A novel luminescent Ir(iii) complex for dual mode imaging: synergistic response to hypoxia and acidity of the tumor microenvironment

Mitochondria-targeted and near-infrared phosphorescent Ir(III) complexes for specific detection of Hg2+ and photodynamic therapy

S: Mercury ions (Hg2+) are highly toxic and prone to bioaccumulation, showing a strong attraction to proteins and enzymes that contain sulfur. Even minute quantities of Hg2+ can lead to severe health issues. Given that mitochondria are a primary target organelle of Hg2+, it is essential to create a probe that can accurately detect Hg2+ within intracellular mitochondria. In this study, we developed two innovative Ir(III) complex probes that emit near-infrared light. The crystal structure of Ir2 was determined using X-ray techniques, which reveals that Ir2 contains a pyridine group capable of recognizing Hg2+ and targeting mitochondria, allowing for the precise identification of Hg2+ both in vitro and within the mitochondria of living cells. Additionally, these two novel near-infrared phosphorescent Ir(III) complexes demonstrate significant capabilities in producing ROS including singlet oxygen, ·O2- and ·OH, which renders them effective photosensitizers under visible light exposure for photodynamic therapy (PDT). This research offers a promising approach for detecting Hg2+ in vitro and in the mitochondrial microenvironment of living cells, which have some implications for the future development of pertinent transition metal complexes for mitochondria-targeted photodynamic therapy in cancer cells.

High Photocytotoxicity Iridium(III) Complex Photosensitizer for Photodynamic Therapy Induces Antitumor Effect Through GPX4-Dependent Ferroptosis

The development of small molecule photosensitizers based on iridium complex is limited by the mismatch between therapeutic effect and systemic toxicity, as well as the incomplete understanding of the molecular mechanism underlying cell death induction. Herein, a small molecule iridium complex IrC with high photocytotoxicity is synthesized, with half maximal inhibitory concentration as low as 91 nm, demonstrating excellent anti-tumor, relief of splenomegaly, and negligible side effects. Starting from the factors of effective photosensitizers, the in-depth theoretical analysis on photon absorption efficiency, energy transfer level matching, and properties of the triplet excited state of IrC is conducted. This also elucidates the feasibility of generating the high singlet oxygen quantum yield. In addition, the death mechanism induced by IrC is focused, innovatively utilizing GPX4-overexpression and GPX4-knockout cells via CRISPR/Cas9 technique to comprehensively verify ferroptosis and its further molecular mechanism. The generation of ROS mediated by IrC, along with the direct inhibition of GPX4 and glutathione, synergistically increased cellular oxidative stress and the level of lipid peroxidation. This study provides an effective approach for small molecule complexes to induce GPX4-dependent ferroptosis at low-dose photodynamic therapy.

Advances in imaging techniques for tumor microenvironment evaluation in hepatocellular carcinoma

The tumor microenvironment (TME) plays a critical role in the development and treatment of liver cancer, which ranks sixth in incidence and third in mortality worldwide, according to the "Global Cancer Statistics 2022". Hepatocellular carcinoma (HCC), the most common form of liver cancer, is heavily influenced by the TME, which affects tumor growth, invasion, metastasis, and the response to various treatments. Despite advancements in surgery, liver transplantation, targeted therapies, and immunotherapy, the complexity of the TME often limits treatment efficacy, especially in advanced-stage HCC cases. The TME consists of a dynamic interaction between tumor cells, immune cells, fibroblasts, blood vessels, and signaling molecules, all of which contribute to cancer progression and therapy resistance. Assessing the HCC TME is essential for designing effective, personalized treatments and improving patient outcomes. Recent research highlights the value of imaging technologies as non-invasive tools to evaluate the TME, offering new possibilities for more targeted therapies and better prognosis monitoring in HCC patients.

Shining New Light on Biological Systems: Luminescent Transition Metal Complexes for Bioimaging and Biosensing Applications

Luminescence imaging is a powerful and versatile technique for investigating cell physiology and pathology in living systems, making significant contributions to life science research and clinical diagnosis. In recent years, luminescent transition metal complexes have gained significant attention for diagnostic and therapeutic applications due to their unique photophysical and photochemical properties. In this Review, we provide a comprehensive overview of the recent development of luminescent transition metal complexes for bioimaging and biosensing applications, with a focus on transition metal centers with a d6, d8, and d10 electronic configuration. We elucidate the structure-property relationships of luminescent transition metal complexes, exploring how their structural characteristics can be manipulated to control their biological behavior such as cellular uptake, localization, biocompatibility, pharmacokinetics, and biodistribution. Furthermore, we introduce the various design strategies that leverage the interesting photophysical properties of luminescent transition metal complexes for a wide variety of biological applications, including autofluorescence-free imaging, multimodal imaging, organelle imaging, biological sensing, microenvironment monitoring, bioorthogonal labeling, bacterial imaging, and cell viability assessment. Finally, we provide insights into the challenges and perspectives of luminescent transition metal complexes for bioimaging and biosensing applications, as well as their use in disease diagnosis and treatment evaluation.

Effects of Photodynamic Therapy on Tumor Metabolism and Oxygenation Revealed by Fluorescence and Phosphorescence Lifetime Imaging

This work was aimed at the complex analysis of the metabolic and oxygen statuses of tumors in vivo after photodynamic therapy (PDT). Studies were conducted on mouse tumor model using two types of photosensitizers-chlorin e6-based drug Photoditazine predominantly targeted to the vasculature and genetically encoded photosensitizer KillerRed targeted to the chromatin. Metabolism of tumor cells was assessed by the fluorescence lifetime of the metabolic redox-cofactor NAD(P)H, using fluorescence lifetime imaging. Oxygen content was assessed using phosphorescence lifetime macro-imaging with an oxygen-sensitive probe. For visualization of the perfused microvasculature, an optical coherence tomography-based angiography was used. It was found that PDT induces different alterations in cellular metabolism, depending on the degree of oxygen depletion. Moderate decrease in oxygen in the case of KillerRed was accompanied by an increase in the fraction of free NAD(P)H, an indicator of glycolytic switch, early after the treatment. Severe hypoxia after PDT with Photoditazine resulted from a vascular shutdown yielded in a persistent increase in protein-bound (mitochondrial) fraction of NAD(P)H. These findings improve our understanding of physiological mechanisms of PDT in cellular and vascular modes and can be useful to develop new approaches to monitoring its efficacy.

Surface-Roughened SERS-Active Single Silver Nanowire for Simultaneous Detection of Intracellular and Extracellular pHs

The simultaneous and accurate detection of intracellular pH (pH_i) and extracellular pH (pH_e) is essential for studying the complex physiological activities of cancer cells and exploring pH-related therapeutic mechanisms. Here, we developed a super-long silver nanowire-based surface-enhanced Raman scattering (SERS) detection strategy for simultaneous sensing of pH_i and pH_e. A surface-roughened silver nanowire (AgNW) with a high aspect ratio is prepared at a nanoelectrode tip using a Cu-mediated oxidation process, which is then modified by pH-sensitive 4-mercaptobenzoic acid (4-MBA) to form 4-MBA@AgNW as a pH sensing probe. With the assistance of a 4D microcontroller, 4-MBA@AgNW is efficient in simultaneously detecting pH_i and pH_e in both 2D and 3D culture cancer cells by SERS, with minimal invasiveness, high sensitivity, and spatial resolution. Further investigation proves that the surface-roughened single AgNW can also be used in monitoring the dynamic variation of pH_i and pH_e of cancer cells upon stimulation with anticancer drugs or under a hypoxic environment.

Aqua-friendly organometallic Ir-Pt complexes: pH-responsive AIPE-guided imaging of bacterial cells

In this work, the aggregation-induced photoluminescence emission (AIPE) of three water-soluble heterobimetallic Ir-Pt complexes was reported with insight into their photophysical and electrochemical properties and imaging of bacterial cells. An alkyne appended Schiff's base L, bridges bis-cyclometalated iridium(III) and platinum(II) terpyridine centre. The Schiff's base (N-N fragment) serves as the ancillary ligand to the iridium(III) centre, while the alkynyl end is coordinated to platinum(II). The pH and ionic strength influence the aggregation kinetics of the alkynylplatinum(II) fragment, leading to metal-metal and π-π interactions with the emergence of a triplet metal-metal-to-ligand charge transfer (³MMLCT) emission. The excellent reversibility and photostability of aggregation-induced emission (AIE) of these aqua-friendly complexes were tested for their ability to sense and selectively image E. coli cells at various pH values.

Afterglow/Fluorescence Dual-Emissive Ratiometric Oxygen Probe for Tumor Hypoxia Imaging

Hypoxia is a common feature of many diseases such as solid tumors. The measurement and imaging of oxygen (O₂) are extremely important for disease diagnosis and therapy evaluation. In this work, the afterglow/fluorescence dual-emissive ratiometric O₂ probe based on a photochemical reaction-based afterglow system is reported. The afterglow is highly sensitive to O₂ because the O₂ content is directly related to the ¹O₂ yield and eventually affects the afterglow intensity. The O₂-insensitive fluorescence of an emitter can serve as an internal reference. As the O₂ concentration changes from 0.08 to 18.5 mg L^-1, the ratio value shows a remarkable 53-fold increase. Compared with the intensity of a single peak, the ratiometric signal can eliminate the interference of the probe concentration to achieve higher accuracy. This afterglow/fluorescence dual-emissive ratiometric O₂ probe is successfully applied to hypoxia imaging in tumor-bearing mice, which may further promote the development of O₂ sensing in the biomedical field.

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.

Cellular imaging properties of phosphorescent iridium(III) complexes substituted with ester or amide groups

We reported four iridium(III) complexes substituted with ester or amide groups as luminescent cellular imaging reagents. While three of the complexes stained the cytoplasm, the other complex showed the exceptional...

pH-Responsive N^C-Cyclometalated Iridium(III) Complexes: Synthesis, Photophysical Properties, Computational Results, and Bioimaging Application

Herein we report four [Ir(N^C)2(L^L)]n+, n = 0,1 complexes (1-4) containing cyclometallated N^C ligand (N^CH = 1-phenyl-2-(4-(pyridin-2-yl)phenyl)-1H-phenanthro[9,10-d]imidazole) and various bidentate L^L ligands (picolinic acid (1), 2,2'-bipyridine (2), [2,2'-bipyridine]-4,4'-dicarboxylic acid (3), and sodium 4,4',4″,4‴-(1,2-phenylenebis(phosphanetriyl))tetrabenzenesulfonate (4). The N^CH ligand precursor and iridium complexes 1-4 were synthesized in good yield and characterized using chemical analysis, ESI mass spectrometry, and NMR spectroscopy. The solid-state structure of 2 was also determined by XRD analysis. The complexes display moderate to strong phosphorescence in the 550-670 nm range with the quantum yields up to 30% and lifetimes of the excited state up to 60 µs in deoxygenated solution. Emission properties of 1-4 and N^CH are strongly pH-dependent to give considerable variations in excitation and emission profiles accompanied by changes in emission efficiency and dynamics of the excited state. Density functional theory (DFT) and time-dependent density functional theory (TD DFT) calculations made it possible to assign the nature of emissive excited states in both deprotonated and protonated forms of these molecules. The complexes 3 and 4 internalize into living CHO-K1 cells, localize in cytoplasmic vesicles, primarily in lysosomes and acidified endosomes, and demonstrate relatively low toxicity, showing more than 80% cells viability up to the concentration of 10 µM after 24 h incubation. Phosphorescence lifetime imaging microscopy (PLIM) experiments in these cells display lifetime distribution, the conversion of which into pH values using calibration curves gives the magnitudes of this parameter compatible with the physiologically relevant interval of the cell compartments pH.

Phosphorescent NIR emitters for biomedicine: applications, advances and challenges

Application of NIR (near-infrared) emitting transition metal complexes in biomedicine is a rapidly developing area of research. Emission of this class of compounds in the "optical transparency windows" of biological tissues and the intrinsic sensitivity of their phosphorescence to oxygen resulted in the preparation of several commercial oxygen sensors capable of deep (up to whole-body) and quantitative mapping of oxygen gradients suitable for in vivo experimental studies. In addition to this achievement, the last decade has also witnessed the increased growth of successful alternative applications of NIR phosphors that include (i) site-specific in vitro and in vivo visualization of sophisticated biological models ranging from 3D cell cultures to intact animals; (ii) sensing of various biologically relevant analytes, such as pH, reactive oxygen and nitrogen species, RedOx agents, etc.; (iii) and several therapeutic applications such as photodynamic (PDT), photothermal (PTT), and photoactivated cancer (PACT) therapies as well as their combinations with other therapeutic and imaging modalities to yield new variants of combined therapies and theranostics. Nevertheless, emerging applications of these compounds in experimental biomedicine and their implementation as therapeutic agents practically applicable in PDT, PTT, and PACT face challenges related to a critically important improvement of their photophysical and physico-chemical characteristics. This review outlines the current state of the art and achievements of the last decade and stresses the most promising trends, major development prospects, and challenges in the design of NIR phosphors suitable for biomedical applications.

Mitochondria targeted near-infrared chemodosimeter for upconversion luminescence bioimaging of hypoxia

We report a mitochondria-targeted near-infrared probe (NRh-O) for frequency upconversion luminescence (FUCL) imaging of hypoxia. Under hypoxic conditions, NRh-O rapidly responds to release the FUCL product NRh (λ_ex/em = 850/825 nm) with high sensitivity and selectivity in mitochondria. This highlights the potential application of a hypoxia-responsive probe in early clinical diagnosis.

Employing hypoxia characterization to predict tumour immune microenvironment, treatment sensitivity and prognosis in hepatocellular carcinoma

The hypoxic microenvironment was recognized as a major driving force of the malignant phenotype in hepatocellular carcinoma (HCC), which contributes to tumour immune microenvironment (TIM) remodeling and tumor progression. Dysregulated hypoxia-related genes (HRGs) result in treatment resistance and poor prognosis by reshaping tumor cellular activities and metabolism. Approaches to identify the relationship between hypoxia and tumor progression provided new sight for improving tumor treatment and prognosis. But, few practical tools, forecasting relationship between hypoxia, TIM, treatment sensitivity and prognosis in HCC were reported. Here, we pooled mRNA transcriptome and clinical pathology data from the International Cancer Genome Consortium (ICGC) and The Cancer Genome Atlas (TCGA), and later developed a hypoxia risk model including four HRGs (DCN, DDIT4, PRKCA and NDRG1). The high-risk group displayed poor clinical characteristics, a malignant phenotype with carcinogenesis/proliferation pathways activation (MTORC1 and E2F) and immunosuppressive TIM (decreased immune cell infiltrations and upregulated immunosuppressive cytokines). Meanwhile, activated B cells, effector memory CD8 T cells and EZH2 deregulation were associated with patient’s survival, which might be the core changes of HCC hypoxia. Finally, we validated the ability of the hypoxia risk model to predict treatment sensitivity and found high hypoxia risk patients had poor responses to HCC treatment, including surgical resection, Sorafenib, Transarterial Chemoembolization (TACE) and immunotherapy. In conclusion, based on 4 HRGs, we developed and validated a hypoxia risk model to reflect pathological features, evaluate TIM landscape, predict treatment sensitivity and compounds specific to hypoxia signatures in HCC patients.

Photoinduced synergistic cytotoxicity towards cancer cells via Ru(ii) complexes

Ru complexes have been recognized as excellent anticancer candidates.