Rationally Designed Long-Wavelength Absorbing Ru(II) Polypyridyl Complexes as Photosensitizers for Photodynamic Therapy

Theranostic Rhenium(I)-Based ER-Phagy Retardant Promotes Immunogenic Cell Death

ER-phagy is a double-edged sword in the occurrence, development, and treatment of cancer; especially, its functions in immunotherapy are still unknown. In this work, we designed a theranostic Re complex (Re1) containing a BODIPY-derived ligand and a β-carboline ligand to target the endoplasmic reticulum (ER) and block ER-phagy at the late stages. Interestingly, as validated both in vitro and in vivo, ER-phagy blockage greatly enhances the capability of Re1 to induce immunogenic cell death (ICD). In summary, we dexterously fused two molecular modules for ER targeting and ER-phagy blockage into a coordination complex to afford a highly effective ICD inducer, which provides clues for designing new cancer immunotherapeutics.

Shift of cell-death mechanisms in primary human neutrophils with a ruthenium photosensitizer

Primary human neutrophils are the most abundant human white blood cells and are central for innate immunity. They act as early responders at inflammation sites, guided by chemotactic gradients to find infection or inflammation sites. Neutrophils can undergo both apoptosis as well as NETosis. NETosis is a form of neutrophil cell death that releases chromatin-based extracellular traps (NETs) to capture and neutralize pathogens. Understanding or controlling the balance between these cell-death mechanisms is crucial. In this study, the chemical synthesis and biologic assessment of a ruthenium complex as a light-activated photosensitizer that creates reactive oxygen species (ROS) in primary human neutrophils is reported. The ruthenium complex remains non-toxic in the dark. However, upon exposure to blue light at 450 nm, it exhibits potent cytotoxic effects in both cancerous and non-cancerous cell lines. Interestingly, the metal complex shifts the cell-death mechanism of primary human neutrophils from NETosis to apoptosis. Cells irradiated directly by the light source immediately undergo apoptosis, whereas those further away from the light source perform NETosis at a slower rate. This indicates that high ROS levels trigger apoptosis and lower ROS levels NETosis. The ability to control the type of cell death undergone in primary human neutrophils could have implications in managing acute and chronic infectious diseases.

A Novel Substituted Benzo[g]quinoxaline-Based Cyclometalated Ru(II) Complex as a Biocompatible Membrane-Targeted PDT Colon Cancer Stem Cell Agent

Herein, we describe and investigate biological activity of three octahedral ruthenium(II) complexes of the type [Ru(C∧N)(phen)2]+, RuL1-RuL3, containing a π-expansive cyclometalating substituted benzo[g]quinoxaline ligand (C∧N ligand) (phen = 1,10-phenanthroline). Compounds RuL1-RuL3 in cervical, melanoma, and colon human cancer cells exhibit high phototoxicity after irradiation with light (particularly blue), with the phototoxicity index reaching 100 for the complex RuL2 in most sensitive HCT116 cells. RuL2 accumulates in the cellular membranes. If irradiated, it induces lipid peroxidation, likely connected with photoinduced ROS generation. Oxidative damage to the fatty acids leads to the attenuation of the membranes, the activation of caspase 3, and the triggering of the apoptotic pathway, thus implementing membrane-localized photodynamic therapy. RuL2 is the first photoactive ruthenium-based complex capable of killing the hardly treatable colon cancer stem cells, a highly resilient subpopulation within a heterogeneous tumor mass, responsible for tumor recurrence and the metastatic progression of cancer.

Charge Engineering of Star-Shaped Organic Photosensitizers Enables Efficient Type-I Radicals for Photodynamic Therapy of Multidrug-Resistant Bacterial Infection

Infection induced by multidrug-resistant bacteria is now the second most common cause of accidental death worldwide. However, identifying a high-performance strategy with good efficiency and low toxicity is still urgently needed. Antibacterial photodynamic therapy (PDT) is considered a non-invasive and efficient approach with minimal drug resistance. Whereas, the precise molecular design for highly efficient oxygen-independent type-I photosensitizers is still undefined. In this work, the regulation of the positive charge of star-shaped NIR-emissive organic photosensitizers can boost radical generation for the efficient treatment of wounds infected with multidrug-resistant bacteria. With positive charge engineering, TPAT-DNN, which has six positive charges, mainly produces hydroxyl radicals via the type-I pathway, while TPAT-DN, which has three positive charges, tends to generate singlet oxygen and superoxide radicals. For multidrug-resistant bacteria, TPAT-DNN exhibited specific killing effects on multidrug-resistant gram-positive bacteria at low concentrations, while TPAT-DN is similar antibacterial effects on both multidrug-resistant gram-negative and gram-positive bacteria. Furthermore, the efficiency and safety of TPAT-DNN for eradicating multidrug-resistant bacteria methicillin-resistant S. aureus (MRSA) infection and accelerating wound healing in an MRSA-infected mouse model are demonstrated. This work offers a new approach toward manipulating efficient type-I photosensitizers for MRSA treatment.

Novel tris-bipyridine based Ru(II) complexes as type-I/-II photosensitizers for antitumor photodynamic therapy through ferroptosis and immunogenic cell death

Ru(II) complexes have attracted attention as photosensitizers for their promising photodynamic properties. Herein, novel tris-bipyridine based Ru(II) complexes (6a-e) were synthesized by introducing saturated heterocycles to improve photodynamic properties and lipid-water partition coefficients. Among them, 6d demonstrated significant phototoxicity towards three cancer cells, with IC50 values of 5.66-7.17 μM, exceeding values in dark (IC50s > 100 μM). Under hypoxic conditions, 6d maintained excellent photodynamic activity in A549 cells, with PI values exceeding 24, highlighting its potential for highly effective type-I/-II photodynamic therapy by inducing ROS generation, oxidative stress, and mitochondrial damage. Additionally, it induced ferroptosis and immunogenic cell death of A549 cells by regulating the expression of relevant markers. Finally, 6d remarkably inhibited the growth of A549 transplanted tumor growth by 95.4 %. This Ru(II) complex shows great potential for cancer treatment with its potent photodynamic activity and diverse mechanisms of tumor cell death.

Intrinsic 77 K Phosphorescence Characteristics and Computational Modeling of Ru(II)-(Bidentate Cyclometalated-Aromatic Ligand) Chromophores: Their Relatively Low Nonradiative Rate Constants Originating from Low Spin-Orbit Coupling Driven Vibronic Coupling Amplitudes between Emitting and Ground States

We investigated the photoinduced relaxation of Kasha-type emitting ruthenium-(bidentate cyclometalated aromatic ligand), Ru-CM, chromophores of [Ru(pzpy)2(CM)]+ ions (CM = 1-phenylisoquinoline, 2,3-diphenylpyrazine, and 1,4-diazatriphenylene and pzpy = 2-pyrazol-1-yl-pyridine). This is the first report of the phosphorescence behavior of pure Ru-(bidentate CM) chromophores. The 77 K photoinduced relaxation characteristics of phosphorescence chromophores showed emission quantum yields higher than those of reference Ru-bpy (bpy = 2,2'-bipyridine) chromophores in the emission region of 670-900 nm. This phenomenon of the Ru-CM chromophores could be attributed to their unusually low magnitudes for 77 K nonradiative rate constants (kNRD), although their radiative rate-constants (kRAD) are not remarkable. In order to examine the 77 K photoinduced behavioral relaxation difference between Ru-CM and Ru-bpy chromophores, we used computational simulation, applying the fundamental formalism of kRAD and temperature-independent kNRD equations, which included calculated spin-orbit coupling values.

Designing Quasi-Intrinsic Photosensitizers with Dual Function of Fluorescence Imaging and Photodynamic Therapy

Photosensitizers (PSs) with effective two-photon absorption in the therapeutic window are the key to two-photon photodynamic therapy. However, the traditional exogenous PSs usually lead to rejection in the body. Besides, the precise visualization of treatments proposes new demands and challenges for the design of PSs. Accordingly, in this work, a series of quasi-intrinsic PSs are obtained based on the artificial base 2-amino-8-(1'-β-d-2'-deoxyribofuranosyl)-imidazo[1,2-α]-1,3,5-triazin-4(8H)-one (P). The calculations show that the structural modification could enhance the two-photon absorption and fluorescence emission, which is beneficial for tumor localization. Furthermore, the reduced singlet-triplet energy gaps and enhanced spin-orbit coupling contribute to the rapid intersystem crossing process, which results in a triplet state with high quantum yields. To ensure the phototherapeutic performance of the newly designed PSs, we also examined the vertical electron affinity and vertical ionization potential for generation of superoxide anions, as well as the T1 energy required to produce singlet oxygen.

Rational Design of Biocompatible Ir(III) Photosensitizer to Overcome Drug-Resistant Cancer via Oxidative Autophagy Inhibition

Autophagy is a crucial quality control mechanism that degrades damaged cellular components through lysosomal fusion with autophagosomes. However, elevated autophagy levels can promote drug resistance in cancer cells, enhancing their survival. Downregulation of autophagy through oxidative stress is a clinically promising strategy to counteract drug resistance, yet precise control of oxidative stress in autophagic proteins remains challenging. Here, a molecular design strategy of biocompatible neutral Ir(III) photosensitizers is demonstrated, B2 and B4, for precise reactive oxygen species (ROS) control at lysosomes to inhibit autophagy. The underlying molecular mechanisms for the biocompatibility and lysosome selectivity of Ir(III) complexes are explored by comparing B2 with the cationic or the non-lysosome-targeting analogs. Also, the biological mechanisms for autophagy inhibition via lysosomal oxidation are explored. Proteome analyses reveal significant oxidation of proteins essential for autophagy, including lysosomal and fusion-mediator proteins. These findings are verified in vitro, using mass spectrometry, live cell imaging, and a model SNARE complex. The anti-tumor efficacy of the precise lysosomal oxidation strategy is further validated in vivo with B4, engineered for red light absorbance. This study is expected to inspire the therapeutic use of spatiotemporal ROS control for sophisticated modulation of autophagy.

Leveraging the Photofunctions of Transition Metal Complexes for the Design of Innovative Phototherapeutics

Despite the advent of various medical interventions for cancer treatment, the disease continues to pose a formidable global health challenge, necessitating the development of new therapeutic approaches for more effective treatment outcomes. Photodynamic therapy (PDT), which utilizes light to activate a photosensitizer to produce cytotoxic reactive oxygen species (ROS) for eradicating cancer cells, has emerged as a promising approach for cancer treatment due to its high spatiotemporal precision and minimal invasiveness. However, the widespread clinical use of PDT faces several challenges, including the inefficient production of ROS in the hypoxic tumor microenvironment, the limited penetration depth of light in biological tissues, and the inadequate accumulation of photosensitizers at the tumor site. Over the past decade, there has been increasing interest in the utilization of photofunctional transition metal complexes as photosensitizers for PDT applications due to their intriguing photophysical and photochemical properties. This review provides an overview of the current design strategies used in the development of transition metal complexes as innovative phototherapeutics, aiming to address the limitations associated with PDT and achieve more effective treatment outcomes. The current challenges and future perspectives on the clinical translation of transition metal complexes are also discussed.

Supramolecular H-Aggregates of Squaraines with Enhanced Type I Photosensitization for Combined Photodynamic and Photothermal Therapy

Combined photodynamic and photothermal therapy (PDT and PTT) can achieve more superior therapeutic effects than the sole mode by maximizing the photon utilization, but there remains a significant challenge in the development of related single-molecule photosensitizers (PSs), particularly those with type I photosensitization. In this study, self-assembly of squaraine dyes (SQs) is shown to be a promising strategy for designing PSs for combined type I PDT and PTT, and a supramolecular PS (TPE-SQ7) has been successfully developed through subtle molecular design of an indolenine SQ, which can self-assemble into highly ordered H-aggregates in aqueous solution as well as nanoparticles (NPs). In contrast to the typical quenching effect of H-aggregates on reactive oxygen species (ROS) generation, our results encouragingly manifest that H-aggregates can enhance type I ROS (•OH) generation by facilitating the intersystem crossing process while maintaining a high PTT performance. Consequently, TPE-SQ7 NPs with ordered H-aggregates not only exhibit superior combined therapeutic efficacy than the well-known PS (Ce6) under both normoxic and hypoxic conditions but also have excellent biosafety, making them have important application prospects in tumor phototherapy and antibacterial fields. This study not only proves that the supramolecular self-assembly of SQs is an effective strategy toward high-performance PSs for combined type I PDT and PTT but also provides a different understanding of the effect of H-aggregates on the PDT performance.

A BODIPY-Naphtholimine-BF2 Dyad for Precision Photodynamic Therapy, Targeting, and Dual Imaging of Endoplasmic Reticulum and Lipid Droplets in Cancer

Currently, effective therapeutic modalities for pancreatic ductal adenocarcinoma (PDAC) are quite limited, leading to gloomy prognosis and ∼6-months median patient survival. Recent advances showed the promise of photodynamic therapy (PDT) for PDAC patients. Next generation photosensitizers (PS) are based on "organelle-targeted-PDT" and provide new paradigm in the field of precision medicines to address the current challenge for treating PDAC. In this investigation, we have constructed a novel PS, named as N b B, for precise and simultaneous targeting of endoplasmic reticulum (ER) and lipid droplets (LDs) in PDAC, based on the fact that malignant PDAC cells are heavily relying on ER for hormone synthesis. Our live cell imaging and fluorescence recovery after photobleaching (FRAP) experiments revealed that N b B is quickly targeted to ER and subsequently to LDs and shows simultaneous dual fluorescence color due to polar and nonpolar milieu of ER and LDs. Interestingly, the same molecule generates triplet state and singlet oxygen efficiently and causes robust ER stress and cellular lipid peroxidation, leading to apoptosis in two different PDAC cells in the presence of light. Together, we present, for the first time, a potential next generation precision medicine for ER-LD organelle specific imaging and PDT of pancreatic cancer.

A Nanoencapsulated Ir(III)-Phthalocyanine Conjugate as a Promising Photodynamic Therapy Anticancer Agent

Despite the potential of photodynamic therapy (PDT) in cancer treatment, the development of efficient and photostable photosensitizing molecules that operate at long wavelengths of light has become a major hurdle. Here, we report for the first time an Ir(III)-phthalocyanine conjugate (Ir-ZnPc) as a novel photosensitizer for high-efficiency synergistic PDT treatment that takes advantage of the long-wavelength excitation and near infrared (NIR) emission of the phthalocyanine scaffold and the known photostability and high phototoxicity of cyclometalated Ir(III) complexes. In order to increase water solubility and cell membrane permeability, the conjugate and parent zinc phthalocyanine (ZnPc) were encapsulated in amphoteric redox-responsive polyurethane-polyurea hybrid nanocapsules (Ir-ZnPc-NCs and ZnPc-NCs, respectively). Photobiological evaluations revealed that the encapsulated Ir-ZnPc conjugate achieved high photocytotoxicity in both normoxic and hypoxic conditions under 630 nm light irradiation, which can be attributed to dual Type I and Type II reactive oxygen species (ROS) photogeneration. Interestingly, PDT treatments with Ir-ZnPc-NCs and ZnPc-NCs significantly inhibited the growth of three-dimensional (3D) multicellular tumor spheroids. Overall, the nanoencapsulation of Zn phthalocyanines conjugated to cyclometalated Ir(III) complexes provides a new strategy for obtaining photostable and biocompatible red-light-activated nano-PDT agents with efficient performance under challenging hypoxic environments, thus offering new therapeutic opportunities for cancer treatment.

Dinuclear Tridentate Ru(II) Complex with Strong Near-Infrared Light-Triggered Anticancer Activity

The design of the dinuclear Ru(II) complex (Ru2) with strong near-infrared (NIR) absorption properties has been reported for efficient anticancer phototherapy. Under 700 nm LED light excitation, Ru2 exhibited remarkable synergistic type I/II photosensitization ability and photocatalytic activity toward intracellular biomolecules. Ru2 showed impressive 700 nm light-triggered anticancer activity under normoxia and hypoxia compared with the clinically used photosensitizer Chlorin e6. The mechanistic studies showed that Ru2 induced intracellular redox imbalance and perturbed the energy metabolism and biosynthesis in A549 cancer cells. Overall, this work provides a new strategy for developing efficient metal-based complexes for anticancer phototherapy under NIR light.

Trinuclear ruthenium(II) polypyridyl complexes: Evaluation as photosensitizers for enhanced cervical cancer treatment

Trinuclear ruthenium(II) polypyridyl complexes anchored to benzimidazole-triazine / trisamine scaffolds were investigated as photosensitizers for photodynamic therapy. The trinuclear complexes were noted to produce a significant amount of singlet oxygen in both DMF and aqueous media, are photostable and show appreciable emission quantum yields (ɸem). In our experimental setting, despite the moderate phototoxic activity in the HeLa cervical cancer cell line, the phototoxic indices (PI) of the trinuclear complexes are superior relative to the PIs of a clinically approved photosensitizer, Photofrin®, and the pro-drug 5-aminolevulinic acid (PI: >7 relative to PI: >1 and PI: 4.4 for 5-aminolevulinic acid and Photofrin®, respectively). Furthermore, the ruthenium complexes were noted to show appreciable long-term cytotoxicity upon light irradiation in HeLa cells in a concentration-dependent manner. Consequently, this long-term activity of the ruthenium(II) polypyridyl complexes embodies their ability to reduce the probability of the recurrence of cervical cancer. Taken together, this presents a strong motivation for the development of polymetallic complexes as anticancer agents.

Aromaticity Tuning of Heavy-Atom-Free Photosensitizers for Singlet Fission-Enhanced Immunogenic Photodynamic Oncotherapy

The quantum yield of reactive oxygen species is of central importance for the development of organic photosensitizers and photodynamic therapy (PDT). A common molecular design approach for optimizing organic photosensitizers involves the incorporation of heavy atoms into their backbones. However, this raises concerns regarding heightened dark cytotoxicity and a shortened triplet-state lifetime. Herein, we demonstrate a heavy-atom-free (HAF) photosensitizer design strategy founded on the singlet fission (SF) mechanism for cancer PDT. Through the "single-atom surgery" approach to deleting oxygen atoms in pyrazino[2,3-g]quinoxaline skeleton photosensitizers, photosensitizers PhPQ and TriPhPQ are produced with Huckel's aromaticity and Baird's aromaticity in the ground state and triplet state, respectively, enabling the generation of two triplet excitons through SF. The SF process endows photosensitizer PhPQ with an ultrahigh triplet-state quantum yield (186%) and an outstanding 1O2 quantum yield (177%). Notably, HAF photosensitizers PhPQ and TriPhPQ enhanced PDT efficacy and potentiated αPD-L1 immune check blockade therapy in vivo, which show their promise for translational oncology treatment.

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).

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.

Phenazine Cations as Anticancer Theranostics†

The biological properties of two water-soluble organic cations based on polypyridyl structures commonly used as ligands for photoactive transition metal complexes designed to interact with biomolecules are investigated. A cytotoxicity screen employing a small panel of cell lines reveals that both cations show cytotoxicity toward cancer cells but show reduced cytotoxicity to noncancerous HEK293 cells with the more extended system being notably more active. Although it is not a singlet oxygen sensitizer, the more active cation also displayed enhanced potency on irradiation with visible light, making it active at nanomolar concentrations. Using the intrinsic luminescence of the cations, their cellular uptake was investigated in more detail, revealing that the active compound is more readily internalized than its less lipophilic analogue. Colocalization studies with established cell probes reveal that the active cation predominantly localizes within lysosomes and that irradiation leads to the disruption of mitochondrial structure and function. Stimulated emission depletion (STED) nanoscopy and transmission electron microscopy (TEM) imaging reveal that treatment results in distinct lysosomal swelling and extensive cellular vacuolization. Further imaging-based studies confirm that treatment with the active cation induces lysosomal membrane permeabilization, which triggers lysosome-dependent cell-death due to both necrosis and caspase-dependent apoptosis. A preliminary toxicity screen in the Galleria melonella animal model was carried out on both cations and revealed no detectable toxicity up to concentrations of 80 mg/kg. Taken together, these studies indicate that this class of synthetically easy-to-access photoactive compounds offers potential as novel therapeutic leads.

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.

Green Light-Triggered Photocatalytic Anticancer Activity of Terpyridine-Based Ru(II) Photocatalysts

The relentless increase in drug resistance of platinum-based chemotherapeutics has opened the scope for other new cancer therapies with novel mechanisms of action (MoA). Recently, photocatalytic cancer therapy, an intrusive catalytic treatment, is receiving significant interest due to its multitargeting cell death mechanism with high selectivity. Here, we report the synthesis and characterization of three photoresponsive Ru(II) complexes, viz., [Ru(ph-tpy)(bpy)Cl]PF6 (Ru1), [Ru(ph-tpy)(phen)Cl]PF6 (Ru2), and [Ru(ph-tpy)(aip)Cl]PF6 (Ru3), where, ph-tpy = 4'-phenyl-2,2':6',2″-terpyridine, bpy = 2,2'-bipyridine, phen = 1,10-phenanthroline, and aip = 2-(anthracen-9-yl)-1H-imidazo[4,5-f][1,10] phenanthroline, showing photocatalytic anticancer activity. The X-ray crystal structures of Ru1 and Ru2 revealed a distorted octahedral geometry with a RuN5Cl core. The complexes showed an intense absorption band in the 440-600 nm range corresponding to the metal-to-ligand charge transfer (MLCT) that was further used to achieve the green light-induced photocatalytic anticancer effect. The mitochondria-targeting photostable complex Ru3 induced phototoxicity with IC50 and PI values of ca. 0.7 μM and 88, respectively, under white light irradiation and ca. 1.9 μM and 35 under green light irradiation against HeLa cells. The complexes (Ru1-Ru3) showed negligible dark cytotoxicity toward normal splenocytes (IC50s > 50 μM). The cell death mechanistic study revealed that Ru3 induced ROS-mediated apoptosis in HeLa cells via mitochondrial depolarization under white or green light exposure. Interestingly, Ru3 also acted as a highly potent catalyst for NADH photo-oxidation under green light. This NADH photo-oxidation process also contributed to the photocytotoxicity of the complexes. Overall, Ru3 presented multitargeting synergistic type I and type II photochemotherapeutic effects.

Investigation of the Nanoparticulation Method and Cell-Killing Effect following the Mitochondrial Delivery of Hydrophobic Porphyrin-Based Photosensitizers

Photodynamic therapy is expected to be a less invasive treatment, and strategies for targeting mitochondria, the main sources of singlet oxygen, are attracting attention to increase the efficacy of photodynamic therapy and reduce its side effects. To date, we have succeeded in encapsulating the photosensitizer rTPA into MITO-Porter (MP), a mitochondria-targeted Drug Delivery System (DDS), aimed at mitochondrial delivery of the photosensitizer while maintaining its activity. In this study, we report the results of our studies to alleviate rTPA aggregation in an effort to improve drug efficacy and assess the usefulness of modifying the rTPA side chain to improve the mitochondrial retention of MITO-Porter, which exhibits high therapeutic efficacy. Conventional rTPA with anionic side chains and two rTPA analogs with side chains that were converted to neutral or cationic side chains were encapsulated into MITO-Porter. Low-MP (MITO-Porter with Low Drug/Lipid) exhibited high drug efficacy for all three types of rTPA, and in Low-MP, charged rTPA-encapsulated MP exhibited high drug efficacy. The cellular uptake and mitochondrial translocation capacities were similar for all particles, suggesting that differences in aggregation rates during the incorporation of rTPA into MITO-Porter resulted in differences in drug efficacy.

Photoactivatable Ruthenium Complexes Containing Minimal Straining Benzothiazolyl-1,2,3-triazole Chelators for Cancer Treatment

Ruthenium(II) complexes containing diimine ligands have contributed to the development of agents for photoactivated chemotherapy. Several approaches have been used to obtain photolabile Ru(II) complexes. The two most explored have been the use of monodentate ligands and the incorporation of steric effects between the bidentate ligands and the Ru(II). However, the introduction of electronic effects in the ligands has been less explored. Herein, we report a systematic experimental, theoretical, and photocytotoxicity study of a novel series of Ru(II) complexes Ru1-Ru5 of general formula [Ru(phen)2(N∧N')]2+, where N∧N' are different minimal strained ligands based on the 1-aryl-4-benzothiazolyl-1,2,3-triazole (BTAT) scaffold, being CH3 (Ru1), F (Ru2), CF3 (Ru3), NO2 (Ru4), and N(CH3)2 (Ru5) substituents in the R4 of the phenyl ring. The complexes are stable in solution in the dark, but upon irradiation in water with blue light (λex = 465 nm, 4 mW/cm2) photoejection of the ligand BTAT was observed by HPLC-MS spectrometry and UV-vis spectroscopy, with t1/2 ranging from 4.5 to 14.15 min depending of the electronic properties of the corresponding BTAT, being Ru4 the less photolabile (the one containing the more electron withdrawing substituent, NO2). The properties of the ground state singlet and excited state triplet of Ru1-Ru5 have been explored using density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations. A mechanism for the photoejection of the BTAT ligand from the Ru complexes, in H2O, is proposed. Phototoxicity studies in A375 and HeLa human cancer cell lines showed that the new Ru BTAT complexes were strongly phototoxic. An enhancement of the emission intensity of HeLa cells treated with Ru5 was observed in response to increasing doses of light due to the photoejection of the BTAT ligand. These studies suggest that BTAT could serve as a photocleavable protecting group for the cytotoxic bis-aqua ruthenium warhead [Ru(phen)2(OH2)2]2+.

Localization of Cancer Cells for Subsequent Robust Photodynamic Therapy by ROS Responsive Polymeric Nanoparticles With Anti-Metastasis Complexes NAMI-A

Photodynamic therapy (PDT), as a new type of light-mediated reactive oxygen species (ROS) cancer therapy, has the advantages of high therapeutic efficiency, non-resistance, and less trauma than traditional cancer therapy such as surgery, radiotherapy, and chemotherapy. However, oxygen-dependent PDT further exacerbates tumor metastasis. To this end, a strategy that circumvents tumor metastasis to improve the therapeutic efficacy of PDT is proposed. Herein, a near-infrared light-activated photosensitive polymer is synthesized and branched the anti-metastatic ruthenium complex NAMI-A on the side, which is further assembled to form nanoparticles (NP2) for breast cancer therapy. NP2 can kill tumor cells by generating ROS under 808 nm radiation (NP2 + L), reduce the expression of matrix metalloproteinases (MMP2/9) in cancer cells, decrease the invasive and migration capacity of cancer cells, and eliminate cancer cells. Further animal experiments show that NP2 + L can inhibit tumor growth and reduce liver and lung metastases. In addition, NP2 + L can activate the immune system in mice to avoid tumor recurrence. In conclusion, a PDT capable of both preventing tumor metastasis and precisely hitting the primary tumor to achieve effective treatment of highly metastatic cancers is developed.

MOF@COF Nanocapsules Enhance Soft Tissue Sarcoma Treatment: Synergistic Effects of Photodynamic Therapy and PARP Inhibition on Tumor Growth Suppression and Immune Response Activation

Soft tissue sarcomas (STS) are highly malignant tumors with limited treatment options owing to their heterogeneity and resistance to conventional therapies. Photodynamic therapy (PDT) and poly-ADP-ribose polymerase (PARP) inhibitors (PARPi) have shown potential for STS treatment, with PDT being effective for sarcomas located on the extremities and body surface and PARPi targeting defects in homologous recombination repair. To address the limitations of PDT and harness the potential of PARPi, herein, a novel therapeutic approach for STS treatment combining nanocapsules bearing integrated metal-organic frameworks (MOFs) and covalent organic frameworks (COFs), i.e., MOF@COF, with PDT and PARPi is proposed. Nanocapsules are designed, referred to as ZTN@COF@poloxamer, which contain a Zr-based MOF and tetrakis (4-carbethoxyphenyl) porphyrin as a photosensitizer, are coated with a COF to improve the sensitizing properties, and are loaded with niraparib to inhibit DNA repair. Experiments demonstrate that this new nanocapsules treatment significantly inhibits STS growth, promotes tumor cell apoptosis, exhibits high antitumor activity with minimal side effects, activates the immune response of the tumor, and inhibits lung metastasis in vivo. Therefore, MOF@COF nanocapsules combined with PARPi offer a promising approach for STS treatment, with the potential to enhance the efficacy of PDT and prevent tumor recurrence.

Probing a Major DNA Weakness: Resolving the Groove and Sequence Selectivity of the Diimine Complex Λ-[Ru(phen)2 phi]2

The grooves of DNA provide recognition sites for many nucleic acid binding proteins and anticancer drugs such as the covalently binding cisplatin. Here we report a crystal structure showing, for the first time, groove selectivity by an intercalating ruthenium complex. The complex Λ-[Ru(phen)2 phi]2+ , where phi=9,10-phenanthrenediimine, is bound to the DNA decamer duplex d(CCGGTACCGG)2 . The structure shows that the metal complex is symmetrically bound in the major groove at the central TA/TA step, and asymmetrically bound in the minor groove at the adjacent GG/CC steps. A third type of binding links the strands, in which each terminal cytosine base stacks with one phen ligand. The overall binding stoichiometry is four Ru complexes per duplex. Complementary biophysical measurements confirm the binding preference for the Λ-enantiomer and show a high affinity for TA/TA steps and, more generally, TA-rich sequences. A striking enantiospecific elevation of melting temperatures is found for oligonucleotides which include the TATA box sequence.

Combining Multiple Photosensitizer Modules into One Supramolecular System for Synergetic Enhanced Photodynamic Therapy

Photodynamic therapy (PDT), as an emerging cancer treatment, requires the development of highly desirable photosensitizers (PSs) with integrated functional groups to achieve enhanced therapeutic efficacy. Coordination-driven self-assembly (CDSA) would provide an alternative approach for combining multiple PSs synergistically. Here, we demonstrate a simple yet powerful strategy of combining conventional chromophores (tetraphenylethylene, porphyrin, or Zn-porphyrin) with pyridinium salt PSs together through condensation reactions, followed by CDSA to construct a series of novel metallo-supramolecular PSs (S1-S3). The generation of reactive oxygen species (ROS) is dramatically enhanced by the direct combination of two different PSs, and further reinforced in the subsequent ensembles. Among all the ensembles, S2 with two porphyrin cores shows the highest ROS generation efficiency, specific interactions with lysosome, and strong emission for probing cells. Moreover, the cellular and living experiments confirm that S2 has excellent PDT efficacy, biocompatibility, and biosafety. As such, this study will enable the development of more efficient PSs with potential clinical applications.

A photo-degradable BODIPY-modified Ru(II) photosensitizer for safe and efficient PDT under both normoxic and hypoxic conditions

Photodynamic therapy (PDT) is promising for cancer treatment but still suffers from some limitations. For instance, PDT based on 1O2 generation (in a type-II mechanism) is heavily dependent on high oxygen concentrations and will be significantly depressed in hypoxic tumors. In addition, the residual photosensitizers after PDT treatment may cause severe side-effects under light irradiation. To solve these problems, herein a BODIPY (boron dipyrromethene)-modified Ru(II) complex [Ru(dip)2(tpy-BODIPY)]2+ (complex 1, dip = 4,7-diphenyl-1,10-phenanthroline, tpy = 2,2':6',2''-terpyridine) was designed and synthesized. Complex 1 exhibited both high singlet oxygen quantum yield (Φ = 0.7 in CH3CN) and excellent superoxide radical (O2˙-) generation, and thus demonstrated efficient PDT activity under both normoxic and hypoxic conditions. Moreover, complex 1 is photo-degradable in water, and greatly loses its ROS generation ability after PDT treatment. These novel properties of complex 1 make it promising for efficient PDT under both normoxic and hypoxic conditions with reduced side-effects.

Review of three-dimensional spheroid culture models of gynecological cancers for photodynamic therapy research

Photodynamic therapy (PDT) is a specific cancer treatment with minimal side effects. However, it remains challenging to apply PDT clinically, partially due to the difficulty of translating research findings to clinical settings as the conventional 2D cell models used for in vitro research are accepted as less physiologically relevant to a solid tumour. 3D spheroids offer a better model for testing PDT mechanisms and efficacy, particularly on photosensitizer uptake, cellular and subcellular distribution and interaction with cellular oxygen consumption. 3D spheroids are usually generated by scaffold-free and scaffold-based methods and are accepted as physiologically relevant models for PDT anticancer research. Scaffold-free methods offer researchers advantages including high efficiency, reproducible, and controlled microenvironment. While the scaffold-based methods offer an extracellular matrix-like 3D scaffold with the necessary architecture and chemical mediators to support the spheroid formation, the natural scaffold used may limit its usage because of low reproducibility due to patch-to-patch variation. Many studies show that the 3D spheroids do offer advantages to gynceologcial cancer PDT investigation. This article will provide a review of the applications of 3D spheroid culture models for the PDT research of gynaecological cancers.

Novel 2-(5-Arylthiophen-2-yl)-benzoazole Cyclometalated Iridium(III) dppz Complexes Exhibit Selective Phototoxicity in Cancer Cells by Lysosomal Damage and Oncosis

A second-generation series of biscyclometalated 2-(5-aryl-thienyl)-benzimidazole and -benzothiazole Ir(III) dppz complexes [Ir(C^N)2(dppz)]+, Ir1-Ir4, were rationally designed and synthesized, where the aryl group attached to the thienyl ring was p-CF3C6H4 or p-Me2NC6H4. These new Ir(III) complexes were assessed as photosensitizers to explore the structure-activity correlations for their potential use in biocompatible anticancer photodynamic therapy. When irradiated with blue light, the complexes exhibited high selective potency across several cancer cell lines predisposed to photodynamic therapy; the benzothiazole derivatives (Ir1 and Ir2) were the best performers, Ir2 being also activatable with green or red light. Notably, when irradiated, the complexes induced leakage of lysosomal content into the cytoplasm of HeLa cancer cells and induced oncosis-like cell death. The capability of the new Ir complexes to photoinduce cell death in 3D HeLa spheroids has also been demonstrated. The investigated Ir complexes can also catalytically photo-oxidate NADH and photogenerate 1O2 and/or •OH in cell-free media.

Unprecedented enantio-selective live-cell mitochondrial DNA super-resolution imaging and photo-sensitizing by the chiral ruthenium polypyridyl DNA "light-switch"

Mitochondrial DNA (mtDNA) is known to play a critical role in cellular functions. However, the fluorescent probe enantio-selectively targeting live-cell mtDNA is rare. We recently found that the well-known DNA 'light-switch' [Ru(phen)2dppz]Cl2 can image nuclear DNA in live-cells with chlorophenolic counter-anions via forming lipophilic ion-pairing complex. Interestingly, after washing with fresh-medium, [Ru(phen)2dppz]Cl2 was found to re-localize from nucleus to mitochondria via ABC transporter proteins. Intriguingly, the two enantiomers of [Ru(phen)2dppz]Cl2 were found to bind enantio-selectively with mtDNA in live-cells not only by super-resolution optical microscopy techniques (SIM, STED), but also by biochemical methods (mitochondrial membrane staining with Tomo20-dronpa). Using [Ru(phen)2dppz]Cl2 as the new mtDNA probe, we further found that each mitochondrion containing 1-8 mtDNA molecules are distributed throughout the entire mitochondrial matrix, and there are more nucleoids near nucleus. More interestingly, we found enantio-selective apoptotic cell death was induced by the two enantiomers by prolonged visible light irradiation, and in-situ self-monitoring apoptosis process can be achieved by using the unique 'photo-triggered nuclear translocation' property of the Ru complex. This is the first report on enantio-selective targeting and super-resolution imaging of live-cell mtDNA by a chiral Ru complex via formation and dissociation of ion-pairing complex with suitable counter-anions.

Research Progress of Metal Anticancer Drugs

Cancer treatments, including traditional chemotherapy, have failed to cure human malignancies. The main reasons for the failure of these treatments are the inevitable drug resistance and serious side effects. In clinical treatment, only 5 percent of the 50 percent of cancer patients who are able to receive conventional chemotherapy survive. Because of these factors, being able to develop a drug and treatment that can target only cancer cells without affecting normal cells remains a big challenge. Since the special properties of cisplatin in the treatment of malignant tumors were accidentally discovered in the last century, metal anticancer drugs have become a research hotspot. Metal anticancer drugs have unique pharmaceutical properties, such as ruthenium metal drugs with their high selectivity, low toxicity, easy absorption by tumor tissue, excretion, and so on. In recent years, efficient and low-toxicity metal antitumor complexes have been synthesized. In this paper, the scientific literature on platinum (Pt), ruthenium (Ru), iridium (Ir), gold (Au), and other anticancer complexes was reviewed by referring to a large amount of relevant literature at home and abroad.

A dipyridophenazine Ni(II) dithiolene complex as a dual-acting cancer phototherapy agent activatable within the phototherapeutic window

A combination of photodynamic therapy (PDT) and photothermal therapy (PTT) within the phototherapeutic window (600-900 nm) can lead to significantly enhanced therapeutic outcomes, surpassing the efficacy observed with PDT or PTT alone in cancer phototherapy. Herein, we report a novel small-molecule mixed-ligand Ni(II)-dithiolene complex (Ni-TDD) with a dipyridophenazine ligand, demonstrating potent red-light PDT and significant near-infrared (NIR) light mild-temperature PTT activity against cancer cells and 3D multicellular tumour spheroids (MCTSs). The four-coordinate square planar complex exhibited a moderately intense absorption band (ε ∼ 3700 M-1cm-1) centered around 900 nm and demonstrated excellent dark and photostability in an aqueous phase. Ni-TDD induced a potent red-light (600-720 nm) PDT effect on HeLa cancer cells (IC50 = 1.8 μM, photo irritation factor = 44), triggering apoptotic cell death through efficient singlet oxygen generation. Ni-TDD showed a significant intercalative binding affinity towards double-helical calf thymus DNA, resulting in a binding constant (Kb) ∼ 106 M-1. The complex induced mild hyperthermia and exerted a significant mild-temperature PTT effect on MDA-MB-231 cancer cells upon irradiation with 808 nm NIR light. Simultaneous irradiation of Ni-TDD-treated HeLa MCTSs with red and NIR light led to a remarkable synergistic inhibition of growth, exceeding the effects of individual irradiation, through the generation of singlet oxygen and mild hyperthermia. Ni-TDD displayed minimal toxicity towards non-cancerous HPL1D and L929 cells, even at high micromolar concentrations. This is the first report of a Ni(II) complex demonstrating red-light PDT activity and the first example of a first-row transition metal complex exhibiting combined PDT and PTT effects within the clinically relevant phototherapeutic window. Our findings pave the way for designing and developing metal-dithiolene complexes as dual-acting cancer phototherapy agents using long wavelength light for treating solid tumors.

Structure-based biological investigations on ruthenium complexes containing 2,2'-bipyridine ligands and their applications in photodynamic therapy as a potential photosensitizer

Ruthenium complexes have been investigated for various biological applications by virtue of their radical scavenging, DNA binding, receptor binding, and cytotoxic abilities; especially the possible potential application of these complexes in photodynamic therapy (PDT). This study focuses on the synthesis, structural characterization and biological application (pertaining to its cytotoxicity and radical generation) of ruthenium complexed with salicylaldehyde fumaryl-dihydrazone (slfhH4 ), salicylaldehyde glutaryl-di-hydrazone (slfgH4 ) and 2,2'-bipyridine (bpy). During the synthesis, the anticipated complex was precipitated out but as serendipity, Ruthenium(II) tris (2,2'-bipyridyl) monochloride nonahydrate {[Ru(bpy)3 ]2+ .Cl.9H2 O} (RBMN) and Ruthenium(II) tris (2,2'-bipyridyl) monochloride septahydrate {[Ru(bpy)3 ]2+ .Cl.7H2 O}(RBMS) were crystallized from the filtrate. The crystal structure of complexes RBMN and RBMS were determined by a single-crystal X-ray diffraction methods and it showed that chlorine anion lies at the crystallographic axis and forms a halogen hydrogen-bonded organic framework (XHOF) to provide the stability. In comparison with similar structures in Cambridge Crystallographic Data Center (CCDC) revealed that the nature of the XHOF framework and the layered packing are conserved. The compounds showed excellent cytotoxic ability (against L6 cells) and the nitro blue tetrazolium (NBT) assay upon irradiation to light revealed its ability to produce reactive oxygen species (ROS). The presence of partially occupied water molecules in the layered organization within the crystal packing mimics the release of ROS resulting in cytotoxicity. The structural results together with the biological data make these complexes interesting candidates for potential photosensitizers for PDT applications.

Ruthenium(II) Polypyridyl-Based Photocages for an Anticancer Phytochemical Diallyl Sulfide: Comparative Dark and Photoreactivity Studies of Caged and Precursor Uncaged Complexes

The spatiotemporal control over the drug's action offered by ruthenium(II) polypyridyl complexes by the selective activation of the prodrug inside the tumor has beaconed toward much-desired selectivity issues in cancer chemotherapy. The photocaging of anticancer bioactive ligands attached synergistically with cytotoxic Ru(II) polypyridyl cores and selective release thereof in cancer cells are a promising modality for more effective drug action. Diallyl sulfide (DAS) naturally found in garlic has anticancer, antioxidant, and anti-inflammatory activities. Herein, we designed two Ru(II) polypyridyl complexes to cage DAS having a thioether-based donor site. For in-depth photocaging studies, we compared the reactivity of the DAS-caged compounds with the uncaged Ru(II)-complexes with the general formula [Ru(ttp)(NN)(L)]+/2+. Here, in the first series, ttp = p-tolyl terpyridine, NN = phen (1,10-phenanthroline), and L = Cl- (1-Cl) and H2O (1-H2O), while for the second series, NN = dpq (pyrazino[2,3-f][1,10]phenanthroline), and L = Cl- (2-Cl) and H2O (2-H2O). The reaction of DAS with 1-H2O and 2-H2O yielded the caged complexes [Ru(ttp)(NN)(DAS)](PF6)2, i.e., 1-DAS and 2-DAS, respectively. The complexes were structurally characterized by X-ray crystallography, and the solution-state characterization was done by 1H NMR and ESI-MS studies. Photoinduced release of DAS from the Ru(II) core was monitored by 1H NMR and UV-vis spectroscopy. When irradiated with a 470 nm blue LED in DMSO, the photosubstitution quantum yields (Φ) of 0.035 and 0.057 were observed for 1-DAS and 2-DAS, respectively. Intriguing solution-state speciation and kinetic behaviors of the uncaged and caged Ru(II)-complexes emerged from 1H NMR studies in the dark, and they are depicted in this work. The caged 1-DAS and 2-DAS complexes remained mostly structurally intact for a reasonably long period in DMSO. The uncaged 1-Cl and 2-Cl complexes, although did not undergo substitution in only DMSO but in the 10% DMSO/H2O mixture, completely converted to the corresponding DMSO-adduct within 16 h. Toward gaining insights into the reactivity with the biological targets, we observed that 1-Cl upon hydrolysis formed an adduct with 5'-GMP, while a small amount of GSSG-adduct was observed when 1-Cl was reacted with GSH in H2O at 323 K. 1-Cl after hydrolysis reacted with l-methionine, although the rate was slightly slower compared with that with DMSO, suggesting varying reaction kinetics with different sulfur-based linkages. Although 1-H2O reacted with sulfoxide and thioether ligands at room temperature, the rate was much faster at higher temperatures obviously, and thiol-based systems needed higher thermal energy for conjugation. Overall, these studies provide insight for thoughtful design of new generation Ru(II) polypyridyl complexes for caging suitable bioactive organic molecules.

Increasing the π-Expansive Ligands in Ruthenium(II) Polypyridyl Complexes: Synthesis, Characterization, and Biological Evaluation for Photodynamic Therapy Applications

Lack of selectivity is one of the main issues with currently used chemotherapies, causing damage not only to altered cells but also to healthy cells. Over the last decades, photodynamic therapy (PDT) has increased as a promising therapeutic tool due to its potential to treat diseases like cancer or bacterial infections with a high spatiotemporal control. Ruthenium(II) polypyridyl compounds are gaining attention for their application as photosensitizers (PSs) since they are generally nontoxic in dark conditions, while they show remarkable toxicity after light irradiation. In this work, four Ru(II) polypyridyl compounds with sterically expansive ligands were studied as PDT agents. The Ru(II) complexes were synthesized using an alternative route to those described in the literature, which resulted in an improvement of the synthesis yields. Solid-state structures of compounds [Ru(DIP)2phen]Cl2 and [Ru(dppz)2phen](PF6)2 have also been obtained. It is well-known that compound [Ru(dppz)(phen)2]Cl2 binds to DNA by intercalation. Therefore, we used [Ru(dppz)2phen]Cl2 as a model for DNA interaction studies, showing that it stabilized two different sequences of duplex DNA. Most of the synthesized Ru(II) derivatives showed very promising singlet oxygen quantum yields, together with noteworthy photocytotoxic properties against two different cancer cell lines, with IC50 in the micro- or even nanomolar range (0.06-7 μM). Confocal microscopy studies showed that [Ru(DIP)2phen]Cl2 and [Ru(DIP)2TAP]Cl2 accumulate preferentially in mitochondria, while no mitochondrial internalization was observed for the other compounds. Although [Ru(dppn)2phen](PF6)2 did not accumulate in mitochondria, it interestingly triggered an impairment in mitochondrial respiration after light irradiation. Among others, [Ru(dppn)2phen](PF6)2 stands out for its very good IC50 values, correlated with a very high singlet oxygen quantum yield and mitochondrial respiration disruption.

Towards Ruthenium(II)-Rhenium(I) Binuclear Complexes as Photosensitizers for Photodynamic Therapy

The search for new metal-based photosensitizers (PSs) for anticancer photodynamic therapy (PDT) is a fast-developing field of research. Knowing that polymetallic complexes bear a high potential as PDT PSs, in this study, we aimed at combining the known photophysical properties of a rhenium(I) tricarbonyl complex and a ruthenium(II) polypyridyl complex to prepare a ruthenium-rhenium binuclear complex that could act as a PS for anticancer PDT. Herein, we present the synthesis and characterization of such a system and discuss its stability in aqueous solution. In addition, one of our complexes prepared, which localized in mitochondria, was found to have some degree of selectivity towards two types of cancerous cells: human lung carcinoma A549 and human colon colorectal adenocarcinoma HT29, with interesting photo-index (PI) values of 135.1 and 256.4, respectively, compared to noncancerous retinal pigment epithelium RPE1 cells (22.4).

In Situ Bioconjugation of a Maleimide-Functionalized Ruthenium-Based Photosensitizer to Albumin for Photodynamic Therapy

Maleimide-containing prodrugs can quickly and selectively react with circulating serum albumin following their injection in the bloodstream. The drug-albumin complex then benefits from longer blood circulation times and better tumor accumulation. Herein, we have applied this strategy to a previously reported highly phototoxic Ru polypyridyl complex-based photosensitizer to increase its accumulation at the tumor, reduce off-target cytotoxicity, and therefore improve its pharmacological profile. Specifically, two complexes were synthesized bearing a maleimide group: one complex with the maleimide directly incorporated into the bipyridyl ligand, and the other has a hydrophilic linker between the ligand and the maleimide group. Their interaction with albumin was studied in-depth, revealing their ability to efficiently bind both covalently and noncovalently to the plasma protein. A crucial finding is that the maleimide-functionalized complexes exhibited significantly lower cytotoxicity in noncancerous cells under dark conditions compared to the nonfunctionalized complex, which is a highly desirable property for a photosensitizer. The binding to albumin also led to a decrease in the phototoxicity of the Ru bioconjugates in comparison to the nonfunctionalized complex, probably due to a decreased cellular uptake. Unfortunately, this decrease in phototoxicity was not compensated by a dramatic increase in tumor accumulation, as was demonstrated in a tumor-bearing mouse model using inductively coupled plasma mass spectrometry (ICP-MS) studies. Consequently, this study provides valuable insight into the future design of in situ albumin-binding complexes for photodynamic therapy in order to maximize their effectiveness and realize their full potential.

Metal-Based Photosensitizers as Inducers of Regulated Cell Death Mechanisms

Over the last few decades, various forms of regulated cell death (RCD) have been discovered and were found to improve cancer treatment. Although there are several reviews on RCD induced by photodynamic therapy (PDT), a comprehensive summary covering metal-based photosensitizers (PSs) as RCD inducers has not yet been presented. In this review, we systematically summarize the works on metal-based PSs that induce different types of RCD, including ferroptosis, immunogenic cell death (ICD), and pyroptosis. The characteristics and mechanisms of each RCD are explained. At the end of each section, a summary of the reported commonalities between different metal-based PSs inducing the same RCD is emphasized, and future perspectives on metal-based PSs inducing novel forms of RCD are discussed at the end of the review. Considering the essential roles of metal-based PSs and RCD in cancer therapy, we hope that this review will provide the stage for future advances in metal-based PSs as RCD inducers.

Phototoxicity of Tridentate Ru(II) Polypyridyl Complex with Expanded Bite Angles toward Mammalian Cells and Multicellular Tumor Spheroids

Tridentate ligand-coordinated ruthenium (II) polypyridyl complexes with large N-Ru-N bite angles have been shown to promote ligand field splitting and reduce singlet-triplet state mixing leading to dramatically extended emission quantum yields and lifetimes under ambient conditions. These effects are anticipated to enhance their photoinduced singlet oxygen production, promoting prospects for such complexes as type II phototherapeutics. In this contribution, we examined this putative effect for [Ru(bqp)(bqpCOOEt)]2+, Ru-bqp-ester, a heteroleptic complex containing bqp = [2,6-bi(quinolin-8-yl)pyridine], a well-established large bite angle tridentate ligand, as well as its peptide conjugates [Ru(bqp)(bqpCONH-ahx-FrFKFrFK(Ac)-CONH2)]5+ (Ru-bqp-MPP) and [Ru(bqp) (bqp)(CONH-ahx-RRRRRRRR-CONH2)]10+ (Ru-bqp-R8) that were prepared in an effort to promote live cell/tissue permeability and targeting of the parent. Membrane permeability of both parent and peptide conjugates were compared across 2D cell monolayers; A549, Chinese hamster ovary, human pancreatic cancer (HPAC), and 3D HPAC multicellular tumor spheroids (MCTS) using confocal microscopy. Both the parent complex and peptide conjugates showed exceptional permeability with rapid uptake in both 2D and 3D cell models but with little distinction in permeability or distribution in cells between the parent or peptide conjugates. Unexpectedly, the uptake was temperature independent and so attributed to passive permeation. Both dark and photo-toxicity of the Ru(II) complexes were assessed across cell types, and the parent showed notably low dark toxicity. In contrast, the parent and conjugates were found to be highly phototoxic, with impressive phototoxic indices (PIs) toward HPAC cell monolayers in particular, with PI values ranging from ∼580 to 760. Overall, our data indicate that the Ru(II) parent complex and its peptide conjugates show promise at both cell monolayers and 3D MCTS as photosensitizers for photodynamic therapy.

Encapsulation of Ru(II) Polypyridine Complexes for Tumor-Targeted Anticancer Therapy

Ru(II) polypyridine complexes have attracted much attention as anticancer agents because of their unique photophysical, photochemical, and biological properties. Despite their promising therapeutic profile, the vast majority of compounds are associated with poor water solubility and poor cancer selectivity. Among the different strategies employed to overcome these pharmacological limitations, many research efforts have been devoted to the physical or covalent encapsulation of the Ru(II) polypyridine complexes into nanoparticles. This article highlights recent developments in the design, preparation, and physicochemical properties of Ru(II) polypyridine complex-loaded nanoparticles for their potential application in anticancer therapy.

Synthesis and Characterization of a Luminescent Cyclic Poly(ethylene oxide)-Polypyridyl Ruthenium Complex

We report the synthesis of a macrocyclic poly(ethylene oxide) (PEO) connected by one [Ru(bpy)₃]²⁺ unit (where bpy = 2,2'-bipyridine), a photoactive metal complex that provides photosensitivity and potential biomedical applications to this polymer structure. The PEO chain provides biocompatibility, water solubility, and topological play. The macrocycles were successfully synthesized by copper-free click cycloaddition between a bifunctional dibenzocyclooctyne (DBCO)-PEO precursor and 4,4'-diazido-2,2'-bipyridine, followed by complexation with [Ru(bpy)₂Cl₂]. The cyclic product accumulated efficiently in MCF7 cancer cells and exhibited a longer fluorescence lifetime than its linear analogue, likely due to differences in the accessibility of the ligand-centered/intraligand states of Ru polypyridyls in both topologies.

In vivo metallophilic self-assembly of a light-activated anticancer drug

Self-assembling molecular drugs combine the easy preparation typical of small-molecule chemotherapy and the tumour-targeting properties of drug-nanoparticle conjugates. However, they require a supramolecular interaction that survives the complex environment of a living animal. Here we report that the metallophilic interaction between cyclometalated palladium complexes generates supramolecular nanostructures in living mice that have a long circulation time (over 12 h) and efficient tumour accumulation rate (up to 10.2% of the injected dose per gram) in a skin melanoma tumour model. Green light activation leads to efficient tumour destruction due to the type I photodynamic effect generated by the self-assembled palladium complexes, as demonstrated in vitro by an up to 96-fold cytotoxicity increase upon irradiation. This work demonstrates that metallophilic interactions are well suited to generating stable supramolecular nanotherapeutics in vivo with exceptional tumour-targeting properties.

Near-infrared metal agents assisting precision medicine: from strategic design to bioimaging and therapeutic applications

Metal agents have made incredible strides in preclinical research and clinical applications in recent years, but their short emission/absorption wavelengths continue to be a barrier to their distribution, therapeutic action, visual tracking, and efficacy evaluation. Nowadays, the near-infrared window (NIR, 650-1700 nm) provides a more accurate imaging and treatment option. Thus, there has been ongoing research focusing on developing multifunctional NIR metal agents for imaging and therapy that have deeper tissue penetration. The design, characteristics, bioimaging, and therapy of NIR metal agents are covered in this overview of papers and reports published to date. To start with, we focus on describing the structure, design strategies, and photophysical properties of metal agents from the NIR-I (650-1000 nm) to NIR-II (1000-1700 nm) region, in order of molecular metal complexes (MMCs), metal-organic complexes (MOCs), and metal-organic frameworks (MOFs). Next, the biomedical applications brought by these superior photophysical and chemical properties for more accurate imaging and therapy are discussed. Finally, we explore the challenges and prospects of each type of NIR metal agent for future biomedical research and clinical translation.

Critical PDT Theory VII: The Saga of Ruthenium

Recent studies involving photosensitizing agents containing ruthenium (Ru) are interpreted as an indication that this approach might be useful for treatment of bladder cancer. The absorbance spectrum of such agents tends to be limited to wavelengths < 600 nm. While this can spare underlying tissues from photodamage, this will limit applications to instances where only a thin layer of malignant cells is present. Among the more potentially interesting results is a protocol that uses only Ru nanoparticles. Other issues in Ru-based photodynamic therapy are discussed including the limited absorbance spectrum, questions relating to methodology and a general lack of details concerning localization and death pathways.

Integration of phosphate functionalized Pt/TiO2 and Ru(bpy)32+ sensitization for ultrasensitive assay of adenosine deaminase activity on a novel split-typed PEC aptasensor

To date, it is still a challenge for high-performance photoelectrochemical (PEC) assay of low-abundance adenosine deaminase (ADA) in fundamental research and clinical diagnosis. Herein, phosphate-functionalized Pt/TiO₂ (termed PO₄^3-/Pt/TiO₂) was prepared as ideal photoactive material to develop a split-typed PEC aptasensor for detection of ADA activity, coupled by a Ru(bpy)₃²⁺ sensitization strategy. We critically studied the effects of the PO₄^3- and Ru(bpy)₃²⁺ on the detection signals, and discussed the signal-amplified mechanism. Specifically, hairpin-structured adenosine (AD) aptamer was splited into single chain via ADA-induced catalytic reaction, and subsequently hybridized with complementary DNA (cDNA, initially coating on magnetic beads). The in-situ formed double-stranded DNA (dsDNA) was further intercalated by more Ru(bpy)₃²⁺ to amplify the photocurrents. The resultant PEC biosensor showed a broader linear range of 0.05-100 U L^-1 and a lower limit of detection (0.019 U L^-1), which can fill the blank for analysis of ADA activity. This research would provide some valuable insights for building advanced PEC aptasensors in ADA-related research and clinical diagnosis.

Low-Temperature Observation of the Excited-State Decay of Ruthenium-(Mono-2,2':6',2″-Terpyridine) Ions with Innocent Ligands: DFT Modeling of an 3MLCT-3MC Intersystem Crossing Pathway

The synthesis, electrochemistry, and photophysical characterization of five 2,2':6',2″-terpyridine ruthenium complexes (Ru-tpy complexes) is reported. The electrochemical and photophysical behavior varied depending on the ligands, i.e., amine (NH₃), acetonitrile (AN), and bis(pyrazolyl)methane (bpm), for this series of Ru-tpy complexes. The target [Ru(tpy)(AN)₃]²⁺ and [Ru(tpy)(bpm)(AN)]²⁺ complexes were found to have low-emission quantum yields in low-temperature observations. To better understand this phenomenon, density functional theory (DFT) calculations were performed to simulate the singlet ground state (S₀), T_e, and metal-centered excited states (³MC) of these complexes. The calculated energy barriers between T_e and the low-lying ³MC state for [Ru(tpy)(AN)₃]²⁺ and [Ru(tpy)(bpm)(AN)]²⁺ provided clear evidence in support of their emitting state decay behavior. Developing a knowledge of the underlying photophysics of these Ru-tpy complexes will allow new complexes to be designed for use in photophysical and photochemical applications in the future.

A Photoisomerizable Zinc (II) Complex Inhibits Microtubule Polymerization for Photoactive Therapy

The photoisomerization-induced cytotoxicity in photopharmacology provides a unique pathway for phototherapy because it is independent of endogenous oxygen. In this study, we developed a biosafe photoisomerizable zinc(II) complex (Zn1), which releases its trans ligand (trans-L1) after being irradiated with blue light. This causes the complex to undergo photoisomerization and produce the toxic cis product (cis-L1) and generate singlet oxygen (¹ O₂ ). The resulting series of events caused impressive phototoxicity in hypoxic A431 skin cancer cells, as well as in a tumor model in vivo. Interestingly, Zn1 was able to inhibit tumor microtubule polymerization, while still showing good biocompatibility and biosafety in vivo. This photoisomerizable zinc(II) complex provides a novel strategy for addressing the oxygen-dependent limitation of traditional photodynamic therapy.

Theoretical Investigation of Ru(II) Complexes with Long Lifetime and a Large Two-Photon Absorption Cross-Section in Photodynamic Therapy

Two-photon photodynamic therapy (TP-PDT), as a new method for cancer, has shown unique advantages in tumors. A low two-photon absorption cross-section (δ) in the biologic spectral window and a short triplet state lifetime are the important issues faced by the current photosensitizers (PSs) in TP-PDT. In this paper, the photophysical properties of a series of Ru(II) complexes were studied by density functional theory and time-dependent density functional theory methods. The electronic structure, one- and two-photon absorption properties, type I/II mechanisms, triplet state lifetime, and solvation free energy were calculated. The results showed that the substitution of methoxyls by pyrene groups greatly improved the lifetime of the complex. Furthermore, the addition of acetylenyl groups subtly enhanced δ. Overall, complex 3b possess a large δ(1376 GM), a long lifetime (136 μs), and better solvation free energy. It is hoped that it can provide valuable theoretical guidance for the design and synthesis of efficient two-photon PSs in the experiment.