Development of ruthenium(II) complexes as topical antibiotics against methicillin resistant Staphylococcus aureus

Visible-Light-Responsive Novel Ru(II)-Metallo-Antibiotics with Potential Antibiofilm and Antibacterial Activity

Growing challenges with antibiotic resistance pose immense challenges in combating microbial infections and biofilm prevention on medical devices. Lately, antibacterial photodynamic therapy (aPDT) is now emerging as an alternative therapy to overcome this problem. Herein, we synthesized and characterized four Ru(II)-complexes, viz., [Ru(ph-tpy)(bpy)Cl]PF6 (Ru1), [Ru(ph-tpy)(dpq)Cl]PF6 (Ru2), [Ru(ph-tpy)(dppz)Cl]PF6 (Ru3), and [Ru(ph-tpy)(dppn)Cl]PF6 (Ru4) (where 4'-phenyl-2,2':6',2″-terpyridine = ph-tpy; 2,2'-bipyridine = bpy; dipyrido[3,2-f:2',3'-h]quinoxaline = dpq; dipyrido[3,2-a:2',3'-c]phenazine = dppz; and Benzo[I]dipyrido[3,2-a:2',3'-c]phenazine = dppn), among which Ru2-Ru4 are novel. Octahedral geometry of the complexes with a RuN5Cl core was evident from the crystal structure of Ru2. Ru1-Ru4 showed an MLCT absorption band in the 450-600 nm region, useful for aPDT performances. Further, optimum triplet excited state energy and excellent photostability of Ru1-Ru4 made them good photosensitizers for aPDT. Ru1-Ru4 demonstrated enhanced antimicrobial activity on visible-light exposure (400-700 nm, 10 J cm-2), confirmed using different antibacterial assays. Mechanistic studies revealed that inhibition of bacterial growth was due to the generation of oxidative stress (via NADH oxidation and ROS generation) upon treatment with Ru2-Ru4, resulting in destruction of the bacterial wall. Ru2 performed best killing performance against both Gram-negative (Escherichia coli) and Gram-positive (Bacillus subtilis) bacteria when exposed to light. Ru2-Ru4, when coated on a polydimethylsiloxane (PDMS) disk, showed long-term reusability and durable antibiofilm properties. Molecular docking confirmed the efficient interaction of Ru2-Ru4 with FabH (regulates fatty acid biosynthesis of E. coli) and PgaB (gives structural stability and helps biofilm formation of E. coli), resulting in probable downregulation. In vivo studies with healthy Wistar rats confirmed the biocompatibility of Ru2. This study shows that these lead complexes (Ru2-Ru4) can be used as potent alternative antimicrobial agents in low concentrations toward bacterial eradication with photodynamic therapy (PDT).

Antifungal activity of ruthenium (II) complex combined with fluconazole against drug-resistant Candida albicans in vitro and its anti-invasive infection in vivo

With the abuse of antibiotics and azoles, drug-resistant Candida albicans infections have increased sharply and are spreading rapidly, thereby significantly reducing the antifungal efficacy of existing therapeutics. Several patients die of fungal infections every year. Therefore, there is an urgent requirement to develop new drugs. Accordingly, we synthesized a series of polypyridyl ruthenium (II) complexes having the formula [Ru (NN)2 (bpm)] (PF6)2 (N-N = 2,2'-bipyridine) (bpy, in Ru1), 1,10-phenanthroline (phen, in Ru2), 4,7-diphenyl-1,10-phenanthroline (DIP, in Ru3) (bpm = 2,2'-bipyrimidine) and studied their antifungal activities. Ru3 alone had no effect on the drug-resistant strains, but Ru3 combined with fluconazole (FLC) exhibited significant antifungal activity on drug-resistant strains. A high-dose combination of Ru3 and FLC exhibited direct fungicidal activity by promoting the accumulation of reactive oxygen species and damaging the cellular structure of C. albicans. Additionally, the combination of Ru3 and FLC demonstrated potent antifungal efficacy in vivo in a mouse model of invasive candidiasis. Moreover, the combination significantly improved the survival state of mice, restored their immune systems, and reduced renal injury. These findings could provide ideas for the development of ruthenium (II) complexes as novel antifungal agents for drug-resistant microbial stains.

Synthesis, structure, theoretical calculation and antibacterial property of two novel Zn(II)/Ni(II) compounds based on 3, 5-dichlorosalicylaldehyde thiocarbamide ligand

Two new compounds namely [Zn(L1)phen]31 and Ni(L1)phen(MeOH) 2 (L1 = 3, 5-dichlorosalicylaldehyde thiosemicarbazone) were synthesized by the slow evaporation method at room temperature. The structure of ligand L1 was determined using 1H NMR and 13C NMR spectra. X-ray single crystal diffraction analysis revealed that compounds 1-2 can form 3D supramolecular network structures through π···π stacking and hydrogen bonding interactions. The DFT calculation shows that the coordination of ligand and metal is in good agreement with the experimental results. Hirshfeld surface analysis revealed that H…H and Cl…H interactions were the predominant interactions in compounds 1-2. Energy framework analysis indicated that dispersion energy played a dominant role in the energy composition of compounds 1-2. The inhibitory effects of compounds 1-2 against Escherichia coli (E. coli) and Methicillin-resistant Staphylococcus aureus (MRSA) were tested using the paper disk diffusion method (1: E. coli: 18 mm, MRSA: 17 mm, 2: E. coli: 15 mm, MRSA: 16 mm). Ion releasing experiments were conducted to assess the ion release capacity of compounds 1-2 (Zn2+, 4 days, 38.33 µg/mL; Ni2+, 4 days, 29.12 µg/mL). Molecular docking demonstrated the interaction modes of compounds 1-2 with UDP-N-acetylenolpyruvoylglucosamine reductase (MurB) and dihydrofolate reductase (DHFR) in bacteria, involving hydrophobic, stacking, hydrogen bonding and halogen bonding interactions. The generation of reactive oxygen species (ROS) in bacteria under the presence of compounds 1-2 were evaluated using a fluorescent dye known as dichlorodihydrofluorescein diacetate (DCFH-DA). Potential antibacterial mechanisms of compounds 1-2 were proposed.

Ru(II) Complex Grafted Ti3C2Tx MXene Nano Sheet with Photothermal/Photodynamic Synergistic Antibacterial Activity

For a long time, the emergence of microbial drug resistance due to the abuse of antibiotics has greatly reduced the therapeutic effect of many existing antibiotics. This makes the development of new antimicrobial materials urgent. Light-assisted antimicrobial therapy is an alternative to antibiotic therapy due to its high antimicrobial efficiency and non-resistance. Here, we develop a nanocomposite material (Ru@MXene) which is based on Ru(bpy)(dcb)²⁺ connected to MXene nanosheets by ester bonding as a photothermal/photodynamic synergistic antibacterial material. The obtained Ru@MXene nanocomposites exhibit a strengthened antimicrobial capacity compared to Ru or MXene alone, which can be attributed to the higher reactive oxygen species (ROS) yield and the thermal effect. Once exposed to a xenon lamp, Ru@MXene promptly achieved almost 100% bactericidal activity against Escherichia coli (200 μg/mL) and Staphylococcus aureus (100 μg/mL). This is ascribed to its synergistic photothermal therapy (PTT) and photodynamic therapy (PDT) capabilities. Consequently, the innovative Ru@MXene can be a prospective non-drug antimicrobial therapy that avoids antibiotic resistance in practice. Notably, this high-efficiency PTT/PDT synergistic antimicrobial material by bonding Ru complexes to MXene is the first such reported model. However, the toxic effects of Ru@MXene materials need to be studied to evaluate them for further medical applications.

Ru(II) Complexes with Enaminone Structures for Rapid Sterilization of Staphylococcus aureus and MRSA with Little Accumulation of Drug Resistance

Drug-resistant bacteria, such as methicillin-resistant Staphylococcus aureus (MRSA), pose a serious threat to human life. Therefore, there is urgent need to develop antibiotics with new chemical structures and antibacterial mechanisms, especially those that elicit little drug resistance after long-term use. Herein we synthesized three novel ruthenium complexes (Ru1-Ru3) containing the enaminone structures for the first time. At a concentration of 5 μM, Ru1-Ru3 can lead to a CFU reduction of about 5 log units towards S. aureus and MRSA. Interestingly, Ru3 displayed rapid bactericidal effects and could decrease the CFU numbers of both pathogens by 5 log units within 40 min. The control compounds (Ru4 and Ru5) without the enaminone structures displayed very poor antibacterial activity under the same conditions. Moreover, S. aureus did not show apparent drug resistance towards Ru3 after 20 passages incubation with a sublethal concentration. These results highlight the critical role of enaminone structures for antibacterial applications.

Synthesis, Characterization, Antibiofilm and Anticancer Activity of New Ruthenium Complexes with 2,2'-Bipyridine-4,4'-Dicarboxamide

Abstract:

New ruthenium complexes bearing bipyridine ligands with different substituents (propyl, hexyl, isobutyl, and benzyl) were synthesized and characterized by MS, NMR, FTIR, and UV/Visible spectroscopy. Moreover, their cytotoxic, anti-carcinogenic, and anti-biofilm activities were evaluated. The electrochemical properties of the complexes have been investigated by cyclic voltammetry. The HOMO and LUMO energy levels of RuL1-RuL4 were found as (-5.45 eV)-(-5.46 eV) and (-2.98 eV)-(-3.01 eV), respectively. Cytotoxic activities of ruthenium complexes were investigated in Caco-2, HepG2, and HEK293 cells. It was found that RuL3 showed a cytotoxic effect on cancer cells without affecting non-cancerous cells at applied doses. The presence of the benzyl group may increase the cytotoxic effect of RuL3 compared to other derivatives that contain the alkyl group. The apoptotic effect of the RuL3 derivative was determined by using Arthur image-based cytometer. It found that RuL3 was induced apoptosis in Caco-2 (5-fold) and HepG2 (2-fold) cancer cells, respectively. All ruthenium complexes inhibited Staphylococcus aureus ATCC 29213 biofilm, but RuL3 had a more pronounced effect. Moreover, RuL3 had biofilm inhibition and biofilm degradation effect while RuL1 and RuL4 demonstrated only biofilm inhibition. The fluorescent microscopy analysis confirmed the antibiofilm effect of ruthenium complexes. All of these results clearly showed that RuL3 showed cytotoxic and apoptotic effects on cancer cells.

A Ru(ii)-arene-ferrocene complex with promising antibacterial activity

The evolution of high virulence bacterial strains has necessitated the development of novel therapeutic agents to treat resistant infections.

An Alkynyl-Dangling Ru(II) Polypyridine Complex for Targeted Antimicrobial Photodynamic Therapy

To realize clinical application of antibacterial photodynamic therapy (aPDT), one of the most arduous challenges is how to render aPDT agents high selectivity against bacterial pathogens. In light of the fact that amino group-containing lipids are rich on the outer surfaces of Gram-positive bacteria, we herein constructed an alkynyl-dangling ruthenium(II) polypyridine complex (Ru2) to preferentially label Staphylococcus aureus (S. aureus) and methicillin-resistant Staphylococcus aureus (MRSA) over mammalian cells via the amino-yne bio-orthogonal click reaction. Thanks to the strong singlet oxygen generation ability, Ru2 could photo-inactivate S. aureus and MRSA effectively and specifically. Phosphatidylethanolamine (PE) molecules also exist in mammalian cells but are not accessible for Ru2, leading to its poor binding/uptake and negligible cytotoxicity in the dark and upon irradiation towards mammalian cells as well as low hemolysis, all favorable for aPDT application.

Antibacterial Activity of 2-Picolyl-polypyridyl-Based Ruthenium (II/III) Complexes on Non-Drug-Resistant and Drug-Resistant Bacteria

A new hexadentate 2-picolyl-polypyridyl-based ligand (4, 4'-(butane-1, 4-diylbis(oxy))bis(N, N-bis(pyridin-2-ylmethyl)aniline)) (2BUT) (1) and its corresponding Ru(II/III) complexes were synthesized and characterized, followed by assessment of their possible bioactive properties towards drug-resistant and non-drug-resistant bacteria. Spectroscopic characterization of the ligand was done using proton NMR, FTIR, and ESI-MS, which showed that the ligand was successfully synthesized. The Ru(II/III) complexes were characterized by FTIR, UV/Vis, elemental analysis, proton NMR, ESI-MS, and magnetic susceptibility studies. The analysis of ESI-MS data of the complexes showed that they were successfully synthesized. Empirical formulae derived from elemental analysis of the complexes also indicated successful synthesis and relative purity of the complexes. The important functional groups of the ligands could be observed after complexation using FTIR. Magnetic susceptibility data and electronic spectra indicated that both complexes adopt a low spin configuration. The disc diffusion assay was used to test the compounds for antibiotic activity on two bacteria species and their drug-resistant counterparts. The compounds displayed antibiotic activity towards the two non-drug-resistant bacteria. As for the drug-resistant organisms, only [Ru₂(2BUT)(DMF)₂(DPA)₂](BH₄)₄3 and 2, 2-dipyridylamine inhibited the growth of MRSA. Gel electrophoresis DNA cleavage studies showed that the ligands had no DNA cleaving properties while all the complexes denatured the bacterial DNA. Therefore, the complexes may have DNA nuclease activity towards the bacterial genomic material.

Development of photoactivable phenanthroline-based manganese(I) CO-Releasing molecules (PhotoCORMs) active against ESKAPE bacteria and bacterial biofilms

The synthesis and biological evaluation of a series of phenanthroline-based visible-light-activated manganese(I) carbon-monoxide-releasing molecules (PhotoCORMs) against ESKAPE bacteria and bacterial biofilms is reported. Four carbonyl compounds of general formula fac-[Mn(N^∧N)(CO)₃(L)] have been synthesized and characterized. Despite being thermally stable in the absence of light, these PhotoCORMs readily release CO upon blue (435-450 nm) LED light irradiation as confirmed by spectrophotometric CO releasing experiments (Mb Assay). The antibacterial activity of the four PhotoCORMs has been investigated against a panel of ESKAPE bacteria. The compounds 1-3 were found to be effective antibacterials at low concentrations against multidrug-resistant Klebsiella pneumoniae and Acinetobacter baumannii when photoactivated with blue-light. In addition, the PhotoCORMs 1-2 were found to inhibit the formation of Klebsiella pneumoniae and Acinetobacter baumannii bacterial biofilms at low concentrations (MIC = 4-8 μg/mL), turning out to be promising candidates to combat antimicrobial resistance. The antibacterial and biofilm inhibitory effect of the PhotoCORMs is plausibly due to the release of CO as well as the formation of phenanthroline photo-by-products as revealed by spectroscopy and microbiology experiments.

Polymeric ruthenium precursor as a photoactivated antimicrobial agent

Ruthenium coordination compounds have demonstrated a promising anticancer and antibacterial activity, but their poor water solubility and low stability under physiological conditions may limit their therapeutic applications. Physical encapsulation or covalent conjugation with polymers may overcome these drawbacks, but generally involve multistep reactions and purification processes. In this work, the antibacterial activity of the polymeric precursor dicarbonyldichlororuthenium (II) [Ru(CO)₂Cl₂]_n has been studied against Escherichia coli and Staphylococcus aureus. This Ru-carbonyl precursor shows minimum inhibitory concentration at nanogram per millilitre, which renders it a novel antimicrobial polymer without any organic ligands. Besides, [Ru(CO)₂Cl₂]_n antimicrobial activity is markedly boosted under photoirradiation, which can be ascribed to the enhanced generation of reactive oxygen species under UV irradiation. [Ru(CO)₂Cl₂]_n has been able to inhibit bacterial growth via the disruption of bacterial membranes and triggering upregulation of stress responses as shown in microscopic measurements. The activity of polymeric ruthenium as an antibacterial material is significant even at 6.6 ng/mL while remaining biocompatible to the mammalian cells at much higher concentrations. This study proves that this simple precursor, [Ru(CO)₂Cl₂]_n, can be used as an antimicrobial compound with high activity and a low toxicity profile in the context of need for new antimicrobial agents to fight bacterial infections.

Organometallic ruthenium (η6-p-cymene) complexes interfering with quorum sensing and biofilm formation: an anti-infective approach to combat multidrug-resistance in bacteria

Organometallic ruthenium complexes of flavonoids as antiquorum sensing agents against pathogens likeChromobacterium violaceumATCC 12472,Pseudomonas aeruginosaPAO1 and methicillin-resistantS. aureus(MRSA).

Encapsulation of a Ru(II) Polypyridyl Complex into Polylactide Nanoparticles for Antimicrobial Photodynamic Therapy

Antimicrobial photodynamic therapy (aPDT) also known as photodynamic inactivation (PDI) is a promising strategy to eradicate pathogenic microorganisms such as Gram-positive and Gram-negative bacteria. This therapy relies on the use of a molecule called photosensitizer capable of generating, from molecular oxygen, reactive oxygen species including singlet oxygen under light irradiation to induce bacteria inactivation. Ru(II) polypyridyl complexes can be considered as potential photosensitizers for aPDT/PDI. However, to allow efficient treatment, they must be able to penetrate bacteria. This can be promoted by using nanoparticles. In this work, ruthenium-polylactide (RuPLA) nanoconjugates with different tacticities and molecular weights were prepared from a Ru(II) polypyridyl complex, RuOH. Narrowly-dispersed nanoparticles with high ruthenium loadings (up to 53%) and an intensity-average diameter < 300 nm were obtained by nanoprecipitation, as characterized by dynamic light scattering (DLS). Their phototoxicity effect was evaluated on four bacterial strains (Staphylococcus aureus, Staphylococcus epidermidis, Escherichia coli and Pseudomonas aeruginosa) and compared to the parent compound RuOH. RuOH and the nanoparticles were found to be non-active towards Gram-negative bacterial strains. However, depending on the tacticity and molecular weight of the RuPLA nanoconjugates, differences in photobactericidal activity on Gram-positive bacterial strains have been evidenced whereas RuOH remained non active.

Antibacterial activity of ruthenium polypyridyl complexes against Staphylococcus aureus and biofilms

There is clearly a need for the development of new classes of antimicrobials to fight against multidrug-resistant bacteria. Here, we designed and synthesized of three ruthenium polypyridyl complexes: [Ru(bpy)₂(BTPIP)](ClO₄)₂ (Ru(II)-1), [Ru(bpy)₂(ETPIP)](ClO₄)₂ (Ru(II)-2) and [Ru(bpy)₂(CAPIP)](ClO₄)₂ (Ru(II)-3) (N-N = bpy = 2,2'-bipyridine), their antimicrobial activities against S. aureus were assessed. The lead complexes of this set, Ru(II)-1(MIC = 0.016 mg/mL), was tested against biofilm. We also investigated whether bacteria can easily develop resistance to Ru(II)-1. The result demonstrated that S. aureus could not easily develop resistance to the ruthenium complexes. In addition, aimed to test whether ruthenium complexes treatment could increase the susceptibility of S. aureus to antibiotics, the synergism between Ru(II)-1 and common antibiotics against S. aureus were investigated using the checkerboard method. Interesting, Ru(II)-1 could increased the susceptibility of S. aureus to some aminoglycoside antibiotics(kanamycin and gentamicin). Finally, in vivo bacterial infection treatment studies were also conducted through murine skin infection model. These results confirmed ruthenium complexes have good antimicrobial activity in vitro and in vivo.

Sousa EH, de França Lopes LG, Teixeira EH, Vasconcelos MA, Martins PHR, Medeiros EJT, Batista AA, Holanda AKM. Biphosphinic ruthenium complexes as the promising antimicrobial agents

There is an urgent need for new antimicrobial compounds to combat the growing threat of widespread antibiotic resistance. Ruthenium compounds have shown promising activities including two biphosphinic compounds as described here.

Fluorination in enhancing photoactivated antibacterial activity of Ru(ii) complexes with photo-labile ligands

Fluorination in the dppz ligand efficiently enhanced the photoactivated antibacterial activity of Ru(ii) complexes with photo-labile ligands against antibiotic-resistant bacteria both under normoxic and hypoxic conditions.

Two ruthenium polypyridyl complexes functionalized with thiophen: synthesis and antibacterial activity against Staphylococcus aureus

Ruthenium polypyridyl complex Ru(ii)-2 could increase the susceptibility of S. aureus to the aminoglycoside antibiotic (kanamycin).

Biguanide Iridium(III) Complexes with Potent Antimicrobial Activity

We have synthesized novel organoiridium(III) antimicrobial complexes containing a chelated biguanide, including the antidiabetic drug metformin. These 16- and 18-electron complexes were characterized by NMR, ESI-MS, elemental analysis, and X-ray crystallography. Several of these complexes exhibit potent activity against Gram-negative bacteria and Gram-positive bacteria (including methicillin-resistant Staphylococcus aureus (MRSA)) and high antifungal potency toward C. albicans and C. neoformans, with minimum inhibitory concentrations (MICs) in the nanomolar range. Importantly, the complexes exhibit low cytotoxicity toward mammalian cells, indicating high selectivity. They are highly stable in broth medium, with a low tendency to generate resistance mutations. On coadministration, they can restore the activity of vancomycin against vancomycin-resistant Enterococci (VRE). Also the complexes can disrupt and eradicate bacteria in mature biofilms. Investigations of reactions with biomolecules suggest that these organometallic complexes deliver active biguanides into microorganisms, whereas the biguanides themselves are inactive when administered alone.

Recent Advances on Octahedral Polypyridyl Ruthenium(II) Complexes as Antimicrobial Agents

Recent developments of therapeutic agents based on transition metals have attracted a great deal of attention. Metal drugs have advantages over other small molecule drugs, and it was demonstrated that, in a number of studies, they played an important role in pharmaceutical chemical research and clinical chemotherapy of cancers. It is worthwhile mentioning that octahedral polypyridyl ruthenium(II) complexes have shown remarkable applications in chemical biology and medicinal chemistry over the last decade. However, only very recently has there been comprehensive interest in their antimicrobial properties due to metal-related toxic concerns or neglected potential roles in microbiological systems. Our review will highlight the recent developments in octahedral polypyridyl ruthenium(III) complexes that have exhibited significant antimicrobial activities and will discuss the relationship between the chemical structure and biological process of ruthenium complexes, in both bacterial and fungal cells.

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).

Recent advances on topical antimicrobials for skin and soft tissue infections and their safety concerns

Antimicrobial resistance of disease-related microorganisms is considered a worldwide prevalent and serious issue which increases the failure of treatment outcomes and leads to high mortality. Considering that the increased resistance to systemic antimicrobial therapy often needs of the use of more toxic agents, topical antimicrobial therapy emerges as an attractive route for the treatment of infectious diseases. The topical antimicrobial therapy is based on the absorption of high drug doses in a readily accessible skin surface, resulting in a reduction of microbial proliferation at infected skin sites. Topical antimicrobials retain the following features: (a) they are able to escape the enzymatic degradation and rapid clearance in the gastrointestinal tract or the first-pass metabolism during oral administration; (b) alleviate the physical discomfort related to intravenous injection; (c) reduce possible adverse effects and drug interactions of systemic administrations; (d) increase patient compliance and convenience; and (e) reduce the treatment costs. Novel antimicrobials for topical application have been widely exploited to control the emergence of drug-resistant microorganisms. This review provides a description of antimicrobial resistance, common microorganisms causing skin and soft tissue infections, topical delivery route of antimicrobials, safety concerns of topical antimicrobials, recent advances, challenges and future prospective in topical antimicrobial development.

Methylation of Ir(iii)-tetrazolato complexes: an effective route to modulate the emission outputs and to switch to antimicrobial properties

Two neutral cyclometalated Ir(iii)-tetrazolato complexes that differ by variations of the substituents on either the phenylpyridine or the tetrazolate ligand have been converted into the corresponding methylated and cationic analogues. NMR (¹H and ¹³C) characterization of the Ir(iii) complexes provided the results in agreement with the chemo- and regioselective character of methylation at the N-3 position of the Ir(iii)-coordinated tetrazolato ring. This evidence was further corroborated by the analysis of the molecular structures of the cationic complexes obtained by X-ray diffraction. In view of the photophysical properties, the addition of a methyl moiety to neutral Ir(iii) tetrazolates, which behave as sky-blue or orange phosphors, caused a systematic red shift of their phosphorescence output. The transformation of neutral Ir(iii) tetrazolates into cationic Ir(iii)-tetrazole complexes was screened for any eventual antimicrobial activity in vitro against Gram negative (E. coli) and Gram positive (D. radiodurans) microorganisms. While both kinds of complexes were not active against E. coli, the conversion of the neutral Ir(iii) tetrazolates into the corresponding methylated and cationic Ir(iii)tetrazole derivatives determined the turn-on of a good to excellent antimicrobial activity toward Gram positive Deinococcus radiodurans, a non-pathogenic bacterium that is listed as one of the toughest microorganisms in light of its outstanding resistance to radiation and oxidative stress.

Combatting AMR: photoactivatable ruthenium(ii)-isoniazid complex exhibits rapid selective antimycobacterial activity

The novel photoactive ruthenium(ii) complex cis-[Ru(bpy)2(INH)2][PF6]2 (1·2PF6, INH = isoniazid) was designed to incorporate the anti-tuberculosis drug, isoniazid, that could be released from the Ru(ii) cage by photoactivation with visible light. In aqueous solution, 1 rapidly released two equivalents of isoniazid and formed the photoproduct cis-[Ru(bpy)2(H2O)2]2+ upon irradiation with 465 nm blue light. We screened for activity against bacteria containing the three major classes of cell envelope: Gram-positive Bacillus subtilis, Gram-negative Escherichia coli, and Mycobacterium smegmatis in vitro using blue and multi-colored LED multi-well arrays. Complex 1 is inactive in the dark, but when photoactivated is 5.5× more potent towards M. smegmatis compared to the clinical drug isoniazid alone. Complementary pump-probe spectroscopy measurements along with density functional theory calculations reveal that the mono-aqua product is formed in <500 ps, likely facilitated by a 3MC state. Importantly, complex 1 is highly selective in killing mycobacteria versus normal human cells, towards which it is relatively non-toxic. This work suggests that photoactivatable prodrugs such as 1 are potentially powerful new agents in combatting the global problem of antibiotic resistance.

Synthesis of platinum(II) and palladium(II) complexes with 9,9-dihexyl-4,5-diazafluorene and their in vivo antitumour activity against Hep3B xenografted mice

Two complexes dichloro(9,9-dihexyl-4,5-diazafluorene)platinum(II) (Pt-DHF) and dichloro(9,9-dihexyl-4,5-diazafluorene)palladium(II) (Pd-DHF) were synthesized and their in vivo antitumour activity was investigated using an athymic nude mice model xenografted with human Hep3B carcinoma cells. Pt-DHF- and Pd-DHF-treated groups showed significant tumour growth inhibition (with about 9-fold and 3-fold tumour growth retardation) when compared with the vehicle control group. The liver toxicology effects on the animals of the two compounds were investigated. Pt-DHF and Pd-DHF-treated groups had a lower alanine transaminase and aspartate transaminase values than those of the vehicle treated group as the animals from the vehicle control group had very heavy hepatoma burden. We assume that both complexes could be further investigated as effective antitumour agents and it is worthwhile to study their underlying working mechanism.

Antimicrobial Activity of the Manganese Photoactivated Carbon Monoxide-Releasing Molecule [Mn(CO)3(tpa-κ(3)N)](+) Against a Pathogenic Escherichia coli that Causes Urinary Infections

AIMS

We set out to investigate the antibacterial activity of a new Mn-based photoactivated carbon monoxide-releasing molecule (PhotoCORM, [Mn(CO)3(tpa-κ(3)N)](+)) against an antibiotic-resistant uropathogenic strain (EC958) of Escherichia coli.

RESULTS

Activated PhotoCORM inhibits growth and decreases viability of E. coli EC958, but non-illuminated carbon monoxide-releasing molecule (CORM) is without effect. NADH-supported respiration rates are significantly decreased by activated PhotoCORM, mimicking the effect of dissolved CO gas. CO from the PhotoCORM binds to intracellular targets, namely respiratory oxidases in strain EC958 and a bacterial globin heterologously expressed in strain K-12. However, unlike previously characterized CORMs, the PhotoCORM is not significantly accumulated in cells, as deduced from the cellular manganese content. Activated PhotoCORM reacts avidly with hydrogen peroxide producing hydroxyl radicals; the observed peroxide-enhanced toxicity of the PhotoCORM is ameliorated by thiourea. The PhotoCORM also potentiates the effect of the antibiotic, doxycycline.

INNOVATION

The present work investigates for the first time the antimicrobial activity of a light-activated PhotoCORM against an antibiotic-resistant pathogen. A comprehensive study of the effects of the PhotoCORM and its derivative molecules upon illumination is performed and mechanisms of toxicity of the activated PhotoCORM are investigated.

CONCLUSION

The PhotoCORM allows a site-specific and time-controlled release of CO in bacterial cultures and has the potential to provide much needed information on the generality of CORM activities in biology. Understanding the mechanism(s) of activated PhotoCORM toxicity will be key in exploring the potential of this and similar compounds as antimicrobial agents, perhaps in combinatorial therapies with other agents. Antioxid. Redox Signal. 24, 765-780.

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.

Coordination chemistry and applications of versatile 4,5-diazafluorene derivatives

This review article highlights the versatile nature of 4,5-diazafluorene derivatives as ligands, and details some recent advances made using this ligand family.

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).

Antifungal study of substituted 4-pyridylmethylene-4′-aniline Schiff bases

Two substituted 4-pyridylmethylene-4′-aniline Schiff bases showed antifungal activity against.Aspergillus nigerandCandida albicans. They may be useful for the development of safe drug candidates forA. nigerandC. albicanscases of otomycosis.

Biological assessment of substituted quinoline based heteroleptic organometallic compounds

Spectral changes accompanying addition of HS DNA to solution of quinoline nucleus based piano stool coordination compounds indicate intercalative mode of binding of the compound between DNA base pairs.

Antimicrobial and toxicological evaluations of binuclear mercury(ii)bis(alkynyl) complexes containing oligothiophenes and bithiazoles

We investigated the antimicrobial activity of bis-(alkynyl)mercury(ii) complexes with oligothiophene and bithiazole linking units against MRSA and C. albicans, and their cytotoxicity was tested on NIH 3T3 cells.

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.

CO-releasing Metal Carbonyl Compounds as Antimicrobial Agents in the Post-antibiotic Era

The possibility of a “post-antibiotic era” in the 21st century, in which common infections may kill, has prompted research into radically new antimicrobials. CO-releasing molecules (CORMs), mostly metal carbonyl compounds, originally developed for therapeutic CO delivery in animals, are potent antimicrobial agents. Certain CORMs inhibit growth and respiration, reduce viability, and release CO to intracellular hemes, as predicted, but their actions are more complex, as revealed by transcriptomic datasets and modeling. Progress is hindered by difficulties in detecting CO release intracellularly, limited understanding of the biological chemistry of CO reactions with non-heme targets, and the cytotoxicity of some CORMs to mammalian cells.

Redox-active cationic organoiron complex: a promising lead structure for developing antimicrobial agents with activity against Gram-positive pathogens including methicillin-resistant Staphylococcus aureus and vancomycin-resistant Enterococcus faecium

A redox-active, cationic organoiron complexes active against multidrug-resistant strain of Gram-positive bacteria is presented as a potential new lead structure for the design of antimicrobial agents.

Synthesis, characterisation and antibacterial activity of [(p-cym)RuX(L)]+/2+ (X = Cl, H2O; L = bpmo, bpms) complexes

Simple replacement of Cl⁻ by H₂O in {(p-cym)Ru^II(L)X]ⁿ⁺ (X = Cl or H₂O) complexes enhances antibacterial activity significantly.

Synthesis of 1,3,5-trisubstituted pyrazoline derivatives and their applications

The biological activities of pyrazoline based Ru(iii) complexes were carried out using gel electrophoresis, absorption titration, cellular level cytotoxicity and molecular docking study. Compounds exhibit potent nuclease and cytotoxicity activity.