Photodynamic antimicrobial studies on a Ruthenium-based metal complex

Systematic review on antibacterial photodynamic therapeutic effects of transition metals ruthenium and iridium complexes

The prevalence of antibiotic-resistant pathogenic bacteria poses a significant threat to public health and ranks among the principal causes of morbidity and mortality worldwide. Antimicrobial photodynamic therapy is an emerging therapeutic technique that has excellent potential to embark upon antibiotic resistance problems. The efficacy of this therapy hinges on the careful selection of suitable photosensitizers (PSs). Transition metal complexes, such as Ruthenium (Ru) and Iridium (Ir), are highly suitable for use as PSs because of their surface plasmonic resonance, crystal structure, optical characteristics, and photonics. These metals belong to the platinum family and exhibit similar chemical behavior due to their partially filled d-shells. Ruthenium and Iridium-based complexes generate reactive oxygen species (ROS), which interact with proteins and DNA to induce cell death. As photodynamic therapeutic agents, these complexes have been widely studied for their efficacy against cancer cells, but their potential for antibacterial activity remains largely unexplored. Our study focuses on exploring the antibacterial photodynamic effect of Ruthenium and Iridium-based complexes against both Gram-positive and Gram-negative bacteria. We aim to provide a comprehensive overview of various types of research in this area, including the structures, synthesis methods, and antibacterial photodynamic applications of these complexes. Our findings will provide valuable insights into the design, development, and modification of PSs to enhance their photodynamic therapeutic effect on bacteria, along with a clear understanding of their mechanism of action.

Controlling Excited State Localization in Bichromophoric Photosensitizers via the Bridging Group

A series of photosensitizers comprised of both an inorganic and an organic chromophore are investigated in a joint synthetic, spectroscopic, and theoretical study. This bichromophoric design strategy provides a means by which to significantly increase the excited state lifetime by isolating the excited state away from the metal center following intersystem crossing. A variable bridging group is incorporated between the donor and acceptor units of the organic chromophore, and its influence on the excited state properties is explored. The Franck-Condon (FC) photophysics and subsequent excited state relaxation pathways are investigated with a suite of steady-state and time-resolved spectroscopic techniques in combination with scalar-relativistic quantum chemical calculations. It is demonstrated that the presence of an electronically conducting bridge that facilitates donor-acceptor communication is vital to generate long-lived (32 to 45 μs), charge-separated states with organic character. In contrast, when an insulating 1,2,3-triazole bridge is used, the excited state properties are dominated by the inorganic chromophore, with a notably shorter lifetime of 60 ns. This method of extending the lifetime of a molecular photosensitizer is, therefore, of interest for a range of molecular electronic devices and photophysical applications.

Ruthenium(II) polypyridyl complexes as emerging photosensitisers for antibacterial photodynamic therapy

Photodynamic therapy (PDT) has recently emerged as a potential valuable alternative to treat microbial infections. In PDT, singlet oxygen is generated in the presence of photosensitisers and oxygen under light irradiation of a specific wavelength, causing cytotoxic damage to bacteria. This review highlights different generations of photosensitisers and the common characteristics of ideal photosensitisers. It also focuses on the emergence of ruthenium and more specifically on Ru(II) polypyridyl complexes as metal-based photosensitisers used in antimicrobial photodynamic therapy (aPDT). Their photochemical and photophysical properties as well as structures are discussed while relating them to their phototoxicity. The use of Ru(II) complexes with recent advancements such as nanoformulations, combinatory therapy and photothermal therapy to improve on previous shortcomings of the complexes are outlined. Future perspectives of these complexes used in two-photon PDT, photoacoustic imaging and sonotherapy are also discussed. This review covers the literature published from 2017 to 2023.

Fe(II) complexes of 2,2':6',2''-terpyridine ligands functionalized with substituted-phenyl groups: synthesis, crystal structures and anticancer potential

With the aim of developing potential anticancer drug candidates, a series of Fe(II) complexes were synthesized using nine 2,2':6',2''-terpyridine ligands functionalized with substituted-phenyl groups, and their biological activities were systematically investigated. Their bis-terpyridine sandwich-like structures were determined by single crystal X-ray crystallography. In vitro antiproliferative experiments based on three human cancer cell lines, including human hepatoma cancer cell line (Bel-7402), human esophageal cancer cell line (Eca-109), and human cervical squamous cancer cell line (SiHa), indicate the high antiproliferation activities of these complexes compared with commercial cisplatin. And their toxicity to normal cells was estimated based on human normal hepatocyte (HL-7702) cell line. In particular, when the phenyl in terpyridine ligand was modified by a carboxyl group, the corresponding complex 3 exhibited much higher antiproliferation to cancer Bel-7402 cells (IC50 = 3.653 μmol L-1) than cisplatin and low toxicity to normal HL-7702 cells (IC50 = 99.92 μmol L-1), implying a significant selectivity for 3 in killing hepatoma cancer cells. Combined with the fact that iron element is more accessible than platin, this series of Fe(II) complexes comprises potential candidates for anticancer drugs with specific inhibition of hepatoma cancer. UV titration experiments and circular dichroism (CD) showed a strong binding affinity between these nine complexes and CT-DNA. However, molecular docking simulation revealed the competitive binding of DNA and protein to these complexes. Further, the interactions between these complexes and bovine serum albumin (BSA) have been studied by fluorescence titration and CD spectroscopy.

Recent Advances in Metal Complexes for Antimicrobial Photodynamic Therapy

Antimicrobial resistance (AMR) is a growing global problem with more than 1 million deaths due to AMR infection in 2019 alone. New and innovative therapeutics are required to overcome this challenge. Antimicrobial photodynamic therapy (aPDT) is a rapidly growing area of research poised to provide much needed help in the fight against AMR. aPDT works by administering a photosensitizer (PS) that is activated only when irradiated with light, allowing high spatiotemporal control and selectivity. The PS typically generates reactive oxygen species (ROS), which can damage a variety of key biological targets, potentially circumventing existing resistance mechanisms. Metal complexes are well known to display excellent optoelectronic properties, and recent focus has begun to shift towards their application in tackling microbial infections. Herein, we review the last five years of progress in the emerging field of small-molecule metal complex PSs for aPDT.

The Enhancement of Antimicrobial Photodynamic Therapy of Escherichia Coli by a Functionalized Combination of Photosensitizers: In Vitro Examination of Single Cells by Quantitative Phase Imaging

The prevention of biofilm formation is crucial for the limitation of bacterial infections typically associated with postoperative infections, complications in bedridden patients, and a short-term prognosis in affected cancer patients or mechanically ventilated patients. Antimicrobial photodynamic therapy (aPDT) emerges as a promising alternative for the prevention of infections due to the inability of bacteria to become resistant to aPDT inactivation processes. The aim of this study was to demonstrate the use of a functionalized combination of Chlorin e6 and Pheophorbide as a new approach to more effective aPDT by increasing the accumulation of photosensitizers (PSs) within Escherichia coli cells. The accumulation of PSs and changes in the dry mass density of single-cell bacteria before and after aPDT treatment were investigated by digital holotomography (DHT) using the refractive index as an imaging contrast for 3D label-free live bacteria cell imaging. The results confirmed that DHT can be used in complex examination of the cell–photosensitizer interaction and characterization of the efficiency of aPDT. Furthermore, the use of Pheophorbide a as an efflux pomp inhibitor in combination with Chlorin e6 increases photosensitizers accumulation within E. coli and overcomes the limited penetration of Gram-negative cells by anionic and neutral photosensitizers.