Hydrazone chelators for the treatment of iron overload disorders: iron coordination chemistry and biological activity

The potentially tridentate ligand 2-pyridinecarbaldehyde isonicotinoyl hydrazone (HPCIH) and its analogues are an emerging class of orally effective Fe chelators that show great promise for the treatment of Fe overload diseases. Herein, we present an extensive study of the Fe coordination chemistry of the HPCIH analogues including the first crystallographically characterised Fe-II complex of these chelators. Unlike most other clinically effective Fe chelators, the HPCIH analogues bind Fe-II and not F-III. In fact, these chelators form low-spin bis-ligand F-II complexes, although NMR suggests that the complexes are close to the high-spin/low-spin crossover. All the Fe complexes show a high potential Fe-III/(II) redox couple (> 500 mV vs. NHE) and cyclic voltammetry in aqueous or mixed aqueous/organic solvents is irreversible as a consequence of a rapid hydration reaction that occurs upon oxidation. A number of the HPCIH analogues show high activity at inducing Fe efflux from cells and also at preventing Fe uptake by cells from the serum Fe transport protein transferrin. As a class of ligands, these chelators are more effective at reducing Fe uptake from transferrin than inducing Fe mobilisation from cells. This may be related to their ability to intercept Fe-II after its release from transferrin within the cell. Our studies indicate that their Fe chelation efficacy is due, at least in part, to the fact that these ligands and their Fe-II complexes are neutral at physiological pH (7.4) and sufficiently lipophilic to permeate cell membranes

New chelating agents for the treatment of thalassemia

The research described in this thesis was carried out in the context of the development and evaluation of new iron chelators for the treatment of iron overload. The chemistry of pyridoxal isonicotinoyl hydrazone (PIH) and structurally related compounds : pyridoxal benzoyl hydrazone (PBH), 3-hydroxyisonicotinaldehyde isonicotinoyl hydrazone (IIH), salicylaldehyde isonicotinoyl hydrazone (SIH) and salicylaldehyde benzoyl hydrazone (SBH) was investigated with emphasis on the chemical characteristics pertinent to the assessment of their value as biological iron chelators. As oral effectiveness is a highly desirable feature of any drug. a detailed study into the behaviour of PIH and related compounds in acidic environment was carried out. The results indicate that these compounds are remarkably stable, as a decomposition of < 3% is observed after 72 hr at pH 2 and 37°C. The acid dissociation constants of the several ionizable groups present in the ligands were determined by potentiometric titration and the formation of their iron complexes was studied subsequently by potentiometry and UV-vis spectrophotometry. All these systems are quite complex due to the number of dissociable protons in both the free and coordinated ligands. the high affinity of these compounds towards iron(III) and the formation of sparingly soluble species at pH~5. The affinity of PIH and related compounds for iron(III) was compared to that of known ligands (including desferrioxamine and transferrin) At pH via the respective pM values. 7.4, [Fe3+ ]= 10“6 M and a 1000-fold excess of ligand, the three chelating agents PIH, PBH and SIH have pM values of 27.7, 39.7 and 50.0 respectively, which, when compared to 25.6 of transferrin, indicates that these compounds are thermodynamically capable of removing iron from transferrin. This is not the case with IIH, which has a pM value of 24.5. The distribution of the complex species as a function of pH shows that in each case a significant fraction is present as the electrically neutral Fe(L)(HL) at pH 7.4. A model of the coordination geometry of this species, supported by spectroscopic data, is proposed. The affinity of PIH for iron(II) is significantly lower than for iron(III), as indicated by a formation constant of 6.98 for [Fe(II)(HL)2] compared to 12.47 for [Fe(III)(HL)2]. The mobilization of ferritin iron by PIH and related compounds was studied by UV-vis spectrophotometry and equilibrium dialysis. Both PIH and its analogs were able to mobilize ferritin iron in significant amounts, up to 80% of that achieved with desferrioxamine. The hydrazones under study possess therefore the chemical features desirable in an effective iron chelating drug. Further studies of their toxicity and metabolism are certainly justified to establish their pharmacological and clinical value

Iron chelators of the pyridoxal isonicotinoyl hydrazone class Part II. Formation constants with iron(III) and iron(II)

Formation constants for the iron(III) complexes of the orally effective iron chelator pyridoxal isonicotinoyl hydrazone (PIH) and three analogues: pyridoxal benzoyl hydrazone (PBH), 3-hydroxy- isonicotinaldehyde isonicotinoyl hydrazone (IIH) and salicylaldehyde isonicotinoyl hydrazone (SIH), have been determined by a combination of spectrophotometry and potentiometry. All four ligands bind iron(III) strongly giving, at physiological pH 7.4, values of pM (−log[uncomplexed metal]) between 27.7 and 50, comparable to or greater than those for transferrin (25.6) and desferrioxamine B (28.6). The complexation of Fe(II) by PIH has also been studied and has been found to be appreciable but very much weaker than that for Fe(III)

Iron chelators of the pyridoxal isonicotinoyl hydrazone class Part I. Ionisation characteristics of the ligands and their relevance to biological properties

The orally effective iron chelator, pyridoxal isonicotinoyl hydrazone (PIH), and five analogues, pyridoxal benzoyl hydrazone (PBH), pyridoxal p-methoxybenzoyl hydrazone ((PpMBH), pyridoxal m-fluorobenzoyl hydrazone (PmFBH), 3-hydroxy- isonicotinaldehyde isonicotinoyl hydrazone (IIH) and salicylaldehyde isonicotinoyl hydrazone (SIH) were synthesised and characterised and their acid dissociation constants measured by glass electrode potentiometry and UV—Vis spectrophotometry. Analysis of the data showed that at physiological pH all of the ligands are predominantly (av. 80%) in the form of the neutral molecule, allowing passage through cell membranes and access to intracellular iron pools. The results are discussed in the context of the development of an orally effective iron chelator for clinical use