Orally active chelators for the treatment of iron overload diseases

Abstract

Iron, although crucial for life, is toxic if present in excessive amounts in the humannbody. Patients suffering from diseases, including hemochromatosis and s-thalassemianmajor, accumulate, either directly or through repeated blood transfusions, high levels ofniron in their vital organs, which rapidly leads to death through cardiac or liver failure if notntreated. Currently, Desferalt (desferrioxamine mesylate, DFO), a hexadentatenhydroxamic acid, is still the only clinically approved iron chelator. This drug is verynexpensive and orally ineffective so patients must endure long periods (12-24 h/day, 5-6ndays/week) of subcutaneous infusion of DFO to excrete these excessive levels of iron.nThese inherent limitations of DFO have prompted research in the search for alternatives.n We have developed a new aroylhydrazone class of iron chelators based on thenparent ligands 2-pyridinecarbaldehyde isonicotinoyl hydrazone (HPCIH) and 2-npyrrolecarbaldehyde isonicotinoyl hydrazone (H2PRCIH). These are tridentate ligandsncapable of coordinating through the pyridine-N (or pyrrole-N), imine-N and carbonyl-0,nforming bis-ligand iron complexes (FeL2) with N4O2 octahedral coordination sphere.nStudies on ligands from the HPCIH series have revealed much greater iron-chelatingnefficacy than DFO in vitro and are promising as orally effective iron chelators. However,nperformance of the H2PRCIH series was unsatisfactory. Besides witnessing a completende-activation of the ligands by substituting the pyridine ring with a pyrrole ring, it wasnalso interesting to note that subtle changes to the non-coordinating pendant aromatic ringnof the ligand resulted in pronounced changes in its iron chelation efficacy. We have alsonobserved an unexpected ligand oxidation in Fll(PCIH)2 under aqueous aerobic conditionsnto give the ligand N-(isonicotinoyl)-N'-(picolinoyl) hydrazine (H2IPH) complexed to Felll, namely Felll(HIPH)(IPH). This has spawned a series of N,N'-diaroylhydrazine chelators,nthe H2IPH series. Our studies have shown that most of the H2IPH analogues exhibitnsimilar iron-chelating efficacy to their hydrazone analogues, with just a couple ofnexceptions.n 13, 6 and 8 analogues of the HPCIH, H2PRCIH and H2IPH series, respectively,nhave been synthesised and screened for iron chelation efficacy in an in vitro environment.nOut of the 27 ligands, 5 of the HPCIH and 5 of the H2IPH analogues have displayednsubstantially greater activity than DFO. To better understand the biological activities ofnthese ligands, we have studied the chemical and physical properties of these ligands andntheir iron complexes. Chiefly, we are concerned with the metal selectivity and affinity ofnthe ligands, their biological barrier permeability in terms of protonation states andnlipophilicity, as well as the redox chemistry of the metal complexes. The protonationnconstants of the ligands as well as the formation constants of their iron complexes werendetermined potentiometrically, and lipophilicity / hydrophilicity of the ligands and theirniron complexes were determined by direct partitioning between 1-octanol and water. Wenhave determined that the H2PRCIH and H2IPH analogues are Felll chelators, whereas thenHPCIH analogues prefer Felll. Hence, HPCIH and HPCAH (2-pyridinecarbaldehyde-4-naminobenzoyl hydrazone) were used as model ligands to study the coordination chemistrynof the HPCIH series with divalent metals of the first transition series, since several ofnthese metals are physiologically important. During the course of our investigation, we have discovered several interestingnaspects of the coordination chemistry of these iron chelators. We have also investigatednthe oxidation of HPCIH to H2IPH catalysed by Felll, and have proposed a plausiblenmechanism for its occurrence.nn