Evaluation of 2-methyl-3-hydroxy-4-pyridinecarboxylic acid as a possible chelating agent for iron and aluminium

Abstract

In view of a possible application to Fe and Al chelation therapy, 2-methyl-3-hydroxy-4-pyridinecarboxylic acid (DT2) was synthesised, and its complex formation, electrochemical and cytotoxic properties were studied. The complexing properties of DT2 towards Fe(III) and Al(III) were investigated in aqueous 0.6 m (Na)Cl at 25 °C by means of potentiometric titrations, UV-vis spectrophotometry, and ¹H NMR spectroscopy. DT2 is a triprotic acid (H₃L⁺) having pK_a1 = 0.47, pK_a2 = 5.64 and pK_a3 = 11.18. The metal–ligand complexes observed in solution and their corresponding stability constants (log β values) are the following: FeLH (19.38), FeL (16.01), FeLH₋₁ (12.28), FeL₂H₂ (37.29), FeL₃H₃ (53.41), FeL₃H₂ (47.99), FeL₃H (41.21) and FeL₃ (34.1); AlLH (17.43), AlL₂H₂ (33.74), AlL₂H (27.6), AlL₃H₃ (48.72), AlL₃H₂ (42.67), AlL₃H (35.8) and AlL₃ (27.92). The complex formation between DT2 and Fe(II) was studied by UV-vis: the weak complex FeLH (log β = 15.8) was detected. DT2 shows a lower complexation efficiency with Fe(III) and Al(III) than that of other available chelators, but higher than that of its non-methylated analogue 3-hydroxy-4-pyridinecarboxylic acid (DT0). The electrochemical behaviour of DT2 was investigated by means of cyclic voltammetry, indicating that the oxidation of the ligand proceeds through a two electron process with a CECE mechanism. Voltammetric curves suggest that the oxidation or the reduction of DT2 in vivo is unlikely. According to the thermodynamic data, also the Fe(III)–DT2 complexes do not undergo redox cycling at physiological pH. Amperometric titrations of solutions containing Fe(III) and DT2 at pH = 5 indicated the same Fe(III) : ligand stoichiometric ratio as calculated from potentiometric data. The toxicity of DT2 and of other simple hydroxypyridinecarboxylic acids was investigated in vitro and no cytotoxic activity was observed (IC₅₀ > 0.1 mM) on cancer cell lines and also on primary human cells, following a three day exposure.