Photoinduced electron transfer (PeT) based fluorescent probes for cellular imaging and disease therapy

Typical PeT-based fluorescent probes are multi-component systems where a fluorophore is connected to a recognition/activating group by an unconjugated linker. PeT-based fluorescent probes are powerful tools for cell imaging and disease diagnosis due to their low fluorescence background and significant fluorescence enhancement towards the target. This review provides research progress towards PeT-based fluorescent probes that target cell polarity, pH and biological species (reactive oxygen species, biothiols, biomacromolecules, etc.) over the last five years. In particular, we emphasise the molecular design strategies, mechanisms, and application of these probes. As such, this review aims to provide guidance and to enable researchers to develop new and improved PeT-based fluorescent probes, as well as promoting the use of PeT-based systems for sensing, imaging, and disease therapy.

Activatable fluorescence molecular imaging and anti-tumor effects investigation of GSH-sensitive BRD4 ligands

Overexpression of bromodomain 4 (BRD4) is closely correlated with a variety of human cancers by regulating the histone post-translational modifications, which renders BRD4 a promising target for pharmacological discoveries of novel therapeutic agents for cancer therapy. We herein present the design, chemical synthesis, cellular imaging and biological assessment of a novel tumor-sensitive BRD4 ligand (compound 4) by introducing anticancer BRD4 inhibitor into naphthalimide moiety (fluorescent reporter) via a sulfonamide unit as glutathione (GSH)-specific cleavable linker. Upon reaction with abundant intramolecular GSH in cancer cells or free GSH in aqueous solution (pH = 7.4), sulfonamide cleavage of 4 occurs, leading to the release of BRD4 inhibitor and concomitant fluorescence-on. This activatable fluorescence molecular imaging was demonstrated to preferentially occur in tumor cells. Moreover, towards cancer cell lines MGC-803 cells and THP-1, compound 4 was identified to show better antitumor efficacy than net BRD4 inhibitor. Collectively, this study presents a drug delivery strategy, wherein the drug release can be directly monitored in the cellular content by fluorescence imaging, and provides a valuable compound 4 as a potential antitumor agent. Compound 4 may represent a useful tool for explorative studies of BRD4 inhibition, such as an improved understanding of BRD4 inhibitor release-related information.

Fluorescence-Raman dual-mode quantitative detection and imaging of small-molecule thiols in cell apoptosis with DNA-modified gold nanoflowers

The monitoring of small-molecule thiols (especially glutathione) has attracted widespread attention due to their involvement in numerous physiological processes in living organisms and cells. In this work, a dual-mode nanosensor was designed to detect small-molecule thiols, which is based on the "on-off" switch of fluorescence resonance energy transfer (FRET) and surface-enhanced Raman scattering (SERS). Briefly, DNA was modified by Cy5 (signal probe) and disulfide bonds (recognition element). Gold nanoflowers (AuNFs) were used as the fluorescence-quenching and SERS-enhancing substrate. However, small-molecule thiols can cleave disulfide bonds and release short Cy5-labeled chains, causing the recovery of the fluorescence signal and a decrease of the SERS signal. The nanosensor showed a sensitive response to small-molecule thiols represented by GSH, with a linear range of 0.01-3 mM and a detection limit of 913 nM. In addition, it competed with other related biological interferences and presented good stability and better selectivity towards small-molecule thiols. Most importantly, the developed nanosensor had been successfully applied to in situ imaging and quantitative monitoring of the concentration of small-molecule thiols which changed during T-2 toxin-induced apoptosis in HeLa cells. Meanwhile, nanosensors are also versatile with their potential applications and can be easily extended to the detection and imaging of other human cell lines. The proposed method combines the dual advantages of fluorescence and SERS, which has broad prospects for in situ studies of physiological processes involving small-molecule thiols in biological systems.

Photodynamic Therapy with Tumor Cell Discrimination through RNA-Targeting Ability of Photosensitizer

Photodynamic therapy (PDT) represents an effective treatment to cure cancer. The targeting ability of the photosensitizer is of utmost importance. Photosensitizers that discriminate cancer cells can avoid the killing of normal cells and improve PDT efficacy. However, the design and synthesis of photosensitizers conjugated with a recognition unit of cancer cell markers is complex and may not effectively target cancer. Considering that the total RNA content in cancer cells is commonly higher than in normal cells, this study has developed the photosensitizer QICY with RNA-targeting abilities for the discrimination of cancer cells. QICY was specifically located in cancer cells rather than normal cells due to their stronger electrostatic interactions with RNA, thereby further improving the PDT effects on the cancer cells. After intravenous injection into mice bearing a xenograft tumor, QICY accumulated into the tumor location through the enhanced permeability and retention effect, automatically targeted cancer cells under the control of RNA, and inhibited tumor growth under 630 nm laser irradiation without obvious side effects. This intelligent photosensitizer with RNA-targeting ability not only simplifies the design and synthesis of cancer-cell-targeting photosensitizers but also paves the way for the further development of highly efficient PDTs.

Discovery of a Fluorogenic Probe for In Situ Pyruvate Kinase M2 Isoform (PKM2) Labeling through Chemoselective SNAr with a Binding Site Lysine Residue

The key challenge of developing reaction-based turn-on probes is to establish latent electrophilic fluorophores exhibiting high reactivity only upon binding to a specific protein(s). Herein, we identified such a fluorophore, 6-arylthioether-substituted 3-cyano-1-oxo-1H-phenalene-2-carboxylate, which chemoselectively labels binding site Cys or Lys residues. Based on this fluorophore, we developed the first reaction-based turn-on pyruvate kinase M2 isoform (PKM2) fluorescent probe AT-OPC1, which selectively labels PKM2 with the binding site Lys305. The latent electrophilic reactivity of the fluorophore endows the probe with precise detection of the expression of PKM2 in situ by means of both in-gel fluorescence imaging at the proteome level and real-time no-wash cell imaging approaches, which has the potential to be applied in cancer diagnoses.

A highly sensitive and selective fluorescence turn-on probe for the sensing of H2S in vitro and in vivo

A fluorescence turn-on probe, 2-butyl-1,3-dioxo-2,3-dihydro-1H-benzo[de]isoquinolin-6-yl 2,4-dinitrobenzenesulfonate (NT-SH), has been constructed for sensing of hydrogen sulfide (H₂S). NT-SH exhibited excellent detection performance including favorable water solubility, low fluorescence background, high enhancement (45-fold), large linear response range (0-50 μM) and low detection limit (80.01 nM) for H₂S in aqueous. In addition, the response mechanism of NT-SH for H₂S was confirmed by the theoretical calculation and mass spectral analysis. More importantly, the imaging experiments of H₂S in vitro and in vivo confirmed that NT-SH had low cytotoxicity, and favorable biocompatibility. In addition, it illustrated that NT-SH was able to detected exogenous H₂S in living cells and zebrafish. These results suggested that NT-SH can be act as a potential molecular tool for detecting of H₂S in aqueous solution, in vitro and in vivo.

Controlling Conjugated Antibodies at the Molecular Level for Active Targeting Nanoparticles toward HER2-Positive Cancer Cells

For active targeting nanodrug delivery systems conjugated with antibodies, both lack of control of the antibody at the molecular level and protein corona formation remarkably decreases targeting efficacy. Herein, we designed a series of silica nanoparticles toward HER2-positive breast cancer cells, with an anti-HER2 Fab-6His density ranging from 50 to 180 molecules per nanoparticle. Through the site-directed immobilization method we developed, the antigen-binding domain of anti-HER2 Fab was mostly accessible to the HER2 receptor. Both polyethylene glycol (PEG) chains and a high density of Fab were shown to suppress protein corona formation and macrophage uptake. The dependency of targeting efficacy and cytotoxicity on Fab density was shown using a series of breast cancer cell lines with different levels of the HER2 expression. The high density of Fab stimulates quick responses from HER2-positive cells. Combined with PEG chains, conjugated antibodies with a well-controlled orientation and density significantly improves delivery performance and sheds light on the design and preparation of an improved active targeting nanodrug delivery system.

Recent Progress of Glutathione (GSH) Specific Fluorescent Probes: Molecular Design, Photophysical Property, Recognition Mechanism and Bioimaging

The selective detection of glutathione (GSH) in vitro and in vivo has attracted great attentions, credited to its important role in life activities and association with a series of diseases. Among all kinds of analytical techniques, the fluorescent probe for GSH detection become prevalent recently because of its ease of operation, high temporal-spatial resolution, visualization and noninvasiveness, etc. The special structural features of GSH, such as the nucleophilicity of sulfhydryl group, the concerted reaction ability of amino group, the negative charged nature, the latent hydrogen bonding ability along with its flexible molecular chain, are all potent factors to be employed to design the specific fluorescent probe for GSH and discriminate it from other bio-species including its analogues cysteine (Cys) and homocysteine (Hcy). This paper reviewed the studies in the last 3 years and was organized based on the reaction mechanism of each probe. According to the reactivity of GSH, various recognition mechanisms including Michael addition, nucleophilic aromatic substitution, ordinary nucleophilic substitution, multi-site reaction, and other unique reactions have been utilized to construct the GSH specific fluorescent probes, and the molecular design strategy, photophysical property, recognition mechanism, and bioimaging application of each reported probe were all discussed here systematically. Great progress has been made in this area, and we believe the analyses and summarization of these excellent studies would provide valuable message and inspiration to researchers to advance the research toward clinic applications.

Evaluation of a GSH-targeting prodrug via a sulfonamide-induced "integrative" platform for selective cancer therapy

A sulfonamide-appended gemcitabine prodrug was newly produced. The prodrug was shown to efficiently distinguish GSH from cysteine and homocysteine. Upon reaction of this prodrug with GSH, which is relatively abundant in tumor cells, sulfonyl group cleavage occurred as well as active release of the drug GMC and a concomitant increase in the innate fluorescence intensity. As a proof of concept, colocalization experiments were carried out; these experiments demonstrated that the probe LHX resulted in, via receptor-mediated endocytosis, significantly improved therapeutic efficacy and few side effects. Thus, these results indicated the theranostic agent to be a promising "integrative" platform for efficient cancer therapy. The agent can be activated in real time, and not only be selectively monitored and localized by specific tumour cells, but also undergo cascaded cleavage to induce both a fluorogenic response and release of an active cytotoxic drug.

A novel near-infrared fluorescent probe for the dynamic monitoring of the concentration of glutathione in living cells

In this study, a water-soluble, near-infrared fluorescent probe (EQR-S) was designed for the measurement of the glutathione (GSH) concentration. Responses of different interfering substances to the developed probe were investigated, and the luminescence mechanism was examined by theoretical calculations. Results revealed that EQR-S can be applied for the rapid, sensitive determination of the GSH concentration with a detection limit of 69 nM. Based on the above advantages, EQR-S was successfully applied to investigate the fluctuation in the GSH concentration of living cells under high-temperature stress.

A highly selective and sensitive fluorescent probe based on the chromone fluorophore for imaging hydrogen sulfide in living cells

A fluorescent probe that was designed based on the 6-amino chromone fluorophore for detecting H₂S can distinguish H₂S from Cys/GSH under acidic conditions.