Fluorogenic and Mitochondria-Localizable Probe Enables Selective Labeling and Imaging of Nitroreductase

A Tumor-Targeting Dual-Modal imaging probe for nitroreductase in vivo

Nitroreductase (NTR) overexpression often occurs in tumors, highlighting the significance of effective NTR detection. Despite the utilization of various optical methods for this purpose, the absence of an efficient tumor-targeting optical probe for NTR detection remains a challenge. In this research, a novel tumor-targeting probe (Cy-Bio-NO2) is developed to perform dual-modal NTR detection using near-infrared fluorescence and photoacoustic techniques. This probe exhibits exceptional sensitivity and selectivity to NTR. Upon the reaction with NTR, Cy-Bio-NO2 demonstrates a distinct fluorescence "off-on" response at 800 nm, with an impressive detection limit of 12 ng/mL. Furthermore, the probe shows on-off photoacoustic signal with NTR. Cy-Bio-NO2 has been successfully employed for dual-modal NTR detection in living cells, specifically targeting biotin receptor-positive cancer cells for imaging purposes. Notably, this probe effectively detects tumor hypoxia through dual-modal imaging in tumor-bearing mice. The strategy of biotin incorporation markedly enhances the probe's tumor-targeting capability, facilitating its engagement in dual-modal imaging at tumor sites. This imaging capacity holds substantial promise as an accurate tool for cancer diagnosis.

Quinone Methide Based Self-Immobilizing Molecular Fluorescent Probes for In Situ Imaging of Enzymes

Enzymes play important roles not only in normal physiological processes but in the development of many diseases. In situ imaging of enzymes with high-resolution in living systems would helpful for clinical diagnosis and treatment. However, many molecular fluorescent probes suffer from the drawback of diffusing away from the reaction site of enzymes even out of the cells, losing the in situ information and resulting in poor imaging resolution. Quinone methide (QM) based self-immobilizing probes allow the fluorescent signal to be immobilized near the target for an extended period without deactivating the target enzymes, ensuring that it will provide amplified signals and in situ information of the target with high resolution. In this review, we summarized the recent progress of QM-based self-immobilizing probes including their design strategies, working mechanisms, classifications and applications in in situ enzyme imaging. This review calls for the development of more activatable QM-based probe with the advantages of high stability in the absence of the target but very high labeling efficiency after activation.

Visual monitoring of biocatalytic processes using small molecular fluorescent probes: strategies-mechanisms-applications

Real-time monitoring of biocatalytic-based processes is significantly improved and simplified when they can be visualized. Visual monitoring can be achieved by integrating a fluorescent unit with the biocatalyst. Herein, we outline the design strategies of fluorescent probes for monitoring biocatalysis: (1) probes for monitoring biocatalytic transfer: γ-glutamine is linked to the fluorophore as both a recognition group and for intramolecular charge transfer (ICT) inhibition; the probe is initially in an off state and is activated via the transfer of the γ-glutamine group and the release of the free amino group, which results in restoration of the "Donor-π-Acceptor" (D-π-A) system and fluorescence recovery. (2) Probes for monitoring biocatalytic oxidation: a propylamine is connected to the fluorophore as a recognition group, which cages the hydroxyl group, leading to the inhibition of ICT; propylamine is oxidized and subsequently β-elimination occurs, resulting in exposure of the hydroxyl group and fluorescence recovery. (3) Probes for monitoring biocatalytic reduction: a nitro group attached to a fluorophore as a fluorescence quenching group, this is converted to an amino group by catalytic reduction, resulting in fluorescence recovery. (4) Probes for monitoring biocatalytic hydrolysis: β-D-galactopyranoside or phosphate acts as a recognition group attached to hydroxyl groups of the fluorophore; the subsequent biocatalytic hydrolysis reaction releases the hydroxyl group resulting in fluorescence recovery. Following these 4 mechanisms, fluorophores including cyanine, coumarin, rhodamine, and Nile-red, have been used to develop systems for monitoring biocatalytic reactions. We anticipate that these strategies will result in systems able to rapidly diagnose and facilitate the treatment of serious diseases.

A self-immolative near-infrared fluorescent probe for identification of cancer cells and facilitating its apoptosis

Hydrogen sulfide (H2S) plays a significant role in the onset and progression of cancer. It has led to increased interest in its potential as a diagnostic tool owing to its overexpression in cancer. However, research into the anti-cancer activity of H2S, particularly its ability to promote apoptosis, is hindered by the lack of effective detection tools. To gain a comprehensive understanding of the targeted efficacy of H2S in promoting cancer cell apoptosis, we designed and synthesized a self-immolative near-infrared fluorescent diagnostic probe, named YH-NO2. The activation of this self-immolative reaction is dependent on the presence of nitroreductase (NTR) overexpressed in tumor cells. The design of YH-NO2 involves releasing fluorophores through the activated self-immolative reaction for detection, while simultaneously releasing H2S-loaded self-immolative spacers to promote cancer cell apoptosis. Consequently, YH-NO2 achieves a seamless integration of recognizing and promoting cancer cell apoptosis through its self-immolative structure. This dual function allows YH-NO2 to recognize NTR activity in cells under varying hypoxia levels and differentiate between normal cells and cancer cells using imaging technology. Notably, YH-NO2 exhibits remarkable stability in cellular environments, providing controlled and selective H2S release, thereby targeting the elimination of cancer cells through the promotion of apoptosis. Furthermore, in vivo experiments have demonstrated that YH-NO2 can accurately identify tumor tissue and effectively reduce its size by utilizing its apoptosis-promoting properties. These findings not only provide further evidence for the anti-cancer activity of H2S but also offer valuable tools for understanding the complex relationship between H2S and cancer.

Visualization of an Endogenous Mitochondrial Azoreductase Activity under Normoxic Conditions Using a Naphthalimide Azo-Based Fluorogenic Probe

In this paper, we demonstrate the existence of an endogenous mitochondrial azoreductase (AzoR) activity that can induce the cleavage of N═N double bonds of azobenzene compounds under normoxic conditions. To this end, 100% OFF-ON azo-based fluorogenic probes derived from 4-amino-1,8-naphthalimide fluorophores were synthesized and evaluated. The in vitro study conducted with other endogenous reducing agents of the cell, including reductases, demonstrated both the efficacy and the selectivity of the probe for AzoR. Confocal experiments with the probe revealed an AzoR activity in the mitochondria of living cells under normal oxygenation conditions, and we were able to demonstrate that this endogenous AzoR activity appears to be expressed at different levels across different cell lines. This discovery provides crucial information for our understanding of the biochemical processes occurring within the mitochondria. It thus contributes to a better understanding of its function, which is implicated in numerous pathologies.

A fluorescent probe for imaging nitroreductase with signal amplification in high-viscosity environments

Herein, we developed a fluorescent probe ENBT for in vitro detection of nitroreductase (NTR) as well as imaging intracellular NTR. ENBT itself is non-fluorescent and it could be catalyzed by NTR to generate a viscosity-sensitive fluorophore EBT. The fluorescence intensity of EBT could be further enhanced in cancer cells with relatively high viscosity due to the inhibition of the twisted intramolecular charge transfer effect. The probe ENBT has a good response to NTR with a detection limit of 36.8 ng mL-1, and EBT has a good response to viscosity. Furthermore, different concentrations of NTR (0-1.4 μg mL-1) were used to react with the probe and the reaction systems were subjected to different viscosity solutions, and the fluorescence signals of the products in the viscosity range of 45.86-163.60 cP were increased up to 1.69-fold. ENBT was successfully used to image NTR in cells under different hypoxic conditions as well as in Staphylococcus aureus. Finally, lipopolysaccharide was added to stimulate an increase in cellular viscosity after ENBT was catalyzed by intracellular NTR into EBT, and the fluorescence signals were observed to increase by 1.72-fold. The signal amplification capability gives ENBT higher sensitivity and immunity to interference. Moreover, it has the advantages of mitochondrial targeting, large Stokes shift (190 nm), high selectivity, and can be easily synthesized.

Recent progress of self-immobilizing and self-precipitating molecular fluorescent probes for higher-spatial-resolution imaging

Flourished in the past two decades, fluorescent probe technology provides researchers with accurate and efficient tools for in situ imaging of biomarkers in living cells and tissues and may play a significant role in clinical diagnosis and treatment such as biomarker detection, fluorescence imaging-guided surgery, and photothermal/photodynamic therapy. In situ imaging of biomarkers depends on the spatial resolution of molecular probes. Nevertheless, the majority of currently available molecular fluorescent probes suffer from the drawback of diffusing from the target region. This leads to a rapid attenuation of the fluorescent signal over time and a reduction in spatial resolution. Consequently, the diffused fluorescent signal cannot accurately reflect the in situ information of the target. Self-immobilizing and self-precipitating molecular fluorescent probes can be used to overcome this problem. These probes ensure that the fluorescent signal remains at the location where the signal is generated for a long time. In this review, we introduce the development history of the two types of probes and classify them in detail according to different design strategies. In addition, we compare their advantages and disadvantages, summarize some representative studies conducted in recent years, and propose prospects for this field.

Diagnosing Orthotopic Lung Tumor Using a NTR-Activatable Near-Infrared Fluorescent Probe by Tracheal Inhalation

Nitroreductase (NTR) is an enzyme that is upregulated under tumor-depleted oxygen conditions. The majority of studies have been conducted on NTR, but many existing fluorescent imaging tools for monitoring NTR inevitably suffer from weak targeting, low sensitivity, and simple tumor models. Research on diagnosing lung tumors has been very popular in recent years, but targeting assays in orthotopic lung tumors is still of great research value, as such models better mimic the reality of cancer in the organism. Here, we developed a novel near-infrared (NIR) fluorescent probe IR-ABS that jointly targets NTR and carbonic anhydrase IX (CAIX). IR-ABS has excellent sensitivity and selectivity and shows exceptional NTR response in spectroscopic tests. The measurements ensured that this probe has good biosafety in both cells and mice. A better NTR response was found in hypoxic tumor cells at the cellular level, distinguishing tumor cells from normal cells. In vivo experiments demonstrated that IR-ABS achieves a hypoxic response at the zebrafish level and enables rapid and accurate tumor margin distinguishment in different mouse tumor models. More importantly, we successfully applied IR-ABS for NTR detection in orthotopic lung tumor models, further combined with tracheal inhalation drug delivery to improve targeting. To the best of our knowledge, we present for the first time a near-infrared imaging method for targeting lung cancerous tumor in situ via tracheal inhalation drug delivery, in contrast to the reported literature. This NIR fluorescence diagnostic strategy for targeting orthotopic lung cancer holds exciting potential for clinical aid in cancer diagnosis.

Fluorescent dyes based on rhodamine derivatives for bioimaging and therapeutics: recent progress, challenges, and prospects

Since their inception, rhodamine dyes have been extensively applied in biotechnology as fluorescent markers or for the detection of biomolecules owing to their good optical physical properties. Accordingly, they have emerged as a powerful tool for the visualization of living systems. In addition to fluorescence bioimaging, the molecular design of rhodamine derivatives with disease therapeutic functions (e.g., cancer and bacterial infection) has recently attracted increased research attention, which is significantly important for the construction of molecular libraries for diagnostic and therapeutic integration. However, reviews focusing on integrated design strategies for rhodamine dye-based diagnosis and treatment and their wide application in disease treatment are extremely rare. In this review, first, a brief history of the development of rhodamine fluorescent dyes, the transformation of rhodamine fluorescent dyes from bioimaging to disease therapy, and the concept of optics-based diagnosis and treatment integration and its significance to human development are presented. Next, a systematic review of several excellent rhodamine-based derivatives for bioimaging, as well as for disease diagnosis and treatment, is presented. Finally, the challenges in practical integration of rhodamine-based diagnostic and treatment dyes and the future outlook of clinical translation are also discussed.

Fast and Bioorthogonal Release of Isocyanates in Living Cells from Iminosydnones and Cycloalkynes

Bioorthogonal click-and-release reactions are powerful tools for chemical biology, allowing, for example, the selective release of drugs in biological media, including inside animals. Here, we developed two new families of iminosydnone mesoionic reactants that allow a bioorthogonal release of electrophilic species under physiological conditions. Their synthesis and reactivities as dipoles in cycloaddition reactions with strained alkynes have been studied in detail. Whereas the impact of the pH on the reaction kinetics was demonstrated experimentally, theoretical calculations suggest that the newly designed dipoles display reduced resonance stabilization energies compared to previously described iminosydnones, explaining their higher reactivity. These mesoionic compounds react smoothly with cycloalkynes under physiological, copper-free reaction conditions to form a click pyrazole product together with a released alkyl- or aryl-isocyanate. With rate constants up to 1000 M^-1 s^-1, this click-and-release reaction is among the fastest described to date and represents the first bioorthogonal process allowing the release of isocyanate electrophiles inside living cells, offering interesting perspectives in chemical biology.

Diagnosis of Bacterial Plant Diseases via a Nitroreductase-Activated Fluorescent Sensor

Plant diseases caused by bacteria have become one of the serious problems that threaten human food security, which led to the remarkable reduction of agricultural yields and economic loss. Nitroreductase (NTR), as an important biomarker, is highly expressed in bacteria, and the level of NTR is closely related to the progression of pathogen infection. Therefore, the design of small-molecule fluorescent sensors targeting NTR is of great significance for the detection and diagnosis of plant pathogenic bacteria. In this study, a new fluorescent sensor targeting NTR was discovered and then successfully applied to the imaging of zebrafish and pathogenic bacteria. Most importantly, the developed sensor achieved the real-time diagnosis of Brassica napus L. infected with bacteria, which provides a promising tool for examining the temporal and spatial infection of plant pathogens in precision agriculture.

Interplay of conformational relaxation and hydrogen bond dynamics in the excited states of fluorescent Schiff base anions

Time resolved fluorescence spectroscopic investigation of four Schiff base anions has established that their excited state dynamics is governed by several solvent properties: polarity, viscosity and hydrogen bond donating ability. With viscous protic solvents like glycerol, fluorescence lifetimes of anions have been found to be markedly longer than those in ethanol, implying that conformational relaxation of molecules plays a key role in their nonradiative relaxation. Surprisingly, the lifetimes in less viscous aprotic solvents, like acetonitrile, are found to be even longer. The only plausible rationalization of this observation is in the light of hydrogen bond-assisted nonradiative phenomena that are operative in protic solvents. This contention draws support from a time evolution of the emission in the red end of the spectrum in low to moderately hydrogen bond donating protic solvents, with regard to an absence of such a rise time in aprotic solvents and strongly hydrogen bond donating solvents, viz., 2,2,2-trifluoroethanol. Rudimentary quantum chemical calculations provide a preliminary idea about the nature of excited state hydrogen bond redistribution involved in the process.

Small-Molecule Fluorescent Probes for Detecting Several Abnormally Expressed Substances in Tumors

Malignant tumors have always been the biggest problem facing human survival, and a huge number of people die from cancer every year. Therefore, the identification and detection of malignant tumors have far-reaching significance for human survival and development. Some substances are abnormally expressed in tumors, such as cyclooxygenase-2 (COX-2), nitroreductase (NTR), pH, biothiols (GSH, Cys, Hcy), hydrogen sulfide (H₂S), hydrogen sulfide (H₂O₂), hypochlorous acid (HOCl) and NADH. Consequently, it is of great value to diagnose and treat malignant tumors due to the identification and detection of these substances. Compared with traditional tumor detection methods, fluorescence imaging technology has the advantages of an inexpensive cost, fast detection and high sensitivity. Herein, we mainly introduce the research progress of fluorescent probes for identifying and detecting abnormally expressed substances in several tumors.