An Octahedral Cobalt(III) Complex with Axial NH3 Ligands that Templates and Selectively Stabilises G-quadruplex DNA

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.

G-quadruplex DNA binding properties of novel nickel Schiff base complexes with four pendant groups

Three new nickel Schiff base complexes were prepared using a two-step procedure that involves initial selective dialkylation of 2,4,6-trihydroxybenzaldehyde, followed by reaction with 1,2-phenylenediamine and nickel(II) acetate. Each of the complexes possessed the same Schiff base core but differed in the identity of the four pendant groups. All complexes were characterised by microanalysis, NMR spectroscopy and ESI mass spectrometry. In addition, two of the complexes were also characterised in the solid state using X-ray crystallography, which confirmed the presence of a square planar geometry around the metal ion. The DNA binding properties of the three nickel complexes with double stranded DNA and a range of G-quadruplex DNA structures were explored using ESI mass spectrometry, CD spectroscopy, UV melting curves, Fluorescence Resonance Energy Transfer (FRET) assays, Fluorescent Indicator Displacement (FID) assays and molecular docking studies. These techniques confirmed the ability of the three nickel complexes to bind to most of the DNA molecules examined, as well as stabilise the latter in several instances. In addition, the results of these investigations provided evidence that pendant groups with morpholine rings generally reduced DNA binding behaviour, whilst pendants featuring piperidine ring systems attached to the Schiff base core by three and not two methylene linkers often showed the greatest extent of binding or DNA stabilisation.

Salphen metal complexes as potential anticancer agents: interaction profile and selectivity studies toward the three G-quadruplex units in the KIT promoter

DNA G-rich sequences can organize in four-stranded structures called G-quadruplexes (G4s). These motifs are enriched in significant sites within the human genomes, including telomeres and promoters of cancer related genes. For instance, KIT proto-oncogene promoter, associated with diverse cancers, contains three adjacent G4 units, namely Kit2, SP, and Kit1. Aiming at finding new and selective G-quadruplex binders, we have synthesized and characterized five non-charged metal complexes of Pt(II), Pd(II), Ni(II), Cu(II) and Zn(II) of a chlorine substituted Salphen ligand. The crystal structure of the Pt(II) and Pd(II) complexes was determined by XRPD. FRET measurements indicated that Pt(II) and Pd(II) compounds stabilize Kit1 and Kit2 G4s but not SP, telomeric and double stranded DNA. Spectroscopic investigations (UV-Vis, circular dichroism and fluorescence) suggested the Cu(II) complex as the most G4-selective compound. Interestingly, docking simulations indicate that the synthesized compounds fit groove binding pockets of both Kit1 and Kit2 G4s. Moreover, they exhibited dose-dependent cytotoxic activity in MCF-7, HepG2 and HeLa cancer cells.

Radiolabelling Pt-based quadruplex DNA binders via click chemistry

Guanine-rich sequences of DNA and RNA can fold into intramolecular tetra-helical assemblies known as G-quadruplexes (G4). Their formation in vivo has been associated to a range of biological functions and therefore they have been identified as potential drug targets. Consequently, a broad range of small molecules have been developed to target G4s. Amongst those are metal complexes with Schiff base ligands. Herein, we report the functionalisation of one of these well-established G4 DNA binders (based on a square planar platinum(II)-salphen complex) with two different radiolabelled complexes. An 111In-conjugate was successfully used to assess its in vivo distribution in a mouse tumour model using single-photon emission computed tomography (SPECT) imaging. These studies highlighted the accumulation of this Pt-salphen-111In conjugate in the tumour.

"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.

Unravelling the binding affinity and selectivity of molybdenum(II) phenanthroline complexes with DNA G-quadruplexes by using linear-scaling DFT studies. The important role of ancillary ligands

We have used near linear-scaling density functional theory (LS-DFT) methods including dispersion, for the first time, to study the interaction of two isomers, equatorial (Eq) and axial (Ax), of the [Mo(η³-C₃H₅)Br(CO)₂(phen)] metal complex with the DNA G-quadruplexes (GQ) to gain insight into its cytotoxicity. The LMKLL/DZDP level of calculation, which includes van der Waals contributions, with the SIESTA software was used to treat by means of first-principles computations the whole biological studied model system with ∼1000 atoms. Computed formation energies point to systems containing the Ax isomer as the most stable although the nearest system in energy containing the Eq isomer is only 7.5 kcal mol^-1 above. On the other hand, the energy decomposition analysis (EDA) favours interaction energies for the systems containing the Eq isomer. However, when solvent effects are taken into account the systems containing the Ax isomer are again the most stable. This Ax isomer was found interacting by means of end-stacking with the GQ and surprisingly totally inside the non-canonical secondary structure, where all the ligands of the metal complex produce several weak interactions with the DNA structure. On the other hand, the Eq isomer prefers to interact from outside by means of intercalation in which the ancillary ligands also have some role in the interaction. Such features and comparison with the results regarding the interaction of the [Mo(η³-C₃H₅)Br(CO)₂(phen)] metal complex with duplex DNA suggest that the [Mo(η³-C₃H₅)Br(CO)₂(phen)] would have a higher affinity and eventual selectivity for non-canonical DNA GQ structures.

The strong in vitro and vivo cytotoxicity of three new cobalt(II) complexes with 8-methoxyquinoline

Three new cobalt(II) complexes, [Co(MQL)₂Cl₂] (CoCl), [Co(MQL)₂Br₂] (CoBr), and [Co(MQL)₂I₂] (CoI), bearing 8-methoxyquinoline (MQL) have been designed for the first time. MTT assays showed that CoCl, CoBr, and CoI exhibit much better antiproliferative activities than cisplatin toward cisplatin-resistant SK-OV-3/DDP and SK-OV-3 ovarian cancer cells, with IC₅₀ values of as low as 0.32-5.49 μM. Further, CoCl and CoI can regulate autophagy-related proteins in SK-OV-3/DDP cells and, therefore, they can induce primarily autophagy-mediated cell apoptosis in the following order: CoCl > CoI. The different antiproliferative activities of the MQL complexes CoCl, CoBr, and CoI could be correlated with the lengths of their Co-X bonds, which adopted the following order: CoI > CoBr > CoCl. The 8-HOMQ complexes CoCl (ca. 60.1%) and CoI (ca. 48.8%) also showed potent in vivo anticancer effects after 15 days of treatment. In summary, the MQL ligand highly enhances the antiproliferative activities of cobalt(II) complexes in comparison to other previously reported 8-hydroxyquinoline metal complexes.

Ruthenium(II) Polypyridyl Complexes and Their Use as Probes and Photoreactive Agents for G-quadruplexes Labelling

Due to their optical and electrochemical properties, ruthenium(II) polypyridyl complexes have been used in a wide array of applications. Since the discovery of the light-switch ON effect of [Ru(bpy)2dppz]2+ when interacting with DNA, the design of new Ru(II) complexes as light-up probes for specific regions of DNA has been intensively explored. Amongst them, G-quadruplexes (G4s) are of particular interest. These structures formed by guanine-rich parts of DNA and RNA may be associated with a wide range of biological events. However, locating them and understanding their implications in biological pathways has proven challenging. Elegant approaches to tackle this challenge relies on the use of photoprobes capable of marking, reversibly or irreversibly, these G4s. Indeed, Ru(II) complexes containing ancillary π-deficient TAP ligands can create a covalently linked adduct with G4s after a photoinduced electron transfer from a guanine residue to the excited complex. Through careful design of the ligands, high selectivity of interaction with G4 structures can be achieved. This allows the creation of specific Ru(II) light-up probes and photoreactive agents for G4 labelling, which is at the core of this review composed of an introduction dedicated to a brief description of G-quadruplex structures and two main sections. The first one will provide a general picture of ligands and metal complexes interacting with G4s. The second one will focus on an exhaustive and comprehensive overview of the interactions and (photo)reactions of Ru(II) complexes with G4s.

Modulation of amyloid-β aggregation by metal complexes with a dual binding mode and their delivery across the blood-brain barrier using focused ultrasound

One of the key hallmarks of Alzheimer's disease is the aggregation of the amyloid-β peptide to form fibrils. Consequently, there has been great interest in studying molecules that can disrupt amyloid-β aggregation. While a handful of molecules have been shown to inhibit amyloid-β aggregation in vitro, there remains a lack of in vivo data reported due to their inability to cross the blood-brain barrier. Here, we investigate a series of new metal complexes for their ability to inhibit amyloid-β aggregation in vitro. We demonstrate that octahedral cobalt complexes with polyaromatic ligands have high inhibitory activity thanks to their dual binding mode involving π-π stacking and metal coordination to amyloid-β (confirmed via a range of spectroscopic and biophysical techniques). In addition to their high activity, these complexes are not cytotoxic to human neuroblastoma cells. Finally, we report for the first time that these metal complexes can be safely delivered across the blood-brain barrier to specific locations in the brains of mice using focused ultrasound.

FeII Metallohelices Stabilize DNA G-Quadruplexes and Downregulate the Expression of G-Quadruplex-Regulated Oncogenes

DNA G-quadruplexes (G4s) have been identified within the promoter regions of many proto-oncogenes. Thus, G4s represent attractive targets for cancer therapy, and the design and development of new drugs as G4 binders is a very active field of medicinal chemistry. Here, molecular biophysics and biology methods were employed to investigate the interaction of chiral metallohelices with a series of four DNA G4s (hTelo, c-myc, c-kit1, c-kit2) that are formed by the human telomeric sequence (hTelo) and in the promoter regions of c-MYC and c-KIT proto-oncogenes. We show that the investigated water-compatible, optically pure metallohelices, which are made by self-assembly of simple nonpeptidic organic components around Fe^II ions and exhibit bioactivity emulating the natural systems, bind with high affinity to G4 DNA and much lower affinity to duplex DNA. Notably, both enantiomers of a metallohelix containing a m-xylenyl bridge (5 b) were found to effectively inhibit primer elongation catalyzed by Taq DNA polymerase by stabilizing G4 structures formed in the template strands containing c-myc and c-kit2 G4-forming sequences. Moreover, both enantiomers of 5 b downregulated the expression of c-MYC and c-KIT oncogenes in human embryonic kidney cells at mRNA and protein levels. As metallohelices also bind alternative nucleic acid structures, they hold promise as potential multitargeted drugs.

G-quadruplex targeting chemical nucleases as a nonperturbative tool for analysis of cellular G-quadruplex DNA

G-quadruplex structures are associated with various biological activities, while in vivo evidence is essential to confirm the formation of G-quadruplexes inside cells. Most conventional agents that recognize G-quadruplex, including antibodies and small-molecule G-quadruplex ligands, either stabilize the G-quadruplex or prevent G-quadruplex unfolding by helicase, thereby artificially increasing the G-quadruplex levels in cells. Unambiguous study of G-quadruplexes at natural cellular levels requires agents that do not enhance the stability of G-quadruplex. Herein, we report the first example of nonperturbative chemical nucleases that do not influence the stability of G-quadruplex telomeric DNA but can selectively cleave G-quadruplex DNA over duplex DNA. These chemical nucleases can be readily taken up by cells and promote selective cleavage of telomeric DNA with low levels of nonselective DNA cleavage of other regions of the genome. The cleavage of G-quadruplex telomeric DNA by nonperturbative chemical nucleases confirms the formation of G-quadruplex telomeric DNA in live cells.

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Novel chemical nucleases exhibit no effect on G-quadruplex telomeric DNA stability

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Selective nucleases cleave G-quadruplex DNA over duplex DNA

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Cleavage of G-quadruplex telomeric DNA motifs confirms their existence in cells

Targeting Guanine Quadruplexes with Luminescent Platinum(II) Complexes Bearing a Pendant Nucleobase

Guanine quadruplexes are tetra-stranded nucleic acid structures currently raising significant interest in the context of the development of potential anticancer therapeutics with a new mode of action. They are composed of planar guanine tetrads, allowing a high-affinity targeting by using molecules with a large π surface. However, the extreme topological versatility of guanine quadruplexes impedes a straightforward targeting of particular preselected guanine-rich sequences. We report here a systematic study of a family of luminescent platinum(II) complexes devised to overcome this challenge. By attaching a pendant adenine or thymine nucleobase as a substituent to one of the ligands at the platinum center, an additional recognition site is introduced with the aim of modulating the affinity of the metal complex to different DNA sequences. By comparing different attached nucleobases and a series of linker moieties, first conclusions are drawn with respect to the scope of this approach.

The effect of isomerism and other structural variations on the G-quadruplex DNA-binding properties of some nickel Schiff base complexes

A series of novel isomeric nickel Schiff base complexes, as well as nickel complexes of related ligands having asymmetric structures have been prepared and characterised using microanalysis, 1H and 13C NMR spectroscopy and ESI-MS. The Schiff base ligands were prepared by condensation reactions involving ethylenediamine and different derivatives of benzophenone. The solid-state structures of eight of the complexes were also determined and revealed that each possessed a regular square planar coordination geometry around the metal ion. Many of the new complexes featured at least one, and in many instances two, protonatable pendant groups that enhance aqueous solubility. This enabled the DNA binding properties of the latter complexes to be explored using a variety of instrumental approaches, including ESI-MS, circular dichroism (CD) spectroscopy, FRET melting assays and FID assays, as well as molecular docking studies. The results of experiments performed using ESI-MS suggested that none of the nickel complexes exhibit a high affinity towards either a double stranded DNA (dsDNA) molecule D2, or the parallel unimolecular quadruplex DNA (qDNA) molecule Q1. In contrast, complexes (8) and (12) both gave spectra which reflected a significant level of binding to the parallel tetramolecular qDNA Q4. The results of binding experiments performed using CD spectroscopy suggested that (12) exhibits a significant level of affinity towards most types of DNA, while (4) shows a preference for interacting with parallel, unimolecular qDNA molecules. Complex (4) produced the lowest values of DC50 in FID assays performed using parallel Q1 or Q4, confirming its affinity for these qDNA molecules. The results of FRET melting experiments provided further evidence that (12), along with (8), can interact extensively with anti-parallel unimolecular qDNA. Experiments which monitored the effect of the nickel complexes on the melting temperature of D2 showed that none had a stabilising effect on this dsDNA molecule.