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

An estrone–salicylaldehyde thiosemicarbazone hybrid (estrone–TSC) containing integrated domains was designed and synthesized with excellent yield via the condensation reaction of thiosemicarbazide and 2-formyl-estrone under optimized microwave reaction conditions. A structurally related bicyclic derivative (thn-TSC) starting from 5, 6, 7, 8-tetrahydro-1-naphtol (th-1-n) was also prepared in addition to their copper(II) complexes. The ligands have somewhat higher pKa values determined for the deprotonation of the hydroxyl group by UV-visible spectrophotometric and fluorometric titrations than the reference compound salicylaldehyde thiosemicarbazone (STSC), and are neutral at physiological pH. The novel conjugates are more lipophilic and possess higher membrane permeability than STSC based on the n-octanol/water partitioning and the parallel artificial membrane permeability assays, respectively. The isolated [Cu(estrone–TSCH−2)] and [Cu(thn-TSCH−2)] complexes were characterized by ESI-MS, UV-visible and EPR spectroscopy and a detailed solution study was performed to reveal their stoichiometry, stability and reduction by glutathione. The crystal structure of the ligand thn-TSC and its complex [Cu(thn-TSCH−1)Cl] was studied by single crystal X-ray diffraction method. The complexes are fairly stable at pH 7.4, the observed stability order is STSC < thn-TSC < estrone–TSC, and are able to oxidize glutathione readily. The novel ligands thn-TSC and estrone–TSC were found to be only moderately cytotoxic against several human cancer cell lines ; however rather low IC50 values were measured in the hormone-responsive MCF-7 breast cancer cell lines (thn-TSC: 3.7 μM, estrone–TSC: 6.4 μM). The copper(II) complexes exhibited high cytotoxicity (IC50 < 0.3–2 μM) and were considerably more cytotoxic than the respective ligands. Low level of reactive oxygen species was measured and a weak GSH depletion was observed for the complexes of thn-TSC and estrone–TSC in SUM159 breast cancer cells, thus their mechanism of action might be related to the induction of oxidative stress

Comparative Solution Equilibrium Studies on Anticancer Estradiol-Based Conjugates and Their Copper Complexes

Steroids are often considered valuable molecular tools for the development of anticancer agents with improved pharmacological properties. Conjugation of metal chelating moieties with a lipophilic sterane backbone is a viable option to obtain novel anticancer compounds. In this work, two estradiol-based hybrid molecules (PMA-E2 and DMA-E2) with an (N,N,O) binding motif and their Cu(II) complexes were developed. The lipophilicity, solubility, and acid-base properties of the novel ligands were determined by the combined use of UV-visible spectrophotometry, pH-potentiometry, and 1H NMR spectroscopy. The solution speciation and redox activity of the Cu(II) complexes were also investigated by means of UV-visible and electron paramagnetic resonance spectroscopy. Two structurally analogous ligands (PMAP and DMAP) were also included in the studies for better interpretation of the solution chemical data obtained. Three pKa values were determined for all ligands, revealing the order of the deprotonation steps: pyridinium-NH+ or NH(CH3)2+, secondary NH2+, and OH. The dimethylamine derivatives (DMA-E2, DMAP) are found in their H2L+ forms in solution at pH 7.4, whereas the fraction of the neutral HL species is significant (34–37%) in the case of the pyridine nitrogen-containing derivatives (PMA-E2, PMAP). Both estradiol derivatives were moderately cytotoxic in human breast (MCF-7) and colon adenocarcinoma (Colo-205) cells (IC50 = 30–63 μM). They form highly stable complexes with Cu(II) ions capable of oxidizing ascorbate and glutathione. These Cu(II) complexes are somewhat more cytotoxic (IC50 = 15–45 μM) than their corresponding ligands and show a better selectivity profile

Estradiol-Based Salicylaldehyde (Thio)Semicarbazones and Their Copper Complexes with Anticancer, Antibacterial and Antioxidant Activities

A series of novel estradiol-based salicylaldehyde (thio)semicarbazones ((T)SCs) bearing (O,N,S) and (O,N,O) donor sets and their Cu(II) complexes were developed and characterized in detail by 1H and ¹³C nuclear magnetic resonance spectroscopy, UV–visible and electron paramagnetic resonance spectroscopy, electrospray ionization mass spectrometry and elemental analysis. The structure of the Cu(II)-estradiol-semicarbazone complex was revealed by X-ray crystallography. Proton dissociation constants of the ligands and stability constants of the metal complexes were determined in 30% (v/v) DMSO/H2O. Estradiol-(T)SCs form mono-ligand complexes with Cu(II) ions and exhibit high stability with the exception of estradiol-SC. The Cu(II) complexes of estradiol-TSC and its N,N-dimethyl derivative displayed the highest cytotoxicity among the tested compounds in MCF-7, MCF-7 KCR, DU-145, and A549 cancer cells. The complexes do not damage DNA according to both in vitro cell-free and cellular assays. All the Cu(II)-TSC complexes revealed significant activity against the Gram-positive Staphylococcus aureus bacteria strain. Estradiol-TSCs showed efficient antioxidant activity, which was decreased by complexation with Cu(II) ions. The exchange of estrone moiety to estradiol did not result in significant changes to physico-chemical and biological properties

Insight into the Anticancer Activity of Copper(II) 5-Methylenetrimethylammonium-Thiosemicarbazonates and Their Interaction with Organic Cation Transporters

A series of four water-soluble salicylaldehyde thiosemicarbazones with a positively charged trimethylammonium moiety ([H2LR]Cl, R = H, Me, Et, Ph) and four copper(II) complexes [Cu(HLR)Cl]Cl (1–4) were synthesised with the aim to study (i) their antiproliferative activity in cancer cells and, (ii) for the first time for thiosemicarbazones, the interaction with membrane transport proteins, specifically organic cation transporters OCT1–3. The compounds were comprehensively characterised by analytical, spectroscopic and X-ray diffraction methods. The highest cytotoxic effect was observed in the neuroblastoma cell line SH-5YSY after 24 h exposure and follows the rank order: 3 > 2 > 4 > cisplatin > 1 >> [H2LR]Cl. The copper(II) complexes showed marked interaction with OCT1–3, comparable to that of well-known OCT inhibitors (decynium 22, prazosin and corticosterone) in the cell-based radiotracer uptake assays. The work paves the way for the development of more potent and selective anticancer drugs and/or OCT inhibitors

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(L1–3)]+ 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

Copper(II) Complexes with Isomeric Morpholine-Substituted 2-Formylpyridine Thiosemicarbazone Hybrids as Potential Anticancer Drugs Inhibiting Both Ribonucleotide Reductase and Tubulin Polymerization: The Morpholine Position Matters

The development of copper(II) thiosemicarbazone complexes as potential anticancer agents, possessing dual functionality as inhibitors of R2 ribonucleotide reductase (RNR) and tubulin polymerization by binding at the colchicine site, presents a promising avenue for enhancing therapeutic effectiveness. Herein, we describe the syntheses and physicochemical characterization of four isomeric proligands H2L3–H2L6, with the methylmorpholine substituent at pertinent positions of the pyridine ring, along with their corresponding Cu(II) complexes 3–6. Evidently, the position of the morpholine moiety and the copper(II) complex formation have marked effects on the in vitro antiproliferative activity in human uterine sarcoma MES-SA cells and the multidrug-resistant derivative MES-SA/Dx5 cells. Activity correlated strongly with quenching of the tyrosyl radical (Y•) of mouse R2 RNR protein, inhibition of RNR activity in the cancer cells, and inhibition of tubulin polymerization. Insights into the mechanism of antiproliferative activity, supported by experimental results and molecular modeling calculations, are presented