Kinetico-mechanistic studies on methemoglobin generation by biologically active thiosemicarbazone iron(III) complexes

Isosteric Replacement of Sulfur to Selenium in a Thiosemicarbazone: Promotion of Zn(II) Complex Dissociation and Transmetalation to Augment Anticancer Efficacy

We implemented isosteric replacement of sulfur to selenium in a novel thiosemicarbazone (PPTP4c4mT) to create a selenosemicarbazone (PPTP4c4mSe) that demonstrates potentiated anticancer efficacy and selectivity. Their design specifically incorporated cyclohexyl and styryl moieties to sterically inhibit the approach of their Fe(III) complexes to the oxy-myoglobin heme plane. Importantly, in contrast to the Fe(III) complexes of the clinically trialed thiosemicarbazones Triapine, COTI-2, and DpC, the Fe(III) complexes of PPTP4c4mT and PPTP4c4mSe did not induce detrimental oxy-myoglobin oxidation. Furthermore, PPTP4c4mSe demonstrated more potent antiproliferative activity than the homologous thiosemicarbazone, PPTP4c4mT, with their selectivity being superior or similar, respectively, to the clinically trialed thiosemicarbazone, COTI-2. An advantageous property of the selenosemicarbazone Zn(II) complexes relative to their thiosemicarbazone analogues was their greater transmetalation to Cu(II) complexes in lysosomes. This latter effect probably promoted their antiproliferative activity. Both ligands down-regulated multiple key receptors that display inter-receptor cooperation that leads to aggressive and resistant breast cancer.

Desulfurization of thiosemicarbazones: the role of metal ions and biological implications

Thiosemicarbazones are biologically active substances whose structural formula is formed by an azomethine, an hydrazine, and a thioamide fragments, to generate a R2C=N-NR-C(=S)-NR2 backbone. These compounds often act as ligands to generate highly stable metal-organic complexes. In certain experimental conditions, however, thiosemicarbazones undergo reactions leading to the cleavage of the chain. Sometimes, the breakage involves desulfurization processes. The present work summarizes the different chemical factors that influence the desulfurization reactions of thiosemicarbazones, such as pH, the presence of oxidant reactants or the establishment of redox processes as those electrochemically induced, the effects of the solvent, the temperature, and the electromagnetic radiation. Many of these reactions require coordination of thiosemicarbazones to metal ions, even those present in the intracellular environment. The nature of the products generated in these reactions, their detection in vivo and in vitro, together with the relevance for the biological activity of these compounds, mainly as antineoplastic agents, is discussed.

Differential transmetallation of complexes of the anti-cancer thiosemicarbazone, Dp4e4mT: effects on anti-proliferative efficacy, redox activity, oxy-myoglobin and oxy-hemoglobin oxidation

The di-2-pyridylthiosemicarbazone (DpT) analogs demonstrate potent and selective anti-proliferative activity against human tumors. The current investigation reports the synthesis and chemical and biological characterization of the Fe(iii), Co(iii), Ni(ii), Cu(ii), Zn(ii), Ga(iii), and Pd(ii) complexes of the promising second generation DpT analog, di-2-pyridylketone-4-ethyl-4-methyl-3-thiosemicarbazone (Dp4e4mT). These studies demonstrate that the Dp4e4mT Co(iii), Ni(ii), and Pd(ii) complexes display distinct biological activity versus those with Cu(ii), Zn(ii), and Ga(iii) regarding anti-proliferative efficacy against cancer cells and a detrimental off-target effect involving oxidation of oxy-myoglobin (oxy-Mb) and oxy-hemoglobin (oxy-Hb). With regards to anti-proliferative activity, the Zn(ii) and Ga(iii) Dp4e4mT complexes demonstrate facile transmetallation with Cu(ii), resulting in efficacy against tumor cells that is strikingly similar to the Dp4e4mT Cu(ii) complex (IC50: 0.003-0.006 μM and 72 h). Relative to the Zn(ii) and Ga(iii) Dp4e4mT complexes, the Dp4e4mT Ni(ii) complex demonstrates kinetically slow transmetallation with Cu(ii) and intermediate anti-proliferative effects (IC50: 0.018-0.076 μM after 72 h). In contrast, the Co(iii) and Pd(ii) complexes demonstrate poor anti-proliferative activity (IC50: 0.262-1.570 μM after 72 h), probably due to a lack of transmetallation with Cu(ii). The poor efficacy of the Dp4e4mT Co(iii), Ni(ii), and Pd(ii) complexes to transmetallate with Fe(iii) markedly suppresses the oxidation of oxy-Mb and oxy-Hb. In contrast, the 2 : 1 Dp4e4mT: Cu(ii), Zn(ii), and Ga(iii) complexes demonstrate facile reactions with Fe(iii), leading to the redox active Dp4e4mT Fe(iii) complex and oxy-Mb and oxy-Hb oxidation. This study demonstrates the key role of differential transmetallation of Dp4e4mT complexes that has therapeutic ramifications for their use as anti-cancer agents.

Steric Blockade of Oxy-Myoglobin Oxidation by Thiosemicarbazones: Structure-Activity Relationships of the Novel PPP4pT Series

The di-2-pyridylketone thiosemicarbazones demonstrated marked anticancer efficacy, prompting progression of DpC to clinical trials. However, DpC induced deleterious oxy-myoglobin oxidation, stifling development. To address this, novel substituted phenyl thiosemicarbazone (PPP4pT) analogues and their Fe(III), Cu(II), and Zn(II) complexes were prepared. The PPP4pT analogues demonstrated potent antiproliferative activity (IC50: 0.009-0.066 μM), with the 1:1 Cu:L complexes showing the greatest efficacy. Substitutions leading to decreased redox potential of the PPP4pT:Cu(II) complexes were associated with higher antiproliferative activity, while increasing potential correlated with increased redox activity. Surprisingly, there was no correlation between redox activity and antiproliferative efficacy. The PPP4pT:Fe(III) complexes attenuated oxy-myoglobin oxidation significantly more than the clinically trialed thiosemicarbazones, Triapine, COTI-2, and DpC, or earlier thiosemicarbazone series. Incorporation of phenyl- and styryl-substituents led to steric blockade, preventing approach of the PPP4pT:Fe(III) complexes to the heme plane and its oxidation. The 1:1 Cu(II):PPP4pT complexes were inert to transmetalation and did not induce oxy-myoglobin oxidation.

Designing Tailored Thiosemicarbazones with Bespoke Properties: The Styrene Moiety Imparts Potent Activity, Inhibits Heme Center Oxidation, and Results in a Novel "Stealth Zinc(II) Complex"

A novel, potent, and selective antitumor agent, namely (E)-3-phenyl-1-(2-pyridinyl)-2-propen-1-one 4,4-dimethyl-3-thiosemicarbazone (PPP44mT), and its analogues were synthesized and characterized and displayed strikingly distinctive properties. This activity was mediated by the inclusion of a styrene moiety, which through steric and electrochemical mechanisms prevented deleterious oxy-myoglobin or oxy-hemoglobin oxidation relative to other potent thiosemicarbazones, i.e., di-2-pyridylketone-4-cyclohexyl-4-methyl-3-thiosemicarbazone (DpC) or di-2-pyridylketone-4,4-dimethyl-3-thiosemicarbazone (Dp44mT). Structure-activity relationship analysis demonstrated specific tuning of PPP44mT electrochemistry further inhibited oxy-myoglobin or oxy-hemoglobin oxidation. Both PPP44mT and its Cu(II) complexes showed conspicuous almost immediate cytotoxicity against SK-N-MC tumor cells (within 3 h). In contrast, [Zn(PPP44mT)₂] demonstrated a pronounced delay in activity, taking 48 h before marked antiproliferative efficacy was apparent. As such, [Zn(PPP44mT)₂] was designated as a "stealth Zn(II) complex" that overcomes the near immediate cytotoxicity of PPP44mT or its copper complexes. Upon examination of the suppression of oncogenic signaling, [Zn(PPP44mT)₂] was superior at inhibiting cyclin D1 expression compared to DpC or Dp44mT.

Liposomal formulations of anticancer copper(II) thiosemicarbazone complexes

α-N-Heterocyclic thiosemicarbazones such as triapine and COTI-2 are currently investigated as anticancer therapeutics in clinical trials. However, triapine was widely inactive against solid tumor types. A likely explanation is the short plasma half-life time and fast metabolism. One promising approach to overcome these drawbacks is the encapsulation of the drug into nanoparticles (passive drug-targeting). In a previous work we showed that it was not possible to stably encapsulate free triapine into liposomes. Hence, in this manuscript we present the successful preparation of liposomal formulations of the copper(II) complexes of triapine and COTI-2. To this end, various drug-loading strategies were examined and the resulting liposomes were physico-chemically characterized. Especially for liposomal Cu-triapine, a decent encapsulation efficacy and a slow drug release behavior could be observed. In contrast, for COTI-2 and its copper(II) complex no stable loading could be achieved. Subsequent in vitro studies in different cell lines with liposomal Cu-triapine showed the expected strongly reduced cytotoxicity and DNA damage induction. Also in vivo distinctly higher copper plasma levels and a continuous release could be observed for the liposomal formulation compared to free Cu-triapine. Taken together, the here presented nanoformulation of Cu-triapine is an important step further to increase the plasma half-life time and tumor targeting properties of anticancer thiosemicarbazones.

A Novel Centrosymmetric Fe(III) Complex with Anionic Bis-pyrazolyl-s-triazine Ligand; Synthesis, Structural Investigations and Antimicrobial Evaluations

Reaction of 2,4-bis(3,5-dimethyl-1H-pyrazol-1-yl)-6-methoxy-1,3,5-triazine (MBPT) pincer ligand with FeCl3 in acidic medium (1:1 v/v) afforded the [Fe(BPT)Cl2(CH3OH)] complex of the hydrolyzed monobasic ligand: 4,6-bis(3,5-dimethyl-1H-pyrazol-1-yl)-1,3,5-triazin-2(1H)-one (HBPT). In this complex, the Fe(III) ion is hexacoordinated with one anionic pincer ligand unit (BPT−1), two chloride ions, and one coordinated methanol molecule. It crystallized in the monoclinic crystal system and centrosymmetric P21/c space group with Z = 2 and unit cell parameters of a = 7.309(2) Å, b = 25.461(8) Å, c = 9.918(3) Å and β = 102.646(7)°. The structure of this complex is stabilized by C–H…Cl intramolecular hydrogen bonding interactions with donor acceptor distances ranging from 3.577(3)–3.609(3) Å while its supramolecular structure is controlled by intermolecular O–H…O, O–H…N, and C–H…Cl hydrogen bonding interactions. Hirshfeld analysis of molecular packing indicates that the percentages of the Cl…H, C…O, O…H, C…C, H…C, and N…H contacts are 21.1, 1.7, 10.2, 2.1, 8.6, and 10.4%, respectively. The nature and relative strength of the different coordination interactions in the [Fe(BPT)Cl2(CH3OH)] complex are discussed based on atoms in molecules theory. Antimicrobial evaluations indicated that the [Fe(BPT)Cl2(CH3OH)] complex showed moderate antibacterial and antifungal activities compared to amoxicillin and ampicillin antibiotics as standard drugs.

Thiosemicarbazone-metal complexes exhibiting cytotoxicity in colon cancer cell lines through oxidative stress

Colorectal cancer is the third most common type of cancer and has a high incidence in developed countries. At present, specific treatments are being required to allow individualized therapy depending on the molecular alteration on which the drug may act. The aim of this project is to evaluate whether HPTSC and HPTSC* thiosemicarbazones (HPTSC = pyridine-2-carbaldehyde thiosemicarbazone and HPTSC* = pyridine-2-carbaldehyde 4N-methylthiosemicarbazone), and their complexes with different transition metal ions as Cu(II), Fe(III) and Co(III), have antitumor activity in colon cancer cells (HT-29 and SW-480), that have different oncogenic characteristics. Cytotoxicity was evaluated and the involvement of oxidative stress in its mechanism of action was analyzed by quantifying the superoxide dismutase activity, redox state by quantification of the thioredoxin levels and reduced/oxidized glutathione rate and biomolecules damage. The apoptotic effect was evaluated by measurements of the levels of caspase 9 and 3 and the index of histones. All the metal-thiosemicarbazones have antitumor activity mediated by oxidative stress. The HPTSC*-Cu was the compound that showed the best antitumor and apoptotic characteristics for the cell line SW480, that is KRAS gene mutated.

Proton-assisted air oxidation mechanisms of iron(ii) bis-thiosemicarbazone complexes at physiological pH: a kinetico-mechanistic study

The kinetics of oxidation of different biologically-active Fe^II bis-thiosemicarbazone complexes in water has been monitored at varying dioxygen concentration, temperature, pressure, and pH. The oxidation reactions observed can be resolved as a single-step process, producing the expected ferric complex, with rates increasing with decreasing pH. From the pH-dependence of the observed rate constants, a rate law with two terms can be derived, one of them being independent of the acid concentration and the other term showing a saturation behaviour with respect to [H⁺]. These results indicate the existence of two parallel pathways for oxidation: the acid-independent pathway is only operative for the complexes with ligands bearing terminal, non-coordinated, unsubstituted amines, whereas the term with a [H⁺]-limiting kinetic behaviour is observed for all the complexes and indicates that the reacting species has to be protonated prior to the oxidation step. From the data collected, the rate law and the thermal and pressure activation parameters have been used to interpret the operating reaction mechanisms. Given the fact that the empirical trends rule out an outer-sphere oxidation process, DFT calculations have been carried out to explain the results and suggest the likely formation, under steady-state very low concentration conditions, of Fe^III superoxo and hydroperoxo intermediates.

Anticancer Thiosemicarbazones: Chemical Properties, Interaction with Iron Metabolism, and Resistance Development

SIGNIFICANCE

During the past decades, thiosemicarbazones were clinically developed for a variety of diseases, including tuberculosis, viral infections, malaria, and cancer. With regard to malignant diseases, the class of α-N-heterocyclic thiosemicarbazones, and here especially 3-aminopyridine-2-carboxaldehyde thiosemicarbazone (Triapine), was intensively developed in multiple clinical phase I/II trials. Recent Advances: Very recently, two new derivatives, namely COTI-2 and di-2-pyridylketone 4-cyclohexyl-4-methyl-3-thiosemicarbazone (DpC) have entered phase I evaluation. Based on the strong metal-chelating/metal-interacting properties of thiosemicarbazones, interference with the cellular iron (and copper) homeostasis is assumed to play an important role in their biological activity.

CRITICAL ISSUES

In this review, we summarize and analyze the data on the interaction of (α-N-heterocyclic) thiosemicarbazones with iron, with the special aim of bridging the current knowledge on their mode of action from chemistry to (cell) biology. In addition, we highlight the difference to classical iron(III) chelators such as desferrioxamine (DFO), which are used for the treatment of iron overload.

FUTURE DIRECTIONS

We want to emphasize that thiosemicarbazones are not solely removing iron from the cells/organism. In contrast, they should be considered as iron-interacting drugs influencing diverse biological pathways in a complex and multi-faceted mode of action. Consequently, in addition to the discussion of physicochemical properties (e.g., complex stability, redox activity), this review contains an overview on the diversity of cellular thiosemicarbazone targets and drug resistance mechanisms.

Kineticomechanistic Study of the Redox pH Cycling Processes Occurring on a Robust Water-Soluble Cyanido-Bridged Mixed-Valence {CoIII/FeII}2 Square

A kineticomechanistic study of reversible electron-transfer processes undergone by the water-soluble, cyanido-bridged mixed-valence [{Co^III{(Me)₂(μ-ET)cyclen}}₂{(μ-NC)₂Fe^II(CN)₄}₂]^2- square has been carried out. The oxidation reaction consists of a two-step process with the participation of a solvent-assisted outer-sphere complex, as a result of the establishment of hydrogen bonds that involve the oxo groups of the oxidant (peroxodisulfate) and the terminal cyanido ligands of the tetrametallic square. The formally endergonic reduction reaction of the fully oxidized ([{Co^III{(Me)₂(μ-ET)cyclen}}₂{(μ-NC)₂Fe^III(CN)₄}₂]) core by water, producing hydrogen peroxide from water even at low pH values, is also a two-step process. Each one of these processes requires a set of two preequilibria involving the association of OH^- and its subsequent deprotonation by a further OH^- anion. The structure of the square compound in its fully protonated form has also been determined by X-ray diffraction and shows the existence of strong hydrogen-bonding interactions, in agreement with the rather high basicity of the terminal cyanido ligands. Likewise, density functional theory calculations on the tetrametallic complex showed zones with negative electrostatic potential around the Fe^II centers of the square that would account for the establishment of the hydrogen bonds found in the solid state. Spectroelectrochemistry experiments demonstrated the singular stability of the {Co^III/Fe^II}₂^2- complex, as well as that of their partially, {Co₂^III/Fe^IIIFe^II}^-, and fully, {Co^III/Fe^III}₂, oxidized counterparts because no hysteresis was observed in these measurements.

Less toxic zinc(ii), diorganotin(iv), gallium(iii) and cadmium(ii) complexes derived from 2-benzoylpyridine N,N-dimethylthiosemicarbazone: synthesis, crystal structures, cytotoxicity and investigations of mechanisms of action

Four metal complexes based on 2-benzoylpyridine N,N-dimethylthiosemicarbazone (Bp44mT) were designed. Free ligand and zinc(ii), diorganotin(iv), gallium(iii) and cadmium(ii) complexes all demonstrated pronounced activity, which was indicated using the growth inhibition test in vitro. Interestingly, most of the compounds were found to be selective against hepatocellular carcinoma (HepG2) cells but had little effect on normal hepatocyte (QSG7701) cells. In particular, Zn(Bp44mT)₂ (1) exhibited toxicity on QSG7701 cells which approximately 12-fold lower than that on HepG2 cells. The studies of mechanisms of action indicated that 1 induced reactive oxygen species (ROS) generation in a dose-dependent manner via the mitochondria transduction pathway. Protein analyses showed that 1 significantly promoted p21 and p53 gene expression, causing caspase-3 activation.

Effects of polar substituents on the biological activity of thiosemicarbazone metal complexes

In this paper, citronellal, vanillin and pyridoxal thiosemicarbazones were modified with polar substituents, namely ethylmorpholine and glucose, to increase their polarity and compare the effects of these moieties on their biological activity. Altogether, nine ligands were synthesized and for each of them also their copper(II) and nickel(II) complexes were prepared and used for the biological tests. Eventually, assays on proliferation inhibition were conducted using leukemic cell line U937, already used as a model for previous citronellal thiosemicarbazone tests. Biological tests were also performed on solid tumor cell line HT29. From the first screenings, two of the metal complexes showed remarkable interesting properties, and, therefore, were also tested for histosensitivity.

pH-Driven preparation of two related platinum(ii) complexes exhibiting distinct cytotoxic properties

A distinct cytotoxic/DNA interaction behaviour has been observed for a pair of aqua/hydroxido/chlorido equilibrium related Pt^II complex structures. The equilibrium is shifted between {Pt^II(NN)} and {Pt^II(NNO)} cores as a function of pCl and pH.

Nanoformulations of anticancer thiosemicarbazones to reduce methemoglobin formation and improve anticancer activity

Triapine and two derivatives were encapsulated into polymeric nanoparticles as well as liposomes. The most stable formulation showed strongly reduced methemoglobin formation and improved anticancer activity.