Detecting Basal Myeloperoxidase Activity in Living Systems with a Near-Infrared Emissive "Turn-On" Probe

Dual-reporter fluorescent probe for precise identification of liver cancer by sequentially responding to carboxylesterase and polarity

Early treatment significantly improves the survival rate of liver cancer patients, so the development of early diagnostic methods for liver cancer is urgent. Liver cancer can develop from viral hepatitis, alcoholic liver, and fatty liver, thus making the above diseases share common features such as elevated viscosity, reactive oxygen species, and reactive nitrogen species. Therefore, accurate differentiation between other liver diseases and liver cancer is both a paramount practical need and challenging. Numerous fluorescent probes have been reported for the diagnosis of liver cancer by detecting a single biomarker, but these probes lack specificity for liver cancer in complex biological systems. Obviously, using multiple liver cancer biomarkers as the basis for judgment can dramatically improve diagnostic accuracy. Herein, we report the first fluorescent probe, LD-TCE, that sequentially detects carboxylesterase (CE) and lipid droplet polarity in liver cancer cells with high sensitivity and selectivity, with linear detection of CE in the range of 0-6 U/mL and a 65-fold fluorescence enhancement in response to polarity. The probe first reacts with CE and releases weak fluorescence, which is then dramatically enhanced due to the decrease in lipid droplet polarity in liver cancer cells. This approach allows the probe to enable specific imaging of liver cancer with higher contrast and accuracy. The probe successfully achieved the screening of liver cancer cells and the precise identification of liver cancer in mice. More importantly, it is not disturbed by liver fibrosis, which is a common pathological feature of many liver diseases. We believe that the LD-TCE is expected to be a powerful tool for early diagnosis of liver cancer.

Ex Tenebris Lux: Illuminating Reactive Oxygen and Nitrogen Species with Small Molecule Probes

Reactive oxygen and nitrogen species are small reactive molecules derived from elements in the air─oxygen and nitrogen. They are produced in biological systems to mediate fundamental aspects of cellular signaling but must be very tightly balanced to prevent indiscriminate damage to biological molecules. Small molecule probes can transmute the specific nature of each reactive oxygen and nitrogen species into an observable luminescent signal (or even an acoustic wave) to offer sensitive and selective imaging in living cells and whole animals. This review focuses specifically on small molecule probes for superoxide, hydrogen peroxide, hypochlorite, nitric oxide, and peroxynitrite that provide a luminescent or photoacoustic signal. Important background information on general photophysical phenomena, common probe designs, mechanisms, and imaging modalities will be provided, and then, probes for each analyte will be thoroughly evaluated. A discussion of the successes of the field will be presented, followed by recommendations for improvement and a future outlook of emerging trends. Our objectives are to provide an informative, useful, and thorough field guide to small molecule probes for reactive oxygen and nitrogen species as well as important context to compare the ecosystem of chemistries and molecular scaffolds that has manifested within the field.

Probing metalloenzyme dynamics in living systems: Contemporary advances in fluorescence imaging tools and applications

Metalloenzymes are essential to cellular function, and their overexpression or enhanced activation are potential therapeutic targets. However, the study of metalloenzymes in vitro presents various challenges, leading many to develop tools to study them in their native cellular environment. Small-molecule fluorescence probes are commonly used to monitor metalloenzyme function, activity, and distribution in situ. These include probes that are activity-based (fluorescence is mediated by enzyme activity) or binding-based (fluorescence is mediated by interactions with the enzyme upon binding its metal cofactor). We discuss recent innovations that overcome key design challenges, such as the rapid diffusion of activity-based probes, the difficulty of probing redox-active enzymes, the selectivity of binding-based probes, and the poor penetration depth of fluorescence, and describe novel applications of these tools.

A Hypochlorite-Activated Theranostic Prodrug for Selective Imaging of High Myeloperoxidase Expression Acute Myeloid Leukemia Cells and Drug Release

Acute myeloid leukemia (AML) is a fatal hematologic disease. Diagnosis and proper treatment are important for prognosis. High myeloperoxidase (MPO) expression AML cells are characterized with high levels of hypochlorite (ClO-). In this study, we report a ClO--activated theranostic agent, FNC, for AML therapy. FNC responds to ClO- specifically in high MPO expression AML cells, resulting in bright fluorescence and chlorambucil release. FNC can be used to quickly distinguish high MPO expression AML cells from other cells, including low MPO expression leukemia and activated inflammatory cells. FNC exhibits selective toxicity to highly MPO expression AML cells and can efficiently inhibit tumor growth. Meanwhile, FNC can be used to indicate differentiation through the detection of ClO-.

Fabrication of CdS quantum dots with egg white and application in the assay of hypochlorous acid and myeloperoxidase activity and inhibition

The myeloperoxidase (MPO)/H2O2-Cl- enzymatic reaction system and its product hypochlorous acid (HOCl) are closely related to many disease processes, and new methods to detect the levels of HOCl and MPO are being focused on. MPO is the only known enzyme for the catalytic production of HOCl in biological systems; therefore, monitoring the HOCl levels is a selective and direct readout of MPO activity. This study reported a simple and efficient fluorescence assay of HOCl and MPO activity and inhibition. Highly fluorescent CdS quantum dots (CdS QDs) were prepared in one pot where NaOH-pretreated egg white served as a stabilizer. These CdS QDs exhibit strong green emission centered at ca. 550 nm and enable rapid and selective fluorescence response to HOCl with a linear detection range of 8.0-250 μM and a limit of detection (LOD) of 2.5 μM. Moreover, the CdS QDs were further applied for sensing MPO based on the fluorescence quenching exerted by its reaction product HOCl. Detection of MPO is accomplished with a linear range from 0.1 to 40 mU mL-1 (1 U is the MPO concentration for catalysis of 1 micromolar substrate per minute) and a LOD of 0.06 mU mL-1. The developed synthesis method can be applied to large-scale synthesis of CdS QDs, and the strategy to sense HOCl and MPO activity and inhibition has potential biomedical applications such as clinical diagnosis and drug screening.

Detection and Imaging of Active Substances in Early Atherosclerotic Lesions Using Fluorescent Probes

Atherosclerosis (AS) is a vascular disease caused by chronic inflammation and lipids that is the main cause of myocardial infarction, stroke and other cardiovascular diseases. Atherosclerosis is often difficult to detect in its early stages due to the absence of clinically significant vascular stenosis. This is not conducive to early intervention or treatment of the disease. Over the past decade, researchers have developed various imaging methods for the detection and imaging of atherosclerosis. At the same time, more and more biomarkers are being found that can be used as targets for detecting atherosclerosis. Therefore, the development of a variety of imaging methods and a variety of targeted imaging probes is an important project to achieve early assessment and treatment of atherosclerosis. This paper provides a comprehensive review of the optical probes used to detect and target atherosclerosis imaging in recent years, and describes the current challenges and future development directions.

Fluorescent molecular probes for imaging and detection of oxidases and peroxidases in biological samples

Oxidases and peroxidases are two subclasses of oxidoreductases. The abnormal expression of oxidases (such as tyrosinase, cytochrome P450 oxidases, and monoamine oxidases) and peroxidases (such as glutathione peroxidase, myeloperoxidase, and eosinophil peroxidase) is relative with some diseases. Therefore, the analysis of oxidases and peroxidases is great important for disease diagnosis and treatment. Fluorescent probes present simple protocol, high sensitivity and good stability in sensing field. Molecule fluorescent probes are constructed with chemical groups that tunes their fluorescence emission in response to binding events, chemical reactions, and the surrounding environment. A fluorescent probe is an efficient tool for visualizing the activity of enzymes in living organisms on the basis of its high specificity, sensitivity, and noninvasiveness characteristics. In this review, we focus on the sensing of oxidases and peroxidases by molecule fluorescent probes, and hope to bring new insight to wide researchers about oxidases and peroxidases in biological samples.

Tracking HOCl by an incredibly simple fluorescent probe with AIE plus ESIPT in vitro and in vivo

Hypochlorous acid is an important active substance involved in a variety of physiological processes in living organisms, while abnormal concentrations of HOCl are strongly associated with a variety of diseases such as cancer, inflammation, atherosclerosis, and Alzheimer's disease. As a result, it's crucial to establish a reliable method for tracking HOCl in vivo in order to investigate its physiological consequences. In this work, we developed a fluorescent probe DFSN with both AIE and ESIPT for imaging HOCl in vivo. DFSN not only has a basic structure and is easy to synthesize, but also has superior performance. The probe responds to HOCl in less than 10 s and has good selectivity and sensitivity to HOCl (DL = 6.3 nM), with a 110-fold increase in fluorescence intensity following response. In addition, DFSN can realize the rapid detection of hypochlorous acid with naked eyes. Moreover, DFSN can be used for the detection of exogenous and endogenous HOCl in RAW264.7 cells, and additionally enables the tracking of HOCl in cancer cells (Hela cells and HepG2 cells). More notably, it has been utilized to image hypochlorous acid in zebrafish with great success. The probe DFSN will be useful in determining the physiological significance of HOCl.

HClO-Activated Near-Infrared Fluorogenic Aza-BODIPY-Bisferrocene Triad with High Turn-on Ratio for In Vivo Biosensing

Optically monitoring hypochlorous acid (HClO) in living body favors diagnosis and study of inflammatory diseases. However, this has been hampered by limited strategies to develop highly fluorogenic tools in the deep-penetration near-infrared spectrum. Herein, a near-infrared aza-BODIPY-bisferrocene triad Fc₂ -CBDP that unexpectedly achieves an exceptionally sensitive and selective fluorescence turn-on (>220-fold) response toward HClO through single-ferrocene oxidation and boron-alkynyl hydrolysis cascade is reported. Mechanism insight shows that Fc₂ -CBDP features "enhanced charge transfer"-caused quenching due to intramolecular bisferrocene electronic coupling, which is decoupled in the reaction with HClO. The utility of Fc₂ -CBDP for intracellular HClO imaging is evaluated and, more importantly, in vivo high-contrast deep-tissue imaging of lymphatic inflammation and colitis is realized. This work provides new insights into both HClO and ferrocene chemistry, and extends the reach of fluorogenic strategies in the near-infrared biosensing.

Targeted Imaging in Atherosclerosis

Atherosclerosis is a chronic inflammatory disease involved in plaque rupture, stroke, thrombosis, and heart attack (myocardial infarction), which is a leading cause of sudden cardiovascular events. In the past decades, various imaging strategies have been designed and employed for the diagnosis of atherosclerosis. Targeted imaging can accurately distinguish pathological tissues from normal tissues and reliably reveal biological information in the occurrence and development of atherosclerosis. By taking advantage of versatile imaging techniques, rationally designed imaging probes targeting biomarkers overexpressed in plaque microenvironments and targeting activated cells by modifying specific ligands accumulated in lesion regions have attracted increasing attention. This Perspective elucidates comprehensively the targeted imaging strategies, current challenges, and future development directions for precise identification and diagnosis of atherosclerosis, which is beneficial to better understand the physiological and pathological progression and exploit novel imaging strategies.

HOCl-Activated Reactive Organic Selenium Delivery Platform for Alleviation of Inflammation

Selenium plays an important role in the biological system and can be used to treat various types of diseases. However, the current selenium delivery systems face the problems of low activity of released Se-containing compounds or nonspecific toxicity of reactive organic selenium donors in living systems. In response to these problems, we constructed a reactive organic selenium delivery platform by the activation of HOCl. Compared with prodrugs without activation capability, the hypochloroselenoite derivatives released from the present platform after activation displayed higher reactivity and could react with various nucleophiles to participate in specific life processes. Taking the selected compound (DHU-Se1) as an example, we found that it could alleviate the process of inflammation by blocking the polarization of macrophages from M0 to M1. Therefore, the development of this system is of great significance for expanding the application of selenium-containing compounds and treating related diseases.

Coumarin-based fluorescent probes toward viscosity in mitochondrion/lysosome

Viscosity is an important microenvironmental indicator that plays an important role in the process of information transmission in various regions. Herein, two coumarin-based viscosity-sensitive fluorescent probes (CHB, CHN) were synthesized and the photophysical properties of the two probes were studied. The fluorescence quantum yields of CHB and CHN in glycerol can be as high as 25.2% and 18.3% respectively. The two probes can linearly detect viscosity in the viscosity logarithm range of 0.83-2.07, which is not interfered with pH, metal ions, anions and biomolecules. Fluorescent confocal cell experiments show CHB and CHN have good targeting ability to mitochondrion, lysosome, Endoplasmic reticulum and Golgi apparatus, and can be used to detect viscosity in mitochondrion/lysosome.

An excipient-free “sugar-coated bullet” for the targeted treatment of orthotopic hepatocellular carcinoma

Several components of traditional nanoformulations that result in structural heterogeneity, poor reproducibility, excipient-trigged biotoxicity, and low retention of antitumor drugs in neoplastic foci are important barriers limiting clinical translation. We report an excipient-free nanoformulation prepared by a reactive oxygen species (ROS)-responsive amphiphilic prodrug (Gal-MB-DOX) for the targeted treatment of orthotopic hepatocellular carcinoma (HCC). Gal-MB-DOX can form monocomponent nanoparticles with a galactose-rich surface similar to a “sugar-coated bullet” through self-assembly in aqueous solution. This nanoformulation can be decomposed quickly by ROS and release free hydrophobic drugs that further precipitate into larger particles, potentially promoting the retention of drugs in tumor cells. These sugar-coated bullets selectively target tumor cells through passive and active targeting, resulting in high in vivo therapeutic efficacy in an orthotopic HCC mouse model. This monocomponent nanomedicine system provides a simple but effective strategy for the treatment of tumors.

An excipient-free nanoformulation is prepared by a reactive oxygen species-responsive prodrug. These sugar-coated bullets selectively target tumor cells, resulting in high in vivo therapeutic efficacy in an orthotopic HCC mouse model.

Aggregation-Induced Emission (AIE) Nanoparticles-Assisted NIR-II Fluorescence Imaging-Guided Diagnosis and Surgery for Inflammatory Bowel Disease (IBD)

The incidence of inflammatory bowel diseases (IBD), including Crohn's diseases and ulcerative colitis, is increasing by time and showing a trend of younger age. Precise diagnosis and effective treatments for IBD have attracted growing attention in recent years. However, diagnosing and locating inflammatory lesions remain a great challenge for IBD. In this study, assisted by a kind of aggregation-induced emission (AIE) nanoprobes (BPN-BBTD nanoparticles [NPs]), the second near-infrared (NIR-II) fluorescence imaging is first utilized to accurately trace inflammatory lesions, monitor inflammation severity and detect the response to the drug intervention in IBD mouse models. Through the advantages of high signal-to-background ratio (SBR) and sharp spatial resolution of bio-imaging in NIR-II region, the NIR-II fluorescence imaging-guided surgery can help to achieve a complete resection of severe inflammatory bowls and a secure surgical anastomosis. In addition, with the help of NIR-II fluorescence wide-field microscopy, the distribution of BPN-BBTD NPs can be directly detected in tissue level and found to be mainly accumulated in mucosa and submucosa layers. This study highlights that AIE NPs-assisted NIR-II fluorescence imaging hold a great potential value for future diagnosis and imaging-guided surgery in IBD.

Enzyme-activated probes in optical imaging: a focus on atherosclerosis

Enzyme-activated probes enable complex biological processes to be studied in real-time. A wide range of enzymes are modulated in diseases, including cancer, inflammatory diseases and cardiovascular disease, and have the potential to act as vital diagnostic and prognostic biomarkers to monitor and report on disease progression. In this perspective article, we discuss suitable design characteristics of enzyme-activated fluorescent probes for ex vivo and in vivo optical imaging applications. With a particular focus on atherosclerosis imaging, we highlight recent approaches to report on the activity of cathepsins (K and B), matrix metalloproteinases (MMP-2 and MMP-9), thrombin, heme oxygenase-1 (HO-1) and myeloperoxidase (MPO).

A Fast-Responsive OFF-ON Near-Infrared-II Fluorescent Probe for In Vivo Detection of Hypochlorous Acid in Rheumatoid Arthritis

Rheumatoid arthritis (RA) is a common chronic autoimmune inflammatory disease, and its etiology is closely related to the overproduction of hypochlorous acid (HClO). However, early detection of RA using an activatable near-infrared-II (NIR-II, 1000-1700 nm) fluorescent probe remains challenging. Herein, we first report an "OFF-ON" NIR-II fluorescent probe named PTA (phenothiazine triphenylamine) for imaging HClO in deep-seated early RA. Electron-rich phenothiazine in the core of PTA was utilized as both an HClO-recognition moiety and a precursor of electron acceptors, displaying a typical donor-acceptor-donor structure with excellent NIR-II emission at 936/1237 nm once reacted with HClO. The probe PTA exhibited good water solubility, high photostability, and rapid response capability toward HClO within 30 s. Moreover, it was able to sensitively and specifically detect exogenous and endogenous HClO in living cells in both visible and NIR-II windows. Notably, PTA enabled the sensitive and rapid visualization of HClO generation in an inflammatory RA mouse model, showing a 4.3-fold higher NIR-II fluorescence intensity than that in normal hindlimb joints. These results demonstrate that PTA holds great promise as a robust platform for diagnosis of HOCl-mediated inflammatory disorders.

HOCl-Activated Aggregation of Gold Nanoparticles for Multimodality Therapy of Tumors

Tumor microenvironment-responsive nanodrugs offer promising opportunities for imaging-guided precision therapy with reduced side effects. Considering that the antitumor effect is closely related to the size of the nanodrugs, it is particularly important to develop a therapeutic system with size adjustability in the tumor microenvironment, which is still a great challenge in the field of nanotheranostics. Herein, a reactive oxygen species (ROS)-activated aggregation strategy is reported for imaging-guided precision therapy of tumors. The ROS-activated nanoplatform is constructed based on gold nanoparticles (AuNPs) coated with an HOCl probe on its surface (namely, Au-MB-PEG NPs). The Au-MB-PEG NPs show high sensitivity toward HOCl, resulting in the modulation of surface charge and rapid aggregation of AuNPs, and simultaneous release of methylene blue as a photosensitizer for photodynamic therapy (PDT). In the tumor environment, the aggregated AuNPs ensure higher tumor accumulation and retention. Furthermore, the redshift of the absorption of aggregated AuNPs leads to activated photoacoustic imaging signals and photothermal therapy (PTT) under near-infrared irradiation. Au-MB-PEG NPs thus efficiently inhibit the tumor growth through combined PTT-PDT therapy. This work contributes to the design of stimuli-induced size-aggregation nanodrugs, thereby attaining advanced performance in cancer diagnosis and treatment.

Highly Efficient Activatable MRI Probe to Sense Myeloperoxidase Activity

Myeloperoxidase (MPO) is a key component of innate immunity but can damage tissues when secreted abnormally. We developed a new generation of a highly efficient MPO-activatable MRI probe (heMAMP) to report MPO activity. heMAMP has improved Gd stability compared to bis-5-HT-Gd-DTPA (MPO-Gd) and demonstrates no significant cytotoxicity. Importantly, heMAMP is more efficiently activated by MPO compared to MPO-Gd, 5HT-DOTA(Gd), and 5HT-DOTAGA-Gd. Molecular docking simulations revealed that heMAMP has increased rigidity via hydrogen bonding intramolecularly and improved binding affinity to the active site of MPO. In animals with subcutaneous inflammation, activated heMAMP showed a 2-3-fold increased contrast-to-noise ratio (CNR) compared to activated MPO-Gd and 4-10 times higher CNR compared to conventional DOTA-Gd. This increased efficacy was further confirmed in a model of unstable atherosclerotic plaque where heMAMP demonstrated a comparable signal increase and responsiveness to MPO inhibition at a 3-fold lower dosage compared to MPO-Gd, further underscoring heMAMP as a potential translational candidate.

A simple pyridine-based highly specific fluorescent probe for tracing hypochlorous acid in lysosomes of living cells

A simple pyridine-based highly specific fluorescent probe was constructed to trace hypochlorous acid in lysosomes of living cells.