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

Abstract

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.

Introduction

The most frequent treatment of haemoglobinopathic disorders such as β-thalassaemia major is to maintain high levels of haemoglobin by regular blood transfusion. Because man lacks a physiological means of eliminating excess iron, iron associated with transfused red cells progressively accumulates, in the liver and other highly perfused organs, leading to tissue damage, organ failure and eventually death.¹ Complications associated with elevated iron levels can be largely avoided by the use of iron-specific chelating agents and in particular desferrioxamine (DFO). Unfortunately, DFO lacks oral activity and has to be administered parenterally. This inevitably leads to poor patient compliance.² In an attempt to overcome the disadvantages associated with DFO, the successful design of an orally active, non-toxic, selective iron chelator has been a much sort after goal. In designing iron chelators for clinical application, metal selectivity and ligand-metal complex stability are of paramount importance.3, 4 A suitable comparator for ligands is the pFe³⁺ value, defined as the negative logarithm of the concentration of the free iron(III) in solution. Typically pFe³⁺ values are calculated for total [ligand]=10⁻⁵ M, total [iron]=10⁻⁶ M at pH 7.4. The comparison of ligands using this parameter is useful, since pFe³⁺, unlike the corresponding stability constants, takes into account the effects of ligand basicity, denticity and degree of protonation, as well as differences in metal–ligand stoichiometries. Chelators with high pFe³⁺ values are predicted not only to scavenge iron more effectively at low ligand concentrations, but also dissociate less readily and therefore form lower concentrations of the partially co-ordinated complexes.

3-Hydroxypyridin-4-ones (HPOs) (1) (Table 1) are currently one of the main candidates for the development of orally active iron chelators.³ Indeed, the 1,2-dimethyl derivative 1a (deferiprone), with an associated pFe³⁺ value of 19.4, is the only orally active iron chelator currently available for clinical use (marketed by Apotex Inc., Toronto, Canada as Ferriprox™). In order to further improve chelation efficacy and minimise drug-induced toxicity, considerable effort has been put into the design of novel hydroxypyridinones with enhanced pFe³⁺ values.5, 6 Novartis has produced a range of bidentate hydroxypyridinone ligands, which possess an aromatic substituent at the 2-position. The aromatic group is reported to stabilise the resulting iron complex and hence increase the pFe³⁺ values.⁵ The lead compound 2 was found to be orally active⁷ and highly effective at removing iron from both the iron-loaded rat and marmoset.⁵ Recently we have demonstrated that the introduction of a 1′-hydroxyalkyl group at the 2-position of 3-hydroxypyridin-4-ones 3 leads to an appreciable enhancement of pFe³⁺ values.6, 8 This effect results from the decrease in pK_a value of the 3 and 4 pyridinone oxygens due to the combined effect of intramolecular hydrogen bonding and electron withdrawal from the pyridinone ring.⁸ Interestingly the Novartis lead compound 2 also possesses a 1′-hydroxyl group at the 2-position and this is almost certainly responsible for the observed enhanced pFe³⁺ value of the molecule. Such enhancement of pFe³⁺ values is associated with a clear improvement of chelator ability to remove iron under in vivo conditions.6, 8 It was reasoned that the introduction of other suitable substituents, e.g., the amido function 4 at the 2-position of 3-hydroxypyridin-4-ones would increase the pFe³⁺ value in a similar manner. A potential advantage of these 2-amido HPOs 4 is their non-chiral nature, which contrasts with the high pFe³⁺ 2-(1′-hydroxyalkyl) HPOs 2 and 3, which possess a chiral centre. In this report, we describe the synthesis, physicochemical properties and in vivo iron mobilisation efficacies of a range of 2-amido-3-hydroxypyridin-4-ones.