A bioactive ligand-conjugated iridium(III) metal-based complex as a Keap1-Nrf2 protein-protein interaction inhibitor against acetaminophen-induced acute liver injury.

Hepatotoxicity caused by an overdose of acetaminophen (APAP) is the leading reason for acute drug-related liver failure. Nuclear factor erythroid-2-related factor 2 (Nrf2) is a protein that helps to regulate redox homeostasis and coordinate stress responses via binding to the Kelch-like ECH-associated protein 1 (Keap1). Targeting the Keap1-Nrf2 interaction has recently emerged as a potential strategy to alleviate liver injury caused by APAP. Here, we designed and synthesized a number of iridium (III) and rhodium (III) complexes bearing ligands with reported activity against oxidative stress, which is associated with Nrf2 transcriptional activation. The iridium (III) complex 1 bearing a bioactive ligand 2,9-dimethyl-1,10-phenanthroline and 4-chloro-2-phenylquinoline, a derivative of the bioactive ligand 2-phenylquinoline, was identified as a direct small-molecule inhibitor of the Keap1-Nrf2 protein-protein interaction. 1 could stabilize Keap1 protein, upregulate HO-1 and NQO1, and promote Nrf2 nuclear translocation in normal liver cells. Moreover, 1 reversed APAP-induced liver damage by disrupting Keap1-Nrf2 interaction and without inducing organ damage and immunotoxicity in mice. Our study demonstrates the identification of a selective and efficacious antagonist of Keap1-Nrf2 interaction possessed good cellular permeability in cellulo and ideal pharmacokinetic parameters in vivo, and, more importantly, validates the feasibility of conjugating metal complexes with bioactive ligands to generate metal-based drug leads as non-toxic Keap1-Nrf2 interaction inhibitors for treating APAP-induced acute liver injury.

A robust photoluminescence screening assay identifies uracil-DNA glycosylase inhibitors against prostate cancer.

Many cancers have developed resistance to 5-FU, due to removal by the enzyme uracil-DNA glycosylase (UDG), a type of base excision repair enzyme (BER) that can excise uracil and 5-fluorouracil (5-FU) from DNA. However, the development of UDG inhibitor screening methods, especially for the rapid and efficient screening of natural product/natural product-like compounds, is still limited so far. We developed herein a robust time-resolved photoluminescence method for screening UDG inhibitors, which could significantly improve sensitivity over the screening method based on the conventional steady-state spectroscopy, reducing the substantial fluorescence background interference. As a proof-of-concept, two potential UDG inhibitors were identified from a database of natural products and approved drugs. Co-treatment of these two compounds with 5-FU showed synergistic cytotoxicity, providing the basis for treating drug-resistant cancers. Overall, this method provides an avenue for the rapid screening of small molecule regulators of other BER enzyme activities that can avoid false negatives arising from the background fluorescence.

An iridium(III) complex/G-quadruplex ensemble for detection of ochratoxin A based on long-lifetime luminescent.

A G-quadruplex-based platform has been developed for the time-resolved monitoring of ochratoxin A (OTA). The simple platform displays good sensitivity for OTA with a detection limit of 40 nM via steady-state emission spectroscopy. Notably, the platform showed a detection limit of 10.8 nM via time-resolved emission spectroscopy (TRES), which is about 4 times more sensitive than steady-state mode. Moreover, the probe showed excellent selectivity for OTA over other mycotoxins. Furthermore, OTA was successfully detected in actual herbal plant extracts samples. Our platform is the first to detect OTA using TRES to distinguish between the target signals versus the auto-fluorescence of real samples. This platform shows improved detection speed, accuracy and sensitivity with simple operation, low cost, and no requirement for complicated pre-processing.

Iridium(III)-based chemosensors for the detection of metal ions.

In recent years, transition metal complexes with their prominent photophysical properties have emerged as versatile chemosensors to probe different target analytes, including metal ions. By incorporating specific metal ion receptors, various iridium(III) complex-based cation sensors have been developed using different mechanisms. In this review, we survey examples of iridium(III) complex-based metal ion chemosensors that have been reported in the literature. Their design, mechanism and outlook will also be discussed.

Rhodium(III)-Based Inhibitor of the JMJD3-H3K27me3 Interaction and Modulator of the Inflammatory Response.

We describe in this study a rhodium(III) complex 1 as a new JMJD3 inhibitor and proinflammatory mediator. Complex 1 selectively inhibited the demethylation of H3K27me3 over other similar substrates, indicating its selectivity for JMJD3 over other histone demethylases, including JMJD2D and KDM5A. In terms of mechanism, complex 1 inhibited the JMJD3-H3K27me3 interaction in mouse macrophage cells and down-regulated the expression of TNF-α. To our knowledge, complex 1 is the first metal-based inhibitor of JMJD3 activity and only the second class of JMJD3 inhibitor reported overall.

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. This review highlights recent examples of iridium-based compounds that have been developed as probes for various environmental analytes. We also discuss the further challenges to be overcome for this class of probes in the future.

Iridium-based probe for luminescent nitric oxide monitoring in live cells.

Nitric oxide (NO) is an intra- and extracellular messenger with important functions during human physiology process. A long-lived luminescent iridium(III) complex probe 1 has been designed and synthesized for the monitoring of NO controllably released from sodium nitroprusside (SNP). Probe 1 displayed a 15-fold switch-on luminescence in the presence of SNP at 580 nm. The probe exhibited a linear response towards SNP between 5 to 25 µM with detection limit at 0.18 µM. Importantly, the luminescent switch-on detection of NO in HeLa cells was demonstrated. Overall, complex 1 has the potential to be applied for NO tracing in complicated cellular environment.

Metalated Chromene and Chromone Complexes: pH Switchable Metal-Carbon Bonding Interaction, Photo-triggerable Chromone Delivery Application, and Antioxidative Activity.

The two families of RuII -chromene and -chromone complexes isolated in this work represent the first examples of metalated chromene and chromone complexes synthesized through transition-metal-mediated cyclization of phenol-tethered ynone. These unprecedented metalated heterocyclic compounds exhibit remarkable features, such as pH-switchable metal-carbon bonding interactions, photo-triggerable release of organic chromone upon visible-light irradiation, and superior antioxidative property to their organic analogue (1,4-benzopyrone). These findings not only offer mechanistic insights into metal-induced activation of functionalized alkynes, but also add a new dimension to rational design of antioxidants and photo-responsive drug delivery systems.

An anti-prostate cancer benzofuran-conjugated iridium(III) complex as a dual inhibitor of STAT3 and NF-κB.

Four benzofuran-conjugated iridium(III) or rhodium (III)-based metal complexes are synthesized to screen as an inhibitor of STAT3 activity in prostate cancer cells. All complexes show the high stability and solubility in the biological system. In this study, an iridium(III) complex engages STAT3 and NF-κB to inhibit their translocation and transcriptional activities. Moreover, complex 1 shows more potential antiproliferative activity against DU145 cells and suppresses tumor growth in a prostate cancer xenograft mouse without observable adverse effects. Complex 1 may provide the basis for developing new therapeutic strategy in vivo and in vitro for the treatment of advanced prostate cancer.

Utilization of G-Quadruplex-Forming Aptamers for the Construction of Luminescence Sensing Platforms.

Aptamers are nucleic acid sequences that can recognize and bind to analytes with high affinity and selectivity. Intriguingly, a number of aptamers undergo a conformational change within the G-quadruplex motif upon ligand binding. This Minireview aims to highlight interesting examples of luminescent G-quadruplex aptamer-based probes that have been developed in recent years. Various mechanisms and sensing modes are described, and the outlook and future directions of this field are also discussed.

A Rhodium(III) Complex as an Inhibitor of Neural Precursor Cell Expressed, Developmentally Down-Regulated 8-Activating Enzyme with in Vivo Activity against Inflammatory Bowel Disease.

We report herein the identification of the rhodium(III) complex [Rh(phq)2(MOPIP)]+ (1) as a potent and selective ATP-competitive neural precursor cell expressed, developmentally down-regulated 8 (NEDD8)-activating enzyme (NAE) inhibitor. Structure-activity relationship analysis indicated that the overall organometallic design of complex 1 was important for anti-inflammatory activity. Complex 1 showed promising anti-inflammatory activity in vivo for the potential treatment of inflammatory bowel disease.

A G-quadruplex-selective luminescent iridium(III) complex and its application by long lifetime.

BACKGROUND: The G-quadruplex motif has been widely used for the construction of analytical detection platforms due to its rich structural polymorphism and flexibility. Luminescent assays are often limited due to the interference from endogenous fluorophores in biological samples. METHODS: To address this challenge, a novel long lifetime iridium(III) complex 1 was synthesized and used to construct a G-quadruplex-based assay for detecting prostate specific antigen (PSA) in aqueous solution. PSA is a common biomarker in serum and used as a model for demonstration in this work. RESULTS: The PSA assay has achieved a detection limit of 40.8pg·mL-1, and shows high selectivity towards PSA over other proteins. Additionally, the assay could function in diluted human serum by using time-resolved luminescent spectroscopy, with good linearity from 1 to 10ng·mL-1 of PSA, which is adequate to detect the PSA levels for physiological (<4ng·mL-1) and clinical (4-10ng·mL-1) applications. CONCLUSIONS: The assay was successfully constructed. As revealed from time-resolved method, the long lifetime property of iridium(III) complex 1 plays an important role in distinguishing phosphorescence signals from short-life auto-fluorescence of human serum. GENERAL SIGNIFICANCE: Luminescent transition metal complexes offer several advantages over other widely used organic fluorophores, such as long phosphorescence lifetime, large Stokes shift and modular syntheses. In addition, the assay could work effectively in diluted human serum using time-resolved luminescent spectroscopy, it therefore could be potentially developed to monitor PSA in biological samples. This article is part of a Special Issue entitled "G-quadruplex" Guest Editor: Dr. Concetta Giancola and Dr. Daniela Montesarchio.

A rhodium(III)-based inhibitor of autotaxin with antiproliferative activity.

BACKGROUND: Cancer of the skin is by far the most common of all cancers. Melanoma accounts for only about 1% of skin cancers but causes a large majority of skin cancer deaths. Autotaxin (ATX), also known as ectonucleotide pyrophosphatase/phosphodiesterase 2 (ENPP2), regulates physiological and pathological functions of lysophosphatidic acid (LPA), and is thus an important therapeutic target. METHODS: We synthesized ten metal-based complexes and a novel cyclometalated rhodium(III) complex 1 was identified as an ATX enzymatic inhibitor using multiple methods, including ATX enzymatic assay, thermal shift assay, western immunoblotting and so on. RESULTS: Protein thermal shift assays showed that 1 increased the melting temperature (Tm) of ATX by 3.5°C. 1 also reduced ATX-LPA mediated downstream survival signal pathway proteins such as ERK and AKT, and inhibited the activation of the transcription factor nuclear factor κB (NF-κB) and signal transducer and activator of transcription 3 (STAT3). 1 also exhibited strong anti-proliferative activity against A2058 melanoma cells (IC50=0.58µM). Structure-activity relationship indicated that both the rhodium(III) center and the auxiliary ligands of complex 1 are important for bioactivity. CONCLUSIONS: 1 represents a promising scaffold for the development of small-molecule ATX inhibitors for anti-tumor applications. To our knowledge, complex 1 is the first metal-based ATX inhibitor reported to date. GENERAL SIGNIFICANCE: Rhodium complexes will have the increased attention in therapeutic and bioanalytical applications.

Antagonizing STAT5B dimerization with an osmium complex.

Targeting STAT5 is an appealing therapeutic strategy for the treatment of hematologic malignancies and inflammation. Here, we present the novel osmium(II) complex 1 as the first metal-based inhibitor of STAT5B dimerization. Complex 1 exhibited superior inhibitory activity against STAT5B DNA binding compared to STAT5A DNA binding. Moreover, 1 repressed STAT5B transcription and blocked STAT5B dimerization via binding to the STAT5B protein, thereby inhibiting STAT5B translocation to the nucleus. Furthermore, 1 was able to selectively inhibit STAT5B phosphorylation without affecting the expression level of STAT5B.

An Ir(III) complex chemosensor for the detection of thiols.

In this study, we report the use of a cyclometalated luminescent iridium(III) complex for the visualization of thiols. The detection of glutathione (GSH) by complex 1 is achieved through the reduction of its phendione N^N donor, which influences the metal-to-ligand charge-transfer (MLCT) of the complex. Complex 1 produced a maximum threefold luminescence enhancement at 587 nm in response to GSH. The linear detection range of 1 for GSH is between 0.2 and 2 M equivalents of GSH, with a detection limit of 1.67 µM. Complex 1 also displays good selectivity for thiols over other amino acids.

Interaction of an Iridium(III) Complex with G-Quadruplex DNA and Its Application in Luminescent Switch-On Detection of Siglec-5.

Sialic acid (Sia) binding immunoglobulin (Ig)-like lectin-5 (Siglec-5) is a type-I transmembrane protein, and it has been demonstrated as a biomarker of granulocytic maturation and acute myeloid leukemia phenotype. Herein we aimed to construct a method that could sensitively detect Siglec-5 by taking advantage of the high affinity and selectivity of the K19 aptamer for its cognate target, and the selective interaction of luminescent iridium(III) transition metal complexes with G-quadruplex DNA. The iridium(III) complex 1 [Ir(tpyd)2(2,9-dmphen)]PF6 (where tpyd =2-(m-tolyl)pyridine; 2,9-dmphen =2,9-dimethyl-1,10-phenanthroline) was synthesized, and it displayed high luminescence for G-quadruplex DNA compared to dsDNA and ssDNA. Additionally, complex 1 exhibited a blue shift luminescence response to c-kit2 G-quadruplex, and the interaction between 1 and G-quadruplexes was discussed based on the results of G-tetrad assay, loop effect assay, and other assays. Then complex 1 was utilized to develop a G-quadruplex-based sensing platform for Siglec-5 in aqueous solution. Upon the addition of Siglec-5, the specific binding of the K19 aptamer sequence results in a conformational change that generates a split G-quadruplex structure, which is then recognized by the G-quadruplex-specific iridium(III) complex with an enhanced luminescent response. Futhermore, the use of the assay for detecting Siglec-5 in cellular debris was demonstrated.

A G-quadruplex-selective luminescent probe with an anchor tail for the switch-on detection of thymine DNA glycosylase activity.

Thymine DNA glycosylase (TDG) performs essential functions in maintaining genetic integrity and epigenetic regulation, which also plays an essential role in DNA demethylation. In this work, the novel iridium(III) complex 1 with an anchor tail was synthesized and employed to construct a G-quadruplex-based assay for detecting TDG activity in aqueous solution by using the mismatched base excising property of TDG with T4 DNA ligase and phi29 DNA polymerase, in concert with the rolling circle amplification (RCA) strategy. The assay achieved a detection limit of 0.048UmL(-)(1) (0.012ngmL(-1)), and showed high selectivity towards TDG even in the presence of other proteins and enzymes. Additionally, the assay could function in diluted cellular debris.

A versatile nanomachine for the sensitive detection of platelet-derived growth factor-BB utilizing a G-quadruplex-selective iridium(III) complex.

The novel iridium(III) complex 1 was found to be highly selective for G-quadruplex DNA, and was employed for the development of a versatile nanomachine. In the nanomachine, the single-stranded DNA (ssDNA) oligonucleotide ON1 hybridizes with the hairpin DNA oligonucleotide ON2, leading to the formation of a 5'-recessed double-stranded DNA (dsDNA) structure and the simultaneous release of a G-rich DNA sequence, which folds into a G-quadruplex motif that is recognized by complex 1. Upon the addition of Exo III, the dsDNA substrate is digested leading to the liberation of ON1 ssDNA, which enters a new cycle of the nanomachine. This nanomachine was successfully used to detect PDGF-BB by combination with a simple pre-procedure module, and exhibited a linear response between luminescence intensity and PDGF-BB concentration in the range of 10 to 300pM (R(2)=0.992), with a limit of detection for PDGF-BB of 10pM. This assay was highly selective for PDGF-BB over other proteins, and exhibited potential use in biological sample analysis. Moreover, the versatility of the nanomachine was demonstrated by adapting the nanomachine for the detection of thrombin by simply changing the pre-procedure module.

Identification of an Iridium(III)-Based Inhibitor of Tumor Necrosis Factor-α.

The novel iridium(III) complex 1 was verified as a potent inhibitor of the TNF-α-TNFR protein-protein interaction in vitro and in cellulo. The iridium(III) center plays a critical role in organizing the structure of the bioactive metal complex, as the isolated ligands were found to be completely inactive. Both iridium enantiomers inhibited TNF-α-induced NF-κB activity and TNF-α-TNFR binding. 1 represents a promising scaffold for the further development of more potent organometallic TNF-α inhibitors.

A novel dinuclear iridium(III) complex as a G-quadruplex-selective probe for the luminescent switch-on detection of transcription factor HIF-1α.

A novel dinuclear Ir(III) complex 5 was discovered to be specific to G-quadruplex DNA, and was utilized in a label-free G-quadruplex-based detection platform for transcription factor activity. The principle of this assay was demonstrated by using HIF-1α as a model protein. Moreover, this HIF-1α detection assay exhibited potential use for biological sample analysis.