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.

Basic 3-hydroxypyridin-4-ones: potential antimalarial agents.

3-Hydroxypyridin-4-ones selectively bind iron under biological conditions and one such compound has found application in the treatment of thalassaemia-linked iron overload. Related molecules have also been demonstrated to possess an antimalarial effect at levels which are non-toxic to mammalian cells. In an attempt to improve the efficiency of such molecules we have investigated the effect of introducing basic nitrogen centres into 3-hydroxypyridin-4-ones in an attempt to achieve targeting to lysosomes and other intracellular acidic vacuoles. Several of the compounds reported in this communication possess enhanced antimalarial activity over that of the simple hydroxypyridinone class.

Structural characterization of chelator-terminated dendrimers and their synthetic intermediates by mass spectrometry.

Herein, we report the structural analysis of a novel family of iron-chelating dendrimers and their synthetic intermediates utilizing matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) and electrospray ionization ion trap (ESI IT) MS. These dendrimers share a benzene tricarbonyl core moiety attached to three tripodal branching units, each linking to three terminal groups, ranging from carboxyl, catechol and 3-hydroxy-6-methyl-pyran-4-one moieties and their protected analogs. In order to monitor the progression of dendrimer synthesis, all intermediates and final products were mass analyzed by conventional MALDI-TOF MS with and without alkali metal spiking. For structural characterization, interpretable post-source decay (PSD) and electrospray ionization ion trap MS/MS spectra were obtained from proton, sodium and potassium adducts of the dendrimers. One major route of dendrimer fragmentation was at or adjacent to the amide bonds either of the terminal chelating groups or near the core moiety. Fragmentation in the latter case was primarily at the N-terminal side of the amide bond and was directed by the proximity of a tertiary carbon of the branching unit. Furthermore, it was found that terminal ester, ether and amide linkages to the protecting and chelating groups could be sequentially broken in a single MS/MS spectrum through multiple cleavages resulting in product ions of decreasing intensity. Moreover, such cleavages could also be induced in a stepwise manner in a multistage ion trap MS(n) experiment.

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 Fe(3+). The fluorescence was quenched to a greater extent by Fe(3+) than by Mn(2+), Co(2+), Zn(2+), Ca(2+), Mg(2+), Na(+) and K(+) and to approximately the same extent as Cu(2+). 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.

Emerging understanding of the advantage of small molecules such as hydroxypyridinones in the treatment of iron overload.

Deferiprone, a hydroxypyridin-4-one, is effective at facilitating iron removal from iron overloaded patients, when administered orally. Some problems associated with deferiprone are discussed. Hydroxypyridinone analogues with improved distribution, metabolism and affinity for iron are described. In particular the "high pFe(3+)" hydroxypyridin-4-ones possess considerable clinical potential.

Oral iron chelators--development and application.

Iron chelation therapy is the only therapeutic approach that leads to enhanced iron excretion in beta-thalassaemia major and other transfusion-dependent patients. Although desferrioxamine has been used in such treatment over the last three decades, it is not an ideal drug due to its poor oral availability. Consequently extensive research effort has been directed towards the identification of non-toxic, orally active iron chelators. An ideal candidate must possess a range of critical physicochemical and biological properties, such as high selectivity and affinity for iron(III), tightly controlled distribution and metabolic profiles and low toxicity. Unfortunately, hexadentate ligands are generally associated with poor oral bioavailability, whereas many tridentate and bidentate molecules are orally active. The tridentate triazoles have been investigated for clinical potential; they are readily absorbed from the gastrointestinal tract and promote iron excretion with high efficacy. In similar fashion, several bidentate hydroxypyridinones have been demonstrated to possess potential as oral chelating agents.

Design, synthesis, and evaluation of novel 2-substituted 3-hydroxypyridin-4-ones: structure-activity investigation of metalloenzyme inhibition by iron chelators.

A range of novel 3-hydroxypyridin-4-ones with different R(2) substitutents has been synthesized for the investigation of the structure-activity relationship between the chemical nature of the ligand and the inhibitory activity of the iron-containing metalloenzyme 5-lipoxygenase. Results indicate that the molecular dimensions, together with the lipophilicity, have a critical impact on the ability of this class of chelator to inhibit 5-lipoxygenase. Hydrophilic ligands with a bulky R(2) substitutent tend to be weak inhibitors; thus 1,6-dimethyl-2-(4'-N-n-propylsuccinamido)methyl-3-hydroxypyridin-4(1H)-one (22b) which has the largest R(2) substitutent only caused 2% inhibition of the enzyme activity after 30 min incubation at 110 microM IBE (iron-binding equivalents), as compared with deferiprone which caused 40% inhibition of the enzyme activity, under the same conditions.

Design of clinically useful iron(III)-selective chelators.

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, nontoxic, 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.