A nitroreductase-responsive fluorescence turn-on photosensitizer for lysosomes imaging and photodynamic therapy

Nitroreductase (NTR) is a frequently used biomarker for the assessment of hypoxia level in tumors. As one of the main sources of enzymes, the dysfunction of lysosomes typically leads to various diseases. In this study, an NTR-triggered lysosome-targeting probe, M-TPE-P, was designed based on a tetraphenylethylene core. DFT calculation indicated that the probe possessed a narrow singlet-triplet energy gap (ΔEST), rendering it an efficient photosensitizer. The docking affinity of M-TPE-P to NTR revealed a strong structural match between them. Photophysical properties demonstrated that the probe exhibited high selectivity and sensitivity in a broad pH rang for detecting NTR with kcat/Km as 2.18 × 104 M-1 s-1. The detection limit was determined to be 53.6 ng/mL in 80 % PBS/DMSO solution. Cell imaging studies showed the probe could trace intracellular NTR behavior with green fluorescence. The colocalization analysis indicated its excellent lysosome-targeting specificity. In addition, the probe exhibited effective ROS generation ability and significant PDT effect after NIR irradiation, positioning it as a promising photosensitizer for cancer treatment.

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

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

Vision for Ratiometric Nanoprobes: In Vivo Noninvasive Visualization and Readout of Physiological Hallmarks

Lesion areas are distinguished from normal tissues surrounding them by distinct physiological characteristics. These features serve as biological hallmarks with which targeted biomedical imaging of the lesion sites can be achieved. Although tremendous efforts have been devoted to providing smart imaging probes with the capability of visualizing the physiological hallmarks at the molecular level, the majority of them are merely able to derive anatomical information from the tissues of interest, and thus are not suitable for taking part in in vivo quantification of the biomarkers. Recent advances in chemical construction of advanced ratiometric nanoprobes (RNPs) have enabled a horizon for quantitatively monitoring the biological abnormalities in vivo. In contrast to the conventional probes whose dependency of output on single-signal profiles restricts them from taking part in quantitative practices, RNPs are designed to provide information in two channels, affording a self-calibration opportunity to exclude the analyte-independent factors from the outputs and address the issue. Most of the conventional RNPs have encountered several challenges regarding the reliability and sufficiency of the obtained data for high-performance imaging. In this Review, we have summarized the recent progresses in developing highly advanced RNPs with the capabilities of deriving maximized information from the lesion areas of interest as well as adapting themselves to the complex biological systems in order to minimize microenvironmental-induced falsified signals. To provide a better outlook on the current advanced RNPs, nanoprobes based on optical, photoacoustic, and magnetic resonance imaging modalities for visualizing a wide range of analytes such as pH, reactive species, and different derivations of amino acids have been included. Furthermore, the physicochemical properties of the RNPs, the major constituents of the nanosystems and the analyte recognition mechanisms have been introduced. Moreover, the alterations in the values of the ratiometric signal in response to the analyte of interest as well as the time at which the highest value is achieved, have been included for most of RNPs discussed in this Review. Finally, the challenges as well as future perspectives in the field are discussed.

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

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

A fluorescent pH probe for evaluating the freshness of chicken breast meat

A fluorescent probe, Nap-MOR, based on the naphthalimide fluorophore, was designed and developed for pH measurement in aqueous solutions. Nap-MOR had a close linear relationship between fluorescence intensity and pH, in the range 4.5-8, which covers the full range of pH found in normal fresh, defective and spoiled meat. pH measurement with Nap-MOR was free from interference by a wide range of ions and biochemicals found in meat and the results were not significantly different in comparison with a pH meter. Therefore, Nap-MOR is a robust and convenient way to evaluate the freshness of chicken breast meat by measuring its pH.

Solvent-controlled regioselective arylation of indoles and mechanistic explorations

A new reaction for the regioselective arylation of indoles at C2 or C3 positions achieved by adjusting the solvent and with P(O)tBu2 as an auxiliary group is reported. And the experimental results and DFT confirmed the process.

9-Nitrobenzo[b]quinolizinium as a fluorogenic probe for the detection of nitroreductase in vitro and in Escherichia coli

The non-fluorescent 9-nitrobenzo[b]quinolizinium is readily reduced by nitroreductase to fluorescent reaction products whose formation depends on the reaction conditions.

Small molecule based fluorescent chemosensors for imaging the microenvironment within specific cellular regions

The microenvironment (local environment), including viscosity, temperature, polarity, hypoxia, and acidic-basic status (pH), plays indispensable roles in cellular processes. Significantly, organelles require an appropriate microenvironment to perform their specific physiological functions, and disruption of the microenvironmental homeostasis could lead to malfunctions of organelles, resulting in disorder and disease development. Consequently, monitoring the microenvironment within specific organelles is vital to understand organelle-related physiopathology. Over the past few years, many fluorescent probes have been developed to help reveal variations in the microenvironment within specific cellular regions. Given that a comprehensive understanding of the microenvironment in a particular cellular region is of great significance for further exploration of life events, a thorough summary of this topic is urgently required. However, there has not been a comprehensive and critical review published recently on small-molecule fluorescent chemosensors for the cellular microenvironment. With this review, we summarize the recent progress since 2015 towards small-molecule based fluorescent probes for imaging the microenvironment within specific cellular regions, including the mitochondria, lysosomes, lipid drops, endoplasmic reticulum, golgi, nucleus, cytoplasmic matrix and cell membrane. Further classifications at the suborganelle level, according to detection of microenvironmental factors by probes, including polarity, viscosity, temperature, pH and hypoxia, are presented. Notably, in each category, design principles, chemical synthesis, recognition mechanism, fluorescent signals, and bio-imaging applications are summarized and compared. In addition, the limitations of the current microenvironment-sensitive probes are analyzed and the prospects for future developments are outlined. In a nutshell, this review comprehensively summarizes and highlights recent progress towards small molecule based fluorescent probes for sensing and imaging the microenvironment within specific cellular regions since 2015. We anticipate that this summary will facilitate a deeper understanding of the topic and encourage research directed towards the development of probes for the detection of cellular microenvironments.

An HDAC8-selective fluorescent probe for imaging in living tumor cell lines and tissue slices

Histone deacetylase 8 (HDAC8) has been used as a therapeutic target for many cancers as it is highly expressed in neuroblastoma cells and breast cancer cells. HDAC8-selective fluorescent probes need to be urgently developed. Herein, two novel fluorescent probes, namely NP-C6-PCI and AM-C6-PCI, based on the conjugation of 1,8-naphthalimide with a highly selective inhibitor of HDAC8 (PCI-34051) were reported. Compared with PCI-34051 (KD = 6.25 × 10-5 M), NP-C6-PCI (KD = 8.05 × 10-6 M) and AM-C6-PCI (KD = 7.42 × 10-6 M) showed great selectivity toward HDAC8. Two fluorescent probes exhibited high fluorescence intensity under λex = 450 nm and a large Stokes shift (100 nm). NP-C6-PCI was selected for cell and tissue imaging due to the similarity in the bioactivity of NP-C6-PCI with PCI-34051. The ability of NP-C6-PCI to target imaging HDAC8 in SH-SY5Y and MDA-MB-231 tumor cells was demonstrated. Furthermore, NP-C6-PCI was applied to imaging SH-SY5Y tumor tissue slices to indicate the relative expression level of HDAC8. Therefore, this HDAC8-selective fluorescent probe can be expected for applications in HDAC8-targeted drug screening as well as in pathologic diagnoses.

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.

A Study on Hypoxia Susceptibility of Organ Tissues by Fluorescence Imaging with a Ratiometric Nitroreductase Probe

Hypoxia, a condition of oxygen deficiency in tissues, features various diseases including solid tumor. Under hypoxia, several reductases such as nitroreductases are elevated. Based on this fact, we have investigated an indirect way to assess the hypoxia susceptibility of different organ tissues (mouse lung, heart, spleen, kidney, and liver) by detecting nitroreductase present within. Among the organs, the kidney showed a notable susceptibility to hypoxia, which was due to the renal medulla, not due to the renal cortex, as observed by ratiometric fluorescence imaging with a probe. The probe features ratiometric signaling, NIR-emitting, two-photon absorbing, and pH-insensitive emission properties, offering a practical tool for studying the nitroreductase activity and, furthermore, hypoxia-associated biological processes.

Rational design of fluorescent probes for targeted in vivo nitroreductase visualization

Three fluorescent probes were made by conjugation of para-, ortho-, and meta-nitrobenzene to the BODIPY core via a thiolether bond. It revealed that the linkage and nitro substituent position significantly influence the capability of NTR detection.