Design of iron chelators with therapeutic application

Skin complications during iron chelation therapy for beta-thalassemia: overview and treatment approach

Thalassemia is an inherited genetic disorder of hemoglobin that affects a large population worldwide, and it is estimated that between 50,000 and 60,000 infants with thalassemia are born each year. The most common treatment for thalassemia is blood transfusion, which leads to iron overload. This in itself is a serious clinical condition, and is commonly managed with iron chelation therapy. However, iron chelators can cause various skin complications, including hyperpigmentation, skin rash, itching, and photosensitivity. These skin side effects can impact patients' quality of life. Therefore, this article provides a comprehensive overview of skin complications caused by iron chelators, along with a proposed comprehensive approach to their management in patients with beta-thalassemia. Key strategies include patient education, regular skin assessment, sun protection measures, symptomatic relief with topical corticosteroids and antihistamines, and consideration of treatment modification if severe complications occur. Collaboration between hematologists and dermatologists, along with psychological support and regular follow-up, is an essential component of this multidisciplinary approach. By implementing these strategies, healthcare providers can optimize skin care for patients with beta-thalassemia treated with iron chelators and improve their quality of life.

Enhancement of growth media for extreme iron limitation in Escherichia coli

Iron is an essential nutrient for microbial growth and bacteria have evolved numerous routes to solubilize and scavenge this biometal, which is often present at very low concentrations in host tissue. We recently used a MOPS-based medium to induce iron limitation in Escherichia coli K-12 during the characterization of novel siderophore-conjugated antibiotics. In this study we confirm that growth media derived from commercially available M9 salts are unsuitable for studies of iron-limited growth, probably through the contamination of the sodium phosphate buffer components with over 100 µM iron. In contrast, MOPS-based media that are treated with metal-binding Chelex resin allow the free iron concentration to be reduced to growth-limiting levels. Despite these measures a small amount of E. coli growth is still observed in these iron-depleted media. By growing E. coli in conditions that theoretically increase the demand for iron-dependent enzymes, namely by replacing the glucose carbon source for acetate and by switching to a microaerobic atmosphere, we can reduce background growth even further. Finally, we demonstrate that by adding an exogeneous siderophore to the growth media which is poorly used by E. coli, we can completely prevent growth, perhaps mimicking the situation in host tissue. In conclusion, this short study provides practical experimental insight into low iron media and how to augment the growth conditions of E. coli for extreme iron-limited growth.

Repurposing iron chelators for accurate positron emission tomography imaging tracking of radiometal-labeled cell transplants

The use of radiolabeled cells for positron emission tomography (PET) imaging tracking has been a promising approach for monitoring cell-based therapies. However, the presence of free radionuclides released from dead cells during tracking can interfere with the signal from living cells, leading to inaccurate results. In this study, the effectiveness of the iron chelators deferoxamine (DFO) and deferiprone in removing free radionuclides 89Zr and 68Ga, respectively, was demonstrated in vivo utilizing PET imaging. The use of DFO during PET imaging tracking of 89Zr-labeled mesenchymal stem cells (MSCs) significantly reduced uptake in bone while preserving uptake in major organs, resulting in more accurate and reliable tracking. Furthermore, the clearance of free 89Zr in vivo resulted in a significant reduction in radiation dose from 89Zr-labeled MSCs. Additionally, the avoidance of free radionuclide accumulation in bone allowed for more precise observation of the homing process and persistence during bone marrow transplantation. The efficacy and safety of this solution suggest this finding has potential for widespread use in imaging tracking studies involving various cells. Moreover, since this method employed iron chelator drugs in clinical use, which makes it is a good prospect for clinical translation.

Design of Tetrazolium Cations for the Release of Antiproliferative Formazan Chelators in Mammalian Cells

Cancer cells generally present a higher demand for iron, which plays crucial roles in tumor progression and metastasis. This iron addiction provides opportunities to develop broad spectrum anticancer drugs that target iron metabolism. In this context, prochelation approaches are investigated to release metal-binding compounds under specific conditions, thereby limiting off-target toxicity. Here, we demonstrate a prochelation strategy inspired by the bioreduction of tetrazolium cations widely employed to assess the viability of mammalian cells. We designed a series of tetrazolium-based compounds for the intracellular release of metal-binding formazan ligands. The combination of reduction potentials appropriate for intracellular reduction and an N-pyridyl donor on the formazan scaffold led to two effective prochelators. The reduced formazans bind as tridentate ligands and stabilize low-spin Fe(II) centers in complexes of 2:1 ligand-to-metal stoichiometry. The tetrazolium salts are stable in blood serum for over 24 h, and antiproliferative activities at micromolar levels were recorded in a panel of cancer cell lines. Additional assays confirmed the intracellular activation of the prochelators and their ability to affect cell cycle progression, induce apoptotic death, and interfere with iron availability. Demonstrating the role of iron in their intracellular effects, the prochelators impacted the expression levels of key iron regulators (i.e., transferrin receptor 1 and ferritin), and iron supplementation mitigated their cytotoxicity. Overall, this work introduces the tetrazolium core as a platform to build prochelators that can be tuned for activation in the reducing environment of cancer cells and produce antiproliferative formazan chelators that interfere with cellular iron homeostasis.

Synthesis of Modular Desferrioxamine Analogues and Evaluation of Zwitterionic Derivatives for Zirconium Complexation

The natural siderophore desferrioxamine B (DFOB) has been used for targeted PET imaging with 89 Zr before. However, Zr-DFOB has a limited stability and a number of derivatives have been developed with improved chelation properties for zirconium. We describe the synthesis of pseudopeptidic analogues of DFOB with azido side chains. These are termed AZA-DFO (hexadentate) and AZA-DFO* (octadentate) and are assembled via a modular synthesis from Orn-β-Ala and Lys-β-Ala. Nine different chelators have been conjugated to zwitterionic moieties by copper-catalyzed azide-alkyne cycloaddition (CuAAC). The resulting water-soluble chelators form Zr complexes under mild conditions (room temperature for 90 min). Transchelation assays with 1000-fold excess of EDTA and 300-fold excess of DFOB revealed that a short spacing of hydroxamates in (Orn-β-Ala)3-4 leads to improved complex stability compared to a longer spacing in (Lys-β-Ala)3-4 . We found that the alignment of amide groups in the pseudopeptide backbone and the presence of zwitterionic sidechains did not compromise the stability of the Zr-complexes with our chelators. We believe that the octadentate derivative AZA-DFO* is particularly valuable for the preparation of new Zr-chelators for targeted imaging which combine tunable pharmacokinetic properties with high complex stability and fast Zr-complexation kinetics.

Siderophore-Linked Ruthenium Catalysts for Targeted Allyl Ester Prodrug Activation within Bacterial Cells

Due to rising resistance, new antibacterial strategies are needed, including methods for targeted antibiotic release. As targeting vectors, chelating molecules called siderophores that are released by bacteria to acquire iron have been investigated for conjugation to antibacterials, leading to the clinically approved drug cefiderocol. The use of small-molecule catalysts for prodrug activation within cells has shown promise in recent years, and here we investigate siderophore-linked ruthenium catalysts for the activation of antibacterial prodrugs within cells. Moxifloxacin-based prodrugs were synthesised, and their catalyst-mediated activation was demonstrated under anaerobic, biologically relevant conditions. In the absence of catalyst, decreased antibacterial activities were observed compared to moxifloxacin versus Escherichia coli K12 (BW25113). A series of siderophore-linked ruthenium catalysts were investigated for prodrug activation, all of which displayed a combinative antibacterial effect with the prodrug, whereas a representative example displayed little toxicity against mammalian cell lines. By employing complementary bacterial growth assays, conjugates containing siderophore units based on catechol and azotochelin were found to be most promising for intracellular prodrug activation.

Hydroxypyrone derivatives in drug discovery: from chelation therapy to rational design of metalloenzyme inhibitors

The versatile structural motif of hydroxypyrone is found in natural products and can be easily converted into hydroxypyridone and hydroxythiopyridone analogues. The favourable toxicity profile and ease of functionalization to access a vast library of compounds make them an ideal structural scaffold for drug design and discovery. This versatile scaffold possesses excellent metal chelating properties that can be exploited for chelation therapy in clinics. Deferiprone [1,2-dimethyl-3-hydroxy-4(1H)-one] was the first orally active chelator to treat iron overload in thalassemia major. Metal complexes of hydroxy-(thio)pyr(id)ones have been investigated as magnetic resonance imaging contrast agents, and anticancer and antidiabetic agents. In recent years, this compound class has demonstrated potential in discovering and developing metalloenzyme inhibitors. This review article summarizes recent literature on hydroxy-(thio)pyr(id)ones as inhibitors for metalloenzymes such as histone deacetylases, tyrosinase and metallo-β-lactamase. Different approaches to the design of hydroxy-(thio)pyr(id)ones and their biological properties against selected metalloenzymes are discussed.

Safety and efficacy of a novel iron chelator (HBED; (N,N'-Di(2-hydroxybenzyl)ethylenediamine-N,N'-diacetic acid)) in equine (Equus caballus) as a model for black rhinoceros (Diceros bicornis)

While iron overload disorder (IOD) and related disease states are not considered a common occurrence in domestic equids, these issues appear prevalent in black rhinoceroses under human care. In addressing IOD in black rhinos, altering dietary iron absorption and excretion may be the most globally practical approach. A main option for treatment used across other species such as humans, is chelation therapy using iron‐specific synthetic compounds. As horses may serve as an appropriate digestive model for the endangered rhinoceros, we evaluated the potential use of the oral iron chelator N,N‐bis(2‐hydroxybenzyl)ethylenediamine‐N,N‐diacetic acid (HBED) in horses for safety and efficacy prior to testing in black rhinoceros. Health and iron digestibility and dynamics were assessed in horses (n = 6) before, and after treatment with HBED (50 mg/kg body weight) for 8 days using a crossover design with serum, faecal and urine collection. A preliminary pharmacokinetic trial was also performed but no trace of HBED was found in serially sampled plasma through 8 h post‐oral dosing. HBED increased urinary iron output in horses compared to control by 0.7% of total iron intake (p < 0.01), for an average of 27 mg urinary iron/day, similar to human chelation goals. Blood chemistry, blood cell counts and overall wellness were not affected by treatment. As healthy horses are able to regulate iron absorption, the lack of change in iron balance is unsurprising. Short‐term HBED administration appeared to be safely tolerated by horses, therefore it was anticipated it would also be safe to administer to black rhinos for the management of iron overload.

Advances in Evaluation of Antioxidant and Toxicological Properties of Stryphnodendron rotundifolium Mart. in Drosophila melanogaster Model

This study investigated the flavonoid content, antioxidant activity, and toxicological properties of the acetone–water fraction of stem bark of Stryphnodendron rotundifolium Mart. (TFSR). The total flavonoid content and antioxidant activity were determined, as typified by DPPH^● and ABTS^●+ radical scavenging abilities, Fe³⁺ reducing antioxidant power (FRAP), relative antioxidant capacity (RAC), and the inhibition of thiobarbituric acid reactive species (TBARs) in Drosophila melanogaster tissue. Toxicity and locomotor functions were evaluated in adult D. melanogaster flies through aging and survival assays, startle-induced negative geotaxis, and centrophobic responses with video-assisted open field motion tracking. The flavonoid content of dry TFSR (DF) was 3.36 mg quercetin/g. Furthermore, the significant antioxidant activity of TFSR was revealed through scavenging 95.3% of the ABTS^●+ radical and 82.4% of the DPPH^● radical, as well reducing 74.7% of Fe³⁺ in the FRAP assay and 80% Mo⁶⁺ in the RAC assay. TFSR conferred 70.25% protection against lipid peroxidation in Drosophila tissue. Survival rates ranged from 84.65 to 103.98% in comparison to the non-supplemented control and no evident deterioration of locomotor functions and centrophobia responses was observed. These results revealed that TFSR has potent antioxidant activity and low toxicity in vivo, profiling TFSR as a promising natural product in the treatment/management of iron overload and associated conditions.

Hydroxypyridinone-Based Metal Chelators towards Ecotoxicity: Remediation and Biological Mechanisms

Hydroxypyridinones (HPs) are recognized as excellent chemical tools for engineering a diversity of metal chelating agents, with high affinity for hard metal ions, exhibiting a broad range of activities and applications, namely in medical, biological and environmental contexts. They are easily made and functionalizable towards the tuning of their pharmacokinetic properties or the improving of their metal complex thermodynamic stabilities. In this review, an analysis of the recently published works on hydroxypyridinone-based ligands, that have been mostly addressed for environmental applications, namely for remediation of hard metal ion ecotoxicity in living beings and other biological matrices is carried out. In particular, herein the most recent developments in the design of new chelating systems, from bidentate mono-HP to polydentate multi-HP derivatives, with a structural diversity of soluble or solid-supported backbones are outlined. Along with the ligand design, an analysis of the relationship between their structures and activities is presented and discussed, namely associated with the metal affinity and the thermodynamic stability of the corresponding metal complexes.

Synthesis, physicochemical characterization and neuroprotective evaluation of novel 1-hydroxypyrazin-2(1H)-one iron chelators in an in vitro cell model of Parkinson's disease

Iron dysregulation, dopamine depletion, cellular oxidative stress and α-synuclein protein mis-folding are key neuronal pathological features seen in the progression of Parkinson's disease. Iron chelators endowed with one or more therapeutic modes of action have long been suggested as disease modifying therapies for its treatment. In this study, novel 1-hydroxypyrazin-2(1H)-one iron chelators were synthesized and their physicochemical properties, iron chelation abilities, antioxidant capacities and neuroprotective effects in a cell culture model of Parkinson's disease were evaluated. Physicochemical properties (log β, log D7.4, pL0.5) suggest that these ligands have a poorer ability to penetrate cell membranes and form weaker iron complexes than the closely related 1-hydroxypyridin-2(1H)-ones. Despite this, we show that levels of neuroprotection provided by these ligands against the catecholaminergic neurotoxin 6-hydroxydopamine in vitro were comparable to those seen previously with the 1-hydroxypyridin-2(1H)-ones and the clinically used iron chelator Deferiprone, with two of the ligands restoring cell viability to ≥89% compared to controls. Two of the ligands were endowed with additional phenol moieties in an attempt to derive multifunctional chelators with dual iron chelation/antioxidant activity. However, levels of neuroprotection with these ligands were no greater than ligands lacking this moiety, suggesting the neuroprotective properties of these ligands are due primarily to chelation and passivation of intracellular labile iron, preventing the generation of free radicals and reactive oxygen species that otherwise lead to the neuronal cell death seen in Parkinson's disease.

Diversity in the Interaction of Amino Acid- and Peptide-Based Hydroxamic Acids with Some Platinum Group Metals in Solution

Complexes that incorporate both ligand(s) and metal(s) exhibiting cytotoxic activity can especially be interesting to develop multifunctional drug molecules with desired activities. In this review, the limited number of solution results collected in our laboratory on the complexes of Pd(II) and two other platinum group metals—the half-sandwich type, [(η⁶-p-cym)Ru(H₂O)₃]²⁺, and [(η⁵-Cp*)Rh(H₂O)₃]²⁺—with hydroxamic acid derivatives of three amino acids, two imidazole analogues, and four small peptides are summarized and evaluated. Unlike the limited number of coordination sites of these metal ions (four and three for Pd(II) and the organometallic cations, respectively), the ligands discussed here offer a relatively high number of donor atoms as well as variation in their position within the ligands, resulting in a large versatility of the likely coordination modes. The review, besides presenting the solution equilibrium results, also discusses the main factors, such as (N,N) versus (O,O) chelate; size of chelate; amino-N versus imidazole-N; primary versus secondary hydroxamic function; differences between hydrolytic ability of the metal ions studied; and hydrolysis of the coordinated peptide hydroxamic acids in their Pd(II) complexes, which all determine the coordination modes present in the complexes formed in measurable concentrations in these systems. The options for the quantitative evaluation of metal binding effectivity and selectivity of the various ligands and the comparison with each other by using solution equilibrium data are also discussed.

Ten Years Progress of Electrical Detection of Heavy Metal Ions (HMIs) Using Various Field-Effect Transistor (FET) Nanosensors: A Review

Heavy metal pollution remains a major concern for the public today, in line with the growing population and global industrialization. Heavy metal ion (HMI) is a threat to human and environmental safety, even at low concentrations, thus rapid and continuous HMI monitoring is essential. Among the sensors available for HMI detection, the field-effect transistor (FET) sensor demonstrates promising potential for fast and real-time detection. The aim of this review is to provide a condensed overview of the contribution of certain semiconductor substrates in the development of chemical and biosensor FETs for HMI detection in the past decade. A brief introduction of the FET sensor along with its construction and configuration is presented in the first part of this review. Subsequently, the FET sensor deployment issue and FET intrinsic limitation screening effect are also discussed, and the solutions to overcome these shortcomings are summarized. Later, we summarize the strategies for HMIs' electrical detection, mechanisms, and sensing performance on nanomaterial semiconductor FET transducers, including silicon, carbon nanotubes, graphene, AlGaN/GaN, transition metal dichalcogenides (TMD), black phosphorus, organic and inorganic semiconductor. Finally, concerns and suggestions regarding detection in the real samples using FET sensors are highlighted in the conclusion.

Quercetin attenuates neurotoxicity induced by iron oxide nanoparticles

Iron oxide nanoparticles (IONPs) have been proposed as targeted carriers to deliver therapeutic molecules in the central nervous system (CNS). However, IONPs may damage neural tissue via free iron accumulation, protein aggregation, and oxidative stress. Neuroprotective effects of quercetin (QC) have been proven due to its antioxidant and anti-inflammatory properties. However, poor solubility and low bioavailability of QC have also led researchers to make various QC-involved nanoparticles to overcome these limitations. We wondered how high doses or prolonged treatment with quercetin conjugated superparamagnetic iron oxide nanoparticles (QCSPIONs) could improve cognitive dysfunction and promote neurogenesis without any toxicity. It can be explained that the QC inhibits protein aggregation and acts against iron overload via iron-chelating activity, iron homeostasis genes regulation, radical scavenging, and attenuation of Fenton/Haber-Weiss reaction. In this review, first, we present brain iron homeostasis, molecular mechanisms of iron overload that induced neurotoxicity, and the role of iron in dementia-associated diseases. Then by providing evidence of IONPs neurotoxicity, we discuss how QC neutralizes IONPs neurotoxicity, and finally, we make a brief comparison between QC and conventional iron chelators. In this review, we highlight that QC as supplementation and especially in conjugated form reduces iron oxide nanoparticles neurotoxicity in clinical application.

Efficient Iron and ROS Nanoscavengers for Brain Protection after Intracerebral Hemorrhage

Intracerebral hemorrhage (ICH) will be accompanied by the overload of iron and reactive oxygen species (ROS) following hematoma clearance. Although deferoxamine (DFO) has been widely utilized as a clinical first-line siderophore to remove the iron overload, the ROS-inducing damage still greatly limits the therapeutic effect of DFO. To address this issue, we designed and fabricated a series of dual-functional macromolecular nanoscavengers featuring high-density DFO units and catechol moieties. Note that the former units could effectively remove the iron overload, while the latter ones could efficiently deplete the ROS. The resulting nanoscavengers efficiently down-regulate the iron and ROS levels as well as significantly reduce the cell death in both iron-overloaded RAW 264.7 cells and the ICH mice model. This work suggests a novel clue for the ICH-ameliorated iron-depleting interventional therapeutic regimen.

High Antiproliferative Activity of Hydroxythiopyridones over Hydroxypyridones and Their Organoruthenium Complexes

Hydroxypyr(id)ones are a pharmaceutically important class of compounds that have shown potential in diverse areas of drug discovery. We investigated the 3-hydroxy-4-pyridones 1a-1c and 3-hydroxy-4-thiopyridones 1d-1f as well as their Ru(η⁶-p-cymene)Cl complexes 2a-2f, and report here the molecular structures of 1b and 1d as determined by X-ray diffraction analysis. Detailed cell biological investigations revealed potent cytotoxic activity, in particular of the 3-hydroxy-4-thiopyridones 1d-1f, while the Ru complexes of both compound types were less potent, despite still showing antiproliferative activity in the low μM range. The compounds did not modulate the cell cycle distribution of cancer cells but were cytostatic in A549 and cytotoxic in NCI-H522 non-small lung cancer cells, among other effects on cancer cells.

Morphological Profiling Identifies a Common Mode of Action for Small Molecules with Different Targets

Unbiased morphological profiling of bioactivity, for example, in the cell painting assay (CPA), enables the identification of a small molecule's mode of action based on its similarity to the bioactivity of reference compounds, irrespective of the biological target or chemical similarity. This is particularly important for small molecules with nonprotein targets as these are rather difficult to identify with widely employed target-identification methods. We employed morphological profiling using the CPA to identify compounds that are biosimilar to the iron chelator deferoxamine. Structurally different compounds with different annotated cellular targets provoked a shared physiological response, thereby defining a cluster based on their morphological fingerprints. This cluster is based on a shared mode of action and not on a shared target, that is, cell-cycle modulation in the S or G2 phase. Hierarchical clustering of morphological fingerprints revealed subclusters that are based on the mechanism of action and could be used to predict target-related bioactivity.

Practical Management of Iron Overload Disorder (IOD) in Black Rhinoceros (BR; Diceros bicornis)

Simple Summary

Black rhinoceros under human care are predisposed to Iron Overload Disorder that is unlike the hereditary condition seen in humans. We aim to address the black rhino caretaker community at multiple perspectives (keeper, curator, veterinarian, nutritionist, veterinary technician, and researcher) to describe approaches to Iron Overload Disorder in black rhinos and share learnings. This report includes sections on (1) background on how iron functions in comparative species and how Iron Overload Disorder appears to work in black rhinos, (2) practical recommendations for known diagnostics, (3) a brief review of current investigations on inflammatory and other potential biomarkers, (4) nutrition knowledge and advice as prevention, and (5) an overview of treatment options including information on chelation and details on performing large volume voluntary phlebotomy. The aim is to use evidence to support the successful management of this disorder to ensure optimal animal health, welfare, and longevity for a sustainable black rhinoceros population.

Abstract

Critically endangered black rhinoceros (BR) under human care are predisposed to non-hemochromatosis Iron Overload Disorder (IOD). Over the last 30 years, BR have been documented with diseases that have either been induced by or exacerbated by IOD, prompting significant efforts to investigate and address this disorder. IOD is a multi-factorial chronic disease process requiring an evidence-based and integrative long-term approach. While research continues to elucidate the complexities of iron absorption, metabolism, and dysregulation in this species, preventive treatments are recommended and explained herein. The aim of this report is to highlight the accumulated evidence in nutrition, clinical medicine, and behavioral husbandry supporting the successful management of this disorder to ensure optimal animal health, welfare, and longevity for a sustainable black rhinoceros population.

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.

HPO iron chelator, CP655, causes the G1/S phase cell cycle block via p21 upregulation

Iron is known not only for its importance in cellular and metabolic pathways but also for its role in causing cellular toxicities such as production of reactive oxygen species and growth of pathogens. The inability of the human body to physiologically excrete excess iron highlights the need to develop a cheap yet effective iron chelator. This study provides initial evidence of the therapeutic and prophylactic properties of 3-hydroxypyridin-4-one (HPO) chelators in murine collagen-induced arthritis. To determine whether these chelators would be effective on human cells, we tested a panel of different HPO chelators and identified 7-diethylamino-N-((5-hydroxy-6-methyl-4-oxo-1,4-dihydropyridin-3-yl)methyl)-N-methyl-2-oxo-chromen-3-carboxamide (CP655) as the most effective compound targeting human CD4+ T cells. Treatment with CP655 causes significant inhibition of cell proliferation and production of inflammatory cytokines such as interferon-γ and interleukin-17. Microarray analysis revealed dysregulation in cell cycle-related genes following CP655 treatment. This was validated by flow cytometry demonstrating a G1/S phase block caused by CP655. Finally, mechanistic experiments revealed that the chelator may be causing an upregulation of the cell cycle inhibitor protein CDKN1A (p21) as a possible mechanism of action. In conclusion, this study demonstrates that HPO chelators could prove to have therapeutic potential for diseases driven by excessive T cell proliferation.

Polyglycerol-Based Macromolecular Iron Chelator Adjuvants for Antibiotics To Treat Drug-Resistant Bacteria

Iron is an essential micronutrient for life. Its redox activity is a key component in a plethora of vital enzymatic reactions that take place in processes such as drug metabolism, DNA synthesis, steroid synthesis, gene regulation, and cellular respiration (oxygen transport and the electron transport chain). Bacteria are highly dependent on iron for their survival and growth and have specific mechanisms to acquire iron. Limiting the availability of iron to bacteria, thereby preventing their growth, provides new opportunities to treat infection in the era of the persistent rise of antibiotic-resistant bacteria. In this work, we have developed macromolecular iron chelators, conjugates of a high-affinity iron chelator (HBEDS) with polyglycerol, in an attempt to sequester iron uptake by bacteria to limit their growth in order to enhance antibiotic activity. The new macromolecular chelators are successful in slowing the growth of Staphylococcus aureus and worked as an efficient bacteriostatic against S. aureus. Further, these cytocompatible macrochelators acted as effective adjuvants to prevent bacterial growth when used in conjunction with antibiotics. The adjuvant activity of the macrochelators depends on their molecular weight and the chelator density on these molecules. These selective macro iron(III) chelators are highly efficient in growth inhibition and killing of methicillin-resistant S. aureus in conjunction with a low concentration of rifampicin.

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Binding and Release of FeIII Complexes from Glucan Particles for the Delivery of T1 MRI Contrast Agents

Yeast-derived β-glucan particles (GPs) are a class of microcarriers under development for the delivery of drugs and imaging agents to immune-system cells for theranostic approaches. However, the encapsulation of hydrophilic imaging agents in the porous GPs is challenging. Here, we show that the unique coordination chemistry of FeIII -based macrocyclic T1 MRI contrast agents permits facile encapsulation in GPs. Remarkably, GPs labeled with the simple FeIII complexes are stable under physiologically relevant conditions, despite the absence of amphiphilic groups. In contrast to the free FeIII coordination complex, the labeled FeIII -GPs have lowered T1 relaxivity and act as a silenced form of the contrast agent. Addition of a fluorescent tag to the FeIII complex produces a bimodal agent to further enable tracking of the nanoparticles and to monitor release. Treatment of the iron-labeled GPs with a maltol chelator or with mildly acidic conditions releases the intact iron complex and restores enhanced T1 relaxation of the water protons.

Thiosemicarbazones as Potent Anticancer Agents and their Modes of Action

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

The Iron-chelator, N,N'-bis (2-hydroxybenzyl) Ethylenediamine-N,N'-Diacetic acid is an Effective Colistin Adjunct against Clinical Strains of Biofilm-Dwelling Pseudomonas aeruginosa

Targeting the iron requirement of Pseudomonas aeruginosa may be an effective adjunctive for conventional antibiotic treatment against biofilm-dwelling P. aeruginosa. We, therefore, assessed the anti-biofilm activity of N,N’-bis (2-hydroxybenzyl) ethylenediamine-N,N’-diacetic acid (HBED), which is a synthetic hexadentate iron chelator. The effect of HBED was studied using short-term (microtitre plate) and longer-term (flow-cell) biofilm models, under aerobic, anaerobic, and microaerobic (flow-cell) conditions and in combination with the polymyxin antibiotic colistimethate sodium (colistin). HBED was assessed against strains of P. aeruginosa from patients with cystic fibrosis and the reference strain PAO1. HBED inhibited growth and biofilm formation of all clinical strains under aerobic and anaerobic conditions, but inhibitory effects against PAO1 were predominantly exerted under anaerobic conditions. PA605, which is a clinical strain with a robust biofilm-forming phenotype, was selected for flow-cell studies. HBED significantly reduced biomass and surface coverage of PA605, and, combined with colistin, HBED significantly enhanced the microcolony killing effects of colistin to result in almost complete removal of the biofilm. HBED combined with colistin is highly effective in vitro against biofilms formed by clinical strains of P. aeruginosa.

Predicting the Thermodynamic Stability of Zirconium Radiotracers

The thermodynamic stability of a metal-ligand complex, as measured by the formation constant (log β), is one of the most important parameters that determines metal ion selectivity and potential applications in, for example, radiopharmaceutical science. The stable coordination chemistry of radioactive ⁸⁹Zr⁴⁺ in an aqueous environment is of paramount importance when developing positron-emitting radiotracers based on proteins (usually antibodies) for use with positron emission tomography. Desferrioxamine B (DFO) remains the chelate of choice for clinical applications of ⁸⁹Zr-labeled proteins, but the coordination of DFO to Zr⁴⁺ ions is suboptimal. Many alternative ligands have been reported, but the challenges in measuring very high log β values with metal ions such as Zr⁴⁺ that tend to hydrolyze mean that accurate thermodynamic data are scarce. In this work, density functional theory (DFT) calculations were used to predict the reaction energetics for metal ion complexation. Computed values of pseudoformation constants (log β') are correlated with experimental data and showed an excellent linear relationship (R² = 0.97). The model was then used to estimate the absolute and relative formation constants of 23 different Zr⁴⁺ complexes using a total of 17 different ligands, including many of the alternative bifunctional chelates that have been reported recently for use in ⁸⁹Zr⁴⁺ radiochemistry. In addition, detailed computational studies were performed on the geometric isomerism and hydration state of Zr-desferrioxamine. Collectively, the results offer new insights into Zr⁴⁺ coordination chemistry that will help guide the synthesis of future ligands. The computational model developed here is straightforward and reproducible and can be readily applied in the design of other metal coordination compounds.

The influence of linkages between 1-hydroxy-2(1H)-pyridinone coordinating groups and a tris(2-aminoethyl)amine core in a novel series of synthetic hexadentate iron(III) chelators on antimicrobial activity

Resistance of pathogens to antimicrobials is a major current healthcare concern. In a series of linked studies, we have investigated synthetic iron chelators based on hydroxy-pyridinone ligands as novel bacteriostatic agents. Herein we describe our synthesis of several useful building blocks based on the 1-hydroxy-2(1H)-pyridinone moiety, including a novel formyl derivative, which were combined with a tris(2-aminoethyl)amine core to obtain a series of new high-affinity hexadentate Fe(III) chelators. The design principle examined by this series is the size and flexibility of the linker between the core and the metal ligands. Measurement of the pKa and stability constants (Fe³⁺ and Cu²⁺) of representative coordinating groups was performed to help rationalise the biological activity of the chelators. The novel chelators were tested on a panel of representative microorganisms with some effectively inhibiting microbial growth. We demonstrate that the nature and position of the linker between the hydroxypyridinone and the tris(2-aminoethyl)amine core has considerable impact upon microbial growth inhibition and that both amide or amine linkages can give efficacious chelators.

Electronic spectroscopic characterization of the formation of iron(III) metal complexes: The 8-HydroxyQuinoline as ligand case study

Synthetic siderophores derivated from 8-HydroxyQuinoline (HQ) present various biological and pharmacological activities, such as anti-neurodegenerative or anti-oxydative. However, their affinity towards iron(III) seems to depend on the position (i.e., 7 or 2) of the HQ substitution by an electron withdrawing group. Two ester-derivatives of HQ at 2- and 7-position are synthesized and their respective iron-complexation is characterized by a joined experimental and theoretical work. By investigating the stability of all the possible accessible spin states of the iron(III) complexes at density-functional theory (DFT) level, we demonstrate that the high-spin (HS) state is the most stable one, and leads to a UV/vis absorption spectrum in perfect match with experiments. From this DFT protocol, and in agreement with the experimental results, we show that the ester functionalization of HQ in 2-position weakens the formation of the iron(III) complex while its substitution in 7-position allows a salicylate coordination of the metal very close to the ideal octahedral environment.

New therapeutic targets for the prevention of infectious acute exacerbations of COPD: role of epithelial adhesion molecules and inflammatory pathways

Chronic respiratory diseases are among the leading causes of mortality worldwide, with the major contributor, chronic obstructive pulmonary disease (COPD) accounting for approximately 3 million deaths annually. Frequent acute exacerbations (AEs) of COPD (AECOPD) drive clinical and functional decline in COPD and are associated with accelerated loss of lung function, increased mortality, decreased health-related quality of life and significant economic costs. Infections with a small subgroup of pathogens precipitate the majority of AEs and consequently constitute a significant comorbidity in COPD. However, current pharmacological interventions are ineffective in preventing infectious exacerbations and their treatment is compromised by the rapid development of antibiotic resistance. Thus, alternative preventative therapies need to be considered. Pathogen adherence to the pulmonary epithelium through host receptors is the prerequisite step for invasion and subsequent infection of surrounding structures. Thus, disruption of bacterial-host cell interactions with receptor antagonists or modulation of the ensuing inflammatory profile present attractive avenues for therapeutic development. This review explores key mediators of pathogen-host interactions that may offer new therapeutic targets with the potential to prevent viral/bacterial-mediated AECOPD. There are several conceptual and methodological hurdles hampering the development of new therapies that require further research and resolution.

Therapeutic Macromolecular Iron Chelators

Iron is a key element for every single living process. On a fundamental level, targeting iron is a valuable approach for the treatment of disorders caused by iron overload. Utilizing iron chelators as therapeutic agents has received expanding consideration in chelation therapy. Approved low molecular weight (MW) iron chelators to treat iron overload may experience short half-lives and toxicities prompting moderately high adverse effects. In recent years, polymeric/macromolecular iron chelators have received attention as therapeutic agents. Polymeric iron chelators show unique pharmaceutical properties that are different to their conventional small molecule counterparts. These polymeric iron chelators possess longer plasma half-lives and reduced toxicities, thus exhibiting a significant supplement to currently using low MW iron chelator therapy. In this review, we have briefly discussed polymeric iron chelators and factors to be considered when designing clinically valuable iron chelators. We have also discussed applications of polymeric iron chelators in the diseases caused by iron overload associated with transfusional hemosiderosis, neurodegenerative disorders, malaria and cancer. With this, research findings for new polymeric iron chelators are also covered.

On the preferences of five-membered chelate rings in coordination chemistry: insights from the Cambridge Structural Database and theoretical calculations

The purpose of this review is to give an overview of three important N-bidentate ligands: 1,10-phenanthroline (phen), 2,2'-bipyridine (bpy), and ethylenediamine (en). We have not attempted to be comprehensive because of the huge amount of activity being done in coordination chemistry using these ligands. Instead we present a full structural and geometrical study by using the Cambridge Structural Database (CSD) combined with theoretical calculations that allow us to parameterize their coordinating properties and ability to coordinate to transition and non-transition metals. More importantly, we illustrate that upon coordination and formation of the five-membered chelate ring, these ligands are able to adapt themselves to the requirements of the different metals by changing the MN distances and NMN angles. Therefore, a redefinition of the preferences of these ligands to metals with large ionic radii is needed. Finally, we will present some facts about the participation of these ligands in inorganic-organic hybrids (IOHs) based on Keggin polyoxometalates (POMs).

Tacrine-deferiprone hybrids as multi-target-directed metal chelators against Alzheimer's disease: a two-in-one drug

Alzheimer's disease (AD) is a severe age-dependent neurodegenerative disorder affecting several million people worldwide. So far, there is no adequate medication to prevent or slow down the progression of the disease, only medication with palliative effects allowing temporary symptomatic reliefs. As part of our continuing efforts into the development of innovative drugs following a polypharmacological strategy, we decided to use a former anti-AD palliative drug (tacrine) and to reposition it by hybridization with a metal chelating drug (deferiprone, DFP). This combination endows the hybrids with good capacity to inhibit acetylcholinesterase (low micromolar range) and self-/Cu-induced Aβ aggregation (up to ca. 90%) as well as a good radical scavenging ability (micromolar range) and metal (M) chelating capacity, with pM (pM = -log[M], C_L/C_M = 10, C_M = 10^-6 M at pH = 7.4, M = Fe, Cu, Zn) values close to those of DFP. The most promising compounds have 2-hydroxypropyl linkers, and a selection of compounds have demonstrated neuroprotective roles in neuroblastoma cells treated with Aβ_1-42 and ascorbate/iron stressors. Consequently, these hybrids can be considered as attractive multipotent therapeutic molecules that will eventually play key roles against AD progression, namely in the control of cholinergic dysfunction, amyloid peptide aggregation, oxidative stress, and metal modulation, besides presenting a good pharmacokinetic profile.

Mechanism Underlying the Effectiveness of Deferiprone in Alleviating Parkinson's Disease Symptoms

Elevation in iron content as well as severe depletion of dopamine (DA) as a result of iron-induced loss of dopaminergic neurons has been recognized to accompany the progression of Parkinson's disease (PD). To better understand the mechanism of the mitigating effect of the iron chelator deferiprone (DFP) on PD, the interplay between iron and DFP was investigated both in the absence and presence of DA. The results show that DFP was extremely efficient in scavenging both aqueous iron and iron that was loosely bound to DA with the entrapment of iron in Fe-DFP complexed form critical to halting the iron catalyzed degradation of DA and associated generation of toxic metabolites. The DFP related scavenging of dopamine semiquinone (DA^•-) and superoxide (O₂^•-) may also contribute to its positive effects in the treatment of PD.

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).

Hydroxypyridinone-benzofuran hybrids with potential protective roles for Alzheimer´s disease therapy

A series of (3-hydroxy-4-pyridinone)-benzofuran hybrids have been developed and studied as potential multitargeting drugs for Alzheimer's disease (AD). Their design envisaged mainly to mimic the donepezil drug, a marketed inhibitor of acetylcholinesterase (AChE), and to endow the conjugate molecules with extra-properties such as metal chelation, radical scavenging and inhibition of amyloid peptide (Aβ) aggregation. Thus, a set of eleven new hybrid compounds was developed and evaluated for chemical and biological properties, in solution and in neuronal cell environment. The results are discussed in terms of the type of substituents on both main moieties and the linker size. The closest similarity with donepezil, in terms of AChE inhibitory activity, was obtained for the O-benzyl-hydroxypyridinone hybrids containing a 2-methylene linker, although still less active than the drug. However, the free-hydroxypyridinone hybrids present higher activity for the Aβ aggregation inhibition, metal chelating capacity and radical scavenging activity. Overall, some compounds demonstrated capacity to exert a multiple action by hitting three- (7d) or four- (8d, 8f) pathophysiological targets of AD. Furthermore, the compounds showed neuroprotective effects in neuronal cells subjected to model stressors of AD, but not significant dependence on the substituent groups. Importantly, the compounds evidenced drug-likeness properties, including good membrane permeability.