LC-UV/MS methods for the analysis of prochelator-boronyl salicylaldehyde isonicotinoyl hydrazone (BSIH) and its active chelator salicylaldehyde isonicotinoyl hydrazone (SIH)

Examination of diverse iron-chelating agents for the protection of differentiated PC12 cells against oxidative injury induced by 6-hydroxydopamine and dopamine

Labile redox-active iron ions have been implicated in various neurodegenerative disorders, including the Parkinson's disease (PD). Iron chelation has been successfully used in clinical practice to manage iron overload in diseases such as thalassemia major; however, the use of conventional iron chelators in pathological states without systemic iron overload remains at the preclinical investigative level and is complicated by the risk of adverse outcomes due to systemic iron depletion. In this study, we examined three clinically-used chelators, namely, desferrioxamine, deferiprone and deferasirox and compared them with experimental agent salicylaldehyde isonicotinoyl hydrazone (SIH) and its boronate-masked prochelator BSIH for protection of differentiated PC12 cells against the toxicity of catecholamines 6-hydroxydopamine and dopamine and their oxidation products. All the assayed chelating agents were able to significantly reduce the catecholamine toxicity in a dose-dependent manner. Whereas hydrophilic chelator desferrioxamine exerted protection only at high and clinically unachievable concentrations, deferiprone and deferasirox significantly reduced the catecholamine neurotoxicity at concentrations that are within their plasma levels following standard dosage. SIH was the most effective iron chelator to protect the cells with the lowest own toxicity of all the assayed conventional chelators. This favorable feature was even more pronounced in prochelator BSIH that does not chelate iron unless its protective group is cleaved in disease-specific oxidative stress conditions. Hence, this study demonstrated that while iron chelation may have general neuroprotective potential against catecholamine auto-oxidation and toxicity, SIH and BSIH represent promising lead molecules and warrant further studies in more complex animal models.

Novel multifunctional iron chelators of the aroyl nicotinoyl hydrazone class that markedly enhance cellular NAD+ /NADH ratios

BACKGROUND AND PURPOSE

Alzheimer's disease (AD) is a multifactorial condition leading to cognitive decline and represents a major global health challenge in ageing populations. The lack of effective AD therapeutics led us to develop multifunctional nicotinoyl hydrazones to target several pathological characteristics of AD.

EXPERIMENTAL APPROACH

We synthesised 20 novel multifunctional agents based on the nicotinoyl hydrazone scaffold, which acts as a metal chelator and a lipophilic delivery vehicle, donating a NAD⁺ precursor to cells, to target metal dyshomeostasis, oxidative stress, β-amyloid (Aβ) aggregation, and a decrease in the NAD⁺ /NADH ratio.

KEY RESULTS

The most promising compound, 6-methoxysalicylaldehyde nicotinoyl hydrazone (SNH6), demonstrated low cytotoxicity, potent iron (Fe)-chelation efficacy, significant inhibition of copper-mediated Aβ aggregation, oxidative stress alleviation, effective donation of NAD⁺ to NAD-dependent metabolic processes (PARP and sirtuin activity) and enhanced cellular NAD⁺ /NADH ratios, as well as significantly increased median Caenorhabditis elegans lifespan (to 1.46-fold of the control); partly decreased BACE1 expression, resulting in significantly lower soluble amyloid precursor protein-β (sAPPβ) and Aβ_1-40 levels; and favourable blood-brain barrier-permeation properties. Structure-activity relationships demonstrated that the ability of these nicotinoyl hydrazones to increase NAD⁺ was dependent on the electron-withdrawing or electron-donating substituents on the aldehyde- or ketone-derived moiety. Aldehyde-derived hydrazones containing the ONO donor set and electron-donating groups were required for NAD⁺ donation and low cytotoxicity.

CONCLUSIONS AND IMPLICATIONS

The nicotinoyl hydrazones, particularly SNH6, have the potential to act as multifunctional therapeutic agents and delivery vehicles for NAD⁺ precursors for AD treatment.

Modifying aroylhydrazone prochelators for hydrolytic stability and improved cytoprotection against oxidative stress

BSIH ((E)-N'-(2-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)benzylidene)isonicotinohydrazide) is a prodrug version of the metal chelator SIH ((E)-N'-(2-hydroxybenzylidene)isonicotinohydrazide) in which a boronate group prevents metal chelation until reaction with hydrogen peroxide releases SIH, which is then available for sequestering iron(III) and inhibiting iron-catalyzed oxidative damage. While BSIH has shown promise for conditionally targeting iron sequestration in cells under oxidative stress, the yield of SIH is limited by the fact that BSIH exists in cell culture media as an equilibrium mixture with its hydrolysis products isoniazid and 2-formylphenyl boronic acid. In the current study, several BSIH analogs were evaluated for their hydrolytic stability, reaction outcomes with H₂O₂, and prochelator-to-chelator conversion efficiency. Notably, the para-methoxy derivative (p-OMe)BSIH ((E)-N'-(5-methoxy-2-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)benzylidene)isonicotinohydrazide) and the meta-, para-double substituted (MD)BSIH ((E)-N'-((6-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)benzo[d][1,3]dioxol-5-yl)methylene)isonicotinohydrazide) showed 1.3- and 1.9-fold improved hydrolytic stability compared to BSIH, respectively, leading to a 22 and 50% increase in chelator released. Moreover, both prochelators were found to protect retinal pigment epithelial cells stressed with either H₂O₂ or paraquat insult.

The hydrolytic susceptibility of prochelator BSIH in aqueous solutions

The prochelator BSIH ((E)-N'-(2-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)benzylidene)isonicotinohydrazide) contains a boronate group that prevents metal coordination until reaction with peroxide releases the iron chelator SIH ((E)-N'-(2-hydroxybenzylidene)isonicotinohydrazide). BSIH exists in aqueous buffer and cell culture media in equilibrium with its hydrolysis products isoniazid and (2-formylphenyl)boronic acid (FBA). The relative concentrations of these species limit the yield of intact SIH available for targeted iron chelation. While the hydrolysis fragments are nontoxic to retinal pigment epithelial cells, these results suggest that modifications to BSIH that improve its hydrolytic stability yet maintain its low inherent cytotoxicity are desirable for creating more efficient prochelators for protection against cellular oxidative damage.

Cardioprotective effects of iron chelator HAPI and ROS-activated boronate prochelator BHAPI against catecholamine-induced oxidative cellular injury

Catecholamines may undergo iron-promoted oxidation resulting in formation of reactive intermediates (aminochromes) capable of redox cycling and reactive oxygen species (ROS) formation. Both of them induce oxidative stress resulting in cellular damage and death. Iron chelation has been recently shown as a suitable tool of cardioprotection with considerable potential to protect cardiac cells against catecholamine-induced cardiotoxicity. However, prolonged exposure of cells to classical chelators may interfere with physiological iron homeostasis. Prochelators represent a more advanced approach to decrease oxidative injury by forming a chelating agent only under the disease-specific conditions associated with oxidative stress. Novel prochelator (lacking any iron chelating properties) BHAPI [(E)-Ń-(1-(2-((4-(4,4,5,5-tetramethyl-1,2,3-dioxoborolan-2-yl)benzyl)oxy)phenyl)ethylidene) isonicotinohydrazide] is converted by ROS to active chelator HAPI with strong iron binding capacity that efficiently inhibits iron-catalyzed hydroxyl radical generation. Our results confirmed redox activity of oxidation products of catecholamines isoprenaline and epinephrine, that were able to activate BHAPI to HAPI that chelates iron ions inside H9c2 cardiomyoblasts. Both HAPI and BHAPI were able to efficiently protect the cells against intracellular ROS formation, depletion of reduced glutathione and toxicity induced by catecholamines and their oxidation products. Hence, both HAPI and BHAPI have shown considerable potential to protect cardiac cells by both inhibition of deleterious catecholamine oxidation to reactive intermediates and prevention of ROS-mediated cardiotoxicity.

Characterization of cytoprotective and toxic properties of iron chelator SIH, prochelator BSIH and their degradation products

Free cellular iron catalyzes the formation of toxic hydroxyl radicals and therefore chelation of iron could be a promising therapeutic approach in pathological states associated with oxidative stress. Salicylaldehyde isonicotinoyl hydrazone (SIH) is a strong intracellular iron chelator with well documented potential to protect against oxidative damage both in vitro and in vivo. Due to the short biological half-life of SIH and risk of toxicity due to iron depletion, boronate prochelator BSIH has been designed. BSIH cannot bind iron until it is activated by certain reactive oxygen species to active chelator SIH. The aim of this study was to examine the toxicity and cytoprotective potential of BSIH, SIH, and their decomposition products against hydrogen peroxide-induced injury of H9c2 cardiomyoblast cells. Using HPLC, we observed that salicylaldehyde was the main decomposition products of SIH and BSIH, although a small amount of salicylic acid was also detected. In the case of BSIH, the concentration of formed salicylaldehyde consistently exceeded that of SIH. Isoniazid and salicylic acid were not toxic nor did they provide any antioxidant protective effect in H9c2 cells. In contrast, salicylaldehyde was able to chelate intracellular iron and significantly preserve cellular viability and mitochondrial inner membrane potential induced by hydrogen peroxide. However it was consistently less effective than SIH. The inherent toxicities of salicylaldehyde and SIH were similar. Hence, although SIH - the active chelating agent formed following the BSIH activation - undergoes rapid hydrolysis, its principal decomposition product salicylaldehyde accounts markedly for both cytoprotective and toxic properties.