Cyclometalated Iridium(III) Bipyridine-Phenylboronic Acid Complexes as Bioimaging Reagents and Luminescent Probes for Sialic Acids

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.

Dynamic Ir(III) Photosensors for the Major Human Drug-Metabolizing Enzyme Cytochrome P450 3A4

Probing the activity of cytochrome P450 3A4 (CYP3A4) is critical for monitoring the metabolism of pharmaceuticals and identifying drug-drug interactions. A library of Ir(III) probes that detect occupancy of the CYP3A4 active site were synthesized and characterized. These probes show selectivity for CYP3A4 inhibition, low cellular toxicity, Kd values as low as 9 nM, and are highly emissive with lifetimes up to 3.8 μs in cell growth media under aerobic conditions. These long emission lifetimes allow for time-resolved gating to distinguish probe from background autofluorescence from growth media and live cells. X-ray crystallographic analysis revealed structure-activity relationships and the preference or indifference of CYP3A4 toward resolved stereoisomers. Ir(III)-based probes show emission quenching upon CYP3A4 binding, then emission increases following displacement with CYP3A4 inhibitors or substrates. Importantly, the lead probes inhibit the activity of CYP3A4 at concentrations as low as 300 nM in CYP3A4-overexpressing HepG2 cells that accurately mimic human hepatic drug metabolism. Thus, the Ir(III)-based agents show promise as novel chemical tools for monitoring CYP3A4 active site occupancy in a high-throughput manner to gain insight into drug metabolism and drug-drug interactions.

Ir(III)-Based Agents for Monitoring the Cytochrome P450 3A4 Active Site Occupancy

Cytochromes P450 (CYPs) are a superfamily of enzymes responsible for biosynthesis and drug metabolism. Monitoring the activity of CYP3A4, the major human drug-metabolizing enzyme, is vital for assessing the metabolism of pharmaceuticals and identifying harmful drug-drug interactions. Existing probes for CYP3A4 are irreversible turn-on substrates that monitor activity at specific time points in end-point assays. To provide a more dynamic approach, we designed, synthesized, and characterized emissive Ir(III) and Ru(II) complexes that allow monitoring of the CYP3A4 active-site occupancy in real time. In the bound state, probe emission is quenched by the active-site heme. Upon displacement from the active site by CYP3A4-specific inhibitors or substrates, these probes show high emission turn-on. Direct probe binding to the CYP3A4 active site was confirmed by X-ray crystallography. The lead Ir(III)-based probe has nanomolar Kd and high selectivity for CYP3A4, efficient cellular uptake, and low toxicity in CYP3A4-overexpressing HepG2 cells.

Aggregation-Induced Emission (AIE) Photosensitizer Combined Polydopamine Nanomaterials for Organelle-Targeting Photodynamic and Photothermal Therapy by the Recognition of Sialic Acid

The construction of organelle-targeting nanomaterials is an effective way to improve tumor imaging and treatment. Here, a new type of composite nanomaterial named as PTTPB is developed. PTTPB is composed of organelle-targeting aggregation-induced emission photosensitizer TTPB and polydopamine nanomaterials. With the functional modification of TTPB, PTTPB can recognize sialic acid on the cell membrane and present mitochondrial targeted capabilities. The intake of PTTPB in cancerous cells can be increased by the recognition process of cell membrane. PTTPB can generate singlet oxygen for photodynamic therapy (PDT), and present good photothermal conversion ability with irradiation. The PTTPB with organelle-targeting imaging-guided can realize the tumor ablation with the synergistic effect of PDT and photothermal therapy.

Phenylboronic-acid-based Functional Chemical Materials for Fluorescence Imaging and Tumor Therapy

Various functional chemical materials have been widely used in imaging and tumor therapy. Targeted ligands such as antibodies, peptides, and small molecules have been combined with functional materials to enhance cellular uptake and are used for active targeting of cancer cells and tumors. Among them, phenylboronic acid (PBA), as a small molecular ligand, has the characteristics of low cytotoxicity and easy modification. PBA improves the cancer cell imaging and tumor treatment effect by binding to glycans on the surface of cancer cells. In this Mini-Review, we introduced the modification strategy and targeting strategy of PBA. We focused on the applications of PBA-based functional materials in fluorescence imaging and tumor therapy. For fluorescence imaging, the potential of PBA-based functional chemical materials in cancer diagnosis and tumor targeting was proved by cell imaging and in vivo imaging. For tumor therapy, we mainly discussed the applications of PBA-based functional chemical materials in chemotherapy, gene therapy, phototherapy, and immunotherapy. PBA-based functional chemical materials provide a useful method for cancer diagnosis and treatment.

Photo- and Electrochemical Dual-Responsive Iridium Probe for Saccharide Detection

Dual detection systems are of interest for rapid, accurate data collection in sensing systems and in vitro testing. We introduce an Ir^III complex with a boronic acid receptor site attached to the 2‐phenylpyridine ligand as an ideal probe with photo‐ and electrochemical signals that is sensitive to monosaccharide binding in aqueous solution. The complex displays orange luminescence at 618 nm, which is reduced by 70 and 40 % upon binding of fructose and glucose, respectively. The electro‐chemiluminescent signal of the complex also shows a direct response to monosaccharide binding. The Ir^III complex shows the same response upon incorporation into hydrogel matrices as in solution, thus demonstrating the potential of its integration into a device, as a nontoxic, simple‐to‐use tool to observe sugar binding over physiologically relevant pH ranges and saccharide concentrations. Moreover, the complex's luminescence is responsive to monosaccharide presence in cancer cells.

Bioresponsive Functional Phenylboronic Acid-Based Delivery System as an Emerging Platform for Diabetic Therapy

The glucose-sensitive self-adjusting drug delivery system simulates the physiological model of the human pancreas-secreting insulin and then precisely regulates the release of hypoglycemic drugs and controls the blood sugar. Thus, it has good application prospects in the treatment of diabetes. Presently, there are three glucose-sensitive drug systems: phenylboronic acid (PBA) and its derivatives, concanavalin A (Con A), and glucose oxidase (GOD). Among these, the glucose-sensitive polymer carrier based on PBA has the advantages of better stability, long-term storage, and reversible glucose response, and the loading of insulin in it can achieve the controlled release of drugs in the human environment. Therefore, it has become a research hotspot in recent years and has been developed very rapidly. In order to further carry out a follow-up study, we focused on the development process, performance, and application of PBA and its derivatives-based glucose-sensitive polymer drug carriers, and the prospects for the development of this field.

[Negative regulation of gastric proton pump by desialylation suggested by fluorescent imaging with the sialic acid-specific nanoprobe]

Gastric proton pump (H⁺,K⁺-ATPase) which is responsible for H⁺ secretion of gastric acid (HCl) in gastric parietal cells is the major therapeutic target for treatment of acid-related diseases. H⁺,K⁺-ATPase consists of two subunits, a catalytic α-subunit (αHK) and a glycosylated β-subunit (βHK). N-glycosylation of βHK is essential for trafficking and stability of αHK in apical membrane of gastric parietal cells. Terminal sialic acid residues on sugar chains have an important role in various cellular functions. Recently, we succeeded in visualizing the sialylation and desialylation dynamics of βHK using a fluorescence bioimaging nanoprobe consisting of biocompatible polymers conjugated with lectins for detecting sialic acid. In H⁺,K⁺-ATPase-expressing cell lines, rat gastric mucosa, and primary culture of rat gastric parietal cells, fluorescence imaging of sialic acid with the nanoprobe showed that sialylation of βHK is regulated by intragastric pH and that inhibition of gastric acid secretion induces desialylation of βHK. In biochemical and pharmacological studies, we revealed that enzyme activity of αHK is negatively regulated by desialylation of βHK. Our studies uncovered a novel negative-feedback mechanism of H⁺,K⁺-ATPase in which sialic acids of βHK positively regulates H⁺,K⁺-ATPase activity, and acidic pH decreases the pump activity by cleaving sialic acids of βHK. In this topic, we introduce the overview of our research using the bioimaging nanoprobe.

Iridium(iii) polypyridine complexes with a disulfide linker as biological sensors and cytotoxic agents

The synthesis, characterization, and photophysical properties of novel iridium(iii) polypyridine complexes as thiol-sensing probes and cytotoxic agents are reported.