Desferrithiocin: a search for clinically effective iron chelators

Targeting Labile Iron-Mediated Ferroptosis Provides a Potential Therapeutic Strategy for Rhabdomyolysis-Induced Acute Kidney Injury

Acute kidney injury (AKI) is a global health problem that occurs in a variety of clinical settings. Despite some advances in supportive clinical care, no medicinal intervention has been demonstrated to reliably prevent AKI thus far. Therefore, it is highly necessary to investigate the pathophysiology and mechanisms involved in AKI for the discovery of therapeutics. In the current study, a robust change in the level of renal malondialdehyde (MDA) and 4-hydroxynonenal (4-HNE) and elevated renal iron levels were observed in murine rhabdomyolysis-induced AKI (RM-AKI), which supports a pathogenic role of labile iron-mediated ferroptosis and provides a chance to utilize iron chelation for RM-AKI prevention. Given that the existing small molecule-based iron chelators did not show promising preventative effects against RM-AKI, we further designed and synthesized a new hydroxypyridinone-based iron chelator to potently inhibit labile iron-mediated ferroptosis. Lead compound AKI-02 was identified, which remarkably protected renal proximal tubular epithelial cells from ferroptosis as well as showed excellent iron chelation ability. Moreover, administration of AKI-02 led to renal function recovery, a result that was substantiated by the decreased contents of BUN and creatinine, as well as the reduced labile iron level and improved histopathology. Thus, our studies highlighted that targeting labile iron-mediated ferroptosis could provide therapeutic benefits against RM-AKI.

Intracellular Iron Binding and Antioxidant Activity of Phytochelators

Phytochelators have been studied as templates for designing new drugs for chelation therapy. This work evaluated key chemical and biological properties of five candidate phytochelators for iron overload diseases: maltol, mimosine, morin, tropolone, and esculetin. Intra- and extracellular iron affinity and antioxidant activity, as well as the ability to scavenge iron from holo-transferrin, were studied in physiologically relevant settings. Tropolone and mimosine (and, to a lesser extent, maltol) presented good binding capacity for iron, removing it from calcein, a high-affinity fluorescent probe. Tropolone and mimosine arrested iron-mediated oxidation of ascorbate with the same efficiency as the standard iron chelator DFO. Also, both were cell permeant and able to access labile pools of iron in HeLa and HepG2 cells. Mimosine was an effective antioxidant in cells stressed by iron and peroxide, being as efficient as the cell-permeant iron chelator deferiprone. These results reinforce the potential of those molecules, especially mimosine, as adjuvants in treatments for iron overload.

Role of Iron in the Molecular Pathogenesis of Diseases and Therapeutic Opportunities

Iron is an essential mineral that serves as a prosthetic group for a variety of proteins involved in vital cellular processes. The iron economy within humans is highly conserved in that there is no proper iron excretion pathway. Therefore, iron homeostasis is highly evolved to coordinate iron acquisition, storage, transport, and recycling efficiently. A disturbance in this state can result in excess iron burden in which an ensuing iron-mediated generation of reactive oxygen species imparts widespread oxidative damage to proteins, lipids, and DNA. On the contrary, problems in iron deficiency either due to genetic or nutritional causes can lead to a number of iron deficiency disorders. Iron chelation strategies have been in the works since the early 1900s, and they still remain the most viable therapeutic approach to mitigate the toxic side effects of excess iron. Intense investigations on improving the efficacy of chelation strategies while being well tolerated and accepted by patients have been a particular focus for many researchers over the past 30 years. Moreover, recent advances in our understanding on the role of iron in the pathogenesis of different diseases (both in iron overload and iron deficiency conditions) motivate the need to develop new therapeutics. We summarized recent investigations into the role of iron in health and disease conditions, iron chelation, and iron delivery strategies. Information regarding small molecule as well as macromolecular approaches and how they are employed within different disease pathogenesis such as primary and secondary iron overload diseases, cancer, diabetes, neurodegenerative diseases, infections, and in iron deficiency is provided.

Is Chelation Therapy a Potential Treatment for Parkinson's Disease?

Iron loading in some brain regions occurs in Parkinson’s Disease (PD), and it has been considered that its removal by iron chelators could be an appropriate therapeutic approach. Since neuroinflammation with microgliosis is also a common feature of PD, it is possible that iron is sequestered within cells as a result of the “anaemia of chronic disease” and remains unavailable to the chelator. In this review, the extent of neuroinflammation in PD is discussed together with the role played by glia cells, specifically microglia and astrocytes, in controlling iron metabolism during inflammation, together with the results of MRI studies. The current use of chelators in clinical medicine is presented together with a discussion of two clinical trials of PD patients where an iron chelator was administered and showed encouraging results. It is proposed that the use of anti-inflammatory drugs combined with an iron chelator might be a better approach to increase chelator efficacy.

Hydroxypyridinone-Based Iron Chelators with Broad-Ranging Biological Activities

Iron plays an essential role in all living cells because of its unique chemical properties. It is also the most abundant trace element in mammals. However, when iron is present in excess or inappropriately located, it becomes toxic. Excess iron can become involved in free radical formation, resulting in oxidative stress and cellular damage. Iron chelators are used to treat serious pathological disorders associated with systemic iron overload. Hydroxypyridinones stand out for their outstanding chelation properties, including high selectivity for Fe³⁺ in the biological environment, ease of derivatization, and good biocompatibility. Herein, we overview the potential for multifunctional hydroxypyridinone-based chelators to be used as therapeutic agents against a wide range of diseases associated either with systemic or local elevated iron levels.

Iron Pathways and Iron Chelation Approaches in Viral, Microbial, and Fungal Infections

Iron is an essential element required to support the health of organisms. This element is critical for regulating the activities of cellular enzymes including those involved in cellular metabolism and DNA replication. Mechanisms that underlie the tight control of iron levels are crucial in mediating the interaction between microorganisms and their host and hence, the spread of infection. Microorganisms including viruses, bacteria, and fungi have differing iron acquisition/utilization mechanisms to support their ability to acquire/use iron (e.g., from free iron and heme). These pathways of iron uptake are associated with promoting their growth and virulence and consequently, their pathogenicity. Thus, controlling microorganismal survival by limiting iron availability may prove feasible through the use of agents targeting their iron uptake pathways and/or use of iron chelators as a means to hinder development of infections. This review will serve to assimilate findings regarding iron and the pathogenicity of specific microorganisms, and furthermore, find whether treating infections mediated by such organisms via iron chelation approaches may have potential clinical benefit.

The Efficacy of Iron Chelators for Removing Iron from Specific Brain Regions and the Pituitary-Ironing out the Brain

Iron chelation therapy, either subcutaneous or orally administered, has been used successfully in various clinical conditions. The removal of excess iron from various tissues, e.g., the liver spleen, heart, and the pituitary, in beta thalassemia patients, has become an essential therapy to prolong life. More recently, the use of deferiprone to chelate iron from various brain regions in Parkinson’s Disease and Friederich’s Ataxia has yielded encouraging results, although the side effects, in <2% of Parkinson’s Disease(PD) patients, have limited its long-term use. A new class of hydroxpyridinones has recently been synthesised, which showed no adverse effects in preliminary trials. A vital question remaining is whether inflammation may influence chelation efficacy, with a recent study suggesting that high levels of inflammation may diminish the ability of the chelator to bind the excess iron.

Suppressive effects of iron chelation in clear cell renal cell carcinoma and their dependency on VHL inactivation

Increasing data implicate iron accumulation in tumorigenesis of the kidney, particularly the clear cell renal cell carcinoma (ccRCC) subtype. The von Hippel Lindau (VHL)/hypoxia inducible factor-α (HIF-α) axis is uniquely dysregulated in ccRCC and is a major regulator and regulatory target of iron metabolism, yet the role of iron in ccRCC tumorigenesis and its potential interplay with VHL inactivation remains unclear. We investigated whether ccRCC iron accumulation occurs due to increased cell dependency on iron for growth and survival as a result of VHL inactivation. Free iron levels were compared between four VHL-mutant ccRCC cell lines (786-0, A704, 769-P, RCC4) and two benign renal tubule epithelial cell lines (RPTEC, HRCEp) using the Phen Green SK fluorescent iron stain. Intracellular iron deprivation was achieved using two clinical iron chelator drugs, deferasirox (DFX) and deferoxamine (DFO), and chelator effects were measured on cell line growth, cell cycle phase, apoptosis, HIF-1α and HIF-2α protein levels and HIF-α transcriptional activity based on expression of target genes CA9, OCT4/POU5F1 and PDGFβ/PDGFB. Similar assays were performed in VHL-mutant ccRCC cells with and without ectopic wild-type VHL expression. Baseline free iron levels were significantly higher in ccRCC cell lines than benign renal cell lines. DFX depleted cellular free iron more rapidly than DFO and led to greater growth suppression of ccRCC cell lines (>90% at ~30-150 µM) than benign renal cell lines (~10-50% at up to 250 µM). Similar growth responses were observed using DFO, with the exception that a prolonged treatment duration was necessary to deplete cellular iron adequately for differential growth suppression of the less susceptible A704 ccRCC cell line relative to benign renal cell lines. Apoptosis and G1-phase cell cycle arrest were identified as potential mechanisms of chelator growth suppression based on their induction in ccRCC cell lines but not benign renal cell lines. Iron chelation in ccRCC cells but not benign renal cells suppressed HIF-1α and HIF-2α protein levels and transcriptional activity, and the degree and timing of HIF-2α suppression correlated with the onset of apoptosis. Restoration of wild-type VHL function in ccRCC cells was sufficient to prevent chelator-induced apoptosis and G1 cell cycle arrest, indicating that ccRCC susceptibility to iron deprivation is VHL inactivation-dependent. In conclusion, ccRCC cells are characterized by high free iron levels and a cancer-specific dependency on iron for HIF-α overexpression, cell cycle progression and apoptotic escape. This iron dependency is introduced by VHL inactivation, revealing a novel interplay between VHL/HIF-α dysregulation and ccRCC iron metabolism. Future study is warranted to determine if iron deprivation using chelator drugs provides an effective therapeutic strategy for targeting HIF-2α and suppressing tumor progression in ccRCC patients.

Chemical features of in use and in progress chelators for iron overload

An excessive amount of iron may become extremely toxic both for its ability to generate reactive oxygen species, and for the lack of regulatory mechanisms for iron excretion in humans. Chelation therapy has been introduced in clinical practice in the 1970's to defend thalassemia patients from the effects of iron overload and it has dramatically changed both life expectancy and quality of life. The disadvantages of the drugs in clinical use make the research for new, more suitable iron chelating agents, urgent. This review defines the requirements of an iron chelator, then points out the principal chemical features of the iron chelators in use. Finally, a survey on the last ten years of the literature relative to iron chelators is done, and the most interesting ligands are presented, with particular emphasis to those that reached clinical trials.

Siderophores as molecular tools in medical and environmental applications

Almost all life forms depend on iron as an essential micronutrient that is needed for electron transport and metabolic processes. Siderophores are low-molecular-weight iron chelators that safeguard the supply of this important metal to bacteria, fungi and graminaceous plants. Although animals and the majority of plants do not utilise siderophores and have alternative means of iron acquisition, siderophores have found important clinical and agricultural applications. In this review, we will highlight the different uses of these iron-chelating molecules.

Design and synthesis of novel pegylated iron chelators with decreased metabolic rate

Background: Deferiprone has proved to be a successful iron selective chelator in a range of pathologies. However, its use is limited by rapid Phase II metabolism, necessitating the administration of large doses. In an attempt to modify metabolic rate of this class of compounds, a range of pegylated 3-hydroxypyridin-4-ones has been synthesized. Experimental: The synthetic route in which the polyethylene glycol counterparts are introduced to a protected pyran ring involves either a Williamson etherification reaction or direct addition leading to polyethylene glycol-containing precursors. Results & discussion: The introduction of the pegylated substituent was found to lead to a relatively low rate of metabolism for some of the derivatives (6a, 6b, 8a and 8b), offering a possible improvement over deferiprone.

Recent advances in cancer treatment by iron chelators

The development of new therapeutic alternatives for cancers is a major public health priority. Among the more promising approaches, the iron depletion strategy based on metal chelation in the tumoral environment has been particularly studied in recent decades. After a short description of the importance of iron for cancer cell proliferation, we will review the different iron chelators developed as potential chemotherapeutics. Finally, the recent efforts to vectorize the chelating agents specifically in the microtumoral environment will be discussed in detail.

Metabolically programmed iron chelators

Extensive structure activity relationship (SAR) studies focused on the desferrithiocin [DFT, (S)-4,5-dihydro-2-(3-hydroxy-2-pyridinyl)-4-methyl-4-thiazolecarboxylic acid] pharmacophore have led to three different DFT analogs being evaluated clinically for the treatment of iron overload diseases, for example, thalassemia. The SAR work revealed that the lipophilicity of a ligand, as determined by its partition between octanol and water, logP(app), could have a profound effect on the drug's iron clearing efficiency (ICE), organ distribution, and toxicity profile. While within a given structural family the more lipophilic a chelator the better the ICE, unfortunately, the more lipophilic ligands are often more toxic. Thus, a balance between lipophilicity, ICE, and toxicity must be achieved. In the current study, we introduce the concept of 'metabolically programmed' iron chelators, that is, highly lipophilic, orally absorbable, effective deferration agents which, once absorbed, are quickly converted to their nontoxic, hydrophilic counterparts.