Searching for new aluminium chelating agents: a family of hydroxypyrone ligands.

Attention is devoted to the role of chelating agents in the treatment of aluminium related diseases. In fact, in spite of the efforts that have drastically reduced the occurrence of aluminium dialysis diseases, they so far constitute a cause of great medical concern. The use of chelating agents for iron and aluminium in different clinical applications has found increasing attention in the last thirty years. With the aim of designing new chelators, we synthesized a series of kojic acid derivatives containing two kojic units joined by different linkers. A huge advantage of these molecules is that they are cheap and easy to produce. Previous works on complex formation equilibria of a first group of these ligands with iron and aluminium highlighted extremely good pMe values and gave evidence of the ability to scavenge iron from inside cells. On these bases a second set of bis-kojic ligands, whose linkers between the kojic chelating moieties are differentiated both in terms of type and size, has been designed, synthesized and characterized. The aluminium(III) complex formation equilibria studied by potentiometry, electrospray ionization mass spectroscopy (ESI-MS), quantum-mechanical calculations and (1)H NMR spectroscopy are here described and discussed, and the structural characterization of one of these new ligands is presented. The in vivo studies show that these new bis-kojic derivatives induce faster clearance from main organs as compared with the monomeric analog.

A new bis-3-hydroxy-4-pyrone as a potential therapeutic iron chelating agent. Effect of connecting and side chains on the complex structures and metal ion selectivity.

This work reports the synthesis, characterization and study of complex formation equilibria of the new ligand 6,6'-(2-(diethylamino)ethylazanediyl)bis(methylene)bis(5-hydroxy-2-hydroxymethyl-4H-pyran-4-one) with Fe(III), Al(III), Cu(II) and Zn(II). On the basis of previous encouraging results with tetradentate bis-kojic acid chelators, this ligand was designed to improve the pharmacokinetic properties: increase the solubility, neutral at physiological pH7.4, and enhancement of membrane crossing ability. Fe(III) and Al(III) complexation gave evidence of high metal-sequestering capacity of L9. Cellular assays showed that the ligand is capable of crossing cellular membranes and it does not present toxic effects. Complex formation equilibria with the essential metal ions Cu(II) and Zn(II) have been furthermore studied to evaluate disturbances of this chelator on the homeostatic equilibria of these essential metal ions. A variety of techniques (potentiometry, UV-visible spectrophotometry, 1D and 2D NMR spectroscopy, ESI-MS (electrospray ionization-mass spectrometry), quantum mechanical calculations and X-ray diffraction) have facilitated the characterization of the ligand, and the corresponding iron and zinc complexes, together with an exhaustive analysis of the protonation and complex equilibria.

Chelation therapy for metal intoxication: comments from a thermodynamic viewpoint.

Chelation therapy plays a prominent role in the clinical treatment of metal intoxication. In this paper the principal causes of metal toxicity are exposed, and the chemical and biomedical requisites of a chelating agent are sketched. The chelating agents currently in use for scavenging toxic metal ions from humans belong to few categories: those characterized by coordinating mercapto groups, by oxygen groups, poliaminocarboxylic acids, and dithiocarbamates. Considering that the complex formation equilibria have been studied for less than 50% of chelators in use, some reflections on the utility of stability constants are presented, together with an evaluation of ligands under the stability profile. The competition between endogenous and toxic target metal ions for the same chelating agent is furthermore examined. A thorough examination of stability constant databases has allowed to select, for each toxic metal, the ligands distinguished by the best pMe values. Even though this selection does not consider the biomedical requisites of a chelating agent, it gives a clear picture both of the pMe values that can be attained, and of the most appropriate chelators for each metal ion.

Iron(III) and aluminium(III) complexes with substituted salicyl-aldehydes and salicylic acids.

The chelating properties toward iron(III) and aluminium(III) of variously substituted salicyl-aldehydes and salicylic acids have been evaluated, together with the effect of methoxy and nitro substituents in ortho and para position with respect to the phenolic group. The protonation and iron and aluminium complex formation equilibria have been studied by potentiometry, UV-visible spectrophotometry and (1)H NMR spectroscopy. The overall results highlight that salicyl-aldehydes present good chelating properties toward iron(III), with pFe ranging from 14.2 with nitro to 15.7 with methoxy substituent, being ineffective toward aluminium; the pFe values for salicylic acids are generally lower than those for salicyl-aldehydes, and about 4 units higher than the corresponding pAl values. The effect of the two substituents on the chelating properties of the ligands can be rationalized in terms of the Swain-Lupton treatment which accounts for the field and resonance effects. The structural characterization of the 1:2 iron complex with p-nitro salicylic acid shows that iron(III) ion exhibits an octahedral surrounding where two salicylate chelating ligands supply two O-phenolate and two O-carboxylate donor atoms in a roughly equatorial plane. The trans-apical sites are occupied by two aqua ligands.

A family of hydroxypyrone ligands designed and synthesized as iron chelators.

The use of chelating agents for iron and aluminum in different clinical applications has found increasing attention in the last thirty years. Desferal, deferiprone and deferasirox, chelating agents nowadays in use, are based on hydroxamic groups, hydroxyl-substituted pyridinones or aromatic ring systems. With the aim of designing new chelators, we synthesized a series of kojic acid derivatives composed by two kojic units joined by linkers variously substituted. The huge advantages of these molecules are that they are easy and cheap to produce. Preliminary works on complex formation equilibria of the first group of ligands with iron and aluminium highlighted extremely good pMe values and gave evidence of the ability to scavenge iron from inside cells. On these bases a second set of bis-kojic ligands, whose linkers between the kojic chelating moieties are differentiated both in terms of type and size, has been designed, synthesized and characterized. The structural characterization of these new ligands is presented, and the protonation and iron(III) complex formation equilibria studied by potentiometry, UV-Visible spectrophotometry, electrospray ionization mass (ESI-MS) and (1)H NMR spectroscopy will be described and discussed.

Thermodynamic and spectroscopic investigation on the role of Met residues in Cu(II) binding to the non-octarepeat site of the human prion protein.

Among the common features shared by neurodegenerative diseases there is the central role played by specific proteins or peptides which accumulate in neurons as insoluble plaques or tangles, containing abnormal amounts of redox-active metal ions, like copper and iron. In the case of transmissible spongiform encephalopathies (TSE), the involved protein is known as "prion protein" (PrP(C)) since "prions" (proteinaceous and infectious) are the agents which make TSE transmissible. It is widely accepted that PrP(C), in its wild-type form, can bind up to six Cu(II) ions, four of them in the so-called "octarepeat domain" and the others in the "fifth (non-octarepeat) binding-site". The latter domain contains two His residues, acting as anchoring sites for Cu(II) ions, and other potential binding residues, such as Lys and Met. While it is widely accepted that Lys residues do not take part in complex-formation, the role of methionines is still debated. In order to shed light on this issue, some peptides have been synthesized, either directly mimicking the sequence of the second half of the fifth binding site of human-PrP(C) (apo-form) or analogues where Met residues have been substituted by n-leucine. In addition, a series of short peptides, containing both His and Met residues in different relative positions, have been investigated, for the sake of comparison. Spectroscopic results, including NMR spectra of systems containing Ni(II) as a probe for the paramagnetic Cu(II) ion, agree on the exclusion of any direct interaction between the sulphur atom of Met residues and the Cu(II) ion already bound to His-imidazole side-chains. However, thermodynamic data show that Met-109 somewhat contributes to stability of complex species and this can be attributed to different electronic and steric effects.