MAY 2007 VOLUME 20, NUMBER 5 © Copyright 2007 by the American Chemical Society
PerspectiVe Future of ToxicologysIron Chelators and Differing Modes of Action and Toxicity: The Changing Face of Iron Chelation Therapy Danuta S. Kalinowski and Des R. Richardson* Iron Metabolism and Chelation Program, Department of Pathology, UniVersity of Sydney, Sydney, New South Wales 2006, Australia ReceiVed February 3, 2007
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. 1. Introduction 2. 2-Pyridylcarboxaldehyde Isonicotinoyl Hydrazone Series 3. Di-2-Pyridylketone Isonicotinoyl Hydrazone Series 4. Di-2-Pyridylketone Thiosemicarbazone Series 5. 3-Aminopyridine-2-carboxaldehyde Thiosemicarbazone 6. Redox Activity and Donor Atoms 7. Conclusions
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1. Introduction Although iron (Fe) chelation therapy was initially designed to alleviate the toxic effects of excess Fe evident in Fe-overload * Corresponding author. Phone: +61-2-9036-6548. Fax: +61-2-90366549. E-mail:
[email protected].
diseases, the novel toxicological properties of some Fe chelator complexes have shifted their intended application to cancer chemotherapy (1, 2). Some Fe chelators are able to bind and inhibit the redox activity of Fe, making them ideal candidates for the treatment of Fe-overload conditions, such as β-thalassemia and Friedreich’s ataxia (3, 4). Such ligands are able to prevent excess Fe from participating in Fenton chemistry and inhibit the formation of reactive oxygen species (ROS1), such as the hydroxyl radical, which initiates oxidative damage (5). Excess free Fe and the resulting ROS generation have many toxicological consequences, resulting in oxidative insults to critical biomolecules including DNA, protein, and lipids (5, 6). This damage, which results in impaired cellular functions, is characteristic of Fe-overload diseases (7, 8). Hence, Fe chelators that remove excess Fe and form redox-inactive complexes provide a useful method of treatment in preventing the toxic effects of Fe overload.
10.1021/tx700039c CCC: $37.00 © 2007 American Chemical Society Published on Web 04/03/2007
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Figure 1. Chemical structures of the iron chelators. (A) Structures of the members of the PCIH series, including 2-pyridylcarboxaldehyde benzoyl hydrazone (PCBH), 2-pyridylcarboxaldehyde isonicotinoyl hydrazone (PCIH), 2-pyridylcarboxaldehyde p-hydroxybenzoyl hydrazone (PCHH), 2-pyridylcarboxaldehyde m-bromobenzoyl hydrazone (PCBBH), 2-pyridylcarboxaldehyde p-aminobenzoyl hydrazone (PCAH), and 2-pyridylcarboxaldehyde thiophenecarboxyl hydrazone (PCTH), and the PKIH series, including di-2-pyridylketone benzoyl hydrazone (PKBH), di-2-pyridylketone isonicotinoyl hydrazone (PKIH), di-2-pyridylketone p-hydroxybenzoyl hydrazone (PKHH), di-2-pyridylketone m-bromobenzoyl hydrazone (PKBBH), di-2-pyridylketone p-aminobenzoyl hydrazone (PKAH), and di-2-pyridylketone thiophenecarboxyl hydrazone (PKTH). (B) General mono-iron complex demonstrating the tridentate binding mode of both aroylhydrazones and thiosemicarbazones. (C) Chemical structures of the DpT series, including di-2-pyridylketone thiosemicarbazone (DpT), di-2-pyridylketone 4-methyl-3-thiosemicarbazone (Dp4mT), di-2-pyridylketone 4,4-dimethyl-3thiosemicarbazone (Dp44mT), di-2-pyridylketone 4-ethyl-3-thiosemicarbazone (Dp4eT), di-2-pyridylketone 4-allyl-3-thiosemicarbazone (Dp4aT), and di-2-pyridylketone 4-phenyl-3-thiosemicarbazone (Dp4pT). (D) Chemical structure of 3-aminopyridine-2-carboxaldehyde-thiosemicarbazone (3-AP).
However, some chelators are capable of enhancing the production of ROS after complexation with Fe. In contrast to those used for Fe-overload diseases, these ligands may promote the toxic effects of Fe for cancer chemotherapy (3, 9). By depleting rapidly proliferating cancer cells of Fe, Fe chelators can also inhibit the activity of ribonucleotide reductase (RR), an Fe-containing enzyme involved in the rate-limiting step of DNA synthesis (10-13). In addition, Fe depletion is known to affect the expression of molecules involved in cell cycle progression and growth, for example, N-myc downstream regulated gene 1, cyclin D1, p21WAF1 etc., leading to G1/S arrest (14-16). These effects, in combination with ROS generation, provide multiple mechanisms of action mediated by Fe chelation to inhibit tumor cell proliferation. This perspective describes the different modes of action and toxicological profiles of various tridentate Fe chelators, including analogues of 2-pyridylcarboxaldehyde isonicotinoyl hydrazone (PCIH, Figure 1A), di-2-pyridylketone isonicotinoyl hydrazone (PKIH, Figure 1A), and di-2-pyridylketone thiosemicarbazone 1 Abbreviations: 3-AP, 3-aminopyridine-2-carboxaldehyde thiosemicarbazone; bd, bi-daily; DpT, di-2-pyridylketone thiosemicarbazone; Dp44mT, di-2-pyridylketone 4,4-dimethyl-3-thiosemicarbazone; Dp4aT, di-2-pyridylketone 4-allyl-3-thiosemicarbazone; Dp4eT, di-2-pyridylketone 4-ethyl3-thiosemicarbazone; Dp4mT, di-2-pyridylketone 4-methyl-3-thiosemicarbazone; Dp4pT, di-2-pyridylketone 4-phenyl-3-thiosemicarbazone; PCAH, 2-pyridylcarboxaldehyde p-aminobenzoyl hydrazone; PCBH, 2-pyridylcarboxaldehyde benzoyl hydrazone; PCBBH, 2-pyridylcarboxaldehyde mbromobenzoyl hydrazone; PCHH, 2-pyridylcarboxaldehyde p-hydroxybenzoyl hydrazone; PCIH, 2-pyridylcarboxaldehyde isonicotinoyl hydrazone; PCTH, 2-pyridylcarboxaldehyde thiophenecarboxyl hydrazone; PKAH, di2-pyridylketone p-aminobenzoyl hydrazone; PKBH, di-2-pyridylketone benzoyl hydrazone; PKBBH, di-2-pyridylketone m-bromobenzoyl hydrazone; PKHH, di-2-pyridylketone p-hydroxybenzoyl hydrazone; PKIH, di2-pyridylketone isonicotinoyl hydrazone; PKTH, di-2-pyridylketone thiophenecarboxyl hydrazone; ROS, reactive oxygen species; RR, ribonucleotide reductase.
(DpT, Figure 1C). In addition, we discuss the tridentate chelator, 3-aminopyridine-2-carboxaldehyde-thiosemicarbazone (3-AP, also known as Triapine, Figure 1D), that has undergone clinical trials for its use as a chemotherapeutic agent against cancer (3). The potential application of all these ligands in the changing face of Fe chelation therapy is discussed.
2. 2-Pyridylcarboxaldehyde Isonicotinoyl Hydrazone Series The PCIH (Figure 1A) analogues were derived from the pyridoxal isonicotinoyl hydrazone series of chelators (3) and represent a class of ligands that are candidates for the treatment of Fe-overload disease because these effectively bind Fe and inhibit its toxic effects (17). This tridentate series use the pyridyl and imine nitrogens and carbonyl oxygen as donor atoms (NNO system, Figure 1B), binding Fe in a meridional fashion (18). Various in Vitro studies using tumor cells and cardiomyocytes illustrated the marked ability of 2-pyridylcarboxaldehyde thiophenecarboxyl hydrazone (PCTH, Figure 1A), 2-pyridylcarboxaldehyde benzoyl hydrazone (Figure 1A), and 2-pyridylcarboxaldehyde m-bromobenzoyl hydrazone (Figure 1A) to remove intracellular Fe and also to prevent the uptake of Fe from the Fe-transport protein, transferrin (19, 20). In addition, studies were designed to investigate the ability of the PCIH analogues to permeate the mitochondrion in order to test their potential to treat the Fe loading observed in the neuro- and cardio-degenerative disease, Friedreich’s ataxia (21). These studies utilized reticulocytes treated with the heme synthesis inhibitor, succinylacetone, which results in a mammalian model of mitochondrial Fe loading in culture (21). This investigation demonstrated that PCIH and PCTH were highly effective at mobilizing mitochondrial Fe and supported their potential for the treatment of Friedreich’s ataxia (21). Also, an in ViVo mouse study
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Figure 2. Iron chelators mediate antiproliferative activity via two mechanisms. Chelators are able to enter tumor cells and bind intracellular iron, depriving these cells of this important metal. Iron chelators can also form redox-active iron complexes, inducing cytotoxicity via Fenton chemistry.
demonstrated the low toxicity of PCTH when it was administered orally at 50 and 100 mg/kg/bi-daily (bd) for 3 weeks (19). Cumulatively, these results illustrate the potential of the PCIH series to act as Fe-chelating agents with low toxicity. In addition, the PCIH series of ligands were found to have low anti-proliferative effects on cells in culture (IC50: 40-50 µM (20)). This is an important criterion for developing Fe chelators that are suitable for the treatment of Fe-overload diseases, given that these agents will probably need to be given daily for the life of the patient. Further investigations revealed that the Fe(III) complexes of these ligands could not effectively redox cycle and thus did not enhance the oxidation of ascorbate, nor the hydroxylation of benzoate, and in fact, acted in a protective manner by inhibiting Fenton chemistry (22). This suggested that Fe complexes formed by the PCIH series are largely redoxinactive and prevent the oxidative damage catalyzed by free Fe. These results were further confirmed by the observation that no significant level of DNA damage was observed in intact human cells treated with these ligands (22). Hence, the low antiproliferative activity of the PCIH series relative to more cytotoxic chelators of similar structure (e.g., PKIH analogues) is probably related to their lack of redox activity. In addition, these compounds are possibly less effective at binding cellular Fe pools that are critical for RR activity and essential metabolic processes. In summary, the PCIH series of chelators effectively mobilize Fe and combat the toxic effects of excess Fe by forming Fe complexes that were not markedly redox-active.
More importantly, these ligands as a group, showed much higher anti-proliferative activity (IC50: 1-42 µM) than their parent PCIH analogues (24). Interestingly, this cytotoxic activity was found to be selective against neoplastic cells and had little effect on the normal fibroblast cell line, MRC-5 (24). The PKIH series were found to act via a mechanism different to that of their PCIH counterparts. The fact that Fe complexes of the PKIH series showed cytotoxic activity similar to that of the free ligands suggested that in addition to Fe depletion, other factors contributed toward their anti-proliferative activity (Figure 2) (24). Although the Fe(III) complexes of these ligands could not increase ascorbate oxidation, their Fe(II) complexes in the presence of H2O2 were able to stimulate benzoate hydroxylation and plasmid DNA degradation (25). Additionally, PKIH was found to increase the intracellular generation of ROS, and their anti-proliferative activity could be attenuated by catalase and superoxide dismutase, which alleviate redox stress (25). Furthermore, cyclic voltammetry of the Fe(II) complexes of the PKIH series revealed their complicated electrochemistry, which included nucleophilic attack on the CdN-Fe group by the hydroxide anion upon oxidation to the Fe(III) state (23). Thus, the potentially high oxidizing power of the PKIH series was attenuated to some degree by such irreversible electrochemistry. Overall, these studies suggest that redox cycling of the PKIH Fe complexes plays a role in their toxicity by enhancing the production of damaging ROS (23, 25).
4. Di-2-Pyridylketone Thiosemicarbazone Series 3. Di-2-Pyridylketone Isonicotinoyl Hydrazone Series Considering the high Fe chelation efficacy of the PCIH analogues and the potential of cytotoxic chelators to act as potent anti-tumor agents (3), a novel series of ligands were developed. As lipophilicity and membrane permeability play a critical role in Fe chelation efficacy, the aldehyde moiety of the PCIH series, namely, 2-pyridylcarboxaldehyde, was replaced with the more lipophilic di-2-pyridylketone moiety. These tridentate chelators, known as the PKIH analogues (Figure 1A), bind Fe in a manner similar to that of the PCIH series, again using the NNO donor atoms (Figure 1B (23)). The PKIH series were found to effectively promote the efflux of intracellular Fe and inhibit the uptake of Fe from transferrin by tumor cells in culture (24).
Recently, on the basis of the results described with the PKIH analogues above, even more effective Fe chelators were rationally designed for the treatment of cancer, namely, the DpT series (Figure 1C). This group of ligands incorporate the thiosemicarbazone moiety, which was observed to confer the potent anti-proliferative efficacy of the R-(N)-heterocyclic carboxaldehyde thiosemicarbazone family of chelators (26-28). To prepare the DpT series of chelators, the thiosemicarbazide group was condensed with the ketone component of the PKIH series, namely, di-2-pyridylketone. Being thiosemicarbazones, the DpT series utilize the pyridyl and imine nitrogens and sulfur as donor atoms (NNS system, Figure 1B), binding Fe in a 2:1 chelator to Fe ratio. These novel and patented thiosemicarba-
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zones were found to have high anti-proliferative activity against a number of tumor cell lines (i.e., for the most effective ligands IC50 was 0.01 µM) and the majority of the compounds showed high Fe chelation efficacy (29, 30). Like the less-active PKIH analogues, these highly cytotoxic chelators illustrated selective activity, having little effect on the growth of normal MRC-5 fibroblasts (29). Significantly, a number of in ViVo studies using mouse models have confirmed the anti-tumor activity of the DpT analogue, di-2-pyridylketone 4,4-dimethyl-3-thiosemicarbazone (Dp44mT, Figure 1C (29, 31)). In an initial study, intravenously administered Dp44mT (0.4 mg/kg/bd for 5 days) significantly decreased the tumor weight in mice bearing a chemotherapyresistant M109 lung carcinoma to 47% of the vehicle control (29). Importantly, there were no marked changes in animal weight nor hematological indices (29). More recently, Dp44mT was shown to have a pronounced effect on the net growth of a variety of human tumor xenografts in nude mice, including lung carcinoma, neuroepithelioma, ovarian carcinoma, and melanoma (31). For example, the growth of a melanoma xenograft in mice treated intravenously with Dp44mT at 0.4 mg/kg/day for 7 weeks was only 8% of that of the vehicle-treated control mice (31). At an optimal dose, no differences in weight loss or hematological indices were observed between Dp44mT-treated and control mice (31). Interestingly, no systemic Fe depletion was detected in Dp44mT-treated animals, probably because of the low doses of Dp44mT required to induce anti-tumor activity (31). These studies illustrated the selective anti-tumor effects of Dp44mT in ViVo and their strong potential for the treatment of cancer. In an attempt to understand the toxicological profile of the DpT series, a number of studies were undertaken to characterize the mechanisms involved in their cytotoxic activity (30). The Fe complexes of the DpT analogues illustrated anti-proliferative activity in cultured tumor cells, although this was significantly lower than that of the free ligand (30). The ability of their Fe complexes to mediate redox reactions, including the oxidation of ascorbate and the hydroxylation of benzoate, was measured and demonstrated that the majority of analogues were redoxactive (30). Some of the DpT ligands were found to induce Fedependent hydroxyl radical-mediated strand breaks in plasmid DNA (30). However, in contrast to the PKIH series, the electrochemistry of the Fe complexes of the DpT analogues showed facile interconversion between the Fe(II) and Fe(III) states, with the Fe(III) complex retaining its oxidizing ability (30). This was confirmed by the isolation of both the Fe(II)and Fe(III)-DpT series complexes, indicating the stable nature of both the ferric and ferrous states (30). The Fe-DpT series redox potentials were found to lie within the range accessible to both cellular oxidants and reductants, that is, E0 ) +153225 mV versus the normal hydrogen electrode, facilitating ROS generation under physiological conditions (30). Hence, the redox activity of the Fe complexes of these DpT analogues was marked. Cumulatively, these studies suggested that the mechanism of toxicity of the DpT series was mediated by the ability of their Fe complexes to form highly redox-active complexes that are able to induce ROS generation and subsequently initiate oxidative damage to important biomolecules (Figure 2).
5. 3-Aminopyridine-2-carboxaldehyde Thiosemicarbazone Another tridentate Fe chelator, known as 3-AP or Triapine (Figure 1D), belongs to the same general class of chelators as that of the DpT series, namely, the thiosemicarbazone family,
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and thus binds Fe using the NNS donor atoms (Figure 1B (32)). In cell culture studies, 3-AP illustrated some ability to mobilize intracellular Fe and prevent the uptake of Fe from transferrin, and was determined to have moderate anti-proliferative activity (IC50: 0.2 µM (32)). Similar to that in the PKIH and DpT chelators, the addition of Fe to 3-AP had no significant effect on its anti-proliferative activity, suggesting a possible role of the Fe complex in the cytotoxicity of 3-AP (32). Further studies were conducted to determine if the Fe complex of 3-AP was able to participate in Fenton chemistry because this could play a role in its anti-tumor activity (32). Importantly, the Fe complex of 3-AP demonstrated increased ability to mediate the oxidation of ascorbate, the hydroxylation of benzoate, and the Fe-dependent hydroxyl radical-mediated strand breaks in plasmid DNA in comparison to that of the control (32). Additionally, spin-trapping experiments using 5,5-dimethyl-1-pyrroline-N-oxide confirmed that the Fe(II) complex of 3-AP was capable of reducing O2 to give ROS (33). Together, these results revealed that the toxicological profile of 3-AP was highly dependent on the ability of its Fe complex to redox cycle, generate free radicals, and mediate oxidative damage (Figure 2) (32). Although 3-AP has only recently been noted to act as an Fe chelator (32), it has been long described as a potent RR inhibitor, and this has been illustrated in a number of studies (32, 3436). Mammalian RR is a tetramer consisting of two non-identical homodimers, R1 and either R2 or p53R2, which are involved in DNA replication or repair, respectively (12, 13, 37). Iron, located in the R2 and p53R2 subunits, stabilizes the tyrosine radical generated in these subunits (13). The reaction of oxygen with the binuclear Fe center of RR results in the formation of a tyrosine radical that gives a characteristic electron paramagnetic resonance (EPR) signal that is directly proportional to RR activity (38). Therefore, EPR measurements have been used to monitor the effects of Fe chelation on RR activity. An in Vitro EPR study demonstrated that 3-AP was able to reduce the tyrosyl radical signal and thus decrease RR activity in intact tumor cells (32). Furthermore, an in Vitro EPR study of R2 and p53R2 showed that the Fe(II) 3-AP complex was able to completely quench the tyrosyl radical signal (33). This finding was supported by an in Vitro activity assay which illustrated that the Fe(II) complex of 3-AP was a much more potent inhibitor of both R1/R2 and R1/p53R2 than 3-AP alone (33). It was also noted that the enzyme activity of RR was maintained with the use of catalase or catalase in combination with superoxide dismutase, which acts to alleviate oxidative stress (33). Additionally, these anti-oxidant enzymes markedly decreased the anti-proliferative activity of 3-AP in KB cells (33). These results suggested that the generation of ROS after the formation of the Fe complex plays a crucial role in the antiproliferative effects of 3-AP and its ability to inhibit RR activity (33). Clearly, as the results above suggest, the ability of thiosemicarbazones to form Fe complexes that participate in Fenton chemistry is a crucial mechanism in their toxicological profiles as anti-cancer agents (30). Various in ViVo studies using mouse models have been conducted, investigating the effects of 3-AP (31, 36). For instance, 3-AP (5 mg/kg) was found to inhibit the growth of L1210 leukemia in ViVo to a greater extent than the classical RR inhibitor, hydroxyurea (70 mg/kg (36)). 3-AP was also able to inhibit the growth of M109 mouse lung and A2780 ovarian carcinoma xenografts and was found to penetrate the bloodbrain barrier, effectively killing >95% of L1210 leukemia cells in the brain (36). More recently, a mouse study comparing the
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in ViVo activity of 3-AP (12 mg/kg) to that of Dp44mT (0.75 mg/kg) found that even at this much higher dose, 3-AP inhibited the growth of tumor xenografts to a similar or lesser extent than Dp44mT (31). Also, these 3-AP-treated mice showed an undesirable increase in serum alkaline phosphatase levels, splenic weight, hematopoeisis, anisocytosis, polychromasia, and liver Fe levels in comparison to the control (31). Mice treated with 3-AP also demonstrated a significant decrease in brain Fe levels (31). These effects are likely due to the considerable doses of 3-AP required to induce effective anti-tumor activity (31). In addition to animal studies, 3-AP has also undergone various clinical trials (39-41). One phase I clinical trial found that 3-AP caused prolonged stabilization of the disease, but no objective responses were observed (41). Moreover, at high doses of 3-AP (160 mg/m2/day), toxicity was evident, including neutropenia and hyperbilirubinemia, whereas a lower dose administered as a 96 h iv infusion (120 mg/m2/day) every 2 weeks was well tolerated (41). Other studies using 3-AP in combination with gemcitabine (2′,2′-difluoro-2′-deoxycytidine) in patients with advanced cancer have also been performed (40). Gemcitabine is a nucleoside analogue that can act as a DNA synthesis and RR inhibitor. Three of the 22 patients treated with the combination of these two drugs were observed to have an objective response and one showed evidence of reduced tumor size (40). Importantly, 3-AP has been suggested to cause the oxidation of hemoglobin to methemoglobin, which may have led to or contributed to hypoxia, acute hypotension, and EKG changes in patients (40). Obviously, the deleterious effects of the oxidation of heme-containing proteins is a factor relating to the redox activity of this compound that must be considered during its administration. The negative side effects observed with 3-AP may not apply to all Fe chelators. Due to the relatively low anti-tumor activity of 3-AP, large doses are necessary to observe an appreciable anti-tumor effect (31). Hence, the development of other Fe chelators, such as Dp44mT with potent anti-tumor activity and a higher therapeutic index, overcomes the problems of 3-AP because much lower doses can be administered (31).
6. Redox Activity and Donor Atoms Importantly, the choice of donor atom identity is a crucial factor in the ability of an Fe chelator complex to redox cycle and consequently determines its toxicological profile. High-spin Fe(III) forms stable bonds with “hard” donor atoms such as oxygen (42). Chelators containing oxygen donor atoms act to stabilize the ferric state and are unlikely to redox cycle under biological conditions (42). This was evident in the previously mentioned PCIH series, which are candidates for the treatment of Fe-overload diseases, forming Fe complexes that are not markedly redox-active (22). Ligands that are able to bind both Fe(II) and Fe(III) have the potential to redox cycle. Iron complexes of chelators utilizing “soft” donor atoms, such as the nitrogen and sulfur donor atoms used in the DpT series, can be enzymatically reduced under physiological conditions (42). The resulting Fe(II) can participate in Fenton chemistry, generating ROS and oxidative damage. Therefore, chelators containing “soft” donor atoms can be targeted for the treatment of cancer. Hence, the choice of donor atoms plays an important part in the toxicology of Fe chelators. Finally, the denticity of the ligands can be important in terms of redox activity. For instance, in hexadentate chelators, the coordination sphere is fully occupied, preventing the access of hydrogen peroxide or oxygen to the Fe center (3). In contrast,
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tridentate ligands can form partially dissociated complexes that render the Fe center exposed, which can lead to radical generation (3). Hence, the chelator-to-Fe ratio for tridentate ligands needs to be greater than that for hexadentate chelators to prevent redox activity.
7. Conclusions In the past, there has been little consideration regarding the mechanisms involved in the toxicological effects of Fe chelators and the consequences of these in terms of designing ligands for the treatment of Fe overload or cancer. As discussed above, it is obvious that some of the toxicological effects are directly related to specific ligand moieties. For example, features such as lipophilicity and donor atom identity impart important chelator characteristics. Increased lipophilicity has previously been shown to increase the cytotoxic effects due to the enhanced ability of the ligand to permeate the cell membrane and gain access to intracellular Fe pools (43). The careful choice of donor atom identity, such as “soft” donors (e.g., N and S), results in redox-active Fe complexes that can be utilized in the design of ligands for the treatment of cancer. However, the use of “hard” donor atoms (e.g., O), which stabilize the ferric state, result in redox-inactive complexes suitable for the treatment of Feoverload diseases. Obviously, such structural characteristics play an important role in the differing modes of action and toxicity of Fe chelators in the changing face of Fe chelation therapy. To comment on this or other Future of Toxicology perspectives, please visit our Perspectives Open Forum at http:// pubs.acs.org/journals/crtoec/openforum. Acknowledgment. We thank Dr. Erika Becker, Dr. David Lovejoy, Dr. Dong Fu, Dr. Robert Sutak, Dr. Daniel Vyoral, Ms. Zaklina Kovacevic, Mr. Yohan Suryo Rahmanto, and Ms. Megan Whitnall for careful assessment of the manuscript prior to submission. The work discussed in this perspective was performed under grants from the NHMRC, ARC, MDA USA, and Friedreich’s Ataxia Research Alliance of Australia and USA. D.S.K. thanks the University of New South Wales and the University of Sydney for an Australian Postgraduate Award.
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