Recent progress in the design principles, sensing mechanisms, and applications of small-molecule probes for nitroreductases

Real-time bioluminescence imaging of nitroreductase in breast cancer bone metastasis

Bone metastasis is a leading cause of mortality in breast cancer patients. Monitoring biomarkers for bone metastasis in breast cancer is crucial for the development of effective interventional treatments. Despite being a highly vascularized tissue, the bone presents a particularly hypoxic environment. Tumor hypoxia is closely linked to increased levels of various reductases, including nitroreductase (NTR). Currently, there are few probes available to detect NTR levels in breast cancer bone metastases. Although bioluminescent imaging is promising due to its specificity and high signal-to-noise ratio, many probes face challenges such as short emission wavelengths, reliance on complex conditions like external adenosine triphosphate, or lack of tissue specificity. In this study, through "caging" the luciferase substrate with an NTR-responsive aromatic nitro recognition group, we developed a highly sensitive and selective NTR-sensitive bioluminescent probe. The resulting probe effectively detects NTR in breast cancer cells and enables real-time monitoring of NTR in a mouse model of breast cancer bone metastasis. Additionally, it can differentiate between primary and bone tumors, and allow continuous monitoring of NTR levels, thus providing valuable insights into bone tumor progression. This work provides a powerful tool for further understanding the biological functions of NTR in breast cancer bone metastasis.

Fluorescent Molecular Probe for Imaging Hypoxia in 2D Cell Culture Monolayers and 3D Tumor Spheroids: The Cell Membrane Partition Model for Predicting Probe Distribution in a Spheroid

Compared to cultured 2D cell monolayers, 3D multicellular spheroids are more realistic tumor models. Nonetheless, spheroids remain under-utilized in preclinical research, in part, because there is a lack of fluorescence sensors that can noninvasively interrogate all the individual cells within a spheroid. This present study describes a deep-red fluorogenic molecular probe for microscopic imaging of cells that contain a high level of nitroreductase enzyme activity as a biomarker of cell hypoxia. A first-generation version of the probe produced "turn-on" fluorescence in a 2D cell monolayer under hypoxic conditions; however, it was not useful in a 3D multicellular tumor spheroid because it only accumulated in the peripheral cells. To guide the probe structural optimization process, an intuitive theoretical membrane partition model was conceived to predict how a dosed probe will distribute within a 3D spheroid. The model identifies three limiting molecular diffusion pathways that are determined by a probe's membrane partition properties. A lipophilic probe with high membrane affinity rapidly becomes trapped in the membranes of the peripheral cells. In contrast, a very hydrophilic probe molecule with negligible membrane affinity diffuses rapidly through the spheroid intercellular space and rarely enters the cells. However, a probe molecule with intermediate membrane affinity undergoes sequential diffusion in and out of cells and distributes to all the cells within a spheroid. Using the model as a predictive tool, a second-generation fluorescent probe was prepared with a smaller and more hydrophilic molecular structure, and optical sectioning using structured illumination or light sheet microscopy revealed roughly even probe diffusion throughout a tumor spheroid. The membrane permeation model is likely to be broadly applicable for the structural optimization of various classes of molecules and nanoparticles to enable even distribution within a tumor spheroid.

Versatile Stimuli-Responsive Controlled Release of Pinanediol-Caged Boronic Esters for Spatiotemporal and Nitroreductase-Selective Glucose Bioimaging

Boronic acids have been widely applied in various biological fields, particularly achieving significant practical progress in boronic acid-based glucose sensing. However, boronic acids exhibit nonspecific binding to other nucleophiles, and the inherent lability of boronic esters in biological systems limits their further applications. Herein, we developed a stimuli-responsive controllable caging strategy to achieve photoresponsive spatiotemporally and nitroreductase-responsive cancer cell-selective glucose sensing. We introduced o-/p-nitroaryl-containing self-immolative linkers onto δ-pinanediol derivatives, effectively caging boronic acids and blocking glucose recognition. Upon triggering by specific stimuli, the caged boronic esters decompose, releasing boronic acids and thereby restoring glucose recognition of the diboronic acid-based sensor. The proof of concept was confirmed through intracellular glucose bioimaging in living cells. Upon regional UV irradiation, we could monitor intracellular glucose with excellent spatiotemporal selectivity. Furthermore, we used the cancer biomarker nitroreductases as the internal stimuli and utilized the caged glucose sensor to selectively label hypoxic cancer cells in a cocultured living cell sample. We believe that our stimuli-responsive caging strategies will hold promising potential for the controlled release of other boronic acids in various biological contexts.

Covalent assembly-based two-photon fluorescent probes for in situ visualizing nitroreductase activities: From cancer cells to human cancer tissues

Nitroreductase (NTR) is widely regarded as a biomarker whose enzymatic activity correlates with the degree of hypoxia in solid malignant tumors. Herein, we utilized 2-dimethylamino-7-hydroxynaphthalene as fluorophore linked diverse nitroaromatic groups to obtain four NTR-activatable two-photon fluorescent probes based on covalent assembly strategy. With the help of computer docking simulation and in vitro assay, the sulfonate-based probe XN3 was proved to be able to identify NTR activity with best performances in rapid response, outstanding specificity, and sensitivity in comparison with the other three probes. Furthermore, XN3 could detect the degree of hypoxia by monitoring NTR activity in kinds of cancer cells with remarkable signal-to-noise ratios. In cancer tissue sections of the breast and liver in mice, XN3 had the ability to differentiate between healthy and tumorous tissues, and possessed excellent fluorescence stability, high tissue penetration and low tissue autofluorescence. Finally, XN3 was successfully utilized for in situ visualizing NTR activities in human transverse colon and rectal cancer tissues, respectively. The findings suggested that XN3 could directly identify the boundary between cancer and normal tissues by monitoring NTR activities, which provides a new method for imaging diagnosis and intraoperative navigation of tumor tissue.

A Nitroreductase-Activatable Metabolic Reporter for Covalent Labeling of Pathological Hypoxic Cells in Tumorigenesis

Aberrant hypoxic stress will initiate a cascade of pathological consequence observed prominently in tumorigenesis. Understanding of hypoxia's role in tumorigenesis is highly essential for developing effective therapeutics, which necessitates reliable tools to specifically distinguish hypoxic tumor cells (or tissues) and correlate their dynamics with the status of disease in complex living settings for precise theranostics. So far, disparate hypoxia-responsive probe molecules and prodrugs were designed via chemical or enzymatic reactions, yet their capability in real-time reporting pathogenesis development is often compromised due to unrestricted diffusion and less selectivity towards the environmental responsiveness. Herein we present an oxygen-insensitive nitroreductase (NTR)-activatable glycan metabolic reporter (pNB-ManNAz) capable of covalently labeling hypoxic tumor cells and tissues. Under pathophysiological hypoxic environments, the caged non-metabolizable precursor pNB-ManNAz exhibited unique responsiveness to cellular NTR, culminating in structural self-immolation and the resultant ManNAz could incorporate onto cell surface glycoproteins, thereby facilitating fluorescence labeling via bioorthogonal chemistry. This NTR-responsive metabolic reporter demonstrated broad applicability for multicellular hypoxia labeling, particularly in the dynamic monitoring of orthotopic tumorigenesis and targeted tumor phototherapy in vivo. We anticipate that this approach holds promise for investigating hypoxia-related pathological progression, offering valuable insights for accurate diagnosis and treatment.

Stimuli-responsive prodrugs with self-immolative linker for improved cancer therapy

Self-immolative prodrugs have gained significant attention as an innovative approach for targeted cancer therapy. These prodrugs are engineered to release the active anticancer agents in response to specific triggers within the tumor microenvironment, thereby improving therapeutic precision while reducing systemic toxicity. This review focuses on the molecular architecture and design principles of self-immolative prodrugs, emphasizing the role of stimuli-responsive linkers and activation mechanisms that enable controlled drug release. Recent advancements in this field include the development of prodrugs that incorporate targeting moieties for enhanced site-specificity. Moreover, the review discusses the incorporation of targeting moieties to achieve site-specific drug delivery, thereby improving the selectivity of treatment. By summarizing key research from the past five years, this review highlights the potential of self-immolative prodrugs to revolutionize cancer treatment strategies and pave the way for their integration into clinical practice.

Dual-Mode Reactive Oxygen Species-Stimulated Carbon Monoxide Release for Synergistic Photodynamic and Gas Tumor Therapy

Controllable carbon monoxide (CO) release simulated by light-generated reactive oxygen species (ROS) represents a promising approach for cancer therapy but is hampered by low CO release rate and low ROS generation of conventional photosensitizers in hypoxia tumor microenvironments. In this study, we developed a highly efficient nanoplatform (TPyNO2-FeCO NPs) through co-encapsulating organic AIE photosensitizers (PSs) and CO prodrug (Fe3(CO)12), which are capable of light-triggered robust ROS generation and CO release for synergistic photodynamic therapy (PDT) and CO gas therapy. The success of this nanoplatform leverages the design of a PS, TPyNO2, with exceptional type I and type II ROS generation capabilities, achieved through the introduction of the α-photoinduced electron transfer (α-PET) process. With the incorporation of a 4-nitrobenzyl unit as a typical PET donor, the intramolecular α-PET process not only suppresses the radiative decay to redirect the excited-state energy to intersystem crossing for more triplet-state formation but also promotes electron separation and transfer processes for radical-type ROS generation. The resultant TPyNO2 demonstrates superior singlet oxygen, superoxide anion, and hydroxyl radial generation capabilities in the aggregate state. Upon light irradiation, TPyNO2-FeCO NPs release CO via the type I and type II dual-mode ROS-mediated processes in a controlled and targeted manner, overcoming the limitations of conventional CO release systems. TPyNO2-FeCO NPs also demonstrate a self-accelerating ROS-CO-ROS loop as the released CO induces intracellular oxidative stress, depolarizes mitochondria membrane potentials, and inhibits ATP production, leading to further intracellular ROS generation. Both in vitro and in vivo experiments validated the excellent antitumor performance of the combined PDT and CO gas therapy. This study provides valuable insights into the development of advanced PSs and establishes TPyNO2-FeCO NPs as promising nanoplatforms for safe and effective antitumor applications.

Design, Synthesis, and Evaluation of Doxifluridine Derivatives as Nitroreductase-Responsive Anticancer Prodrugs

Antimetabolite antitumor drugs interfere with nucleic acid and DNA synthesis, causing cancer cell death. However, they also affect rapidly dividing normal cells and cause serious side effects. Doxifluridine (5'-deoxy-5-fluorouridine [5'-DFUR]), a 5-fluorouracil (5-FU) prodrug converted to 5-FU by thymidine phosphorylase (TP), exerts antitumor effects. Since TP is distributed in tumor and normal tissues, 5'-DFUR features side effects. Here we designed a series of novel 5'-DFUR derivatives based on high nitroreductase (NTR) levels in the hypoxic microenvironment of tumor tissues by introducing nitro-containing moieties into the 5'-DFUR structure. These derivatives exert their antitumor effects by producing 5-FU under the dual action of TP and NTR in the tumor microenvironment. The derivatives were synthesized and their stability, release, and cytotoxicity evaluated in vitro and antitumor activity evaluated in vivo. Compound 2c, featuring nitrofuran fragments, was stable in phosphate-buffered saline and plasma at different pH values and reduced rapidly in the presence of NTR. The in vitro cytotoxicity evaluation indicated that compound 2c showed excellent selectivity in the MCF-7 and HT29 cell lines. Moreover, it exhibited antitumor effects comparable to those of 5'-DFUR in vivo without significant toxic side effects. These results suggest that compound 2c is a promising antitumor prodrug.

Recent Advances in the Application of Nitro(het)aromatic Compounds for Treating and/or Fluorescent Imaging of Tumor Hypoxia

This review delves into recent advancements in the field of nitro(het)aromatic bioreductive agents tailored for hypoxic environments. These compounds are designed to exploit the low-oxygen conditions typically found in solid tumors, making them promising candidates for targeted cancer therapies. Initially, this review focused on their role as gene-directed enzyme prodrugs, which are inert until activated by specific enzymes within tumor cells. Upon activation, these prodrugs undergo chemical transformations that convert them into potent cytotoxic agents, selectively targeting cancerous tissue while sparing healthy cells. Additionally, this review discusses recent developments in prodrug conjugates containing nitro(het)aromatic moieties, designed to activate under low-oxygen conditions within tumors. This approach enhances their efficacy and specificity in cancer treatment. Furthermore, this review covers innovative research on using nitro(het)aromatic compounds as fluorescent probes for imaging hypoxic tumors. These probes enable non-invasive visualization of low-oxygen regions within tumors, providing valuable insights for the diagnosis, treatment planning, and monitoring of therapeutic responses. We hope this review will inspire researchers to design and synthesize improved compounds for selective cancer treatment and early diagnostics.

Recent advances and applications of nitroreductase activable agents for tumor theranostic

Nitroreductase activable agents offer a personalized and targeted approach to cancer theranostics by selectively activating prodrugs within the tumor microenvironment. These agents enable non-invasive tumor imaging, image-guided drug delivery, and real-time treatment monitoring. By leveraging the enzymatic action of tumor-specific nitroreductase enzymes, cytotoxic drugs are delivered directly to cancer cells while minimizing systemic toxicity. This review highlights the key features, mechanisms of action, diagnostic applications, therapeutic potentials, and future directions of nitroreductase activable agents for tumor theranostics. Integration with imaging modalities, advanced drug delivery systems, immunotherapy combinations, and theranostic biomarkers shows promise for optimizing treatment outcomes and improving patient survival in oncology. Continued research and innovation in this field are crucial for advancing novel theranostic strategies and enhancing patient care. Nitroreductase activable agents represent a promising avenue for personalized cancer therapy and have the potential to transform cancer diagnosis and treatment approaches.

A Novel Fluorescent Probe for Nitroreductase Detection and Imaging of cancer Cells under Hypoxia Conditions

Nitroreductase (NTR) is to be pivotal in the biodegradation of nitroaromatics. NTR is produced in tumor tissues under hypoxic conditions, which is one of the markers for early tumor diagnosis. In this study, a novel probe FD-NTR was developed for NTR detection. Probe FD-NTR can exhibit a specific reaction with NTR in the presence of NADH. The probe displayed satisfactory selectivity and sensitivity towards NTR with a calculated detection limit of 12 ng/mL. Under the conditions of low cytotoxic hypoxia, the FD-NTR probe has shown successful application in imaging both MCF-7 cells and tumor tissues, which indicated that the FD-NTR probe holds promising application prospects for detecting NTR in tumors.

Nitroreductase-responsive nanoparticles for in situ fluorescence imaging and synergistic antibacterial therapy of bacterial keratitis

As bacterial keratitis progresses rapidly, prompt intervention is necessary. Current diagnostic processes are time-consuming and invasive, leading to improper antibiotics for treatment. Therefore, innovative strategies for diagnosing and treating bacterial keratitis are urgently needed. In this study, Cu2-xSe@BSA@NTRP nanoparticles were developed by loading nitroreductase-responsive probes (NTRPs) onto Cu2-xSe@BSA. These nanoparticles exhibited integrated fluorescence imaging and antibacterial capabilities. In vitro and in vivo experiments showed that the nanoparticles produced responsive fluorescence signals in bacteria within 30 min due to an interaction between the released NTRP and bacterial endogenous nitroreductase (NTR). When combined with low-temperature photothermal therapy (PTT), the nanoparticles effectively eliminated E. coli and S. aureus, achieved antibacterial efficacy above 95% and facilitated the re-epithelialization process at the corneal wound site in vivo. Overall, the Cu2-xSe@BSA@NTRP nanoparticles demonstrated potential for rapid, noninvasive in situ diagnosis, treatment, and visualization assessment of therapy effectiveness in bacterial keratitis.

A near-infrared fluorescence probe with large Stokes shift for selectively monitoring nitroreductase in living cells and mouse tumor models

Hypoxia is commonly regarded as a typical feature of solid tumors, which originates from the insufficient supply of oxygen. Herein, the development of an efficient method for assessing hypoxia levels in tumors is strongly desirable. Nitroreductase (NTR) is an overexpressed reductase in the solid tumors, has been served as a potential biomarker to evaluate the degrees of hypoxia. In this work, we elaborately synthesized a new near-infrared (NIR) fluorescence probe (MR) to monitor NTR activity for assessment of hypoxia levels in living cells and in tumors. Upon exposure of NTR, the nitro-unit of MR could be selectively reduced to amino-moiety with the help of nicotinamide adenine dinucleotide. Moreover, the obtained fluorophore emitted a prominent NIR fluorescence, because it possessed a classical "push-pull" structure. The MR displayed several distinguished characters toward NTR, including intense NIR fluorescent signals, large Stokes shift, high selectivity and low limit of detection (46 ng/mL). Furthermore, cellular confocal fluorescence imaging results validated that the MR had potential of detecting NTR levels in hypoxic cells. Significantly, using the MR, the elevated of NTR levels were successfully visualized in the tumor-bearing mouse models. Therefore, this detecting platform based on this probe may be tactfully constructed for monitoring the variations of NTR and estimating the degrees of hypoxia in tumors.

Ratiometric near-infrared fluorescent probe for nitroreductase activity enables 3D imaging of hypoxic cells within intact tumor spheroids

Fluorescent molecular probes that report nitroreductase activity have promise as imaging tools to elucidate the biology of hypoxic cells and report the past hypoxic history of biomedical tissue. This study describes the synthesis and validation of a "first-in-class" ratiometric, hydrophilic near-infrared fluorescent molecular probe for imaging hypoxia-induced nitroreductase activity in 2D cell culture monolayers and 3D multicellular tumor spheroids. The probe's molecular structure is charge-balanced and the change in ratiometric signal is based on Förster Resonance Energy Transfer (FRET) from a deep-red, pentamethine cyanine donor dye (Cy5, emits ∼660 nm) to a linked near-infrared, heptamethine cyanine acceptor dye (Cy7, emits ∼780 nm). Enzymatic reduction of a 4-nitrobenzyl group on the Cy7 component induces a large increase in Cy7/Cy5 fluorescence ratio. The deep penetration of near-infrared light enables 3D optical sectioning of intact tumor spheroids, and visualization of individual hypoxic cells (i.e., cells with raised Cy7/Cy5 ratio) as a new way to study tumor spheroids. Beyond preclinical imaging, the near-infrared fluorescent molecular probe has high potential for ratiometric imaging of hypoxic tissue in living subjects.

Development of a Fluorescence Probe for Detecting Nitroreductase Activity Based on Steric Repulsion-Induced Twisted Intramolecular Charge Transfer (sr-TICT)

Twisted intramolecular charge transfer (TICT) is a phenomenon involving intramolecular charge transfer together with intramolecular rotation upon photoexcitation, and in general this excited state of fluorescent dyes undergoes non-radiative decay (producing no fluorescence). We recently discovered that the magnitude of TICT in rhodamine derivatives could be regulated by altering the size of the substituents on the xanthene moiety, generating differing degrees of intramolecular steric repulsion. To further illustrate the usefulness and generality of this strategy, we describe here an application of quinone methide chemistry, which is widely used as a fluorescence off/on switching reaction for fluorescence probes detecting enzymatic activity, to construct a steric repulsion-induced (sr)-TICT-based fluorescence probe targeting nitroreductase (NTR) activity. The developed probe was almost non-fluorescent in phosphate-buffered saline (PBS) due to strong induction of the TICT state. On the other hand, when the probe was incubated with NTR and nicotinamide adenine dinucleotide (NADH), a large fluorescence increase was observed over time. We confirmed that the enzymatic reaction proceeded as expected, i.e., the nitro group of the probe was reduced to the corresponding amino group, followed by spontaneous elimination of iminoquinone methide. These results suggest that our simple design strategy based on the sr-TICT mechanism, i.e., controlling intramolecular steric repulsion, would be applicable to the development of fluorescence probes for a variety of enzymes.

Unveiling the hypoxia-induced mitophagy process through two-channel real-time imaging of NTR and viscosity under the same excitation

Mitophagy is an essential physiological process that eliminates damaged mitochondria via lysosomes. It is reported that hypoxia, inflammatory stimuli or other stress conditions could lead to mitochondrial damage and mitochondrial dysfunction, which induces the process of mitophagy. Herein, we report a novel fluorescent probe PC-NTR for imaging hypoxia-induced mitophagy by monitoring the change of nitroreductase and viscosity simultaneously. To our delight, PC-NTR could respond simultaneously to nitroreductase and viscosity at different fluorescence channels with no mutual interference under the same excitation wavelength. The fluorescence emission around 535 nm was enhanced dramatically after addition of nitroreductase while the fluorescence emission around 635 nm heightened as the viscosity increased. The probe would be able to selectively targeting of mitochondria in cells because of the positively charged pyridine salt structure of PC-NTR. The probe was successfully applied to assess the different levels of hypoxia and real-time imaging of mitochondrial autophagy in live cells. More importantly, using dual channel imaging, PC-NTR could be used to distinguish cancer cells from normal cells and was successfully applied to imaging experiments in HeLa-derived tumor-bearing nude mice. Therefore, PC-NTR would be an important molecular tool for hypoxia imaging and detecting solid tumors in vivo.

A highly selective ratio-metric fluorescent sensor for visualizing nitroreductase in hypoxic cells

Herein, we have developed a novel single-molecular probe (NORP) for selective and accurate determination of NTR in living cells. It was discovered that up-regulation of endogenous NTR occurred in response to hypoxic stimulation, and there was a dependence between the NTR levels and the degree of hypoxia.

Camptothecin-based prodrug nanomedicines for cancer therapy

Camptothecin (CPT) is a cytotoxic alkaloid that attenuates the replication of cancer cells via blocking DNA topoisomerase 1. Despite its encouraging and wide-spectrum antitumour activity, its application is significantly restricted owing to its instability, low solubility, significant toxicity, and acquired tumour cell resistance. This has resulted in the development of many CPT-based therapeutic agents, especially CPT-based nanomedicines, with improved pharmacokinetic and pharmacodynamic profiles. Specifically, smart CPT-based prodrug nanomedicines with stimuli-responsive release capacity have been extensively explored owing to the advantages such as high drug loading, improved stability, and decreased potential toxicity caused by the carrier materials in comparison with normal nanodrugs and traditional delivery systems. In this review, the potential strategies and applications of CPT-based nanoprodrugs for enhanced CPT delivery toward cancer cells are summarized. We appraise in detail the chemical structures and release mechanisms of these nanoprodrugs and guide materials chemists to develop more powerful nanomedicines that have real clinical therapeutic capacities.

Synchronized activating therapeutic nano-agent: Enhancement and tracing for hypoxia-induced chemotherapy

Prodrug is a potential regime to overcome serious adverse events and off-target effects of chemotherapy agents. Among various prodrug activators, hypoxia stands out owing to the generalizability and prominence in tumor micro-environment. However, existing hypoxia activating prodrugs generally face the limitations of stringent structural requirements, the lack of feedback and the singularity of therapeutic modality, which is imputed to the traditional paradigm that recognition groups must be located at the terminus of prodrugs. Herein, a multifunctional nano-prodrug Mal@Cy-NTR-CB has been designed. In this nano-prodrug, a self-destructive tether is introduced to break the mindset, and achieves the activation by hypoxia of chemotherapy based on Chlorambucil (CB), whose efficacy can be augmented and traced by photodynamic therapy (PDT) and fluorescence from Cyanine dyes (Cy). In addition, Maleimide (Mal) carried by the nano-shells can regulate glutathione (GSH) content, preventing 1O2 scavenging, so as to realize PDT sensitization. Experiments demonstrate that Mal@Cy-NTR-CB specifically responds to hypoxic tumors, and achieve synchronous activation, enhancement and feedback of chemotherapy and PDT, inhibiting the tumor growth effectively. This study broadens the design ideas of activatable prodrugs and provides the possibility of multifunctional nano-prodrugs to improve the generalization and prognosis in precision oncology.

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.

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.

Supramolecular Host-Guest Strategy for the Accelerating Detection of Nitroreductase

Identifying nitroreductase (NTR) with fluorescent techniques has become a research hotspot, due to its good sensitivity and selectivity toward the early-stage cancer diagnosis and monitoring. Herein, a host-guest reporter (NAQA⊂Zn-MPPB) is successfully achieved by encapsulating the NTR probe NAQA into a new NADH-functioned metal-organic cage Zn-MPPB, which makes the reporter for ultrafast detection of NTR within dozens of seconds in solution. The host-guest strategy fuses the Zn-MPPB and NAQA to form a pseudomolecule material, which changes the reaction process of NTR and NAQA from a double substrates mechanism to a single substrate one, and accelerates the reduction efficiency of NAQA. This advantage make the new host-guest reporter exhibit a linear relationship between emission changes and NTR concentration, and it shows better sensitively toward NTR than that of NAQA. Additionally, the positively charged water-soluble metal-organic cage can encapsulate NAQA in the cavity, promote it to dissolve in an aqueous environment, and facilitate their accumulation into tumor cells. As expected, such host-guest reporter displays a fast and high efficiently imaging capability toward NTR in tumor cells and tumor-bearing mice, and flow cytometry assay is conducted to corroborate the capability as well, implying the considerably potential of host-guest strategy for early tumor diagnosis and treatment.

An edoplasmic reticulum-targeted NIR fluorescent probe with a large Stokes shift for hypoxia imaging

Hypoxia is closely linked to various diseases, including solid tumors. The level of nitroreductase (NTR) is usually abnormally upregulated in hypoxic conditions, which can be a biomarker of hypoxia. Herein, the first endoplasmic reticulum-targeting NIR fluorescent probe, ISO-NTR, was developed for highly selective and sensitive detection of NTR. It shows a large Stokes shift (185 nm) and a 5-fold increases in fluorescence intensity. Meanwhile, the ISO-NTR probe with a dicyanoisophorone derivative has excellent endoplasmic reticulum targeting in living systems with high Pearson's correlation coefficients (R_r = 0.9489). Molecular docking calculations and high binding energy between the probe and NTR (-10.78 kcal·mol^-1) may explain the high selectivity of ISO-NTR. Additionally, it has been successfully applied to NTR imaging in vitro and vivo due to its good sensitivity, high selectivity and large Stokes shift, which may provide an effective method for studying the physiological and pathological functions of NTR in living systems. This probe could be developed as a potential imaging tool to further explore the pathogenesis of hypoxia-related diseases in endoplasmic reticulum stress.

A minireview of recent developments in ozone detection using optical chemodosimeters

The development of optical chemodosimeters for ozone detection has been an important research subject in recent years because of the environmental and biological relevance of ozone. The design and development of ozone chemodosimeters, as well as their numerous applications from 2009 to 2022, have all been thoroughly covered in this minireview. In this review, chemodosimeters are categorised according to their distinctive reaction mechanism with ozone. The comparative data for each of these chemodosimeters have also been provided here. We have also discussed the difficulties and potential prospects of this fast-evolving discipline. To the best of our knowledge, this is the first review that has comprehensively analysed the progress made in the development of ozone chemodosimeters.

Precise Monitoring and Assessing Treatment Response of Sepsis-Induced Acute Lung Hypoxia with a Nitroreductase-Activated Golgi-Targetable Fluorescent Probe

Sepsis-induced acute lung injury (ALI) is mostly attributed to an outbreak of reactive oxygen species (ROS), which makes leukocytes infiltrate into the lung and results in lung hypoxia. Nitroreductase (NTR) is significantly upregulated under hypoxia, which is commonly regarded as a potential biomarker for assessing sepsis-induced acute lung hypoxia. Increasing evidence shows that NTR in the Golgi apparatus could be induced in sepsis-induced ALI. Meanwhile, the prolyl hydroxylase (PHD) inhibitor (dimethyloxalylglycine, DMOG) attenuated sepsis-induced ALI through further increasing the level of Golgi NTR by improving hypoxia inducible factor-1α (HIF-1α) activity, but as yet, no Golgi-targetable probe has been developed for monitoring and assessing treatment response of sepsis-induced ALI. Herein, we report a Golgi-targetable probe, Gol-NTR, for monitoring and assessing treatment response of sepsis-induced ALI through mapping the generation of NTR. The probe displayed high sensitivity with a low detection limit of 54.8 ng/mL and good selectivity to NTR. In addition, due to the excellent characteristics of Golgi-targetable, Gol-NTR was successfully applied in mapping the change of Golgi NTR in cells and zebrafish caused by various stimuli. Most importantly, the production of Golgi NTR in the sepsis-induced ALI and the PHD inhibitor (DMOG) against sepsis-induced ALI were visualized and precisely assessed for the first time with the assistance of Gol-NTR. The results demonstrated the practicability of Gol-NTR for the precise monitoring and assessing of the personalized treatment response of sepsis-induced ALI.

Nitroreductase-induced bioorthogonal ligation for prodrug activation: A traceless strategy for cancer-specific imaging and therapy

Prodrug development is of great interest in cancer therapy. From bio-friendly standpoints, traceless prodrug activation would be an ideal approach for cancer treatment owning to the avoidance of byproduct which might induce side effects in living system. Here, we report a fully traceless strategy for cancer imaging and therapy via a metal-free bioorthogonal ligation triggered by nitroreductase (NTR) overexpressed in solid tumors. The reduction of nitro substrates to amines by NTR and further condensation of amines with aldehydes can be seamlessly combined to yield imine-based resveratrol (RSV) with water as the only byproduct. In comparison with RSV, this precursor exhibited not only the same level of anticancer efficiency both in vitro and in vivo under hypoxia, but also a high sensitivity to hypoxia and much lower perturbation towards normal cells, which holds a great potential of theranostic prodrug for cancer therapy.

Near-Infrared Fluorescent Probes with Amine-Incorporated Xanthene Platforms for the Detection of Hypoxia

Three fluorescent probes A, B, and C that function in the near-infrared wavelengths and utilize pseudo xanthene platforms with an oxygen atom at the 10-position replaced by a [Me-N]2- group have been created to identify hypoxia via nitroreductase determinations at the 0.04, 0.10, and 0.19 ng/mL levels. Theoretical calculations suggest that the probes are not planar due to steric interactions. Absorptions of photons result in the transition of electron density from the indoline moieties to delocalized orbitals on the anthranilic section, ending up on the nitro groups of the electron-poor (i.e., nonreduced) probes (i.e., A, B, and C), whereas those for the more electron-rich (i.e., reduced) probes consisted of movement from the indoline groups to the right side of the anthranilic sections, resulting in a shift in absorption.

A Highly Selective Fluorescent Probe for the Detection of Nitroreductase Based on a Naphthalimide Scaffold

The development of fluorescent probes for nitroreductase (NTR) has received intense attention because of its biological significance and wide application. In this work, a novel fluorescent probe for the detection of NTR in aqueous solution was designed and synthesized on a 1,8-naphthalimide scaffold. In the presence of NTR and nicotinamide adenine dinucleotide (NADH) under physiological conditions, the probe was converted into a 4-hydroxy-1,8-naphthalimide derivative and exhibited a sharp fluorescence enhancement at 550 nm, with a high selectivity for NTR over various analytes. The detection limit for NTR was determined to be 9.8 ng/ml by this probe. Due to its low signal background, this probe showed > 70-fold fluorescence enhancement. Theoretical calculations revealed that the reason for the fluorescence quenching of this probe is the photoinduced electron transfer (PET) from both the nitrobenzene and morpholine groups to the naphthalimide fluorophore.

Structure-Activity Studies of Nitroreductase-Responsive Near-Infrared Heptamethine Cyanine Fluorescent Probes

Two new classes of near-infrared molecular probes were prepared and shown to exhibit "turn on" fluorescence when cleaved by the nitroreductase enzyme, a well-known biomarker of cell hypoxia. The fluorescent probes are heptamethine cyanine dyes with a central 4'-carboxylic ester group on the heptamethine chain that is converted by a self-immolative fragmentation mechanism to a 4'-caboxylate group that greatly enhances the fluorescence brightness. Each compound was prepared by ring opening of a Zincke salt. The chemical structures have either terminal benzoindolinenes or propargyloxy auxochromes, which provide favorable red-shifted absorption/emission wavelengths and a hyperchromic effect that enhances the photon output when excited by 808 nm light. A fluorescent probe with terminal propargyloxy-indolenines exhibited less self-aggregation and was rapidly activated by nitroreductase with large "turn on" fluorescence; thus, it is the preferred choice for translation towards in vivo applications.

Fluorescent probes for biomolecule detection under environmental stress

The use of fluorescent probes in visible detection has been developed over the last several decades. Biomolecules are essential in the biological processes of organisms, and their distribution and concentration are largely influenced by environmental factors. Significant advances have occurred in the applications of fluorescent probes for the detection of the dynamic localization and quantity of biomolecules during various environmental stress-induced physiological and pathological processes. Herein, we summarize representative examples of small molecule-based fluorescent probes that provide bimolecular information when the organism is under environmental stress. The discussion includes strategies for the design of smart small-molecule fluorescent probes, in addition to their applications in biomolecule imaging under environmental stresses, such as hypoxia, ischemia-reperfusion, hyperthermia/hypothermia, organic/inorganic chemical exposure, oxidative/reductive stress, high glucose stimulation, and drug treatment-induced toxicity. We believe that comprehensive insight into the beneficial applications of fluorescent probes in biomolecule detection under environmental stress should enable the further development and effective application of fluorescent probes in the biochemical and biomedical fields.

Significance of Specific Oxidoreductases in the Design of Hypoxia-Activated Prodrugs and Fluorescent Turn Off–On Probes for Hypoxia Imaging

Simple Summary

Hypoxia-activated prodrugs (HAPs), selectively reduced by specific oxidoreductases under hypoxic conditions, form cytotoxic agents damaging the local cancer cells. On the basis of the reported clinical data concerning several HAPs, one can draw conclusions regarding their preclinical attractiveness and, regrettably, the low efficacy of Phase III clinical trials. Clinical failure may be explained, inter alia, by the lack of screening of patients on the basis of tumor hypoxia and low availability of specific oxidoreductases involved in HAP activation. There is surprisingly little information on the quantification of these enzymes in cells or tissues, compared to the advanced research associated with the use of HAPs. Our knowledge about the expression and activity of these enzymes in various cancer cell lines under hypoxic conditions is inadequate. Only in a few cases were researchers able to demonstrate the differences in the expression or activity of selected oxidoreductases, depending on the oxygen concentration. Additionally, it was cell line dependent. More systematic studies are required. The optical probes, based on turning on the fluorescence emission upon irreversible reduction catalyzed by the overexpressed oxidoreductases, can be helpful in this type of research. Ultimately, such sensors can estimate both the oxidoreductase activity and the degree of oxygenation in one step. To achieve this goal, their response must be correlated with the expression or activity of enzymes potentially involved in turning on their emissions, as determined by biochemical methods. In conclusion, the incorporation of biomarkers to identify hypoxia is a prerequisite for successful HAP therapies. However, it is equally important to assess the level of specific oxidoreductases required for their activation.

Abstract

Hypoxia is one of the hallmarks of the tumor microenvironment and can be used in the design of targeted therapies. Cellular adaptation to hypoxic stress is regulated by hypoxia-inducible factor 1 (HIF-1). Hypoxia is responsible for the modification of cellular metabolism that can result in the development of more aggressive tumor phenotypes. Reduced oxygen concentration in hypoxic tumor cells leads to an increase in oxidoreductase activity that, in turn, leads to the activation of hypoxia-activated prodrugs (HAPs). The same conditions can convert a non-fluorescent compound into a fluorescent one (fluorescent turn off–on probes), and such probes can be designed to specifically image hypoxic cancer cells. This review focuses on the current knowledge about the expression and activity of oxidoreductases, which are relevant in the activation of HAPs and fluorescent imaging probes. The current clinical status of HAPs, their limitations, and ways to improve their efficacy are briefly discussed. The fluorescence probes triggered by reduction with specific oxidoreductase are briefly presented, with particular emphasis placed on those for which the correlation between the signal and enzyme expression determined with biochemical methods is achievable.

Novel Lysosome-Targeting Fluorescence Off-On Photosensitizer for Near-Infrared Hypoxia Imaging and Photodynamic Therapy In Vitro and In Vivo

Photodynamic therapy (PDT) has emerged as a new antitumor modality. Hypoxia, a vital characteristic of solid tumors, can be explored to stimulate the fluorescence response of photosensitizers (PSs). Considering the characteristics of PDT, the targeting of organelles employing PS would enhance antitumor effects. A new multifunctional cyanine-based PS (CLN) comprising morpholine and nitrobenzene groups was prepared and characterized. It generated fluorescence in the near-infrared (NIR) region in the presence of sodium dithionite (Na₂S₂O₄) and nitroreductase (NTR). The response mechanism of CLN was well investigated, thus revealing that its obtained reduction product was CLNH. The obtained fluorescence and singlet oxygen quantum yield of CLNH were 8.65% and 1.60%, respectively. Additionally, the selective experiment for substrates indicated that CLN exhibited a selective response to NTR. Thus, CLN fluorescence could be selectively switched on and its fluorescence intensity increased, following a prolonged stay in hypoxic cells. Furthermore, fluorescence colocalization demonstrated that CLN could effectively target lysosomes. CLN could generate reactive oxygen species and kill tumor cells (IC50 for 4T1 cells was 7.4 μM under a hypoxic condition), following its response to NTR. NIR imaging and targeted PDT were finally applied in vivo.

Insight into the spatial interaction of D-π-A bridge derived cyanines and nitroreductase for fluorescent cancer hypoxia detection

Nitroreductase (NTR) detection in tumor is critical because NTR level is correlated with hypoxia degree and cancer prognosis. With the feature of high sensitivity and selectivity, fluorescence organic probes for NTR detection exhibited a promising future for tumor hypoxia detection. However, the discovery and design of such probes have been impeded due to the lack of the understanding of spatial match and mismatch of these probes with NTR. Here, we have developed two new nitrophenyl-functionalized trimethincyanine (Cy3) probes with para- or meta- positions of nitro-group in phenyl ring. Para-nitrophenyl substituted Cy3 (pNP-Cy3) exhibited a remarkable response to NTR (20-fold fluorescence enhancement) with good selectivity and sensitivity. Experimental and theoretical analysis verified that the substituent position of nitro group on phenyl ring of dyes altered the spatial arrangement of nitro-substituent group, thereby modulated the spatial match and mismatch between Cy3 dyes and binding domain of NTR, and consequently led to a different fluorescent turn-on response. In tumor-bearing mice model, hypoxia status of A549 xenografted tumor of mice was successfully delineated by using pNP-Cy3. These results may provide a clue for designing new cyanine-derived NTR probe to monitor NTR-overexpressed hypoxia cancer cells.

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

Nitroreductase (NTR), one of the flavin-dependent enzymes and an upregulated enzyme under tumor hypoxia, has been studied for decades. Many fluorescent probes were developed to detect NTR activity; however, these probes tend to diffuse away from their reaction site (NTR) inevitably, leading to inappropriate sample fixation, lower accuracy of NTR localization, and weaker signal-to-noise ratio. Herein, we present the design, synthesis, in vitro evaluation, and biological applications of an NTR-activatable fluorogenic and labeling probe FY. By integrating with quinone methide (QM) proximity-based protein labeling, the additional fluoromethyl group on FY serves as a potential origin of QM. Compared with conventional fluorescent probes, this new NTR probe not only offers mitochondrial localizable and fluorogenic response but also achieves permanent retention on the site of activation with an enhanced spatial resolution to improve the detection sensitivity even after cell fixation. We believe our work could offer an expandable synthetic approach to develop these permanent labeling and imaging fluorescence probes for deciphering complex biological events.

Two-Photon Small-Molecule Fluorogenic Probes for Visualizing Endogenous Nitroreductase Activities from Tumor Tissues of a Cancer Patient

Nitroreductase (NTR), a common enzymatic biomarker of hypoxia, is widely used to evaluate tumor microenvironments. To date, numerous optical probes have been reported for NTRs detection. Approaches capable of concisely guiding the probe design of NTRs suitable for deep-tissue imaging, however, are still lacking. As such, direct optical imaging of endogenous NTR activities from tumors derived from cancer patients is thus far not possible. Herein, aided by computational calculations, the authors have successfully developed a series of two-photon (TP) small-molecule fluorogenic probes capable of sensitively detecting general NTR activities from various biological samples; by optimizing the distance between the recognition moiety and the reactive site of NTRs from different sources, the authors have discovered and experimentally proven that X4 displays the best performance in both sensitivity and selectivity. Furthermore, X4 shows excellent TP excited fluorescence properties capable of directly monitoring/imaging endogenous NTR activities from live mammalian cells, growing zebrafish, and tumor-bearing mice. Finally, with an outstanding TP tissue-penetrating imaging property, X4 is used, for the first time, to successfully detect endogenous NTR activities from the liver lysates and cardia tissues of a cancer patient. The work may provide a universal strategy to design novel TP small-molecule enzymatic probes in future clinical applications.

Recent advances in bioprobes and biolabels based on cyanine dyes

As a functional dye, cyanine dye promotes the widespread application of bioprobes in the fields of medicine, genetics and environment, owing to its advantages of good photophysical properties, excellent biocompatibility and low toxicity to biological systems. Nowadays, it is mainly used in the fields of life sciences such as fluorescent labeling of biological macromolecules, disease diagnosis, immunoassay and DNA detection, all of which lie at the core of this review. First, we briefly introduced the characteristics and principles of the cyanine dye bioprobe. Afterward, we paid attention to the recent progress of cyanine dye bioprobes widely used in the 10 years from 2010 to 2020. The application of cyanine dyes as bioprobes with different identification elements, including enzymes, organelles, immunity and DNAs, was mainly summarized. Finally, this review gave an outlook on the future development trend of cyanine dye bioprobes. This facilitates the construction of a new type of multifunctional fluorescent probe and promotes its clinical application.

Visualizing nitroreductase activity in living cells and tissues under hypoxia and hepatic inflammation

Detecting nitroreductase (NTR) activity in hypoxic cells and tissues in situ represents an important step toward accurate delineation of hypoxic disease loci. However, it remains challenging to develop fluorescent probes with the necessary attributes of selectivity, sensitivity, precise targeting and aqueous solubility. Herein, two kinds of fluorescent probes (NNP and cRGD-NNP) built on a 2-nitroimidazole sensing platform were synthesized for the detection of NTR activity in cell and in vivo models of hypoxia. In the presence of NADH, NNP displayed high selectivity for NTR, a strong fluorescence enhancement (108 fold), and a low detection limit (3.6 ng mL^-1). Benefiting from the hydrophilic structure and tumor-targeting properties of the cRGD cyclopeptide group, the probe cRGD-NNP efficiently detected NTR activity in MCF cancer cells under hypoxia. In addition, the liposome-encapsulated probe was successfully applied to visualize NTR during liver inflammation in mice.

Small-molecule fluorescent probes: big future for specific bacterial labeling and infection detection

Bacterial infections remain a global healthcare problem that is particularly attributed to the spread of antibiotic resistance and the evolving pathogenicity. Accurate and swift approaches for infection diagnosis are urgently needed to facilitate antibiotic stewardship and effective medical treatment. Direct optical imaging for specific bacterial labeling and infection detection offers an attractive prospect of precisely monitoring the infectious disease status and therapeutic response in real time. This feature article focuses on the recent advances of small-molecule probes developed for fluorescent imaging of bacteria and infection, which covers the probe design, responsive mechanisms and representative applications. In addition, the perspective and challenges to advance small-molecule fluorescent probes in the field of rapid drug-resistant bacterial detection and clinical diagnosis of bacterial infections are discussed. We envision that the continuous advancement and clinical translations of such a technique will have a strong impact on future anti-infective medicine.

Research on endoplasmic reticulum-targeting fluorescent probes and endoplasmic reticulum stress-mediated nanoanticancer strategies: A review

Subcellular localization of organelles can achieve accurate drug delivery and maximize drug efficacy. As the largest organelle in eukaryotic cells, the endoplasmic reticulum (ER) plays an important role in protein synthesis, folding, and posttranslational modification; lipid biosynthesis; and calcium homeostasis. Observing the changes in various metal ions, active substances, and the microenvironment in the ER is crucial for diagnosing and treating many diseases, including cancer. Excessive endoplasmic reticulum stress (ERS) can have a killing effect on malignant cells and can mediate cell apoptosis, proper modulation of ERS can provide new perspectives for the treatment of many diseases, including cancer. Therefore, the ER is used as a new anticancer target in cancer treatment. This review discusses ER-targeting fluorescent probes and ERS-mediated nanoanticancer strategies.

A General Approach to Design Dual Ratiometric Fluorescent and Photoacoustic Probes for Quantitatively Visualizing Tumor Hypoxia Levels In Vivo

Herein, we describe an energy balance strategy between fluorescence and photoacoustic effects by sulfur substitution to transform existing hemicyanine dyes (Cy) into optimized NIRF/PA dual ratiometric scaffolds. Based on this optimized scaffold, we reported the first dual-ratio response of nitroreductase probe AS-Cy-NO₂ , which allows quantitative visualization of tumor hypoxia in vivo. AS-Cy-NO₂ , composed of a new NIRF/PA scaffold thioxanthene-hemicyanine (AS-Cy-1) and a 4-nitrobenzene moiety, showed a 10-fold ratiometric NIRF enhancement (I₇₇₃ /I₇₃₃ ) and 2.4-fold ratiometric PA enhancement (PA₇₃₀ /PA₆₇₀ ) upon activation by a biomarker (nitroreductase, NTR) associated with tumor hypoxia. Moreover, the dual ratiometric NIRF/PA imaging accurately quantified the hypoxia extent with high sensitivity and high imaging depth in xenograft breast cancer models. More importantly, the 3D maximal intensity projection (MIP) PA images of the probe can precisely differentiate the highly heterogeneous oxygen distribution in solid tumor. Thus, this study provides a promising NIRF/PA scaffold that may be generalized for the dual ratiometric imaging of other disease-relevant biomarkers.

Generalizable synthesis of bioresponsive near-infrared fluorescent probes: sulfonated heptamethine cyanine prototype for imaging cell hypoxia

Continued advancement in bioresponsive fluorescence imaging requires new classes of activatable fluorescent probes that emit near-infrared fluorescence with wavelengths above 740 nm. Heptamethine cyanine dyes (Cy7) have suitable fluorescence properties but it is challenging to create activatable probes because Cy7 dyes have a propensity for self-aggregation and fluorescence quenching. A new synthetic strategy is employed to create a generalizable class of hydrophilic bioresponsive near-infrared fluorescent probes with appended sulfonates that provide excellent physiochemical properties. A prototype version is triggered by nitroreductase enzyme to undergo self-immolative cleavage with a large enhancement in fluorescence signal at 780 nm and the probe enables microscopic imaging of cell hypoxia with "turn on" fluorescence. Near-infrared fluorescence imaging of hypoxia is potentially useful in many different areas of biomedical research and clinical treatment.

Design, synthesis and evaluation of enzyme-responsive fluorogenic probes based on pyridine-flanked diketopyrrolopyrrole dyes

The ever-growing demand for fluorogenic dyes usable in the rapid construction of analyte-responsive fluorescent probes, has recently contributed to a revival of interest in the chemistry of diketopyrrolopyrrole (DPP) pigments. In this context, we have explored the potential of symmetrical and unsymmetrical DPP derivatives bearing two or one 4-pyridyl substituents acting as optically tunable group(s). The unique fluorogenic behavior of these molecules, closely linked to N-substitution/charge state of their pyridine unit (i.e., neutral pyridine or cationic pyridinium), has been used to design DPP-based fluorescent probes for detection of hypoxia-related redox enzymes and penicillin G acylase (PGA). In this paper, we describe synthesis, spectral characterization and bioanalytical validations of these probes. Dramatic differences in terms of aqueous stability and enzymatic fluorescence activation were observed. This systematic study enables to delineate the scope of application of pyridine-flanked DPP fluorophores in the field of enzyme biosensing.

Nitroreductase-responsive polymeric micelles based on 4-nitrobenzyl and AIE moieties for intracellular doxorubicin release

An amphiphilic polymer TNP demonstrated the formation of a nitroreductase-responsive DOX delivery nanoplatform with high sensitivity and selectivity.

Discovery of a highly efficient nitroaryl group for detection of nitroreductase and imaging of hypoxic tumor cells

Hypoxia is a pathological hallmark of solid tumors. Detection of hypoxia is therefore of great interest for tumor diagnosis and treatment. As a well-established biomarker of hypoxia, nitroreductase (NTR) has been widely exploited in the development of hypoxia-responsive fluorescent probes on the basis of its enzymatic activity to reduce nitroaryl groups. However, studies on the relationship between the nitroaryl structure and the probe performance for optimal probe design are still rare. Here we report a comparative investigation of nitroaryl groups and identification of the optimal nitroaryl structure for developing new fluorescent probes with extremely high efficiency in the detection of NTR and the imaging of hypoxic tumor cells. Specifically, we synthesized a series of resorufin-based fluorescent probes containing different nitroaryl groups, compared their fluorescence responses to NTR, and identified 2-nitro-N-methyl-imidazolyl as the optimal nitroaryl group that is much more efficient than the most widely used 4-nitrophenyl for NTR detection. The structure-performance relationship was then studied by theoretical molecular docking, revealing the unique features of 2-nitro-N-methyl-imidazolyl in binding and reaction with NTR. We further incorporated the 2-nitro-N-methyl-imidazolyl group into a near-infrared (NIR) hemicyanine fluorophore and developed a NIR fluorescent probe NFP-7 for the detection of NTR and hypoxic tumor cells. NFP-7 exhibits a strong fluorescence increase toward NTR in vitro with an ultrafast (within 40 seconds to fluorescence maximum) and ultrasensitive (0.2 ng mL-1 detection limit) response. NFP-7 has also been demonstrated for imaging the degree of hypoxia in live tumor cells and, more importantly, in a murine tumor model. Our study provides important insights into hypoxia probe development and new tools for hypoxia imaging.