Mechanisms underlying reductant-induced reactive oxygen species formation by anticancer copper(II) compounds

Synthesis, characterization and comparative biological activity of a novel set of Cu(II) complexes containing azole-based ligand frames

The synthesis and spectroscopic characterization of three complexes containing a substituted 2-(2-pyridyl)benzothiazole (PyBTh) group in the ligand frame are reported along with the comparative biological activity. The ligands have been substituted at the 6-position with either a methoxy (Py(OMe)BTh) or a methyl group (Py(Me)BTh). Reaction of Py(OMe)BTh with either CuCl2 or Cu(NO3)2·2.5 H2O yielded the monomeric [Cu(Py(OMe)BTh))2(NO3)]NO3·1.5 MeOH, (1·1.5 MeOH) complex or the dimeric [Cu(Py(OMe)BTh)Cl2]2 (2), respectively, with the nuclearity of the complex dependent on the starting Cu(II) salt. Reaction between the methyl substituted ligand and Cu(NO3)2·2.5 H2O resulted in the isolation of Cu(Py(Me)BTh)(NO3)2·0.5 THF (3·0.5 THF). Complexes 1-3 were fully characterized. Cyclic voltammetry measurements were performed on all three complexes as well as on [Cu(PyBTh)2(H2O)](BF4)2 (4), a compound previously reported by us which contains the unsubstituted 2-(2-pyridyl)benzothiazole ligand. The biological activity was studied and included concentration dependent DNA binding and cleavage, antibacterial activity, and cancer cell toxicity. All complexes exhibited DNA cleavage activity, however 2 and 4 were found to be the most potent. Mechanistic studies revealed that the nuclease activity is dependent on an oxidative mechanism reliant principally on O2-. Antibacterial studies revealed complex 4 was more potent compared to 1-3. Cancer cell toxicity studies were carried out on HeLa, PC-3, and MCF7 cells with 1-4, Cu(QBTh)(NO3)2(H2O) and Cu(PyBIm)3(BF4)2. The differences in the observed toxicities suggests the importance of the ligand and its substituents in modulating cell death.

Mixed-ligand copper(ii)-diimine complexes of 3-formylchromone-N 4-phenyl thiosemicarbazone: 5,6-dmp co-ligand confers enhanced cytotoxicity

The promising biological applications of thiosemicarbazone derivatives have inspired the design, synthesis, and study of their Cu(ii) complexes for anticancer therapeutic applications. Herein, we have evaluated the DNA/protein binding, DNA cleaving, and cytotoxic properties of four mixed-ligand Cu(ii) complexes of the type [Cu(L)(diimine)](NO3) 1-4, where HL is 4-oxo-4H-chromene-3-carbaldehyde-4(N)-phenylthiosemicarbazone and diimine is 2,2'-bipyridine (bpy, 1) 1,10-phenanthroline (phen, 2), 5,6-dimethyl-1,10-phenanthroline (5,6-dmp, 3), or dipyrido-[3,2-f:2',3'-h]-quinoxaline (dpq, 4). Interestingly, complex 3 with higher lipophilicity shows stronger DNA binding and oxidative DNA cleavage, higher ROS production, and more reversible redox behaviour, resulting in its remarkable cytotoxicity (IC50, 1.26 μM) against HeLa cervical cancer cells, and rendering it 5 times more potent than the widely used drug cisplatin. The same complex induces enhanced apoptotic cell death on HeLa cells but lower toxicity towards the non-cancerous PBMC cells. Molecular docking studies suggest that all the complexes bind in the minor groove of DNA and subdomain II of HSA, which is in close agreement with the experimental results. Also, 3 shows cytotoxicity higher than the analogous mixed ligand Cu(ii) complexes, reported already, emphasizing the importance of co-ligand in tuning the anticancer activity.

Synthesis and Systematic Investigation of Lepidiline A and Its Gold(I), Silver(I), and Copper(I) Complexes Using In Vitro Cancer Models and Multipotent Stem Cells

The imidazole alkaloid lepidiline A from the root of Lepidium meyenii has a moderate to low in vitro anticancer effect. Our aim was to extend cytotoxicity investigations against a panel of cancer cells, including multidrug-resistant cancer cells, and multipotent stem cells. Lepidiline A is a N-heterocyclic carbene precursor, therefore a suitable ligand source for metal complexes. Thus, we synthesized lepidiline A and its copper(I), gold(I), and silver(I) complexes and tested them against ovarian, gastrointestinal, breast, and uterine cancer cells and bone marrow-derived and adipose-derived mesenchymal stem cells. Lepidiline A and its copper complex demonstrated moderate cytotoxicity, while silver and gold complexes exhibited significantly enhanced and consistent cytotoxicity against both cancer and stem cell lines. ABCB1 in the multidrug-resistant uterine sarcoma line conferred significant resistance against lepidiline A and the copper-lepidiline A complex, but not against the silver and gold complexes. Our results indicate that only the copper complex induced a significant and universal increase in the production of reactive oxygen species within cells. In summary, binding of metal ions to lepidiline A results in enhanced cytotoxicity with the nature of the metal ion playing a critical role in determining its properties.

L-Citrullinato-Bipyridine and L-Citrullinato-Phenanthroline Mixed Copper Complexes: Synthesis, Characterization and Potential Anticancer Activity

Citrulline (C6H13N3O3) is an amino acid found in the body as a zwitterion. This means its carboxylic and amine groups can act as Lewis donors to chelate metal cations. In addition, citrulline possesses a terminal ureido group on its aliphatic chain, which also appears to coordinate. Here, two new mixed complexes of citrulline were made with 1,10-phenanthroline and 2,2'-bipyridine. These compounds, once dissolved in water, gave aquo-complexes that were subject to DFT studies and in vitro toxicity studies on cancer cell lines (HeLa, MDA-MB-231, HCT 15, and MCF7) showed promising results. Docking studies with DNA were also conducted, indicating potential anticancer properties.

Proving the Antimicrobial Therapeutic Activity on a New Copper-Thiosemicarbazone Complex

The misuse and overdose of antimicrobial medicines are fostering the emergence of novel drug-resistant pathogens, providing negative repercussions not only on the global healthcare system due to the rise of long-term or chronic patients and inefficient therapies but also on the world trade, productivity, and, in short, to the global economic growth. In view of these scenarios, novel action plans to constrain this antibacterial resistance are needed. Thus, given the proven antiproliferative tumoral and microbial features of thiosemicarbazone (TSCN) ligands, we have here synthesized a novel effective antibacterial copper-thiosemicarbazone complex, demonstrating both its solubility profile and complex stability under physiological conditions, along with their safety and antibacterial activity in contact with human cellular nature and two most predominant bacterial strains, respectively. A significant growth inhibition (17% after 20 h) is evidenced over time, paving the way toward an effective antibacterial therapy based on these copper-TSCN complexes.

A Comparison of In Vitro Studies between Cobalt(III) and Copper(II) Complexes with Thiosemicarbazone Ligands to Treat Triple Negative Breast Cancer

Metal complexes have gained significant attention as potential anti-cancer agents. The anti-cancer activity of [Co(phen)2(MeATSC)](NO3)3•1.5H2O•C2H5OH 1 (where phen = 1,10-phenanthroline and MeATSC = 9-anthraldehyde-N(4)-methylthiosemicarbazone) and [Cu(acetylethTSC)Cl]Cl•0.25C2H5OH 2 (where acetylethTSC = (E)-N-ethyl-2-[1-(thiazol-2-yl)ethylidene]hydrazinecarbothioamide) was investigated by analyzing DNA cleavage activity. The cytotoxic effect was analyzed using CCK-8 viability assay. The activities of caspase 3/7, 9, and 1, reactive oxygen species (ROS) production, cell cycle arrest, and mitochondrial function were further analyzed to study the cell death mechanisms. Complex 2 induced a significant increase in nicked DNA. The IC50 values of complex 1 were 17.59 μM and 61.26 μM in cancer and non-cancer cells, respectively. The IC50 values of complex 2 were 5.63 and 12.19 μM for cancer and non-cancer cells, respectively. Complex 1 induced an increase in ROS levels, mitochondrial dysfunction, and activated caspases 3/7, 9, and 1, which indicated the induction of intrinsic apoptotic pathway and pyroptosis. Complex 2 induced cell cycle arrest in the S phase, ROS generation, and caspase 3/7 activation. Thus, complex 1 induced cell death in the breast cancer cell line via activation of oxidative stress which induced apoptosis and pyroptosis while complex 2 induced cell cycle arrest through the induction of DNA cleavage.

New copper(II) and oxidovanadium(IV) complexes with a vitamin B6 Schiff base: mechanism of action and synergy studies on 2D and 3D human osteosarcoma cell models

We report the synthesis, characterization and anticancer activity of a new Schiff base (H2L) derived from the condensation of pyridoxamine with pyridoxal and its novel copper(II) and oxidovanadium(IV) complexes: [Cu(HL)Cl] (1), [Cu(LH2)(phen)]Cl2 (2), [Cu(LH2)(amphen)]Cl2 (3), [VIVO(HL)Cl] (4), and [VIVO(LH2)(phen)]Cl2 (5), where phen is 1,10-phenanthroline and amphen is its 5-amino derivative. All compounds were characterized by analytical and spectroscopic techniques, namely FTIR, UV-vis and EPR spectroscopy. Their stability in aqueous media was evaluated, revealing that the presence of the phen co-ligand significantly increases the stability. The ternary Cu(II) complexes (2 and 3) impaired cell viability of osteosarcoma cells (MG-63) (IC50 values of 3.6 ± 0.6 and 7 ± 1.9 μM for 2 and 3), while 1 and the VIVO complexes did not show relevant anticancer activity. Complexes 2 and 3 are also more active than cisplatin (CDDP). Synergistic studies between 2 and sorafenib showed significant synergism on MG-63 cells for the following combinations: 2 (2.0 μM) + sorafenib (10.0 μM) and 2 (2.5 μM) + sorafenib (12.5 μM), whilst the combination of 2 and CDDP did not show synergy. Complex 2 interacts with DNA, inducing significant genotoxic effects on MG-63 cells from 1.0 to 2.5 μM and it increases the ROS levels 880% over basal. Moreover, 2 induces apoptosis at 1.0 and 2.0 μM, while its combination with sorafenib induces apoptosis and necrosis. Finally, compound 2 reduces the cell viability of MG-63 spheroids showing an IC50 value 7-fold lower than that of CDDP (8.5 ± 0.4 μM vs. 65 ± 6 μM). The combination of 2 and sorafenib also showed synergism on spheroids, suggesting that the combination of these drugs improves the anticancer effect against bone cancer cells.

Addressing the gaps in homeostatic mechanisms of copper and copper dithiocarbamate complexes in cancer therapy: a shift from classical platinum-drug mechanisms

The platinum drug, cisplatin, is considered as among the most successful medications in cancer treatment. However, due to its inherent toxicity and resistance limitations, research into other metal-based non-platinum anticancer medications with diverse mechanisms of action remains an active field. In this regard, copper complexes feature among non-platinum compounds which have shown promising potential as effective anticancer drugs. Moreover, the interesting discovery that cancer cells can alter their copper homeostatic processes to develop resistance to platinum-based treatments leads to suggestions that some copper compounds can indeed re-sensitize cancer cells to these drugs. In this work, we review copper and copper complexes bearing dithiocarbamate ligands which have shown promising results as anticancer agents. Dithiocarbamate ligands act as effective ionophores to convey the complexes of interest into cells thereby influencing the metal homeostatic balance and inducing apoptosis through various mechanisms. We focus on copper homeostasis in mammalian cells and on our current understanding of copper dysregulation in cancer and recent therapeutic breakthroughs using copper coordination complexes as anticancer drugs. We also discuss the molecular foundation of the mechanisms underlying their anticancer action. The opportunities that exist in research for these compounds and their potential as anticancer agents, especially when coupled with ligands such as dithiocarbamates, are also reviewed.

Exploring a new family of designer copper(II) complexes of anthracene-appended polyfunctional organic assembly displaying potential anticancer activity via cytochrome c mediated mitochondrial apoptotic pathway

The present article describes the systematic study on design and synthesis, physicochemical properties and spectroscopic features, and potential anticancer activities of a family of novel copper(II)-based designer metal complexes [Cu₂(acdp)(μ-Cl)(H₂O)₂] (1), [Cu₂(acdp)(μ-NO₃)(H₂O)₂] (2) and [Cu₂(acdp)(μ-O₂CCF₃)(H₂O)₂] (3) of anthracene-appended polyfunctional organic assembly, H₃acdp (H₃acdp = N,N'-bis[anthracene-2-ylmethyl]-N,N'-bis[carboxymethyl]-1,3-diaminopropan-2-ol). Synthesis of 1-3 was accomplished under facile experimental conditions, preserving their overall integrity in solution. The incorporation of polycyclic anthracene skeleton within the backbone of organic assembly increases lipophilicity of resulting complexes, thereby dictating the degree of cellular uptake with improved biological activity. Complexes 1-3 were characterized by elemental analysis, molar conductance, FTIR, UV-Vis absorption/fluorescence emission titration spectroscopy, PXRD and TGA/DTA studies, including DFT calculations. The cellular cytotoxicity of 1-3 when studied in HepG2 cancer cell line showed substantial cytotoxic effects, whereas no such cytotoxicity was observed when exposed to normal L6 skeletal muscle cell line. Thereafter, the signaling factors involved in the process of cytotoxicity in HepG2 cancer cells were investigated. Alteration of cytochrome c and Bcl-2 protein expression levels along with modulation of mitochondrial membrane potential (MMP) in the presence of 1-3, strongly suggested the possibility of activating mitochondria-mediated apoptotic pathway involved in halting the cancer cell propagation. However, when a comparative assessment on their bio-efficacies was made, 1 showed higher cytotoxicity, nuclear condensation, DNA binding and damage, ROS generation and lower rate of cell proliferation compared to 2 and 3 in HepG2 cell line, indicating that the anticancer activity of 1 is significantly higher than that of 2 and 3.

Dual Role of Glutathione as a Reducing Agent and Cu-Ligand Governs the ROS Production by Anticancer Cu-Thiosemicarbazone Complexes

α-Pyridyl thiosemicarbazones (TSC) such as Triapine (3AP) and Dp44mT are a promising class of anticancer agents. Contrary to Triapine, Dp44mT showed a pronounced synergism with Cu^II, which may be due to the generation of reactive oxygen species (ROS) by Dp44mT-bound Cu^II ions. However, in the intracellular environment, Cu^II complexes have to cope with glutathione (GSH), a relevant Cu^II reductant and Cu^I-chelator. Here, aiming at rationalizing the different biological activity of Triapine and Dp44mT, we first evaluated the ROS production by their Cu^II-complexes in the presence of GSH, showing that Cu^II-Dp44mT is a better catalyst than Cu^II-3AP. Furthermore, we performed density functional theory (DFT) calculations, which suggest that a different hard/soft character of the complexes could account for their different reactivity with GSH.

Copper-Catalyzed Glutathione Oxidation is Accelerated by the Anticancer Thiosemicarbazone Dp44mT and Further Boosted at Lower pH

Glutathione (GSH) is the most abundant thiol in mammalian cells and plays a crucial role in maintaining redox cellular homeostasis. The thiols of two GSH molecules can be oxidized to the disulfide GSSG. The cytosolic GSH/GSSG ratio is very high (>100), and its reduction can lead to apoptosis or necrosis, which are of interest in cancer research. Cu^II ions are very efficient oxidants of thiols, but with an excess of GSH, Cu^I_n(GS)_m clusters are formed, in which Cu^I is very slowly reoxidized by O₂ at pH 7.4 and even more slowly at lower pH. Here, the aerobic oxidation of GSH by Cu^II was investigated at different pH values in the presence of the anticancer thiosemicarbazone Dp44mT, which accumulates in lysosomes and induces lysosomal membrane permeabilization in a Cu-dependent manner. The results showed that Cu^II-Dp44mT catalyzes GSH oxidation faster than Cu^II alone at pH 7.4 and hence accelerates the production of very reactive hydroxyl radicals. Moreover, GSH oxidation and hydroxyl radical production by Cu^II-Dp44mT were accelerated at the acidic pH found in lysosomes. To decipher this unusually faster thiol oxidation at lower pH, density functional theory (DFT) calculations, electrochemical and spectroscopic studies were performed. The results suggest that the acceleration is due to the protonation of Cu^II-Dp44mT on the hydrazinic nitrogen, which favors the rate-limiting reduction step without subsequent dissociation of the Cu^I intermediate. Furthermore, preliminary biological studies in cell culture using the proton pump inhibitor bafilomycin A1 indicated that the lysosomal pH plays a role in the activity of Cu^II-Dp44mT.

The coordination modes of (thio)semicarbazone copper(II) complexes strongly modulate the solution chemical properties and mechanism of anticancer activity

Thiosemicarbazones are promising candidates for anticancer therapy and their mechanism of action is often linked to their metal chelating ability. In this study, five (thio)semicarbazones with different donor sets (NNS, NNO, ONS, ONO) were selected and their behaviour in aqueous solution, the stability of their copper(II) complexes in addition to their cytotoxicity, DNA-binding, DNA cleavage ability and inhibition of topoisomerase IIα were investigated and compared. We aimed to reveal relationships between the structural variations, the significantly different physico-chemical properties, solution speciation and biological activity. The cytotoxicity of the ligands did not show correlation with the solubility, lipophilicity and permeability; and the decreased activity of the oxygen donor containing compounds was explained by their stronger preference towards chelation of iron(III) over iron(II). Meanwhile, among the copper complexes the most lipophilic species with the highest stability and membrane permeability exhibited the highest cytotoxicity. The studied copper(II) complexes interact with DNA, and reaction with glutathione led to heavy DNA cleavage in the case of the highly stable complexes which could be reduced in a reversible reaction with moderate rate. All the tested copper complexes inhibited topoisomerase IIα, however, this property of the complexes with low stability is most probably linked to the liberated free copper(II).

Mechanistic Insights of Chelator Complexes with Essential Transition Metals: Antioxidant/Pro-Oxidant Activity and Applications in Medicine

The antioxidant/pro-oxidant activity of drugs and dietary molecules and their role in the maintenance of redox homeostasis, as well as the implications in health and different diseases, have not yet been fully evaluated. In particular, the redox activity and other interactions of drugs with essential redox metal ions, such as iron and copper, need further investigation. These metal ions are ubiquitous in human nutrition but also widely found in dietary supplements and appear to exert major effects on redox homeostasis in health, but also on many diseases of free radical pathology. In this context, the redox mechanistic insights of mainly three prototype groups of drugs, namely alpha-ketohydroxypyridines (alpha-hydroxypyridones), e.g., deferiprone, anthraquinones, e.g., doxorubicin and thiosemicarbazones, e.g., triapine and their metal complexes were examined; details of the mechanisms of their redox activity were reviewed, with emphasis on the biological implications and potential clinical applications, including anticancer activity. Furthermore, the redox properties of these three classes of chelators were compared to those of the iron chelating drugs and also to vitamin C, with an emphasis on their potential clinical interactions and future clinical application prospects in cancer, neurodegenerative and other diseases.

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.

Interaction of the potent antitumoral compounds Casiopeinas® with blood serum and cellular bioligands

Casiopeinas® are among the few Cu^II compounds patented for their antitumor activity, but their mode of action has not been fully elucidated yet. One of them, Cas II-gly, is formed by 4,7-dimethyl-1,10-phenanthroline (Me₂phen) and glycinato (Gly). In blood and cells, Cas II-gly can keep its identity or form mixed species with serum or cytosol bioligands (bL or cL) with composition Cu^II-Me₂phen-bL/cL, Cu^II-Gly-bL/cL, or Cu^II-bL/cL. In this study, the binding of Cas II-gly with low molecular mass bioligands of blood serum (citric, L-lactic acid, and L-histidine) and cytosol (reduced glutathione (GSH), reduced nicotinamide adenine dinucleotide (NADH), adenosine triphosphate (ATP), and l-ascorbic acid) was examined through the application of instrumental (ElectroSpray Ionization-Mass Spectrometry and Electron Paramagnetic Resonance) and computational (Density Functional Theory) methods. The results indicated that mixed species Cu^II-Me₂phen-bL/cL are formed, with the bioligands replacing glycinato. The formation of these adducts may participate in the copper transport toward the target organs and facilitate the cellular uptake or, in constrast, preclude it. In the systems with GSH, NADH and L-ascorbate, a redox reaction occurs with the partial oxidation of cL to the corresponding oxidized form (GSSG, NAD⁺ and dehydroascorbate) which interact with Cu^II. Formed Cu^I ion does not give complexation reactions with reduced or oxidized form of bioligands for its 'soft' character and low affinity for oxygen and nitrogen donors compared to Cu^II. However, Cu^I could promote Fenton-like reactions with production of reactive oxygen species (ROS) related to the antitumor activity of Casiopeinas®.

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.

Chemodynamic therapy agents Cu(II) complexes of quinoline derivatives induced ER stress and mitochondria-mediated apoptosis in SK-OV-3 cells

Three Cu(II) complexes of quinoline derivatives as cancer chemodynamic therapy agents were synthesized and characterized. These complexes were heavily taken up by cells and reacted with cellular glutathione (GSH) to reduce Cu²⁺ to Fenton-like Cu⁺, which catalyzed endogenous H₂O₂ to produce the highly toxic hydroxyl radicals (•OH) to kill cancer cells. Cu1 and Cu2 initiated CAT activity declines, mitochondrial membrane potential and ATP concentration decrease, mitochondrial Ca²⁺ overload and ER stress response, leading to cell cycle arrest in sub-G1 and cancer cell caspase-dependent apoptosis. On account of the high GSH and H₂O₂ specific properties of the tumor microenvironment, Cu1 and Cu2 exhibited higher in vitro anticancer activity and lower toxicity to normal cells. Cu1 and Cu2 efficiently inhibited tumor growth in the SK-OV-3 xenograft mouse model without obvious systemic toxicity.

Binding Models of Copper(II) Thiosemicarbazone Complexes with Human Serum Albumin: A Speciation Study

Copper(II) complexes of thiosemicarbazones (TSCs) often exhibit anticancer properties, and their pharmacokinetic behavior can be affected by their interaction with blood transport proteins. Interaction of copper(II) complexes of an {N,N,S} donor α-N-pyridyl TSC (Triapine) and an {O,N,S} donor 2-hydroxybenzaldehyde TSC (STSC) with human serum albumin (HSA) was investigated by UV–visible and electron paramagnetic resonance spectroscopy at physiological pH. Asp-Ala-His-Lys and the monodentate N-methylimidazole were also applied as binding models. Conditional formation constants were determined for the ternary copper(II)-TSC complexes formed with HSA, DAHK, and N-methylimidazole based on the spectral changes of both charge transfer and d-d bands. The neutral N-methylimidazole displays a similar binding affinity to both TSC complexes. The partially negatively charged tetrapeptide binds stronger to the positively charged Triapine complex in comparison to the neutral STSC complex, while the opposite trend was observed for HSA, which demonstrates the limitations of the use of simple ligands to model the protein binding. The studied TSC complexes are able to bind to HSA in a fast process, and the conditional constants suggest that their binding strength is only weak-to-moderate.

The first copper(I) complex of anthrahydrazone with potential ROS scavenging activity showed significant in vitro anticancer activity by inducing apoptosis and autophagy

Based on the anticancer pharmacophore of anthrahydrazone and quinoline, a new quinolylanthrahydrazone ligand, 9-AQH (anthracene-9-quinolylhydrazone), was synthesized to further afford four metal complexes, [Co^II(9-AQH)(NO₃)₂(H₂O)] (1), [Ni^II(9-AQH)₂(H₂O)₂]·2NO₃ (2), [Cu^I(9-AQH)₂]·NO₃ (3), [Zn^II(9-AQH)₂(NO₃)]·NO₃ (4), determined by X-ray single crystal diffraction analysis. The reaction of Cu(NO₃)₂ with 9-AQH formed the stable and repeatable copper(I) complex 3. In vitro screening demonstrated only 3 showed significant and broad-spectrum anticancer activity, indicating that Cu(I) played a key role in exerting the anticancer activity. In solution, Cu(I) was not naturally oxidized to Cu(II) suggested by ¹H-NMR (Nuclear Magnetic Resonance) and EPR (Electron Paramagnetic Resonance) analysis. The presence of 3 could also catalyze the H₂O₂ system to give hydroxyl free radicals, suggested by further EPR and electrophoresis assay. At the cellular level, although no obvious Cu(II) signals were detected and the total ROS (Reactive Oxygen Species) scavenging in the tumor cells treated with 3, the potential redox property between Cu(I)/Cu(II), as a key role, should not be denied for the significant anticancer activity of 3, considering the much complicated circumstance and other reductive substances in cells. The anticancer mechanism of 3 on the most sensitive MGC-803 cells pointed to significant cell apoptosis through mitochondrial pathway, rather than cell cycle arrest. While the autophagy observed in tumor cells treated by 3 suggested its complicated anticancer mechanism, and whether there was an intrinsic correlation still needed to be further investigated.

A novel biocompatible formate bridged 1D-Cu(ii) coordination polymer induces apoptosis selectively in human lung adenocarcinoma (A549) cells

Copper compounds are promising candidates for next-generation metal anticancer drugs. Therefore, we synthesized and characterized a formate bridged 1D coordination polymer [Cu(L)(HCOO)₂]_n, (L = 2-methoxy-6-methyl-3-((quinolin-8-ylimino)methyl)chroman-4-ol), PCU1, wherein the Cu(ii) center adopts a square pyramidal coordination environment with adjacent CuCu distances of 5.28 Å. Primarily, in vitro DNA interaction studies revealed a metallopolymer which possesses high DNA binding propensity and cleaves DNA via the oxidative pathway. We further analysed its potential on cancerous cells MCF-7, HeLa, A549, and two non-tumorigenic cells HEK293 and HBE. The selective cytotoxicity potential of PCU1 against A549 cells driven us to examine the mechanistic pathways comprehensively by carrying out various assays viz, cell cycle arrest, Annexin V-FTIC/PI assay, autophagy, intercellular localization, mitochondrial membrane potential 'MMP', antiproliferative assay, and gene expression of TGF-β and MMP-2.

Relation of Metal-Binding Property and Selective Toxicity of 8-Hydroxyquinoline Derived Mannich Bases Targeting Multidrug Resistant Cancer Cells

Simple Summary

Effective treatment of cancer is often limited by the resistance of cancer cells to chemotherapy. A well-described mechanism supporting multidrug resistance (MDR) relies on the efflux of toxic drugs from cancer cells, mediated by P-glycoprotein (Pgp). Circumventing Pgp-mediated resistance is expected to make a significant contribution to improved therapy of malignancies. Interestingly, MDR cells exhibit paradoxical hypersensitivity towards a diverse set of anticancer chelators. In this study we explore the relation of chemical and structural properties influencing metal binding and toxicity of a set of 8-hydroxyquinoline derivatives to reveal key characteristics governing “MDR-selective” activity. We find that subtle changes in the stability and redox activity of the biologically relevant metal complexes significantly influence MDR-selective toxicity. Our results underline the importance of chelation in MDR-selective toxicity, suggesting that the collateral sensitivity of MDR cells may be targeted by preferential iron deprivation or the formation of redox-active copper(II) complexes.

Abstract

Resistance to chemotherapeutic agents is a major obstacle in cancer treatment. A recently proposed strategy is to target the collateral sensitivity of multidrug resistant (MDR) cancer. Paradoxically, the toxicity of certain metal chelating agents is increased, rather than decreased, by the function of P-glycoprotein (Pgp), which is known to confer resistance by effluxing chemotherapeutic compounds from cancer cells. We have recently characterized and compared the solution’s chemical properties including ligand protonation and the metal binding properties of a set of structurally related 8-hydroxyquinoline derived Mannich bases. Here we characterize the impact of the solution stability and redox activity of their iron(III) and copper(II) complexes on MDR-selective toxicity. Our results show that the MDR-selective anticancer activity of the studied 8-hydroxyquinoline derived Mannich bases is associated with the iron deprivation of MDR cells and the preferential formation of redox-active copper(II) complexes, which undergo intracellular redox-cycling to induce oxidative stress.

Anticancer activity of a new copper(II) complex with a hydrazone ligand. Structural and spectroscopic characterization, computational simulations and cell mechanistic studies on 2D and 3D breast cancer cell models

We report here the synthesis, crystal structure, characterization and anticancer activity of a copper(ii)-hydrazone complex, [Cu(MeBHoVa)(H2O)2](NO3) (for short, CuHL), against human breast cancer cells on monolayer (2D) and spheroids/mammospheres (3D). The solid-state molecular structure of the complex has been determined by X-ray diffraction methods. The conformational space was searched and geometries were optimized both in the gas phase and including solvent effects by computational methods based on DFT. The compound has been characterized in the solid state and in solution by spectroscopic (FTIR, Raman, UV-vis) methods. The results were compared with those obtained for the hydrazone ligand and complemented with DFT calculations. Cell viability assays on MCF7 (IC50(CuHL) = 1.7 ± 0.1 μM, IC50(CDDP) = 42.0 ± 3.2 μM) and MDA-MB-231 (IC50(CuHL) = 1.6 ± 0.1 μM, IC50(CDDP) = 131.0 ± 18 μM) demonstrated that the complex displays higher antitumor activity than cisplatin (CDDP) on 2D and 3D human breast cancer cell models. Molecular docking and molecular dynamics simulations showed that CuHL could interacts with DNA, inducing a significant genotoxic effect on both breast cancer cells from 0.5 to 1 μM. On the other hand, CuHL increases the ROS production and induces cell programmed death on breast cancer cells at very low micromolar concentrations (0.5-1.0 μM). Moreover, the compound decreased the amount of breast CSCs on MCF7 and MDA-MB-231 cells reducing the percentage of CD44+/CD24-/low cells from 0.5 to 1.5 μM. In addition, CuHL overcame CDDP with an IC50 value 65-fold lower against breast multicellular spheroids ((IC50(CuHL) = 2.2 ± 0.3 μM, IC50(CDDP) = 125 ± 4.5 μM)). Finally, CuHL reduced mammosphere formation capacity, hence affecting the size and number of mammospheres and showing that the complex exhibits antitumor properties on monolayer (2D) and spheroids (3D) derived from human breast cancer cells.

Complex formation and cytotoxicity of Triapine derivatives: a comparative solution study on the effect of the chalcogen atom and NH-methylation

α-N-Heterocyclic thiosemicarbazones are an important class of investigational anticancer drugs. The most prominent representative is 3-aminopyridine-2-carboxaldehyde thiosemicarbazone (Triapine), which has shown promising results in clinical trials and is currently evaluated in phase III. In this study, we investigated the influence of a chalcogen atom exchange from S (Triapine) to O (O-Triapine) and Se (Se-Triapine) and the methylation of the hydrazonic NH moiety (Me-Triapine) on their complexation with Fe(ii), Fe(iii) and Cu(ii) ions and their cytotoxicity. The main aim of this study was to characterize and compare the most feasible chemical forms in solution, their stability and redox properties, as well as to reveal the relationships of the solution speciation and kinetic data with cytotoxic activity. The complex equilibria and redox properties of the complexes were characterized by the combined use of pH-potentiometry, UV-visible spectrophotometry, electron paramagnetic resonance spectroscopy, and cyclic voltammetry. These revealed that Se-Triapine forms Cu(ii) complexes with higher, and O-Triapine with lower stability as compared with Triapine. Me-Triapine, which is not able to coordinate via the typical (N,N,S-) donor set, nevertheless coordinates to Cu(ii) with unexpected high stability. The Cu(ii) complexes of Se-Triapine and Me-Triapine can be relatively slowly reduced by glutathione at pH 7.4 (but not by ascorbate), similarly to Cu(ii)-Triapine. In contrast, the Cu(ii)-O-Triapine complex can be reduced by both reducing agents in rapid redox reactions. Se-Triapine and Triapine form high stability complexes with both Fe(ii) and Fe(iii) ions, while O-Triapine has a much stronger preference towards Fe(iii) and Me-Triapine towards Fe(ii). This difference in the iron preference of the ligands seems to have a strong impact on their cytotoxic effects, which was measured in a human uterine sarcoma cell line (MES-SA) and its multidrug-resistant subline (MES-SA/Dx5). The Cu(ii) complexes of these calcogensemicarbazones are moderately toxic, and the highest level of ROS generation was found for the Cu(ii) complex of O-Triapine, which is the most reducible.

Cancer Cell Resistance Against the Clinically Investigated Thiosemicarbazone COTI-2 Is Based on Formation of Intracellular Copper Complex Glutathione Adducts and ABCC1-Mediated Efflux

COTI-2 is a novel anticancer thiosemicarbazone in phase I clinical trial. However, the effects of metal complexation (a main characteristic of thiosemicarbazones) and acquired resistance mechanisms are widely unknown. Therefore, in this study, the copper and iron complexes of COTI-2 were synthesized and evaluated for their anticancer activity and impact on drug resistance in comparison to metal-free thiosemicarbazones. Investigations using Triapine-resistant SW480/Tria and newly established COTI-2-resistant SW480/Coti cells revealed distinct structure-activity relationships. SW480/Coti cells were found to overexpress ABCC1, and COTI-2 being a substrate for this efflux pump. This was unexpected, as ABCC1 has strong selectivity for glutathione adducts. The recognition by ABCC1 could be explained by the reduction kinetics of a ternary Cu-COTI-2 complex with glutathione. Thus, only thiosemicarbazones forming stable, nonreducible copper(II)-glutathione adducts are recognized and, in turn, effluxed by ABCC1. This reveals a crucial connection between copper complex chemistry, glutathione interaction, and the resistance profile of clinically relevant thiosemicarbazones.

NLRP3 Inflammasome and Allergic Contact Dermatitis: A Connection to Demystify

Allergic contact dermatitis is a common occupational disease that manifests as a cell-mediated hypersensitivity reaction following skin exposure to small reactive chemicals termed haptens. Haptens penetrate the stratum corneum and covalently modify proteins in the epidermis, inducing intracellular stress, which further leads to the release of damage-associated molecular patterns (DAMPs), such as uric acid, reactive oxygen species, hyaluronic acid fragments and extracellular adenosine triphosphate (ATP). These DAMPs are recognized by pattern recognition receptors (PRRs) in innate immune cells, namely dendritic cells (DCs), leading to their maturation and migration to the draining lymph nodes where they activate naïve T lymphocytes. Among all PRRs, several studies emphasize the role of NOD-, LRR- and pyrin domain-containing protein 3 (NLRP3) inflammasome on the allergic contact dermatitis (ACD) sensitization phase. However, skin allergens-danger signals-NLRP3 inflammasome axis is yet to be completely elucidated. Therefore, in this review, we sought to discuss the molecular mechanisms underlying DAMPs release and NLRP3 inflammasome activation triggered by skin allergens. The elucidation of these key events might help to identify novel therapeutic strategies for ACD, as well as the development of nonanimal alternative methods for the identification and potency categorization of skin sensitizers.

High Copper Complex Stability and Slow Reduction Kinetics as Key Parameters for Improved Activity, Paraptosis Induction, and Impact on Drug-Resistant Cells of Anticancer Thiosemicarbazones

Aims: Due to their significant biological activity, thiosemicarbazones (TSCs) are promising candidates for anticancer therapy. In part, the efficacy of TSCs is linked to their ability to chelate essential metal ions such as copper and iron. Triapine, the best-studied anticancer TSC, has been tested clinically with promising results in hematological diseases. During the past few years, a novel subclass of TSCs with improved anticancer activity was found to induce paraptosis, a recently characterized form of cell death. The aim of this study was to identify structural and chemical properties associated with anticancer activity and paraptosis induction of TSCs. Results: When testing a panel of structurally related TSCs, compounds with nanomolar anticancer activity and paraptosis-inducing properties showed higher copper(II) complex solution stability and a slower reduction rate, which resulted in reduced redox activity. In contrast, TSCs with lower anticancer activity induced higher levels of superoxide that rapidly stimulated superoxide dismutase expression in treated cells, effectively protecting the cells from drug-induced redox stress. Innovation: Consequently, we hypothesize that in the case of close Triapine derivatives, intracellular reduction leads to rapid dissociation of intracellularly formed copper complexes. In contrast, TSCs characterized by highly stable, slowly reducible copper(II) complexes are able to reach new intracellular targets such as the endoplasmic reticulum-resident protein disulfide isomerase. Conclusion: The additional modes of actions observed with highly active TSC derivatives are based on intracellular formation of stable copper complexes, offering a new approach to combat (drug-resistant) cancer cells.

Isoxazole-Derived Aroylhydrazones and Their Dinuclear Copper(II) Complexes Show Antiproliferative Activity on Breast Cancer Cells with a Potentially Alternative Mechanism Of Action

This paper reports the design, synthesis and cytotoxicity studies of two new isoxazole-derived aroylhydrazone ligands and their dinuclear copper(II) complexes. Compounds were fully characterized by various spectroscopic and analytical techniques. The molecular structures of four derivatives were confirmed by X-ray crystallography. The stability of the ligands and the complexes in aqueous medium was monitored spectroscopically. Both the ligands and the complexes were shown to interact with calf thymus DNA (ct-DNA). Additionally, structures containing a phenol pendant arm were significantly more cytotoxic than those carrying a pendant pyridine substituent, reaching sub-micromolar IC₅₀ values on the triple-negative human breast cancer cell line MDA-MB-231. The metal chelation and transchelation ability of the compounds towards Fe^II , Fe^III and Zn^II ions was explored as a possible mechanism of action of these compounds.

Thiosemicarbazones as Potent Anticancer Agents and their Modes of Action

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Thiosemicarbazones (TSCs) are a class of Schiff bases usually obtained by the condensation of thiosemicarbazide with a suitable aldehyde or ketone. TSCs have been the focus of chemists and biologists due to their wide range of pharmacological effects. One of the promising areas in which these excellent metal chelators are being developed is their use against cancer. TSCs have a wide clinical antitumor spectrum with efficacy in various tumor types such as leukemia, pancreatic cancer, breast cancer, non-small cell lung cancer, cervical cancer, prostate cancer and bladder cancer. To obtain better activity, different series of TSCs have been developed by modifying the heteroaromatic system in their molecules. These compounds possessed significant antineoplastic activity when the carbonyl attachment of the side chain was located at a position α to the ring nitrogen atom, whereas attachment of the side chain β or γ to the heterocyclic N atom resulted in inactive antitumor agents. In addition, replacement of the heterocyclic ring N with C also resulted in a biologically inactive compound suggesting that a conjugated N,N,S-tridentate donor set is essential for the biological activities of thiosemicarbazones. Several possible mechanisms have been implemented for the anticancer activity of thiosemicarbazones.

β-Carboline copper complex as a potential mitochondrial-targeted anticancer chemotherapeutic agent: Favorable attenuation of human breast cancer MCF7 cells via apoptosis

The development of preferentially selective cancer chemotherapeutics is a new trend in drug research. Thus, we designed and synthesized novel ternary complexes, [Cu(tryp)(Hnor)₂(DMSO)]NO₃ (1) and [Zn(tryp)(Hnor)₂(DMSO)]NO₃(2) (tryp = DL-Tryptophane; Hnor = Norharmane, β-carboline; DMSO = Dimethyl sulfoxide), characterized with elemental analysis, FTIR, UV–vis, FL, NMR, ESI-MS, and molar conductivity. Furthermore, the TD-DFT studies with UV–vis and FTIR validated the proposed structures of 1 and 2. Moreover, we evaluated the HOMO-LUMO energy gap and found that 1 has a smaller energy gap than 2. Then, 1 and 2 were assessed for anticancer chemotherapeutic potential against cancer cell lines MCF7 (human breast cancer) and HepG2 (human liver hepatocellular carcinoma) as well as the non-tumorigenic HEK293 (human embryonic kidney) cells. The MTT assay illustrated the preferentially cytotoxic behavior of 1 when compared with that of 2 and cisplatin (standard drug) against MCF7 cells. Moreover, 1 was exposed to MCF7 cells, and the results indicated the arrest of the G2/M phases, which followed the apoptotic pathway predominantly. Generation of ROS, GSH depletion, and elevation in LPO validated the redox changes prompted by 1. These studies establish the great potential of 1 as a candidate for anticancer therapeutics.

In vitro and in vivo anticancer activity of tridentate thiosemicarbazone copper complexes: Unravelling an unexplored pharmacological target

Certain metal complexes can have a great antitumor activity, as the use of cisplatin in therapy has been demonstrating for the past fifty years. Copper complexes, in particular, have attracted much attention as an example of anticancer compounds based on an endogenous metal. In this paper we present the synthesis and the activity of a series of copper(II) complexes with variously substituted salicylaldehyde thiosemicarbazone ligands. The in vitro activity of both ligands and copper complexes was assessed on a panel of cell lines (HCT-15, LoVo and LoVo oxaliplatin resistant colon carcinoma, A375 melanoma, BxPC3 and PSN1 pancreatic adenocarcinoma, BCPAP thyroid carcinoma, 2008 ovarian carcinoma, HEK293 non-transformed embryonic kidney), highlighting remarkable activity of the metal complexes, in some cases in the low nanomolar range. The copper(II) complexes were also screened, with good results, against 3D spheroids of colon (HCT-15) and pancreatic (PSN1) cancer cells. Detailed investigations on the mechanism of action of the copper(II) complexes are also reported: they are able to potently inhibit Protein Disulfide Isomerase, a copper-binding protein, that is recently emerging as a new therapeutic target for cancer treatment. Good preliminary results obtained in C57BL mice indicate that this series of metal-based compounds could be a very promising weapon in the fight against cancer.

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.

Cu(ii) phenanthroline–phenazine complexes dysregulate mitochondrial function and stimulate apoptosis

Herein we report the central role of the mitochondria in the cytotoxicity of four developmental cytotoxic copper(ii) complexes [Cu(phen)₂]²⁺, [Cu(DPQ)(Phen)]²⁺, [Cu(DPPZ)(Phen)]²⁺and [Cu(DPPN)(Phen)]²⁺superior to cisplatin and independent of resistance in a range of cells.

Salicylaldehyde thiosemicarbazone copper complexes: impact of hybridization with estrone on cytotoxicity, solution stability and redox activity

Copper complex of a novel estrone–thiosemicarbazone hybrid with significant cytotoxicity, lipophilicity and solution stability in addition to its structurally related bicyclic analogue.

Copper(II) Coordination and Translocation in Luteolin and Effect on Radical Scavenging

Luteolin differs as a radical scavenger dramatically from apigenin in response to Cu(II) coordination despite a minor structural difference. Coordination of Cu(II) increases the radical scavenging efficiency of luteolin, especially at low pH, while decreases the efficiency of apigenin at both low and higher pH as studied by ABTS^•+ radical scavenging. Luteolin forms a 1:1 complex with Cu(II) binding to 4-carbonyl and 5-phenol for pH <6 and to 3',4'-catechol for pH >6. Apigenin forms a 1:2 complex independent of pH coordinated to 4-carbonyl and 5-hydroxylyl. Cu(II) coordinated to luteolin, as studied by pH jump stopped-flow, translocates with rate constants of 11.1 ± 0.3 s^-1 from 4,5 to 3',4' sites and 1.0 ± 0.1 s^-1 from 3',4' to 4,5 sites independent of Cu(II) concentration, pointing toward the dissociation of Cu(II) from an intermediate with two Cu(II) coordination as rate determining. 3',4'-Catechol is suggested to be a switch for Cu(II) translocation with deprotonation initiating 4,5 to 3',4' translocation and protonation initiating 3',4' to 4,5 translocation. For dicoordinated apigenin, the coordination symmetry balances an electron withdrawal effect of Cu(II) resulting in a decrease of phenol acidity and less radical scavenging efficiency compared to parent apigenin. Compared to that of parent luteolin, the radical scavenging rate of both 4,5 and 3',4' Cu(II)-coordinated luteolin is enhanced through increased phenol acidity by electron withdrawal by Cu(II), as confirmed by density functional theory (DFT) calculations. Coordination and translocation of Cu(II) accordingly increases the antioxidant activity of luteolin at pH approaching the physiological level and is discovered as a novel class of natural molecular machinery derived from plant polyphenols, which seems to be of importance for protection against oxidative stress.

Copper-functionalized nanostructured silica-based systems: Study of the antimicrobial applications and ROS generation against gram positive and gram negative bacteria

A series of copper-functionalized SBA-15 (Santa Barbara Amorphous) materials containing the ligands triethoxysilylpropylmaleamic acid (maleamic) or triethoxy-3-(2-imidazolin-1-yl)propylsilane (imidazoline) have been prepared. The nanostructured silica-based systems SBA-maleamic, SBA-imidazoline, SBA-maleamic-Cu and SBA-imidazoline-Cu were characterized by several methods observing that the functionalization took place mainly inside the pores of the mesoporous system. The antimicrobial behaviour of the synthesized materials against Staphylococcus aureus and Escherichia coli was tested observing a very potent activity of the copper-functionalized systems (minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) values for SBA-maleamic-Cu of ca. 31.25 μg/mL, which correspond with ca. 1.13 μg/mL of Cu). A study of the oxidative stress promoted by the synthesized materials showed that the SBA-maleamic-Cu and the SBA-imidazoline-Cu were able to increase the reactive oxygen species (ROS) production in S. aureus by 427% and 373%, respectively, while this increase was slightly lower in E. coli (387 and 324%, respectively). Furthermore, an electrochemical study was carried out in order to determine if these materials interact with lysine or alanine to validate a potential antimicrobial mechanism based on the inhibition of the synthesis of the peptidoglycan of the bacterial wall. Finally, these studies were also performed to determine the potential interaction of the copper-containing materials with glutathione in order to assess if they are able to perturb the metabolism of this tripeptide.

Synthesis, Characterization, and in Vitro Anticancer Activity of Copper and Zinc Bis(Thiosemicarbazone) Complexes

A series of eight bis(thiosemicarbazone) ligands and 16 of their respective copper(II) and zinc(II) complexes containing a combination of hydrogen, methyl, pyridyl, phenyl, and/or ethyl substituents at the diimine position of the ligand backbone were synthesized and characterized. The objective of this study was to identify the structure-activity relationships within a series of analogues with different substituents at the diimine position of the backbone and at the terminal N atom. The Cu(II) complexes Cu(GTSM₂), Cu(GTSCM), Cu(PyTSM₂), Cu(EMTSM₂) and Cu(PGTSM₂) demonstrated a distorted square planar geometry, while the Zn(II) complexes Zn(ATSM₂)(DMSO), Zn(PyTSM₂)(DMSO), and Zn(PGTSM₂)(H₂O) formed a distorted square pyramidal geometry. Cyclic voltammetry showed that the Cu(II) complexes display quasi-reversible electrochemistry. Of the agents, Cu(II) glyoxal bis(4,4-dimethyl-3-thiosemicarbazone) [Cu(GTSM₂)] and Cu(II) diacetyl bis(4,4-dimethyl-3-thiosemicarbazone) [Cu(ATSM₂)] demonstrated the greatest antiproliferative activity against tumor cells. Substitutions at the diimine position and at the terminal N atom with hydrophobic moieties markedly decreased their antiproliferative activity. Complexation of the bis(thiosemicarbazones) with Zn(II) generally decreased their antiproliferative activity, suggesting the Zn(II) complex did not act as a chaperone to deliver the ligand intracellularly, in contrast to similar bis(thiosemicarbazone) cobalt(III) complexes [King et al. Inorg. Chem. 2017, 56, 6609-6623]. However, five of the eight bis(thiosemicarbazone) Cu(II) complexes maintained or increased their antiproliferative activity, relative to the ligand alone, and a mechanism of Cu-induced oxidative stress is suggested. Surprisingly, relative to normoxic growth conditions, hypoxia that is found in the tumor microenvironment decreased the antiproliferative efficacy of most bis(thiosemicarbazones) and their copper complexes. This was independent of the potential hypoxia-selectivity mediated by Cu(II/I) redox potentials. These results provide structure-activity relationships useful for the rational design of bis(thiosemicarbazone) anticancer agents.

Anticancer Function and ROS-Mediated Multi-Targeting Anticancer Mechanisms of Copper (II) 2-hydroxy-1-naphthaldehyde Complexes

Multi-targeting of oncoproteins by a single molecule represents an effectual, rational, and an alternative approach to target therapy. We carried out a systematic study to reveal the mechanisms of action of newly synthesized Cu²⁺ compounds of 2-naphthalenol and 1-(((2-pyridinylmethyl)imino)methyl)- (C1 and C2). The antiproliferative activity of the as-synthesized complexes in three human cancer cell lines indicates their potential as multi-targeted antitumor agents. Relatively, C1 and C2 showed better efficacy in vitro relative to Cisplatin and presented promising levels of toxicity against A-549 cells. On the whole, the Cu²⁺ complexes exhibited chemotherapeutic effects by generating reactive oxygen species (ROS) and arresting the cell cycle in the G₀/G₁ phase by competent regulation of cyclin and cyclin-dependent kinases. Fascinatingly, the Cu²⁺ complexes were shown to activate the apoptotic and autophagic pathways in A-549 cells. These complexes effectively induced endoplasmic reticulum stress-mediated apoptosis, inhibited topoisomerase-1, and damaged cancer DNA through a ROS-mediated mechanism. The synthesized Cu²⁺ complexes established ROS-mediated targeting of multiple cell signaling pathways as a fabulous route for the inhibition of cancer cell growth.

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.