Reversible off-on fluorescence probe for hypoxia and imaging of hypoxia-normoxia cycles in live cells

An overview on recent advances of reversible fluorescent probes and their biological applications

Due to the simplicity and low detection limit, fluorescent probes are widely used in both analytical sensing and optical imaging. Compared to conventional fluorescent probes, reversibility endows the reversible fluorescent probe outstanding advantages and special properties, making reversible fluorescent probes with capable of quantitative, repetitive or circulatory. Reversible fluorescent probes can also monitor the concentration dynamics of target analytes in real time, such as metal ions, proteins and enzymes, as well as intracellular redox processes, which have been widely applied in various fields. This review summarized the types and excellent properties of reversible fluorescent probes designed and developed in recent years. It also summarized the applications of reversible fluorescent probe in fluorescence imaging, biological testing, monitoring redox cycles, and proposed the remaining challenges and future development directions of the reversible fluorescent probe. This review provided comprehensive overview of reversible fluorescent probe, which may provide valuable references for the design and fabrication of the reversible fluorescent probe.

Biodegradable and switchable near-infrared fluorescent probes for hypoxia detection

Aims: Among solid tumors, hypoxia is a common characteristic and responsible for chemotherapeutic resistance. Hypoxia-sensitive imaging probes are therefore essential for early tumor detection, growth monitoring and drug-response evaluation. Despite significant efforts, detecting hypoxic oxygen levels remains challenging. Materials & methods: This paper demonstrates the use of an amine-rich carbon dot probe functionalized with an imidazole group that exhibits reversible fluorescence switching in normoxic and hypoxic environments. Results & conclusion: We demonstrate the ability to emit near-infrared light only under hypoxic conditions. The probes are found to be biodegradable in the presence of human digestive enzymes such as lipase. Ex vivo tissue imaging experiments revealed promising near-infrared signals even at a depth of 5 mm for the probe under ex vivo imaging conditions.

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

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

An aggregation-induced emission fluorescence probe for evaluating the effect of CYP450 changes under tumor chemotherapy

Cancer is a complex disease with very high incidence and mortality rates every year. However, cancer drug resistance greatly mitigates the cure rates of tumors, and cytochrome P450 (CYP450) plays an important role in the development of cisplatin resistance. We developed the aggregation-induced emission luminogen (AIEgen) TPE-CYP to monitor the changes in CYP450. The TPE-CYP fluorescent probe was successfully used to assess CYP450 levels in tumor cells and tumor tissue sections. This study presented that CYP450 level in HepG2/DDP cells (cisplatin-resistant cells) was higher than that in HepG2 cells, and the inhibition of CYP450 by 1-ABT effectively improved the tumor resistance. Thus, CYP450 plays a key role in the development of tumor resistance. The synergistic effect of 1-ABT and the chemotherapeutic agent cisplatin was superior to that of cisplatin alone in tumor-bearing mice. The TPE-CYP probe will provide an idea for the clinical implementation of individualized tumor treatment strategies, through the accurate monitoring of CYP450.

Imaging the oxygen wave with a single bioluminescent bacterium

We developed a capability of a monolayer of bioluminescent (BL) bacteria for spatiotemporally visualizing the heterogeneous distribution and dynamic evolution of interfacial oxygen concentration, resulting in the discovery of spontaneous and stochastic oxygen waves at the interface between the substrate and an undisturbed, apparently still solution. Wild type bacteria, P. phosphoreum, spontaneously emit light during the native metabolism processes, i.e., bioluminescence. The emission intensity is sensitively regulated by oxygen concentration. By taking the electrolysis of water as a model, it was demonstrated that time-lapsed BL imaging of a bacterial monolayer allowed for visualizing the dynamic distribution of oxygen. The results were quantitatively understood with a physical model involving the diffusion equation and Michaelis-Menten equation. Unexpectedly, further study uncovered a spontaneous and stochastic oxygen wave in a standard well of a microtiter plate, which was subsequently attributed to the inevitable micro-convections induced by inhomogeneous evaporation and thermal fluctuation. Because of the wide application of microtiter plates, this study sheds new light to better understand the apparent heterogeneity in cell-culture and bio-assays.

Rational construction of a reversible arylazo-based NIR probe for cycling hypoxia imaging in vivo

Reversible NIR luminescent probes with negligible photocytotoxicity are required for long-term tracking of cycling hypoxia in vivo. However, almost all of the reported organic fluorescent hypoxia probes reported until now were irreversible. Here we report a reversible arylazo-conjugated fluorescent probe (HDSF) for cycling hypoxia imaging. HDSF displays an off-on fluorescence switch at 705 nm in normoxia-hypoxia cycles. Mass spectroscopic and theoretical studies confirm that the reversible sensing behavior is attributed to the two electron-withdrawing trifluoromethyl groups, which stabilizes the reduction intermediate phenylhydrazine and blocks the further reductive decomposition. Cycling hypoxia monitoring in cells and zebrafish embryos is realized by HDSF using confocal imaging. Moreover, hypoxic solid tumors are visualized and the ischemia-reperfusion process in mice is monitored in real-time. This work provides an effective strategy to construct organic fluorescent probes for cycling hypoxia imaging and paves the way for the study of cycling hypoxia biology.

Quantitative In Vivo Monitoring of Hypoxia and Vascularization of Patient-Derived Murine Xenografts of Mantle Cell Lymphoma Using Photoacoustic and Ultrasound Imaging

Tumor oxygenation and vascularization are important parameters that determine the aggressiveness of the tumor and its resistance to cancer therapies. We introduce dual-modality ultrasound and photoacoustic imaging (US-PAI) for the direct, non-invasive real-time in vivo evaluation of oxygenation and vascularization of patient-derived xenografts (PDXs) of B-cell mantle cell lymphomas. The different optical properties of oxyhemoglobin and deoxyhemoglobin make it possible to determine oxygen saturation (sO₂) in tissues using PAI. High-frequency color Doppler imaging enables the visualization of blood flow with high resolution. Tumor oxygenation and vascularization were studied in vivo during the growth of three different subcutaneously implanted patient-derived xenograft (PDX) lymphomas (VFN-M1, VFN-M2 and VFN-M5 R1). Similar values of sO₂ (sO₂ Vital), determined from US-PAI volumetric analysis, were obtained in small and large VFN-M1 tumors ranging from 37.9 ± 2.2 to 40.5 ± 6.0 sO₂ Vital (%) and 37.5 ± 4.0 to 35.7 ± 4.6 sO₂ Vital (%) for small and large VFN-M2 PDXs. In contrast, the higher sO₂ Vital values ranging from 57.1 ± 4.8 to 40.8 ± 5.7 sO₂ Vital (%) (small to large) of VFN-M5 R1 tumors corresponds with the higher aggressiveness of that PDX model. The different tumor percentage vascularization (assessed as micro-vessel areas) of VFN-M1, VFN-M2 and VFN-M5 R1 obtained by color Doppler (2.8 ± 0.1%, 3.8 ± 0.8% and 10.3 ± 2.7%) in large-stage tumors clearly corresponds with their diverse growth and aggressiveness. The data obtained by color Doppler were validated by histology. In conclusion, US-PAI rapidly and accurately provided relevant and reproducible information on tissue oxygenation in PDX tumors in real time without the need for a contrast agent.

Floxuridine Oligomers Activated under Hypoxic Environment

Floxuridine oligomers are anticancer oligonucleotide drugs composed of a number of floxuridine residues. They show enhanced cytotoxicity per floxuridine monomer because the nuclease degradation of floxuridine oligomers directly releases highly active floxuridine monophosphate in cells. However, their clinical use is limited by the low selectivity against cancer cells. To address this limitation, we herein report floxuridine oligomer prodrugs that are active under hypoxia conditions, which is one of the distinguishing features of the microenvironment of all solid tumors. We designed and synthesized two types of floxuridine oligomer prodrugs that possess hypoxia-responsive moieties on nucleobases. The floxuridine oligomer prodrugs showed lower cytotoxicity under normoxia conditions (O2 = 20%), while the parent floxuridine oligomer showed similar anticancer effects under hypoxia conditions (O2 = 1%). The floxuridine oligomer prodrug enabled tumor growth suppression in live mice. This would be the first example demonstrating the conditional control of the medicinal efficacy of oligomerized nucleoside anticancer drugs.

Optical and magnetic resonance imaging approaches for investigating the tumour microenvironment: state-of-the-art review and future trends

The tumour microenvironment (TME) strongly influences tumorigenesis and metastasis. Two of the most characterized properties of the TME are acidosis and hypoxia, both of which are considered hallmarks of tumours as well as critical factors in response to anticancer treatments. Currently, various imaging approaches exist to measure acidosis and hypoxia in the TME, including magnetic resonance imaging (MRI), positron emission tomography and optical imaging. In this review, we will focus on the latest fluorescent-based methods for optical sensing of cell metabolism and MRI as diagnostic imaging tools applied both in vitro and in vivo. The primary emphasis will be on describing the current and future uses of systems that can measure intra- and extra-cellular pH and oxygen changes at high spatial and temporal resolution. In addition, the suitability of these approaches for mapping tumour heterogeneity, and assessing response or failure to therapeutics will also be covered.

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.

Photoexcited molecular probes for selective and revertible imaging of cellular reactive oxygen species

Redox homeostasis is key to maintaining the normal physiological status of living cells.

Responsive fluorinated nanoemulsions for 19F magnetic resonance detection of cellular hypoxia

We report two highly fluorinated Cu-based imaging agents, CuL1 and CuL2, for detecting cellular hypoxia as nanoemulsion formulations. Both complexes retained their initial quenched 19F MR signals due to paramagnetic Cu2+; however, both complexes displayed a large signal increase when the complex was reduced. DLS studies showed that the CuL1 nanoemulsion (NECuL1) had a hydrodiameter of approximately 100 nm and that it was stable for four weeks post-preparation. Hypoxic cells incubated with NECuL1 showed that 40% of the Cu2+ taken up was reduced in low oxygen environments.

Förster resonance energy transfer (FRET)-based small-molecule sensors and imaging agents

In this tutorial review, we will explore recent advances in the construction and application of Förster resonance energy transfer (FRET)-based small-molecule fluorescent probes. The advantages of FRET-based fluorescent probes include: a large Stokes shift, ratiometric sensing and dual/multi-analyte responsive systems. We discuss the underlying energy donor-acceptor dye combinations and emphasise their applications for the detection or imaging of cations, anions, small neutral molecules, biomacromolecules, cellular microenvionments and dual/multi-analyte responsive systems.

Quantitative Mapping of Liver Hypoxia in Living Mice Using Time-Resolved Wide-Field Phosphorescence Lifetime Imaging

Hypoxia has been identified to contribute the pathogenesis of a wide range of liver diseases, and therefore, quantitative mapping of liver hypoxia is important for providing critical information in the diagnosis and treatment of hepatic diseases. However, the existing imaging methods are unsuitable to quantitatively assess liver hypoxia due to the need of liver-specific contrast agents and be easily affected by other imaging factors. Here, a time-resolved lifetime-based imaging method is established for quantitative mapping of the distribution of hypoxia in the livers of mice by combining a wide-field luminescence lifetime imaging system with an oxygen-sensitive nanoprobe. It is shown that the method is suitable for real-time quantification of the change of oxygen pressure in the process of hepatic ischemia-reperfusion of the mouse. Moreover, the developed lifetime imaging methodology is used to quantitatively map liver hypoxia regions in the mouse model of orthotopic liver tumor, where the average oxygen pressure in tumorous liver is far below the normal liver.

Cellular and Extracellular Components in Tumor Microenvironment and Their Application in Early Diagnosis of Cancers

Tumors are surrounded by complex environmental components, including blood and lymph vessels, fibroblasts, endothelial cells, immune cells, cytokines, extracellular vesicles, and extracellular matrix. All the stromal components together with the tumor cells form the tumor microenvironment (TME). In addition, extracellular physical and chemical factors, including extracellular pH, hypoxia, elevated interstitial fluid pressure, and fibrosis, are closely associated with tumor progression, metastasis, immunosuppression, and drug resistance. Cellular and extracellular components in TME contribute to nearly all procedures of carcinogenesis. By summarizing the recent work in this field, we make a comprehensive review on the role of cellular and extracellular components in the process of carcinogenesis and their potential application in early diagnosis of cancer. We hope that a systematic review of the diverse aspects of TME will help both research scientists and clinicians in this field.

Construction of a 1-D Sm(iii) coordination polymer with a long-chain Schiff base ligand: dual-emissive response to metal ions

A 1-D coordination polymer [Sm(HL)₂(NO₃)]_n was constructed using a new flexible Schiff base ligand that has a long-chain (CH₂)₁₀ backbone, and it shows dual-emissive response to metal ions, especially to Co²⁺ at the ppm level.

A fluorescent naphthalimide NADH mimic for continuous and reversible sensing of cellular redox state

A new naphthalimide based NADH mimic that functions as a fully reversible fluorescent “on off” probe for redox state has been synthesised and evaluated.

Hypoxia-responsive nanoparticle based drug delivery systems in cancer therapy: An up-to-date review

Hypoxia is a salient feature observed in most solid malignancies that holds a pivotal role in angiogenesis, metastasis and resistance to conventional cancer therapeutic approaches, and thus enables cancer progression. However, the typical characteristics of hypoxic cells such as low oxygen levels and highly bio-reductive environment can offer stimuli-responsive drug release to aid in tumor-specific chemo, radio, photodyanamic and sonodynamic therapies. This approach based on targeting the poorly oxygenated tumor habitats offers the prospective to overcome the difficulties that arises due to heterogenic nature of tumor and could be possibly used in the design of diagnostic as well as therapeutic nanocarriers for targeting various types of solid cancers. Consequently, hypoxia triggered nanoparticle based drug delivery systems is a rapidly progressing research area in developing effective strategies to combat drug-resistance in solid tumors. The present review presents the recent advances in the development of hypoxia-responsive nanovehicles for drug delivery to heterogeneous tumors. The initial sections of the article provides insights into the development of hypoxia in growing cancer and its role in disease progression. The current limitations and the future prospective of hypoxia-stimulated nanomachines for cancer treatment are also discussed.

A dual-responsive probe for detecting cellular hypoxia using 19F magnetic resonance and fluorescence

We report the first dual-responsive 19F MRI and fluorescence imaging probe for cellular hypoxia. The Cu2+-based probe exhibits no 19F MR signal and reduced fluorescence signal due to paramagnetic quenching; however, the probe turns-on in both modes following reduction to Cu+. This bimodal agent can differentiate hypoxic and normoxic cells in both modalities.

Naphthalimides in fluorescent imaging of tumor hypoxia - An up-to-date review

Hypoxia is a distinctive characteristic of advanced solid malignancies that results from a disparity between oxygen supply and its consumption. The degree of hypoxia is believed to have adverse prognostic significance. Therefore detecting cellular hypoxia can potentially offer insights into the grade of tumour as well as its evolution towards a progressive malignant phenotype, which clinically translates to greater metastatic potential and treatment resistance. Fluorescence imaging to visualize hypoxia in biological systems is a minimally-invasive method. Recently there are several reports on interdisciplinary research that aims at developing functional probes that can be efficiently used for non-invasive imaging of hypoxic tumours. Upregulated levels of nitroreductase (NTR) is detected in hypoxic solid malignancies, and this characteristic feature is increasingly utilized in the development of NTR-targeted fluorescent molecules to selectively sense hypoxia in vivo. The present review summarizes various reports published on the design concepts of nitro naphthalimide-based bio-reductive fluorescent sensors that can be applied noninvasively to image hypoxia in cancer.

Simultaneous spatiotemporal tracking and oxygen sensing of transient implants in vivo using hot-spot MRI and machine learning

A varying oxygen environment is known to affect cellular function in disease as well as activity of various therapeutics. For transient structures, whether they are unconstrained therapeutic transplants, migrating cells during tumor metastasis, or cell populations induced by an immunological response, the role of oxygen in their fate and function is known to be pivotal albeit not well understood in vivo. To address such a challenge in the case of generation of a bioartificial pancreas, we have combined fluorine magnetic resonance imaging and unsupervised machine learning to monitor over time the spatial arrangement and the oxygen content of implants encapsulating pancreatic islets that are unconstrained in the intraperitoneal (IP) space of healthy and diabetic mice. Statistically significant trends in the postimplantation temporal dependence of oxygen content between aggregates of 0.5-mm or 1.5-mm alginate microcapsules were identified in vivo by looking at their dispersity as well as arrangement in clusters of different size and estimating oxygen content on a pixel-by-pixel basis from thousands of 2D images. Ultimately, we found that this dependence is stronger for decreased implant capsule size consistent with their tendency to also induce a larger immunological response. Beyond the bioartificial pancreas, this work provides a framework for the simultaneous spatiotemporal tracking and oxygen sensing of other cell populations and biomaterials that change over time to better understand and improve therapeutic design across diverse applications such as cellular transplant therapy, treatments preventing metastatic formation, and modulators for improving immunologic response, for all of which oxygen is a major mechanistic component.

Construction of a crystalline 14-metal Zn–Nd rectangular nanocluster with a dual-emissive response towards metal ions

A crystalline 14-metal Zn–Nd cluster [Zn₆Nd₈L₂(OAc)₂₀(O)₂(NO₃)₄(OC₂H₅)₄] (1) was constructed using a flexible Schiff base ligand with long-chain (CH₂)₂O(CH₂)₂O(CH₂)₂ backbone. 1 exhibits a nanoscale rectangular structure (10 × 14 × 18 Å). Besides the visible ligand-centered emission, 1 displays typical NIR luminescence of Nd³⁺. Interestingly, 1 shows both a ligand-centered and lanthanide fluorescence response towards metal ions, especially to K⁺ and Co²⁺ at the ppm level.

A crystalline 14-metal Zn–Nd nanocluster was constructed, and it shows ligand-centered and lanthanide fluorescence response towards metal ions.

Hypoxia-triggered single molecule probe for high-contrast NIR II/PA tumor imaging and robust photothermal therapy

Hypoxia is a common characteristic of solid tumors. This important feature is associated with resistance to radio-chemotherapy, which results in poor prognosis and probability of tumor recurrence. Taking advantage of background-free NIR II fluorescence imaging and deeper-penetrating photoacoustic (PA) imaging, we developed a hypoxia-triggered and nitroreductase (NTR) enzyme-responsive single molecule probe for high-contrast NIR II/PA tumor imaging and hypoxia-activated photothermal therapy (PTT), which will overcome cellular resistance during hypoxia. Methods: The single molecule probe IR1048-MZ was synthesized by conjugating a nitro imidazole group as a specific hypoxia trigger with an IR-1048 dye as a NIR II/PA signal reporter. We investigated the NIR II fluorescence, NIR absorbance and photothermal effect in different hypoxia conditions in vitro, and performed NIR II/PA tumor imaging and hypoxia-activated photothermal therapy in mice. Results: This versatile molecular probe IR1048-MZ not only realized high-contrast tumor visualization with a clear boundary by NIR II fluorescence imaging, but also afforded deep-tissue penetration at the centimeter level by 3D PA imaging. Moreover, after being activated by NTR that is overexpressed in hypoxic tumors, the probe exhibited a significant photothermal effect for curative tumor ablation with no recurrence. Conclusions: We have developed the first hypoxia-triggered and NTR enzyme-responsive single molecule probe for high-contrast NIR II/PA tumor imaging and hypoxia-activated photothermal therapy. By tracing the activity of NTR, IR1048-MZ may be a promising contrast agent and theranostic formulation for other hypoxia-related diseases (such as cancer, inflammation, stroke, and cardiac ischemia).

Efficient Synthesis of a Wide-Range Absorbing Azaphthalocyanine Dark Quencher and Its Application to Dual-Labeled Oligonucleotide Probes for Quantitative Real-Time Polymerase Chain Reactions

Unsymmetrical dialkylamino-substituted zinc azaphthalocyanine (AzaPc) exhibits unique spectral and photophysical properties for dark quenchers of fluorescence in DNA hybridization probes. The panchromatic light absorption of AzaPc from 300 nm up to at least 700 nm and its lack of fluorescence make it an ideal candidate for a universal dark quencher. To prove this experimentally, oligodeoxyribonucleotide probes were labeled at the 3'-end by this AzaPc and at the 5'-end by a fluorophore used in the polymerase chain reaction (PCR)-that is, fluorescein, CAL Fluor Red 610, and Cy5. AzaPc showed a significantly higher quenching efficiency compared to the commercially available dark quenchers (BHQ-1, BHQ-2, BBQ-650) in a developed model of TaqMan PCR assay. The AzaPc-labeled probe proved to also be useful in a practical PCR assay for the quantification of the SLCO2B1 transporter gene expression. The constructed calibration curves indicated linearity in the range from 10² to 10⁷ of target copies.

A nitroreductase and acidity detecting dual functional ratiometric fluorescent probe for selectively imaging tumor cells

A dual functional ratiometric fluorescent probe can obviously distinguish acidity, nitroreductase, and nitroreductase in an acidic environment. Confocal fluorescence imaging of A549 cells indicates the probe can detect acidity and expressed nitroreductase in living cells.

Nanodiamond-based non-canonical autophagy inhibitor synergistically induces cell death in oxygen-deprived tumors

Blockage of autophagic flux by nanodiamonds induces apoptosis in hypoxic tumor cells with minimal toxicity to normal tissues and enhances the effects of anti-angiogenic therapy.

A bioreducible N-oxide-based probe for photoacoustic imaging of hypoxia

Hypoxia occurs when limited oxygen supply impairs physiological functions and is a pathological hallmark of many diseases including cancer and ischemia. Thus, detection of hypoxia can guide treatment planning and serve as a predictor of patient prognosis. Unfortunately, current methods suffer from invasiveness, poor resolution and low specificity. To address these limitations, we present Hypoxia Probe 1 (HyP-1), a hypoxia-responsive agent for photoacoustic imaging. This emerging modality converts safe, non-ionizing light to ultrasound waves, enabling acquisition of high-resolution 3D images in deep tissue. HyP-1 features an N-oxide trigger that is reduced in the absence of oxygen by heme proteins such as CYP450 enzymes. Reduction of HyP-1 produces a spectrally distinct product, facilitating identification via photoacoustic imaging. HyP-1 exhibits selectivity for hypoxic activation in vitro, in living cells, and in multiple disease models in vivo. HyP-1 is also compatible with NIR fluorescence imaging, establishing its versatility as a multimodal imaging agent.

Hypoxia is a hallmark of many diseases including cancer and ischemia, and detection can be invasive and of low resolution and specificity. Here the authors show a hypoxia probe that converts non-ionizing light to ultrasound, which enables the acquisition of high-resolution 3D images in deep tissue.

Development of Photoacoustic Probes for in Vivo Molecular Imaging

Photoacoustic (PA) imaging is an emerging, non-invasive imaging modality that encompasses attributes of both optical and ultrasound imaging. Because of the combination of optical excitation and acoustic detection, PA imaging enables high contrast and high resolution within deep tissue (centimeter depths). Recent advances in PA probe development have allowed for stimulus-responsive imaging in a variety of biological models with implications for basic, translational, and clinical sciences. This perspective highlights recent progress in the development of PA probes and their application to live-animal molecular imaging.

A universal fluorogenic switch for Fe(ii) ion based on N-oxide chemistry permits the visualization of intracellular redox equilibrium shift towards labile iron in hypoxic tumor cells

Iron (Fe) species play a number of biologically and pathologically important roles. In particular, iron is a key element in oxygen sensing in living tissue where its metabolism is intimately linked with oxygen metabolism. Regulation of redox balance of labile iron species to prevent the generation of iron-catalyzed reactive oxygen species (ROS) is critical to survival. However, studies on the redox homeostasis of iron species are challenging because of a lack of a redox-state-specific detection method for iron, in particular, labile Fe2+. In this study, a universal fluorogenic switching system is established, which is responsive to Fe2+ ion based on a unique N-oxide chemistry in which dialkylarylamine N-oxide is selectively deoxygenized by Fe2+ to generate various fluorescent probes of Fe2+-CoNox-1 (blue), FluNox-1 (green), and SiRhoNox-1 (red). All the probes exhibited fluorescence enhancement against Fe2+ with high selectivity both in cuvette and in living cells. Among the probes, SiRhoNox-1 showed an excellent fluorescence response with respect to both reaction rate and off/on signal contrast. Imaging studies were performed showing the intracellular redox equilibrium shift towards labile iron in response to reduced oxygen tension in living cells and 3D tumor spheroids using SiRhoNox-1, and it was found that the hypoxia induction of labile Fe2+ is independent of iron uptake, hypoxia-induced signaling, and hypoxia-activated enzymes. The present studies demonstrate the feasibility of developing sensitive and specific fluorescent probes for Fe2+ with refined photophysical characteristics that enable their broad application in the study of iron in various physiological and pathological conditions.

Hypoxia-Responsive 19F MRI Probes with Improved Redox Properties and Biocompatibility

¹⁹F magnetic resonance imaging (MRI), an emerging modality in biomedical imaging, has shown promise for in vitro and in vivo preclinical studies. Here we present a series of fluorinated Cu(II)ATSM derivatives for potential use as ¹⁹F magnetic resonance agents for sensing cellular hypoxia. The synthesized complexes feature a hypoxia-targeting Cu²⁺ coordination core, nine equivalent fluorine atoms connected via a variable-length poly(ethylene glycol) linker. Introduction of the fluorine moiety maintains the planar coordination geometry of the Cu²⁺ center, while the linker length modulates the Cu^2+/+ reduction potential, ¹⁹F NMR relaxation properties, and lipophilicity. In particular, the ¹⁹F NMR relaxation properties were quantitatively evaluated by the Solomon-Bloembergen model, revealing a regular pattern of relaxation enhancement tuned by the distance between Cu²⁺ and F atoms. Finally, the potential utility of these complexes for sensing reductive environments was demonstrated using both ¹⁹F MR phantom imaging and ¹⁹F NMR, including experiments in intact live cells.

Context-dependent intravital imaging of therapeutic response using intramolecular FRET biosensors

Intravital microscopy represents a more physiologically relevant method for assessing therapeutic response. However, the movement into an in vivo setting brings with it several additional considerations, the primary being the context in which drug activity is assessed. Microenvironmental factors, such as hypoxia, pH, fibrosis, immune infiltration and stromal interactions have all been shown to have pronounced effects on drug activity in a more complex setting, which is often lost in simpler two- or three-dimensional assays. Here we present a practical guide for the application of intravital microscopy, looking at the available fluorescent reporters and their respective expression systems and analysis considerations. Moving in vivo, we also discuss the microscopy set up and methods available for overlaying microenvironmental context to the experimental readouts. This enables a smooth transition into applying higher fidelity intravital imaging to improve the drug discovery process.