The iron chelator pyridoxal isonicotinoyl hydrazone (PIH) and its analogues prevent damage to 2-deoxyribose mediated by ferric iron plus ascorbate

Iron chelating agents are essential for treating iron overload in diseases such as beta -thalassemia and are potentially useful for therapy in non-iro n overload conditions, including free radical mediated tissue injury. Defer oxamine (DFO), the only drug available for iron chelation therapy, has a nu mber of disadvantages (e.g., lack of intestinal absorption and high cost). The tridentate chelator pyridoxal isonicotinoyl hydrazone (PIH) has high ir on chelation efficacy in vitro and in vivo with high selectivity and affini ty for iron. It is relatively non-toxic, economical to synthesize and orall y effective. We previously demonstrated that submillimolar levels of PIH an d some of its analogues inhibit lipid peroxidation, ascorbate oxidation, 2- deoxyribose degradation, plasmid DNA strand breaks and 5, 5-dimethylpyrroli ne-N-oxide (DMPO) hydroxylation mediated by either Fe(II) plus H2O2 or Fe(I II)-EDTA plus ascorbate. To further characterize the mechanism of PIH actio n, we studied the effects of PIH and some of its analogues on the degradati on of 2-deoxyribose induced by Fe(III)-EDTA plus ascorbate. Compared with h ydroxyl radical scavengers (DMSO, salicylate and mannitol), PIH was about t wo orders of magnitude more active in protecting 2-deoxyribose from degrada tion, which was comparable with some of its analogues and DFO. Competition experiments using two different concentrations of 2-deoxyribose (15 vs. 1.5 mM) revealed that hydroxyl radical scavengers (at 20 or 60 mM) were signif icantly less effective in preventing degradation of 2-deoxyribose at 15 mM than 2-deoxyribose at 1.5 mM. In contrast, 400 muM PIH was equally effectiv e in preventing degradation of both 15 mM and 1.5 mM 2-deoxyribose. At a fi xed Fe(III) concentration, increasing the concentration of ligands (either EDTA or NTA) caused a significant reduction in the protective effect of PIH towards 2-deoxyribose degradation. We also observed that PIH and DFO preve nt 2-deoxyribose degradation induced by hypoxanthine, xanthine oxidase and Fe(III)-EDTA. The efficacy of PIH or DFO was inversely related to the EDTA concentration. Taken together, these results indicate that PIH land its ana logues) works by a mechanism different than the hydroxyl radical scavengers . It is likely that PIH removes Fe(III) from the chelates (either Fe(III)-E DTA or Fe(III)-NTA) and forms a Fe(III)-PIH2 complex that does not catalyze oxyradical formation. (C) 2000 Elsevier Science B.V. All rights reserved.