Ru-terpyridine complexes containing clotrimazole as potent photoactivatable selective antifungal agents

The overuse of antimicrobial agents in medical and veterinary applications has led to the development of antimicrobial resistance in some microorganisms and this is now one of the major concerns in modern society. In this context, the use of transition metal complexes with photoactivatable properties, which can act as drug delivery systems triggered by light, could become a potent strategy to overcome the problem of resistance. In this work several Ru complexes with terpyridine ligands and the clotrimazole fragment, which is a potent antimycotic drug, were synthesized. The main goal was to explore the potential photoactivated activity of the complexes as antifungal agents and evaluate the effect of introducing different substituents on the terpyridine ligand. The complexes were capable of delivering the clotrimazole unit upon irradiation with visible light in a short period of time. The influence of the substituents on the photodissociation rate was explained by means of TD-DFT calculations. The complexes were tested against three different yeasts, which were selected based on their prevalence in fungal infections. The complex in which a carboxybenzene unit was attached to the terpyridine ligand showed the best activity against the three species under light, with minimal inhibitory concentration values of 0.88 μM and a phototoxicity index of 50 achieved. The activity of this complex was markedly higher than that of free clotrimazole, especially upon irradiation with visible light (141 times higher). The complexes were more active on yeast species than on cancer cell lines.

TAR RNA selective targeting ruthenium(II) complexes as HIV-1 reverse transcriptase inhibitors: On exploring structure-activity relationships of multiple positions

HIV-1 reverse transcriptase (RT) inhibitors play a crucial role in the treatment of HIV by preventing the activity of the enzyme responsible for the replication of the virus. The HIV-1 Tat protein binds to transactivation response (TAR) RNA and recruits host factors to stimulate HIV-1 transcription. We have created a small library consisting of 4 × 6 polypyridyl Ru(II) complexes that selectively bind to TAR RNA, with targeting groups specific to HIV-1 TAR RNA. The molecule design was conducted by introducing hydroxyl or methoxy groups into an established potent TAR binder. The potential TAR binding ability was analysis from nature charge population and electrostatic potential by quantum chemistry calculations. Key modifications were found to be R1 and R3 groups. The most potent and selective TAR RNA binder was a3 with R1 = OH, R2 = H and R3 = Me. Through molecular recognition of hydrogen bonds and electrostatic attraction, they were able to firmly and selectively bind HIV-1 TAR RNA. Furthermore, they efficiently obstructed the contact between TAR RNA and Tat protein, and inhibited the reverse transcription activity of HIV-1 RT. The polypyridyl Ru(II) complexes were chemical and photo-stable, and sensitive and selective spectroscopic responses to TAR RNA. They exhibited little toxicity towards normal cells. Hence, this study might offer significant drug design approaches for researching AIDS and other illnesses associated with RT, including HCV, EBOV, and SARS-CoV-2. Moreover, it could contribute to fundamental research on the interactions of inorganic transition metal complexes with biomolecules.

A monoadduct generating Ru(ii) complex induces ribosome biogenesis stress and is a molecular mimic of phenanthriplatin

Ruthenium complexes are often investigated as potential replacements for platinum-based chemotherapeutics in hopes of identifying systems with improved tolerability in vivo and reduced susceptibility to cellular resistance mechanisms. Inspired by phenanthriplatin, a non-traditional platinum agent that contains only one labile ligand, monofunctional ruthenium polypyridyl agents have been developed, but until now, few demonstrated promising anticancer activity. Here we introduce a potent new scaffold, based on [Ru(tpy)(dip)Cl]Cl (tpy = 2,2':6',2''-terpyridine and dip = 4,7-diphenyl-1,10-phenanthroline) in pursuit of effective Ru(ii)-based monofunctional agents. Notably, the extension of the terpyridine at the 4' position with an aromatic ring resulted in a molecule that was cytotoxic in several cancer cell lines with sub-micromolar IC50 values, induced ribosome biogenesis stress, and exhibited minimal zebrafish embryo toxicity. This study demonstrates the successful design of a Ru(ii) agent that mimics many of the biological effects and phenotypes seen with phenanthriplatin, despite numerous differences in both the ligands and metal center structure.

Metals to combat antimicrobial resistance

Bacteria, similar to most organisms, have a love-hate relationship with metals: a specific metal may be essential for survival yet toxic in certain forms and concentrations. Metal ions have a long history of antimicrobial activity and have received increasing attention in recent years owing to the rise of antimicrobial resistance. The search for antibacterial agents now encompasses metal ions, nanoparticles and metal complexes with antimicrobial activity ('metalloantibiotics'). Although yet to be advanced to the clinic, metalloantibiotics are a vast and underexplored group of compounds that could lead to a much-needed new class of antibiotics. This Review summarizes recent developments in this growing field, focusing on advances in the development of metalloantibiotics, in particular, those for which the mechanism of action has been investigated. We also provide an overview of alternative uses of metal complexes to combat bacterial infections, including antimicrobial photodynamic therapy and radionuclide diagnosis of bacterial infections.

Neutral Re(I) Complex Platform for Live Intracellular Imaging

Luminescent metal complexes are a valuable platform for the generation of cell imaging agents. However, many metal complexes are cationic, a factor that can dominate the intracellular accumulation to specific organelles. Neutral Re(I) complexes offer a more attractive platform for the development of bioconjugated imaging agents, where charge cannot influence their intracellular distribution. Herein, we report the synthesis of a neutral complex (ReAlkyne), which was used as a platform for the generation of four carbohydrate-conjugated imaging agents via Cu(I)-catalyzed azide-alkyne cycloaddition. A comprehensive evaluation of the physical and optical properties of each complex is provided, together with a determination of their utility as live cell imaging agents in H9c2 cardiomyoblasts. Unlike their cationic counterparts, many of which localize within mitochondria, these neutral complexes have localized within the endosomal/lysosomal network, a result consistent with examples of dinuclear carbohydrate-appended neutral Re(I) complexes that have been reported. This further demonstrates the utility of these neutral Re(I) complex imaging platforms as viable imaging platforms for the development of bioconjugated cell imaging agents.

Ruthenium Complexes in the Fight against Pathogenic Microorganisms. An Extensive Review

The widespread use of antibiotics has resulted in the emergence of drug-resistant populations of microorganisms. Clearly, one can see the need to develop new, more effective, antimicrobial agents that go beyond the explored 'chemical space'. In this regard, their unique modes of action (e.g., reactive oxygen species (ROS) generation, redox activation, ligand exchange, depletion of substrates involved in vital cellular processes) render metal complexes as promising drug candidates. Several Ru (II/III) complexes have been included in, or are currently undergoing, clinical trials as anticancer agents. Based on the in-depth knowledge of their chemical properties and biological behavior, the interest in developing new ruthenium compounds as antibiotic, antifungal, antiparasitic, or antiviral drugs has risen. This review will discuss the advantages and disadvantages of Ru (II/III) frameworks as antimicrobial agents. Some aspects regarding the relationship between their chemical structure and mechanism of action, cellular localization, and/or metabolism of the ruthenium complexes in bacterial and eukaryotic cells are discussed as well. Regarding the antiviral activity, in light of current events related to the Covid-19 pandemic, the Ru (II/III) compounds used against SARS-CoV-2 (e.g., BOLD-100) are also reviewed herein.

Surface-Enhanced Raman Scattering Detection of Nucleic Acids Exhibiting Sterically Accessible Guanines Using Ruthenium-Polypyridyl Reagents

Here, we report the application of surface-enhanced Raman scattering (SERS) spectroscopy as a rapid and practical tool for assessing the formation of coordinative adducts between nucleic acid guanines and ruthenium polypyridyl reagents. The technology provides a practical approach for the wash-free and quick identification of nucleic acid structures exhibiting sterically accessible guanines. This is demonstrated for the detection of a quadruplex-forming sequence present in the promoter region of the c-myc oncogene, which exhibits a nonpaired, reactive guanine at a flanking position of the G-quartets.

A Novel Ruthenium Based Coordination Compound Against Pathogenic Bacteria

The current epidemic of antibiotic-resistant infections urges to develop alternatives to less-effective antibiotics. To assess anti-bacterial potential, a novel coordinate compound (RU-S4) was synthesized using ruthenium-Schiff base-benzimidazole ligand, where ruthenium chloride was used as the central atom. RU-S4 was characterized by scanning electron microscope (SEM), energy-dispersive X-ray spectroscopy (EDS), and Raman spectroscopy. Antibacterial effect of RU-S4 was studied against Staphylococcus aureus (NCTC 8511), vancomycin-resistant Staphylococcus aureus (VRSA) (CCM 1767), methicillin-resistant Staphylococcus aureus (MRSA) (ST239: SCCmecIIIA), and hospital isolate Staphylococcus epidermidis. The antibacterial activity of RU-S4 was checked by growth curve analysis and the outcome was supported by optical microscopy imaging and fluorescence LIVE/DEAD cell imaging. In vivo (balb/c mice) infection model prepared with VRSA (CCM 1767) and treated with RU-S4. In our experimental conditions, all infected mice were cured. The interaction of coordination compound with bacterial cells were further confirmed by cryo-scanning electron microscope (Cryo-SEM). RU-S4 was completely non-toxic against mammalian cells and in mice and subsequently treated with synthesized RU-S4.

Mitochondrial targeted osmium polypyridyl probe shows concentration dependent uptake, localisation and mechanism of cell death

A symmetric osmium(ii) [bis-(4'-(4-carboxyphenyl)-2,2':6',2''-terpyridine)] was prepared and conjugated to two mitochondrial-targeting peptide sequences; FrFKFrFK (r = d-arginine). The parent and conjugate complexes showed strong near infra-red emission centred at λmax 745 nm that was modestly oxygen dependent in the case of the parent and oxygen independent in the case of the conjugate, attributed in the latter case, surprisingly, to a shorter emission lifetime of the conjugate compared to the parent. Confocal fluorescence imaging of sub-live HeLa and MCF 7 cells showed the parent complex was cell impermeable whereas the conjugate was rapidly internalised into the cell and distributed in a concentration dependent manner. At concentrations below approximately 30 μmol, the conjugate localised to the mitochondria of both cell types where it was observed to trigger apoptosis induced by the collapse of the mitochondrial membrane potential (MPP). At concentrations exceeding 30 μmol the conjugate was similarly internalised rapidly but distributed throughout the cell, including to the nucleus and nucleolus. At these concentrations, it was observed to precipitate a caspase-dependent apoptotic pathway. The combination of concentration dependent organelle targeting, NIR emission coincident with the biological window, and distribution dependent cytotoxicity offers an interesting approach to theranostics with the possibility of eliciting site dependent therapeutic effect whilst monitoring the therapeutic effect with luminescence imaging.

Metal complexes as a promising source for new antibiotics

There is a dire need for new antimicrobial compounds to combat the growing threat of widespread antibiotic resistance. With a currently very scarce drug pipeline, consisting mostly of derivatives of known antibiotics, new classes of antibiotics are urgently required. Metal complexes are currently in clinical development for the treatment of cancer, malaria and neurodegenerative diseases. However, only little attention has been paid to their application as potential antimicrobial compounds. We report the evaluation of 906 metal-containing compounds that have been screened by the Community for Open Antimicrobial Drug Discovery (CO-ADD) for antimicrobial activity. Metal-bearing compounds display a significantly higher hit-rate (9.9%) when compared to the purely organic molecules (0.87%) in the CO-ADD database. Out of 906 compounds, 88 show activity against at least one of the tested strains, including fungi, while not displaying any cytotoxicity against mammalian cell lines or haemolytic properties. Herein, we highlight the structures of the 30 compounds with activity against Gram-positive and/or Gram-negative bacteria containing Mn, Co, Zn, Ru, Ag, Eu, Ir and Pt, with activities down to the nanomolar range against methicillin resistant S. aureus (MRSA). 23 of these complexes have not been reported for their antimicrobial properties before. This work reveals the vast diversity that metal-containing compounds can bring to antimicrobial research. It is important to raise awareness of these types of compounds for the design of truly novel antibiotics with potential for combatting antimicrobial resistance.

Metal Complexes, an Untapped Source of Antibiotic Potential?

With the widespread rise of antimicrobial resistance, most traditional sources for new drug compounds have been explored intensively for new classes of antibiotics. Meanwhile, metal complexes have long had only a niche presence in the medicinal chemistry landscape, despite some compounds, such as the anticancer drug cisplatin, having had a profound impact and still being used extensively in cancer treatments today. Indeed, metal complexes have been largely ignored for antibiotic development. This is surprising as metal compounds have access to unique modes of action and exist in a wider range of three-dimensional geometries than purely organic compounds. These properties make them interesting starting points for the development of new drugs. In this perspective article, , the encouraging work that has been done on antimicrobial metal complexes, mainly over the last decade, is highlighted. Promising metal complexes, their activity profiles, and possible modes of action are discussed and issues that remain to be addressed are emphasized.

Making the Right Link to Theranostics: The Photophysical and Biological Properties of Dinuclear RuII-ReI dppz Complexes Depend on Their Tether

The synthesis of new dinuclear complexes containing linked Ru^II(dppz) and Re^I(dppz) moieties is reported. The photophysical and biological properties of the new complex, which incorporates a N,N'-bis(4-pyridylmethyl)-1,6-hexanediamine tether ligand, are compared to a previously reported Ru^II/Re^I complex linked by a simple dipyridyl alkane ligand. Although both complexes bind to DNA with similar affinities, steady-state and time-resolved photophysical studies reveal that the nature of the linker affects the excited state dynamics of the complexes and their DNA photocleavage properties. Quantum-based DFT calculations on these systems offer insights into these effects. While both complexes are live cells permeant, their intracellular localizations are significantly affected by the nature of the linker. Notably, one of the complexes displayed concentration-dependent localization and possesses photophysical properties that are compatible with SIM and STED nanoscopy. This allowed the dynamics of its intracellular localization to be tracked at super resolutions.

Rationally designed curcumin based ruthenium(ii) antimicrobials effective against drug-resistant Staphylococcus aureus

Two new curcumin containing octahedral ruthenium(ii) polypyridyl complexes, viz. [Ru(NN)2(cur)](PF6) [NN = bpy (1), phen (2)], were designed to explore the antimicrobial activity against ESKAPE pathogens, especially with the Gram-positive drug resistant S. aureus. Solid-state structural characterization by single-crystal X-ray crystallography shows the RuII-center in a distorted octahedral {RuN4O2} geometry. The tested compounds showed significant inhibitory activity and high selectivity (MIC = 1 μg mL-1, SI = 80) against a wide variety of methicillin and vancomycin-resistant S. aureus strains. Compound 1 exhibited strong anti-biofilm activity (48% reduction of biofilm) at 10× MIC compared to the other approved drugs. The murine model of Staphylococcus infection significantly reduced the mean bacterial counts when treated with complex 1 compared to vancomycin, demonstrating its antimicrobial potential in vivo.

Ganoderma lucidum Polysaccharides Prevent Palmitic Acid-Evoked Apoptosis and Autophagy in Intestinal Porcine Epithelial Cell Line via Restoration of Mitochondrial Function and Regulation of MAPK and AMPK/Akt/mTOR Signaling Pathway

Ganoderma lucidum polysaccharide (GLP) extracted from Ganoderma lucidum (Leyss. ex Fr.) Karst, a traditional Chinese medicine, is a biologically active substance reported to possess anti-oxidative, anti-apoptotic, and neurological protection. However, it is unknown whether GLP have any protective effect against high-fat constituents-induced epithelial cell injury. The aim of this study was to investigate the protection and molecular mechanism of GLP on injury induced by palmitic acid (PA) in the intestinal porcine epithelial cell line (IPEC-J2). First, we tested whether the treatment of GLP attenuate PA-induced IPEC-J2 cell death. GLP markedly blocked PA-caused cytotoxicity and apoptosis in IPEC-J2 cells. Moreover, GLP recovered the decreased mitochondrial function and inhibited activation of caspase-dependent apoptotic pathway. Interestingly, PA promoted cell apoptosis and autophagy through stimulation of phosphorylation of mitogen-activated protein kinases (MAPKs), AMP-activated protein kinase (AMPK), and inhibition of phosphorylation of Akt and mammalian target of rapamycin (mTOR), which was reversed by GLP. Taken together, this study revealed a protective effect of GLP against PA-evoked IPEC-J2 cell death through anti-apoptotic and anti-autophagic properties.

Synthesis and biological properties of tetranuclear ruthenium complexes containing the bis[4(4′-methyl-2,2′-bipyridyl)]-1,7-heptane ligand

The non-linear polypyridylruthenium(ii) complex (Rubb7-TNL) exhibited good antimicrobial activity, but surprisingly was also highly active against cancer cells. The results suggestRubb7-TNLmay have potential as a new anticancer agent.

Eukaryotic Cell Toxicity and HSA Binding of [Ru(Me4phen)(bb7)]2+ and the Effect of Encapsulation in Cucurbit[10]uril

The toxicity (IC₅₀) of a series of mononuclear ruthenium complexes containing bis[4(4′-methyl-2,2′-bipyridyl)]-1,n-alkane (bb_n) as a tetradentate ligand against three eukaryotic cell lines—BHK (baby hamster kidney), Caco-2 (heterogeneous human epithelial colorectal adenocarcinoma) and Hep-G2 (liver carcinoma)—have been determined. The results demonstrate that cis-α-[Ru(Me₄phen)(bb₇)]²⁺ (designated as α-Me₄phen-bb₇, where Me₄phen = 3,4,7,8-tetramethyl-1,10-phenanthroline) showed little toxicity toward the three cell lines, and was considerably less toxic than cis-α-[Ru(phen)(bb₁₂)]²⁺ (α-phen-bb₁₂) and the dinuclear complex [{Ru(phen)₂}₂{μ-bb₁₂}]⁴⁺. Fluorescence spectroscopy was used to study the binding of the ruthenium complexes with human serum albumin (HSA). The binding of α-Me₄phen-bb₇ to the macrocyclic host molecule cucurbit[10]uril (Q[10]) was examined by NMR spectroscopy. Large upfield ¹H NMR chemical shift changes observed for the methylene protons in the bb₇ ligand upon addition of Q[10], coupled with the observation of several intermolecular ROEs in ROESY spectra, indicated that α-Me₄phen-bb₇ bound Q[10] with the bb₇ methylene carbons within the cavity and the metal center positioned outside one of the portals. Simple molecular modeling confirmed the feasibility of the binding model. An α-Me₄phen-bb₇-Q[10] binding constant of 9.9 ± 0.2 × 10⁶ M⁻¹ was determined by luminescence spectroscopy. Q[10]-encapsulation decreased the toxicity of α-Me₄phen-bb₇ against the three eukaryotic cell lines and increased the binding affinity of the ruthenium complex for HSA. Confocal microscopy experiments indicated that the level of accumulation of α-Me₄phen-7 in BHK cells is not significantly affected by Q[10]-encapsulation. Taken together, the combined results suggest that α-Me₄phen-7 could be a good candidate as a new antimicrobial agent, and Q[10]-encapsulation could be a method to improve the pharmacokinetics of the ruthenium complex.

The development of ruthenium(ii) polypyridyl complexes and conjugates for in vitro cellular and in vivo applications

Ruthenium(ii) [Ru(ii)] polypyridyl complexes have been the focus of intense investigations since work began exploring their supramolecular interactions with DNA. In recent years, there have been considerable efforts to translate this solution-based research into a biological environment with the intention of developing new classes of probes, luminescent imaging agents, therapeutics and theranostics. In only 10 years the field has expanded with diverse applications for these complexes as imaging agents and promising candidates for therapeutics. In light of these efforts this review exclusively focuses on the developments of these complexes in biological systems, both in cells and in vivo, and hopes to communicate to readers the diversity of applications within which these complexes have found use, as well as new insights gained along the way and challenges that researchers in this field still face.

Synthesis, isomerisation and biological properties of mononuclear ruthenium complexes containing the bis[4(4'-methyl-2,2'-bipyridyl)]-1,7-heptane ligand

A series of mononuclear ruthenium(ii) complexes containing the tetradentate ligand bis[4(4'-methyl-2,2'-bipyridyl)]-1,7-heptane have been synthesised and their biological properties examined. In the synthesis of the [Ru(phen')(bb₇)]²⁺ complexes (where phen' = 1,10-phenanthroline and its 5-nitro-, 4,7-dimethyl- and 3,4,7,8-tetramethyl- derivatives), both the symmetric cis-α and non-symmetric cis-β isomers were formed. However, upon standing for a number of days (or more quickly under harsh conditions) the cis-β isomer converted to the more thermodynamically stable cis-α isomer. The minimum inhibitory concentrations (MIC) and the minimum bactericidal concentrations (MBC) of the ruthenium(ii) complexes were determined against six strains of bacteria: Gram-positive Staphylococcus aureus (S. aureus) and methicillin-resistant S. aureus (MRSA); and the Gram-negative Escherichia coli (E. coli) strains MG1655, APEC, UPEC and Pseudomonas aeruginosa (P. aeruginosa). The results showed that the [Ru(5-NO₂phen)(bb₇)]²⁺ complex had little or no activity against any of the bacterial strains. By contrast, for the other cis-α-[Ru(phen')(bb₇)]²⁺ complexes, the antimicrobial activity increased with the degree of methylation. In particular, the cis-α-[Ru(Me₄phen)(bb₇)]²⁺ complex showed excellent and uniform MIC activity against all bacteria. By contrast, the MBC values for the cis-α-[Ru(Me₄phen)(bb₇)]²⁺ complex varied considerably across the bacteria and even within S. aureus and E. coli strains. In order to gain an understanding of the relative antimicrobial activities, the DNA-binding affinity, cellular accumulation and water-octanol partition coefficients (log P) of the ruthenium complexes were determined. Interestingly, all the [Ru(phen')(bb₇)]²⁺ complexes exhibited stronger DNA binding affinity (K_a ≈ 1 × 10⁷ M^-1) than the well-known DNA-intercalating complex [Ru(phen)₂(dppz)]²⁺ (where dppz = dipyrido[3,2-a:2',3'-c]phenazine).

Far-red fluorescent probes for canonical and non-canonical nucleic acid structures: current progress and future implications

Our review presents the recent progress on far-red fluorescent probes of canonical and non-canonical nucleic acid (NA) structures, critically discusses the design principles, applications, limitations and outline the future prospects of developing newer probes with target-specificity for different NA structures.

Fluorescence sensing platform based on ruthenium(II) complexes as high 3S (sensitivity, specificity, speed) and "on-off-on" sensors for the miR-185 detection

Inspired by the enormous importance attributed to the biological function of miRNA, we pour our attention into the design and synthesis of four ruthenium(II) complexes and evaluate their applications as miR-185 detection agents by spectroscopic measurements. It was found that all complexes can form sensing platform for the detection of the complementary target miR-185 through the introduction of carboxyfluorescein (FAM) labeled single stranded DNA (P-DNA), giving the detection limits of 0.42nM for Ru 1, 0.28nM for Ru 2, 0.32nM for Ru 3, 0.85nM for Ru 4, all with instantaneous detection time in 1min. The results of the binding constant, fluorescence anisotropy (FA) and polyacrylamide gel electrophoresis experiments (PAGE) revealed that the ruthenium(II) complexes prefer to bind P-DNA other than hybrid duplexes DNA@RNA upon recognition, resulting in the detection of miR-185. These results provide useful suggestions in the new type of metal-based miRNA detection agents.

The Microbiology of Ruthenium Complexes

Ruthenium is seldom mentioned in microbiology texts, due to the fact that this metal has no known, essential roles in biological systems, nor is it generally considered toxic. Since the fortuitous discovery of cisplatin, first as an antimicrobial agent and then later employed widely as an anticancer agent, complexes of other platinum group metals, such as ruthenium, have attracted interest for their medicinal properties. Here, we review at length how ruthenium complexes have been investigated as potential antimicrobial, antiparasitic and chemotherapeutic agents, in addition to their long and well-established roles as biological stains and inhibitors of calcium channels. Ruthenium complexes are also employed in a surprising number of biotechnological roles. It is in the employment of ruthenium complexes as antimicrobial agents and alternatives or adjuvants to more traditional antibiotics, that we expect to see the most striking developments in the future. Such novel contributions from organometallic chemistry are undoubtedly sorely needed to address the antimicrobial resistance crisis and the slow appearance on the market of new antibiotics.

Monomeric and dimeric coordinatively saturated and substitutionally inert Ru(ii) polypyridyl complexes as anticancer drug candidates

Monomeric and dimeric coordinatively saturated and substitutionally inert Ru(ii) polypyridyl complexes with anticancer properties are reviewed.

Biological processing of dinuclear ruthenium complexes in eukaryotic cells

The biological processing - mechanism of cellular uptake, effects on the cytoplasmic and mitochondrial membranes, intracellular sites of localisation and induction of reactive oxygen species - of two dinuclear polypyridylruthenium(ii) complexes has been examined in three eukaryotic cells lines. Flow cytometry was used to determine the uptake of [{Ru(phen)2}2{μ-bb12}](4+) (Rubb12) and [Ru(phen)2(μ-bb7)Ru(tpy)Cl](3+) {Rubb7-Cl, where phen = 1,10-phenanthroline, tpy = 2,2':6',2''-terpyridine and bbn = bis[4(4'-methyl-2,2'-bipyridyl)]-1,n-alkane} in baby hamster kidney (BHK), human embryonic kidney (HEK-293) and liver carcinoma (HepG2) cell lines. The results demonstrated that the major uptake mechanism for Rubb12 and Rubb7-Cl was active transport, although with a significant contribution from carrier-assisted diffusion for Rubb12 and passive diffusion for Rubb7-Cl. Flow cytometry coupled with Annexin V/TO-PRO-3 double-staining was used to compare cell death by membrane damage or apoptosis. Rubb12 induced significant direct membrane damage, particularly with HepG2 cells, while Rubb7-Cl caused considerably less membrane damage but induced greater levels of apoptosis. Confocal microscopy, coupled with JC-1 assays, demonstrated that Rubb12 depolarises the mitochondrial membrane, whereas Rubb7-Cl had a much smaller affect. Cellular localisation experiments indicated that Rubb12 did not accumulate in the mitochondria, whereas significant mitochondrial accumulation was observed for Rubb7-Cl. The effect of Rubb12 and Rubb7-Cl on intracellular superoxide dismutase activity showed that the ruthenium complexes could induce cell death via a reactive oxygen species-mediated pathway. The results of this study demonstrate that Rubb12 predominantly kills eukaryotic cells by damaging the cytoplasmic membrane. As this dinuclear ruthenium complex has been previously shown to exhibit greater toxicity towards bacteria than eukaryotic cells, the results of the present study suggest that metal-based cationic oligomers can achieve selective toxicity against bacteria, despite exhibiting a non-specific membrane damage mechanism of action.

The Acinetobacter baumannii Two-Component System AdeRS Regulates Genes Required for Multidrug Efflux, Biofilm Formation, and Virulence in a Strain-Specific Manner

The opportunistic pathogen Acinetobacter baumannii is able to persist in the environment and is often multidrug resistant (MDR), causing difficulties in the treatment of infections. Here, we show that the two-component system AdeRS, which regulates the production of the AdeABC multidrug resistance efflux pump, is required for the formation of a protective biofilm in an ex vivo porcine mucosal model, which mimics a natural infection of the human epithelium. Interestingly, deletion of adeB impacted only on the ability of strain AYE to form a biofilm on plastic and only on the virulence of strain Singapore 1 for Galleria mellonella. RNA-Seq revealed that loss of AdeRS or AdeB significantly altered the transcriptional landscape, resulting in the changed expression of many genes, notably those associated with antimicrobial resistance and virulence interactions. For example, A. baumannii lacking AdeRS displayed decreased expression of adeABC, pil genes, com genes, and a pgaC-like gene, whereas loss of AdeB resulted in increased expression of pil and com genes and decreased expression of ferric acinetobactin transport system genes. These data define the scope of AdeRS-mediated regulation, show that changes in the production of AdeABC mediate important phenotypes controlled by AdeRS, and suggest that AdeABC is a viable target for antimicrobial drug and antibiofilm discovery.

Acinetobacter baumannii is a nosocomial pathogen and is an increasing problem in hospitals worldwide. This organism is often multidrug resistant, can persist in the environment, and forms a biofilm on environmental surfaces and wounds. Overproduction of efflux pumps can allow specific toxic compounds to be pumped out of the cell and can lead to multidrug resistance. This study demonstrates the role of the A. baumannii efflux pump AdeB, and its regulator AdeRS, in multidrug resistance, epithelial cell killing, and biofilm formation. Deletion of the genes encoding these systems led to increased susceptibility to antibiotics, decreased biofilm formation on biotic and abiotic surfaces, and decreased virulence. Our data suggest that inhibition of AdeB could prevent biofilm formation or colonization in patients by A. baumannii and provides a good target for drug discovery.

Oligonuclear polypyridylruthenium(II) complexes: selectivity between bacteria and eukaryotic cells

OBJECTIVES

The objectives of this study were to: (i) determine the in vitro activities of a series of di-, tri- and tetra-nuclear ruthenium complexes (Rubbn, Rubbn-tri and Rubbn-tetra) against a range of Gram-positive and -negative bacteria and compare the antimicrobial activities with the corresponding toxicities against eukaryotic cells; and (ii) compare MIC values with achievable in vivo serum concentrations for the least toxic ruthenium complex.

METHODS

The in vitro activities were determined by MIC assays and time-kill curve experiments, while the toxicities of the ruthenium complexes were determined using the Alamar blue cytotoxicity assay. A preliminary pharmacokinetic study was undertaken to determine the Rubb12 serum concentration in mice as a function of time after administration.

RESULTS

Rubb12, Rubb12-tri and Rubb12-tetra are highly active, with MIC values of 1-2 mg/L (0.5-1.5 μM) for a range of Gram-positive strains, but showed variable activities against a panel of Gram-negative bacteria. Time-kill experiments indicated that Rubb12, Rubb12-tri and Rubb12-tetra are bactericidal and kill bacteria within 3-8 h. The di-, tri- and tetra-nuclear complexes were ∼50 times more toxic to Gram-positive bacteria and 25 times more toxic to Gram-negative strains, classified as susceptible, than to liver and kidney cells. Preliminary pharmacokinetic experiments established that serum concentrations higher than MIC values can be obtained for Rubb12 with an administered dose of 32 mg/kg.

CONCLUSIONS

The ruthenium complexes, particularly Rubb12, have potential as new antimicrobial agents. The structure of the dinuclear ruthenium complex can be readily further modified in order to increase the selectivity for bacteria over eukaryotic cells.

Fingerprinting the junctions of RNA structure by an open-paddlewheel diruthenium compound

RNA function is determined by its structural organization. The RNA structure consists of the combination of distinct secondary structure motifs connected by junctions that play an essential role in RNA folding. Selective 2'-hydroxyl acylation analyzed by primer extension (SHAPE) probing is an established methodology to analyze the secondary structure of long RNA molecules in solution, which provides accurate data about unpaired nucleotides. However, the residues located at the junctions of RNA structures usually remain undetected. Here we report an RNA probing method based on the use of a novel open-paddlewheel diruthenium (OPW-Ru) compound [Ru2Cl2(µ-DPhF)3(DMSO)] (DPhF = N,N'-diphenylformamidinate). This compound has four potential coordination sites in a singular disposition to establish covalent bonds with substrates. As a proof of concept, we have analyzed the reactivity of OPW-Ru toward RNA using two viral internal ribosome entry site (IRES) elements whose function depends on the structural organization of the molecule. Our study suggests that the compound OPW-Ru preferentially attacks at positions located one or two nucleotides away from junctions or bulges of the RNA structure. The OPW-Ru fingerprinting data differ from that obtained by other chemical reagents and provides new information about RNA structure features.

Fluorescent active ruthenium(ii) complex units containing bpy or phen or dmp ligands anchored on branched poly(ethylenimine): DNA binding and in vitro biological assessment

UV light irradiation visible responses of polymer ruthenium complexes.

Dinuclear ruthenium(ii) complexes containing one inert metal centre and one coordinatively-labile metal centre: syntheses and biological activities

Dinuclear ruthenium(ii) complexes containing one inert and one labile metal centre have been synthesised and their biological properties examined in bacterial and eukaryotic cells.

Antimicrobial Properties of Mono- and Di-fac-rhenium Tricarbonyl 2-Pyridyl-1,2,3-triazole Complexes

A family of mono- and di-fac-rhenium tricarbonyl 2-pyridyl-1,2,3-triazole complexes with different aliphatic and aromatic substituents was synthesized in good-to-excellent yields (46–99 %). The complexes were characterized by 1H and 13C NMR spectroscopy, infrared spectroscopy, electronic (UV-visible) spectroscopy, high-resolution electrospray mass spectrometry, and elemental analyses. In four examples, the solid-state structures of the rhenium(i) complexes were confirmed by X-ray crystallography. The family of the mono- and di-rhenium(i) complexes and the corresponding 2-pyridyl-1,2,3-triazole was tested for antimicrobial activity in vitro against both Gram-positive (Staphylococcus aureus) and Gram-negative (Escherichia coli) microorganisms. Agar-based disk diffusion assays indicated that most of the rhenium(i) complexes were active against Staphylococcus aureus and that the cationic rhenium(i) complexes were more active than the related neutral systems. However, in all cases, the minimum inhibitory concentrations for all the complexes were modest (i.e. 16–1024 µg mL–1).

Ruthenium complexes as antimicrobial agents

One of the major advances in medical science has been the development of antimicrobials; however, a consequence of their widespread use has been the emergence of drug-resistant populations of microorganisms. There is clearly a need for the development of new antimicrobials--but more importantly, there is the need for the development of new classes of antimicrobials, rather than drugs based upon analogues of known scaffolds. Due to the success of the platinum anticancer agents, there has been considerable interest in the development of therapeutic agents based upon other transition metals--and in particular ruthenium(II/III) complexes, due to their well known interaction with DNA. There have been many studies of the anticancer properties and cellular localisation of a range of ruthenium complexes in eukaryotic cells over the last decade. However, only very recently has there been significant interest in their antimicrobial properties. This review highlights the types of ruthenium complexes that have exhibited significant antimicrobial activity and discusses the relationship between chemical structure and biological processing--including site(s) of intracellular accumulation--of the ruthenium complexes in both bacterial and eukaryotic cells.

New imaging reagents for lipid dense regions in live cells and the nucleus in fixed MCF-7 cells

Two new uracil (U) and 5-flurouracil (5-FU) labeled ruthenium(ii)-polypyridyl based cellular imaging reagents are reported. Confocal laser scanning microscopic images with live and paraformaldehyde (PFA) fixed MCF-7 cells are examined using these two low-cytotoxic reagents. Experimental results show that these two complexes, appropriately functionalized with U (1) and 5-FU (2), have specific affinity for the lipid dense regions like the endoplasmic reticulum, cell membrane, and cytoplasmic vacuoles in live MCF-7 cells, and dye internalization in these regions happened following an endocytosis pathway. Interestingly, these two complexes are found to be localized in the nucleus of the PFA fixed cells. For fixed cells, presumably the lipid layer disruption helped in the explicit localization of the complexes 1 and 2 in the cell nucleus through specific interaction with cellular DNA. Poor and non-specific internalization of an analogous model complex 3, without having a U or 5-FU moiety, reveals the definite influence of U or 5-FU as well as the role of lipophilicity of the respective complex 1 and 2 in the cellular internalization process. Apart from these, a large Stokes shift (∼160 nm) and an appreciably long lived ³MLCT excited state (∼320 ns) in aq. buffer medium (pH 7.4) are other key features for complexes 1 and 2. Unlike the common nuclear DNA staining reagents like DAPI, these low-cytotoxic reagents are found to be highly stable towards photo-bleaching upon irradiation with 455 nm at the MLCT band for these complexes.