Conjugates of desferrioxamine B (DFOB) with derivatives of adamantane or with orally available chelators as potential agents for treating iron overload

Combining Desferriferrioxamine B and 1-Hydroxy-2-Piperidone ((PIPO)H) to Chelate Zirconium. Solution Structure of a Model Complex of the [89Zr]Zr-DFOcyclo*-mAb Radioimmunoconjugate

89Zr-immunoPET is a hot topic as 89Zr cumulates the advantages of 64Cu and 124I without their drawbacks. We report the synthesis of a model ligand of a chiral bioconjugable tetrahydroxamic chelator combining the desferriferrioxamine B siderophore and 1-hydroxy-2-piperidone ((PIPO)H), a chiral cyclic hydroxamic acid derivative, and the study by NMR spectroscopy of its zirconium complex. Nuclear Overhauser effect measurements (ROESY) indicated that the complex exists in the form of two diastereomers, in 77 : 23 ratio, resulting from the combination of the central chiralities at the 3-C of the (PIPO)H component and at the Zr4+ cation. The 44 lowest energy structures out of more than 1000 configurations/conformations returned by calculations based on density functional theory were examined. Comparison of the ROESY data and the calculated interatomic H⋅⋅⋅H distances allowed us to select the most probable configuration and conformations of the major complex.

Evaluation of Candidate Theranostics for 227Th/89Zr Paired Radioimmunotherapy of Lymphoma

²²⁷Th is a promising radioisotope for targeted α-particle therapy. It produces 5 α-particles through its decay, with the clinically approved ²²³Ra as its first daughter. There is an ample supply of ²²⁷Th, allowing for clinical use; however, the chemical challenges of chelating this large tetravalent f-block cation are considerable. Using the CD20-targeting antibody ofatumumab, we evaluated chelation of ²²⁷Th⁴⁺ for α-particle-emitting and radiotheranostic applications. Methods: We compared 4 bifunctional chelators for thorium radiopharmaceutical preparation: S-2-(4-Isothiocyanatobenzyl)-1,4,7,10-tetraazacyclododecane tetraacetic acid (p-SCN-Bn-DOTA), 2-(4-isothicyanatobenzyl)-1,2,7,10,13-hexaazacyclooctadecane-1,4,7,10,13,16-hexaacetic acid (p-SCN-Bn-HEHA), p-isothiacyanatophenyl-1-hydroxy-2-oxopiperidine-desferrioxamine (DFOcyclo*-p-Phe-NCS), and macrocyclic 1,2-HOPO N-hydroxysuccinimide (L804-NHS). Immunoconstructs were evaluated for yield, purity, and stability in vitro and in vivo. Tumor targeting of the lead ²²⁷Th-labeled compound in vivo was performed in CD20-expressing models and compared with a companion ⁸⁹Zr-labeled PET agent. Results: ²²⁷Th-labeled ofatumumab-chelator constructs were synthesized to a radiochemical purity of more than 95%, excepting HEHA. ²²⁷Th-HEHA-ofatumumab showed moderate in vitro stability. ²²⁷Th-DFOcyclo*-ofatumumab presented excellent ²²⁷Th labeling efficiency; however, high liver and spleen uptake was revealed in vivo, indicative of aggregation. ²²⁷Th-DOTA-ofatumumab labeled poorly, yielding no more than 5%, with low specific activity (0.08 GBq/g) and modest long-term in vitro stability (<80%). ²²⁷Th-L804-ofatumumab coordinated ²²⁷Th rapidly and efficiently at high yields, purity, and specific activity (8 GBq/g) and demonstrated extended stability. In vivo tumor targeting confirmed the utility of this chelator, and the diagnostic analog, ⁸⁹Zr-L804-ofatumumab, showed organ distribution matching that of ²²⁷Th to delineate SU-DHL-6 tumors. Conclusion: Commercially available and novel chelators for ²²⁷Th showed a range of performances. The L804 chelator can be used with potent radiotheranostic capabilities for ⁸⁹Zr/²²⁷Th quantitative imaging and α-particle therapy.

Screen-printed Gold Electrode Functionalized with Deferoxamine for Iron(III) Detection

Deferoxamine (DFO), a hydroxamic siderophore with a high affinity for Fe(III), is immobilized as a functionalized self-assembled monolayer of a thiol (SAM) on the gold surface of a screen-printed cell to develop a voltammetric sensor for iron(III). The surface of the working electrode was characterized, before and after functionalization, by determining surface properties such as the area and the double-layer capacitance. The Fe(III) detection was performed by DPV analysis after preconcentration of the cation at the open circuit potential in solution at pH = 1 for two minutes. The method was applied to the iron(III) quantification in water samples giving promising results.

Acetyl-CoA-Mediated Post-Biosynthetic Modification of Desferrioxamine B Generates N- and N-O-Acetylated Isomers Controlled by a pH Switch

Biosynthesis of the hydroxamic acid siderophore desferrioxamine D₁ (DFOD₁, 6), which is the N-acetylated analogue of desferrioxamine B (DFOB, 5), has been delineated. Enzyme-independent Ac-CoA-mediated N-acetylation of 5 produced 6, in addition to three constitutional isomers containing an N-O-acetyl group installed at either one of the three hydroxamic acid groups of 5. The formation of N-Ac-DFOB (DFOD₁, 6) and the composite of N-O-acetylated isomers N-O-Ac-DFOB[001] (6a), N-O-Ac-DFOB[010] (6b), and N-O-Ac-DFOB[100] (6c) (defined as the N-O-Ac motif positioned within the terminal amine, internal, or N-acetylated region of 5, respectively), was pH-dependent, with 6a-6c dominant at pH < 8.5 and 6 dominant at pH > 8.5. The trend in the pH dependence was consistent with the pK_a values of the NH₃⁺ (pK_a ∼ 10) and N-OH (pK_a ∼ 8.5-9) groups in 5. The N- and N-O-acetyl motifs can be conceived as a post-biosynthetic modification (PBM) of a nonproteinaceous secondary metabolite, akin to a post-translational modification (PTM) of a protein. The pH-labile N-O-acetyl group could act as a reversible switch to modulate the properties and functions of secondary metabolites, including hydroxamic acid siderophores. An alternative (most likely minor) biosynthetic pathway for 6 showed that the nonribosomal peptide synthetase-independent siderophore synthetase DesD was competent in condensing N'-acetyl-N-succinyl-N-hydroxy-1,5-diaminopentane (N'-Ac-SHDP, 7) with the dimeric hydroxamic acid precursor (AHDP-SHDP, 4) native to 5 biosynthesis to generate 6. The strategy of diversifying protein structure and function using PTMs could be paralleled in secondary metabolites with the use of PBMs.

Deferoxamine-Modified Hybrid Materials for Direct Chelation of Fe(III) Ions from Aqueous Solutions and Indication of the Competitiveness of In Vitro Complexing toward a Biological System

Deferoxamine (DFO) is one of the most potent iron ion complexing agent belonging to a class of trihydroxamic acids. The extremely high stability constant of the DFO-Fe complex (log β = 30.6) prompts the use of deferoxamine as a targeted receptor for scavenging Fe(III) ions. The following study aimed at deferoxamine immobilization on three different supports: poly(methyl vinyl ether-alt-maleic anhydride), silica particles, and magnetite nanoparticles, leading to a class of hybrid materials exhibiting effectiveness in ferric ion adsorption. The formed deferoxamine-loaded hybrid materials were characterized with several analytical techniques. Their adsorptive properties toward Fe(III) ions in aqueous samples, including pH-dependence, isothermal, kinetic, and thermodynamic experiments, were investigated. The materials were described with high values of maximal adsorption capacity q _m, which varied between 87.41 and 140.65 mg g^-1, indicating the high adsorptive potential of the DFO-functionalized materials. The adsorption processes were also described as intense, endothermic, and spontaneous. Moreover, an exemplary magnetically active deferoxamine-modified material has been proven for competitive in vitro binding of ferric ions from the biological complex protoporphyrin IX-Fe(III), which may lead to a further examination of the materials' biological or medical applicability.

Conjugates of desferrioxamine and aromatic amines improve markers of iron-dependent neurotoxicity

Alzheimer's Disease (AD) is a complex neurodegenerative disorder associated in some instances with dyshomeostasis of redox-active metal ions, such as copper and iron. In this work, we investigated whether the conjugation of various aromatic amines would improve the pharmacological efficacy of the iron chelator desferrioxamine (DFO). Conjugates of DFO with aniline (DFOANI), benzosulfanylamide (DFOBAN), 2-naphthalenamine (DFONAF) and 6-quinolinamine (DFOQUN) were obtained and their properties examined. DFOQUN had good chelating activity, promoted a significant increase in the inhibition of β-amyloid peptide aggregation when compared to DFO, and also inhibited acetylcholinesterase (AChE) activity both in vitro and in vivo (Caenorhabditis elegans). These data indicate that the covalent conjugation of a strong iron chelator to an AChE inhibitor offers a powerful approach for the amelioration of iron-induced neurotoxicity symptoms.

Cell Permeable Imidazole-Desferrioxamine Conjugates: Synthesis and In Vitro Evaluation

Desferrioxamine (DFO), a clinically approved iron chelator used for iron overload, is unable to chelate labile plasma iron (LPI) because of its limited cell permeability. Herein, alkyl chain modified imidazolium cations with varied hydrophobicities have been conjugated with DFO. The iron binding abilities and the antioxidant properties of the conjugates were found to be similar to DFO. The degree of cellular internalization was much higher in the octyl-imidazolium-DFO conjugate (IV) compared with DFO, and IV was able to chelate LPI in vitro. This opens up a new avenue in using N-alkyl imidazolium salts as a delivery vector for hydrophilic cell-impermeable drugs.

Analogues of desferrioxamine B (DFOB) with new properties and new functions generated using precursor-directed biosynthesis

Desferrioxamine B (DFOB) is a siderophore native to Streptomyces pilosus biosynthesised by the DesABCD enzyme cluster as a high affinity Fe(III) chelator. Although DFOB has a long clinical history for the treatment of chronic iron overload, limitations encourage the development of new analogues. This review describes a recent body of work that has used precursor-directed biosynthesis (PDB) to access new DFOB analogues. PDB exploits the native biosynthetic machinery of a producing organism in culture medium augmented with non-native substrates that compete against native substrates during metabolite assembly. The method allows access to analogues of natural products using benign methods, compared to multistep organic synthesis. The disadvantages of PDB are the production of metabolites in low yield and the need to purify complex mixtures. Streptomyces pilosus medium was supplemented with different types of non-native diamine substrates to compete against native 1,5-diaminopentane to generate DFOB analogues containing alkene bonds, fluorine atoms, ether or thioether functional groups, or a disulfide bond. All analogues retained function as Fe(III) chelators and have properties that could broaden the utility of DFOB. These PDB studies have also added knowledge to the understanding of DFOB biosynthesis.

Fluorinated Analogues of Desferrioxamine B from Precursor-Directed Biosynthesis Provide New Insight into the Capacity of DesBCD

The siderophore desferrioxamine B (DFOB, 1) native to Streptomyces pilosus is biosynthesized by the DesABCD enzyme cluster. DesA-mediated decarboxylation of l-lysine gives 1,5-diaminopentane (DP) for processing by DesBCD. S. pilosus culture medium was supplemented with rac-1,4-diamino-2-fluorobutane ( rac-FDB) to compete against DP to generate fluorinated analogues of DFOB, as agents of potential clinical interest. LC-MS/MS analysis identified fluorinated analogues of DFOB with one, two, or three DP units (binary notation: 0) exchanged for one (DFOA-F₁[001] (2), DFOA-F₁[010] (3), DFOA-F₁[100] (4)), two (DFOA-F₂[011] (5), DFOA-F₂[110] (6), DFOA-F₂[101] (7)), or three (DFOA-F₃[111] (8)) rac-FDB units (binary notation: 1). The two sets of constitutional isomers 2-4 and 5-7 arose from the position of the substrates in the N-acetyl, internal, or amine-containing regions of the DFOB trimer. N-Acetylated fluorinated DFOB analogues were formed where the rac-FDB substrate was positioned in the amine region ( e.g., N-Ac-DFOA-F₁[001] (2a)). Other analogues contained two hydroxamic acid groups and three amide bonds. Experiments using rac-FDB, R-FDB, or S-FDB showed a similar species profile between rac-FDB and R-FDB. These data are consistent with the following. (i) DesB can act on rac-FDB. (ii) DesC can act directly on rac-FDB. (iii) The products of DesBC or DesC catalysis of rac-FDB can undergo a second round of DesC catalysis at the free amine. (iv) DesD catalysis of these products gives N, N'-diacetylated compounds. (v) A minimum of two hydroxamic acid groups is required to form a viable DesD-substrate(s) precomplex. (vi) One or more DesBCD-catalyzed steps in DFOB biosynthesis is enantioselective. This work has provided a potential path to access fluorinated analogues of DFOB and new insight into its biosynthesis.

Polymeric Nanoparticles Enhance the Ability of Deferoxamine To Deplete Hepatic and Systemic Iron

Chelators are commonly used to remove excess iron in iron-loading disorders. Deferoxamine (DFO) is an effective and safe iron chelator but an onerous parenteral administration regimen limits its routine use. To develop more effective methods for delivering iron chelators, we examined whether amphiphilic copolymer nanoparticles (NPs) could deliver DFO more efficiently. Physical characterization showed a uniform and stable preparation of DFO nanoparticles (DFO-NPs) with an average diameter of 105.3 nm. In macrophage (RAW264.7) and hepatoma (HepG2) cell lines, DFO-NPs proved more effective at depleting iron than free DFO. In wild-type mice previously loaded with iron dextran, as well as Hbb ^{th3 /+} and Hfe ^-/- mice, which are predisposed to iron loading, DFO-NPs (40 mg/kg DFO; alternate days; 4 weeks) reduced hepatic iron levels by 71, 46, and 37%, respectively, whereas the equivalent values for free DFO were 53, 7, and 15%. Staining for tissue iron and urinary iron excretion confirmed these findings. Pharmacokinetic analysis showed that NP-encapsulated DFO had a much longer elimination half-life than free DFO (48.63 ± 28.80 vs 1.46 ± 0.59 h), and that DFO-NPs could be readily taken up by tissues and in particular by hepatic Kupffer cells. In vitro, DFO-NPs were less toxic to several cell lines than free DFO, and in vivo they did not elicit any specific inflammatory responses or histological changes. Our results suggest that using a nanoformulation of DFO is a valuable strategy for improving its efficiency as an iron chelator and that this could broaden its clinical use for the treatment of human iron overload disorders.

Advances in the Chemical Biology of Desferrioxamine B

Desferrioxamine B (DFOB) was discovered in the late 1950s as a hydroxamic acid metabolite of the soil bacterium Streptomyces pilosus. The exquisite affinity of DFOB for Fe(III) identified its potential for removing excess iron from patients with transfusion-dependent hemoglobin disorders. Many studies have used semisynthetic chemistry to produce DFOB adducts with new properties and broad-ranging functions. More recent approaches in chemical biology have revealed some nuances of DFOB biosynthesis and discovered new DFOB-derived drugs and radiometal imaging agents. The current and potential applications of DFOB continue to inspire a rich body of chemical biology research focused on this bacterial metabolite.

Triphenylphosphonium-desferrioxamine as a candidate mitochondrial iron chelator

Cell-impermeant iron chelator desferrioxamine (DFO) can have access to organelles if appended to suitable vectors. Mitochondria are important targets for the treatment of iron overload-related neurodegenerative diseases. Triphenylphosphonium (TPP) is a delocalized lipophilic cation used to ferry molecules to mitochondria. Here we report the synthesis and characterization of the conjugate TPP-DFO as a mitochondrial iron chelator. TPP-DFO maintained both a high affinity for iron and the antioxidant activity when compared to parent DFO. TPP-DFO was less toxic than TPP alone to A2780 cells (IC₅₀ = 135.60 ± 1.08 and 4.34 ± 1.06 μmol L^-1, respectively) and its native fluorescence was used to assess its mitochondrial localization (Rr = +0.56). These results suggest that TPP-DFO could be an interesting alternative for the treatment of mitochondrial iron overload e.g. in Friedreich's ataxia.

Structural Basis for Xenosiderophore Utilization by the Human Pathogen Staphylococcus aureus

Staphylococcus aureus produces a cocktail of metallophores (staphylopine, staphyloferrin A, and staphyloferrin B) to scavenge transition metals during infection of a host. In addition, S. aureus displays the extracellular surface lipoproteins FhuD1 and FhuD2 along with the ABC transporter complex FhuCBG to facilitate the use of hydroxamate xenosiderophores such as desferrioxamine B (DFOB) for iron acquisition. DFOB is used as a chelation therapy to treat human iron overload diseases and has been linked to an increased risk of S. aureus infections. We used a panel of synthetic DFOB analogs and a FhuD2-selective trihydroxamate sideromycin to probe xenosiderophore utilization in S. aureus and establish structure-activity relationships for Fe(III) binding, FhuD2 binding, S. aureus growth promotion, and competition for S. aureus cell entry. Fe(III) binding assays and FhuD2 intrinsic fluorescence quenching experiments revealed that diverse chemical modifications of the terminal ends of linear ferrioxamine siderophores influences Fe(III) affinity but not FhuD2 binding. Siderophore-sideromycin competition assays and xenosiderophore growth promotion assays revealed that S. aureus SG511 and ATCC 11632 can distinguish between competing siderophores based exclusively on net charge of the siderophore-Fe(III) complex. Our work provides a roadmap for tuning hydroxamate xenosiderophore scaffolds to suppress (net negative charge) or enhance (net positive or neutral charge) uptake by S. aureus for applications in metal chelation therapy and siderophore-mediated antibiotic delivery, respectively.

Exploiting the biosynthetic machinery of Streptomyces pilosus to engineer a water-soluble zirconium(iv) chelator

A combined microbiology-chemistry approach has been used to generate a water-soluble chain-extended octadentate hydroxamic acid designed as a high affinity and selective Zr(iv) ligand.

Imidazole-modified deferasirox encapsulated polymeric micelles as pH-responsive iron-chelating nanocarrier for cancer chemotherapy

Modified deferasirox encapsulated polymeric micelles demonstrate pH-responsive and ON–OFF release behavior to deplete the iron level in cancer cells. The cellular iron deficiency is a novel strategy for cancer treatment.

Analogues of desferrioxamine B designed to attenuate iron-mediated neurodegeneration: synthesis, characterisation and activity in the MPTP-mouse model of Parkinson's disease

One dual-function (2) and one first-generation (9) conjugate of the Fe(iii) chelator desferrioxamine B (DFOB,1) showed significant rescue of neurons in the MPTP mouse model of Parkinson's disease.

Cell penetrating peptide (CPP)-conjugated desferrioxamine for enhanced neuroprotection: synthesis and in vitro evaluation

Iron overload causes progressive and sometimes irreversible damage due to accelerated production of reactive oxygen species. Desferrioxamine (DFO), a siderophore, has been used clinically to remove excess iron. However, the applications of DFO are limited because of its inability to access intracellular labile iron. Cell penetrating peptides (CPPs) have become an efficient delivery vector for the enhanced internalization of drugs into the cytosol. We describe, herein, an efficient method for covalently conjugating DFO to the CPPs TAT(47-57) and Penetratin. Both conjugates suppressed the redox activity of labile plasma iron in buffered solutions and in iron-overloaded sera. Enhanced access to intracellular labile iron compared to the parent siderophore was achieved in HeLa and RBE4 (a model of blood-brain-barrier) cell lines. Iron complexes of both conjugates also had better permeability in both cell models. DFO antioxidant and iron binding properties were preserved and its bioavailability was increased upon CPP conjugation, which opens new therapeutic possibilities for neurodegenerative processes associated with brain iron overload.

Unsaturated macrocyclic dihydroxamic acid siderophores produced by Shewanella putrefaciens using precursor-directed biosynthesis

To acquire iron essential for growth, the bacterium Shewanella putrefaciens produces the macrocyclic dihydroxamic acid putrebactin (pbH2; [M + H(+)](+), m/zcalc 373.2) as its native siderophore. The assembly of pbH2 requires endogenous 1,4-diaminobutane (DB), which is produced from the ornithine decarboxylase (ODC)-catalyzed decarboxylation of l-ornithine. In this work, levels of endogenous DB were attenuated in S. putrefaciens cultures by augmenting the medium with the ODC inhibitor 1,4-diamino-2-butanone (DBO). The presence in the medium of DBO together with alternative exogenous non-native diamine substrates, (15)N2-1,4-diaminobutane ((15)N2-DB) or 1,4-diamino-2(E)-butene (E-DBE), resulted in the respective biosynthesis of (15)N-labeled pbH2 ((15)N4-pbH2; [M + H(+)](+), m/zcalc 377.2, m/zobs 377.2) or the unsaturated pbH2 variant, named here: E,E-putrebactene (E,E-pbeH2; [M + H(+)](+), m/zcalc 369.2, m/zobs 369.2). In the latter system, remaining endogenous DB resulted in the parallel biosynthesis of the monounsaturated DB-E-DBE hybrid, E-putrebactene (E-pbxH2; [M + H(+)](+), m/zcalc 371.2, m/zobs 371.2). These are the first identified unsaturated macrocyclic dihydroxamic acid siderophores. LC-MS measurements showed 1:1 complexes formed between Fe(III) and pbH2 ([Fe(pb)](+); [M](+), m/zcalc 426.1, m/zobs 426.2), (15)N4-pbH2 ([Fe((15)N4-pb)](+); [M](+), m/zcalc 430.1, m/zobs 430.1), E,E-pbeH2 ([Fe(E,E-pbe)](+); [M](+), m/zcalc 422.1, m/zobs 422.0), or E-pbxH2 ([Fe(E-pbx)](+); [M](+), m/zcalc 424.1, m/zobs 424.2). The order of the gain in siderophore-mediated Fe(III) solubility, as defined by the difference in retention time between the free ligand and the Fe(III)-loaded complex, was pbH2 (ΔtR = 8.77 min) > E-pbxH2 (ΔtR = 6.95 min) > E,E-pbeH2 (ΔtR = 6.16 min), which suggests one possible reason why nature has selected for saturated rather than unsaturated siderophores as Fe(III) solubilization agents. The potential to conduct multiple types of ex situ chemical conversions across the double bond(s) of the unsaturated macrocycles provides a new route to increased molecular diversity in this class of siderophore.

First-principles electronic structure study of rhizoferrin and its Fe(III) complexes

We have determined the structure and coordination chemistry of rhizoferrin (Rf), which is a particular type of siderophore, and its Fe(III) complexes using density functional theory calculations. Our results show that the Fe(III) ion binds in an octahedral coordination, with a low-spin (S = 1/2) charge-neutral chiral complex having the largest binding energy of the investigated complexes. We have also calculated nuclear magnetic resonance parameters, such as chemical shifts for (1)H and (13)C, and indirect nuclear spin-spin couplings for (1)H-(1)H and (13)C-(1)H in free Rf and in a low-spin neutral Rf metal complex, as well as nuclear quadrupole interaction parameters, such as asymmetry parameter and nuclear quadrupole coupling constants for (14)N. Our calculated values for the chemical shifts for free Rf are in excellent agreement with experimental data while the calculated NMR parameters for Fe(III) complexes are predictions for future experimental work.

Lipophilic adamantyl- or deferasirox-based conjugates of desferrioxamine B have enhanced neuroprotective capacity: implications for Parkinson disease

Parkinson disease (PD) is a neurodegenerative disease characterized by death of dopaminergic neurons in the substantia nigra region of the brain. Iron content is also elevated in this region in PD and is implicated in the pathobiology of the disease. Desferrioxamine B (DFOB) is a high-affinity iron chelator and has shown efficacy in animal models of Parkinson disease. The high water solubility of DFOB, however, attenuates its ability to enter the brain. In this study, we have conjugated DFOB to derivatives of adamantane or the clinical iron chelator deferasirox to produce lipophilic compounds designed to increase the bioavailability of DFOB to brain cells. We found that the novel compounds are highly effective in preventing iron-mediated paraquat and hydrogen peroxide toxicity in neuronal-like BE2-M17 dopaminergic cells, primary neurons, and iron-loaded or glutathione-depleted primary astrocytes. The compounds also alleviated paraquat toxicity in BE2-M17 cells that express the PD-causing A30P mutation of α-synuclein. This protection was ∼66-fold more potent than DFOB alone and also more effective than other cell-permeative metal chelators, clioquinol and phenanthroline. These results demonstrate that increasing the bioavailability of DFOB through the conjugation of lipophilic fragments greatly enhances its protective capacity. These novel compounds have potential as therapeutics for the treatment of PD and other conditions of Fe dyshomeostasis.

Memantine-sulfur containing antioxidant conjugates as potential prodrugs to improve the treatment of Alzheimer's disease

The approved treatments for Alzheimer's disease (AD) exploit mainly a symptomatic approach based on the use of cholinesterase inhibitors or N-methyl-D-aspartate (NMDA) receptor antagonists. Natural antioxidant compounds, able to pass through the blood-brain barrier (BBB), have been extensively studied as useful neuroprotective agents. A novel approach towards excitotoxicity protection and oxidative stress associated with excess β amyloid (Aβ) preservation in AD is represented by selective glutamatergic antagonists that possess as well antioxidant capabilities. In the present work, GSH (1) or (R)-α-lipoic acid (LA) (2) have been covalently linked with the NMDA receptor antagonists memantine (MEM). The new conjugates, proposed as potential antialzheimer drugs, should act both as glutamate receptor antagonists and radical scavenging agents. The physico-chemical properties and "in vitro" membrane permeability, the enzymatic and chemical stability, the demonstrated "in vitro" antioxidant activity associated to the capacity to inhibit Aβ(1-42) aggregation makes at least compound 2 a promising candidate for treatment of AD patients.

Iron(III)-templated macrolactonization of trihydroxamate siderophores

A method was developed to synthesize macrocyclic trihydroxamate siderophores using optimized Yamaguchi macrolactonization conditions. The natural ability of siderophores to bind iron(III) was exploited to template the reactions and allowed for rapid reaction rates, high product conversions, and the formation of large macrolactone rings up to 35 atoms. An X-ray structure of a 33-membered macrolactone siderophore-Fe(III) complex is presented.

Halogenated 2'-benzoylpyridine thiosemicarbazone (XBpT) chelators with potent and selective anti-neoplastic activity: relationship to intracellular redox activity

Iron chelators of the 2'-benzoylpyridine thiosemicarbazone (BpT) class show substantial potential as anticancer agents. To explore structure-activity relationships, new BpT analogues were designed that incorporated halogen substituents on the noncoordinating phenyl group (XBpTs). These XBpT ligands exhibited potent antiproliferative activity with some analogues exceeding that of the parent BpT compound. Importantly, there was an appreciable therapeutic index in vitro, as mortal cells were significantly less affected by these chelators relative to neoplastic cells. The addition of a halogen led to a halogen-specific increase in the redox potential of XBpT-Fe complexes. Probing for chelator-induced intracellular reactive oxygen species (ROS) with the fluorescent probe, 2',7'-dichlorofluorescein, revealed a 1.5-4.7-fold increase in fluorescence upon incorporation of Cl, Br, or I to the parent analogues. Furthermore, an important structure-activity relationship was deduced where the addition of halogens led to a positive correlation between intracellular ROS generation and antiproliferative activity in the more hydrophilic BpT parent compounds.

Comparing the potential renal protective activity of desferrioxamine B and the novel chelator desferrioxamine B-N-(3-hydroxyadamant-1-yl)carboxamide in a cell model of myoglobinuria

Accumulating Mb (myoglobin) in the kidney following severe burns promotes oxidative damage and inflammation, which leads to acute renal failure. The potential for haem-iron to induce oxidative damage has prompted testing of iron chelators [e.g. DFOB (desferrioxamine B)] as renal protective agents. We compared the ability of DFOB and a DFOB-derivative {DFOB-AdAOH [DFOB-N-(3-hydroxyadamant-1-yl)carboxamide]} to protect renal epithelial cells from Mb insult. Loading kidney-tubule epithelial cells with dihydrorhodamine-123 before exposure to 100 μM Mb increased rhodamine-123 fluorescence relative to controls (absence of Mb), indicating increased oxidative stress. Extracellular Mb elicited a reorganization of the transferrin receptor as assessed by monitoring labelled transferrin uptake with flow cytometry and inverted fluorescence microscopy. Mb stimulated HO-1 (haem oxygenase-1), TNFα (tumour necrosis factor α), and both ICAM (intercellular adhesion molecule) and VCAM (vascular cell adhesion molecule) gene expression and inhibited epithelial monolayer permeability. Pre-treatment with DFOB or DFOB-AdAOH decreased Mb-mediated rhodamine-123 fluorescence, HO-1, ICAM and TNFα gene expression and restored monolayer permeability. MCP-1 (monocyte chemotactic protein 1) secretion increased in cells exposed to Mb-insult and this was abrogated by DFOB or DFOB-AdAOH. Cells treated with DFOB or DFOB-AdAOH alone showed no change in permeability, MCP-1 secretion or HO-1, TNFα, ICAM or VCAM gene expression. Similarly to DFOB, incubation of DFOB-AdAOH with Mb plus H2O2 yielded nitroxide radicals as detected by EPR spectroscopy, indicating a potential antioxidant activity in addition to metal chelation; Fe(III)-loaded DFOB-AdAOH showed no nitroxide radical formation. Overall, the chelators inhibited Mb-induced oxidative stress and inflammation and improved epithelial cell function. DFOB-AdAOH showed similar activity to DFOB, indicating that this novel low-toxicity chelator may protect the kidney after severe burns.

Complexes formed in solution between vanadium(IV)/(V) and the cyclic dihydroxamic acid putrebactin or linear suberodihydroxamic acid

An aerobic solution prepared from V(IV) and the cyclic dihydroxamic acid putrebactin (pbH(2)) in 1:1 H(2)O/CH(3)OH at pH = 2 turned from blue to orange and gave a signal in the positive ion electrospray ionization mass spectrometry (ESI-MS) at m/z(obs) 437.0 attributed to the monooxoV(V) species [V(V)O(pb)](+) ([C(16)H(26)N(4)O(7)V](+), m/z(calc) 437.3). A solution prepared as above gave a signal in the (51)V NMR spectrum at δ(V )= -443.3 ppm (VOCl(3), δ(V) = 0 ppm) and was electron paramagnetic resonance silent, consistent with the presence of [V(V)O(pb)](+). The formation of [V(V)O(pb)](+) was invariant of [V(IV)]:[pbH(2)] and of pH values over pH = 2-7. In contrast, an aerobic solution prepared from V(IV) and the linear dihydroxamic acid suberodihydroxamic acid (sbhaH(4)) in 1:1 H(2)O/CH(3)OH at pH values of 2, 5, or 7 gave multiple signals in the positive and negative ion ESI-MS, which were assigned to monomeric or dimeric V(V)- or V(IV)-sbhaH(4) complexes or mixed-valence V(V)/(IV)-sbhaH(4) complexes. The complexity of the V-sbhaH(4) system has been attributed to dimerization (2[V(V)O(sbhaH(2))](+) ↔ [(V(V)O)(2)(sbhaH(2))(2)](2+)), deprotonation ([V(V)O(sbhaH(2))](+) - H(+) ↔ [V(V)O(sbhaH)](0)), and oxidation ([V(IV)O(sbhaH(2))](0) -e(-) ↔ [V(V)O(sbhaH(2))](+)) phenomena and could be described as the sum of two pH-dependent vectors, the first comprising the deprotonation of hydroxamate (low pH) to hydroximate (high pH) and the second comprising the oxidation of V(IV) (low pH) to V(V) (high pH). Macrocyclic pbH(2) was preorganized to form [V(V)O(pb)](+), which would provide an entropy-based increase in its thermodynamic stability compared to V(V)-sbhaH(4) complexes. The half-wave potentials from solutions of [V(IV)]:[pbH(2)] (1:1) or [V(IV)]:[sbhaH(4)] (1:2) at pH = 2 were E(1/2) -335 or -352 mV, respectively, which differed from the expected trend (E(1/2) [VO(pb)](+/0) < V(V/IV)-sbhaH(4)). The complex solution speciation of the V(V)/(IV)-sbhaH(4) system prevented the determination of half-wave potentials for single species. The characterization of [V(V)O(pb)](+) expands the small family of documented V-siderophore complexes relevant to understanding V transport and assimilation in the biosphere.

Synthesis and characterization of a triazine dendrimer that sequesters iron(III) using 12 desferrioxamine B groups

The synthesis of a third generation triazine dendrimer, 1, containing multiple, iron-sequestering desferrioxamine B (DFO) groups is described. Benzoylation of the hydroxamic acid groups of DFO and formation of a reactive dichlorotriazine provide the intermediate for reaction with the second generation dendrimer displaying twelve amines. This strategy further generalizes the 'functional monomer' approach to generate biologically active triazine dendrimers. Dendrimer 1 is prepared in seven steps in 35% overall yield and displays 12 DFO groups making it 56% drug by weight. Spectrophotometric titrations (UV-vis) show that 1 sequesters iron(III) atoms with neither cooperativity nor significant interference from the dendrimer backbone. Evidence from NMR spectroscopy and mass spectrometry reveals a limitation to this functional monomer approach: trace amounts of O-to-N acyl migration from the protected hydroxamic acids to the amine-terminated dendrimer occurs during the coupling step leading to N-benzoylated dendrimers displaying fewer than 12 DFO groups.