglycemic rats. Both aerobic incubation of homogenates for 1 h at 37°C and inclusion of EDTA in the homogenization medium led to further increases in the iron level. In order to demonstrate the deleterious role of iron in brain ischemia, the effect of treatment with bipyridyl, an iron-chelating agent, was assessed by measuring regional brain edema by the specific gravity method, 24 h following induction of thrombotic brain infarction. The treatment significantly attenuated the development of brain edema, reducing the water content of the infarcted area by about 2.5%. Taken together, these results support the hypothesis that a significant component of brain ischemic injury involves an iron-dependent mechanism.     

Selenium-containing naphthalimides as anticancer agents: design, synthesis and bioactivity

Selenium analogues (4b-4h, and 4j) of two known sulfur compounds were synthesized and tested their anticancer activities. The selenium compound 4b had comparable activity with its sulfur analogue 4a, while DNA-binding study showed these two compounds had similar interaction with ct-DNA, the K(b) was 8.23 and 2.36, respectively. The primary results showed that most compounds had moderate anticancer activities with IC(50) values between 10(-6) and 10(-5) M. Another selenium analogue 4j showed the highest activity with the IC(50) values around 5.3 μM against K562 and MCF-7 cell lines. More importantly, the organochalcogen compounds exhibited stronger anticancer activities against K562 cell line than the other cell lines tested.     

Beta-thalassaemia: emergence of new and improved iron chelators for treatment

Beta-thalassaemia is an inherited blood disorder which through repeated blood transfusions and enhanced iron uptake from the gastrointestinal tract, results in marked iron overload. Untreated, the iron accumulation results in the dysfunction of vital organs such as the heart and liver. At present, the most effective treatment for beta-thalassaemia is the use of the iron chelator, desferrioxamine, which is expensive, orally inactive and requires long subcutaneous infusions. In this concise review, we will focus on novel chelators which show therapeutic potential to replace desferrioxamine. Furthermore, we will discuss the potential of combined iron chelation therapy and the principle that, in the future, the use of more than just one chelator may be beneficial in tailoring individual iron chelation regimens.     

A robust and convergent synthesis of dipeptide-DOTAM conjugates as chelators for lanthanide ions: new PARACEST MRI agents

A generally applicable synthetic approach to dipeptide-DOTAM conjugates has been developed which is based on the peralkylation of 1,4,7,10-tetraazacyclododecane (cyclen) with protected N-iodoacetyl dipeptides. Standardized procedures were used for the alkylation, metalation, and purification of the resultant lanthanide complexes. Using this approach, we have been able to rapidly and reliably prepare and screen five different ligands each with up to six lanthanide ions. This preliminary investigation has identified several paramagnetic compounds with strong chemical exchange saturation transfer (PARACEST) properties in water at physiological temperature and pH. Extension of the synthetic approach to a wide variety of amino acids is possible.     

Induction of porphyrin synthesis in etiolated bean leaves by chelators of iron

Primary leaves of 7- to 9-day-old etiolated seedlings of Phaseolus vulgaris L. var. Red Kidney infiltrated in darkness with aqueous solutions of alpha, alpha'-dipyridyl, o-phenanthroline, pyridine-2-aldoxime, pyridine-2-aldehyde, 8-hydroxyquinoline, or picolinic acid synthesize large amounts of magnesium protoporphyrin monomethyl ester and lesser amounts of magnesium protoporphyrin, protoporphyrin, and protochlorophyllide. Pigment formation proceeds in a linear manner for up to 21 hours after vacuum infiltration with 10 mm alpha, alpha'-dipyridyl. Etiolated tissues of Zea mays L., Cucumis sativus L., and Pisum sativum L. respond in the same way to dipyridyl treatment. Compounds active in eliciting this response are aromatic heterocyclic nitrogenous bases which also act as bidentate chelators and form extremely stable complexes with iron; other metal ion chelators, such as ethylenediaminetetraacetic acid, salicylaldoxime, and sodium diethyldithiocarbamate, do not elicit any pigment synthesis. The ferrous, ferric, cobaltous, and zinc chelates of alpha, alpha'-dipyridyl are similarly ineffective. If levulinic acid is supplied to etiolated bean leaves together with alpha, alpha'-dipyridyl, porphyrin production is inhibited and delta-aminolevulinic acid accumulates in the tissue. Synthesis of porphyrins proceeds in the presence of 450 micrograms per milliliter chloramphenicol or 50 micrograms per milliliter cycloheximide with only partial diminution. We propose that heme or an iron-protein complex blocks the action of the enzyme(s) governing the synthesis of delta-aminolevulinic acid in etiolated leaves in the dark and that iron chelators antagonize this inhibition, leading to the biosynthesis of delta-aminolevulinic acid and porphyrins.     

Enabling large-scale design, synthesis and validation of small molecule protein-protein antagonists

Although there is no shortage of potential drug targets, there are only a handful known low-molecular-weight inhibitors of protein-protein interactions (PPIs). One problem is that current efforts are dominated by low-yield high-throughput screening, whose rigid framework is not suitable for the diverse chemotypes present in PPIs. Here, we developed a novel pharmacophore-based interactive screening technology that builds on the role anchor residues, or deeply buried hot spots, have in PPIs, and redesigns these entry points with anchor-biased virtual multicomponent reactions, delivering tens of millions of readily synthesizable novel compounds. Application of this approach to the MDM2/p53 cancer target led to high hit rates, resulting in a large and diverse set of confirmed inhibitors, and co-crystal structures validate the designed compounds. Our unique open-access technology promises to expand chemical space and the exploration of the human interactome by leveraging in-house small-scale assays and user-friendly chemistry to rationally design ligands for PPIs with known structure.     

Design and synthesis of naphthoquinone derivatives as antiproliferative agents and 20S proteasome inhibitors

Fourteen naphthoquinone derivatives (1-14) were designed based on a putative proteasome inhibitor PI-083. These compounds were synthesized and evaluated against A549, DU145, KB, and KBvin tumor cell lines. Six compounds (2, 4, 8, 9, 10, and 13) showed antiproliferative activities comparable to that of PI-083. Among them, compound 8 was confirmed as a 20S proteasome inhibitor in both in vitro and cell-based assays. These findings endorse further optimization efforts based on this structural phenotype to develop potential anticancer drug candidates.     

Multi-receptor drug design: Haloperidol as a scaffold for the design and synthesis of atypical antipsychotic agents

Using haloperidol as a scaffold, new agents were designed to investigate the structural contributions of various groups to binding at CNS receptors associated with atypical antipsychotic pharmacology. It is clear that each pharmacophoric group, the butyrophenone, the piperidine and the 4-chlorophenyl moieties contributes to changes in binding to the receptors of interest. This strategy has resulted in the identification of several new agents, compounds 16, 18, 19, 23, 24 and 25, with binding profiles which satisfy our stated criteria for agents to act as potential atypical antipsychotics. This research demonstrates that haloperidol can serve as a useful lead in the identification and design of new agents that target multiple receptors associated with antipsychotic pharmacology.     

1,2,3-Triazole-nimesulide hybrid: Their design,synthesis and evaluation as potential anticancer agents

A new hybrid template has been designed by integrating the structural features of nimesulide and the 1,2,3-triazole moiety in a single molecular entity at the same time eliminating the problematic nitro group of nimesulide. The template has been used for the generation of a library of molecules as potential anticancer agents. A mild and greener CuAAC approach has been used to synthesize these compounds via the reaction of 4-azido derivative of nimesulide and terminal alkynes in water. Three of these compounds showed promising growth inhibition (IC₅₀ ～6–10 μM) of A549, HepG2, HeLa and DU145 cancer cell lines but no significant effects on HEK293 cell line. They also inhibited PDE4B in vitro (60–70% at 10 μM) that was supported by the docking studies (PLP score 87–94) in silico.     

Development of potential iron chelators for the treatment of Friedreich's ataxia: ligands that mobilize mitochondrial iron

Friedreich's ataxia (FA) is a crippling neurodegenerative disease that is due to iron (Fe) overload within the mitochondrion. One therapeutic intervention may be the development of a chelator that could remove mitochondrial Fe. We have implemented the only well characterized model of mammalian mitochondrial Fe overload to examine the Fe chelation efficacy of novel chelators of the 2-pyridylcarboxaldehyde isonicotinoyl hydrazone (PCIH) class. In this model we utilize reticulocytes treated with the haem synthesis inhibitor succinylacetone which results in mitochondrial Fe-loading. Our experiments demonstrate that in contrast to desferrioxamine, several of the PCIH analogues show very high activity at mobilizing (59)Fe from (59)Fe-loaded reticulocytes. Further studies on these ligands in animals are clearly warranted considering their potential to treat FA.     

2-Hydroxypyridine-N-oxides: effective new chelators in iron mobilisation

The 2-hydroxypyridine-N-oxide derivatives, 2-hydroxypyridine-N-oxide, 2,4-dihydroxypyridine-N-oxide, 2-hydroxy-4-methoxypyridine-N-oxide and 2-hydroxy-4-(2'-methoxyethoxy)pyridine-N-oxide have been shown to remove iron from human transferrin and horse spleen ferritin at pH 7.4 at levels higher than those caused by desferrioxamine. Their reactions with transferrin were mainly biphasic and took 2-5 h to reach completion but iron mobilisation from ferritin was slower and their reactions continued after 40 h of incubation. The intraperitoneal and intragastric administration of 2,4-dihydroxypyridine-N-oxide to two iron-loaded 59Fe-labelled mice caused an increase in 59Fe excretion which is comparable to that caused by desferrioxamine intraperitoneally. These results increase the prospects for the use of these chelators as probes for studying iron metabolism and in the treatment of iron overload and other diseases of iron imbalance.     

Tuning the antiproliferative activity of biologically active iron chelators: characterization of the coordination chemistry and biological efficacy of 2-acetylpyridine and 2-benzoylpyridine hydrazone ligands

2-Pyridinecarbaldehyde isonicotinoyl hydrazone (HPCIH) and di-2-pyridylketone isonicotinoyl hydrazone (HPKIH) are two Fe chelators with contrasting biological behavior. HPCIH is a well-tolerated Fe chelator with limited antiproliferative activity that has potential applications in the treatment of Fe-overload disease. In contrast, the structurally related HPKIH ligand possesses significant antiproliferative activity against cancer cells. The current work has focused on understanding the mechanisms of the Fe mobilization and antiproliferative activity of these hydrazone chelators by synthesizing new analogs (based on 2-acetylpyridine and 2-benzoylpyridine) that resemble both series and examining their Fe coordination and redox chemistry. The Fe mobilization activity of these compounds is strongly dependent on the hydrophobicity and solution isomeric form of the hydrazone (E or Z). Also, the antiproliferative activity of the hydrazone ligands was shown to be influenced by the redox properties of the Fe complexes. This indicated that toxic Fenton-derived free radicals are important for the antiproliferative activity for some hydrazone chelators. In fact, we show that any substitution of the H atom present at the imine C atom of the parent HPCIH analogs leads to an increase in antiproliferative efficacy owing to an increase in redox activity. These substituents may deactivate the imine R–C=N–Fe (R is Me, Ph, pyridyl) bond relative to when a H atom is present at this position preventing nucleophilic attack of hydroxide anion, leading to a reversible redox couple. This investigation describes novel structure–activity relationships of aroylhydrazone chelators that will be useful in designing new ligands or fine-tuning the activity of others. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Uptake of ferritin and iron bound to ferritin by rat hepatocytes: modulation by apotransferrin, iron chelators and chloroquine

Rat liver ferritin is an effective donor of iron to rat hepatocytes. Uptake of iron from ferritin by the cells is partially inhibited by including apotransferrin in the culture medium, but not by inclusion of diferric transferrin. This inhibition is dependent on the concentration of apotransferrin, with a 30% depression in iron incorporation in the cells detected at apotransferrin concentrations above 40 micrograms/ml. However, apotransferrin does not interfere with uptake of 125I-labeled ferritin, suggesting that apotransferrin decreases retention of iron taken up from ferritin by hepatocytes by sequestering a portion of released iron before it has entered the metabolic pathway of the cells. The iron chelators desferrioxamine (100 microM), citrate (10 mM) and diethylenetriaminepentaacetate (100 microM) reduce iron uptake by the cells by 35, 25 and 8%, respectively. In contrast, 1 mM ascorbate increases iron accumulation by 20%. At a subtoxic concentration of 100 microM, chloroquine depresses ferritin and iron uptake by hepatocytes by more than 50% after 3 h incubation. Chloroquine presumably acts by retarding lysosomal degradation of ferritin and recycling of ferritin receptors.     

Isobrassinin and its analogues: novel types of antiproliferative agents

Isobrassinin (2-(S-methyldithiocarbamoylaminomethyl)indole (7a), a regioisomer of the cruciferous phytoalexin brassinin (1), exerted marked antiproliferative effects on the HeLa, A431 and MCF7 cell lines (>78.6% inhibition at 30 μM). For structure–activity relationships, further analogues were synthesized. The highest cytotoxic effect was displayed by 2-phenylimino-1,3-thiazino[5,6-b]indole (10) (10 μM, 76.8%—HeLa and 46.3%—MCF7). The effect of the natural phytoalexin brassinin was also determined.     

Polymeric gadolinium chelate magnetic resonance imaging contrast agents: design, synthesis, and properties.

We have synthesized and evaluated five series of polymeric gadolinium chelates which are of interest as potential MRI blood pool contrast agents. The polymers were designed so that important physical properties including molecular weight, relaxivity, metal content, viscosity, and chelate stability could be varied. We have shown that, by selecting polymers of the appropriate MW, extended blood pool retention can be achieved. In addition, relaxivity can be manipulated by changing the polymer rigidity, metal content affected by monomer selection, viscosity by polymer shape, and chelate stability by chelator selection.