Novel half Salphen cobalt(III) complexes: synthesis, DNA binding and anticancer studies.

While platinum(II)-based drugs continue to be employed in cancer treatments, the escalating occurrence of severe side effects has spurred researchers to explore novel sources for potential therapeutic agents. Notably, cobalt(III) has emerged as a subject of considerable interest due to its ubiquitous role in human physiology. Several studies investigating the anticancer effects of Salphen complexes derived from cobalt(III) have unveiled intriguing antiproliferative properties. In a bid to enhance our understanding of this class of compounds, we synthesized and characterized two novel half Salphen cobalt(III) complexes. Both compounds exhibited notable stability, even in the presence of physiologically relevant concentrations of glutathione. The application of spectroscopic and computational methodologies unravelled their interactions with duplex and G4-DNAs, suggesting an external binding affinity for these structures, with preliminary indications of selectivity trends. Importantly, antiproliferative assays conducted on 3D cultured SW-1353 cancer cells unveiled a compelling anticancer activity at low micromolar concentrations, underscoring the potential therapeutic efficacy of this novel class of cobalt(III) complexes.

How the Metal Ion Affects the 1H NMR Chemical Shift Values of Schiff Base Metal Complexes: Rationalization by DFT Calculations.

The chemical shift (CS) values obtained by 1H NMR spectroscopy for the hydrogen atoms of a tetradentate N2O2-substituted Salphen ligand (H2L1) are differently shifted in its complexes of nickel(II), palladium(II), platinum(II), and zinc(II), all bearing the same charge on the metal ions. To rationalize the observed trends, DFT calculations have been performed in the implicit d6-DMSO solvent in terms of the electronic effects induced by the metal ion and of the nature and strength of the metal-N and metal-O bonds. Overall, the results obtained point out that, in the complexes involving group 10 elements, the CS values show the greater shift when considering the two hydrogen atoms at a shorter distance from the coordinated metal center and follow the decreasing metal charge in the order Ni > Pd > Pt. This trend suggests a more covalent character of the ligand-metal bonds with the increase of the metal atomic number. Furthermore, a slightly poorer agreement between experimental and calculated data is observed in the presence of the nickel(II) ion. Such discrepancy is explained by the formation of stacked oligomers, aimed at minimizing the repulsive interactions with the polar DMSO solvent.

Exploring the Anticancer Activity of Tamoxifen-Based Metal Complexes Targeting Mitochondria.

Two new 'hybrid' metallodrugs of Au(III) (AuTAML) and Cu(II) (CuTAML) were designed featuring a tamoxifen-derived pharmacophore to ideally synergize the anticancer activity of both the metal center and the organic ligand. The compounds have antiproliferative effects against human MCF-7 and MDA-MB 231 breast cancer cells. Molecular dynamics studies suggest that the compounds retain the binding activity to estrogen receptor (ERα). In vitro and in silico studies showed that the Au(III) derivative is an inhibitor of the seleno-enzyme thioredoxin reductase, while the Cu(II) complex may act as an oxidant of different intracellular thiols. In breast cancer cells treated with the compounds, a redox imbalance characterized by a decrease in total thiols and increased reactive oxygen species production was detected. Despite their different reactivities and cytotoxic potencies, a great capacity of the metal complexes to induce mitochondrial damage was observed as shown by their effects on mitochondrial respiration, membrane potential, and morphology.

"Dynamical Docking" of Cyclic Dinuclear Au(I) Bis-N-heterocyclic Complexes Facilitates Their Binding to G-Quadruplexes.

With the aim to improve the design of metal complexes as stabilizers of noncanonical DNA secondary structures, namely, G-quadruplexes (G4s), a series of cyclic dinuclear Au(I) N-heterocyclic carbene complexes based on xanthine and benzimidazole ligands has been synthesized and characterized by various methods, including X-ray diffraction. Fluorescence resonance energy transfer (FRET) and CD DNA melting assays unraveled the compounds' stabilization properties toward G4s of different topologies of physiological relevance. Initial structure-activity relationships have been identified and recognize the family of xanthine derivatives as those more selective toward G4s versus duplex DNA. The binding modes and free-energy landscape of the most active xanthine derivative (featuring a propyl linker) with the promoter sequence cKIT1 have been studied by metadynamics. The atomistic simulations evidenced that the Au(I) compound interacts noncovalently with the top G4 tetrad. The theoretical results on the Au(I) complex/DNA Gibbs free energy of binding were experimentally validated by FRET DNA melting assays. The compounds have also been tested for their antiproliferative properties in human cancer cells in vitro, showing generally moderate activity. This study provides further insights into the biological activity of Au(I) organometallics acting via noncovalent interactions and underlines their promise for tunable targeted applications by appropriate chemical modifications.

Highly-fluorescent BODIPY-functionalised metallacages as drug delivery systems: synthesis, characterisation and cellular accumulation studies.

With the aim of designing new metallosupramolecular architectures for drug delivery, research has focused on porous 3-dimensional (3D)-metallacages able to encapsulate cytotoxic agents protecting them from metabolism while targeting them to cancer sites. Here, two self-assembled [Pd2L4]4+ cages (CG1 and CG2) featuring 3,5-bis(3-ethynylpyridine)phenyl ligands (L) exo-functionalised with dipyrromethene (BODIPY) groups have been synthesised and characterised by different methods, including NMR spectroscopy and mass spectrometry. 1H NMR spectroscopy studies shows that the cages are able to encapsulate the anticancer drug cisplatin in their hydrophobic cavity, as evidenced by electrostatic potential (ESP) analysis based on XRD studies. The stability of the cages in an aqueous environment, and in the presence of the intracellular reducing agent glutathione, has been confirmed by UV-visible absorption spectroscopy. The luminescence properties of the cages enabled the investigation of their cellular uptake and intracellular localisation in human cancer cells by confocal laser scanning microscopy. In melanoma A375 cells, cage CG1 is taken up via active transport and endocytic trafficking studies show little evidence of transport through the early endosome while the cages accumulated in melanosomes rather than lysosomes. The antiproliferative activity of the lead cage was investigated in A375 together with two breast cancer cell lines, SK-BR-3 and MCF7. While the cage per se is non-cytotoxic, very different antiproliferative effects with respect to free cisplatin were evidenced for the [(cisplatin)2⊂CG1·BF4] complex in the various cell lines, which correlate with its different intracellular localisation profiles. The obtained preliminary results provide a new hypothesis on how the subcellular localisation of the cage affects the cisplatin intracellular release.

Competitive profiling of ligandable cysteines in Staphylococcus aureus with an organogold compound.

With the idea of exploiting metal templated C-S bond forming reactions to achieve modification of cysteines in bacterial proteins, a cyclometalated Au(III) compound was explored in a competitive chemoproteomic approach in S. aureus cell extracts. More than 100 ligandable cysteines were identified, of which more than 50% were not engaged by organic α-chloroacetamides in a previous study, indicating that organometallic compounds expand the ligandable space in bacteria. A selected interaction was validated using an enzyme activity assay, and intact protein mass spectrometry showed cysteine arylation of an unprecedented target. The obtained results demonstrate that this family of organogold compounds has potential for therapeutic protein targeting via selective, covalent modification of cysteine residues in bacteria.

Mechanisms of irreversible aquaporin-10 inhibition by organogold compounds studied by combined biophysical methods and atomistic simulations.

The inhibition of glycerol permeation via human aquaporin-10 (hAQP10) by organometallic gold complexes has been studied by stopped-flow fluorescence spectroscopy, and its mechanism has been described using molecular modelling and atomistic simulations. The most effective hAQP10 inhibitors are cyclometalated Au(III) C^N compounds known to efficiently react with cysteine residues leading to the formation of irreversible C-S bonds. Functional assays also demonstrate the irreversibility of the binding to hAQP10 by the organometallic complexes. The obtained computational results by metadynamics show that the local arylation of Cys209 in hAQP10 by one of the gold inhibitors is mapped into a global change of the overall free energy of glycerol translocation across the channel. Our study further pinpoints the need to understand the mechanism of glycerol and small molecule permeation as a combination of local structural motifs and global pore conformational changes, which are taking place on the scale of the translocation process and whose study, therefore, require sophisticated molecular dynamics strategies.

Antiproliferative Properties and G-Quadruplex-Binding of Symmetrical Naphtho[1,2-b:8,7-b']dithiophene Derivatives.

Background: G-quadruplex (G4) forming sequences are recurrent in telomeres and promoter regions of several protooncogenes. In normal cells, the transient arrangements of DNA in G-tetrads may regulate replication, transcription, and translation processes. Tumors are characterized by uncontrolled cell growth and tissue invasiveness and some of them are possibly mediated by gene expression involving G-quadruplexes. The stabilization of G-quadruplex sequences with small molecules is considered a promising strategy in anticancer targeted therapy. Methods: Molecular virtual screening allowed us identifying novel symmetric bifunctionalized naphtho[1,2-b:8,7-b']dithiophene ligands as interesting candidates targeting h-Telo and c-MYC G-quadruplexes. A set of unexplored naphtho-dithiophene derivatives has been synthesized and biologically tested through in vitro antiproliferative assays and spectroscopic experiments in solution. Results: The analysis of biological and spectroscopic data highlighted noteworthy cytotoxic effects on HeLa cancer cell line (GI50 in the low µM range), but weak interactions with G-quadruplex c-MYC promoter. Conclusions: The new series of naphtho[1,2-b:8,7-b']dithiophene derivatives, bearing the pharmacophoric assumptions necessary to stabilize G-quadruplexes, have been designed and successfully synthesized. The interesting antiproliferative results supported by computer aided rational approaches suggest that these studies are a significant starting point for a lead optimization process and the isolation of a more efficacious set of G-quadruplexes stabilizers.

C-C Cross-Couplings from a Cyclometalated Au(III) C ∧ N Complex: Mechanistic Insights and Synthetic Developments.

In recent years, the reactivity of gold complexes was shown to extend well beyond π-activation and to hold promises to achieve selective cross-couplings in several C-C and C-E (E=heteroatom) bond forming reactions. Here, with the aim of exploiting new organometallic species for cross-coupling reactions, we report on the Au(III)-mediated C(sp2 )-C(sp) occurring upon reaction of the cyclometalated complex [Au(CCH2 N)Cl2 ] (1, CCH2 N=2-benzylpyridine) with AgPhCC. The reaction progress has been monitored by NMR spectroscopy, demonstrating the involvement of a number of key intermediates, whose structures have been unambiguously ascertained through 1D and 2D NMR analyses (1 H, 13 C, 1 H-1 H COSY, 1 H-13 C HSQC and 1 H-13 C HMBC) as well as by HR-ESI-MS and X-ray diffraction studies. Furthermore, crystallographic studies have serendipitously resulted in the authentication of zwitterionic Au(I) complexes as side-products arising from cyclization of the coupling product in the coordination sphere of gold. The experimental work has been paralleled and complemented by DFT calculations of the reaction profiles, providing valuable insight into the structure and energetics of the key intermediates and transition states, as well as on the coordination sphere of gold along the whole process. Of note, the broader scope of the cross-coupling at the Au(III) CCH2 N centre has also been demonstrated studying the reaction of 1 with C(sp2 )-based nucleophiles, namely vinyl and heteroaryl tin and zinc reagents. These reactions stand as rare examples of C(sp2 )-C(sp2 ) cross-couplings at Au(III).

An Organogold Compound as Potential Antimicrobial Agent against Drug-Resistant Bacteria: Initial Mechanistic Insights.

The rise of antimicrobial resistance has necessitated novel strategies to efficiently combat pathogenic bacteria. Metal-based compounds have been proven as a possible alternative to classical organic drugs. Here, we have assessed the antibacterial activity of seven gold complexes of different families. One compound, a cyclometalated Au(III) C^N complex, showed activity against Gram-positive bacteria, including multi-drug resistant clinical strains. The mechanism of action of this compound was studied in Bacillus subtilis. Overall, the studies point towards a complex mode of antibacterial action, which does not include induction of oxidative stress or cell membrane damage. A number of genes related to metal transport and homeostasis were upregulated upon short treatment of the cells with gold compound. Toxicity tests conducted on precision-cut mouse tissue slices ex vivo revealed that the organogold compound is poorly toxic to mouse liver and kidney tissues, and may thus, be treated as an antibacterial drug candidate.

Identifying and validating the presence of Guanine-Quadruplexes (G4) within the blood fluke parasite Schistosoma mansoni.

Schistosomiasis is a neglected tropical disease that currently affects over 250 million individuals worldwide. In the absence of an immunoprophylactic vaccine and the recognition that mono-chemotherapeutic control of schistosomiasis by praziquantel has limitations, new strategies for managing disease burden are urgently needed. A better understanding of schistosome biology could identify previously undocumented areas suitable for the development of novel interventions. Here, for the first time, we detail the presence of G-quadruplexes (G4) and putative quadruplex forming sequences (PQS) within the Schistosoma mansoni genome. We find that G4 are present in both intragenic and intergenic regions of the seven autosomes as well as the sex-defining allosome pair. Amongst intragenic regions, G4 are particularly enriched in 3´ UTR regions. Gene Ontology (GO) term analysis evidenced significant G4 enrichment in the wnt signalling pathway (p<0.05) and PQS oligonucleotides synthetically derived from wnt-related genes resolve into parallel and anti-parallel G4 motifs as elucidated by circular dichroism (CD) spectroscopy. Finally, utilising a single chain anti-G4 antibody called BG4, we confirm the in situ presence of G4 within both adult female and male worm nuclei. These results collectively suggest that G4-targeted compounds could be tested as novel anthelmintic agents and highlights the possibility that G4-stabilizing molecules could be progressed as candidates for the treatment of schistosomiasis.

An Organometallic Gold(I) Bis-N-Heterocyclic Carbene Complex with Multimodal Activity in Ovarian Cancer Cells.

The organometallic AuI bis-N-heterocyclic carbene complex [Au(9-methylcaffeine-8-ylidene)2 ]+ (AuTMX2 ) was previously shown to selectively and potently stabilise telomeric DNA G-quadruplex (G4) structures. This study sheds light on the molecular reactivity and mode of action of AuTMX2 in the cellular context using mass spectrometry-based methods, including shotgun proteomics in A2780 ovarian cancer cells. In contrast to other metal-based anticancer agents, this organogold compound is less prone to form coordinative bonds with biological nucleophiles and is expected to exert its drug effects mainly by non-covalent interactions. Global protein expression changes of treated cancer cells revealed a multimodal mode of action of AuTMX2 by alterations in the nucleolus, telomeres, actin stress-fibres and stress-responses, which were further supported by pharmacological assays, fluorescence microscopy and cellular accumulation experiments. Proteomic data are available via ProteomeXchange with identifier PXD020560.

Comparing the Antileishmanial Activity of Gold(I) and Gold(III) Compounds in L. amazonensis and L. braziliensis in Vitro.

A series of mononuclear coordination or organometallic AuI /AuIII complexes (1-9) have been comparatively studied in vitro for their antileishmanial activity against promastigotes and amastigotes, the clinically relevant parasite form, of Leishmania amazonensis and Leishmania braziliensis. One of the cationic AuI bis-N-heterocyclic carbenes (3) has low EC50 values (ca. 4 µM) in promastigotes cells and no toxicity in host macrophages. Together with two other AuIII complexes (6 and 7), the compound is also extremely effective in intracellular amastigotes from L. amazonensis. Initial mechanistic studies include an evaluation of the gold complexes' effect on L. amazonensis' plasma membrane integrity.

Exploring the Chemoselectivity towards Cysteine Arylation by Cyclometallated AuIII Compounds: New Mechanistic Insights.

To gain more insight into the factors controlling efficient cysteine arylation by cyclometallated AuIII complexes, the reaction between selected gold compounds and different peptides was investigated by high-resolution liquid chromatography electrospray ionization mass spectrometry (HR-LC-ESI-MS). The deduced mechanisms of C-S cross-coupling, also supported by density functional theory (DFT) and quantum mechanics/molecular mechanics (QM/MM) calculations, evidenced the key role of secondary peptidic gold binding sites in favouring the process of reductive elimination.

Fluorescent metal-based complexes as cancer probes.

The ability to track drugs inside of cells and tumours has been highly valuable in cancer research and diagnosis. Metal complexes add attractive features to fluorescent drugs, such as targeting and specificity, solubility and uptake or photophysical properties. This review focuses on the latest fluorescent metal-based complexes, their cellular targets, photophysical properties and possible anticancer effects.

DNA-binding of zinc(II) and nickel(II) salphen-like complexes extrapolated at 1 M salt concentration: Removing the ionic strength bias in physiological conditions.

The DNA-binding of two salphen-like metal complexes of nickel(II) (1) and zinc(II) (2) was investigated in different ionic strength solutions by absorption spectroscopy. The data analysis allowed us to obtain the values of their extrapolated DNA-binding constant in physiological conditions, with DNA-binding strength in the order Ni > Zn, and to give relative weight to the electrostatic and non-electrostatic contributions to their DNA-interaction.

Carbon-Phosphorus Coupling from C

With the aim of exploiting new organometallic species for cross-coupling reactions, we report here on the AuIII -mediated Caryl -P bond formation occurring upon reaction of C^N cyclometalated AuIII complexes with phosphines. The [Au(C^N)Cl2 ] complex 1 featuring the bidentate 2-benzoylpyridine (CCO N) scaffold was found to react with PTA (1,3,5-triaza-7-phosphaadamantane) under mild conditions, including in water, to afford the corresponding phosphonium 5 through C-P reductive elimination. A mechanism is proposed for the title reaction based on in situ 31 P{1 H} NMR and HR-ESI-MS analyses combined with DFT calculations. The C-P coupling has been generalized to other C^N cyclometalated AuIII complexes and other tertiary phosphines. Overall, this work provides new insights into the reactivity of cyclometalated AuIII compounds and establishes initial structure-activity relationships to develop AuIII -mediated C-P cross-coupling reactions.

Comparative biological evaluation and G-quadruplex interaction studies of two new families of organometallic gold(I) complexes featuring N-heterocyclic carbene and alkynyl ligands.

Experimental organometallic gold(I) compounds hold promise for anticancer therapy. This study reports the synthesis of two novel families of gold(I) complexes, including N1-substituted bis-N-heterocyclic carbene (NHC) complexes of general formula [Au(N1-TBM)2]BF4 (N1-TBM = N1-substituted 9-methyltheobromin-8-ylidene) and mixed gold(I) NHC-alkynyl complexes, [Au(N1-TBM)alkynyl]. The compounds were fully characterised for their structure and stability in aqueous environment and in the presence of N-acetyl cysteine by nuclear magnetic resonance (NMR) spectroscopy. The structures of bis(1-ethyl-3,7,9-trimethylxanthin-8-ylidene)gold(I), (4-ethynylpyridine)(1,9-dimethyltheobromine-8-ylidene)gold(I) and of (2,8-Diethyl-10-(4-ethynylphenyl)-5,5-difluoro-1,3,7,9-tetramethyl-5H-4λ4,5λ4-dipyrrolo[1,2-c:2',1'-f][1,3,2]diazaborinine)(1,3,7,9-tetramethylxanthin-8-ylidene)gold(I) were also confirmed by X-ray diffraction analysis. The compounds were studied for their properties as DNA G-quadruplex (G4 s) stabilizers by fluorescence resonance energy transfer (FRET) DNA melting. Only the cationic [Au(N1-TBM)2]BF4 family showed moderate G4 stabilization properties with respect to the previously reported benchmark compound [Au(9-methylcaffein-8-ylidene)2]+ (AuTMX2). However, the compounds also showed marked selectivity for binding to G4 structures with respect to duplex DNA in competition experiments. For selected complexes, the interactions with G4 s were also confirmed by circular dichroism (CD) studies. Furthermore, the gold(I) complexes were assessed for their antiproliferative effects in human cancer cells in vitro, displaying moderate activity. Of note, among the mixed gold(I) NHC-alkynyl compounds, one features a fluorescent boron-dipyrromethene (BODIPY) moiety which allowed determining its uptake into the cytoplasm of cancer cells by fluorescence microscopy.

Cyclometalated AuIII Complexes for Cysteine Arylation in Zinc Finger Protein Domains: towards Controlled Reductive Elimination.

With the aim of exploiting the use of organometallic species for the efficient modification of proteins through C-atom transfer, the gold-mediated cysteine arylation through a reductive elimination process occurring from the reaction of cyclometalated AuIII C^N complexes with a zinc finger peptide (Cys2 His2 type) is here reported. Among the four selected AuIII cyclometalated compounds, the [Au(CCO N)Cl2 ] complex featuring the 2-benzoylpyridine (CCO N) scaffold was identified as the most prone to reductive elimination and Cys arylation in buffered aqueous solution (pH 7.4) at 37 °C by high-resolution LC electrospray ionization mass spectrometry. DFT and quantum mechanics/molecular mechanics (QM/MM) studies permitted to propose a mechanism for the title reaction that is in line with the experimental results. Overall, the results provide new insights into the reactivity of cytotoxic organogold compounds with biologically important zinc finger domains and identify initial structure-activity relationships to enable AuIII -catalyzed reductive elimination in aqueous media.

On the Mechanism of Gold/NHC Compounds Binding to DNA G-Quadruplexes: Combined Metadynamics and Biophysical Methods.

The binding modes and free-energy landscape of two AuI /N-heterocyclic carbene complexes interacting with G-quadruplexes, namely a human telomeric (hTelo) and a promoter sequence (C-KIT1), are studied here for the first time by metadynamics. The theoretical results are validated by FRET DNA melting assays and provide an accurate estimate of the absolute gold complex/DNA binding free energy. This advanced in silico approach is valuable to achieve rational drug design of selective G4 binders.