Theranostic Fluorescent Probes

The ability to gain spatiotemporal information, and in some cases achieve spatiotemporal control, in the context of drug delivery makes theranostic fluorescent probes an attractive and intensely investigated research topic. This interest is reflected in the steep rise in publications on the topic that have appeared over the past decade. Theranostic fluorescent probes, in their various incarnations, generally comprise a fluorophore linked to a masked drug, in which the drug is released as the result of certain stimuli, with both intrinsic and extrinsic stimuli being reported. This release is then signaled by the emergence of a fluorescent signal. Importantly, the use of appropriate fluorophores has enabled not only this emerging fluorescence as a spatiotemporal marker for drug delivery but also has provided modalities useful in photodynamic, photothermal, and sonodynamic therapeutic applications. In this review we highlight recent work on theranostic fluorescent probes with a particular focus on probes that are activated in tumor microenvironments. We also summarize efforts to develop probes for other applications, such as neurodegenerative diseases and antibacterials. This review celebrates the diversity of designs reported to date, from discrete small-molecule systems to nanomaterials. Our aim is to provide insights into the potential clinical impact of this still-emerging research direction.

Recent Advancements on Self-Immolative System Based on Dynamic Covalent Bonds for Delivering Heterogeneous Payloads

The precisely spatial-temporal delivery of heterogeneous payloads from a single system with the same pulse is in great demand in realizing versatile and synergistic functions. Very few molecular architectures can satisfy the strict requirements of dual-release translated from single triggers, while the self-immolative systems based on dynamic covalent bonds represent the "state-of-art" of ultimate solution strategy. Embedding heterogeneous payloads symmetrically onto the self-immolative backbone with dynamic covalent bonds as the trigger, can respond to the quasi-bio-orthogonal hallmarks which are higher at the disease's microenvironment to simultaneously yield the heterogeneous payloads (drug A/drug B or drug/reporter). In this review, the modular design principles are concentrated to illustrate the rules in tailoring useful structures, then the rational applications are enumerated on the aspects of drug codelivery and visualized drug-delivery. This review, hopefully, can give the general readers a comprehensive understanding of the self-immolative systems based on dynamic covalent bonds for delivering heterogeneous payloads.

Near-Infrared Fluorescence Probe with a New Recognition Moiety for Specific Detection and Imaging of Aldehyde Dehydrogenase Expecting the Identification and Isolation of Cancer Stem Cells

Aldehyde dehydrogenase (ALDH) is a vital enzyme that converts aldehyde to acetic acid during alcohol metabolism. ALDH is also a cellular marker of cancer stem cells (CSCs), which plays an important role in cancer diagnosis and prognosis assessment. Therefore, there is a need to explore convenient, selective, and sensitive methods for the detection and imaging of ALDH. Because of the low background fluorescence and high penetration, near-infrared (NIR) fluorescent probes are powerful tools for the detection of ALDH. Until now, only one NIR fluorescent probe has been reported for detecting ALDH. Hence, we synthesized a novel NIR fluorescent probe, Probe-ALDH, by linking the new specific recognition moiety 4-hydroxymethyl benzaldehyde with NIR fluorophore AXPI. Compared with the existing ALDH fluorescent probes, Probe-ALDH has excellent properties, such as a new specific recognition moiety without the substitution of benzaldehyde, a simple synthesis method, emission wavelength in the NIR region, reaction time of only 30 min, and a detection limit as low as 0.03 U·mL^-1, which is better than those of the previously reported probes. The probe effectively eliminates the interference from reactive oxygen species (ROS), amino acids, and amines. More importantly, the flow cytometry results showed that Probe-ALDH has great potential applications in the identification and isolation of CSCs. Ultimately, it was successfully applied to the imaging analysis of endogenous ALDH in HepG2 cells by the addition of inhibitor disulfiram. The excellent performance of Probe-ALDH makes it a promising candidate for drug discovery, cancer diagnosis, and so forth.

Noncovalent Dual-Locked Near-Infrared Fluorescent Probe for Precise Imaging of Tumor via Hypoxia/Glutathione Activation

Stimulus-responsive fluorescent probes have broad applications in the early detection and treatment of tumors and thus promote the personalized treatment of tumors and improve patient survival. Among the repertoires of probes, dual-locked near-infrared (NIR) fluorescent probes are of great significance due to their improved specificity and multiplex detection in tumor imaging but remain to be explored. In this work, a facile noncovalent strategy for constructing dual-locked probes was proposed. A glutathione (GSH)-activatable single-locked probe CySS (first lock) was preloaded into a hypoxia-responsive molecular container CF3C4A (second lock) through a host-guest interaction to form the dual-locked probe CF3C4A-CySS. Under physiological conditions, CF3C4A-CySS binds strongly to avoid undesired leakage in normal tissues. We have proven that CF3C4A-CySS can be activated and "turn on" its NIR fluorescent signal under the dual key stimulation of hypoxia and GSH in the tumor microenvironment, which enables precise tumor imaging with enhanced accuracy and specificity. Both in vitro and in vivo results indicated the superiority of CF3C4A-CySS in tumor imaging. This work not only provides an effective tool for tumor imaging but also proposes a promising strategy for dual-locked imaging agent construction.

A self-triggered radioligand therapy agent for fluorescence imaging of the treatment response in prostate cancer

PURPOSE

Radioligand therapy (RLT) targeting prostate-specific membrane antigen (PSMA) is emerging as an effective treatment option for metastatic castration-resistant prostate cancer (mCRPC). An imaging-based method to quantify early treatment responses can help to understand and optimize RLT.

METHODS

We developed a self-triggered probe 2 targeting the colocalization of PSMA and caspase-3 for fluorescence imaging of RLT-induced apoptosis.

RESULTS

The probe binds to PSMA potently with a K_i of 4.12 nM, and its fluorescence can be effectively switched on by caspase-3 with a K_m of 67.62 μM. Cellular and in vivo studies demonstrated its specificity for imaging radiation-induced caspase-3 upregulation in prostate cancer. To identify the detection limit of our method, we showed that probe 2 could achieve 1.79 times fluorescence enhancement in response to ¹⁷⁷Lu-RLT in a medium PSMA-expressing 22Rv1 xenograft model.

CONCLUSION

Probe 2 can potently bind to PSMA, and the fluorescence signal can be sensitively switched on by caspase-3 both in vitro and in vivo. This method may provide an effective tool to investigate and optimize PSMA-RLT.

Self-Assembly of Upconversion Nanoparticles Based Materials and Their Emerging Applications

In the past few decades, significant progress of the conventional upconversion nanoparticles (UCNPs) based nanoplatform has been achieved in many fields, and with the development of nanoscience and nanotechnology, more and more complex situations need a UCNPs based nanoplatform having multifunctions for specific multimodal or multiplexed applications. Through self-assembly, different UCNPs or UCNPs with other materials could be combined together within an entity. It is more like an ideal UCNPs nanoplatform, a unique system with the properties defined by its individual components as well as by the morphology of the composite. Various designs can show their different desired properties depending on the application situation. This review provides a complete summary on the optimization of the synthesis method for the recently designed UCNPs assemblies and summarizes various applications, including dual-modality cell imaging, molecular delivery, detection, and programmed control therapy. The challenges and limitations the UCNPs assembly faces and the potential solutions in this field are also presented.

A novel near-infrared theranostic probe for accurate cancer chemotherapy in vivo by a dual activation strategy

A novel theranostic probe called CX-B-DF is constructed for precise chemotherapy guided by near-infrared (NIR) fluorescence imaging. Moreover, the theranostic probe shows high cytotoxicity to cancer cells under dual activation (H2O2 and TP), which causes the accuracy of drug release to be improved and the toxic side effects to be reduced.

Hemicyanine-Based Near-Infrared Activatable Probes for Imaging and Diagnosis of Diseases

Molecular activatable probes with near-infrared (NIR) fluorescence play a critical role in in vivo imaging of biomarkers for drug screening and disease diagnosis. With structural diversity and high fluorescence quantum yields, hemicyanine dyes have emerged as a versatile scaffold for the construction of activatable optical probes. This Review presents a survey of hemicyanine-based NIR activatable probes (HNAPs) for in vivo imaging and early diagnosis of diseases. The molecular design principles of HNAPs towards activatable optical signaling against various biomarkers are discussed with a focus on their broad applications in the detection of diseases including inflammation, acute organ failure, skin diseases, intestinal diseases, and cancer. This progress not only proves the unique value of HNAPs in preclinical research but also highlights their high translational potential in clinical diagnosis.

Cancer cell identification by facile imaging of intracellular reductive substances with fluorescent nanosensor

Ascorbic acid (AA) and glutathione (GSH), the most abundant intracellular reductive substances, have been widely used as biomarkers for cancer cells identification. The current methods relying on imaging of AA or GSH alone to identify cancer cells may cause systematic errors, since a mutual conversion relationship exists between AA and GSH. In this work, we propose a fluorescent nanosensor for the simultaneous imaging of intracellular reductive substances including AA and GSH. Biocompatible fluorescent silicon nanoparticles (SiNPs) with rich surface amine and carboxyl groups were synthesized. The fluorescence of the SiNP was initially quenched by chelation of Fe³⁺ ions, forming SiNP/Fe³⁺ complex as the fluorescent nanosensor. Upon the redox reaction with reductive substances, the nanosensor showed sensitively fluorescent recovery. Moreover, benefited from the efficient cellular uptake of the SiNP/Fe³⁺ and the overexpressed intracellular reductive substances in cancer cells, the fluorescent nanosensor was used to accurately identify the human breast carcinoma (MCF-7) cells from normal mammary epithelial (MCF-10A) cells by imaging of intracellular AA and GSH simultaneously. This strategy would be promising in imaging-guided precision cancer diagnosis.

Quantification of Drug Release Degree In Vivo Using Antibody-Guided, Dual-NIR-Dye Ratiometric System

Fluorescent dyes linked to drug delivery systems provide important real-time information on the efficacy of drug delivery. However, the quantitative monitoring of drug distribution is challenging because of interferences from the biological sample and instrumental setup. To improve quantification of anticancer drug delivery followed by drug release in tumor, we equipped an antibody-drug conjugate (ADC) with a turn-on near-infrared (NIR) dye, sensitive to drug release, and a reference NIR dye. In this study, chlorambucil (CLB) was chosen as a model anticancer drug and Trastuzumab monoclonal antibody specific to Her2 receptors overexpressed in many tumors was taken as the carrier. The advantage of the obtained dual-dye ratiometric system for drug release monitoring was demonstrated in mice model.

Visualizing the Underlying Signaling Pathway Related to Nitric Oxide and Glutathione in Cardiovascular Disease Therapy by a Sequentially Activated Fluorescent Probe

Clarifying the signaling pathway associated with nitric oxide (NO) and glutathione (GSH) in cardiovascular disease therapy is important for understanding its physiological and pathological processes but is challenging due to the lack of efficient analytical techniques. Herein, we report a BODIPY-based fluorescent probe for recognition of NO and GSH in sequence with high sensitivity and selectivity. The probe exhibits turn-on fluorescence triggered by NO, followed by red-shifted emission in the presence of GSH. The sequentially activated mechanism allows the visualization of NO-induced GSH upregulation in drug-treated endothelial cells and zebrafish for the first time, revealing a signal pathway during the therapy. We hope that it can be used as a convenient and efficient tool for the study of the interplay between NO and GSH and for the screening of effective drugs for cardiovascular disease therapy.

Fluorescence imaging-guided cancer photothermal therapy using polydopamine and graphene quantum dot-capped Prussian blue nanocubes

In recent years, imaging-guided photothermal tumor ablation has attracted intense research interest as one of the most exciting strategies for cancer treatment. Herein, we prepared polydopamine and graphene quantum dot-capped Prussian blue nanocubes (PB@PDA@GQDs, PBPGs) with high photothermal conversion efficiency and excellent fluorescence performance for imaging-guided cancer treatment. Transmission electron microscopy (TEM), UV-vis absorption spectroscopy (UV-vis), fluorescence spectroscopy, and X-ray photoelectron spectroscopy (XPS) were employed to characterize their morphology and structures. The photothermal conversion efficiency and therapeutic effect were evaluated in vitro and in vivo. Results revealed that this nanoagent had excellent biocompatibility and enhanced the photothermal effect compared to blue nanocubes (PBs) and polydopamine-capped Prussian blue nanocubes (PB@PDA, PBPs). Therefore, our study may open a new path for the production of PB-based nanocomposites as theranostic nanoagents for imaging-guided photothermal cancer treatment.

Detection of Tyrosinase in Real Food Samples and Living Cells by a Novel Near-Infrared Fluorescence Probe

A new near-infrared fluorescence probe was developed and applied to the fluorescence detection of tyrosinase in real food samples and living cells. The probe (E)-2-(2-(6-((3-hydroxybenzyloxy)carbonylamino)-2,3-dihydro-1H-xanthen-4-yl)vinyl)-3,3-dimethyl-1-propyl-3H-indolium (1) was designed and synthesized by coupling 3-hydroxybenzyl alcohol via carbamate bond with an amino hemicyanine skeleton, based on the high anti-interference ability of 3-hydroxybenzyl alcohol to reactive oxygen species and its binding affinity to tyrosinase. Compared with the existing tyrosinase probes, the proposed probe exhibits superior analytical performance, such as high selectivity, high sensitivity, superior spatiotemporal sampling ability, fluorescence signal switching at 706 nm, and low detection limit of 0.36 U mL^-1. More importantly, the probe has been successfully used to monitor tyrosinase in the browning of apple slices for the first time, and the results indicated that the strongest fluorescence intensity could be achieved at 2.5 h to realize precise visual recognition of tyrosinase. Notably, the probe determined tyrosinase in real food samples (apple, banana, cheese, and red wine) with a stable average recovery range of 95.7-108.3% and has been successfully used to monitor tyrosinase in the living B16 cells. The superior properties of the probe make it of great potential use in food nutritional value evaluation and clinical diagnosis of melanin-related diseases.

A near-infrared fluorogenic probe with fast response for detecting sodium dithionite in living cells

Developing a reliable fluorescence probe is crucial for accurately monitoring sodium dithionite (Na₂S₂O₄, SDT) in biosystems, but the current reported azo-based ones suffers from short excitation/emission wavelengths and relative slow response speed. To address this issue, we herein present a novel near-infrared emissive fluorescence probe for SDT, namely DCM-MQ, consisting of a dicyanomethylene-benzopyran fluorogenic reporter and a 1-methylquinolinium as recognition moiety. On the basis of the specific reduction mechanism, DCM-MQ exhibited a rapid colorimetric and fluorescent recognition for SDT (less than 3 s) with large Stokes shift (112 nm) and high sensitivity (detection limit was 19 nM). The fluorescence imaging results demonstrate that DCM-MQ is competent for monitoring SDT in living systems.

NIR-Light-Activated Ratiometric Fluorescent Hybrid Micelles for High Spatiotemporally Controlled Biological Imaging and Chemotherapy

Intelligent-responsive imaging-therapy strategy has shown great significance for biomedicine. However, it is still a challenge to construct spatiotemporally controlled imaging-therapy systems triggered by near infrared (NIR) light. In this work, NIR-light-activated ratiometric fluorescent hybrid micelles (RFHM) are prepared via the co-assembly of upconversion nanoparticles (UCNPs), doxorubicin (DOX), and UV-light-responsive amphiphilic block copolymer for the spatiotemporally controlled imaging and chemotherapy. Upon NIR light irradiation, UCNPs can convert NIR light to UV light. The emitted UV light induces the photoreaction of copolymer to further trigger ratiometric fluorescence imaging and degradation of hybrid micelles, resulting in rapid DOX release from hybrid micelles for antitumor therapy. The animal experiments reveal that NIR light can not only remotely regulate the ratiometric fluorescence imaging of RFHM in tumor tissue, but also trigger DOX release from RFHM to inhibit tumor growth. Therefore, this study provides a new strategy to achieve high spatial-temporal-controlled biological imaging and chemotherapy.