Interactions of the pyridine-2-carboxaldehyde isonicotinoyl hydrazone class of chelators with iron and DNA: implications for toxicity in the treatment of iron overload disease

One-pot multicomponent synthesis of novel pyridine derivatives for antidiabetic and antiproliferative activities

Background: Due to the close relationship of diabetes with hypertension reported in various research, a set of pyridine derivatives with US FDA-approved drug cores were designed and integrated by artificial intelligence. Methods: Novel pyridines were designed and synthesized. Compounds MNS-1-MNS-4 were evaluated for their structure and were screened for their in vitro antidiabetic (α-amylase) activity and anticancer (HepG2) activity by methyl thiazolyl tetrazolium assay. Comparative 3D quantitative structure-activity relationship analysis and pharmacophore generation were carried out. Results: The study revealed MNS-1 and MNS-4 as good alternatives to acarbose as antidiabetic agents, and MNS-2 as a more viable, better alternative to doxorubicin in the methyl thiazolyl tetrazolium assay. Conclusion: This combination of studies identifies new and more active analogs of existing FDA-approved drugs for the treatment of diabetes.

Pyridine: the scaffolds with significant clinical diversity

The nitrogen-bearing heterocycle pyridine in its several analogous forms occupies an important position as a precious source of clinically useful agents in the field of medicinal chemistry research. This privileged scaffold has been consistently incorporated in a diverse range of drug candidates approved by the FDA (Food and Drug Administration). This moiety has attracted increasing attention from several disease states owing to its ease of parallelization and testing potential pertaining to the chemical space. In the next few years, a larger share of novel pyridine-based drug candidates is expected. This review unifies the current advances in novel pyridine-based molecular frameworks and their unique clinical relevance as reported over the last two decades. It highlights an inclination to the use of pyridine-based molecules in drug crafting and the subsequent emergence of several potent and eligible candidates against a range of diversified diseases.

The nitrogen-bearing heterocycle pyridine in its several analogous forms occupies an important position as a precious source of clinically useful agents in the field of medicinal chemistry research.

Green Synthesis, SC-XRD, Non-Covalent Interactive Potential and Electronic Communication via DFT Exploration of Pyridine-Based Hydrazone

Ultrasound-based synthesis at room temperature produces valuable compounds greener and safer than most other methods. This study presents the sonochemical fabrication and characterization of a pyridine-based halogenated hydrazone, (E)-2-((6-chloropyridin-2-yl)oxy)-N′-(2-hydroxybenzylidene) acetohydrazide (HBPAH). The NMR spectroscopic technique was used to determine the structure, while SC-XRD confirmed its crystalline nature. Our structural studies revealed that strong, inter-molecular attractive forces stabilize this crystalline organic compound. Moreover, the compound was optimized at the B3LYP/6-311G(d,p) level using the Crystallographic Information File (CIF). Natural bonding orbital (NBO) and natural population analysis (NPA) were performed at the same level using optimized geometry. Time-dependent density functional theory (DFT) was performed at the B3LYP/6-311G (d,p) method to calculate the frontier molecular orbitals (FMOs) and molecular electrostatic potential (MEP). The global reactivity descriptors were determined using HOMO and LUMO energy gaps. Theoretical calculations based on the Quantum Theory of Atoms in Molecules (QT-AIM) and Hirshfeld analyses identified the non-covalent and covalent interactions of the HBPAH compound. Consequently, QT-AIM and Hirshfeld analyses agree with experimental results.

Thiosemicarbazones as Potent Anticancer Agents and their Modes of Action

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Thiosemicarbazones (TSCs) are a class of Schiff bases usually obtained by the condensation of thiosemicarbazide with a suitable aldehyde or ketone. TSCs have been the focus of chemists and biologists due to their wide range of pharmacological effects. One of the promising areas in which these excellent metal chelators are being developed is their use against cancer. TSCs have a wide clinical antitumor spectrum with efficacy in various tumor types such as leukemia, pancreatic cancer, breast cancer, non-small cell lung cancer, cervical cancer, prostate cancer and bladder cancer. To obtain better activity, different series of TSCs have been developed by modifying the heteroaromatic system in their molecules. These compounds possessed significant antineoplastic activity when the carbonyl attachment of the side chain was located at a position α to the ring nitrogen atom, whereas attachment of the side chain β or γ to the heterocyclic N atom resulted in inactive antitumor agents. In addition, replacement of the heterocyclic ring N with C also resulted in a biologically inactive compound suggesting that a conjugated N,N,S-tridentate donor set is essential for the biological activities of thiosemicarbazones. Several possible mechanisms have been implemented for the anticancer activity of thiosemicarbazones.

How iron is handled in the course of heme catabolism: Integration of heme oxygenase with intracellular iron transport mechanisms mediated by poly (rC)-binding protein-2

Heme and iron are essential to almost all forms of life. The strict maintenance of heme and iron homeostasis is essential to prevent cellular toxicity and the existence of systemic and intracellular regulation is fundamental. Cytosolic heme can be catabolized and detoxified by heme oxygenases (HOs). Interestingly, free heme detoxification through HOs results in the production of free ferrous iron, which can be potentially hazardous for cells. Recently, the intracellular iron chaperone, poly (rC)-binding protein 2 (PCBP2), has been identified, which can be involved in accepting iron after heme catabolism as well as intracellular iron transport. In fact, HO1, NADPH-cytochrome P450 reductase, and PCBP2 form a functional unit that integrates the catabolism of heme with the binding and transport of iron by PCBP2. In this review, we provide an overview of our understanding of the iron chaperones and discuss the mechanism how iron chaperones bind iron released during the process of heme degradation.

l-Proline catalyzed one-pot synthesis of polysubstituted pyridine system incorporating benzothiazole moiety via sustainable sonochemical approach

The efficient, highly convenient synthesis of polysubstituted pyridine derivatives was established via the reaction of N-(benzothiazol-2-yl)-2-cyanoacetamides with an assortment of arylidene malononitriles and arylidene ethyl cyanoacetates in the presence of l-proline as an efficient organocatalyst for this type of ultrasonic-mediated Michael addition. The mechanistic pathway and factors affecting this reaction were also established. The main characteristics of this procedure are high yields, use of a cost-effective catalyst, and easy work-up and purification.

α-N-heterocyclic thiosemicarbazone Fe(III) complex: Characterization of its antitumor activity and identification of anticancer mechanism

We synthesized an α-N-heterocyclic thiosemicarbazone ligand (L) and its Fe complex (C1) and assessed their chemical and biological properties in order to understand their marked activity. Electrochemical studies and ascorbate oxidation studies demonstrated that C1 shows considerable redox activity, and Fe(III/II) redox potentials was within the range accessible to cellular oxidants and reductants. Absorption spectral, emission spectral and viscosity analysis reveal that L and C1 interacted with DNA through intercalation and C1 exhibited a higher DNA binding ability. Agarose gel electrophoresis experiments indicated that C1 exhibited the highest pBR322 DNA cleaving ability. In vitro, C1 showed significantly more anticancer activity than the ligand alone. Moreover, C1 induces production of reactive oxygen species (ROS) and DNA damage, resulting in activation of the p53 pathway, cell cycle arrest at the S phase, and mitochondria-mediated apoptosis by regulating the expression of Bcl-2 family proteins.

Antitumor activity of a 2-pyridinecarboxaldehyde 2-pyridinecarboxylic acid hydrazone copper complex and the related mechanism

In the present study, 2-pyridinecarboxaldehyde 2-pyridinecarboxylic acid hydrazone (PPAH) was prepared and its antitumor activity was evaluated. The inhibition of proliferation of PPAH against the HepG2 and HCT-116 cell lines was less effective, yet in the presence of copper ions, the mixture demonstrated excellent antitumor activity (IC50 at 2.75±0.30 µM for the HepG2 cell line, and 1.90±0.20 µM for the HCT-116 cell line, respectively) and the new active species was confirmed to be a PPAH copper complex with a 1:1 ratio by spectral analysis. The excellent antitumor activity of the copper complex prompted us to investigate the underlying mechanism. RT-PCR was performed to detect the changes in the expression of apoptotic genes induced by PPAH and its copper complex. However, no changes were observed when the cells were treated by the agents for 24 or 48 h, indicating that ROS were unlikely involved. Cell cycle analysis showed that both PPAH and its copper complex led to S phase arrest of the cells. The sDNA relaxation assay revealed that the PPAH-copper complex displayed dual topoisomerase inhibition for type I and II. The data suggest that the inhibition of proliferation exhibited by the PPAH copper complex may stem from its dual topoisomerase inhibition, which is rarely observed for a metal complex.

A family of mixed-ligand oxidovanadium(v) complexes with aroylhydrazone ligands: a combined experimental and computational study on the electronic effects of para substituents of hydrazone ligands on the electronic properties, DNA binding and nuclease activities

Substituents at 5-position in the acetophenone ring of the hydrazone ligands in a family of mixed-ligand oxidovanadium(v) complexes show marked influence on the electronic properties, DNA binding ability and nuclease activity.

Oxidovanadium(v) complexes of aroylhydrazones incorporating heterocycles: synthesis, characterization and study of DNA binding, photo-induced DNA cleavage and cytotoxic activities

The interaction of four neutral oxidovanadium(v) complexes with DNA and their cytotoxic activities have been reported.

Evaluation of the cell cytotoxicity and DNA/BSA binding and cleavage activity of some dioxidovanadium(V) complexes containing aroylhydrazones

Three dioxidovanadium(V) complexes [VO2L(1-3)] (1-3) [HL(1)=1-napthoyl hydrazone of 2-acetyl pyridine, HL(2)=2-furoyl hydrazone of 2-acetyl pyridine and H2L(3)=isonicotinoyl hydrazone of 2-hydroxy benzaldehyde] have been reported. All the complexes were characterized by various spectroscopy (IR, UV-visible and NMR) and the molecular structures of 1 and 2 were characterized by single crystal X-ray diffraction technique. Structural report established five-coordinate geometries, distorted toward square pyramidal for each of 1 and 2, based on a tridentate -O,N,N coordinating anion and two oxido-O atoms. The experimental results show that the complexes interact with calf-thymus DNA (CT-DNA) possibly by a groove binding mode, with binding constants of ~10(5)M(-1). All complexes show good photo-induced cleavage of pUC19 supercoiled plasmid DNA with complex 1 showing the highest photo-induced DNA cleavage activity of ~68%. 1-3 also exhibit moderate binding affinity in the range of 10(3)-10(4)M(-1) towards bovine serum albumin (BSA), while all the complexes show good photo-induced BSA cleavage activity. Moreover the antiproliferative activity of all these complexes was studied, which reveal all compounds are significantly cytotoxic towards the HeLa cell line.

Structure-activity relationships of novel salicylaldehyde isonicotinoyl hydrazone (SIH) analogs: iron chelation, anti-oxidant and cytotoxic properties

Salicylaldehyde isonicotinoyl hydrazone (SIH) is a lipophilic, tridentate iron chelator with marked anti-oxidant and modest cytotoxic activity against neoplastic cells. However, it has poor stability in an aqueous environment due to the rapid hydrolysis of its hydrazone bond. In this study, we synthesized a series of new SIH analogs (based on previously described aromatic ketones with improved hydrolytic stability). Their structure-activity relationships were assessed with respect to their stability in plasma, iron chelation efficacy, redox effects and cytotoxic activity against MCF-7 breast adenocarcinoma cells. Furthermore, studies assessed the cytotoxicity of these chelators and their ability to afford protection against hydrogen peroxide-induced oxidative injury in H9c2 cardiomyoblasts. The ligands with a reduced hydrazone bond, or the presence of bulky alkyl substituents near the hydrazone bond, showed severely limited biological activity. The introduction of a bromine substituent increased ligand-induced cytotoxicity to both cancer cells and H9c2 cardiomyoblasts. A similar effect was observed when the phenolic ring was exchanged with pyridine (i.e., changing the ligating site from O, N, O to N, N, O), which led to pro-oxidative effects. In contrast, compounds with long, flexible alkyl chains adjacent to the hydrazone bond exhibited specific cytotoxic effects against MCF-7 breast adenocarcinoma cells and low toxicity against H9c2 cardiomyoblasts. Hence, this study highlights important structure-activity relationships and provides insight into the further development of aroylhydrazone iron chelators with more potent and selective anti-neoplastic effects.

Iron Chelating Agents for Iron Overload Diseases

Although iron is an essential element for life, an excessive amount may become extremely toxic both for its ability to generate reactive oxygen species, and for the lack in humans of regulatory mechanisms for iron excretion. Chelation therapy has been introduced in clinical practice in the seventies of last century to defend thalassemic patients from the effects of iron overload and, in spite of all its limitations, it has dramatically changed both life expectancy and quality of life of patients. It has to be considered that the drugs in clinical use present some disadvantages too, this makes urgent new more suitable chelating agents. The requirements of an iron chelator have been better and better defined over the years and in this paper they will be discussed in detail. As a final point the most interesting ligands studied in the last years will be presented.

Iron chelators with topoisomerase-inhibitory activity and their anticancer applications

SIGNIFICANCE

Iron and topoisomerases are abundant and essential cellular components. Iron is required for several key processes such as DNA synthesis, mitochondrial electron transport, synthesis of heme, and as a co-factor for many redox enzymes. Topoisomerases serve as critical enzymes that resolve topological problems during DNA synthesis, transcription, and repair. Neoplastic cells have higher uptake and utilization of iron, as well as elevated levels of topoisomerase family members. Separately, the chelation of iron and the cytotoxic inhibition of topoisomerase have yielded potent anticancer agents.

RECENT ADVANCES

The chemotherapeutic drugs doxorubicin and dexrazoxane both chelate iron and target topoisomerase 2 alpha (top2α). Newer chelators such as di-2-pyridylketone-4,4,-dimethyl-3-thiosemicarbazone and thiosemicarbazone -24 have recently been identified as top2α inhibitors. The growing list of agents that appear to chelate iron and inhibit topoisomerases prompts the question of whether and how these two distinct mechanisms might interplay for a cytotoxic chemotherapeutic outcome.

CRITICAL ISSUES

While iron chelation and topoisomerase inhibition each represent mechanistically advantageous anticancer therapeutic strategies, dual targeting agents present an attractive multi-modal opportunity for enhanced anticancer tumor killing and overcoming drug resistance. The commonalities and caveats of dual inhibition are presented in this review.

FUTURE DIRECTIONS

Gaps in knowledge, relevant biomarkers, and strategies for future in vivo studies with dual inhibitors are discussed.

Halogenated 2'-benzoylpyridine thiosemicarbazone (XBpT) chelators with potent and selective anti-neoplastic activity: relationship to intracellular redox activity

Iron chelators of the 2'-benzoylpyridine thiosemicarbazone (BpT) class show substantial potential as anticancer agents. To explore structure-activity relationships, new BpT analogues were designed that incorporated halogen substituents on the noncoordinating phenyl group (XBpTs). These XBpT ligands exhibited potent antiproliferative activity with some analogues exceeding that of the parent BpT compound. Importantly, there was an appreciable therapeutic index in vitro, as mortal cells were significantly less affected by these chelators relative to neoplastic cells. The addition of a halogen led to a halogen-specific increase in the redox potential of XBpT-Fe complexes. Probing for chelator-induced intracellular reactive oxygen species (ROS) with the fluorescent probe, 2',7'-dichlorofluorescein, revealed a 1.5-4.7-fold increase in fluorescence upon incorporation of Cl, Br, or I to the parent analogues. Furthermore, an important structure-activity relationship was deduced where the addition of halogens led to a positive correlation between intracellular ROS generation and antiproliferative activity in the more hydrophilic BpT parent compounds.

Chlorella vulgaris aldehyde reductase is capable of functioning as ferric reductase and of driving the fenton reaction in the presence of free flavin

The free flavin-dependent Fenton reaction was detected in cell-free extracts of Chlorella. The corresponding enzyme was purified to homogeneity, and its N-terminal sequence was highly homologous to those of aldo-keto reductase family enzymes. The purified enzyme displayed aldehyde reductase activity in the presence of NADPH. Additionally, it showed ferric reductase activity and drove the Fenton reaction in the presence of free FAD and NADH.

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.

Antitumor activity and mechanism of action of the iron chelator, Dp44mT, against leukemic cells

Iron chelators have been reported to induce apoptosis and cell cycle arrest in cancer cells. Recent studies suggest broad and selective antitumor activity of the new iron chelator, di-2-pyridylketone-4,4-dimethyl-3-thiosemicarbazone (Dp44mT; Whitnall et al., Proc Natl Acad Sci USA 2006;103:14901-14906). However, little is known concerning its effects on hematological malignancies. Using acute leukemia cells, the effect of Dp44mT on apoptosis, cell cycle, caspase-3 activation, and mitochondrial trans-membrane potential has been examined by flow cytometry. Dp44mT acted to induce a G(1)/S arrest in NB4 promyelocytic leukemia cells at low concentrations (0.5-2.5 microM), being far more effective than the clinically used chelator, desferrioxamine (DFO). Moreover, Dp44mT induced apoptosis of NB4 cells in a dose- and time-dependent manner with markedly less effect on nonproliferating cells. The apoptosis-inducing activity of Dp44mT was significantly more effective than DFO. Furthermore, this study also showed that Dp44mT had broad activity, inducing apoptosis in several types of acute leukemia and also multiple myeloma cell lines. Additional studies examining the cytotoxic mechanisms of Dp44mT showed that a reduction in the mitochondrial trans-membrane potential and caspase-3 activation could be involved in the mechanism of apoptosis. Our results suggest that Dp44mT possesses potential as an effective cytotoxic agent for the chemotherapeutic treatment of acute leukemia.

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.

Protection against hydrogen peroxide-mediated cytotoxicity in Friedreich's ataxia fibroblasts using novel iron chelators of the 2-pyridylcarboxaldehyde isonicotinoyl hydrazone class

Iron-loading diseases remain an important problem because of the toxicity of iron-catalyzed redox reactions. Iron loading occurs in the mitochondria of Friedreich's ataxia (FA) patients and may play a role in its pathogenesis. This suggests that iron chelation therapy could be useful. We developed previously the lipophilic iron chelators known as the 2-pyridylcarboxaldehyde isonicotinoyl hydrazone (PCIH) ligands and identified 2-pyridylcarboxaldehyde 2-thiophenecarboxyl hydrazone (PCTH) as the most promising analog. Hence, this study assessed the efficacy of PCTH and other PCIH analogs compared with various chelators, including deferiprone and desferrioxamine (DFO). Age- and sex-matched control and FA fibroblasts were preincubated with iron chelators and subsequently challenged with 50 microM H2O2 for up to 24 h. The current study demonstrates an interesting structure-activity relationship among the closely related PCIH series of ligands, with only PCTH being highly effective at preventing H2O2-induced cytotoxicity. PCTH increased FA fibroblast cell viability by up to 70%, whereas DFO rescued viability by 1 to 5% only. Hence, PCTH, which was well tolerated by cells was far more effective than DFO at preventing oxidative stress. It is noteworthy that kinetic studies demonstrated PCTH to rapidly penetrate cells to induce 59Fe efflux, whereas DFO, PCIH, 2-pyridylcarboxaldehyde benzoyl hydrazone, and 2-pyridylcarboxaldehyde m-bromobenzoyl hydrazone were far slower, indicating it is the rate of chelator permeation that is crucial for protection against H2O2. In addition, PCTH was found to be as effective as or more effective than conventional radical scavengers or the antioxidant idebenone (which has undergone clinical trials) at protecting cells against H2O2-mediated cytotoxicity. These findings further indicate the potential of PCTH for treatment of iron overload.

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.

Design, synthesis, and characterization of novel iron chelators: structure-activity relationships of the 2-benzoylpyridine thiosemicarbazone series and their 3-nitrobenzoyl analogues as potent antitumor agents

Previously, we demonstrated that the potent antiproliferative activity of the di-2-pyridylketone thiosemicarbazone (DpT) series of Fe chelators was due to their ability to induce Fe depletion and form redox-active Fe complexes (Richardson, D. R.; et al. J. Med. Chem. 2006, 49, 6510-6521). We now examine the role of aromatic substituents on the antiproliferative and redox activity of novel DpT analogues, namely, the 2-benzoylpyridine thiosemicarbazone (BpT) and 2-(3-nitrobenzoyl)pyridine thiosemicarbazone (NBpT) series. Both series exhibited selective antiproliferative effects, with the majority having greater antineoplastic activity than their DpT homologues. This makes the BpT chelators the most active anticancer agents developed within our laboratory. The BpT series Fe complexes exhibit lower redox potentials than their corresponding DpT and NBpT complexes, highlighting their enhanced redox activity. The increased ability of BpT-Fe complexes to catalyze ascorbate oxidation and benzoate hydroxylation, relative to their DpT and NBpT analogues, suggested that redox cycling plays an important role in their antiproliferative activity.

Future of toxicology--iron chelators and differing modes of action and toxicity: the changing face of iron chelation therapy

Iron (Fe) chelation therapy was initially designed to alleviate the toxic effects of excess Fe evident in Fe-overload diseases. However, the novel toxicological properties of some Fe chelator-metal complexes have shifted appreciable focus to their application in cancer chemotherapy. Redox-inactive Fe chelator complexes are well suited for the treatment of Fe-overload diseases, whereas Fe chelator complexes with high redox activity have shown promising results as chemotherapeutics against cancer. Within this perspective, we discuss the different modes of action and toxicological profiles of Fe chelators, including analogues of 2-pyridylcarboxaldehyde isonicotinoyl hydrazone, di-2-pyridylketone isonicotinoyl hydrazone, di-2-pyridylketone thiosemicarbazone, and the clinically trialed chelator 3-aminopyridine-2-carboxaldehyde thiosemicarbazone. The potential application of these agents in the changing face of Fe chelation therapy is discussed.

Hydrazone chelators for the treatment of iron overload disorders: iron coordination chemistry and biological activity

The potentially tridentate ligand 2-pyridinecarbaldehyde isonicotinoyl hydrazone (HPCIH) and its analogues are an emerging class of orally effective Fe chelators that show great promise for the treatment of Fe overload diseases. Herein, we present an extensive study of the Fe coordination chemistry of the HPCIH analogues including the first crystallographically characterised Fe(II) complex of these chelators. Unlike most other clinically effective Fe chelators, the HPCIH analogues bind Fe(II) and not Fe(III). In fact, these chelators form low-spin bis-ligand Fe(II) complexes, although NMR suggests that the complexes are close to the high-spin/low-spin crossover. All the Fe complexes show a high potential Fe(III/II) redox couple (> 500 mV vs. NHE) and cyclic voltammetry in aqueous or mixed aqueous/organic solvents is irreversible as a consequence of a rapid hydration reaction that occurs upon oxidation. A number of the HPCIH analogues show high activity at inducing Fe efflux from cells and also at preventing Fe uptake by cells from the serum Fe transport protein transferrin. As a class of ligands, these chelators are more effective at reducing Fe uptake from transferrin than inducing Fe mobilisation from cells. This may be related to their ability to intercept Fe(II) after its release from transferrin within the cell. Our studies indicate that their Fe chelation efficacy is due, at least in part, to the fact that these ligands and their Fe(II) complexes are neutral at physiological pH (7.4) and sufficiently lipophilic to permeate cell membranes.

Desferrioxamine (DFX) has genotoxic effects on cultured human lymphocytes and induces the p53-mediated damage response

Desferrioxamine (DFX), which is an iron chelator, mimics hypoxia by enhancing HIF1-alpha accumulation and upregulating inflammatory mediators. DFX is usually beneficial, with preventive effects related primarily to its ability to scavenge reactive oxygen species. However, toxic effects on skeletal and ocular organs have been reported. The cytokinesis block micronucleus test and alkaline single-cell gel (Comet) assay were used to evaluate the genotoxic effects of DFX on human blood lymphocytes. Cultured human lymphocytes treated with 130microM DFX for various periods of time showed significant differences in the incidence of micronucleated binucleate cells, as well as in the length and moment of the comet tail. Western blot analysis using antibodies to proteins involved in the p53-mediated response to DNA damage revealed that p53 was accumulated and DNA damage checkpoint kinases were activated in lymphocytes treated with DFX. On the other hand, the p53 downstream target proteins p21 and bax were not affected, which indicates that DFX does not promote the transactivational activity of p53. Apoptosis assays demonstrated DFX-induced apoptosis of lymphocytes via the caspase cascade. The observed increase in the sub-G1 fraction and enhanced caspase-3 activity indicate that DFX can promote apoptosis in human lymphocytes, and these results were confirmed by protein immunoblot analysis. As apoptotic cell death is preceded by the collapse of the mitochondrial membrane potential, we also measured the mitochondrial membrane potential (Deltapsi(m)) using DiOC6, which is a fluorescent membrane potential probe. The fluorescence intensity of DiOC6 in lymphocytes was significantly reduced in a time-dependent manner after DFX treatment. Taken together, these results indicate that DFX activates p53-mediated checkpoint signals and induces apoptosis via mitochondrial damage in human peripheral blood lymphocytes.

Dipyridyl thiosemicarbazone chelators with potent and selective antitumor activity form iron complexes with redox activity

There has been much interest in the development of iron (Fe) chelators for the treatment of cancer. We developed a series of di-2-pyridyl ketone thiosemicarbazone (HDpT) ligands which show marked and selective antitumor activity in vitro and in vivo. In this study, we assessed chemical and biological properties of these ligands and their Fe complexes in order to understand their marked activity. This included examination of their solution chemistry, electrochemistry, ability to mediate redox reactions, and antiproliferative activity against tumor cells. The higher antiproliferative efficacy of the HDpT series of chelators relative to the related di-2-pyridyl ketone isonicotinoyl hydrazone (HPKIH) analogues can be ascribed, in part, to the redox potentials of their Fe complexes which lead to the generation of reactive oxygen species. The most effective HDpT ligands as antiproliferative agents possess considerable lipophilicity and were shown to be charge neutral at physiological pH, allowing access to intracellular Fe pools.

The Evolution of Iron Chelators for the Treatment of Iron Overload Disease and Cancer

The evolution of iron chelators from a range of primordial siderophores and aromatic heterocyclic ligands has lead to the formation of a new generation of potent and efficient iron chelators. For example, various siderophore analogs and synthetic ligands, including ICL670A [4-[3,5-bis-(hydroxyphenyl)-1,2,4-triazol-1-yl]-benzoic acid], 4'-hydroxydesazadesferrithiocin, and Triapine, have been developed from predecessors and illustrate potent iron-mobilizing or antineoplastic activities. This review focuses on the evolution of iron chelators from initial lead compounds through to the development of novel chelating agents, many of which show great potential to be clinically applied in the treatment of iron overload disease and cancer.

Novel diaroylhydrazine ligands as iron chelators: coordination chemistry and biological activity

The search for orally effective drugs for the treatment of iron overload disorders is an important goal in improving the health of patients suffering diseases such as beta-thalassemia major. Herein, we report the syntheses and characterization of some new members of a series of N-aroyl-N'-picolinoyl hydrazine chelators (the H2IPH analogs). Both 1:1 and 1:2 Fe(III):L complexes were isolated and the crystal structures of Fe(HPPH)Cl2, Fe(4BBPH)Cl2, Fe(HAPH)(APH) and Fe(H3BBPH)(3BBPH) were determined (H2PPH=N,N'-bis-picolinoyl hydrazine; H2APH=N-4-aminobenzoyl-N'-picolinoyl hydrazine, H23BBPH=N-3-bromobenzoyl-N'-picolinoylhydrazine and H24BBPH=N-(4-bromobenzoyl)-N'-(picolinoyl)hydrazine). In each case, a tridentate N,N,O coordination mode of each chelator with Fe was observed. The Fe(III) complexes of these ligands have been synthesized and their structural, spectroscopic and electrochemical characterization are reported. Five of these new chelators, namely H2BPH (N-(benzoyl)-N'-(picolinoyl)hydrazine), H2TPH (N-(2-thienyl)-N'-(picolinoyl)-hydrazine), H2PPH, H23BBPH and H24BBPH, showed high efficacy at mobilizing 59Fe from cells and inhibiting 59Fe uptake from the serum Fe transport protein, transferrin (Tf). Indeed, their activity was much greater than that found for the chelator in current clinical use, desferrioxamine (DFO), and similar to that observed for the orally active chelator, pyridoxal isonicotinoyl hydrazone (H2PIH). The ability of the chelators to inhibit 59Fe uptake could not be accounted for by direct chelation of 59Fe from 59Fe-Tf. The most effective chelators also showed low antiproliferative activity which was similar to or less than that observed with DFO, which is important in terms of their potential use as agents to treat Fe-overload disease.

Tachpyridine, a metal chelator, induces G2 cell-cycle arrest, activates checkpoint kinases, and sensitizes cells to ionizing radiation

Iron is critical for cell growth and proliferation. Iron chelators are being explored for a number of clinical applications, including the treatment of neurodegenerative disorders, heart disease, and cancer. To uncover mechanisms of action of tachpyridine, a chelator currently undergoing preclinical evaluation as an anticancer agent, cell-cycle analysis was performed. Tachpyridine arrested cells at G2, a radiosensitive phase of the cell cycle, and enhanced the sensitivity of cancer cells but not nontransformed cells to ionizing radiation. G2 arrest was p53 independent and was accompanied by activation of the checkpoint kinases CHK1 and CHK2. G2 arrest was blocked by UCN-01, a CHK1 inhibitor, but proceeded in CHK2 knock-out cells, indicating a critical role for CHK1 in G2 arrest. Tachpyridine-induced cell-cycle arrest was abrogated in cells treated with caffeine, an inhibitor of the ataxia-telangiectasia mutated/ataxia-telangiectasia-mutated and Rad3-related (ATM/ATR) kinases. Further, G2 arrest proceeded in ATM-deficient cells but was blocked in ATR-deficient cells, implicating ATR as the proximal kinase in tachpyridine-mediated G2 arrest. Collectively, our results suggest that iron chelators may function as antitumor and radioenhancing agents and uncover a previously unexplored activity of iron chelators in activation of ATR and checkpoint kinases.

Potent antitumor activity of novel iron chelators derived from di-2-pyridylketone isonicotinoyl hydrazone involves fenton-derived free radical generation

PURPOSE

The development of novel and potent iron chelators as clinically useful antitumor agents is an area of active interest. Antiproliferative activity of chelators often relates to iron deprivation or stimulation of iron-dependent free radical damage. Recently, we showed that novel iron chelators of the di-2-pyridylketone isonicotinoyl hydrazone (PKIH) class have potent and selective antineoplastic activity (E. Becker, et al., Br. J. Pharmacol., 138: 819-30, 2003). In this study, we assessed the effects of the PKIH analogues on the redox activity of iron in terms of understanding their antitumor activity.

EXPERIMENTAL DESIGN

We tested the PKIH analogues for their ability to promote iron-mediated ascorbate oxidation, benzoate hydroxylation, and plasmid degradation. Subsequent experiments assessed their ability to bind DNA, inhibit topoisomerase I, and cause DNA damage. To measure intracellular reactive oxygen species, we used the redox-sensitive probe, 2',7'-dichloro-fluorescein-diacetate, to measure intracellular PKIH-dependent redox activity.

RESULTS

The PKIH analogues had relatively little effect on ascorbate oxidation in the presence of Fe(III) but stimulated benzoate hydroxylation and plasmid DNA degradation in the presence of Fe(II) and H2O2. These ligands could not inhibit DNA topoisomerase I or cause DNA damage in intact cells. PKIH markedly increased the intracellular generation of reactive oxygen species, and this was inhibited by catalase. This enzyme also decreased the antiproliferative effect of PKIH, indicating H2O2 played a role in its cytotoxic activity.

CONCLUSIONS

Our results suggest that the antiproliferative effects of these chelators relates to intracellular iron chelation, followed by the stimulation of iron-mediated free radical generation via the so-formed iron complex.