γ-Glutamyltranspeptidase-Triggered Intracellular Gadolinium Nanoparticle Formation Enhances the T2-Weighted MR Contrast of Tumor

Recent advances in stimuli-responsive in situ self-assembly of small molecule probes for in vivo imaging of enzymatic activity

Stimuli-responsive in situ self-assembly of small molecule probes into nanostructures has been promising for the construction of molecular probes for in vivo imaging. In the past few years, a number of intelligent molecular imaging probes with fluorescence, magnetic resonance imaging (MRI), positron electron tomography (PET) or photoacoustic imaging (PA) modality have been developed based on the in situ self-assembly strategy. In this minireview, we summarize the recent advances in the development of different modality imaging probes through controlling in situ self-assembly for in vivo imaging of enzymatic activity. This review starts from the brief introduction of two different chemical approaches amenable for in situ self-assembly, including (1) stimuli-mediated proteolysis and (2) stimuli-triggered biocompatible reaction. We then discuss their applications in the design of fluorescence, MRI, PET, PA, and bimodality imaging probes for in vivo imaging of different enzymes, such as caspase-3, furin, gelatinase and phosphatase. Finally, we discuss the current and prospective challenges in the stimuli-responsive in situ self-assembly strategy for in vivo imaging.

Introducing charge tag via click reaction in living cells for single cell mass spectrometry

For single living cell mass spectrometry measurement, sensitivity is of great significance due to the extremely complicated chemical components of the cytoplasm. Higher sensitivity is always highly desired, especially for chemicals with low concentrations or poor mass spectrometry responses. Here, a quaternary ammonium salt group-based charge tag was designed to enhance the analytical performance for cysteine within single cells using induced nanoelectrospray mass spectrometry. While the charge tag was coupled to the analyte via biocompatible click reaction, viability of the living cells was maintained during in situ derivatization and following analysis. Enhanced sensitivity under physiological conditions for cysteine, at pH 7.4 and with highly concentrated salts, was achieved due to higher ionization efficiency of the charge tag. Therefore, the cysteine levels in single living HeLa cells and HepG2 cells were found to be in the range of 62.0 ± 3.4 μM and 49.6 ± 7.2 μM, respectively. Furthermore, the low cysteine levels in living single HeLa cells could be monitored, in the presence of cystine transporter inhibitor. Thus, this method provides a general strategy for in situ chemical derivatization for signal amplification in the field of single cell mass spectrometry.

Sulfenic Acid-Mediated on-Site-Specific Immobilization of Mitochondrial-Targeted NIR Fluorescent Probe for Prolonged Tumor Imaging

Mitochondria plays pivotal roles in energy production and apoptotic pathways. Mitochondria-targeting strategy has been recognized as a promising way for cancer theranostics. Thus, spatiotemporally manipulating the prolonged retention of theranostic agents within mitochondria is considerably significant in cancer diagnosis and therapy. Herein, as a proof-of concept, we for the first time report a sulfenic acid-responsive platform on controlled immobilization of probes within mitochondria for prolonged tumor imaging. A novel near-infrared (NIR) probe DATC constructed with a NIR dye (Cy5) as signal unit, a cationic triphenylphosphonium (TPP) for mitochondria targeting, and a sulfenic acid-reactive group (1,3-cyclohexanedione) for mitochondrial fixation was rationally designed and synthesized. This probe displayed good target ability to mitochondria and could act as a promising fluorescent probe for specific visualization of endogenous protein sulfenic acids expressed in the mitochondria. Moreover, the probe could be spontaneously fixed on site through the specific reaction and covalent binding to the sulfenic acids of oxidized proteins under oxidative stress, resulting in enhanced intracellular uptake and prolonged retention. We thus believe that this mitochondria-targeted and locational immobilization strategy may offer a new insight for long-term tumor imaging and effective therapy.

Recent Advances of Bioresponsive Nano-Sized Contrast Agents for Ultra-High-Field Magnetic Resonance Imaging

The ultra-high-field magnetic resonance imaging (MRI) nowadays has been receiving enormous attention in both biomaterial research and clinical diagnosis. MRI contrast agents are generally comprising of T1-weighted and T2-weighted contrast agent types, where T1-weighted contrast agents show positive contrast enhancement with brighter images by decreasing the proton's longitudinal relaxation times and T2-weighted contrast agents show negative contrast enhancement with darker images by decreasing the proton's transverse relaxation times. To meet the incredible demand of MRI, ultra-high-field T2 MRI is gradually attracting the attention of research and medical needs owing to its high resolution and high accuracy for detection. It is anticipated that high field MRI contrast agents can achieve high performance in MRI imaging, where parameters of chemical composition, molecular structure and size of varied contrast agents show contrasted influence in each specific diagnostic test. This review firstly presents the recent advances of nanoparticle contrast agents for MRI. Moreover, multimodal molecular imaging with MRI for better monitoring is discussed during biological process. To fasten the process of developing better contrast agents, deep learning of artificial intelligent (AI) can be well-integrated into optimizing the crucial parameters of nanoparticle contrast agents and achieving high resolution MRI prior to the clinical applications. Finally, prospects and challenges are summarized.

Enzyme-instructed self-aggregation of Fe3O4 nanoparticles for enhanced MRI T2 imaging and photothermal therapy of tumors

The aggregation of superparamagnetic iron oxide (SPIO) nanoparticles (NPs) can greatly enhance magnetic resonance imaging (MRI) T2-weighted imaging and near-infrared (NIR) absorption in experiments. In this study, an Ac-Arg-Val-Arg-Arg-Cys(StBu)-Lys-CBT probe was designed and coupled with monodispersed carboxyl-decorated SPIO NPs to form SPIO@1NPs, which use it for intracellular self-aggregation. In vitro experiments showed that the self-aggregation of SPIO@1NPs was induced by a condensation reaction mediated by the enzyme furin in furin-overexpressing tumor cells. Moreover, the NPs in the aggregated state showed significantly higher MR r2 values and photothermal conversion efficiency than the NPs in the monodisperse state. Then, the in vivo SPIO@1NP self-aggregation in tumors can facilitate accurate MRI T2 imaging-guided photothermal therapy for effectively killing cancer cells. We believe that this basic technique, based on tumor-specific enzyme-instructed intracellular self-aggregation of NPs, could be useful for the rational synthesis of other inorganic NPs for use in the fields of tumor diagnosis and treatment.

A New γ-Glutamyltranspeptidase-Based Intracellular Self-Assembly of Fluorine-18 Labeled Probe for Enhancing PET Imaging in Tumors

γ-Glutamyltranspeptidase (GGT) is a cell -membrane-associated enzyme which has been recognized as a promising biomarker for the diagnosis of many malignant tumors. Herein, we rationally designed a fluorine-18 labeled small-molecule probe, [¹⁸F]γ-Glu-Cys(StBu)-PPG(CBT)-AmBF₃ (¹⁸F-1G), by applying a biocompatible CBT-Cys condensation reaction and ingeniously decorating it with a GGT-recognizable substrate, γ-glutamate (γ-Glu), for enhancing PET imaging to detect GGT level of tumors in living nude mice. The probe had exceptional stability at physiological conditions, but could be efficiently cleaved by GGT, followed by a reduction-triggered self-assembly and formation of nanoparticles (NPs) progressively that could be directly observed by transmission electron microscopy (TEM). In in vitro cell experiments, ¹⁸F-1G showed GGT-targeted uptake contrast of 2.7-fold to that of ¹⁸F-1 for the detection of intracellular GGT level. Moreover, the higher uptake in GGT overexpressed HCT116 tumor cells (∼4-fold) compared to GGT-deficient L929 normal cells demonstrated that ¹⁸F-1G was also capable of distinguishing some tumor cells from normal cells. In vivo PET imaging revealed enhanced and durable radioactive signal in tumor regions after ¹⁸F-1G coinjecting with 1G, thus allowing real-time detection of endogenous GGT level with high sensitivity and noninvasive effect. We anticipated that our probe could serve as a new tool to investigate GGT-related diseases in the near future.

Nanoparticle-based Cell Trackers for Biomedical Applications

The continuous or real-time tracking of biological processes using biocompatible contrast agents over a certain period of time is vital for precise diagnosis and treatment, such as monitoring tissue regeneration after stem cell transplantation, understanding the genesis, development, invasion and metastasis of cancer and so on. The rationally designed nanoparticles, including aggregation-induced emission (AIE) dots, inorganic quantum dots (QDs), nanodiamonds, superparamagnetic iron oxide nanoparticles (SPIONs), and semiconducting polymer nanoparticles (SPNs), have been explored to meet this urgent need. In this review, the development and application of these nanoparticle-based cell trackers for a variety of imaging technologies, including fluorescence imaging, photoacoustic imaging, magnetic resonance imaging, magnetic particle imaging, positron emission tomography and single photon emission computing tomography are discussed in detail. Moreover, the further therapeutic treatments using multi-functional trackers endowed with photodynamic and photothermal modalities are also introduced to provide a comprehensive perspective in this promising research field.

A peptide–drug hydrogel to enhance the anti-cancer activity of chlorambucil

The CRB–FFF–cyclen could transform into a hydrogel via a heating–cooling process. The resulting hydrogel could be protonated in a tumor environment, which is beneficial for cellular uptake and anti-tumor activity.

A supramolecular protein chaperone for vaccine delivery

Rationale: Nanomaterials capable of specifically interacting with proteins are very important for protein storage and vaccine delivery. Supramolecular hydrogels based on peptides have emerged as promising vaccine adjuvants because of their good compatibility, ease of antigen incorporation and display, and efficiency in activating immune responses. Methods: We synthesized a self-assembling peptide (Fbp-GDFDFDYDK(γE)2-NH2, Comp. 1 ) serving as a supramolecular protein chaperone for protein antigen delivery. The gelation was triggered by simply mixing Comp. 1 and proteins. The vaccine adjuvant potential of Comp. 1 was demonstrated by using two protein antigens, ovalbumin (OVA) and hepatitis B surface antigen (HBsAg). Results: The peptide derivative Comp. 1 exhibited high protein binding capacity. Upon contacting proteins, Comp. 1 rapidly formed coassembled nanofibers/hydrogels with the proteins, which greatly delayed the release of protein antigens. Our supramolecular protein chaperone significantly stimulated specific antibody titers by assisting protein delivery to antigen-presenting cells, promoting dendritic cell (DC) maturation, prolonging antigen accumulation and retention in the lymph nodes, and eliciting the secretion of cytokines. Most importantly, our supramolecular protein chaperone strongly stimulated the cellular immune response and significantly retarded tumor growth. Conclusion: Our study demonstrated the great potential of the supramolecular protein chaperone in protein storage and delivery, vaccine production and tumor immunotherapy.

Tandem Molecular Self-Assembly Selectively Inhibits Lung Cancer Cells by Inducing Endoplasmic Reticulum Stress

The selective formation of nanomaterials in cancer cells and tumors holds great promise for cancer diagnostics and therapy. Until now, most strategies rely on a single trigger to control the formation of nanomaterials in situ. The combination of two or more triggers may provide for more sophisticated means of manipulation. In this study, we rationally designed a molecule (Comp. 1) capable of responding to two enzymes, alkaline phosphatase (ALP), and reductase. Since the A549 lung cancer cell line showed elevated levels of extracellular ALP and intracellular reductase, we demonstrated that Comp. 1 responded in a stepwise fashion to those two enzymes and displayed a tandem molecular self-assembly behavior. The selective formation of nanofibers in the mitochondria of the lung cancer cells led to the disruption of the mitochondrial membrane, resulting in an increased level of reactive oxygen species (ROS) and the release of cytochrome C (Cyt C). ROS can react with proteins, resulting in endoplasmic reticulum (ER) stress and the unfolded protein response (UPR). This severe ER stress led to disruption of the ER, formation of vacuoles, and ultimately, apoptosis of the A549 cells. Therefore, Comp. 1 could selectively inhibit lung cancer cells in vitro and A549 xenograft tumors in vivo. Our study provides a novel strategy for the selective formation of nanomaterials in lung cancer cells, which is powerful and promising for the diagnosis and treatment of lung cancer.

Furin-Guided Intracellular 68Ga Nanoparticle Formation Enhancing Tumor MicroPET Imaging

Positron-emission tomography (PET) is routinely used in the clinic for tumor imaging with ultrahigh sensitivity, but tumor-targeted PET imaging probes are quite few. In this work, we rationally designed a furin-responsive radiotracer Acetyl-Arg-Val-Arg-Arg-Cys(StBu)-Lys(DOTA-⁶⁸Ga)-CBT (CBT-⁶⁸Ga) and demonstrated that coinjection of the radiotracer with its cold analogue CBT-Ga instructed the formation of ⁶⁸Ga nanoparticles in furin-overexpressing MDA-MB-468 cancer cells, which significantly enhanced microPET imaging of the tumor in vivo. In vitro results showed that CBT-Ga subjected to furin-initiated CBT-Cys condensation reaction and self-assembly to form the nanoparticles CBT-Ga-NPs with an average diameter of 258.3 nm. In vivo microPET imaging results indicate that the mice coinjected with CBT-⁶⁸Ga and CBT-Ga, which warrants ⁶⁸Ga nanoparticle formation in their MDA-MB-468 tumors, had a tumor/liver ratio 9.1-fold of that of the mice only injected with CBT-⁶⁸Ga. We envisioned that, by replacing the RVRR substrate of CBT-⁶⁸Ga with other enzyme-specific ones and using the strategy of intracellular nanoparticle formation, a series of radioactive probes could be developed for more sensitive and precise tumor microPET imaging in the near future.

NaCeF4:Gd,Tb Scintillator as an X-ray Responsive Photosensitizer for Multimodal Imaging-Guided Synchronous Radio/Radiodynamic Therapy

Photosensitizers (PSs) that are directly responsive to X-ray for radiodynamic therapy (RDT) with desirable imaging abilities have great potential applications in cancer therapy. Herein, the cerium (Ce)-doped NaCeF₄:Gd,Tb scintillating nanoparticle (ScNP or scintillator) is first reported. Due to the sensitization effect of the Ce ions, Tb ions can emit fluorescence under X-ray irradiation to trigger X-ray excited fluorescence (XEF). Moreover, Ce and Tb ions can absorb the energy of secondary electrons generated by X-ray to produce reactive oxide species (ROS) for RDT. With the intrinsic absorption of X-ray by lanthanide elements, the NaCeF₄:Gd,Tb ScNPs also act as a computed tomography (CT) imaging contrast agent and radiosensitizers for radiotherapy (RT) sensitization synchronously. Most importantly, the transverse relaxation time of Gd³⁺ ions is shortened due to the doping of Ce and Tb ions, leading to the excellent performance of our ScNPs in T₂-weighted MR imaging for the first time. Both in vitro and in vivo studies verify that our synthesized ScNPs have good performance in XEF, CT, and T₂-weighted MR imaging, and a synchronous RT/RDT is achieved with significant suppression on tumor progression under X-ray irradiation. Importantly, no systemic toxicity is observed after intravenous injection of ScNPs. Our work highlights that ScNPs have potential in multimodal imaging-guided RT/RDT of deep tumors.

Molecular Magnetic Resonance Imaging with Gd(III)-Based Contrast Agents: Challenges and Key Advances

In an era of personalized medicine, the clinical community has become increasingly focused on understanding diseases at the cellular and molecular levels. Magnetic resonance imaging (MRI) is a powerful imaging modality for acquiring anatomical and functional information. However, it has limited applications in the field of molecular imaging due to its low sensitivity. To expand the capability of MRI to encompass molecular imaging applications, we introduced bioresponsive Gd(III)-based magnetic resonance contrast agents (GBCAs) in 1997. Since that time, many research groups across the globe have developed new examples of bioresponsive GBCAs. These contrast agents have shown great promise for visualizing several biochemical processes, such as gene expression, neuronal signaling, and hormone secretion. They are designed to be conditionally retained, or activated, in vivo in response to specific biochemical events of interest. As a result, an observed MR signal change can serve as a read-out for molecular events. A significant challenge for these probes is how to utilize them for noninvasive diagnostic and theranostic applications. This Perspective focuses on the design strategies that underlie bioresponsive probes, and describes the key advances made in recent years that are facilitating their application in vivo and ultimately in clinical translation. While the field of bioresponsive agents is embryonic, it is clear that many solutions to the experimental and clinical radiologic problems of today will be overcome by the probes of tomorrow.

Self-assembling peptide-based nanodrug delivery systems

Self-assembling peptide-based nanodrug delivery systems (NDDs), consisting of naturally occurring amino acids, not only share the advantages of traditional nanomedicine but also possess the unique properties of excellent biocompatibility, biodegradability, flexible responsiveness, specific biological function, and synthetic feasibility. Physical methods, enzymatic reaction, chemical reaction, and biosurface induction can yield versatile peptide-based NDDs; flexible responsiveness is their main advantage. Different functional peptides and abundant covalent modifications endow such systems with precise controllability and multifunctionality. Inspired by the above merits, researchers have taken advantage of the self-assembling peptide-based NDDs and achieved the accurate delivery of drugs to the lesion site. The present review outlines the methods for designing self-assembling peptide-based NDDs for small-molecule drugs, with an emphasis on the different drug delivery strategies and their applications in using peptides and peptide conjugates.

Activatable NIR Fluorescence/MRI Bimodal Probes for in Vivo Imaging by Enzyme-Mediated Fluorogenic Reaction and Self-Assembly

Stimuli-responsive in situ self-assembly of small molecules to form nanostructures in living subjects has produced promising tools for molecular imaging and tissue engineering. However, controlling the self-assembly process to simultaneously activate multimodality imaging signals in a small-molecule probe is challenging. In this paper, we rationally integrate a fluorogenic reaction into enzyme-responsive in situ self-assembly to design small-molecule-based activatable near-infrared (NIR) fluorescence and magnetic resonance (MR) bimodal probes for molecular imaging. Using alkaline phosphatase (ALP) as a model target, we demonstrate that probe (P-CyFF-Gd) can be activated by endogenous ALP overexpressed on cell membranes, producing membrane-localized assembled nanoparticles (NPs) that can be directly visualized by cryo-SEM. Simultaneous enhancements in NIR fluorescence (>70-fold at 710 nm) and r₁ relaxivity (∼2.3-fold) enable real-time, high-sensitivity, high-spatial-resolution imaging and localization of the ALP activity in live tumor cells and mice. P-CyFF-Gd can also delineate orthotopic liver tumor foci, facilitating efficient real-time, image-guided surgical resection of tumor tissues in intraoperative mice. This strategy combines activatable NIR fluorescence via a fluorogenic reaction and activatable MRI via in situ self-assembly to promote ALP activity imaging, which could be applicable to design other activatable bimodal probes for in vivo imaging of enzyme activity and locations in real time.

An Albumin-Binding T1- T2 Dual-Modal MRI Contrast Agents for Improved Sensitivity and Accuracy in Tumor Imaging

Magnetic resonance imaging (MRI) diagnosis is better assisted by contrast agents that can augment the signal contrast in the imaging appearance. However, this technique is still limited by the inherently low sensitivity on the recorded signal changes in conventional T₁ or T₂ MRI in a qualitative manner. Here, we provide a new paradigm of MRI diagnosis using T₁- T₂ dual-modal MRI contrast agents for contrast-enhanced postimaging computations on T₁ and T₂ relaxation changes. An albumin-binding molecule (i.e., truncated Evans blue) chelated with paramagnetic manganese ion was developed as a novel T₁- T₂ dual-modal MRI contrast agent at high magnetic field (7 T). Furthermore, the postimaging computations on T₁- T₂ dual-modal MRI led to greatly enhanced signal-to-noise ratios (SNR) and contrast-to-noise ratios (CNR) in both subcutaneous and orthotopic brain tumor models compared with traditional MRI methods. The T₁- T₂ dual-modal MRI computations have great potential to eliminate suspicious artifacts and false-positive signals in mouse brain imaging. This study may open new avenues for contrast-enhanced MRI diagnosis and holds great promise for precision medicine.