An Electrochemical Study of Antineoplastic Gallium, Iron and Ruthenium Complexes with Redox Noninnocent α-N-Heterocyclic Chalcogensemicarbazones

Ternary Phenolate-Based Thiosemicarbazone Complexes of Copper(II): Magnetostructural Properties, Spectroscopic Features and Marked Selective Antiproliferative Activity against Cancer Cells

The new diprotic ligand 3,5-di-tert-butylsalicylaldehyde 4-ethyl-3-thiosemicarbazone, abbreviated H2(3,5-t-Bu2)-sal4eT, exists as the thio-keto tautomer and adopts the E-configuration with respect to the imine double bond, as evidenced by single-crystal X-ray analysis and corroborated by spectroscopic characterisation. Upon treatment with Cu(OAc)2·H2O in the presence of either 2,9-dimethyl-1,10-phenanthroline (2,9-Me2-phen) or 1,10-phenanthroline (phen) as a co-ligand in MeOH, this thiosemicarbazone undergoes conformational transformation (relative donor-atom orientations: syn,anti → syn,syn) concomitantly with tautomerisation and double deprotonation to afford the ternary copper(II) complexes [Cu{(3,5-t-Bu2)-sal4eT}(2,9-Me2-phen)] (1) and [Cu2{3,5-t-Bu2)-sal4eT}2(phen)] (2). Crystallographic elucidation has revealed that complex 1 is a centrosymmetric dimer of mononuclear copper(II) complex molecules brought about by intermolecular H-bonding. The coordination geometry at the copper(II) centre is best described as distorted square pyramidal in accordance with the trigonality index (τ = 0.14). The co-ligand adopts an axial-equatorial coordination mode; hence, there is a disparity between its two Cu-N coordinate bonds arising from weakening of the apical one as a consequence of the tetragonal distortion. The axial X-band ESR spectrum of complex 1 is consistent with retention of this structure in solution. Complex 2 is a centrosymmetric dimer of dinuclear copper(II) complex molecules exhibiting intermolecular H-bonding and π-π-stacking interactions. The two copper(II) centres, which are 4.8067(18) Å apart and bridged by the thio-enolate nitrogen of the quadridentate thiosemicarbazonate ligand, display two different coordination geometries, one distorted square planar (τ4 = 0.082) and the other distorted square pyramidal (τ5 = 0.33). Such dinuclear copper(II) thiosemicarbazone complexes, which are crystallographically characterised, are extremely rare. In vitro, complexes 1 and 2 outperform cisplatin as antiproliferative agents in terms of potency and selectivity towards HeLa and MCF-7 cancer cell lines.

Acylselenoureas, selenosemicarbazones and selenocarbamate esters: Versatile ligands in coordination chemistry

Acylselenoureas, selenosemicarbazones and selenocarbamate esters from complexes with various transition- and main-group-metals, adopting several coordination modes.

Triapine Analogues and Their Copper(II) Complexes: Synthesis, Characterization, Solution Speciation, Redox Activity, Cytotoxicity, and mR2 RNR Inhibition

Three new thiosemicarbazones (TSCs) HL¹–HL³ as triapine analogues bearing a redox-active phenolic moiety at the terminal nitrogen atom were prepared. Reactions of HL¹–HL³ with CuCl₂·2H₂O in anoxic methanol afforded three copper(II) complexes, namely, Cu(HL¹)Cl₂ (1), [Cu(L²)Cl] (2′), and Cu(HL³)Cl₂ (3), in good yields. Solution speciation studies revealed that the metal-free ligands are stable as HL¹–HL³ at pH 7.4, while being air-sensitive in the basic pH range. In dimethyl sulfoxide they exist as a mixture of E and Z isomers. A mechanism of the E/Z isomerization with an inversion at the nitrogen atom of the Schiff base imine bond is proposed. The monocationic complexes [Cu(L^1–3)]⁺ are the most abundant species in aqueous solutions at pH 7.4. Electrochemical and spectroelectrochemical studies of 1, 2′, and 3 confirmed their redox activity in both the cathodic and the anodic region of potentials. The one-electron reduction was identified as metal-centered by electron paramagnetic resonance spectroelectrochemistry. An electrochemical oxidation pointed out the ligand-centered oxidation, while chemical oxidations of HL¹ and HL² as well as 1 and 2′ afforded several two-electron and four-electron oxidation products, which were isolated and comprehensively characterized. Complexes 1 and 2′ showed an antiproliferative activity in Colo205 and Colo320 cancer cell lines with half-maximal inhibitory concentration values in the low micromolar concentration range, while 3 with the most closely related ligand to triapine displayed the best selectivity for cancer cells versus normal fibroblast cells (MRC-5). HL¹ and 1 in the presence of 1,4-dithiothreitol are as potent inhibitors of mR2 ribonucleotide reductase as triapine.

Three triapine analogues HL¹−HL³ bearing a phenolic redox-active moiety showed moderate antiproliferative activity, while one of the oxidation products HL^2c′·CH3COOH revealed high cytotoxicity in Colo205 and Colo320 cancer cell lines. Coordination of HL¹−HL³ to copper(II) increased strongly the cytotoxicity, with complex 2′ showing IC₅₀ values of 0.181 and 0.159, respectively. The highest cytotoxicity of 2′ is likely due to the highest thermodynamic stability, more negative reduction potential, and the lowest rate of reduction by GSH.

Thiosemicarbazone Complexes of Transition Metals as Catalysts for Cross-Coupling Reactions

Catalysis of cross-coupling reactions under phosphane-free conditions represents an important ongoing challenge. Although transition metal complexes based on the thiosemicarbazone unit have been known for a very long time, their use in homogeneous catalysis has been studied only relatively recently. In particular, reports of cross-coupling catalytic reactions with such complexes have appeared only in the last 15 years. This review provides a survey of the research in this area and a discussion of the prospects for future developments.

CuII (atsm) inhibits ferroptosis: Implications for treatment of neurodegenerative disease

BACKGROUND AND PURPOSE

Diacetyl-bis(4-methyl-3-thiosemicarbazonato)copper^II (Cu^II (atsm)) ameliorates neurodegeneration and delays disease progression in mouse models of amyotrophic lateral sclerosis (ALS) and Parkinson's disease (PD), yet the mechanism of action remains uncertain. Promising results were recently reported for separate Phase 1 studies in ALS patients and PD patients. Affected tissue in these disorders shares features of elevated Fe, low glutathione and increased lipid peroxidation consistent with ferroptosis, a novel form of regulated cell death. We therefore evaluated the ability of Cu^II (atsm) to inhibit ferroptosis.

EXPERIMENTAL APPROACH

Ferroptosis was induced in neuronal cell models by inhibition of glutathione peroxidase-4 activity with RSL3 or by blocking cystine uptake with erastin. Cell viability and lipid peroxidation were assessed and the efficacy of Cu^II (atsm) was compared to the known antiferroptotic compound liproxstatin-1.

KEY RESULTS

Cu^II (atsm) protected against lipid peroxidation and ferroptotic lethality in primary and immortalised neuronal cell models (EC₅₀ : ≈130 nM, within an order of magnitude of liproxstatin-1). Ni^II (atsm) also prevented ferroptosis with similar potency, whereas ionic Cu^II did not. In cell-free systems, Cu^II (atsm) and Ni^II (atsm) inhibited Fe^II -induced lipid peroxidation, consistent with these compounds quenching lipid radicals.

CONCLUSIONS AND IMPLICATIONS

The antiferroptotic activity of Cu^II (atsm) could therefore be the disease-modifying mechanism being tested in ALS and PD trials. With potency in vitro approaching that of liproxstatin-1, Cu^II (atsm) possesses favourable properties such as oral bioavailability and entry into the brain that make it an attractive investigational product for clinical trials of ferroptosis-related diseases.

iClick Reactions of Square-Planar Palladium(II) and Platinum(II) Azido Complexes with Electron-Poor Alkynes: Metal-Dependent Preference for N1 vs N2 Triazolate Coordination and Kinetic Studies with 1H and 19F NMR Spectroscopy

Two square-planar palladium(II) and platinum(II) azido complexes [M(N₃)(L)] with L = N-phenyl-2-[1-(2-pyridinyl)ethylidene]hydrazine carbothioamide reacted with four different electron-poor alkynes R-C≡C-R' with R = R' = COOCH₃, COOEt, COOCH₂CH₂OCH₃ or R = CF₃, R' = COOEt in a [3 + 2] cycloaddition "iClick" reaction. The resulting triazolate complexes [M(triazolate^R,R')(L)] were isolated by simple precipitation and/or washing in high purity and good yield. Six out of the eight new compounds feature the triazolate ligand coordinated to the metal center via the N2 nitrogen atom, but fortuitous solubility properties allowed isolation of the N1 isomer in two cases from acetone. When the solvent was changed to DMSO, the N1 → N2 isomerization could be studied by NMR spectroscopy and took several days to complete. ¹⁹F NMR studies of the iClick reaction with F₃C-C≡C-COOEt led to identification of a putative early linear intermediate in addition to the N1 and N2 isomers, however with the latter as the final product. Rate constants determined by ¹H or ¹⁹F NMR spectroscopy increased in the order Pd > Pt and CF₃/COOEt > COOR/COOR with R = CH₃, Et, CH₂CH₂OCH₃. The second-order rate constant k₂ > 3.7 M^-1 s^-1 determined for the reaction of [Pd(N₃)(L)] with F₃C-C≡C-COOEt is the fastest observed for an iClick reaction so far and compares favorably with that of the most evolved strained alkynes reported for the SPAAC (strain-promoted azide-alkyne cycloaddition) to date. Selected title compounds were evaluated for their anticancer activity on the GaMG human glioblastoma brain cancer cell line and gave EC₅₀ values in the low micromolar range (2-16 μM). The potency of the Pd(II) complexes increased with the chain length of the substituents in the 4- and 5-positions of the triazolate ligand.

Potential Diagnostic Imaging of Alzheimer's Disease with Copper-64 Complexes That Bind to Amyloid-β Plaques

Amyloid-β plaques, consisting of aggregated amyloid-β peptides, are one of the pathological hallmarks of Alzheimer's disease. Copper complexes formed using positron-emitting copper radionuclides that cross the blood-brain barrier and bind to specific molecular targets offer the possibility of noninvasive diagnostic imaging using positron emission tomography. New thiosemicarbazone-pyridylhydrazone based ligands that incorporate pyridyl-benzofuran functional groups designed to bind amyloid-β plaques have been synthesized. The ligands form stable complexes with copper(II) ( K_d = 10^-18 M) and can be radiolabeled with copper-64 at room temperature. Subtle changes to the periphery of the ligand backbone alter the metabolic stability of the complexes in mouse and human liver microsomes, and influenced the ability of the complexes to cross the blood-brain barrier in mice. A lead complex was selected based on possessing the best metabolic stability and brain uptake in mice. Synthesis of this lead complex with isotopically enriched copper-65 allowed us to show that the complex bound to amyloid-β plaques present in post-mortem human brain tissue using laser ablation-inductively coupled plasma-mass spectrometry. This work provides insight into strategies to target metal complexes to amyloid-β plaques, and how small modifications to ligands can dramatically alter the metabolic stability of metal complexes as well as their ability to cross the blood-brain barrier.

Co(iii) complexes of (1,3-selenazol-2-yl)hydrazones and their sulphur analogues

The first Co(iii) complexes with (1,3-selenazol-2-yl)hydrazones as an unexplored class of ligands were prepared and characterized by NMR spectroscopy and X-ray diffraction analysis. The novel ligands act as NNN tridentate chelators forming octahedral Co(iii) complexes. The impact of structural changes on ligands' periphery as well as that of isosteric replacement of sulphur with selenium on the electrochemical and electronic absorption features of complexes are explored. To support the experimental data, density functional theory (DFT) calculations were also conducted. Theoretical NMR chemical shifts, the relative energies and natural bond orbital (NBO) analysis are calculated within the DFT approach, while the singlet excited state energies and HOMO-LUMO energy gap were calculated with time-dependent density functional theory (TD-DFT). The electrophilic f^- and nucleophilic f⁺ Fukui functions are well adapted to find the electrophile and nucleophile centres in the molecules. Both (1,3-selenazol-2-yl)- and (1,3-thiazol-2-yl)hydrazone Co(iii) complexes showed potent antimicrobial and antioxidant activity. A significant difference among them was a smaller cytotoxicity of selenium compounds.

Spectrophotometric determination of Cu(II) in soil and vegetable samples collected from Abraha Atsbeha, Tigray, Ethiopia using heterocyclic thiosemicarbazone

Two selective and sensitive reagents, 2-acetylpyridine thiosemicarbazone (2-APT) and 3-acetylpyridine thiosemicarbazone (3-APT) were used for the spectrophotometric determination of Cu(II). Both reagents gave yellowish Cu(II) complex at a pH range of 8.0-10.0. Beer's law was obeyed for Cu(II)-2-APT and Cu(II)-3-APT in the concentration range of 0.16-1.3 and 0.44-1.05 µg/mL, respectively. The molar absorptivity and of Cu(II)-2-APT and Cu(II)-3-APT were 2.14 × 10(4) at 370 nm, and 6.7 × 10(3) L/mol cm at 350 nm, respectively, while the Sandell's sensitivity were 0.009 and 0.029 µg/cm(2) in that order. The correlation coefficient of the standard curves of Cu(II)-2-APT and Cu(II)-3-APT were 0.999 and 0.998, respectively. The detection limit of the Cu(II)-2-APT and Cu(II)-3-APT methods were 0.053 and 0.147 µg/mL, respectively. The results demonstrated that the procedure is precise (relative standard deviation <2 %, n = 10). The method was tested for Cu(II) determination in soil and vegetable samples. Comparisons of the results with those obtained using a flame atomic absorption spectrophotometer for Cu(II) determination also tested the validity of the method using paired sample t test at the 0.05 level showing a good agreement between them.

Development of ruthenium-based complexes as anticancer agents: toward a rational design of alternative receptor targets

In the search for novel anticancer agents, the development of metal-based complexes that could serve as alternatives to cisplatin and its derivatives has received considerable attention in recent years. This becomes necessary because, at present, cisplatin and its derivatives are the only coordination complexes being used as anticancer agents in spite of inherent serious side effects and their limitation against metastasized platinum-resistant cancer cells. Although many metal ions have been considered as possible alternatives to cisplatin, the most promising are ruthenium (Ru) complexes and two Ru compounds, KP1019 and NAMI-A, which are currently in phase II clinical trials. The major obstacle against the rational design of these compounds is the fact that their mode of action in relation to their therapeutic activities and selectivity is not fully understood. There is an urgent need to develop novel metal-based anticancer agents, especially Ru-based compounds, with known mechanism of actions, probable targets, and pharmacodynamic activity. In this paper, we review the current efforts in developing metal-based anticancer agents based on promising Ru complexes and the development of compounds targeting receptors and then examine the future prospects.

Zn(ii) complex with 2-quinolinecarboxaldehyde selenosemicarbazone: synthesis, structure, interaction studies with DNA/HSA, molecular docking and caspase-8 and -9 independent apoptose induction

A new Zn(ii)-based complex shows a concentration-dependent apoptotic response in highly resistant pancreatic adenocarcinoma cells with extensive activation of caspase-8 and -9.

Nickel(ii) radical complexes of thiosemicarbazone ligands appended by salicylidene, aminophenol and aminothiophenol moieties

The neutral nickel(ii) complexes are chameleon pro-radical compounds: under their one-electron oxidized form they feature an iminosemiquinonate (or iminothiosemiquinonate) radical, while under their reduced form they are α-diimine π-radicals.

Iron-targeting antitumor activity of gallium compounds and novel insights into triapine(®)-metal complexes

SIGNIFICANCE

Despite advances made in the treatment of cancer, a significant number of patients succumb to this disease every year. Hence, there is a great need to develop new anticancer agents.

RECENT ADVANCES

Emerging data show that malignant cells have a greater requirement for iron than normal cells do and that proteins involved in iron import, export, and storage may be altered in cancer cells. Therefore, strategies to perturb these iron-dependent steps in malignant cells hold promise for the treatment of cancer. Recent studies show that gallium compounds and metal-thiosemicarbazone complexes inhibit tumor cell growth by targeting iron homeostasis, including iron-dependent ribonucleotide reductase. Chemical similarities of gallium(III) with iron(III) enable the former to mimic the latter and interpose itself in critical iron-dependent steps in cellular proliferation. Newer gallium compounds have emerged with additional mechanisms of action. In clinical trials, the first-generation-compound gallium nitrate has exhibited activity against bladder cancer and non-Hodgkin's lymphoma, while the thiosemicarbazone Triapine(®) has demonstrated activity against other tumors.

CRITICAL ISSUES

Novel gallium compounds with greater cytotoxicity and a broader spectrum of antineoplastic activity than gallium nitrate should continue to be developed.

FUTURE DIRECTIONS

The antineoplastic activity and toxicity of the existing novel gallium compounds and thiosemicarbazone-metal complexes should be tested in animal tumor models and advanced to Phase I and II clinical trials. Future research should identify biologic markers that predict tumor sensitivity to gallium compounds. This will help direct gallium-based therapy to cancer patients who are most likely to benefit from it.

Coordination behavior of ligand based on NNS and NNO donors with ruthenium(III) complexes and their catalytic and DNA interaction studies

Reactions of 2-acetylpyridine-thiosemicarbazone HL(1), 2-acetylpyridine-4-methyl-thiosemicarbazone HL(2), 2-acetylpyridine-4-phenyl-thiosemicarbazone HL(3) and 2-acetylpyridine-semicarbazone HL(4) with ruthenium(III) precursor complexes were studied and the products were characterized by analytical and spectral (FT-IR, electronic, EPR and EI-MS) methods. The ligands coordinated with the ruthenium(III) ion via pyridine nitrogen, azomethine nitrogen and thiolate sulfur/enolate oxygen. An octahedral geometry has been proposed for all the complexes based on the studies. All the complexes are redox active and display an irreversible and quasireversible metal centered redox processes. Further, the catalytic activity of the new complexes has been investigated for the transfer hydrogenation of ketones in the presence of isopropanol/KOH and the Kumada-Corriu coupling of aryl halides with aryl Grignard reagents. The DNA cleavage efficiency of new complexes has also been tested.

Interaction of Triapine and related thiosemicarbazones with iron(III)/(II) and gallium(III): a comparative solution equilibrium study

Stoichiometry and stability of Ga(III), Fe(III), Fe(II) complexes of Triapine and five related α-N heterocyclic thiosemicarbazones with potential antitumor activity have been determined by pH-potentiometry, UV-vis spectrophotometry, (1)H NMR spectroscopy, and spectrofluorimetry in aqueous solution (with 30% DMSO), together with the characterization of the proton dissociation processes. Additionally, the redox properties of the iron complexes were studied by cyclic voltammetry at various pH values. Formation of high stability bis-ligand complexes was found in all cases, which are predominant at physiological pH with Fe(III)/Fe(II), whilst only at the acidic pH range with Ga(III). The results show that among the thiosemicarbazones with various substituents the N-terminal dimethylation does not exert a measurable effect on the redox potential, but has the highest impact on the stability of the complexes as well as the cytotoxicity, especially in the absence of a pyridine-NH(2) group in the molecule. In addition the fluorescence properties of the ligands in aqueous solution and their changes caused by Ga(III) were studied.

Copper complexes of bis(thiosemicarbazones): from chemotherapeutics to diagnostic and therapeutic radiopharmaceuticals

The molecules known as bis(thiosemicarbazones) derived from 1,2-diones can act as tetradentate ligands for Cu(II), forming stable, neutral complexes. As a family, these complexes possess fascinating biological activity. This critical review presents an historical perspective of their progression from potential chemotherapeutics through to more recent applications in nuclear medicine. Methods of synthesis are presented followed by studies focusing on their potential application as anti-cancer agents and more recent investigations into their potential as therapeutics for Alzheimer's disease. The Cu(II) complexes are of sufficient stability to be used to coordinate copper radioisotopes for application in diagnostic and therapeutic radiopharmaceuticals. Detailed understanding of the coordination chemistry has allowed careful manipulation of the metal based properties to engineer specific biological activities. Perhaps the most promising complex radiolabelled with copper radioisotopes to date is Cu(II)(atsm), which has progressed to clinical trials in humans (162 references).

Ruthenium- and Osmium-Arene Complexes of 2-Substituted Indolo[3,2-c]quinolines: Synthesis, Structure, Spectroscopic Properties, and Antiproliferative Activity

The synthesis of new modified indolo[3,2-c]quinoline ligands L¹−L⁸ with metal-binding sites is reported. By coordination to ruthenium− and osmium−arene moieties 16 complexes of the type [(η⁶-p-cymene)M(L)Cl]Cl (1a,b−8a,b), where M is Ru^II or Os^II and L is L¹−L⁸, have been prepared. All compounds were comprehensively characterized by elemental analysis, electrospray ionization mass spectrometry, IR, UV−vis, and NMR spectroscopy, thermogravimetric analysis, and single-crystal X-ray diffraction (2a, 4a, 4b, 5a, 7a, and 7b). The complexes were tested for antiproliferative activity in vitro in three human cancer cell lines, namely, CH1 (ovarian carcinoma), SW480 (colon adenocarcinoma), and A549 (non-small-cell lung cancer), yielding IC₅₀ values in the submicromolar or low micromolar range.

A hexadentate bis(thiosemicarbazonato) ligand: rhenium(V), iron(III) and cobalt(III) complexes

A new 1,3-diaminopropane bridged bis(thiosemicarbazone) ligand (H(4)L) has been synthesised. The new hexadentate ligand is capable of forming six coordinate complexes with rhenium(V), iron(III) and cobalt(III). In the case of the iron(III) and cobalt(III) complexes the ligand doubly deprotonates to give the monocations [Fe(III)(H(2)L)](+) and [Co(III)(H(2)L)](+) in which the metal ion is in a distorted octahedral environment. In the rhenium(V) complex the ligand loses four protons by deprotonation of both secondary amine nitrogen atoms to give [Re(V)(L)](+) with the metal ion in a distorted trigonal prismatic coordination environment. [Re(V)(L)](+) represents a rare example of a rhenium(V) complex that does not contain one of the ReO(3+), ReN(2+) or Re(NPh)(2+) cores. The new ligand and metal complexes have been characterised by a combination of NMR spectroscopy, X-ray crystallography, mass spectrometry and microanalysis. The electrochemistry of [Fe(III)(H(2)L)](+), [Co(III)(H(2)L)](+) and [Re(V)(L)](+) has been investigated by cyclic voltammetry with each complex undergoing a single electron reduction event. It is possible to prepare the rhenium(V) complex from ReOCl(3)(PPh(3))(2) or directly from [ReO(4)](-) with the addition of a reductant, which suggests the new ligand may be of interest in the development of rhenium radiopharmaceuticals.