Mitochondrial Nitroreductase Activity Enables Selective Imaging and Therapeutic Targeting

Site-Specific Antimicrobial Activity of a Dual-Responsive Ciprofloxacin Prodrug

Bacterial infections create distinctive microenvironments with a unique mix of metabolites and enzymes compared with healthy tissues that can be used to trigger the activation of antibiotic prodrugs. Here, a single and dual prodrug masking the C3 carboxylate and C7 piperazine of the fluoroquinolone, ciprofloxacin, responsive to nitroreductase (NTR) and/or hydrogen sulfide (H2S), was developed. Masking both functional groups reduced the activity of the prodrug against Staphylococcus aureus and Escherichia coli, increasing its minimum inhibitory concentration (MIC) by ∼512-fold (S. aureus) and ∼8000-fold (E. coli strains), while masking a single group only increased the MIC by ∼128-fold. Bacteria subjected to prolonged prodrug exposure did not show any increase in resistance. Triggering assays demonstrated the conversion of prodrugs to ciprofloxacin, and in a murine infection model, responsive prodrugs showed antibacterial activity comparable to that of ciprofloxacin, suggesting in vivo activation of prodrugs. Thus, the potential for site-specific antibiotic treatment with reduced threat of resistance is demonstrated.

Hemicyanine-Based Highly Water-Soluble Probe for Extracellular Nitroreductase

Nitroreductase (NTR) has long been a target of interest for its important role involved in the nitro compounds metabolism. Various probes have been reported for NTR analysis, but rarely able to distinguish the extracellular NTR from intracellular ones. Herein we reported a new NTR sensor, HCyS-NO2, which was a hemicyanine molecule with one nitro and two sulfo groups attached. The nitro group acted as the reporting group to respond NTR reduction. Direct linkage of nitro group into the hemicyanine π conjugate system facilitated the intramolecular electron transfer (IET) process and thus quenched the fluorescence of hemicyanine core. Upon reduction with NTR, the nitro group was rapidly converted into the hydroxylamino and then the amino group, eliminating IET process and thus restoring the fluorescence. The sulfo groups installed significantly increased the hydrophilicity of the molecule, and introduced negative charges at physiological pH, preventing the diffusion into bacteria. Both gram-negative and gram-positive bacteria were able to turn on the fluorescence of HCyS-NO2, without detectable diffusion into cells, providing a useful tool to probe the extracellular reduction process.

Aryl-Modified Pentamethyl Cyanine Dyes at the C2' Position: A Tunable Platform for Activatable Photosensitizers

Pentamethyl cyanine dyes are promising fluorophores for fluorescence sensing and imaging. However, advanced biomedical applications require enhanced control of their excited-state properties. Herein, a synthetic approach for attaching aryl substituents at the C2' position of the thio-pentamethine cyanine (TCy5) dye structure is reported for the first time. C2'-aryl substitution enables the regulation of both the twisted intramolecular charge transfer (TICT) and photoinduced electron transfer (PET) mechanisms to be regulated in the excited state. Modulation of these mechanisms allows the design of a nitroreductase-activatable TCy5 fluorophore for hypoxic tumor photodynamic therapy and fluorescence imaging. These C2'-aryl TCy5 dyes provide a tunable platform for engineering cyanine dyes tailored to sophisticated biological applications, such as photodynamic therapy.

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.

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.

Smart chemistry for traceless release of anticancer therapeutics

In the design of delivery strategies for anticancer therapeutics, the controlled release of intact cargo at the destined tumor and metastasis locations is of particular importance. To this end, stimuli-responsive chemical linkers have been extensively investigated owing to their ability to respond to tumor-specific physiological stimuli, such as lowered pH, altered redox conditions, increased radical oxygen species and pathological enzymatic activities. To prevent premature action and off-target effects, anticancer therapeutics are chemically modified to be transiently inactivated, a strategy known as prodrug development. Prodrugs are reactivated upon stimuli-dependent release at the sites of interest. As most drugs and therapeutic proteins have the optimal activity when released from carriers in their native and original forms, traceless release mechanisms are increasingly investigated. In this review, we summarize the chemical toolkit for developing innovative traceless prodrug strategies for stimuli-responsive drug delivery and discuss the applications of these chemical modifications in anticancer treatment including cancer immunotherapy.

Hypoxia-Directed and Self-Immolative Theranostic Agent: Imaging and Treatment of Cancer and Bacterial Infections

The impact of bacteria on cancer progression and treatment is becoming increasingly recognized. Cancer-associated bacteria are linked to metastases, reduced efficacy, and survival challenges. In this study, we present a sensitive hypoxia-activated prodrug, NR-NO2, which comprises an antibiotic combined with a chemotherapeutic. This prodrug demonstrates rapid and robust fluorescence enhancement and exhibits potent antibacterial activity against both Gram-positive and Gram-negative bacteria as well as tumor cells. Upon activation, NR-NO2 produces a distinct "fluorescence-on" signal, enabling real-time drug release monitoring. By leveraging elevated nitroreductase in cancer cells, NR-NO2 gives rise to heightened bacterial cytotoxicity while sparing normal cells. In A549 solid tumor-bearing mice, NR-NO2 selectively accumulated at tumor sites, displaying fluorescence signals under hypoxia superior to those of a corresponding prodrug-like control. These findings highlight the potential of NR-NO2 as a promising cancer therapy prodrug that benefits from targeted release, antibacterial impact, and imaging-based guidance.

In Situ Photoacoustic Visualization of Pneumonia Induced by MRSA and Specific Identifying Tumor-Homing Bacteria

Optical imaging holds great promise for monitoring bacterial infectious processes and drug resistance with high temporal-spatial resolution. Currently, the diagnosis of deep-seated bacterial infections in vivo with fluorescence imaging, including near-infrared (NIR) fluorescence imaging technology, remains a significant challenge due to its limited tissue penetration depth. In this study, we developed a highly specific targeting probe, Cy7-Neo-NO2, by conjugating a bacterial 16S rRNA-targeted moiety, neomycin, with a bacterial nitroreductase (NTR)-activated NIR photoacoustic (PA) scaffold using our previously developed caged photoinduced electron transfer (a-PeT) approach. This conjugation effectively resolved probe aggregation issues in physiological conditions and substantially enhanced its reactivity toward bacterial NTR. Notably, Cy7-Neo-NO2 enabled the first in situ photoacoustic imaging of pneumonia induced by methicillin-resistant Staphylococcus aureus (MRSA), as well as the detection of bacteria within tumors. Furthermore, upon NIR irradiation, Cy7-Neo-NO2 successfully inhibited MRSA growth through a synergistic effect combining photothermal therapy and photodynamic therapy. Our results provided an effective tool for obtaining exceptional PA agents for accurate diagnosis, therapeutic evaluation of deep-seated bacterial infections in vivo, and intratumoral bacteria-specific recognition.

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.

Exploration of NIR Squaraine Contrast Agents Containing Various Heterocycles: Synthesis, Optical Properties and Applications

Squaraine dye is a popular class of contrast near-infrared (NIR) dyes. Squaraine dyes have shown the ability to be modified with various heterocycles. The indole moiety is the most notable heterocycle incorporated in squaraine dyes. A tremendous amount of work has gone into developing indole-based squaraine dyes and determining their applications. The optical properties of squaraine dyes containing an indole moiety facilitate high quantum yields and molar absorptivity, but the absorbance maxima is capped near 700 nm. This is the major limitation of indole-based squaraine dyes. In comparison, other heterocycles with larger conjugated systems such as quinoline and perimidine have demonstrated promising optical properties and immense potential for modifications, albeit with limited development. Quinoline- and perimidine-based squaraine dyes have molar extinction coefficients over 100,000 M-1 cm-1 and absorbances over 800 nm. This report will look at indole-, quinoline-, and perimidine-based squaraine dyes. Due to the sheer number of reported dyes, the search for indole-based squaraine dyes has been limited to reports from the past five years (2018-2023). For quinoline- and perimidine-based squaraine dyes, a holistic search was performed to analyze the optical properties and applications, due to the abovementioned limitation. This report will evaluate the three different classes of squaraines: indole-, quinoline-, and perimidine-based, to evaluate their optical properties and applications, with the goal of encouraging the exploration of other heterocycles for use in squaraine dyes.

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.

Hemicyanine-Based Near-Infrared Fluorescence Off-On Probes for Imaging Intracellular and In Vivo Nitroreductase Activity

Nitroreductase (NTR) has the ability to activate nitro group-containing prodrugs and decompose explosives; thus, the evaluation of NTR activity is specifically important in pharmaceutical and environmental areas. Numerous studies have verified effective fluorescent methods to detect and image NTR activity; however, near-infrared (NIR) fluorescence probes for biological applications are lacking. Thus, in this study, we synthesized novel NIR probes (NIR-HCy-NO₂ 1-3) by introducing a nitro group to the hemicyanine skeleton to obtain fluorescence images of NTR activity. Additionally, this study was also designed to propose a different water solubility and investigate the catalytic efficiency of NTR. NIR-HCy-NO₂ inherently exhibited a low fluorescence background due to the interference of intramolecular charge transfer (ICT) by the nitro group. The conversion from the nitro to amine group by NTR induced a change in the absorbance spectra and lead to the intense enhancement of the fluorescence spectra. When assessing the catalytic efficiency and the limit of detection (LOD), including NTR activity imaging, it was demonstrated that NIR-HCy-NO₂ 1 was superior to the other two probes. Moreover, we found that NIR-HCy-NO₂ 1 reacted with type I mitochondrial NTR in live cell imaging. Conclusively, NIR-HCy-NO₂ demonstrated a great potential for application in various NTR-related fields, including NTR activity for cell imaging in vivo.

Targeting Pathogenic Formate-Dependent Bacteria with a Bioinspired Metallo-Nitroreductase Complex

Nitroreductases (NTRs) constitute an important class of oxidoreductase enzymes that have evolved to metabolize nitro-containing compounds. Their unique characteristics have spurred an array of potential uses in medicinal chemistry, chemical biology, and bioengineering toward harnessing nitro caging groups and constructing NTR variants for niche applications. Inspired by how they carry out enzymatic reduction via a cascade of hydride transfer reactions, we sought to develop a synthetic small-molecule NTR system based on transfer hydrogenation mediated by transition metal complexes harnessing native cofactors. We report the first water-stable Ru-arene complex capable of selectively and fully reducing nitroaromatics into anilines in a biocompatible buffered aqueous environment using formate as the hydride source. We further demonstrated its application to activate nitro-caged sulfanilamide prodrug in formate-abundant bacteria, specifically pathogenic methicillin-resistant Staphylococcus aureus. This proof of concept paves the way for a new targeted antibacterial chemotherapeutic approach leveraging on redox-active metal complexes for prodrug activation via bioinspired nitroreduction.

Fluorescent imaging to provide visualized evidences for mercury induced hypoxia stress

As one typical toxic and dangerous heavy metal, mercury brings incalculable hazards to the environment and human, the mechanism at the molecular level is unclear. There is no visualized evidence to support directly that mercury ions (Hg²⁺) exposure may induce secondary stress, which is associated with the risk of hypoxia microenvironment in biological systems. Hypoxia occurs in many physiological and pathophysiological processes in the living system, accompanying overexpression of various biomarkers, such as nitroreductase (NTR). Hence, we had successfully developed two NTR-selective fluorescent probes with excellent performance for evaluating the hypoxia degree in vivo and in vitro. We visualized and qualitatively monitored the fluctuations of the endogenous NTR levels in living cells and zebrafish. The imaging results exhibited that different doses of Hg²⁺ exposure elevated the NTR levels and the same trend in changes of NTR as extrinsic hypoxia exposure, suggesting that Hg²⁺ exposure induced microenvironmental changes resulting in the hypoxia stress. This is the first time to provide visual evidence to support that Hg²⁺ stress may involve in the intracellular hypoxia microenvironment through monitoring the dynamic of NTR levels in the living systems. Our results may provide a novel insight into the molecular mechanisms of typical heavy metal element induced toxicity.

Stimulus-Responsive Amino Acids Behind In Situ Assembled Bioactive Peptide Materials

In situ self-assembly of peptides into well-defined nanostructures represents one of versatile strategies for creation of bioactive materials within living cells with great potential in disease diagnosis and treatment. The intimate relationship between amino acid sequences and the assembling propensity of peptides has been thoroughly elucidated over the past few decades. This has inspired development of various controllable self-assembling peptide systems based on stimuli-responsive naturally occurring or non-canonical amino acids, including redox-, pH-, photo-, enzyme-responsive amino acids. This review attempts to summarize the recent progress achieved in manipulating in situ self-assembly of peptides by controllable reactions occurring to amino acids. We will highlight the systems containing non-canonical amino acids developed in our laboratory during the past few years, primarily including acid/enzyme-responsive 4-aminoproline, redox-responsive (seleno)methionine, and enzyme-responsive 2-nitroimidazolyl alanine. Utilization of the stimuli-responsive assembling systems in creation of bioactive materials will be specifically introduced to emphasize their advantages for addressing the concerns lying in disease theranostics. Eventually, we will provide the perspectives for the further development of stimulus-responsive amino acids and thereby demonstrating their great potential in development of next-generation biomaterials.

Nitroreductase-instructed supramolecular assemblies for microbiome regulation to enhance colorectal cancer treatments

The development of human microbiome has collectively correlated the sophisticated interactions between Fusobacterium nucleatum and colorectal cancers (CRCs). However, the treatment of CRC via disruption of gastrointestinal flora remains less explored. Aiming at the up-regulated activity of nitroreductase in F. nucleatum-infected tumors, here, we developed the nitroreductase-instructed supramolecular self-assembly. The designed assembly precursors underwent enzymatic transformation to form assemblies, which agglutinated F. nucleatum and eradicated the targeted bacteria. These assemblies with anti-F. nucleatum activity could further alleviate the bacteria-induced drug resistance effect, thus sensitizing CRC cells against chemo-drugs. Eventually, in mice bearing F. nucleatum-infected CRC, the local introduction of nitroreductase-instructed assemblies could efficiently inhibit the tumor growth. Overall, this study incorporated nitroreductase to broaden the toolbox of enzyme-instructed supramolecular self-assembly. The local introduction of nitroreductase-instructed assemblies could target F. nucleatum to eliminate its contribution to CRC drug resistance and ameliorate chemotherapy outcomes.

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.

Monitoring mitochondrial nitroreductase activity in tumors and a hind-limb model of ischemia in mice using a novel activatable NIR fluorescent probe

We report a mitochondria-targeted nitroreductase (NTR)-activated near-infrared fluorescent probe: CS-NO2. Overexpressed NTR in mitochondria was measured with high sensitivity. More importantly, the probe CS-NO2 successfully monitored NTR activity in solid tumors and a hind-limb model of ischemia in mice. This novel finding indicates the promising function of our probe for the diagnosis of solid tumors and hypoxia-associated diseases.

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.

Enhancing the Contrast of Tumor Imaging for Image-Guided Surgery Using a Tumor-Targeting Probiotic with the Continuous Expression of a Biomarker

Tumor recurrence commonly results from tumor-positive resection margins and metastatic lesions. The complete removal of tumor-positive margins is particularly essential in clinics. Thus, we designed a strategy based on Escherichia coli Nissle 1917 (EcN) nitroreductase (NTR) with a polyethylene glycol (PEG) polymer coating (PC-EcN-NTR) to specifically target and colonize in tumors for high-contrast tumor imaging by providing a large amount of NTR as biomarkers in situ. NTR is a favorable biomarker for tumor detection and imaging. The nfsB-encoding plasmid with a 16S promoter was transfected into EcN for the continuous and stable expression of NTR (E. coli. NfsB). PC-EcN-NTR can accumulate and proliferate for a long time in tumors to substantially express NTR. When the NTR-activated fluorescence (FL) probe was sprayed on the tumor, the tumor region showed fluorescence signals within 5 min. Compared to the tumor without colonization with bacteria, the PC-EcN-NTR-colonized tumors displayed 3.15× enhanced fluorescence signals. Furthermore, the fluorescence signals of the whole tumor can last at least 3 h, which is suitable for a long and meticulous surgical operation. More importantly, in the PC-EcN-NTR-harboring tumor, obvious FL appeared even at the very edge (approximately 200 μm away from the edge) of the tumor tissue. A TCF-Based near-infrared-II fluorescent probe (probe 2) was designed and synthesized. Results similar to those of probe 1 were observed when probe 2 was used for in vivo tumor imaging, which further proved the generality of the enhancing ability of the tumor-targeting probiotic. This strategy will hopefully guide the surgical resection of tumors via monitoring intense NTR activity. It may spur the use of tumor-targeting probiotic and enzyme-activated fluorescent probes for the processes of tumor diagnosis and image-guided surgery.

Potentiating hypoxic microenvironment for antibiotic activation by photodynamic therapy to combat bacterial biofilm infections

Traditional antibiotic treatment has limited efficacy for the drug-tolerant bacteria present in biofilms because of their unique metabolic conditions in the biofilm infection microenvironment. Modulating the biofilm infection microenvironment may influence the metabolic state of the bacteria and provide alternative therapeutic routes. In this study, photodynamic therapy is used not only to eradicate methicillin-resistant Staphylococcus aureus biofilms in the normoxic condition, but also to potentiate the hypoxic microenvironment, which induces the anaerobic metabolism of methicillin-resistant Staphylococcus aureus and activates the antibacterial activity of metronidazole. Moreover, the photodynamic therapy-activated chemotherapy can polarize the macrophages to a M2-like phenotype and promote the repair of the biofilm infected wounds in mice. This biofilm infection microenvironment modulation strategy, whereby the hypoxic microenvironment is potentiated to synergize photodynamic therapy with chemotherapy, provides an alternative pathway for efficient treatment of biofilm-associated infections.

Bacteria in biofilms present unique metabolic conditions that limit the traditional antibiotic treatment. Here, the authors show a photodynamic therapy-activated chemotherapy potentiating the hypoxia of biofilms of methicillin-resistant Staphylococcus aureus, by developing hyaluronic acid nanoparticles functionalized with chlorin e6 and metronidazole.

Prediction and Design of Nanozymes using Explainable Machine Learning

An abundant number of nanomaterials have been discovered to possess enzyme-like catalytic activity, termed nanozymes. It is identified that a variety of internal and external factors influence the catalytic activity of nanozymes. However, there is a lack of essential methodologies to uncover the hidden mechanisms between nanozyme features and enzyme-like activity. Here, a data-driven approach is demonstrated that utilizes machine-learning algorithms to understand particle-property relationships, allowing for classification and quantitative predictions of enzyme-like activity exhibited by nanozymes. High consistency between predicted outputs and the observations is confirmed by accuracy (90.6%) and R² (up to 0.80). Furthermore, sensitive analysis of the models reveals the central roles of transition metals in determining nanozyme activity. As an example, the models are successfully applied to predict or design desirable nanozymes by uncovering the hidden relationship between different periods of transition metals and their enzyme-like performance. This study offers a promising strategy to develop nanozymes with desirable catalytic activity and demonstrates the potential of machine learning within the field of material science.

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.

Ultra-sensitive responsive near-infrared fluorescent nitroreductase probe with strong specificity for imaging tumor and detecting the invasiveness of tumor cells

Hypoxia plays an important role in cancer progression, which is a characteristic feature of the tumor micro-environment and reflects the invasiveness of tumor cells. Nitroreductase (NTR) is overexpressed in hypoxic tumors, which making it an efficient target for detecting the hypoxic state in tumor. In this work, a new type of nitro-based fluorescent probe, named HNT-NTR, has been proposed, HNT-NTR could detect specifically and rapidly the NTR degree, which reflects the level of hypoxia in bidimensional (2D) tumor cells, three-dimensional (3D) tumor spheres and even the real tumors in vivo without biological toxicity. Most importantly, according to the research, HNT-NTR even could distinguish tumor cells from other normal cells in vivo and reflect the invasiveness of tumor cells by the near-infrared fluorescence intensity, which provides a new way of clinical pathologic diagnosis. All in all, HNT-NTR not only is proven to be an ideal probe for detecting solid tumors in vivo, but also has great potential to distinguish if cells are benign or malignant and even guide therapeutic applications in the clinic.

Dual-Color Fluorescence Switch-On Probe for Imaging G-Quadruplex and Double-Stranded DNA in Living Cells

A tripodal quinone-cyanine dye having one donor and three acceptors, that is, one quinone and three N-methylbenzothiazolium moieties, QCy(MeBT)₃, was synthesized by simple Knoevenagel condensation between 2-hydroxybenzene-1,3,5-tricarbaldehyde and N-methyl-2-methylbenzothiazolium iodide. The 700 nm (λ_ex, 570 nm) and 600 nm (λ_ex, 470 nm) fluorescence emission of QCy(MeBT)₃ was significantly and individually enhanced with the addition of G-quadruplex (G4) DNA and double-stranded DNA (dsDNA), respectively. The results of docking simulations and the response against the viscosity change revealed that the dual-fluorescence response was caused by the difference in the binding mode of QCy(MeBT)₃ depending on the DNA structure. The results of fluorescence microscopy imaging experiments using QCy(MeBT)₃ suggested that G4 DNAs and dsDNAs in the cell nucleus can be imaged with near-infrared (NIR, 700 nm) and red (600 nm) fluorescence emissions. Furthermore, pyridostatin-induced G4 formation in the living cells can be imaged with NIR fluorescence. The results indicated that QCy(MeBT)₃ has huge potential to be a NIR-fluorescent molecular probe for analyzing the structural dynamics of nucleic acids in living cells with a normal fluorescence microscope.

A nitroreductase-responsive near-infrared phototheranostic probe for in vivo imaging of tiny tumor and photodynamic therapy

The hypoxia-activated and nitroreductase-responsive phototheranostic probe has been developed by incorporating a nitro group into a hemicyanine fluorophore. The probe displays extremely sensitive and selective near-infrared fluorescence enhancement to nitroreductase with the detection limit of 2.10 ng/mL. The detection mechanism relies on the nitroreductase-catalyzed reduction of the nitro group to an amino group, along with the generation of the fluorophore. The availability of the probe in fluorescence imaging and photodynamic therapy was demonstrated at cellular level and in vivo. The probe can image endogenous nitroreductase and the hypoxia status of living cells. The probe also exhibits significant phototoxicity to hypoxia tumor cells under the 660 nm laser irradiation. More importantly, the probe has been successfully utilized in imaging tiny tumor (about 6 mm³) and tumor photodynamic therapy in vivo. The proposed probe integrates accurate near-infrared fluorescence imaging and photodynamic therapy into the same molecule, which probably become a promising agent in the early diagnosis and therapy of tumors.

Targeted contrast agents and activatable probes for photoacoustic imaging of cancer

Photoacoustic (PA) imaging has emerged as a powerful technique for the high resolution visualization of biological processes within deep tissue. Through the development and application of exogenous targeted contrast agents and activatable probes that can respond to a given cancer biomarker, researchers can image molecular events in vivo during cancer progression. This information can provide valuable details that can facilitate cancer diagnosis and therapy monitoring. In this tutorial review, we provide a step-by-step guide to select a cancer biomarker and subsequent approaches to design imaging agents for in vivo use. We envision this information will be a useful summary to those in the field, new members to the community, and graduate students taking advanced imaging coursework. We also highlight notable examples from the recent literature, with emphasis on the molecular designs and their in vivo PA imaging performance. To conclude, we provide our outlook and future perspective in this exciting field.

Progress in the past five years of small organic molecule dyes for tumor microenvironment imaging

The tumor microenvironment (TME) is the survival environment for tumor cell proliferation and metastasis in deep tissues.

A nitroreductase responsive and photoactivated fluorescent probe for dual-controlled tumor hypoxia imaging

Tumor hypoxia has great importance in tumor progression and resistance to antitumor therapies. To precisely monitor tumor hypoxia, a controllable hypoxia imaging method is meaningful but still lacking. Herein, we develop a dual-controlled tumor hypoxia probe (TNB) by introducing a nitrophenol group and methyltetrazine group to the boron-dipyrromethene (BODIPY) dye. The fluorescence-quenching group nitrophenol is reduced to aminophenol by upregulated nitroreductase in hypoxic tumors, and the photocage methyltetrazine is cleaved by light irradiation. Hence the fluorescence of TNB is dual-controlled by hypoxia and photoactivation. We first evaluated TNB's potential for controllable hypoxia imaging in solution and tumor cells. The fluorescence of TNB under nitroreductase incubation and photoactivation increased more than 60 fold over that which was untreated or only treated with nitroreductase. Furthermore, results validate that TNB possesses photo-controllable activation features in tumor sections. We believe that the probe design based on enzyme and photoactivation responsiveness provides potential for spatiotemporal detection of other biomarkers.

Tumor hypoxia has great importance in tumor progression and resistance to antitumor therapies.

Conditionally Activatable Photoredox Catalysis in Living Systems

The transformational effect of photoredox catalytic chemistries has inspired new opportunities, enabling us to interrogate nature in ways that are not possible otherwise and to unveil new biotechnologies in therapy and diagnosis. However, the deployment of artificial photoredox catalysis in living systems remains challenging, mired by the off-target risk and safety concerns of photocatalyst toxicity. Here, we present an appealing approach, namely conditionally activatable photoredox catalysis (ConAPC), and as a proof of concept design the first ConAPC architecture (Se-NO₂) based upon classic self-immolative chemistry, in which the inherent photocatalytic properties can be temporarily caged while the species becomes active only at the tumor sites via sensing to specific biomarkers. Such a masking strategy allows a spatial-temporal control of photoresponsivity in vitro and in vivo. In particular, for ConAPC design, a new biologically benign metal-free photocatalyst (Se-NH₂), which is able to initiate NIR photoredox catalysis to manipulate the cellular electron pool in an O₂-independent mechanism of action, is identified. With this unique strategy, potent tumor-specific targeting photocatalytic eradication (TGI: 95%) is obtained in a mouse model. Impressively, favorable features such as high-resolution tumor recognition (SBR: 33.6) and excellent biocompatibility and safety are also achieved. This work therefore offers a new possibility for chemists to leverage artificial photocatalytic reactions toward the development of facile and intelligent photocatalytic theranostics.

Optical/electrochemical methods for detecting mitochondrial energy metabolism

This review highlights the biological importance of mitochondrial energy metabolism and the applications of multiple optical/electrochemical approaches to determine energy metabolites. Mitochondria, the main sites of oxidative phosphorylation and adenosine triphosphate (ATP) biosynthesis, provide the majority of energy required by aerobic cells for maintaining their physiological activity. They also participate in cell growth, differentiation, information transmission, and apoptosis. Multiple mitochondrial diseases, caused by internal or external factors, including oxidative stress, intense fluctuations of the ionic concentration, abnormal oxidative phosphorylation, changes in electron transport chain complex enzymes and mutations in mitochondrial DNA, can occur during mitochondrial energy metabolism. Therefore, developing accurate, sensitive, and specific methods for the in vivo and in vitro detection of mitochondrial energy metabolites is of great importance. In this review, we summarise the mitochondrial structure, functions, and crucial energy metabolic signalling pathways. The mechanism and applications of different optical/electrochemical methods are thoroughly reviewed. Finally, future research directions and challenges are proposed.

Recent Strategies to Develop Innovative Photosensitizers for Enhanced Photodynamic Therapy

This review presents a robust strategy to design photosensitizers (PSs) for various species. Photodynamic therapy (PDT) is a photochemical-based treatment approach that involves the use of light combined with a light-activated chemical, referred to as a PS. Attractively, PDT is one of the alternatives to conventional cancer treatment due to its noninvasive nature, high cure rates, and low side effects. PSs play an important factor in photoinduced reactive oxygen species (ROS) generation. Although the concept of photosensitizer-based photodynamic therapy has been widely adopted for clinical trials and bioimaging, until now, to our surprise, there has been no relevant review article on rational designs of organic PSs for PDT. Furthermore, most of published review articles in PDT focused on nanomaterials and nanotechnology based on traditional PSs. Therefore, this review aimed at reporting recent strategies to develop innovative organic photosensitizers for enhanced photodynamic therapy, with each example described in detail instead of providing only a general overview, as is typically done in previous reviews of PDT, to provide intuitive, vivid, and specific insights to the readers.

Nitroreductase-Induced Aggregation of Gold Nanoparticles for "Off-On" Photoacoustic Imaging of Tumor Hypoxia

Hypoxia is an important phenomenon due to insufficient oxygen supply in tumor tissue, and nitroreductase (NTR) is a characteristic enzyme used for evaluating hypoxia level in tumors. In this work, we designed a smart gold nanoparticle (AuNPs), modified by 16-mercaptoundecanoic acid (MHDA) and hypoxia-responsive 11-(2-nitro-1H-imidazol-1-yl)undecane-1-thiol (NI) ligand, that responds to the hypoxic environment in tumor sites. With proper surface ligand composition, the responsive nanoprobe exhibited aggregation through the bioreduction of the nitro group on NI ligands under hypoxic conditions and the UV-vis absorption peak maximum would shift to 630 nm from 530 nm, which acts as an "off-on" contrast agent for tumor hypoxic photoacoustic (PA) imaging. In vitro and in vivo experiments revealed that AuNPs@MHDA/NO₂ exhibited an enhanced PA signal in hypoxic conditions. This study demonstrates the potential of hypoxia-responsive AuNPs as novel and sensitive diagnostic agents, which lays a firm foundation for precise cancer treatment in the future.

Noncanonical Amino Acids for Hypoxia-Responsive Peptide Self-Assembly and Fluorescence

Design of endogenous stimuli-responsive amino acids allows for precisely modulating proteins or peptides under a biological microenvironment and thereby regulating their performance. Herein we report a noncanonical amino acid 2-nitroimidazol-1-yl alanine and explore its functions in creation of the nitroreductase (NTR)-responsive peptide-based supramolecular probes for efficient hypoxia imaging. On the basis of the reduction potential of the nitroimidazole unit, the amino acid was synthesized via the Mitsunobu reaction between 2-nitroimidazole and a serine derivate. We elucidated the relationship between the NTR-responsiveness of the amino acid and the structural feature of peptides involving a series of peptides. This eventually facilitates development of aromatic peptides undergoing NTR-responsive self-assembly by rationally optimizing the sequences. Due to the intrinsic role of 2-nitroimidazole in the fluorescence quench, we created a morphology-transformable supramolecular probe for imaging hypoxic tumor cells based on NTR reduction. We found that the resulting supramolecular probes penetrated into solid tumors, thus allowing for efficient fluorescence imaging of tumor cells in hypoxic regions. Our findings demonstrate development of a readily synthesized and versatile amino acid with exemplified properties in creating fluorescent peptide nanostructures responsive to a biological microenvironment, thus providing a powerful toolkit for synthetic biology and development of novel biomaterials.

A Novel NIR Fluorescent Probe for Highly Selective Detection of Nitroreductase and Hypoxic-Tumor-Cell Imaging

Nitroreductase as a potential biomarker for aggressive tumors has received extensive attention. In this work, a novel NIR fluorescent probe for nitroreductase detection was synthesized. The probe Py-SiRh-NTR displayed excellent sensitivity and selectivity. Most importantly, the confocal fluorescence imaging demonstrated that HepG-2 cells treated with Py-SiRh-NTR under hypoxic conditions showed obvious enhanced fluorescence, which means that the NTR was overexpressed under hypoxic conditions. Moreover, the probe showed great promise that could help us to study related anticancer mechanisms research.

Small-molecule probes for fluorescent detection of cellular hypoxia-related nitroreductase

Nitroreductase is a reductase that catalyzes nitro aromatic compounds to aromatic amines. It effectively reduces nitro to hydroxylamine or amino when in the presence of nicotinamide adenine dinucleotide or nicotinamide adenine dinucleotide phosphate. In terms of tumor, nitroreductase is upregulated in hypoxic tumor cells, and its content is directly related to the degree of hypoxia. Therefore, effective detection of nitroreductase is important not only for the study of cellular hypoxia, but also for the diagnosis and treatment of tumors and related diseases. In this review, we summarized the latest advances in small-molecule fluorescent probes for nitroreductase detection based on different fluorescence mechanisms, with a focus on research conducted between May 2018 and December 2020. The development trends and application prospect in this rapidly developing field were also highlighted.

Light-Driven Cascade Mitochondria-to-Nucleus Photosensitization in Cancer Cell Ablation

Nuclei and mitochondria are the only cellular organelles containing genes, which are specific targets for efficient cancer therapy. So far, several photosensitizers have been reported for mitochondria targeting, and another few have been reported for nuclei targeting. However, none have been reported for photosensitization in both mitochondria and nucleus, especially in cascade mode, which can significantly reduce the photosensitizers needed for maximal treatment effect. Herein, a light-driven, mitochondria-to-nucleus cascade dual organelle cancer cell ablation strategy is reported. A functionalized iridium complex, named BT-Ir, is designed as a photosensitizer, which targets mitochondria first for photosensitization and subsequently is translocated to a cell nucleus for continuous photodynamic cancer cell ablation. This strategy opens new opportunities for efficient photodynamic therapy.

Disruption of mitochondrial redox homeostasis by enzymatic activation of a trialkylphosphine probe

Redox homeostasis is essential for cell function and its disruption is associated with multiple pathologies. Redox balance is largely regulated by the relative concentrations of reduced and oxidized glutathione. In eukaryotic cells, this ratio is different in each cell compartment, and disruption of the mitochondrial redox balance has been specifically linked to metabolic diseases. Here, we report a probe that is selectively activated by endogenous nitroreductases, and releases tributylphosphine to trigger redox stress in mitochondria. Mechanistic studies revealed that, counterintuitively, release of a reducing agent in mitochondria rapidly induced oxidative stress through accumulation of superoxide. This response is mediated by glutathione, suggesting a link between reductive and oxidative stress. Furthermore, mitochondrial redox stress activates a cellular response orchestrated by transcription factor ATF4, which upregulates genes involved in glutathione catabolism.

Azo-Based Hypoxia-Responsive Self-Assembly Near-Infrared Fluorescent Nanoprobe for In Vivo Real-Time Bioimaging of Tumors

Hypoxia is an obvious characteristic of cancer, especially solid tumors. which may give rise to the expansion of invasion and metastasis. Exploring near-infrared (NIR) nanoprobes that could accurately evaluate the degree of hypoxia will contribute to the assessment of the degree of malignant neoplasms, so as to adopt more accurate and individualized treatment options Here, we have developed a self-assembled NIR organic nanoprobe to specifically and authoritatively detect the oxygen concentration in vivo and in vitro to evaluate the level of hypoxia. The organic nanoprobe mainly contains two motifs: a fluorophore moiety NRh-NH₂ for NIR fluorescence imaging and hypoxia-sensitive moiety Azonaphthalene derivatives for quenching NIR emissions, detecting oxygen in hypoxic regions and improving the hydrophilicity. The nanoprobes were used for detection of oxygen in a variety of living cells under different conditions and real-time bioimaging of neoplasms in live mice. This design strategy provides ideas for the development of nanoprobes for the diagnosis of tumors and other hypoxia-related diseases.

Precise imaging of mitochondria in cancer cells by real-time monitoring of nitroreductase activity with a targetable and activatable fluorescent probe

An activatable and mitochondrial-targetable fluorescent probe was developed. This designed probe showed ratiometric fluorescence and light-up near-infrared emission responsiveness to nitroreductase, achieving precise imaging of mitochondria in cancer cells by real-time monitoring of nitroreductase activity.

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.

Nano versus Molecular: Optical Imaging Approaches to Detect and Monitor Tumor Hypoxia

Hypoxia is a ubiquitous feature of solid tumors, which plays a key role in tumor angiogenesis and resistance development. Conventional hypoxia detection methods lack continuous functional detection and are generally less suitable for dynamic hypoxia measurement. Optical sensors hereby provide a unique opportunity to noninvasively image hypoxia with high spatiotemporal resolution and enable real-time detection. Therefore, these approaches can provide a valuable tool for personalized treatment planning against this hallmark of aggressive cancers. Many small optical molecular probes can enable analyte triggered response and their photophysical properties can also be fine-tuned through structural modification. On the other hand, optical nanoprobes can acquire unique intrinsic optical properties through nanoconfinement as well as enable simultaneous multimodal imaging and drug delivery. Furthermore, nanoprobes provide biological advantages such as improving bioavailability and systemic delivery of the sensor to enhance bioavailability. This review provides a comprehensive overview of the physical, chemical, and biological analytes for cancer hypoxia detection and focuses on discussing the latest nano- and molecular developments in various optical imaging approaches (fluorescence, phosphorescence, and photoacoustic) in vivo. Finally, this review concludes with a perspective toward the potentials of these optical imaging approaches in hypoxia detection and the challenges with molecular and nanotechnology design strategies.

Selective colorimetric and fluorescence detection of nitroreductase enzymes in living cells

Rhodamine dyes bearing aromatic nitro group has been synthesized for nitroreductase enzyme chemosensing applications. The probe is showing very selective turn-on fluorescent response towards nitroreductase enzymes and in hypoxic conditions. The sensor displays a remarkable fluorescent enhancement at 557 nm (λ_ex = 500 nm) without the interference of other biologically relevant species under hypoxic conditions in a physiological medium. The nitro group in the sensor is reduced by the nitroreductase enzyme to the amino group, resulting in the hydrolysis of the probe and subsequent release of highly fluorescent rhodamine 6G dye is observed. This rhodamine based fluorescent probe has been utilized for the imaging of nitroreductase enzymes as well as hypoxia in live cells.