Mitochondrial dynamics tracking with iridium(III) complexes

Mitochondria-localized dinuclear iridium(III) complexes for two-photon photodynamic therapy

Photodynamic therapy (PDT), as a non-invasive cancer treatment, offers significant advantages including high temporal-spatial selectivity, minimal surgical intervention, and low toxicity, thereby garnering considerable research interest from across the world. In this study, we have developed a series of dinuclear cyclometalated Ir(III) complexes as potential two-photon photodynamic anticancer agents. These Ir(III) complexes demonstrate significant two-photon absorption (2PA) cross-sections (σ2 = 66-166 GM) and specifically target mitochondria. Amongst them, N-Ir4 manifests an IC50 value of 2.0 μM and a phototoxicity index (PI) of 24. Under two-photon excitation, N-Ir4 efficiently generates reactive oxygen species (ROS), leading to mitochondrial damage and cell death. Our study reveals drastically enhanced optical properties forged by forming a dinuclear complex bridged by two conjugated rigid planar moieties and sheds light on a potential paradigm to boost 2PA cross-sections.

Triphenylphosphine-modified cyclometalated iridiumIII complexes as mitochondria-targeting anticancer agents with enhanced selectivity

This study presents the development and evaluation of triphenylphosphine-modified cyclometalated iridiumIII complexes as selective anticancer agents targeting mitochondria. By leveraging the mitochondrial localization capability of the triphenylphosphine group, these complexes displayed promising cytotoxicity in the micromolar range (3.12-7.24 μM) against A549 and HeLa cancer cells, these complexes exhibit significantly higher activity compared to their unmodified counterparts lacking the triphenylphosphine moiety. Moreover, they demonstrate improved specificity for cancer cells over normal cells, achieving selectivity index in the range of 5.46-14.83. Mechanistic studies confirmed that these complexes selectively target mitochondria rather than DNA, as shown by confocal microscopy and flow cytometry, where they accumulate to induce mitochondrial dysfunction. This disruption leads to mitochondrial membrane depolarization (MMP), elevated reactive oxygen species (ROS) levels, and activation of intrinsic apoptosis pathways. Furthermore, the complexes induce cell cycle arrest at the G2/M phase and suppress the migration of A549 cells.

Dual-emitting cyclometalated Ir(III) complexes with salicylaldimine-bound fluorophores for ratiometric oxygen sensing

A new class of oxygen-sensing dual-emitting cyclometalated Ir(III) complexes is described. They function as ratiometric sensors that combine the blue fluorescence from coumarin as a self-referenced internal standard with yellow to red phosphorescence from bis-cyclometalated iridium complexes. The compounds have phosphorescence quantum yields up to 15%, lifetimes ranging from 0.23 to 9.4 μs, and are capable of sensing a wide range of O2 partial pressures.

Cyclometalated iridium(III) tetrazine complexes for mitochondria-targeted two-photon photodynamic therapy

The fast-moving field of photodynamic therapy (PDT) has provided fresh opportunities to expand the potential of metallodrugs to combat cancers in a light-controlled manner. As such, in the present study, a series of cyclometalated Ir(III) complexes modified with a tetrazine functional group (namely, Ir-ppy-Tz, Ir-pbt-Tz, and Ir-dfppy-Tz) are developed as potential two-photon photodynamic anticancer agents. These complexes target mitochondria but exhibit low toxicity towards HLF primary lung fibroblast normal cells in the dark. When receiving a low-dose one- or two-photon PDT, they become highly potent towards A549 lung cancer cells (with IC50 values ranging from 24.0 nM to 96.0 nM) through the generation of reactive oxygen species (ROS) to induce mitochondrial damage and subsequent apoptosis. Our results indicated that the incorporation of tetrazine with cyclometalated Ir(III) matrices would increase the singlet oxygen (1O2) quantum yield (ΦΔ) and, meanwhile, enable a type I PDT mechanism. Ir-pbt-Tz, with the largest two-photon absorption (TPA) cross-section (σ2 = 102 GM), shows great promise in serving as a two-photon PDT agent for phototherapy.

Dual-Emissive Iridium(III) Complexes and Their Applications in Biological Sensing and Imaging

Phosphorescent iridium(III) complexes are widely recognized for their unique properties in the excited triplet state, making them crucial for various applications including biological sensing and imaging. Most of these complexes display single phosphorescence emission from the lowest-lying triplet state after undergoing highly efficient intersystem crossing (ISC) and ultrafast internal conversion (IC) processes. However, in cases where these excited-state processes are restricted, the less common phenomenon of dual emission has been observed. This dual emission phenomenon presents an opportunity for developing biological probes and imaging agents with multiple emission bands of different wavelengths. Compared to intensity-based biosensing, where the existence and concentration of an analyte are indicated by the brightness of the probe, the emission profile response involves modifications in emission color. This enables quantification by utilizing the intensity ratio of different wavelengths, which is self-calibrating and unaffected by the probe concentration and excitation laser power. Moreover, dual-emissive probes have the potential to demonstrate distinct responses to multiple analytes at separate wavelengths, providing orthogonal detection capabilities. In this concept, we focus on iridium(III) complexes displaying fluorescence-phosphorescence or phosphorescence-phosphorescence dual emission, along with their applications as biological probes for sensing and imaging.

Synthesis and mitochondria-localized iridium (III) complexes induce cell death through pyroptosis and ferroptosis pathways

This paper introduces a new ligand, 4,6-dichloro-5-(1H-imidazo [4,5-f]phenanthroline-2-yl)pyrimidin-2-amine (DPPA), and its corresponding new iridium(III) complexes: [Ir(ppy)2(DPPA)](PF6) (2a) (where ppy represents deprotonated 2-phenylpyridine), [Ir(bzq)2(DPPA)](PF6) (2b) (with bzq indicating deprotonated benzo[h]quinoline), and [Ir(piq)2(DPPA)](PF6) (2c) (piq denoting deprotonated 1-phenylisoquinoline). The cytotoxic effects of both DPPA and 2a, 2b, and 2c were evaluated against human lung carcinoma A549, melanoma B16, colorectal cancer HCT116, human hepatocellular carcinoma HepG2 cancer cell lines, as well as the non-cancerous LO2 cell line using the 3-(4,5-dimethylthiazole-2-yl)-2,5-diphenyltetrazolium bromide (MTT) method. While DPPA exhibited moderate anticancer activity toward A549, B16, HCT116 and HepG2 cells, complexes 2a, 2b, and 2c displayed remarkable efficacy against A549, B16, and HCT116 cells. The cell colonies and wound healing were investigated. Moreover, various aspects of the anticancer mechanisms were explored. The cell cycle analyses revealed that the complexes block cell proliferation of A549 cells during the S phase. Complex 2c induce an early apoptosis, while 2a and 2b cause a late apoptosis. The interaction of 2a, 2b and 2c with endoplasmic reticulum and mitochondria was identified, leading to elevated ROS and Ca2+ amounts. This resulted in a reduced mitochondrial membrane potential, mitochondrial permeability transition pore opening, and an increase of cytochrome c. Also, ferroptosis was investigated through measurements of intracellular glutathione (GSH), malondialdehyde (MDA), and recombinant glutathione peroxidase (GPX4) protein expression. The pyroptosis was explored via cell morphology, release of lactate dehydrogenase (LDH) and expression of pyroptosis-related proteins. RNA sequencing was applied to examine the signaling pathways. Western blot analyses illuminated that the complexes regulate the expression of Bcl-2 family proteins. Additionally, an in vivo antitumor study demonstrated that complex 2c exhibited a remarkable inhibitory rate of 58.58% in restraining tumor growth. In summary, the findings collectively suggest that the iridium(III) complexes induce cell death via ferroptosis, apoptosis by a ROS-mediated mitochondrial dysfunction pathway and GSDMD-mediated pyroptosis.

N-Heterocyclic Carbene-Iridium Complexes as Photosensitizers for In Vitro Photodynamic Therapy to Trigger Non-Apoptotic Cell Death in Cancer Cells

A series of seven novel iridium complexes were synthetized and characterized as potential photosensitizers for photodynamic therapy (PDT) applications. Among them, four complexes were evaluated in vitro for their anti-proliferative activity with and without irradiation on a panel of five cancer cell lines, namely PC-3 (prostate cancer), T24 (bladder cancer), MCF7 (breast cancer), A549 (lung cancer) and HeLa (cervix cancer), and two non-cancerous cell models (NIH-3T3 fibroblasts and MC3T3 osteoblasts). After irradiation at 458 nm, all tested complexes showed a strong selectivity against cancer cells, with a selectivity index (SI) ranging from 8 to 34 compared with non-cancerous cells. The cytotoxic effect of all these complexes was found to be independent of the anti-apoptotic protein Bcl-xL. The compound exhibiting the best selectivity, complex 4a, was selected for further investigations. Complex 4a was mainly localized in the mitochondria. We found that the loss of cell viability and the decrease in ATP and GSH content induced by complex 4a were independent of both Bcl-xL and caspase activation, leading to a non-apoptotic cell death. By counteracting the intrinsic or acquired resistance to apoptosis associated with cancer, complex 4a could be an interesting therapeutic alternative to be studied in preclinical models.

Ir(III) Complexes with AIE Characteristics for Biological Applications

Both biological process detection and disease diagnosis on the basis of luminescence technology can provide comprehensive insights into the mechanisms of life and disease pathogenesis and also accurately guide therapeutics. As a family of prominent luminescent materials, Ir(III) complexes with aggregation-induced emission (AIE) tendency have been recently explored at a tremendous pace for biological applications, by virtue of their various distinct advantages, such as great stability in biological media, excellent fluorescence properties and distinctive photosensitizing features. Significant breakthroughs of AIE-active Ir(III) complexes have been achieved in the past few years and great progress has been witnessed in the construction of novel AIE-active Ir(III) complexes and their applications in organelle-specific targeting imaging, multiphoton imaging, biomarker-responsive bioimaging, as well as theranostics. This review systematically summarizes the basic concepts, seminal studies, recent trends and perspectives in this area.

Mitochondria-Targeted Luminescent Organotin(IV) Complexes: Synthesis, Photophysical Characterization, and Live Cell Imaging

Five fluorescent ONO donor-based organotin(IV) complexes, [Sn^IV(L^1-5)Ph₂] (1-5), were synthesized by the one-pot reaction method and fully characterized spectroscopically including the single-crystal X-ray diffraction studies of 2-4. Detailed photophysical characterization of all compounds was performed. All the compounds exhibited high luminescent properties with a quantum yield of 17-53%. Additionally, the results of cellular permeability analysis suggest that they are lipophilic and easily absorbed by cells. Confocal microscopy was used to examine the live cell imaging capability of 1-5, and the results show that the compounds are mostly internalized in mitochondria and exhibit negligible cytotoxicity at imaging concentration. Also, 1-5 exhibited high photostability as compared to the commercial dye and can be used in long-term real-time tracking of cell organelles. Also, it is found that the probes (1-5) are highly tolerable during the changes in mitochondrial morphology. Thus, this kind of low-toxic organotin-based fluorescent probe can assist in imaging of mitochondria within living cells and tracking changes in their morphology.

Organelle Interaction and Drug Discovery: Towards Correlative Nanoscopy and Molecular Dynamics Simulation

The inter-organelle interactions, including the cytomembrane, endoplasmic reticulum, mitochondrion, lysosome, dictyosome, and nucleus, play the important roles in maintaining the normal function and homeostasis of cells. Organelle dysfunction can lead to a range of diseases (e.g., Alzheimer's disease (AD), Parkinson's disease (PD), and cancer), and provide a new perspective for drug discovery. With the development of imaging techniques and functional fluorescent probes, a variety of algorithms and strategies have been developed for the ever-improving estimation of subcellular structures, organelle interaction, and organelle-related drug discovery with accounting for the dynamic structures of organelles, such as the nanoscopy technology and molecular dynamics (MD) simulations. Accordingly, this work summarizes a series of state-of-the-art examples of the recent progress in this rapidly changing field and uncovering the drug screening based on the structures and interactions of organelles. Finally, we propose the future outlook for exciting applications of organelle-related drug discovery, with the cooperation of nanoscopy and MD simulations.

Novel NIR-Phosphorescent Ir(III) Complexes: Synthesis, Characterization and Their Exploration as Lifetime-Based O2 Sensors in Living Cells

A series of [Ir(N^C)2(N^N)]+ NIR-emitting orthometalated complexes (1-7) has been prepared and structurally characterized using elemental analysis, mass-spectrometry, and NMR spectroscopy. The complexes display intense phosphorescence with vibrationally structured emission bands exhibiting the maxima in the range 713-722 nm. The DFT and TD DFT calculations showed that the photophysical characteristics of these complexes are largely determined by the properties of the metalating N^C ligands, with their major contribution into formation of the lowest S1 and T1 excited states responsible for low energy absorption and emission, respectively. Emission lifetimes of 1-7 in degassed methanol solution vary from 1.76 to 5.39 µs and show strong quenching with molecular oxygen to provide an order of magnitude lifetime reduction in aerated solution. The photophysics of two complexes (1 and 7) were studied in model physiological media containing fetal bovine serum (FBS) and Dulbecco's Modified Eagle Medium (DMEM) to give linear Stern-Volmer calibrations with substantially lower oxygen-quenching constants compared to those obtained in methanol solution. These observations were interpreted in terms of the sensors' interaction with albumin, which is an abundant component of FBS and cell media. The studied complexes displayed acceptable cytotoxicity and preferential localization, either in mitochondria (1) or in lysosomes (7) of the CHO-K1 cell line. The results of the phosphorescence lifetime imaging (PLIM) experiments demonstrated considerable variations of the sensors' lifetimes under normoxia and hypoxia conditions and indicated their applicability for semi-quantitative measurements of oxygen concentration in living cells. The complexes' emission in the NIR domain and the excitation spectrum, extending down to ca. 600 nm, also showed that they are promising for use in in vivo studies.

Mitochondrial structural variations in the process of mitophagy

Mitochondrion is one of significant organelles inside cells because it serves as a hub for energy management and intracellular signaling. Internal/external damages on mitochondria would lead to mitochondrial stresses with the malfunctions, accompanying with the changes of morphological structure and abnormal local environments (pH values). Mitophagy is capable of degradation of damaged mitochondrial segments to restore its normal metabolism, dynamics, and biogenesis. The dynamic structural visualization and pH quantification can be helpful for the understanding of mitochondrial functions as well as the diagnosis of disorders linking with this process. In this work, we use confocal laser scanning microscopy, STED super-resolution nanoscopy and fluorescence lifetime imaging microscopy, in conjunction with a mitochondrial probe to image the dynamic changes in the mitochondrial morphology and microenvironmental pH values during mitophagy in live cells, in particular, the structural changes of mitochondrial cristae beyond optical diffraction can be distinguished by STED nanoscopy with/without treatment by CCCP, which will provide a new view for the diagnosis and personalized treatment of mitochondrial dysfunction-related diseases.

Dithiocarbazate-FeIII, -CoIII, -NiII, and -ZnII Complexes: Design, Synthesis, Structure, and Anticancer Evaluation

Non-platinum-metal complexes show great potential as anticancer agents. Herein, a series of dithiocarbazate non-Pt-metal complexes, including [Fe^III(L)₂]·Cl·2H₂O 1, [Co^III(L)₂]·NO₃·2.5H₂O 2, [Ni^II(L)₂] 3, and [Zn^II(L)₂] 4, have been designed and evaluated for their efficacy as antineoplastic agents. Among them, complex 2 exhibited higher anticancer efficacy than complexes 1, 3, 4, and cisplatin against several cancer cell lines. Hemolysis assays revealed that complex 2 showed comparable hemolysis with cisplatin. In vivo anticancer evaluations showed that complex 2 could retard tumor xenograft growth effectively with low systemic toxicity. Further studies revealed that complex 2 suppressed cancer cells by triggering multiple mechanisms involving the simultaneous inhibition of mitochondria and glycolytic bioenergetics. Overall, our study provides new insights into the anticancer mechanism of Co complexes, which can be used as a good strategy to overcome the flexibility of cancer cells to chemotherapy adaptation.

Challenges and opportunities in the development of metal-based anticancer theranostic agents

Around 10 million fatalities were recorded worldwide in 2020 due to cancer and statistical projections estimate the number to increase by 60% in 2040. With such a substantial rise in the global cancer burden, the disease will continue to impose a huge socio-economic burden on society. Currently, the most widely used clinical treatment modality is cytotoxic chemotherapy using platinum drugs which is used to treat variety of cancers. Despite its clinical success, critical challenges like resistance, off-target side effects and cancer variability often reduce its overall therapeutic efficiency. These challenges require faster diagnosis, simultaneous therapy and a more personalized approach toward cancer management. To this end, small-molecule ‘theranostic’ agents have presented a viable solution combining diagnosis and therapy into a single platform. In this review, we present a summary of recent efforts in the design and optimization of metal-based small-molecule ‘theranostic’ anticancer agents. Importantly, we highlight the advantages of a theranostic candidate over the purely therapeutic or diagnostic agent in terms of evaluation of its biological properties.

Design and synthesis of aptamer-cyclometalated iridium(III) complex conjugate targeting cancer cells

Targeted therapy showed broad application prospects in the treatment of various types of cancer. Through carriers such as aptamers, antibodies, proteins and peptides, targeted therapy can selectively deliver drugs into tumor cells. Compared with traditional treatment methods such as chemo- and radiotherapy, targeted drug delivery systems can reduce the toxic effects of drugs on normal cells and avoid adverse reactions. Herein, an aptamer-cyclometalated iridium(III) complex conjugate (ApIrC) has been designed and developed as a targeted anticancer agent. Owing to the targeting ability of aptamers, ApIrC specifically bound to nucleolin over-expressed on the surface of cancer cells and showed strong fluorescence signal for tumor imaging and diagnosis. ApIrC had more substantial cellular uptake in cancer cells than the iridium complex alone and exhibited favorable low toxicity to normal cells. After uptake by cells through endocytosis, ApIrC can selectively accumulated in mitochondria and induced caspase-3/7-dependent cell death. Remarkably, ApIrC can also specifically target 3D multicellular spheroids (MCSs) and show excellent tumor permeability. So, it can effectively reach the interior of MCSs and cause cell damage. To our knowledge, this is the first report of the aptamer-cyclometalated iridium(III) complex conjugate which studied for cancer targeted therapy. The developed conjugate has great potential to be developed as novel therapeutics for effective and low-toxic cancer treatment.

Luminescent rhenium(I) perfluorobiphenyl complexes as site-specific labels for peptides to afford photofunctional bioconjugates

We report herein new luminescent rhenium(I) perfluorobiphenyl complexes that reacted specifically with the cysteine residue of the π-clamp sequence (FCPF) to afford novel peptide-based imaging reagents, photosensitisers for singlet oxygen and enzyme sensors.

Luminescent Neutral Cyclometalated Iridium(III) Complexes Featuring a Cubic Polyhedral Oligomeric Silsesquioxane for Lipid Droplet Imaging and Photocytotoxic Applications

New neutral iridium(III) complexes featuring a cubic polyhedral oligomeric silsesquioxane (POSS) unit, [Ir(N^∧C)₂(L1-POSS)] [HN^∧C = 2-phenylpyridine (Hppy; 1), 2-phenylbenzothioazole (Hbt; 2), and 2-(1-naphthyl)benzothiazole (Hbsn; 3); L1-POSS = (E)-4-[(2-hydroxybenzylidene)amino]benzyl 3-heptakis(isobutyl)POSS-propyl carbamate], were designed and synthesized. Their POSS-free counterparts, [Ir(N^∧C)₂(L1)] [L1 = (E)-N-(4-hydroxymethylphenyl)-1-(2-hydroxyphenyl)methanimine; HN^∧C = Hppy (1a), Hbt (2a), and Hbsn (3a)], and the poly(ethylene glycol) (PEG) derivatives [Ir(N^∧C)₂(L1-PEG)] [L1-PEG = (E)-4-[(2-hydroxybenzylidene)amino]benzyl 3-[2-[ω-methoxypoly(1-oxapropyl)]ethyl]carbamate; HN^∧C = Hppy (1b), Hbt (2b), and Hbsn (3b)] were also prepared. The photophysical, photochemical, and biological properties of the POSS complexes were compared with those of their POSS-free and PEG-modified counterparts. Upon irradiation, all of these complexes displayed orange-to-red emission and long emission lifetimes under ambient conditions. The bsn complexes 3, 3a, and 3b exhibited the highest singlet oxygen (¹O₂) generation quantum yields (Φ_Δ = 0.85-0.86) in aerated CH₃CN. Laser-scanning confocal microscopy images revealed that complexes 1-3 and 1a-3a showed exclusive lipid-droplet staining upon cellular uptake, while the PEG derivatives 1b-3b displayed lysosomal localization. Complex 3 was utilized to study various lipid-droplet-related biological events including lipid-droplet accumulation under oleic acid stimulation, the movement of lipid droplets, and preadipocyte differentiation. Notably, 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assays indicated that the ppy complexes 1 and 1b and the bt complexes 2 and 2b were noncytotoxic both in the dark and upon irradiation at 450 nm for 5 min (IC₅₀ > 200 μM), while the bsn complexes 3, 3a, and 3b showed low dark cytotoxicity (IC₅₀ = 52.9 to >200 μM) and high photocytotoxicity (IC₅₀ = 1.1-5.3 μM). The cellular uptake, internalization mechanisms, and cell death pathways of these complexes were also investigated. This work not only offers promising luminescent probes for lipid droplets through the structural modification of iridium(III) complexes but also paves the way to the construction of new reagents for theranostics.

Cancer cell membrane camouflaged iridium complexes functionalized black-titanium nanoparticles for hierarchical-targeted synergistic NIR-II photothermal and sonodynamic therapy

The diagnosis and treatment of cancer is one of the biggest medical challenges of the century. Despite significant improvements, there remains an urgent need for novel anticancer procedures. Among the most promising approaches, increasing attention has been devoted towards photothermal and sonodynamic therapy in which sensitizers are activated upon light/ultrasound radiation to generate a cytotoxic effect. While these methods have undoubtedly shown a high therapeutic success, these techniques are intrinsically limited. Herein, the functionalization of black-titanium nanoparticle with iridium complexes and cancer cell membranes in a nanoplatform for hierarchical targeted synergistic photothermal and sonodynamic cancer imaging and therapy is proposed. The particles showed to generate efficiently heat upon irradiation and catalytically form reactive oxygen species upon ultrasound radiation. The nanoparticle formulation demonstrated to selectively localize in the mitochondria as well as to preferentially accumulate in cancerous over non-cancerous cells as well as in the tumor inside a mouse model, presenting a hierarchical targeting strategy. Upon synergistic irradiation at 1064 nm and ultrasound radiation, the nanoparticles were able to act as an imaging agent and identify the tumor site with high spatial resolution as well as act as a therapeutic agent and completely eradicate a tumor inside a mouse model.

Nano-assembly of ruthenium(II) photosensitizers for endogenous glutathione depletion and enhanced two-photon photodynamic therapy

Photodynamic therapy (PDT) is a promising noninvasive cancer treatment. PDT in the clinic faces several hurdles due to the unique tumor environment, a feature of which is high levels of glutathione (GSH). An excess amount of GSH consumes reactive oxygen species (ROS) generated by photosensitizers (PSs), reducing PDT efficiency. Herein, nano-photosensitizers (RuS1 NPs and RuS2 NPs) are reported. These consist of ruthenium complexes joined by disulfide bonds forming GSH sensitive polymer nanoparticles. The NPs achieve enhanced uptake compared to their constituent monomers. Inside cancer cells, high levels of GSH break the S-S bonds releasing PS molecules in the cell. The level of GSH is also then reduced leading to excellent PDT activity. Furthermore, RuS2 NPs functionalized with tumor targeting hyaluronic acid (HA@RuS2 NPs) assessed in vivo were highly effective with minimal side effects. To the best of our knowledge, RuS NPs are the first metal complex-based nano-assembled photosensitizers which exhibit enhanced specificity and consume endogenous GSH simultaneously, thus achieving excellent two-photon PDT efficiency in vitro and in vivo.

Cyclometalated iridium(III) complexes as mitochondria-targeted anticancer and antibacterial agents to induce both autophagy and apoptosis

Mitochondrial damage will hinder the energy production of cells and produce excessive ROS (reactive oxygen species), resulting in cell death through autophagy or apoptosis. In this paper, four cyclometalated iridium(III) complexes (Ir1: [Ir(piq)₂L]PF₆; Ir2: [Ir(bzq)₂L]PF₆; Ir3: [Ir(dfppy)₂L]PF₆; Ir4: [Ir(thpy)₂L]PF₆; piq = 1-phenylisoquinoline; bzq = benzo[h]quinoline; dfppy = 2-(2,4-difluorophenyl)pyridine;thpy = 2-(2-thienyl)pyridine; L = 1,10-phenanthroline-5-amine) were synthesized and characterized. Cytotoxicity tests show that these complexes have excellent cytotoxicity to cancer cells, and mechanism studies indicatethat these complexes can specifically target mitochondria. Complexes Ir1 and Ir2 can damage the function of mitochondria, subsequently increasing intracellular levels of ROS, decreasing MMP (mitochondrial membrane potential), and interfering with ATP energy production, which leads to autophagy and apoptosis. Furthermore, autophagy induced by Ir1 and Ir2 can promote cell death in coordination with apoptosis. Surprisingly, these four complexes also showed moderate antibacterial activity to S. aureusand P. aeruginosa.

Metal complexes as optical probes for DNA sensing and imaging

Transition and lanthanide metal complexes have rich photophysical properties that can be used for cellular imaging, biosensing and phototherapy. One of the applications of such luminescent compounds is the detection and visualisation of nucleic acids. In this brief review, we survey the recent literature on the use of luminescent metal complexes (including Re^I, Ru^II, Os^II, Ir^III, Pt^II, Eu^III and Tb^III) as DNA optical probes, including examples of compounds that bind selectively to non-duplex DNA topologies such as quadruplex, i-motif and DNA mismatches. We discuss the applications of metal-based luminescent complexes in cellular imaging, including time-resolved microscopy and super-resolution techniques. Their applications in biosensing and phototherapy are briefly mentioned in the relevant sections.

Cyclometalated Iridium(III) Complexes as Mitochondria-targeting Photosensitizers against Cisplatin-resistant Cells†

Four iridium (III) complexes Ir1-Ir4 were synthesized and characterized. Possessing high singlet oxygen production ability and specific mitochondria-localization, Ir1 was developed as a mitochondria-targeting photosensitizer. Ir1 exhibited strong phototoxicity against cancer cell line A549 and its corresponding cisplatin-resistant one A549R. In contrast, Ir1 showed low cytotoxicity toward normal cell HLF. This selectivity resulted from the different uptake amount. With 405 nm irradiation, Ir1 induced mitochondria-mediated cell death in A549R cells, achieving the overcome of drug-resistant.

Phosphorescent metal complexes as theranostic anticancer agents: combining imaging and therapy in a single molecule

Phosphorescent metal complexes are a new kind of multifunctional antitumor compounds that can integrate imaging and antitumor functions in a single molecule. In this minireview, we summarize the recent research progress in this field, concentrating on the theranostic applications of phosphorescent iridium(iii), ruthenium(ii) and rhenium(i) complexes. The molecular design that affords these complexes with tumour- or subcellular organelle-targeting properties is elucidated. The potential of these complexes to induce and monitor the dynamic behavior of subcellular organelles and the changes in microenvironment during the process of therapy is demonstrated. Moreover, the potential and advantages of applying new technologies, such as super-resolution imaging and phosphorescence lifetime imaging, are also described. Finally, the challenges faced in the development of novel theranostic metallo-anticancer complexes for possible clinical translation are proposed.

A mitochondrial-targeting iridium(iii) complex for H2O2-responsive and oxidative stress amplified two-photon photodynamic therapy

The mitochondrial-targeting iridium(iii) complex Ir–B(OH)2 is activated in the presence of endogenous H2O2 to release the two-photon photosensitizer Ir–OH and the GSH scavenger methylquinone for amplified two-photon photodynamic therapy.

A mitochondria-targeting magnetothermogenic nanozyme for magnet-induced synergistic cancer therapy

Magnetic hyperthermia therapy (MHT) and chemodynamic therapy (CDT) are non-invasive in situ treatments without depth limitations and with minimum adverse effects on surrounding healthy tissue. We herein report a mitochondria-targeting magnetothermogenic nanozyme (Ir@MnFe2O4 NPs) for highly efficient cancer therapy. An iridium(III) complex (Ir) acts as a mitochondria-targeting agent on the surface of MnFe2O4 NPs. On exposure to an alternating magnetic field (AMF), the Ir@MnFe2O4 NPs induce a localized increase in temperature causing mitochondrial damage (MHT effect). Meanwhile glutathione (GSH) reduces Fe(III) to Fe(II) on the NPs surface, which in turn catalyzes the conversion of H2O2 to cytotoxic •OH (CDT effect). The depletion of GSH (a •OH scavenger) increases CDT efficacy, while the localized increase in temperature increases the rate of conversion of both Fe(III) to Fe(II) and H2O2 to •OH further enhancing the CDT effect. In addition, the disruption of cellular redox homeostasis due to CDT, leads to greater sensitivity of the cell towards MHT. This nanoplatform integrates these excellent therapeutic properties, with two-photon microscopy (TPM) (demonstrated in vitro) and magnetic resonance imaging (MRI) (demonstrated in vivo) to enable the precise and effective treatment of cancer.

Synthesis, Photophysics, and Reverse Saturable Absorption of trans-Bis-cyclometalated Iridium(III) Complexes (C^N^C)Ir(R-tpy)+ (tpy = 2,2':6',2″-Terpyridine) with Broadband Excited-State Absorption

Extending the bandwidth of triplet excited-state absorption in transition-metal complexes is appealing for developing broadband reverse saturable absorbers. Targeting this goal, five bis-terdentate iridium(III) complexes (Ir1-Ir5) bearing trans-bis-cyclometalating (C^N^C) and 4'-R-2,2':6',2″-terpyridine (4'-R-tpy) ligands were synthesized. The effects of the structural variation in cyclometalating ligands and substituents at the tpy ligand on the photophysics of these complexes have been systematically explored using spectroscopic methods (i.e., UV-vis absorption, emission, and transient absorption spectroscopy) and time-dependent density functional theory (TDDFT) calculations. All complexes exhibited intensely structured ¹π,π* absorption bands at <400 nm and broad charge transfer (¹CT)/¹π,π* transitions at 400-600 nm. Ligand structural variations exerted a very small effect on the energies of the ¹CT/¹π,π* transitions; however, they had a significant effect on the molar extinction coefficients of these absorption bands. All complexes emitted featureless deep red phosphorescence in solutions at room temperature and gave broad-band and strong triplet excited-state absorption ranging from the visible to the near-infrared (NIR) spectral regions, with both originating from the ³π,π*/³CT states. Although alteration of the ligand structures influenced the emission energies slightly, these changes significantly affected the emission lifetimes and quantum yields, transient absorption spectral features, and the triplet excited-state quantum yields of the complexes. Except for Ir3, the other four complexes all manifested reverse saturable absorption (RSA) upon nanosecond laser pulse excitation at 532 nm, with the decreasing trend of RSA following Ir2 ≈ Ir4 > Ir1 > Ir5 > Ir3. The RSA trend corresponded well with the strength of the excited-state and ground-state absorption differences (ΔOD) at 532 nm for these complexes.

Boosting two-photon photodynamic therapy with mitochondria-targeting ruthenium-glucose conjugates

Herein, we present a series of dual-targeted ruthenium-glucose conjugates that can function as two-photon absorption (TPA) PDT agents to effectively destroy tumors by preferentially targeting both tumor cells and mitochondria. The in vivo experiments revealed an excellent tumor inhibitory efficiency of the dual-targeted TPA PSs.

Photostable NIR emitting ruthenium(II) conjugates; uptake and biological activity in live cells

A photostable Ru(2,2-biquinoline)₂(3-(2-pyridyl)-5-(4-carboxyphenyl)-1,2,4-triazolate) (Ru(biq)₂(trzbenzCOOH)) complex that exhibits near-infrared (NIR) emission centred at 786 nm is reported. The parent complex was conjugated via amide coupling to a cell-penetrating peptide sequence octa-arginine (R8), and two signal peptide sequences; the nuclear localizing sequence (NLS) VQRKRQKLMP and the mitochondria penetrating peptide (MPP) FrFKFrFK(Ac) (r = D isomer of arginine, Ac = terminal lysine amine acetyl blocked). Notably, none of the peptide conjugates were cell-permeable as chloride salts but efficient and rapid membrane permeation was observed post ion exchange with perchlorate counterion. Also, surprisingly, all three peptide conjugates exhibited potent dark cytotoxicity in both CHO and HeLa cell lines. The peptide conjugates induce cell death through a caspase dependent apoptotic pathway. At the minimum concentration of dye (approx. 15 μM) required for cell imaging, only 20% of the cells were viable after a 24 h incubation period. To overcome cytotoxicity, the parent complex was PEGylated; this dramatically decreased cytotoxicity, where 50% of cells were viable even at 150 μM concentration after 24 h. Confocal luminescence microscopy indicated that all four bioconjugates, peptides in perchlorate form and polyethylene glycol (PEG) in chloride form, were rapidly internalized within the cell. However, interestingly the precise localisation by the signal peptides observed in related complexes was not observed here and the peptide conjugates were unsuitable as luminescent probes for cell microscopy due to their high cell toxicity. The poor targeting of signal peptides in this instance is attributed to the high lipophilicity of the metal centre.

Highly cytotoxic, cyclometalated iridium(iii)-5-fluoro-8-quinolinol complexes as cancer cell mitochondriotropic agents

Ir-3 and Ir-4 kill HeLa cells and trigger caspase-mediated mitochondrial dysfunction apoptosis pathways.

Dinuclear metal complexes: multifunctional properties and applications

Dinuclear metal complexes have enabled breakthroughs in OLEDs, photocatalytic water splitting and CO₂reduction, DSPEC, chemosensors, biosensors, PDT and smart materials.

Imaging mitochondria and plasma membrane in live cells using solvatochromic styrylpyridines

Investigating the dynamics of different biomolecules in the cellular milieu through microscopic imaging has gained paramount importance in the last decade. Continuous developments in the field of microscopy are paralleled by the design and synthesis of fluorophores that target specific compartments within a cell. In this study, we have synthesized four fluorescent styrene derivatives, a neutral styrylpridine, three cationic styrylpyridinium probes with and without cholesterol tether, and investigated their absorption, emission, and cellular imaging properties. The fluorophores show solvatochromic emission attributed to intramolecular charge transfer from donor to acceptor with an emission range of 500-600 nm. The fluorescent cholesterol conjugate labels plasma membrane effectively while the fluorophores devoid of the cholesterol tether label mitochondria. Cholesterol conjugate also shows strong interaction with liposome membrane. Furthermore, the fluorophores alsotrack the mitochondria in live cells with high specificity. Cell viability assay showed overall non-toxic nature of the probes even at higher fluorophore concentrations. Through sidearm modifications, keeping the fluorescent core intact, we successfully targeted specific subcellular compartments of neuronal (N2a) and non-neuronal (HeLa) mammalian cell lines. This strategy of using a single molecular scaffold with subtle substitutions could be ideal in generating a variety of fluorophores targeting other subcellular compartments.

Anticancer Cyclometalated Iridium(III) Complexes with Planar Ligands: Mitochondrial DNA Damage and Metabolism Disturbance

Emerging studies have shown that mitochondrial DNA (mtDNA) is a potential target for cancer therapy. Herein, six cyclometalated Ir(III) complexes Ir1-Ir6 containing a series of extended planar diimine ligands have been designed and assessed for their efficacy as anticancer agents. Ir1-Ir6 show much higher cytotoxicity than cisplatin and they can effectively localize to mitochondria. Among them, complexes Ir3 and Ir4 with dipyrido[3,2- a:2',3'- c]phenazine (dppz) ligands can bind to DNA tightly in vitro, intercalate to mtDNA in situ, and induce mtDNA damage. Ir3- and Ir4-impaired mitochondria exhibit decline of mitochondrial membrane potential, disability of adenosine triphosphate generation, disruption of mitochondrial energetic and metabolic status, which subsequently cause protective mitophagy, G₀/G₁ phase cell cycle arrest, and apoptosis. In vivo antitumor evaluations also show that Ir4 can inhibit tumor xenograft growth effectively. Overall, our work proves that targeting the mitochondrial genome may present an effective strategy to develop metal-based anticancer agents to overcome cisplatin resistance.

A phosphorescent iridium probe for sensing polarity in the endoplasmic reticulum and in vivo

A phosphorescent iridium complex for in situ tracking endoplasmic reticulum polarity variations during ER stress and in vivo.

Super-Resolution Tracking of Mitochondrial Dynamics with An Iridium(III) Luminophore

Combining luminescent transition metal complex with super-resolution microscopy is an excellent strategy for the long-term visualization of the dynamics of subcellular structures in living cells. However, it remains unclear whether iridium(III) complexes are applicable for a particular type of super-resolution technique, structured illumination microscopy (SIM), to image subcellular structures. Herein, an iridium(III) dye, to track mitochondrial dynamics in living cells under SIM is described. The dye demonstrates excellent specificity and photostability and satisfactory cell permeability. While using SIM to image mitochondria, an ≈80 nm resolution is achieved that allows the clear observation of the structure of mitochondrial cristae. The dye is used to monitor and quantify mitochondrial dynamics relative to lysosomes, including fusion involved in mitophagy, and newly discovered mitochondria-lysosome contact (MLC) under different conditions. The MLC remains intact and fusion vanishes when five receptors, p62, NDP52, OPTN, NBR1, and TAX1BP1, are knocked out, suggesting that these two processes are independent.

Recent progress and developments of iridium-based compounds as probes for environmental analytes

Metal complexes based on iridium metal centers have attracted attention as probes due to their tunable biological and chemical characteristics.

Tracking mitochondrial dynamics during apoptosis with phosphorescent fluorinated iridium(iii) complexes

A series of phosphorescent fluorinated Ir(iii) complexes, which exhibit low cytotoxicity, excellent photostability and specificity of mitochondria-targeting, were used for tracking mitochondrial dynamics during apoptosis.