Coumarin-based fluorescent ‘AND’ logic gate probes for the detection of homocysteine and a chosen biological analyte

With this research we set out to develop a number of coumarin-based ‘AND’ logic fluorescence probes that were capable of detecting a chosen analyte in the presence of HCys. Probe JEG-CAB was constructed by attaching the ONOO⁻ reactive unit, benzyl boronate ester, to a HCys/Cys reactive fluorescent probe, CAH. Similarly, the core unit CAH was functionalised with the nitroreductase (NTR) reactive p-nitrobenzyl unit to produce probe JEG-CAN. Both, JEG-CAB and JEG-CAN exhibited a significant fluorescence increase when exposed to either HCys and ONOO⁻ (JEG-CAB) or HCys and NTR (JEG-CAN) thus demonstrating their effectiveness to function as AND logic gates for HCys and a chosen analyte.

With this research we set out to develop of a number of coumarin-based ‘AND’ logic fluorescence probes that were capable of detecting a chosen analyte in the presence of HCys.

Lysosome-targeting turn-on red/NIR BODIPY probes for imaging hypoxic cells

Two lysosome-targeting turn-on red/NIR BODIPY probes for imaging hypoxic cells were rationally designed.

Development of a red-light emission hypoxia-sensitive two-photon fluorescent probe for in vivo nitroreductase imaging

The overexpression of nitroreductase (NTR) in hypoxia has been recognized as a biomarker of highly aggressive disease, and the development of a hypoxia-sensitive two-photon (TP) bioimaging probe with both excitation and emission wavelengths in the red-light region provides favorable deep-tissue imaging with a low background fluorescence signal. Although quite a few TP hypoxia-sensitive fluorescent probes have been reported for NTR detection, their short emission wavelength (<550 nm) limits their application. Herein, we report a red light emissive TP hypoxia-sensitive turn-on probe (NRP) by employing Nile Red as a red-emitting fluorophore and p-nitrobenzene as an NTR recognition group with improved sensitivity. The NRP probe showed obvious strong red-fluorescence enhancement in the presence of NTR and high selectivity toward NTR in aqueous solution. Our in vitro experimental results illustrated that the NRP loaded tumor cells treated under hypoxia display remarkably strong fluorescence in both OP and TP microscopy at 655 nm with 45-fold enhancement, which affords deep-tissue penetration ability. The NRP probe was also successfully applied for imaging NTR in liver tissue slices and a 4T1-bearing mice model, which is important for bioimaging applications.

Endoplasmic reticulum-targeted two-photon turn-on fluorescent probe for nitroreductase in tumor cells and tissues

Hypoxia conditions could increase the activity of intracellular nitroreductase (NTR) and lead to many malignant diseases. Therefore, monitoring the activity of NTR is of great significance to study the related diseases. Organelles play crucial roles in the metabolism of living cells. In these organelles, the endoplasmic reticulum (ER) possesses single membrane structure, and it is the largest organelle in the cell. ER performs the synthesis, processing and modification of proteins and lipid, stabilizing the intracellular Ca²⁺ concentration and other physiological functions in living cells. Therefore, it is of great significance to develop ER-target probes in living system. Toward this goal, a new endoplasmic reticulum-targeted two-photon fluorescence turn-on NTR probe Na-NTR-ER is designed and synthesized. Probe Na-NTR-ER has been proved to display high sensitivity (36 ng/mL) and selectivity to NTR. Particularly, probe Na-NTR-ER has been successfully applied for the monitoring of NTR in ER with a high the Pearson's colocalization coefficient as 0.90 in HeLa cells and cancerous mouse tissues up to the depth of 100 μm with significant fluorescence signals.

COX-2 Inhibition mediated anti-angiogenic activatable prodrug potentiates cancer therapy in preclinical models

Anti-angiogenesis, i.e., blocking the angiogenic pathway, has been considered as an important component in current cancer therapeutic modalities. However, the associated benefits have proven to be modest as tumor angiogenesis and regrowth persist, probably due to other ill-defined complex angiogenic mechanisms. Herein, we developed an indomethacin (IMC) incorporating system to mediate hypoxia responsive prodrug (TA) and diagnostic agent (DA) in cancer theranostic applications. Cyclooxygenase 2 (COX-2) elevated expression in several cancer types is closely associated with severe tumor supporting vascularization factors. Our strategy utilizing COX-2 inhibition augmented the anti-angiogenetic induced hypoxia responsive prodrug activation well. Both in vitro and in vivo results proved that DA and TA exhibited specificity towards COX-2 positive (+ve) HeLa and A549 cancer cell lines and activation under hypoxic conditions. Compared with controls (R1, and anticancer drug SN-38), TA displayed prolonged tumor retention and enhanced therapeutic efficacy in xenograft mouse models at a reduced dosage. Our results significantly highlighted the importance of COX-2 blockade mediated anti-angiogenesis in complementing the hypoxia-responsive drug delivery systems (DDSs) and could to beneficial for the rapid development of more efficacious antitumor therapeutics.

Imaging of oxygen and hypoxia in cell and tissue samples

Molecular oxygen (O2) is a key player in cell mitochondrial function, redox balance and oxidative stress, normal tissue function and many common disease states. Various chemical, physical and biological methods have been proposed for measurement, real-time monitoring and imaging of O2 concentration, state of decreased O2 (hypoxia) and related parameters in cells and tissue. Here, we review the established and emerging optical microscopy techniques allowing to visualize O2 levels in cells and tissue samples, mostly under in vitro and ex vivo, but also under in vivo settings. Particular examples include fluorescent hypoxia stains, fluorescent protein reporter systems, phosphorescent probes and nanosensors of different types. These techniques allow high-resolution mapping of O2 gradients in live or post-mortem tissue, in 2D or 3D, qualitatively or quantitatively. They enable control and monitoring of oxygenation conditions and their correlation with other biomarkers of cell and tissue function. Comparison of these techniques and corresponding imaging setups, their analytical capabilities and typical applications are given.

Oxygen Sensing, Hypoxia Tracing and in Vivo Imaging with Functional Metalloprobes for the Early Detection of Non-communicable Diseases

Hypoxia has been identified as one of the hallmarks of tumor environments and a prognosis factor in many cancers. The development of ideal chemical probes for imaging and sensing of hypoxia remains elusive. Crucial characteristics would include a measurable response to subtle variations of pO2 in living systems and an ability to accumulate only in the areas of interest (e.g., targeting hypoxia tissues) whilst exhibiting kinetic stabilities in vitro and in vivo. A sensitive probe would comprise platforms for applications in imaging and therapy for non-communicable diseases (NCDs) relying on sensitive detection of pO2. Just a handful of probes for the in vivo imaging of hypoxia [mainly using positron emission tomography (PET)] have reached the clinical research stage. Many chemical compounds, whilst presenting promising in vitro results as oxygen-sensing probes, are facing considerable disadvantages regarding their general application in vivo. The mechanisms of action of many hypoxia tracers have not been entirely rationalized, especially in the case of metallo-probes. An insight into the hypoxia selectivity mechanisms can allow an optimization of current imaging probes candidates and this will be explored hereby. The mechanistic understanding of the modes of action of coordination compounds under oxygen concentration gradients in living cells allows an expansion of the scope of compounds toward in vivo applications which, in turn, would help translate these into clinical applications. We summarize hereby some of the recent research efforts made toward the discovery of new oxygen sensing molecules having a metal-ligand core. We discuss their applications in vitro and/or in vivo, with an appreciation of a plethora of molecular imaging techniques (mainly reliant on nuclear medicine techniques) currently applied in the detection and tracing of hypoxia in the preclinical and clinical setups. The design of imaging/sensing probe for early-stage diagnosis would longer term avoid invasive procedures providing platforms for therapy monitoring in a variety of NCDs and, particularly, in cancers.

A new aggregation-induced emission fluorescent probe for rapid detection of nitroreductase and its application in living cells

The biological activity of nitroreductase (NTR) is closely related to biological hypoxia status in organisms. The development of effective methods for monitoring the activity of NTR is of great significance for medical diagnosis and tumor research. Toward this goal, we have developed a new aggregation-induced emission (AIE) fluorescence NTR probe TPE-HY used the tetraphenylethene as the fluorophore, and used the nitro group as the NTR recognition site. The probe TPE-HY has many excellent properties, including rapid response, AIE characteristics, high sensitivity and selectivity, and low cytotoxicity. Importantly, the probe TPE-HY is successfully applied to monitor endogenous NTR in living HeLa cells.

Excited-state intramolecular proton-transfer (ESIPT) based fluorescence sensors and imaging agents

We review recent advances in the design and application of excited-state intramolecular proton-transfer (ESIPT) based fluorescent probes. These sensors and imaging agents (probes) are important in biology, physiology, pharmacology, and environmental science.

Dimeric BODIPY-loaded liposomes for dual hypoxia marker imaging and activatable photodynamic therapy against tumors

This work reports a dimeric BODIPY (BDP)-loaded liposome with conjugation of anti-HIF antibodies for dual hypoxia marker imaging and nitroreductase (NTR)-activatable photodynamic therapy (PDT) against hypoxic tumors.

Real time detection of ESKAPE pathogens by a nitroreductase-triggered fluorescence turn-on probe

The identification of bacterial pathogens is the critical first step in conquering infection diseases. A novel turn-on fluorescent probe for the selective sensing of nitroreductase (NTR) activity and its initial applications in rapid, real-time detection and identification of ESKAPE pathogens have been reported.

2-Nitroimidazole based fluorescent probes for nitroreductase; monitoring reductive stress in cellulo

Two 2-nitroimidazole-1,8-naphthalimide conjugates, 1 and 2, have been synthesised as fluorescence probes for the detection of reductive stress in HeLa cells. The 4-substituted derivative 1 was shown to act as a highly sensitive and selective substrate for nitroreductase where it exhibited a clear blue to green ratiometric fluorescence response visible to the naked eye. Moreover, biological studies demonstrated 1 could be activated in cellulo where the impact of reductive stress was easily monitored using confocal microscopy and flow cytommetry.

CYP450 Enzymes Effect Oxygen-Dependent Reduction of Azide-Based Fluorogenic Dyes

Azide-containing compounds have broad utility in organic synthesis and chemical biology. Their use as powerful tools for the labeling of biological systems in vitro has enabled insights into complex cellular functions. To date, fluorogenic azide-containing compounds have primarily been employed in the context of click chemistry and as sensitive functionalities for hydrogen sulfide detection. Here, we report an alternative use of this functionality: as fluorogenic probes for the detection of depleted oxygen levels (hypoxia). Oxygen is imperative to all life forms, and probes that enable quantification of oxygen tension are of high utility in many areas of biology. Here we demonstrate the ability of an azide-based dye to image hypoxia in a range of human cancer cell lines. We have found that cytochrome P450 enzymes are able to reduce these probes in an oxygen-dependent manner, while hydrogen sulfide does not play an important role in their reduction. These data indicate that the azide group is a new bioreductive functionality that can be employed in prodrugs and dyes. We have uncovered a novel mechanism for the cellular reduction of azides, which has implications for the use of click chemistry in hypoxia.

Synthesis and cell imaging applications of fluorescent mono/di/tri-heterocyclyl-2,6-dicyanoanilines

Synthesis of 3,4,5-triheterocyclyl-2,6-dicyanoanilines, starting from heterocyclic aldehydes and 1,2-diheterocycle-substituted ethanones, is described. 2,6-Dicyanoanilines with one or two heterocyclic substituents have also been synthesized. It was found that some of these molecules have selective cell-staining properties useful for cell imaging applications. The compounds 1g, 10f and 11 were found to stain cytoplasm of the cells in contact but not the nucleus while the compound 12 showed affinity to apoptotic cells resulting in blue fluorescence. The cell imaging results with compound 12 were similar to Annexin V-FITC, a known reagent containing recombinant Annexin V conjugated to green-fluorescent FITC dye, used for detection of apoptotic cells. These compounds were found to be non-cytotoxic and have potential application as cell imaging agents.

Mitochondrial Nitroreductase Activity Enables Selective Imaging and Therapeutic Targeting

Nitroreductase (NTR) activities have been known for decades, studied extensively in bacteria and also in systems as diverse as yeast, trypanosomes, and hypoxic tumors. The putative bacterial origin of mitochondria prompted us to explore the possible existence of NTR activity within this organelle and to probe its behavior in a cellular context. Presently, by using a profluorescent near-infrared (NIR) dye, we characterize the nature of NTR activity localized in mammalian cell mitochondria. Further, we demonstrate that this mitochondrially localized enzymatic activity can be exploited both for selective NIR imaging of mitochondria and for mitochondrial targeting by activating a mitochondrial poison specifically within that organelle. This constitutes a new mechanism for mitochondrial imaging and targeting. These findings represent the first use of mitochondrial enzyme activity to unmask agents for mitochondrial fluorescent imaging and therapy, which may prove to be more broadly applicable.