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.

Recent Advances in Organometallic NIR Iridium(III) Complexes for Detection and Therapy

Iridium(III) complexes are emerging as a promising tool in the area of detection and therapy due to their prominent photophysical properties, including higher photostability, tunable phosphorescence emission, long-lasting phosphorescence, and high quantum yields. In recent years, much effort has been devoted to develop novel near-infrared (NIR) iridium(III) complexes to improve signal-to-noise ratio and enhance tissue penetration. In this review, we summarize different classes of organometallic NIR iridium(III) complexes for detection and therapy, including cyclometalated ligand-enabled NIR iridium(III) complexes and NIR-dye-conjugated iridium(III) complexes. Moreover, the prospects and challenges for organometallic NIR iridium(III) complexes for targeted detection and therapy are discussed.

Synthesis and luminescence monitoring of iridium(III) complex-functionalized gold nanoparticles and their application for determination of gold(III) ions

A new method is presented for the one-step synthesis and real-time monitoring of iridium(III) complex-functionalized AuNPs from the precursor gold(III) chloride (AuCl₃). The functionalized AuNPs with an average size of 8 - 20 nm were obtained by the reduction of Au³⁺ ions by the alkyne group of iridium(III) complexes, which was accompanied by the anchoring iridium(III) complexes on the surface of the nanoparticles. Meanwhile, the luminescence of the iridium(III) complexes was effectively quenched due to distance-dependent fluorescence quenching by AuNPs, thereby enabling luminescence monitoring of the formation process of the functionalized AuNPs and obtaining scattering information and spectral information in real time. Moreover, this method was applied to the determination of Au³⁺ ions in buffer with a limit of detection of 0.38 μM at 700 nm in luminescence mode, while the detection limit for absorbance was 10.04 μM. Importantly, the multimodal detection strategy alleviates interference from other metal ions. Furthermore, the iridium(III) alkyne complexes were capable of imaging mitochondrial Au³⁺ ions in living cells. Taken together, this work opens a new avenue for convenient synthesis and monitoring formation of functionalized AuNPs, and also provides a tool for selective determination of Au³⁺ ions in solution and in cellulo.

Self-Assembly of Small Organic Molecules into Luminophores for Cancer Theranostic Applications

Self-assembled biomaterials have been widely explored for real-time fluorescence imaging, imaging-guided surgery, and targeted therapy for tumors, etc. In particular, small molecule-based self-assembly has been established as a reliable strategy for cancer theranostics due to the merits of small-sized molecules, multiple functions, and ease of synthesis and modification. In this review, we first briefly introduce the supramolecular chemistry of small organic molecules in cancer theranostics. Then, we summarize and discuss advanced small molecule-based self-assembly for cancer theranostics based on three types, including peptides, amphiphilic molecules, and aggregation-induced emission luminogens. Finally, we conclude with a perspective on future developments of small molecule-based self-assembled biomaterials integrating diagnosis and therapy for biomedical applications. These applications highlight the opportunities arising from the rational design of small organic molecules with self-assembly properties for precision medicine.

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.

Inhibition of the CDK9-cyclin T1 protein-protein interaction as a new approach against triple-negative breast cancer

Cyclin-dependent kinase 9 (CDK9) activity is correlated with worse outcomes of triple-negative breast cancer (TNBC) patients. The heterodimer between CDK9 with cyclin T1 is essential for maintaining the active state of the kinase and targeting this protein-protein interaction (PPI) may offer promising avenues for selective CDK9 inhibition. Herein, we designed and generated a library of metal complexes bearing the 7-chloro-2-phenylquinoline CˆN ligand and tested their activity against the CDK9-cyclin T1 PPI. Complex 1 bound to CDK9 via an enthalpically-driven binding mode, leading to disruption of the CDK9-cyclin T1 interaction in vitro and in cellulo. Importantly, complex 1 showed promising anti-metastatic activity against TNBC allografts in mice and was comparably active compared to cisplatin. To our knowledge, 1 is the first CDK9-cyclin T1 PPI inhibitor with anti-metastatic activity against TNBC. Complex 1 could serve as a new platform for the future design of more efficacious kinase inhibitors against cancer, including TNBC.

Iridium tetrazolato complexes as efficient protein staining agents

Iridium tetrazolato complexes have been illustrated as one kind of efficient protein staining agent.

In vitro studies on the selective cytotoxic effect of luminescent Ru(ii)-p-cymene complexes of imidazo-pyridine and imidazo quinoline ligands

In recent years, Ru(ii) complexes have gained high importance in medicinal chemistry due to their significant anti-cancer activities, which are directly related to their DNA binding ability.

Mitochondria-targeted artesunate conjugated cyclometalated iridium(iii) complexes as potent anti-HepG2 hepatocellular carcinoma agents

Hepatocellular carcinoma (HCC) poses a serious threat to people's health worldwide. Artesunate (ART), one of the classical antimalarial drugs, has recently been shown to exert significant cytotoxicity in various cancers, but its bioavailability is low. Cyclometalated iridium(iii) complexes have emerged as a promising class of anticancer therapeutic agents. Herein, through conjugation of two of them, three novel Ir(iii)-ART conjugates, [Ir(C-N)2(bpy-ART)](PF6) (bpy = 2,2'-bipyridine, C-N = 2-phenylpyridine (ppy, Ir-ART-1), 2-(2-thienyl)pyridine (thpy, Ir-ART-2), and 2-(2,4-difluorophenyl)pyridine (dfppy, Ir-ART-3)) have been synthesized, and their potential as anti-HCC agents was evaluated. We demonstrate that Ir-ART-1-3 display higher cytotoxicity against HCC cell lines than normal liver cells, and they can especially locate to mitochondria of HepG2 cells and induce a series of mitochondria-mediated apoptosis events. Moreover, Ir-ART-1-3 can regulate the cell cycle and inhibit metastasis of HepG2 cells. Finally, in vivo antitumor evaluation also demonstrates the inhibitory activity of Ir-ART-1 on tumor growth. Taken together, these Ir(iii)-ART conjugates have the potential to become drug candidates for future anti-HCC treatments.

Structure-guided discovery of a luminescent theranostic toolkit for living cancer cells and the imaging behavior effect

Dual-functional theranostics are powerful tools that can allow for the in-field understanding of cancer pathology, yet their use is held back by the paucity of suitable theranostics for living systems. Moreover, typical in vitro screening conditions for probe molecules do not necessarily generate candidates that can function effectively in the natural in cellulo environment, limiting their follow-up use in living systems. We introduce herein a general strategy for the development of an iridium(iii) theranostic by grafting a well-known inhibitor as a "binding unit" onto an iridium(iii) complex precursor as a "signaling unit". To further optimize their emissive properties, we explored the effect of imaging behavior by incorporating different substituents onto the parental "signaling unit". This design concept was validated by a series of tailored iridium(iii) theranostics 2a-2h for the visualization and inhibition of EGFR in living cancer cells. By comprehensively assessing the theranostic potency of 2a-2h in both in vitro and in cellulo contexts, probe 2f containing electron-donating methoxy groups on the "signaling unit" was discovered to be the most promising candidate theranostic with desirable photophysical/chemical properties. Probe 2f selectively bound to EGFR in vitro and in cellulo, enabling it to selectively discriminate living EGFR-overexpressing cancer cells from normal cells that express low levels of EGFR with an "always-on" luminescence signal output. In particular, its long-lived lifetime enabled its luminescence signal to be readily distinguished from the interfering fluorescence of organic dyes by using time-resolved techniques. Complex 2f simultaneously visualized and inhibited EGFR in a dose-dependent manner, leading to a reduction in the phosphorylation of downstream proteins ERK and MEK, and inhibition of the activity of downstream transcription factor AP1. Notably, complex 2f is comparable to the parental EGFR inhibitor 1b, in terms of both inhibitory activity against EGFR and cytotoxicity against EGFR-overexpressing cancer cells. This tailored dual-functional iridium(iii) theranostic toolkit provides an alternative strategy for the personalized diagnosis and treatment of cancers.

The anti-infection drug furazolidone inhibits NF-κB signaling and induces cell apoptosis in small cell lung cancer

Targeting nuclear factor kappa B (NF-κB) signaling pathway has become a promising strategy for the development of new antitumor drugs. In this paper, we found that anti-infection drug furazolidone (FZD) could significantly inhibit NF-κB-driven luciferase activity, and FZD could markedly inhibit both of the constitutive and tumor necrosis factor-α (TNFα)-triggered phosphorylation of NF-κB p65 in small cell lung cancer (SCLC). Further studies revealed that FZD inhibited the expression of inhibitor of kappa B kinase β (IKKβ) in SCLC cells. In addition, we found that FZD had significant antitumor activities in SCLC cells. FZD could markedly suppress the cell viability of SCLC cells dose-dependently, and FZD could significantly induce the cleavages of poly ADP-ribose polymerase (PARP) and Caspase3, the biomarkers of cell apoptosis, in SCLC cells. The flow cytometry also revealed that FZD induced cell apoptosis in SCLC cells. Finally, we also found that overexpression of constitutively activated IKKβ could significantly abolish FZD-induced cell growth inhibition in SCLC cells, which further confirmed that FZD displayed its anti-SCLC activity through regulating NF-κB signaling pathway.

Anticancer IrIII -Aspirin Conjugates for Enhanced Metabolic Immuno-Modulation and Mitochondrial Lifetime Imaging

The chemo-anti-inflammatory strategy is attracting ever more attention for the treatment of cancer. Here, two cyclometalated Ir^III complexes Ir2 and Ir3 formed by conjugation of Ir1 with two antiphlogistics (aspirin and salicylic acid) have been designed. Ir2 and Ir3 exhibit higher antitumor and anti-inflammatory potencies than a mixture of Ir1 and aspirin/salicylic acid. We show that they can be hydrolyzed, accumulate in mitochondria, and induce mitochondrial dysfunction. Due to their intense long-lived phosphorescence, Ir2 and Ir3 can track mitochondrial morphological changes. Phosphorescence lifetime imaging shows that Ir2 and Ir3 can aggregate during mitochondrial dysfunction. As expected, Ir2 and Ir3 exhibit immunomodulatory properties by regulating the activity of immune factors. Both Ir2 and Ir3 can induce caspase-dependent apoptosis and caspase-independent paraptosis and inhibit several events related to metastasis. Moreover, Ir2 and Ir3 show potent tumor growth inhibition in vivo. Our study demonstrates that the combination of mitochondrial-targeting and immunomodulatory activities is feasible to develop multifunctional metal-based anticancer agents.

Caspase-1 Specific Light-Up Probe with Aggregation-Induced Emission Characteristics for Inhibitor Screening of Coumarin-Originated Natural Products

Caspase-1 is a key player in pyroptosis and inflammation. Caspase-1 inhibition is found to be beneficial to various diseases. Coumarin-originated natural products have an anti-inflammation function, but their direct inhibition effect to caspase-1 remains unexplored. To evaluate their interactions, the widely used commercial coumarin-based probe (Ac-YVAD-AMC) is not suitable, as the background signal from coumarin-originated natural products could interfere with the screening results. Therefore, fluorescent probes using a large Stokes shift could help solve this problem. In this work, we chose the fluorophore of tetraphenylethylene-thiophene (TPETH) with aggregation-induced emission characteristics and a large Stokes shift of about 200 nm to develop a molecular probe. Bioconjugation between TPETH and hydrophilic peptides (DDYVADC) through a thiol-ene reaction generated a light-up probe, C1-P3. The probe has little background signal in aqueous media and exerts a fluorescent turn-on effect in the presence of caspase-1. Moreover, when evaluating the inhibition potency of coumarin-originated natural products, the new probe could generate a true and objective result but not for the commercial probe (Ac-YVAD-AMC), which is evidenced by HPLC analysis. The quick light-up response and accurate screening results make C1-P3 very useful in fundamental study and inhibitior screening toward caspase-1.

Veratramine modulates AP-1-dependent gene transcription by directly binding to programmable DNA

Because the transcription factor activator protein-1 (AP-1) regulates a variety of protein-encoding genes, it is a participant in many cellular functions, including proliferation, transformation, epithelial mesenchymal transition (EMT), and apoptosis. Inhibitors targeting AP-1 have potential use in the treatment of cancer and other inflammatory diseases. Here, we identify veratramine as a potent natural modulator of AP-1, which selectively binds to a specific site (TRE 5'-TGACTCA-3') of the AP-1 target DNA sequence and regulates AP-1-dependent gene transcription without interfering with cystosolic signaling cascades that might lead to AP-1 activation. Moreover, RNA-seq experiments demonstrate that veratramine does not act on the Hedgehog signaling pathway in contrast to its analogue, cyclopamine, and likely does not harbor the same teratogenicity and toxicity. Additionally, veratramine effectively suppresses EGF-induced AP-1 transactivation and transformation of JB6 P+ cells. Finally, we demonstrate that veratramine inhibits solar-ultraviolet-induced AP-1 activation in mice. The identification of veratramine and new findings in its specific regulation of AP-1 down stream genes pave ways to discovering and designing regulators to regulate transcription factor.

Nitroolefin-modified cyclometalated iridium(iii) complexes for tunable detection of biothiols with deep-red emission

Two nitroolefin-modified cyclometalated iridium(iii) complexes were employed as turn-on probes for the rapid (1 min) detection of biothiols with tunable emission.

Construction of a Nano Biosensor for Cyanide Anion Detection and Its Application in Environmental and Biological Systems

We have developed a Ag@Au core-shell nanoparticle (NP)/iridium(III) complex-based sensing platform for the sensitive luminescence "turn-on" sensing of cyanide ions, an acutely toxic pollutant. The assay is based on the quenching effect of Ag@Au NPs on the emission of complex 1, but luminescence is restored after the addition of cyanide anions due to their ability to dissolve the Au shell. Our sensing platform exhibited a high sensitivity toward cyanide anions with a detection limit of 0.036 μM, and also showed high selectivity for cyanide over 10-fold excess amounts of other anions. The sensing platform was also successfully applied to monitor cyanide anions in drinking water and in living cells.

Ultrasensitive Quantitation of Plasma Membrane Proteins via isRTA

Quantitation of plasma membrane proteins (PMPs) is fundamental and frequently performed daily in the lab. However, challenged by the inherent/interacting heterostructures and complex surroundings of the PMPs in lipid membrane, quantitative techniques for PMP often require complex treatments (e.g., labeling, isolation, purification, and determination), and the sensitivity is usually not satisfactory. To address this problem, we have proposed a novel method that enables quantitation of PMPs with extremely high sensitivity, in an easier-to-manipulate and more streamlined way. This method is based on the design of an in situ rolling cycling replication-templated amplification strategy (isRTA). In fact, two rounds of DNA cascade isothermal amplifications have been conducted. The first round of amplification can provide templates for the second round of amplification; thus, significant enhancement of quantitative signals can be achieved. In this way, PMPs are quantified with ultrahigh sensitivity; as few as 25 copies of PMPs can be detected per cell. Moreover, the advantages of isRTA have been demonstrated by simultaneous identification of several PMP biomarkers (MUC1, EpCAM, and HER2) that are expressed over a wide distribution range on breast cancer cells. The precise typing of breast cancer cell subsets is thus possible because of the "quantitative-to-qualitative" strategy. Therefore, the unprecedented sensitivity and high usability of the isRTA method may present significant prospects for delving into membrane proteins and their related biofunctions in many research fields.

Preparation of metalated azine complexes of iridium(iii)

The preparation of half-sandwich κ¹-aldazine and κ¹-ketazine complexes of iridium and their metalation reaction affording chelate κ²-azine derivatives are described. The photoluminescence properties of the chelate complexes are also described.