Ru(II)-Complex-Based DNA Photocleaver Having Intense Absorption in the Phototherapeutic Window

A combined solid state, solution and DFT study of a dimethyl-cyclen-Pd(II) complex

A new palladium(II) complex containing the previously synthesized 4,10-bis[(3-hydroxy-4-pyron-2-yl)methyl]-1,7-dimethyl-1,4,7,10-tetraazacyclododecane ligand maltonis was prepared and characterized both in solution and in the solid state. Hirshfeld surface and energy framework analyses were also performed. Because maltonis already showed antineoplastic activity, the complexation of Pd(II), chosen as an alternative to Pt(II), was investigated to study its possible biological activity. UV-vis and NMR studies confirmed the formation and stability of the complex in aqueous solution at physiological pH. X-ray diffraction data revealed a structure where the Pd(II) ion is lodged in the dimethyl-cyclen cavity, with maltol rings facing each other (closed shape) even if they are not involved in the coordination. DFT analysis was performed in order to understand the most stable shape of the complex. In view of evaluating its possible bioactive conformation, the DFT study suggested a slight energetic preference for the closed one. The resulting closed complex was stabilized in the X-ray structure by intermolecular interactions that replace the intramolecular interactions present in the optimized complex. According to the DFT calculated formation energies, notwithstanding its rarity, the Pd(II) complex of maltonis is the thermodynamically preferred one among analogous complexes containing different metal ions (Pt(II), Co(II), and Cu(II)). Finally, to study its possible biological activity, the interaction between the Pd(II) complex of maltonis and nucleosides was evaluated through NMR and DFT calculations, revealing a possible interaction with purines via the maltol moieties.

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).

Modulating Excited State Properties and Ligand Ejection Kinetics in Ruthenium Polypyridyl Complexes Designed to Mimic Photochemotherapeutics

Ruthenium(II) polypyridyl complexes have gained significant interest as photochemotherapeutics (PCTs) due to their synthetic viability, strong light absorption, well understood excited state properties, and high phototoxicity indexes. Herein, we report the synthesis, characterization, electrochemical, spectrochemical, and preliminary cytotoxicity analyses of three series of ruthenium(II) polypyridyl complexes designed to mimic PCTs. The three series have the general structure of [Ru(bpy)2(N-N)]2+ (Series 1), [Ru(bpy)(dmb)(N-N)]2+ (Series 2), and [Ru(dmb)2(N-N)]2+ (Series 3, where N-N is a bidentate polypyridyl ligand, bpy = 2,2'-bipyridine, and dmb = 6,6'-dimethyl-2,2'-bipyridine). In the three series, the N-N ligand was systematically modified to incorporate increased conjugation and/or electronegative heteroatoms to increase dπ-π* backbonding, red-shifting the lowest energy metal-to-ligand charge transfer (MLCT) absorptions from λmax = 454 to λmax = 580 nm, nearing the therapeutic window for PCTs (600-1100 nm). In addition, steric bulk was systematically introduced through the series, distorting the Ru(II) octahedra, making the dissociative 3dd* state thermally accessible at room and body temperatures. This resulted in a 4 orders of magnitude increase in photoinduced ligand ejection kinetics, and demonstrates the ability to modulate both the MLCT* and dd* manifolds in the complexes, which is critical in PCT drug design. Preliminary cell viability assays suggest that the increased steric bulk to lower the 3dd* states may interfere with the cytotoxicity mechanism, limiting photoinitiated toxicity of the complexes. This work demonstrates the importance of understanding both the MLCT* and dd* manifolds and how they impact the ability of a complex to act as a PCT agent.

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.

Computational Assessment of a Dual-Action Ru(II)-Based Complex: Photosensitizer in Photodynamic Therapy and Intercalating Agent for Inducing DNA Damage

A combined quantum-mechanical and classical molecular dynamics study of a recent Ru(II) complex with potential dual anticancer action is reported here. The main basis for the multiple action relies on the merocyanine ligand, whose electronic structure allows the drug to be able to absorb within the therapeutic window and in turn efficiently generate ¹O₂ for photodynamic therapy application and to intercalate within two nucleobases couples establishing reversible electrostatic interactions with DNA. TDDFT outcomes, which include the absorption spectrum, triplet states energy, and spin-orbit matrix elements, evidence that the photosensitizing activity is ensured by an MLCT state at around 660 nm, involving the merocyanine-based ligand, and by an efficient ISC from such state to triplet states with different characters. On the other hand, the MD exploration of all the possible intercalation sites within the dodecamer B-DNA evidences the ability of the complex to establish several electrostatic interactions with the nucleobases, thus potentially inducing DNA damage, though the simulation of the absorption spectra for models extracted by each MD trajectory shows that the photosensitizing properties of the complex remain unaltered. The computational results support that the anti-tumor effect may be related to multiple mechanisms of action.

Photosensitizer-Polypeptide Conjugate for Effective Elimination of Candida albicans Biofilm

Persistent fungal infections caused by biofilms seriously endanger human health. In this study, a photosensitizer-polypeptide conjugate (PPa-cP) comprising a photosensitizer, pyropheophorbide a (PPa), and a cationic polypeptide (cP) is readily synthesized for effective antifungal and antibiofilm treatment. Compared with free PPa, the cationic PPa-cP shows enhanced binding ability to the negatively charged surface of Candida albicans (C. albicans) through electrostatic interactions. As a result, PPa-cP exhibits effective antifungal efficiency against both C. albicans and fluconazole-resistant C. albicans in vitro under light irradiation. The minimum inhibitory concentration (MIC) of PPa-cP for both C. albicans and fluconazole-resistant C. albicans is 1 µm. In addition, PPa-cP also shows improved penetration in a C. albicans biofilm, thus effectively eliminating the C. albicans biofilm by photodynamic effects. More importantly, PPa-cP demonstrats significantly enhanced therapeutic effects in a fluconazole-resistant C. albicans-infected rat model with minimal side effects. In conclusion, the current work presents an effective strategy to combat biofilm infections associated with biomedical equipment.

Balancing the interplay between ligand ejection and therapeutic window light absorption in ruthenium polypyridyl complexes

Ruthenium polypyridyl complexes have gained significant interest as photochemotherapies (PCTs) where their excited-state properties play a critical role in the photo-cytotoxicity mechanism and efficacy. Herein we report a systematic electrochemical, spectrochemical, and photophysical analysis of a series of ruthenium(II) polypyridyl complexes of the type [Ru(bpy)₂(N-N)]²⁺ (where bpy = 2,2'-bipyridine; N-N is a bidentate polypyridyl ligand) designed to mimic PCTs. In this series, the N-N ligand was modified through increased conjugation and/or incorporation of electronegative heteroatoms to shift the metal-to-ligand charge-transfer (MLCT) absorptions near the therapeutic window for PCTs (600-1100 nm) while incorporating steric bulk to trigger photoinduced ligand dissociation. The lowest energy MLCT absorptions were red-shifted from λ_max = 454 nm to 564 nm, with emission energies decreasing from λ_max = 620 nm to 850 nm. Photoinduced ligand ejection and temperature-dependent emission studies revealed an important interplay between red-shifting MLCT absorptions and accessing the dissociative ³dd* states, with energy barriers between the ³MLCT* and ³dd* states ranging from 850 cm^-1 to 2580 cm^-1 for the complexes measured. This work demonstrates the importance of understanding both the MLCT manifold and ³dd* state energy levels in the future design of ligands and complexes for PCT.

Recent advances in noble metal complex based photodynamic therapy

Photodynamic therapy (PDT) utilizes light-activated photosensitizers (PSs) to generate toxic species for therapeutics. It has become an emerging solution for cancer treatment because of its specific spatiotemporal selectivity and minimal invasiveness. Noble metal (Ru, Ir and Pt) complexes are of increasing interest as photosensitizers for their excellent photophysical, photochemical, and photobiological properties. In this review, we highlight recent advancements in the development of noble metal complex photosensitizers for PDT during the last 5 years. We will summarize the design strategies of noble metal complexes for efficient and precise PDT, including increasing the light penetration depth, reducing the oxygen-dependent nature and improving target ability. Finally, we summarize recent efforts for the development of noble-based PSs and discuss the limitations of such PSs in clinical application and future perspectives in this field, such as the combination of PDT with other treatment modalities.

Nuclease-like metalloscissors: Biomimetic candidates for cancer and bacterial and viral infections therapy

Despite the extensive and rapid discovery of modern drugs for treatment of cancer, microbial infections, and viral illnesses; these diseases are still among major global health concerns. To take inspiration from natural nucleases and also the therapeutic potential of metallopeptide antibiotics such as the bleomycin family, artificial metallonucleases with the ability of promoting DNA/RNA cleavage and eventually affecting cellular biological processes can be introduced as a new class of therapeutic candidates. Metal complexes can be considered as one of the main categories of artificial metalloscissors, which can prompt nucleic acid strand scission. Accordingly, biologists, inorganic chemists, and medicinal inorganic chemists worldwide have been designing, synthesizing and evaluating the biological properties of metal complexes as artificial metalloscissors. In this review, we try to highlight the recent studies conducted on the nuclease-like metalloscissors and their potential therapeutic applications. Under the light of the concurrent Covid-19 pandemic, the human need for new therapeutics was highlighted much more than ever before. The nuclease-like metalloscissors with the potential of RNA cleavage of invading viral pathogens hence deserve prime attention.

Exploring the use of a Ruthenium complex incorporated into a methacrylate-based dental material for antimicrobial photodynamic therapy

OBJECTIVES

To evaluate the effects of a blue light photosensitizer (PS), Ruthenium II complex (Ru), on the chemical, physical, mechanical, and antimicrobial properties of experimental dental resin blends.

METHODS

The experimental resin (BisEMA, TEEGDMA, HPMA, ethanol, and photoinitiator) was loaded with Ru at 0.00%, 0.07%, 0.14%, 0.28%, 0.56%, 1.12%, 1.2%, 1.5%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, or 10% w/w. Samples were evaluated for the degree of conversion (DC) after 30 and 60 s curing-time (n = 6). Selected formulations (0.00%, 0.28%, 0.56%, 1.12%) were further tested for shear bond strength (SBS) (n = 15); flexural strength (FS) (n = 12); and antimicrobial properties (CFUs), in dark and light conditions. These latter tests were performed on specimens stored for 24-h or 2-month in 37°C water. Water sorption (WS) and solubility (SL) tests were also performed (n = 12). Data were analyzed either by a one- or two-factor general linear model (α = 0.05).

RESULTS

Overall, Ru concentration above 1.2% resulted in reduced DC. In SBS results, only the 1.12%Ru resin blend samples had statistically lower values compared to the 0.00%Ru resin blend at 24-h storage (p = 0.004). In addition, no differences in SBS were detected among the experimental groups after 2-month storage in water. Meanwhile, FS increased for all experimental groups under similar aging conditions (p < 0.001). Antimicrobial properties were improved upon inclusion of Ru and application of light (p < 0.001 for both) at 24-h and 2-month storage. Lastly, no detectable changes in WS or SL were observed for the Ru-added resins compared to the 0.00%Ru resin blend. However, the 0.28% Ru blend presented significantly higher WS compared to the 0.56% Ru blend (p = 0.007).

CONCLUSIONS

Stable SBS, improved FS, and sustained antimicrobial properties after aging gives significant credence to our approach of adding the Ruthenium II complex into dental adhesive resin blends intended for an aPDT approach.

Recent Strategies to Develop Innovative Photosensitizers for Enhanced Photodynamic Therapy

This review presents a robust strategy to design photosensitizers (PSs) for various species. Photodynamic therapy (PDT) is a photochemical-based treatment approach that involves the use of light combined with a light-activated chemical, referred to as a PS. Attractively, PDT is one of the alternatives to conventional cancer treatment due to its noninvasive nature, high cure rates, and low side effects. PSs play an important factor in photoinduced reactive oxygen species (ROS) generation. Although the concept of photosensitizer-based photodynamic therapy has been widely adopted for clinical trials and bioimaging, until now, to our surprise, there has been no relevant review article on rational designs of organic PSs for PDT. Furthermore, most of published review articles in PDT focused on nanomaterials and nanotechnology based on traditional PSs. Therefore, this review aimed at reporting recent strategies to develop innovative organic photosensitizers for enhanced photodynamic therapy, with each example described in detail instead of providing only a general overview, as is typically done in previous reviews of PDT, to provide intuitive, vivid, and specific insights to the readers.

Red-Light-Responsive Ru Complex Photosensitizer for Lysosome Localization Photodynamic Therapy

Photoresponsive ruthenium (Ru) complexes have been extensively studied in the photodynamic therapy (PDT) of cancer. The metal-to-ligand charge transfer (MLCT) absorption maximum of most Ru complexes is located in the short-wavelength visible region, which is well suited for superficial tumors but shows inefficient therapeutic effects for more deep-seated ones. Moreover, Ru complexes are primarily located in the mitochondria or nucleus, always resulting in high levels of dark toxicity and DNA mutation. Herein, we reported a new ruthenium complex (Ru-I) for red-light-triggered PDT. The activation wavelength of Ru-I is successfully extended to 660 nm. Importantly, the complex photosensitizer can be quickly taken up by cancer cells and selectively accumulated in the lysosome, an ideal localization for PDT purposes. Intratumoral injection of Ru-I into tumor-bearing mice achieved excellent therapeutic effects and thus holds great promise for applications in lysosome localization photodynamic therapy.

Exploring the potential of highly charged Ru(II)- and heteronuclear Ru(II)/Cu(II)-polypyridyl complexes as antimicrobial agents

The antimicrobial potential of two ruthenium(II) polypyridyl complexes, [Ru(phen)₂L1]²⁺ and [Ru(phen)₂L2]²⁺ (phen = 1,10-phenanthroline) containing the 4,4'-(2,5,8,11,14-pentaaza[15])-2,2'-bipyridilophane (L1) and the 4,4'-bis-[methylen-(1,4,7,10-tetraazacyclododecane)]-2,2' bipyridine (L2) units, is herein investigated. These peculiar polyamine frameworks afford the formation of highly charged species in solution, influence the DNA-binding and cleavage properties of compounds, but they do not undermine their singlet oxygen sensitizing capacities, thus making these complexes attractive ¹O₂ generators in aqueous solution. L1 and L2 also permit to stably host Fenton -active Cu²⁺ ion/s, leading to the formation of mixed Ru²⁺/Cu²⁺ forms capable to further strengthen the oxidative damages to biological targets. Herein, following a characterization of the Cu²⁺ binding ability by [Ru(phen)₂L2]²⁺, the water-octanol distribution coefficients, the DNA binding, cleavage and ¹O₂ sensitizing properties of [Ru(phen)₂L2]²⁺ and [Cu₂Ru(phen)₂L2]⁶⁺ were analysed and compared with those of [Ru(phen)₂L1]²⁺ and [CuRu(phen)₂L1]⁴⁺. The antimicrobial activity of all compounds was evaluated against B. subtilis, chosen as a model for gram-positive bacteria, both under dark and upon light-activation. Our results unveil a notable phototoxicity of [Ru(phen)₂L2]²⁺ and [Cu₂Ru(phen)₂L2]⁶⁺, with MIC (minimal inhibitory concentrations) values of 3.12 μM. This study highlights that the structural characteristics of polyamine ligands gathered on highly charged Ru(II)-polypyridyl complexes are versatile tools that can be exploited to achieve enhanced antibacterial strategies.

Ruthenium-based Photoactive Metalloantibiotics†

Antibiotic resistance is one of the world's most urgent public health problems. Antimicrobial photodynamic therapy (aPDT) is a promising therapy to combat the growing threat of antibiotic resistance. The aPDT combines a photosensitizer and light to generate reactive oxygen species to induce bacterial inactivation. Ruthenium polypyridyl complexes are significant because they possess unique photophysical properties that allow them to produce reactive oxygen species upon photoirradiation, which leads to cytotoxicity. These antimicrobial agents cause bacterial cell death by DNA and cytoplasmic membrane damage. This article presents a comprehensive review of photoactive antimicrobial properties of kinetically inert and labile ruthenium complexes, nanoparticles coupled photoactive ruthenium complexes, and photoactive ruthenium nanoparticles. Additionally, limitations of current ruthenium-based photoactive antimicrobial agents and future directions for the development of antibiotic-resistant photoactive antimicrobial agents are discussed. It is important to raise awareness for the ruthenium-based aPDT agents in order to develop a new class of photoactive metalloantibiotics capable of combating antibiotic resistance.

Light-Triggered Nitric Oxide Release by a Photosensitizer to Combat Bacterial Biofilm Infections

Bacterial biofilms are a serious global health concern, often responsible for persistent infections. New strategies to prevent and treat bacterial infections by eradication of the biofilms are urgently needed. A novel ruthenium-based compound is reported in this study that functions as both a boronic acid-decorated photosensitizer (PS) and a light-triggered nitric oxide (NO) releasing agent. The compound can selectively attach to the bacterial membrane and biofilms and it is highly potent at eradicating Pseudomonas aeruginosa biofilms through the simultaneous release of NO and reactive oxygen species (ROS). The compound, which is more effective than clinical antibiotic tobramycin, also has excellent bacterial specificity and shows no significant cytotoxicity to human cells. The results reveal potential applications of this innovative dual-functional photoactivated ruthenium compound to combat bacterial biofilm infections.

Visible Light Photochemical Reactions for Nucleic Acid-Based Technologies

The expanding scope of chemical reactions applied to nucleic acids has diversified the design of nucleic acid-based technologies that are essential to medicinal chemistry and chemical biology. Among chemical reactions, visible light photochemical reaction is considered a promising tool that can be used for the manipulations of nucleic acids owing to its advantages, such as mild reaction conditions and ease of the reaction process. Of late, inspired by the development of visible light-absorbing molecules and photocatalysts, visible light-driven photochemical reactions have been used to conduct various molecular manipulations, such as the cleavage or ligation of nucleic acids and other molecules as well as the synthesis of functional molecules. In this review, we describe the recent developments (from 2010) in visible light photochemical reactions involving nucleic acids and their applications in the design of nucleic acid-based technologies including DNA photocleaving, DNA photoligation, nucleic acid sensors, the release of functional molecules, and DNA-encoded libraries.

Light-Activated Rhenium Complexes with Dual Mode of Action against Bacteria

New antibiotics and innovative approaches to kill drug-resistant bacteria are urgently needed. Metal complexes offer access to alternative modes of action but have only sparingly been investigated in antibacterial drug discovery. We have developed a light-activated rhenium complex with activity against drug-resistant S. aureus and E. coli. The activity profile against mutant strains combined with assessments of cellular uptake and synergy suggest two distinct modes of action.

Ruthenium Picolinate Complex as a Redox Photosensitizer With Wide-Band Absorption

Ruthenium(II) picolinate complex, [Ru(dmb)₂(pic)]⁺ (Ru(pic); dmb = 4,4′-dimethyl-2,2′-bipyridine; Hpic = picolinic acid) was newly synthesized as a potential redox photosensitizer with a wider wavelength range of visible-light absorption compared with [Ru(N^∧N)₃]²⁺ (N^∧N = diimine ligand), which is the most widely used redox photosensitizer. Based on our investigation of its photophysical and electrochemical properties, Ru(pic) was found to display certain advantageous characteristics of wide-band absorption of visible light (λ_abs < 670 nm) and stronger reduction ability in a one-electron reduced state (E1/2red = −1.86 V vs. Ag/AgNO₃), which should function favorably in photon-absorption and electron transfer to the catalyst, respectively. Performing photocatalysis using Ru(pic) as a redox photosensitizer combined with a Re(I) catalyst reduced CO₂ to CO under red-light irradiation (λ_ex > 600 nm). TON_CO reached 235 and Φ_CO was 8.0%. Under these conditions, [Ru(dmb)₃]²⁺ (Ru(dmb)) is not capable of working as a redox photosensitizer because it does not absorb light at λ > 560 nm. Even in irradiation conditions where both Ru(pic) and Ru(dmb) absorb light (λ_ex > 500 nm), using Ru(pic) demonstrated faster CO formation (TOF_CO = 6.7 min⁻¹) and larger TON_CO (2347) than Ru(dmb) (TOF_CO = 3.6 min⁻¹; TON_CO = 2100). These results indicate that Ru(pic) is a superior redox photosensitizer over a wider wavelength range of visible-light absorption.

Mitochondria targeted and NADH triggered photodynamic activity of chloromethyl modified Ru(ii) complexes under hypoxic conditions

Three chloromethyl-modified Ru(ii) complexes were designed and synthesized as mitochondria targeting photosensitizers, which can generate carbon radicals in the presence of NADH under visible light irradiation, cause DNA cleavage and covalent binding in Ar-saturated solutions, and lead to apoptosis of human ovarian carcinoma SKOV-3 cells under hypoxic conditions (3% O2), demonstrating a new mode of type I mechanism to overcome the limitation of hypoxia in photodynamic therapy (PDT).

A mitochondria-targeted iridium(iii)-based photoacid generator induces dual-mode photodynamic damage within cancer cells

An Ir(iii)-based photoacid generator was developed as a dual-mode photodynamic therapy agent to kill cancer cells under hypoxic conditions.

Stimuli-Responsive Therapeutic Metallodrugs

The success of platinum-based anticancer agents has motivated the exploration of novel metal-based drugs for several decades, whereas problems such as drug-resistance and systemic toxicity hampered their clinical applications and efficacy. Stimuli-responsiveness of some metal complexes offers a good opportunity for designing site-specific prodrugs to maximize the therapeutic efficacy and minimize the side effect of metallodrugs. This review presents a comprehensive and up-to-date overview on the therapeutic stimuli-responsive metallodrugs that have appeared in the past two decades, where stimuli such as redox, pH, enzyme, light, temperature, and so forth were involved. The compounds are classified into three major categories based on the nature of stimuli, that is, endo-stimuli-responsive metallodrugs, exo-stimuli-responsive metallodrugs, and dual-stimuli-responsive metallodrugs. Representative examples of each type are discussed in terms of structure, response mechanism, and potential medical applications. In the end, future opportunities and challenges in this field are tentatively proposed. With diverse metal complexes being introduced, the foci of this review are pointed to platinum and ruthenium complexes.

Correlation of intracellular oxygen and cell metabolism by simultaneous PLIM of phosphorescent TLD1433 and FLIM of NAD(P)H

During photodynamic therapy (PDT), disruption of cell respiration and metabolic changes could be one of the first events. Photophysical characteristics of the photosensitizer (PS) and its specific redox potential define consumption of molecular oxygen followed by generation of reactive oxygen species. The potential PS TLD1433 is based on transition metal Ru(II) and possess an oxygen-dependent luminescence. This enables the study of oxygen consumption by PS-phosphorescence lifetime imaging (PLIM) and simultaneously changes the cellular metabolic state by nicotinamide adenine dinucleotide (NAD(P)H)-fluorescence lifetime imaging (FLIM). Within this study, localization and cellular function of TLD1433 is investigated in bladder carcinoma cells using time-resolved and confocal laser scanning microscopy. Simultaneous FLIM/PLIM of NAD(P)H and TLD1433 during PDT correlated oxygen consumption, redox state and cellular energy metabolism. Our investigations aimed to provide a personalized protocol in theranostic PDT procedures and demonstrate the potential use of TLD1433 PDT also under hypoxic conditions, which are otherwise difficult to treat.

Ruthenium(II)-Polypyridyl Compounds with π-Extended Nitrogen Donor Ligands Induce Apoptosis in Human Lung Adenocarcinoma (A549) Cells by Triggering Caspase-3/7 Pathway

Ru(II)-polypyridyl complexes exhibit antitumor properties that can be systematically tailored by means of adjusting the ligand environment. In this work, the effect of incorporating π-extended moieties into anionic N^∧O^- based chelating ligands on the cytotoxic properties of Ru compounds is explored. Four new Ru(II) complexes, [Ru(bpy)₂(dphol)][PF₆] (1; bpy = 2,2'-bipyridine, dphol = dibenzo[ a, c]phenazin-10-olate), [Ru(phen)₂(dphol)][PF₆] (2; phen = 1,10-phenanthroline), [Ru(bpy)₂(hbtz)][PF₆] (3; hbtz = 2-(benzo[ d]thiazol-2-yl)phenolate), and [Ru(phen)₂(hbtz)][PF₆] (4) were synthesized and thoroughly characterized. In vitro cytotoxicity was investigated in human lung adenocarcinoma (A549) cells, which revealed that 4 is the most cytotoxic compound (IC₅₀ = 0.8 μM) in the series including a control compound [Ru(bpy)₂(quo)][PF₆] (5; quo = 8-hydroxyquinolinate) and is nearly 8-fold more cytotoxic than cisplatin. An investigation of the mechanism of cell death led to the finding that compounds 1-4 disrupt the mitochondrial transmembrane potential (ΔΨ_m) in a concentration-dependent fashion, which is an event associated with the intrinsic pathway of apoptosis. Moreover, compound 4 triggers the activity of caspase-3/7, which eventually induces the apoptotic cellular death of A549 cells. Thus, increasing the overall lipophilicity of the Ru compounds by introducing π-extended moieties in the anionic N^∧O^- ligand is a successful strategy for realizing a new family of pro-apoptotic compounds with a [Ru^IIN₅O]⁺ coordination environment.

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.

Critical Overview of the Use of Ru(II) Polypyridyl Complexes as Photosensitizers in One-Photon and Two-Photon Photodynamic Therapy

Photodynamic Therapy (PDT) is an emerging technique to treat certain types of cancer, bacterial, fungal, and viral infections, and skin diseases. In past years, different research groups developed new ruthenium-containing photosensitizers (PSs) with tuned photophysical and biological properties to better fit the requirements of PDT. In this Account, we report and discuss the latest results in this research area, emphasizing particularly our own research. For example, inspired by the DNA intercalating complex [Ru(bpy)₂(dppz)]²⁺ (bpy = 2,2'-bipyridine; dppz = (dipyrido[3,2-a:2',3'-c]phenazine), a series of ruthenium complexes bearing differently functionalized dppz ligands were synthesized to target DNA. The introduction of the substituents on the dppz ligand did not reduce much the affinity of the complexes to DNA but highly affected their cellular uptake. The most effective complex in this series, [Ru(bpy)₂(dppz-7-OMe)]²⁺, showed IC₅₀ values in the low micromolar range against several types of cancer cells upon light irradiation and, importantly, a high phototoxic index (PI) of >150. This value is comparable to or even better than several PSs used in clinics under comparable experimental conditions. This compound was found to localize in the nucleus and to induce DNA damage in HeLa cells upon light irradiation. Interestingly, cells in the mitotic phase were found to be more affected and to have a different mechanism of cell death (apoptosis) upon light irradiation than those in the interphase (paraptosis). To take advantage of that, the PS was combined with a cell cycle inhibitor to synchronize cells in the mitotic phase, further improving the phototoxicity by a factor of 3.6. In addition, our group recently demonstrated that [Ru(bphen)₂(benzene-1,2-dislufinate)] (bphen = 4,7-diphenyl-1,10-phenanthroline) localizes in mitochondria and has an IC₅₀ value of 0.62 μM with a PI of over 80 in HeLa cells upon light irradiation at 420 nm. Interestingly, this complex was also found to efficiently kill Gram-positive Staphylococcus aureus under light irradiation. Antimicrobial PDT (aPDT) is another field of research where Ru(II) polypyridyl complexes can play an interesting role to fight antibiotics resistance. [Ru(dqpCO₂Me)(ptpy)]²⁺ (dqpCO₂Me = 4-methylcarboxy-2,6-di(quinolin-8-yl)pyridine), ptpy = 4'-phenyl-2,2':6',2″-terpyridine) is additionally efficient against Gram-negative Escherichia coli. The efficacy of positively charged Ru(II) PSs is related to their affinity to the negatively charged membrane of Gram-negative bacteria. A drawback of many Ru(II) polypyridyl PSs is their low absorption in the biological optical window (600-900 nm) where light penetration depth into tissue is the highest. The lowest energy transition in the UV/Vis spectra of Ru(II) polypyridyl complexes is usually a metal-to-ligand charge-transfer band. To shift the absorption into this range, tuning of the ligand system, for example, by extending π-systems, has been described in the literature. Another approach to make excitation in the optical biological window possible is Two-Photon Absorption (2PA). High photon density is needed and usually confocal laser beams are used for excitation. In collaboration with the Chao group, a series of homoleptic Ru(II) complexes bearing tertiary alkyl ammonium substituted bipyridine ligands with two photon cross sections between 185 and 250 GM at around 800 nm was tested in vitro. They showed IC₅₀ values in the micromolar range and PIs between 103 and 313. The highly positive-charged complexes were found to enter the cell via endocytosis and to target lysosomes. Studies on 3D tumor cell spheroids, a model closer to real tumors than commonly used 2D cell monolayers, were also performed. It could be demonstrated that 2P-PDT treatment with 800 nm laser irradiation was significantly more effective than that with 450 nm laser irradiation.