Therapeutic iron chelators and their potential side-effects

Iron and copper in progressive demyelination--New lessons from Skogholt's disease

The pathophysiological mechanisms of progressive demyelinating disorders including multiple sclerosis are incompletely understood. Increasing evidence indicates a role for trace metals in the progression of several neurodegenerative disorders. The study of Skogholt disease, a recently discovered demyelinating disease affecting both the central and peripheral nervous system, might shed some light on the mechanisms underlying demyelination. Cerebrospinal fluid iron and copper concentrations are about four times higher in Skogholt patients than in controls. The transit into cerebrospinal fluid of these elements from blood probably occurs in protein bound form. We hypothesize that exchangeable fractions of iron and copper are further transferred from cerebrospinal fluid into myelin, thereby contributing to the pathogenesis of demyelination. Free or weakly bound iron and copper ions may exert their toxic action on myelin by catalyzing production of oxygen radicals. Similarities to demyelinating processes in multiple sclerosis and other myelinopathies are discussed.

Mitochondrial metals as a potential therapeutic target in neurodegeneration

Transition metals are critical for enzyme function and protein folding, but in excess can mediate neurotoxic oxidative processes. As mitochondria are particularly vulnerable to oxidative damage due to radicals generated during ATP production, mitochondrial biometal homeostasis must therefore be tightly controlled to safely harness the redox potential of metal enzyme cofactors. Dysregulation of metal functions is evident in numerous neurological disorders including Alzheimer's disease, stroke, Parkinson's disease, Huntington's disease, amyotrophic lateral sclerosis and Friedrich's ataxia. This review describes the mitochondrial metal defects in these disorders and highlights novel metal-based therapeutic approaches that target mitochondrial metal homeostasis in neurological disorders.

Targeting metallo-β-lactamase enzymes in antibiotic resistance

The β-lactam antibiotics are essential for the treatment of a wide range of human bacterial diseases. However, a class of zinc-dependent hydrolases known as the metallo-β-lactamase (MBL) can confer bacteria with extended spectrum β-lactam resistance. To date, there are no clinically approved MBL inhibitors, making these enzymes a serious threat to human health. In this review, a structural approach is taken to outline some of the more promising MBL inhibitors and shed light on how the resistance conferred by this emerging class of enzymes may be circumvented in the future.

Iron redistribution as a therapeutic strategy for treating diseases of localized iron accumulation

Defective iron utilization leading to either systemic or regional misdistribution of the metal has been identified as a critical feature of several different disorders. Iron concentrations can rise to toxic levels in mitochondria of excitable cells, often leaving the cytosol iron-depleted, in some forms of neurodegeneration with brain accumulation (NBIA) or following mutations in genes associated with mitochondrial functions, such as ABCB7 in X-linked sideroblastic anemia with ataxia (XLSA/A) or the genes encoding frataxin in Friedreich's ataxia (FRDA). In anemia of chronic disease (ACD), iron is withheld by macrophages, while iron levels in extracellular fluids (e.g., plasma) are drastically reduced. One possible therapeutic approach to these diseases is iron chelation, which is known to effectively reduce multiorgan iron deposition in iron-overloaded patients. However, iron chelation is probably inappropriate for disorders associated with misdistribution of iron within selected tissues or cells. One chelator in clinical use for treating iron overload, deferiprone (DFP), has been identified as a reversed siderophore, that is, an agent with iron-relocating abilities in settings of regional iron accumulation. DFP was applied to a cell model of FRDA, a paradigm of a disorder etiologically associated with cellular iron misdistribution. The treatment reduced the mitochondrial levels of labile iron pools (LIP) that were increased by frataxin deficiency. DFP also conferred upon cells protection against oxidative damage and concomitantly mediated the restoration of various metabolic parameters, including aconitase activity. Administration of DFP to FRDA patients for 6 months resulted in selective and significant reduction in foci of brain iron accumulation (assessed by T2* MRI) and initial functional improvements, with only minor changes in net body iron stores. The prospects of drug-mediated iron relocation versus those of chelation are discussed in relation to other disorders involving iron misdistribution, such as ACD and XLSA/A.

Selective acetylcholinesterase inhibitor activated by acetylcholinesterase releases an active chelator with neurorescuing and anti-amyloid activities

The finding that acetylcholinesterase (AChE) colocalizes with β-amyloid (Aβ) and promotes and accelerates Aβ aggregation has renewed an intense interest in developing new multifunctional AChE inhibitors as potential disease-modifying drugs for Alzheimer's therapy. To this end, we have developed a new class of selective AChE inhibitors with site-activated chelating activity. The identified lead, HLA20A, exhibits little affinity for metal (Fe, Cu, and Zn) ions but can be activated following inhibition of AChE to liberate an active chelator, HLA20. HLA20 has been shown to possess neuroprotective and neurorescuing activities in vitro and in vivo with the ability to lower amyloid precursor holoprotein (APP) expression and Aβ generation and inhibit Aβ aggregation induced by metal (Fe, Cu, and Zn) ion. HLA20A inhibited AChE in a time and concentration dependent manner with an HLA20A-AChE complex constant (K(i)) of 9.66 × 10(-6) M, a carbamylation rate (k(+2)) of 0.14 min(-1), and a second-order rate (k(i)) of 1.45 × 10 (4) M(-1) min(-1), comparable to those of rivastigmine. HLA20A showed little iron-binding capacity and activity against iron-induced lipid peroxidation (LPO) at concentrations of 1-50 μM, while HLA20 exhibited high potency in iron-binding and in inhibiting iron-induced LPO. At a concentration of 10 μM, HLA20A showed some activity against monoamine oxidase (MAO)-A and -B when tested in rat brain homogenates. Defined restrictively by Lipinski's rules, both HLA20A and HLA20 satisfied drug-like criteria and possible oral and brain permeability, but HLA20A was more lipophilic and considerably less toxic in human SHSY5Y neuroblastoma cells at high concentrations (25 or 50 μM). Together our data suggest that HLA20A may represent a promising lead for further development for Alzheimer's disease therapy.

Anti-tumor and radiosensitization activities of the iron chelator HDp44mT are mediated by effects on intracellular redox status

A novel iron chelator, HDp44mT, has been reported to have potent anti-proliferative effects on cancer cells; however, the underlying mechanism of action is not well understood. In this study, we characterized the cytotoxic effect of HDp44mT in a chemo- and radio-resistant cell line (PC-3) of prostatic cancer origin. The activity of HDp44mT at nM concentrations was dependent on the intracellular GSH and atmospheric O(2) concentration, rather than iron deprivation. HDp44mT also radiosensitized PC-3 cells in a GSH-dependent manner. Interestingly, this radiosensitizing effect was observed under aerobic and, to a larger extent, hypoxic conditions, suggesting its potential utility as a radiosensitizer for some radioresistant tumors.

Chelation in metal intoxication

Chelation therapy is the preferred medical treatment for reducing the toxic effects of metals. Chelating agents are capable of binding to toxic metal ions to form complex structures which are easily excreted from the body removing them from intracellular or extracellular spaces. 2,3-Dimercaprol has long been the mainstay of chelation therapy for lead or arsenic poisoning, however its serious side effects have led researchers to develop less toxic analogues. Hydrophilic chelators like meso-2,3-dimercaptosuccinic acid effectively promote renal metal excretion, but their ability to access intracellular metals is weak. Newer strategies to address these drawbacks like combination therapy (use of structurally different chelating agents) or co-administration of antioxidants have been reported recently. In this review we provide an update of the existing chelating agents and the various strategies available for the treatment of heavy metals and metalloid intoxications.

Mapping brain metals to evaluate therapies for neurodegenerative disease

The brain is rich in metals and has a high metabolic rate, making it acutely vulnerable to the toxic effects of endogenously produced free radicals. The abundant metals, iron and copper, transfer single electrons as they cycle between their reduced (Fe(2+) , Cu(1+) ) and oxidized (Fe(3+) , Cu(2+) ) states making them powerful catalysts of reactive oxygen species (ROS) production. Even redox inert zinc, if present in excess, can trigger ROS production indirectly by altering mitochondrial function. While metal chelators seem to improve the clinical outcome of several neurodegenerative diseases, their mechanisms of action remain obscure and the effects of long-term use are largely unknown. Most chelators are not specific to a single metal and could alter the distribution of multiple metals in the brain, leading to unexpected consequences over the long-term. We show here how X-ray fluorescence will be a valuable tool to examine the effect of chelators on the distribution and amount of metals in the brain.

Redistribution of accumulated cell iron: a modality of chelation with therapeutic implications

Various pathologies are characterized by the accumulation of toxic iron in cell compartments. In anemia of chronic disease, iron is withheld by macrophages, leaving extracellular fluids iron-depleted. In Friedreich ataxia, iron levels rise in the mitochondria of excitable cells but decrease in the cytosol. We explored the possibility of using deferiprone, a membrane-permeant iron chelator in clinical use, to capture labile iron accumulated in specific organelles of cardiomyocytes and macrophages and convey it to other locations for physiologic reuse. Deferiprone's capacity for shuttling iron between cellular organelles was assessed with organelle-targeted fluorescent iron sensors in conjunction with time-lapse fluorescence microscopy imaging. Deferiprone facilitated transfer of iron from extracellular media into nuclei and mitochondria, from nuclei to mitochondria, from endosomes to nuclei, and from intracellular compartments to extracellular apotransferrin. Furthermore, it mobilized iron from iron-loaded cells and donated it to preerythroid cells for hemoglobin synthesis, both in the presence and in the absence of transferrin. These unique properties of deferiprone underlie mechanistically its capacity to alleviate iron accumulation in dentate nuclei of Friedreich ataxia patients and to donate tissue-chelated iron to plasma transferrin in thalassemia intermedia patients. Deferiprone's shuttling properties could be exploited clinically for treating diseases involving regional iron accumulation.

Synthesis, Characterization, and Metal Coordinating Ability of Multifunctional Carbohydrate-Containing Compounds for Alzheimer's Therapy

Dysfunctional interactions of metal ions, especially Cu, Zn, and Fe, with the amyloid-beta (A beta) peptide are hypothesized to play an important role in the etiology of Alzheimer's disease (AD). In addition to direct effects on A beta aggregation, both Cu and Fe catalyze the generation of reactive oxygen species (ROS) in the brain further contributing to neurodegeneration. Disruption of these aberrant metal-peptide interactions via chelation therapy holds considerable promise as a therapeutic strategy to combat this presently incurable disease. To this end, we developed two multifunctional carbohydrate-containing compounds N,N'-bis[(5-beta-D-glucopyranosyloxy-2-hydroxy)benzyl]-N,N'-dimethyl-ethane-1,2-diamine (H2GL1) and N,N'-bis[(5-beta-D-glucopyranosyloxy-3-tert-butyl-2-hydroxy)benzyl]-N,N'-dimethyl-ethane-1,2-diamine (H2GL2) for brain-directed metal chelation and redistribution. Acidity constants were determined by potentiometry aided by UV-vis and 1H NMR measurements to identify the protonation sites of H2GL1,2. Intramolecular H bonding between the amine nitrogen atoms and the H atoms of the hydroxyl groups was determined to have an important stabilizing effect in solution for the H2GL1 and H2GL2 species. Both H2GL1 and H2GL2 were found to have significant antioxidant capacity on the basis of an in vitro antioxidant assay. The neutral metal complexes CuGL1, NiGL1, CuGL2, and NiGL2 were synthesized and fully characterized. A square-planar arrangement of the tetradentate ligand around CuGL2 and NiGL2 was determined by X-ray crystallography with the sugar moieties remaining pendant. The coordination properties of H2GL1,2 were also investigated by potentiometry, and as expected, both ligands displayed a higher affinity for Cu2+ over Zn2+ with H2GL1 displaying better coordinating ability at physiological pH. Both H2GL1 and H2GL2 were found to reduce Zn2+- and Cu2+- induced Abeta1-40 aggregation in vitro, further demonstrating the potential of these multifunctional agents as AD therapeutics.

Novel neuroprotective neurotrophic NAP analogs targeting metal toxicity and oxidative stress: potential candidates for the control of neurodegenerative diseases

A large body of data indicates that a cascade of events contributes to the neurodegeneration in Alzheimer's disease (AD) and Parkinson's disease (PD). Metal (Fe, Cu, Zn) dyshomeostasis and oxidative stress are believed to play a pivotal role in the pathogenesis of these diseases. Accordingly, multifunctional compounds combining metal chelating and antioxidative activity hold a great promise as potential drugs for treating AD and PD. In this study, two novel NAPVSIPQ (NAP) analogs (M98 and M99) with potential antioxidant-metal chelating ability were designed and investigated, aiming to improve the poor metal chelating and antioxidative activity of NAP. Our studies showed that both M98 and M99 formed stable metal (Fe, Cu, Zn) complexes in water and demonstrated good metal (Fe, Cu, Zn) chelating properties as opposed to the poor metal (Fe, Cu, Zn) chelating properties of their parent peptide NAP. M98 and M99 exhibited significant inhibition of iron-induced lipid peroxidation in rat brain homogenates at concentrations of > or = 30 microM, while NAP failed to show any inhibition even at 100 microM. In human neuroblastoma cell (SH-SY5Y) culture, M98 and M99 at 1 microM completely protected against 6-hydroxydopamine (6OHDA) toxicity with potency similar to NAP and desferal (DFO), a strong iron chelator and a highly potent radical scavenger. In PC12 cell culture, M98 at the range of 0.001-1 microM displayed potent protection against 6-OHDA toxicity, comparable to NAP and DFO. These results suggest that M98 and M99 deserve further investigation as potential drug candidates for neuroprotection.

Caged-Iron Chelators a Novel Approach towards Protecting Skin Cells against UVA-Induced Necrotic Cell Death

Exposure of human skin cells to solar UVA radiation leads to an immediate dose-dependent increase of labile iron that subsequently promotes oxidative damage and necrotic cell death. Strong iron chelators have been shown to suppress cell damage and necrotic cell death by moderating the amount of labile iron pool (LIP), but chronic use would cause severe side effects owing to systemic iron depletion. Prodrugs that become activated in skin cells at physiologically relevant doses of UVA, such as "caged-iron chelators", may provide dose- and context-dependent release. Herein, we describe prototypical iron chelator compounds derived from salicylaldehyde isonicotinoyl hydrazone and pyridoxal isonicotinoyl hydrazone and demonstrate that the intracellular LIP and subsequent necrotic cell death of human skin fibroblasts is significantly decreased upon exposure to a combination of the prototypical compounds and physiologically relevant UVA doses. Iron regulatory protein bandshift and calcein fluorescence assays reveal decreased intracellular LIP following irradiation of caged-chelator-treated cells, but not in control samples where either UVA light, or caged-chelator is absent. Furthermore, flow cytometry shows that these compounds have no significant toxicity in the skin fibroblasts. This novel light-activated prodrug strategy may therefore be used to protect skin cells against the deleterious effects of sunlight.

Novel multifunctional neuroprotective iron chelator-monoamine oxidase inhibitor drugs for neurodegenerative diseases: in vitro studies on antioxidant activity, prevention of lipid peroxide formation and monoamine oxidase inhibition

Iron-dependent oxidative stress, elevated levels of iron and of monoamine oxidase (MAO)-B activity, and depletion of antioxidants in the brain may be major pathogenic factors in Parkinson's disease, Alzheimer's disease and related neurodegenerative diseases. Accordingly, iron chelators, antioxidants and MAO-B inhibitors have shown efficacy in a variety of cellular and animal models of CNS injury. In searching for novel antioxidant iron chelators with potential MAO-B inhibitory activity, a series of new iron chelators has been designed, synthesized and investigated. In this study, the novel chelators were further examined for their activity as antioxidants, MAO-B inhibitors and neuroprotective agents in vitro. Three of the selected chelators (M30, HLA20 and M32) were the most effective in inhibiting iron-dependent lipid peroxidation in rat brain homogenates with IC50 values (12-16 microM), which is comparable with that of desferal, a prototype iron chelator that is not has orally active. Their antioxidant activities were further confirmed using electron paramagnetic resonance spectroscopy. In PC12 cell culture, the three novel chelators at 0.1 microM were able to attenuate cell death induced by serum deprivation and by 6-hydroxydopamine. M30 possessing propargyl, the MAO inhibitory moiety of the anti-Parkinson drug rasagiline, displayed greater neuroprotective potency than that of rasagiline. In addition, in vitro, M30 was a highly potent non-selective MAO-A and MAO-B inhibitor (IC50 < 0.1 microM). However, HLA20 was more selective for MAO-B but had poor MAO inhibition, with an IC50 value of 64.2 microM. The data suggest that M30 and HLA20 might serve as leads in developing drugs with multifunctional activities for the treatment of various neurodegenerative disorders.

Novel potential neuroprotective agents with both iron chelating and amino acid-based derivatives targeting central nervous system neurons

Antioxidants and iron chelating molecules are known as neuroprotective agents in animal models of neurodegenerative disorders such as Alzheimer's disease (AD) and Parkinson's disease (PD). In this study, we designed and synthesized a novel bifunctional molecule (M10) with radical scavenging and iron chelating ability on an amino acid carrier likely to be a substrate for system L, thus targeting the compound to the central nervous system (CNS). M10 had a moderate iron affinity in HEPES buffer (pH 7.4) with logK(3)=12.25+/-0.55 but exhibited highly inhibitory action against iron-induced lipid peroxidation, with an IC(50) value (12microM) comparable to that of desferal (DFO). EPR studies indicated that M10 was a highly potent *OH scavenger with an IC(50) of about 0.3 molar ratio of M10 to H(2)O(2). In PC12 cell culture, M10 was at least as potent as the anti-Parkinson drug rasagiline in protecting against cell death induced by serum-deprivation and by 6-hydroxydopamine (6-OHDA). These results suggest that M10 deserves further investigation as a potential agent for the treatment of neurodegenerative disorders such as AD and PD.

Design, synthesis, and evaluation of novel bifunctional iron-chelators as potential agents for neuroprotection in Alzheimer's, Parkinson's, and other neurodegenerative diseases

Several novel antioxidant-iron chelators bearing 8-hydroxyoxyquinoline moiety were synthesized, and various properties related to their iron chelation, and neuroprotective action were investigated. All the chelators exhibited strong iron(III) chelating and high antioxidant properties. Chelator 9 (HLA20), having good permeability into K562 cells and moderate selective MAO-B inhibitory activity (IC50 110 microM), displayed the hightest protective effects against differentiated P19 cell death induced by 6-hydroxydopamine. EPR studies suggested that Chelator 9 also act as radical scavenger to directly scavenge hydroxyl radical.

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.

Mechanisms underlying the cytotoxic effects of Tachpyr--a novel metal chelator

Tachpyr (N,N'N"-tris(2-pyridylmethyl)-cis,cis-1,3,5-triaminocyclohexane), a novel metal chelator, was previously shown to deplete intracellular iron and exert a cytotoxic effect on cultured bladder cancer cells. Tachpyr binds Fe(II) and readily reduces Fe(III). The iron(II)-Tachpyr chelate undergoes intramolecular oxidative dehydrogenation resulting in mono- and diimino Fe(II) complexes. The present study investigates the redox-activity of the Tachpyr-iron complex to better define the mechanism of Tachpyr's cytotoxicity. Tachpyr's mechanism of cytotoxicity was studied using cell-free solutions, isolated DNA, and cultured mammalian cells by employing UV-VIS spectrophotometry, oximetry, spin-trapping technique, and electron paramagnetic resonance (EPR) spectrometry. The results show that: (1) Tachpyr by itself after 24 h of incubation had a cytotoxic effect on cultured cells; (2) fully oxidized Tachpyr had no cytotoxic effects on cultured cells even after 24 h of incubation; (3) Tachpyr protected isolated DNA against H(2)O(2)-induced damage, but not against HX/XO-induced damage; and (4) Tachpyr-Fe(II) chelate slows down but does not block oxidation of Fe(II), allows O*(-)(2)-induced or Tachpyr-induced reduction of Fe(III), and consequently promotes production of *OH through the Haber-Weiss reaction cycle. The results indicate that Tachpyr can protect cells against short-term, metal-mediated damage. However, upon prolonged incubation, Tachpyr exerts cytotoxic effects. Therefore, in addition to iron depletion, low-level oxidative stress, which in part occurs because of redox cycling of the coordinated iron ion, may contribute to the cytotoxic effects of Tachpyr.

Design of clinically useful iron(III)-selective chelators

Iron overload is a serious clinical condition which can be largely prevented by the use of iron-specific chelating agents. Desferrioxamine-B, the most widely used iron chelator in haematology over the past 30 years, has a major disadvantage of being orally inactive. Consequently, the successful design of an orally active, nontoxic, selective iron chelator has become a much sought after goal. In order to identify an ideal iron chelator for clinical use, a range of specifications must be considered such as metal selectivity and affinity, kinetic stability of the complex, bioavailability and toxicity. A wide range of chelator types bind iron(III) and of these, hexa-, tri-, and bidentate are capable of providing iron(III) with the favoured octahedral environment. In this review, the comparative properties of such ligands are discussed, examples being selected from hydroxamates, aminocarboxylates, hydroxypyridinones, orthosubstituted phenols and triazoles.

Cisplatin ototoxicity: involvement of iron and enhanced formation of superoxide anion radicals

Since there are indications that iron influences cisplatin nephrotoxicity, we studied the role of iron in cisplatin ototoxicity in an in vitro model of the neurosensory epithelium of the guinea pig cochlea. Viability tests showed that Deiters and Hensen cells were not damaged and inner hair cells were only slightly damaged by cisplatin (50 microM). The outer hair cells were most sensitive to cisplatin toxicity. The iron chelator 2,2'-dipyridyl provided partial protection against cisplatin-induced cell death. In addition, we studied the influence of the iron chelators 2,2'-dipyridyl and deferoxamine on the chelatable iron pool in the various cells of the neurosensory epithelium using the fluorescent iron indicator Phen Green SK. Both chelators decreased the chelatable iron accessible to Phen Green SK, although the effect of deferoxamine was weaker because it entered the cells more slowly. The cellular concentration of the chelatable iron was measured using Phen Green SK and quantitative laser scanning microscopy. The concentration of chelatable iron in the inner ear cells ranged from 1.3 +/- 0.4 microM iron in inner hair cells to 3.7 +/- 1.7 microM iron in Hensen cells and did not correlate with the various cell types' susceptibility to cisplatin. Furthermore, cisplatin did not raise the intracellular chelatable iron concentration but enhanced the production of superoxide anions inside the neurosensory epithelium, especially inside the hair cells, as detected by the nitrotetrazolium blue reduction assay. Our conclusion is that cisplatin ototoxicity is partially mediated by an iron-dependent pathway and is associated with an enhanced formation of superoxide anions.

Multifunctional antioxidant activity of HBED iron chelator

The use of N,N'-bis (2-hydroxybenzyl) ethylenediamine-N,N'-diacetic acid (HBED) for iron chelation therapy is currently being tested. Besides its affinity for iron, bioavailability, and efficacy in relieving iron overload, it is important to assess its anti- and/or pro-oxidant activity. To address these questions, the antioxidant/pro-oxidant effects of HBED in a cell-free solution and on cultured Chinese hamster V79 cells were studied using UV-VIS spectrophotometry, oximetry, spin trapping, and electron paramagnetic resonance (EPR) spectrometry. The results indicate that HBED facilitates Fe(II) oxidation but blocks O2(.-)-induced reduction of Fe(III) and consequently pre-empts production of .OH or hypervalent iron through the Haber-Weiss reaction cycle. The efficacy of HBED as a 1-electron donor (H-donation) was demonstrated by reduction of the 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonate)-derived nitrogen-centered radical cation (ABTS(.+)), accompanied by formation of a short-lived phenoxyl radical. HBED also provided cytoprotection against toxicity of H2O2 and t-BuOOH. Our results show that HBED can act both as a H-donating antioxidant and as an effective chelator lacking pro-oxidant capacity, thus substantiating its promising use in iron chelation therapy.

Iron metabolism and HIV infection: reciprocal interactions with potentially harmful consequences?

Humans with advanced human immunodeficiency virus (HIV) infection present some evidence suggestive of iron accumulation. Ferritin concentrations increase with HIV disease progression, and iron accumulates in several tissues. Iron excess may exert negative effects in individuals with HIV. Indeed, iron accumulation seems to be associated with shorter survival, and a number of investigations point to an iron-mediated oxidative stress in subjects with HIV infection. The observations on humans infected with HIV are in part supported by in-vitro findings. Indeed, in-vitro HIV infection is associated with changes in iron metabolism, and an iron-mediated oxidative stress is likely to contribute to viral cytopathogenicity. Furthermore, it is interesting to point out that the interaction between iron and HIV may be reciprocal, since viruses with a life-cycle involving a DNA phase require chelatable iron for optimum replication. This combined evidence suggests that iron metabolism is an important area for virus/host interaction. These observations may be relevant to both laboratory monitoring and clinical treatment of individuals with HIV.

Protection of U937 cells against oxidative injury by a novel series of iron chelators

A new series of iron chelators designed to protect tissues against iron-catalysed oxidative damage is described. These compounds are aminocarboxylate derivatives bearing pendant aromatic groups. They were designed to have a relatively low affinity for both ferrous and ferric iron and to be site-specifically oxidizable by hydrogen peroxide through intramolecular aromatic hydroxylation into species with strong iron binding capacity which do not catalyse hydroxyl radical formation. Thus, at the cellular level, oxidative injury is used to convert weak iron chelators into strong iron chelators in order to promote cell survival. The purpose of this local activation process is to minimise toxicity compared to strong iron chelators which may interfere with normal iron metabolism. Compounds within this series were evaluated in vitro in view of their capacity to undergo intramolecular hydroxylation and to protect cultured cells against oxidative injury. Results show that the intramolecular aromatic hydroxylation capacity is critically dependent upon the amino carboxylate chelating moieties and the substituents of the aromatic rings. Cell protection against oxidative injury is only observed with compounds possessing sufficient lipophilicity. The monohydroxylation product of N,N'-dibenzylethylenediamine N,N'-diacetic acid, protects cells against both H2O2 and tBuOOH toxicity with IC50's of 12 and 60 microM, respectively, in agreement with the oxidative activation concept. These results represent the first step toward the development of a new strategy to safe iron chelation for the prevention of oxidative damage.