In vitro antioxidant properties of the iron chelator pyridoxal isonicotinoyl hydrazone and some of its analogs

Iron chelates in the anticancer therapy

Iron plays a significant role in the metabolism of cancer cells. In comparison with normal cells, neoplastic ones exhibit enhanced vulnerability to iron. Ferric ions target tumor via the ferroptotic death pathway—a process involving the iron-mediated lipid oxidation. Ferric ion occurs in complex forms in the physiological conditions. Apart from iron, ligands are the other factors to affect the biological activity of the iron complexes. In recent decades the role of iron chelates in targeting the growth of the tumor was extensively examined. The ligand may possess a standalone activity to restrict cancer’s growth. However, a wrong choice of the ligand might lead to the enhanced cancer cell’s growth in in vitro studies. The paper aims to review the role of iron complex compounds in the anticancer therapy both in the experimental and clinical applications. The anticancer properties of the iron complex rely both on the stability constant of the complex and the ligand composition. When the stability constant is high, the properties of the drug are unique. However, when the stability constant remains low, both components—ferric ions and ligands, act separately on the cells. In the paper we show how the difference in complex stability implies the action of ligand and ferric ions in the cancer cell. Iron complexation strategy is an interesting attempt to transport the anticancer Fe2+/3+ ions throughout the cell membrane and release it when the pH of the microenvironment changes. Last part of the paper summarizes the results of clinical trials and in vitro studies of novel iron chelates such as: PRLX 93,936, Ferumoxytol, Talactoferrin, DPC, Triapine, VLX600, Tachypyridine, Ciclopiroxamine, Thiosemicarbazone, Deferoxamine and Deferasirox.

Iron modulatory property of a polysaccharide from Indian medicinal plant Ocimum sanctum

Despite being an essential element for normal functioning of cells and organisms, iron, in excess, can induce oxidative stress by generating reactive oxygen species. A water-soluble, non-toxic iron chelator can reduce the iron-induced oxidative stress in the body as well as help in extricating excess iron. Herein, we report an Ocimum sanctum-derived antioxidant polysaccharide (OSP) that inhibits the deleterious effect of iron. Ocimum sanctum is a widely acknowledged medicinal plant contributing toward several biological benefits. Besides showing good hydroxyl radical scavenging activity, OSP could bind to ferric and ferrous ions to prevent their participation in redox reactions as revealed from modified 2-deoxyribose assays, carried out under various conditions. It also acted as an iron modulator to prevent site-specific damage and was effective in protecting mouse fibroblast L929 cells against iron induced death.

A Study of Potential Toxic Effects After Repeated 10-Week Administration of a New Iron Chelator – Salicylaldehyde Isonicotinoyl Hydrazone (SIH) to Rabbits

Salicylaldehyde Isonicotinoyl Hydrazone (SIH) – a Pyridoxal Isonicotinoyl Hydrazone (PIH) analogue – is an effective iron chelator with antioxidant and antimalarial effects, as documented in numerous in vitro studies. However, no toxicological data obtained from in vivo studies have been made available yet. In this study, the potential toxic effects of repeated administration of SIH (50 mg/kg, once weekly, 10 weeks, i.p.), partially dissolved in a 10 % Cremophor solution, on various biochemical, haematological, and cardiovascular parameters and on morphology of selected tissues were investigated in rabbits. The obtained values were compared with data from the control (saline, 1 ml/kg, i.v.) and the Cremophor (10 % Cremophor solution, 2 ml/kg, i.p.) groups. In this study, SIH did not induced marked signs of toxicity: No premature deaths occurred, the body weight increase was comparable with the control and Cremophor groups. Only few and mild changes in some biochemical and haematological parameters could be determined, most of them were noticed also in the control or Cremophor groups. The morphological changes in the kidney were mild and did not manifest in the biochemical examination. The cardiac function was also not affected markedly – the values of left ventricular ejection fraction and systolic time interval did not differ from the values of control group. Only an increased left ventricular contractility (dP/dtmax) was noticed in the SIH group at the end of the experiment as compared to the controls (13354±1191 vs. 9339±647 mmHg/s, resp.). These results seem to be promising from the standpoint of possible clinical use of SIH.

The interaction of pyridoxal isonicotinoyl hydrazone (PIH) and salicylaldehyde isonicotinoyl hydrazone (SIH) with iron

The interaction of pyridoxal isonicotinoyl hydrazone (PIH) and salicylaldehyde isonicotinoyl hydrazone (SIH), two important biologically active chelators, with iron has been investigated by spectrophotometric methods. High iron(III) affinity constants were determined for PIH, logβ₂=37.0 and SIH, logβ₂=37.6. The associated redox potentials of the iron complexes were determined using cyclic voltammetry at pH7.4 as +130mV (vs normal hydrogen electrode, NHE) for PIH and +136mV(vs NHE) for SIH. These redox potentials are much higher than those corresponding to iron chelators in clinical use, namely deferiprone, -620mV; desferasirox, -600mV and desferrioxamine, -468mV. Although the positive redox potentials of SIH and PIH are similar to that of EDTA, namely +120mV, the iron complexes of these two hydrazone chelators, unlike the iron complex of EDTA, do not redox cycle in the presence of vitamin C. These properties render PIH and SIH as excellent scavengers of iron, under biological conditions. Both SIH and PIH scavenge mononuclear iron(II) and iron(III) rapidly. These fast kinetic properties of the hydrazone-based chelators provide a ready explanation for the adoption of SIH in fluorescence-based methods for the quantification of cytosolic iron(II).

Oxidative stress mediates toxicity of pyridoxal isonicotinoyl hydrazone analogs

Pyridoxal isonicotinoyl hydrazone (PIH) and many of its analogs are effective iron chelators in vivo and in vitro, and are of interest for the treatment of secondary iron overload. Because previous work has implicated the Fe(3+)-chelator complexes as a determinant of toxicity, the role of iron-based oxidative stress in the toxicity of PIH analogs was assessed. The Fe(3+) complexes of PIH analogs were reduced by K562 cells and the physiological reductant, ascorbate. Depletion of the antioxidant, glutathione, sensitized Jurkat T lymphocytes to the toxicity of PIH analogs and their Fe(3+) complexes, and toxicity of the chelators increased with oxygen tension. Fe(3+) complexes of pyridoxal benzoyl hydrazone (PBH) and salicyloyl isonicotinoyl hydrazone (SIH) caused lipid peroxidation and toxicity in K562 cells loaded with eicosapentenoic acid (EPA), a readily oxidized fatty acid, whereas Fe(PIH)(2) did not. The lipophilic antioxidant, vitamin E, completely prevented both the toxicity and lipid peroxidation caused by Fe(PBH)(2) in EPA-loaded cells, indicating a causal relationship between oxidative stress and toxicity. PBH also caused concomitant lipid peroxidation and toxicity in EPA-loaded cells, both of which were reversed as its concentration increased. In contrast, PIH was inactive, while SIH was equally toxic toward control and EPA-loaded cells, without causing lipid peroxidation, indicating a much smaller contribution of oxidative stress to the mechanism of toxicity of these analogs. In summary, PIH analogs and their Fe(3+) complexes are redox active in the intracellular environment. The contribution of oxidative stress to the overall mechanism of toxicity varies across the series.

Pyridoxal isonicotinoyl hydrazone inhibits iron-induced ascorbate oxidation and ascorbyl radical formation

Previous work from our laboratory demonstrated that pyridoxal isonicotinoyl hydrazone (PIH) has in vitro antioxidant activity against iron plus ascorbate-induced 2-deoxyribose degradation due to its ability to chelate iron; the resulting Fe(III)-PIH(2) complex is supposedly unable to catalyze oxyradical formation. A putative step in the antioxidant action of PIH is the inhibition of Fe(III)-mediated ascorbate oxidation, which yields the Fenton reagent Fe(II) [Biochim. Biophys. Acta 1523 (2000) 154]. In this work, we demonstrate that PIH inhibits Fe(III)-EDTA-mediated ascorbate oxidation (measured at 265 nm) and the formation of ascorbyl radical (in electron paramagnetic resonance (EPR) studies). The efficiency of PIH against ascorbate oxidation, ascorbyl radical formation and 2-deoxyribose degradation was dose dependent and directly proportional to the period of preincubation of PIH with Fe(III)-EDTA. The efficiency of PIH in inhibiting ascorbate oxidation and ascorbyl radical formation was also inversely proportional to the Fe(III)-EDTA concentration in the media. When EDTA was replaced by the weaker iron ligand nitrilotriacetic acid (NTA), PIH was much more effective in preventing ascorbate oxidation, ascorbyl radical formation and 2-deoxyribose degradation. Moreover, the replacement of EDTA with citrate, a physiological chelator with a low affinity for iron, also resulted in PIH having a higher efficiency in inhibiting iron-mediated ascorbate oxidation and 2-deoxyribose degradation. These results demonstrate that PIH removes iron from EDTA (or from either NTA or citrate), forming an iron-PIH complex that cannot induce ascorbate oxidation effectively, thus inhibiting iron-mediated oxyradical formation. These results are of pharmacological relevance because PIH has been considered for experimental chelating therapy in iron-overload diseases.

Lipophilicity of analogs of pyridoxal isonicotinoyl hydrazone (PIH) determines the efflux of iron complexes and toxicity in K562 cells

Iron overload secondary to beta-thalassemia and other iron-loading anemias is the most serious obstacle to be overcome in the treatment of these diseases, since there is no physiological mechanism for excretion of the excess iron acquired by chronic blood transfusion. Due to the inconvenience and cost of the current iron chelation therapy, the search for an orally available iron chelator is ongoing. Pyridoxal isonicotinoyl hydrazone (PIH) and many of its analogs are effective at mobilizing iron in vivo and in vitro at doses that are not toxic. PIH analogs were approximately equally effective at binding 59Fe within K562 cells; their efficacy depended upon the kinetics of release of the iron-chelator complex from the cell, which was correlated inversely with the lipophilicity of the chelators. Addition of BSA, which has a well-characterized affinity for lipophilic species, to the extracellular medium enhanced iron-chelator efflux, such that all analogs caused 59Fe release from the cells as quickly as it was chelated; this suggests that BSA acts as an extracellular sink for the iron-chelator complexes, many of which are highly lipophilic. The toxicity of the free chelators varied over two orders of magnitude, and was correlated with the amount of intracellular 59Fe-chelator complexes, implicating the complexes in the mechanism of toxicity of the chelators. Understanding the structural features that determine the efficacy and toxicity of iron chelators in biological systems is of value in the selection of PIH analogs for in vivo examination.

Pyridoxal isonicotinoyl hydrazone (PIH) prevents copper-mediated in vitro free radical formation

Pyridoxal isonicotinoyl hydrazone (PIH) is an iron chelator with antioxidant activity, low toxicity and is useful in the experimental treatment of iron-overload diseases. Previous studies on x-ray diffraction have revealed that PIH also forms a complex with Cu(II). Since the main drug of choice for the treatment of Wilson's disease, d-penicillamine, causes a series of side effects, there is an urgent need for the development of alternative copper chelating agents for clinical use. These chelators must also have antioxidant activity because oxidative stress is associated with brain and liver copper-overload. In this work we tested the ability of PIH to prevent in vitro free radical formation mediated by Cu(II), ascorbate and dissolved O2. Degradation of 2-deoxyribose mediated by 10 microM Cu(II) and 3 mM ascorbate was fully inhibited by 10 microM PIH (I50 = 6 microM) or 20 microM d-penicillamine (I50 = 10 microM). The antioxidant efficiency of PIH remained unchanged with increasing concentrations (from 1 to 15 mM) of the hydroxyl radical detector molecule, 2-deoxyribose, indicating that PIH does not act as a hydroxyl scavenger. On the other hand, the efficiency of PIH (against copper-mediated 2-deoxyribose degradation and ascorbate oxidation) was inversely proportional to the Cu(II) concentration, suggesting a competition between PIH and ascorbate for complexation with Cu(lI). An almost full inhibitory effect by PIH was observed when the ratio PIH:copper was 1:1. A similar result was obtained with the measurement of copper plus ascorbate-mediated O2 uptake. Moreover, spectral studies of the copper and PIH interaction showed a peak at 455 nm and also indicated the formation of a stable Cu(II) complex with PIH with a 1:1 ratio. These data demonstrated that PIH prevents hydroxyl radical formation and oxidative damage to 2-deoxyribose by forming a complex with Cu(II) that is not reactive with ascorbate (first step of the reactions leading to hydroxyl radical formation from Cu(II), ascorbate and O2) and does not participate in Haber-Weiss reactions.

The iron chelator pyridoxal isonicotinoyl hydrazone inhibits mitochondrial lipid peroxidation induced by Fe(II)-citrate

Pyridoxal isonicotinoyl hydrazone (PIH) is able to prevent iron-mediated hydroxyl radical formation by means of iron chelation and inhibition of redox cycling of the metal. In this study, we investigated the effect of PIH on Fe(II)-citrate-mediated lipid peroxidation and damage to isolated rat liver mitochondria. Lipid peroxidation was quantified by the production of thiobarbituric acid-reactive substances (TBARS) and by antimycin A-insensitive oxygen consumption. PIH at 300 microM induced full protection against 50 microM Fe(II)-citrate-induced loss of mitochondrial transmembrane potential (deltapsi) and mitochondrial swelling. In addition, PIH prevented the Fe(II)-citrate-dependent formation of TBARS and antimycin A-insensitive oxygen consumption. The antioxidant effectiveness of 100 microM PIH (on TBARS formation and mitochondrial swelling) was greater in the presence of 20 or 50 microM Fe(II)-citrate than in the presence of 100 microM Fe(II)-citrate, suggesting that the mechanism of PIH antioxidant action is linked with its Fe(II) chelating property. Finally, PIH increased the rate of Fe(II) autoxidation by sequestering iron from the Fe(II)-citrate complex, forming a Fe(III)-PIH, complex that does not participate in Fenton-type reactions and lipid peroxidation. These results are of pharmacological relevance since PIH is a potential candidate for chelation therapy in diseases related to abnormal intracellular iron distribution and/or iron overload.

The iron chelator pyridoxal isonicotinoyl hydrazone (PIH) and its analogues prevent damage to 2-deoxyribose mediated by ferric iron plus ascorbate

Iron chelating agents are essential for treating iron overload in diseases such as beta-thalassemia and are potentially useful for therapy in non-iron overload conditions, including free radical mediated tissue injury. Deferoxamine (DFO), the only drug available for iron chelation therapy, has a number of disadvantages (e.g., lack of intestinal absorption and high cost). The tridentate chelator pyridoxal isonicotinoyl hydrazone (PIH) has high iron chelation efficacy in vitro and in vivo with high selectivity and affinity for iron. It is relatively non-toxic, economical to synthesize and orally effective. We previously demonstrated that submillimolar levels of PIH and some of its analogues inhibit lipid peroxidation, ascorbate oxidation, 2-deoxyribose degradation, plasmid DNA strand breaks and 5,5-dimethylpyrroline-N-oxide (DMPO) hydroxylation mediated by either Fe(II) plus H(2)O(2) or Fe(III)-EDTA plus ascorbate. To further characterize the mechanism of PIH action, we studied the effects of PIH and some of its analogues on the degradation of 2-deoxyribose induced by Fe(III)-EDTA plus ascorbate. Compared with hydroxyl radical scavengers (DMSO, salicylate and mannitol), PIH was about two orders of magnitude more active in protecting 2-deoxyribose from degradation, which was comparable with some of its analogues and DFO. Competition experiments using two different concentrations of 2-deoxyribose (15 vs. 1.5 mM) revealed that hydroxyl radical scavengers (at 20 or 60 mM) were significantly less effective in preventing degradation of 2-deoxyribose at 15 mM than 2-deoxyribose at 1.5 mM. In contrast, 400 microM PIH was equally effective in preventing degradation of both 15 mM and 1.5 mM 2-deoxyribose. At a fixed Fe(III) concentration, increasing the concentration of ligands (either EDTA or NTA) caused a significant reduction in the protective effect of PIH towards 2-deoxyribose degradation. We also observed that PIH and DFO prevent 2-deoxyribose degradation induced by hypoxanthine, xanthine oxidase and Fe(III)-EDTA. The efficacy of PIH or DFO was inversely related to the EDTA concentration. Taken together, these results indicate that PIH (and its analogues) works by a mechanism different than the hydroxyl radical scavengers. It is likely that PIH removes Fe(III) from the chelates (either Fe(III)-EDTA or Fe(III)-NTA) and forms a Fe(III)-PIH(2) complex that does not catalyze oxyradical formation.

Polyphenol tannic acid inhibits hydroxyl radical formation from Fenton reaction by complexing ferrous ions

Tannic acid (TA), a plant polyphenol, has been described as having antimutagenic, anticarcinogenic and antioxidant activities. Since it is a potent chelator of iron ions, we decided to examine if the antioxidant activity of TA is related to its ability to chelate iron ions. The degradation of 2-deoxyribose induced by 6 microM Fe(II) plus 100 microM H2O2 was inhibited by TA, with an I50 value of 13 microM. Tannic acid was over three orders of magnitude more efficient in protecting against 2-deoxyribose degradation than classical *OH scavengers. The antioxidant potency of TA was inversely proportional to Fe(II) concentration, demonstrating a competition between H2O2 and AT for reaction with Fe(II). On the other hand, the efficiency of TA was nearly unchanged with increasing concentrations of the *OH detector molecule, 2-deoxyribose. These results indicate that the antioxidant activity of TA is mainly due to iron chelation rather than *OH scavenging. TA also inhibited 2-deoxyribose degradation mediated by Fe(III)-EDTA (iron = 50 microM) plus ascorbate. The protective action of TA was significantly higher with 50 microM EDTA than with 500 microM EDTA, suggesting that TA removes Fe(III) from EDTA and forms a complex with iron that cannot induce *OH formation. We also provided evidence that TA forms a stable complex with Fe(II), since excess ferrozine (14 mM) recovered 95-96% of the Fe(II) from 10 microM TA even after a 30-min exposure to 100-500 microM H2O2. Addition of Fe(III) to samples containing TA caused the formation of Fe(II)n-TA, complexes, as determined by ferrozine assays, indicating that TA is also capable of reducing Fe(III) ions. We propose that when Fe(II) is complexed to TA, it is unable to participate in Fenton reactions and mediate *OH formation. The antimutagenic and anticarcinogenic activity of TA, described elsewhere, may be explained (at least in part) by its capacity to prevent Fenton reactions.

The iron chelator pyridoxal isonicotinoyl hydrazone (PIH) protects plasmid pUC-18 DNA against *OH-mediated strand breaks

Pyridoxal isonicotinoyl hydrazone (PIH) has previously been studied for use in iron chelation therapy in iron-overload diseases. It is an efficient in vitro antioxidant due to its Fe(III) complexing activity (Schulman, H. M., et al. Redox Report 1:373-378; 1995). Pathologies associated with iron-overload include hepatic and other cancers. Since oxidative alterations of DNA can be linked to the development of cancer, we decided to study whether PIH protects DNA against in vitro oxidative stress. We report here that pUC-18 plasmid DNA is damaged by *OH radicals generated from Fe(II) plus H2O2 or from Fe(II) plus hypoxanthine/xanthine oxidase. The DNA damage was quantified by determining the diminution of supercoiled DNA forms after oxidative attack using agar gel electrophoresis. Micromolar amounts of PIH (20-30 microM) were able to half-protect DNA from iron (1-7.5 microM)-mediated *OH formation. The antioxidant capacity of PIH was significantly higher than that of some of its analogs and desferrioxamine. PIH and some of its analogues could also inhibit the oxidative degradation of 2-deoxyribose caused by Fenton reagents. Since we observed that PIH enhances the Fe(II) autoxidation rate, measured by the ferrozine technique, PIH may limit *OH formation and consequently DNA damage by decreasing the amount of Fe(II) available to catalyze Fenton reactions.

Development of iron chelators to treat iron overload disease and their use as experimental tools to probe intracellular iron metabolism

The development of an orally effective iron (Fe) chelator for the treatment of Fe overload diseases such as beta-thalassemia has been a difficult challenge. Even though the drug in current clinical use, desferrioxamine (DFO), is efficient and remarkably free of toxicity, it suffers from not being orally effective and requiring long subcutaneous infusion to mobilize sufficient quantities of Fe. In addition, DFO is very expensive, which precludes it from treating most of the world's thalassemic population. Therefore, the development of an economical and orally effective Fe chelator is of great importance. Despite the screening of a wide range of structurally diverse ligands from both natural and synthetic sources, few compounds have been promising enough to proceed to clinical trials. In the current review, the properties of an ideal chelator are discussed, followed by a description of the most successful ligands that have been identified. Apart from the use of Fe chelators as therapeutic agents, some of these compounds have also been useful as experimental probes to investigate cellular Fe metabolism. We describe here the most important of these studies.

Pyridoxal isonicotinoyl hydrazone and its analogs: potential orally effective iron-chelating agents for the treatment of iron overload disease

At present, the only iron (Fe) chelator in clinical use for the treatment of Fe overload disease is the tris-hydroxamate deferoxamine (DFO). However, DFO suffers from a number of disadvantages, including the need for subcutaneous infusion (12 to 24 hours a day, 5 or 6 times per week), its poor intestinal absorption, and high cost. Therefore, there is an urgent need for an efficient, economical, and orally effective Fe chelator. Pyridoxal isonicotinoyl hydrazone (PIH) is a tridentate Fe-chelating agent that shows high Fe chelation efficacy both in vitro in cell culture models and also in vivo in rats and mice. In addition, this chelator is relatively nontoxic, economical to synthesize, and orally effective, and it shows high selectivity and affinity for Fe. However, over the last 10 years the development of PIH and its analogs has largely been ignored because of justifiable interest in other ligands such as 1,2-dimethyl-3-hydroxypyrid-4-one (L1). Unfortunately, recent clinical trials have shown that significant complications occur with L1 therapy, and it is controversial whether this chelator is effective at reducing hepatic Fe levels in patients. Because of the current lack of a clinically useful Fe chelator to replace DFO, PIH and its analogs appear to be potential candidate compounds that warrant further investigation. In this review we will discuss the studies that have been performed to characterize these chelators at the chemical and biologic levels as effective agents for treating Fe overload. The evidence from the literature suggests that these ligands deserve further careful investigation as potential orally effective Fe chelators.

Selective inhibition of l kappaB alpha phosphorylation and HIV-1 LTR-directed gene expression by novel antioxidant compounds

Oxidative stress activates the NF-kappaB/Rel transcription factors which are involved in the activation of numerous immunoregulatory genes and the human immunodeficiency virus type 1 (HIV-1) long terminal repeat (LTR). In the present study, we examined the effects of established and novel compounds including antioxidants, ribonucleotide reductase inhibitors, and iron chelators on NF-kappaB activation and HIV LTR-mediated gene expression induced by TNF-alpha. N-Acetylcysteine (NAC), pyrrolidinedithiocarbamate (PDTC), and Trimidox (TD) at various concentrations inhibited TNF-alpha-induced NF-kappaB binding in Jurkat cells. Pretreatment of cells with these compounds prior to stimulation prevented I kappaB alpha degradation. Phosphorylation of I kappaB alpha, a prerequisite for its signal-induced degradation, was abrogated in these cells, indicating that oxidative stress is an essential step in the NF-kappaB activation pathway. On the other hand, iron chelators desferrioxamine, pyridoxal isonicotinoyl hydrazone (PIH), and salicylaldehyde isonicotinoyl hydrazone (SIH) showed no inhibition of TNF-alpha-induced NF-kappaB DNA-binding activity. Synergistic induction of HIV-1 LTR-mediated gene expression by TNF-alpha and the HIV-1 transactivator Tat in Jurkat cells was significantly suppressed in the presence of NAC and TD, but not PDTC. The inhibition of NAC and TD on LTR-directed gene expression was diminished when NF-kappaB-binding sites in the LTR were deleted, indicating that these compounds affected the NF-kappaB component of the synergism. Iron chelators PIH and SIH also showed some inhibitory effect on LTR-mediated gene activation, presumably through an NF-kappaB-independent mechanism. These experiments demonstrate that TD, at concentration 50 times lower than the effective concentration of NAC, potently inhibits NF-kappaB activity and suppresses HIV LTR expression.

Mobilization of iron from neoplastic cells by some iron chelators is an energy-dependent process

Iron (Fe) chelators of the pyridoxal isonicotinoyl hydrazone (PIH) class may be useful agents to treat Fe overload disease and also cancer. These ligands possess high activity at mobilizing 59Fe from neoplastic cells, and the present study has been designed to examine whether their marked activity may be related to an energy-dependent transport process across the cell membrane. Initial experiments examined the release of 59Fe from SK-N-MC neuroblastoma (NB) cells prelabelled for 3 h at 37 degrees C with 59Fe-transferrin (1.25 microM) and then reincubated in the presence and absence of the chelators for 3 h at 4 degrees C or 37 degrees C. Prelabelled cells released 4-5% of total cellular 59Fe when reincubated in minimum essential medium at 4 degrees C or 37 degrees C. When the chelators desferrioxamine (DFO; 0.1 mM) or PIH (0.1 mM) were reincubated with labelled cells at 4 degrees C, they mobilized only 4-5% of cellular 59Fe, whereas as 37 degrees C, these ligands mobilized 21% and 48% of cell 59Fe, respectively. The lipophilic PIH analogue, 311 (2-hydroxy-1-naphthylaldehyde isonicotinoyl hydrazone; 0.1 mM), which exhibits high anti-proliferative activity, released 10% and 53% of cellular 59Fe when reincubated with prelabelled cells at 4 degrees C and 37 degrees C, respectively. Almost identical results were obtained using the SK-Mel-28 melanoma cell line. These data suggest that perhaps temperature-dependent mechanisms are essential for 59Fe mobilization from these cells. Interestingly, the metabolic inhibitors, 2,4-dinitrophenol, oligomycin, rotenone, and sodium azide, markedly decreased 59Fe mobilization mediated by PIH, but had either no effect or much less effect on 59Fe release by 311. Considering that an ATP-dependent process was involved in 59Fe release by PIH, further studies examined 4 widely used inhibitors of the multi-drug efflux pump P-glycoprotein (P-gp). All of these inhibitors, namely, verapamil (Ver), cyclosporin A (CsA), reserpine (Res) and quinine (Qui), decreased 59Fe mobilization by PIH but had little or no effect on 59Fe release mediated by analogue 311. Further, both CsA and Ver increased the proportion of ethanol-soluble 59Fe within cells in the presence of PIH, suggesting inhibited transport of the 59Fe complex from the cell. However, when PIH-mediated 59Fe release was compared between a well characterized Chinese hamster ovary cell line (CHRB30) expressing high levels of P-gp and the relevant control cell line (AuxB1), no appreciable difference in the kinetics of 59Fe release were found. In contrast, it was intriguing that the CHRB30 cells released more 59Fe into control medium (i.e., without PIH) than the AuxB1 control line (16.7% compared to 5.9%, respectively). In summary, the results suggest that a temperature- and energy-dependent process was involved in the efflux of the PIH-59Fe complex from the cells. In contrast, 59Fe release mediated by 311 was temperature-dependent but not energy-dependent, and could occur by simple diffusion or passive transport. Further studies investigating the membrane transport of Fe chelators may be useful in designing regimes that potentiate their anti-neoplastic effects.

Prevention of postasphyxia electroretinal dysfunction with a pyridoxal hydrazone

The newborn retina is particularly sensitive and frequently subjected to peroxidative stresses that result in visual sequelae. We compared two iron chelators, deferoxamine and a newer compound, pyridoxal isonicotinoyl hydrazone (PIH), in protecting the retina of newborn pigs (1-3 d old) from asphyxia-reoxygenation insults. Animals were treated IV with either saline, deferoxamine 15.2 mumol/kg (10 mg/kg) or PIH 34.8 mumol/kg (10 mg/kg); n = 10 in each treatment group. Scotopic and photopic electroretinograms (ERG) were recorded before and 40 min after drug treatment as well as 45 min following a 5-min period of asphyxia by interrupting ventilation. In separate animals the indices of peroxidation, malondialdehyde (MDA: TBARS) and hydroperoxides, were measured in retina at the same times. In saline-treated animals, there was a marked increase in MDA and hydroperoxide concentrations in the retina following the asphyxia-reoxygenation period. This was associated with a decrease in the a- (photoreceptor generated) and b-wave (generated by Müller and bipolar cells) amplitudes measured under photopic (cone-mediated response) and scotopic (rod-mediated response) conditions, and an increase in their implicit times. PIH and deferoxamine prevented the postasphyxial increase in MDA and hydroperoxides. However, only PIH prevented the postasphyxial changes in a- and b-wave amplitudes and implicit times, whereas deferoxamine markedly altered the preasphyxial ERG and provided only partial postasphyxial protection simply to the retinal outer segment. Our findings indicate that the iron chelator PIH effectively inhibits peroxidation and retinal electrophysiological alterations secondary to asphyxia-reoxygenation-induced oxidative stresses to newborn animals, whereas deferoxamine adversely affects retinal function; hence, PIH may be a preferred alternative to deferoxamine.