Zu D. Liu

Design of iron chelators with therapeutic application

Abstract Iron overload is a serious clinical condition which can be largely prevented by the use of iron-specific chelating agents. Desferrioxamine-B, the most widely used iron chelator in haematology over the past 30 years, has a major disadvantage of being orally inactive. Consequently, the successful design of an orally active, non-toxic, selective iron chelator has become a much sought after goal. In order to identify an ideal iron chelator for clinical use, a range of specifications must be considered such as metal selectivity and affinity, kinetic stability of the complex, bioavailability and toxicity. A wide range of chelator types bind iron(III) and of these, hexa-, tri-, and bidentate are capable of providing iron(III) with the favoured octahedral environment. In this review, the comparative properties of such ligands are discussed, examples being selected from hydroxamates, aminocarboxylates, hydroxypyridinones, orthosubstituted phenols and triazoles.

Synthesis of 2-amido-3-hydroxypyridin-4(1H)-ones: novel iron chelators with enhanced pFe3+ values.

The synthesis of a range of 2-amido-3-hydroxypyridin-4-ones as bidentate iron(III) chelators with potential for oral administration is described. The pKa values of the ligands together with the stability constants of their iron(III) complexes have been determined. Results indicate that the introduction of an amido substituent at the 2-position leads to an appreciable enhancement of the pFe3+ values. The ability of these novel 3-hydroxypyridin-4-ones to facilitate the iron excretion in bile was investigated using a 59Fe-ferritin loaded rat model. The optimal effect was observed with the N-methyl amido derivative 15b, which has an associated pFe3+ value of 21.7, more than two orders of magnitude higher than that of deferiprone (1,2-dimethyl-3-hydroxypyridin-4-one) 1a (pFe3+ = 19.4). Dose response studies suggest that chelators with high pFe3+ values scavenge iron more effectively at lower doses when compared with simple dialkyl substituted hydroxypyridinones.

Amido-3-hydroxypyridin-4-ones as iron(III) ligands.

The synthesis and physicochemical properties of a range of 2- and 6-amido-3-hydroxypyridin-4-ones are described. All the amido-substituted 3-hydroxypyridin-4-ones have lower pK(a) values than 1,2-dimethyl-3-hydroxypyridin-4-one (deferiprone). This is due to the inductive effect of the amido group. Furthermore, the pK(a) values of the 3-hydroxy group in 1-nonsubstituted pyridinones are dramatically lower than those of the corresponding 1-alkyl analogues, indicating that a strong hydrogen bond exists between the 2-amido function and the 3-oxygen anion, which stabilises the anion. As a result of the decreased competition with protons, the pFe(3+) values of this group of molecules are higher than that of deferiprone. The distribution coefficients of these molecules are also increased despite the lack of a hydrophobic 1-alkyl substituent and this is ascribed to the intramolecular hydrogen bond. X-ray diffraction studies confirm the existence of the intramolecular hydrogen bond.

Design, synthesis and properties of novel iron(III)-specific fluorescent probes

Bidentate chelators such as hydroxypyridinones and hydroxypyranones are highly iron selective. The synthesis of two novel fluorescent probes N‐[2‐(3‐hydroxy‐2‐methyl‐4‐oxopyridin‐1(4H)‐yl)ethyl]‐2‐(7‐methoxy‐2‐oxo‐2H‐chromen‐4‐yl)acetamide (CP600) and N‐[(3‐hydroxy‐6‐methyl‐4‐oxo‐4H‐pyran‐2‐yl)methyl]‐2‐(7‐methoxy‐2‐oxo‐2H‐chromen‐4‐yl)acetamide (CP610) is reported. The method involves coupling the bidentate ligands, 3‐hydroxypyridin‐4‐one and 3‐hydroxypyran‐4‐one, with the well‐characterised fluorescent probe methoxycoumarin. Fluorescence emission of both probes at 380 nm is readily quenched by Fe3+. The fluorescence was quenched to a greater extent by Fe3+ than by Mn2+, Co2+, Zn2+, Ca2+, Mg2+, Na+ and K+ and to approximately the same extent as Cu2+. Comparison of the fluorescence‐quenching ability by a range of metal ions on CP600 and CP610 and the hexadentate chelator, calcein, under in‐vitro conditions, demonstrated advantages of the two novel fluorescent probes with respect to both iron(III) sensitivity and selectivity. Chelation of iron(III) by CP600 and CP610 leads to the formation of a complex with a metal‐to‐ligand ratio of 1:3. Fluorescence is quenched on formation of such complexes. These probes possess a molecular weight less than 400 and thus they are predicted to permeate biological membranes by passive diffusion, and have potential for reporting intracellular organelle labile iron levels.

Novel synthetic approach to 2-(1′-hydroxyalkyl)- and 2-amido-3-hydroxypyridin-4-ones

Abstract Novel methods for the synthesis of high pFe 3+ iron chelators, 2-(1′-hydroxyalkyl)- and 2-amido-3-hydroxypyridin-4-ones, have been developed. The products are obtained, via N -oxide intermediates, from either maltol or ethyl maltol.

Iron Chelator Chemistry

Iron overload is a serious clinical condition which can be largely prevented by the use of iron-specific chelating agents. Desferrioxamine-B(1)the most widely used iron chelator in haematology over the past thirty years, has a major disadvantage of being orally inactive’. Consequently, the successful design of an orally active, non-toxic, selective iron chelator has become a much sought after goal.

Design, synthesis and evaluation of N-basic substituted 3-hydroxypyridin-4-ones : Orally active iron chelators with lysosomotrophic potential

To investigate the possibility of targeting chelators into the lysosomal iron pool, nine bidentate 3‐hydroxypyridin‐4‐ones with basic chains have been synthesized. As the turnover of ferritin iron is centred in the lysosome, such strategy is predicted to increase chelator efficacy of bidentate ligands.

Gradient ion-pair high-performance liquid chromatographic method for analysis of 3-hydroxypyridin-4-one iron chelators

A gradient ion-pair HPLC separation of highly hydrophilic 3-hydroxypyridin-4-one (HPO) iron chelators is described. The separation of HPOs was performed using a reversed-phase polymer HPLC column (PLRP-S 100 A, 15x0.46 cm ID, 5 microm). The ion-pair buffer contained 1-heptanesulfonic acid (sodium salt) (5 mM) and the pH was adjusted to 2.0 using HCl. The gradient was 2%-35% CH3CN in 20 min and post-run was followed for 5 min using 2% CH3CN and 98% buffer. The flow-rate was 1 ml/min and the analytes were monitored at 280 nm. The retention times of 30 hydrophilic HPOs fell in the range of 10-18 min with sharp peak shapes, although these iron chelators possess various functional groups and distribution coefficients. The application of this HPLC method in the analysis of HPO chelators and their metabolites in rat bile and urine is described.