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

Light-mediated intracellular polymerization

The synthesis of synthetic intracellular polymers offers groundbreaking possibilities in cellular biology and medical research, allowing for novel experiments in drug delivery, bioimaging and targeted cancer therapies. These macromolecules, composed of biocompatible monomers, are pivotal in manipulating cellular functions and pathways due to their bioavailability, cytocompatibility and distinct chemical properties. This protocol details two innovative methods for intracellular polymerization. The first one uses 2-hydroxy-4'-(2-hydroxyethoxy)-2-methylpropiophenone (Irgacure 2959) as a photoinitiator for free radical polymerization under UV light (365 nm, 5 mW/cm2). The second method employs photoinduced electron transfer-reversible addition-fragmentation chain-transfer polymerization with visible light (470 nm, 100 mW/cm2). We further elaborate on isolating these intracellular polymers by streptavidin/biotin interaction or immobilized metal ion affinity chromatography for polymers tagged with biotin or histidine. The entire process, from polymerization to isolation, takes ~48 h. Moreover, the intracellular polymers thus generated demonstrate significant potential in enhancing actin polymerization, in bioimaging applications and as a novel avenue in cancer treatment strategies. The protocol extends to animal models, providing a comprehensive approach from cellular to systemic applications. Users are advised to have a basic understanding of organic synthesis and cell biology techniques.

Controlled In Situ Self-Assembly of Biotinylated Trans-Cyclooctene Nanoparticles for Orthogonal Dual-Pretargeted Near-Infrared Fluorescence and Magnetic Resonance Imaging

A pretargeted strategy that decouples targeting vectors from radionuclides has shown promise for nuclear imaging and/or therapy in vivo. However, the current pretargeted approach relies on the use of antibodies or nanoparticles as the targeting vectors, which may be compromised by poor tissue penetration and limited accumulation of targeting vectors in the tumor tissues. Herein, we present an orthogonal dual-pretargeted approach by combining stimuli-triggered in situ self-assembly strategy with fast inverse electron demand Diels-Alder (IEDDA) reaction and strong biotin-streptavidin (SA) interaction for near-infrared fluorescence (NIR FL) and magnetic resonance (MR) imaging of tumors. This approach uses a small-molecule probe (P-Cy-TCO&Bio) containing both biotin and trans-cyclooctene (TCO) as a tumor-targeting vector. P-Cy-TCO&Bio can efficiently penetrate subcutaneous HeLa tumors through biotin-assisted targeted delivery and undergo in situ self-assembly to form biotinylated TCO-bearing nanoparticles (Cy-TCO&Bio NPs) on tumor cell membranes. Cy-TCO&Bio NPs exhibited an "off-on" NIR FL and retained in the tumors, offering a high density of TCO and biotin groups for the concurrent capture of Gd-chelate-labeled tetrazine (Tz-Gd) and IR780-labeled SA (SA-780) via the orthogonal IEDDA reaction and SA-biotin interaction. Moreover, Cy-TCO&Bio NPs offered multiple-valent binding modes toward SA, which additionally regulated the cross-linking of Cy-Gd&Bio NPs into microparticles (Cy-Gd&Bio/SA MPs). This process could significantly (1) increase r1 relaxivity and (2) enhance the accumulation of Tz-Gd and SA-780 in the tumors, resulting in strong NIR FL, bright MR contrast, and an extended time window for the clear and precise imaging of HeLa tumors.

Self-assembly of peptides in living cells for disease theranostics

The past few decades have witnessed substantial progress in biomedical materials for addressing health concerns and improving disease therapeutic and diagnostic efficacy. Conventional biomedical materials are typically created through an ex vivo approach and are usually utilized under physiological environments via transfer from preparative media. This transfer potentially gives rise to challenges for the efficient preservation of the bioactivity and implementation of theranostic goals on site. To overcome these issues, the in situ synthesis of biomedical materials on site has attracted great attention in the past few years. Peptides, which exhibit remarkable biocompability and reliable noncovalent interactions, can be tailored via tunable assembly to precisely create biomedical materials. In this review, we summarize the progress in the self-assembly of peptides in living cells for disease diagnosis and therapy. After a brief introduction to the basic design principles of peptide assembly systems in living cells, the applications of peptide assemblies for bioimaging and disease treatment are highlighted. The challenges in the field of peptide self-assembly in living cells and the prospects for novel peptide assembly systems towards next-generation biomaterials are also discussed, which will hopefully help elucidate the great potential of peptide assembly in living cells for future healthcare applications.

Activity-Based Fluorescence Diagnostics for Cancer

Fluorescence imaging is one of the most promising approaches to achieve intraoperative assessment of the tumor/normal tissue margins during cancer surgery. This is critical to improve the patients' prognosis, and therefore various molecular fluorescence imaging probes have been developed for the identification of cancer lesions during surgery. Among them, "activatable" fluorescence probes that react with cancer-specific biomarker enzymes to generate fluorescence signals have great potential for high-contrast cancer imaging due to their low background fluorescence and high signal amplification by enzymatic turnover. Over the past two decades, activatable fluorescence probes employing various fluorescence control mechanisms have been developed worldwide for this purpose. Furthermore, new biomarker enzymatic activities for specific types of cancers have been identified, enabling visualization of various types of cancers with high sensitivity and specificity. This Review focuses on recent advances in the design, function and characteristics of activatable fluorescence probes that target cancer-specific enzymatic activities for cancer imaging and also discusses future prospects in the field of activity-based diagnostics for cancer.

Advances and Perspectives of Responsive Probes for Measuring γ-Glutamyl Transpeptidase

Gamma-glutamyltransferase (GGT) is a plasma-membrane-bound enzyme that is involved in the γ-glutamyl cycle, like metabolism of glutathione (GSH). This enzyme plays an important role in protecting cells from oxidative stress, thus being tested as a key biomarker for several medical conditions, such as liver injury, carcinogenesis, and tumor progression. For measuring GGT activity, a number of bioanalytical methods have emerged, such as chromatography, colorimetric, electrochemical, and luminescence analyses. Among these approaches, probes that can specifically respond to GGT are contributing significantly to measuring its activity in vitro and in vivo. This review thus aims to highlight the recent advances in the development of responsive probes for GGT measurement and their practical applications. Responsive probes for fluorescence analysis, including "off-on", near-infrared (NIR), two-photon, and ratiometric fluorescence response probes, are initially summarized, followed by discussing the advances in the development of other probes, such as bioluminescence, chemiluminescence, photoacoustic, Raman, magnetic resonance imaging (MRI), and positron emission tomography (PET). The practical applications of the responsive probes in cancer diagnosis and treatment monitoring and GGT inhibitor screening are then highlighted. Based on this information, the advantages, challenges, and prospects of responsive probe technology for GGT measurement are analyzed.

In situ peptide assemblies for bacterial infection imaging and treatment

Bacterial infections, especially antibiotic-resistant ones, remain a major threat to human health. Advances in nanotechnology have led to the development of numerous antimicrobial nanomaterials. Among them, in situ peptide assemblies, formed by biomarker-triggered self-assembly of peptide-based building blocks, have received increasing attention due to their unique merits of good spatiotemporal controllability and excellent disease accumulation and retention. In recent years, a variety of "turn on" imaging probes and activatable antibacterial agents based on in situ peptide assemblies have been developed, providing promising alternatives for the treatment and diagnosis of bacterial infections. In this review, we introduce representative design strategies for in situ peptide assemblies and highlight the bacterial infection imaging and treatment applications of these supramolecular materials. Besides, current challenges in this field are proposed.

An Intelligent DNA Nanoreactor for Easy-to-Read In Vivo Tumor Imaging and Precise Therapy

Nanomaterial-based in vivo tumor imaging and therapy have attracted extensive attention; however, they suffer from the unintelligent "always ON" or single-parameter responsive signal output, substantial off-target effects, and high cost. Therefore, achieving in vivo easy-to-read tumor imaging and precise therapy in a multi-parameter responsive and intelligent manner remains challenging. Herein, an intelligent DNA nanoreactor (iDNR) was constructed following the "AND" Boolean logic algorithm to address these issues. iDNR-mediated in situ deposition of photothermal substance polydopamine (PDA) can only be satisfied in tumor tissues with abundant membrane protein biomarkers "AND" hydrogen peroxide (H2 O2 ). Therefore, intelligent temperature-based in vivo easy-to-read tumor imaging is realized without expensive instrumentation, and its diagnostic performance matches with that of flow cytometry, and photoacoustic imaging. Moreover, precise photothermal therapy (PTT) of tumors could be achieved via intelligent heating of tumor tissues. The precise PTT of primary tumors in combination with immune checkpoint blockade (ICB) therapy suppresses the growth of distant tumors and inhibits tumor recurrence. Therefore, highly programmable iDNR is a powerful tool for intelligent biomedical applications.

Size-Related Pathway Flux Analysis of Ultrasmall Iron Oxide Nanoparticles in Macrophage Cell RAW264.7 for Safety Evaluation

The endocytosis, intracellular transport, and exocytosis of different-sized nanoparticles were reported to greatly affect their efficacy and biosafety. The quantitation of endocytosis and exocytosis as well as subcellular distribution of nanoparticles might be an effective approach based on transport pathway flux analysis. Thus, the key parameters that could present the effects of three different-sized ultrasmall iron oxide nanoparticles (USIONPs) were systematically investigated in RAW264.7 cells. The endocytosis and exocytosis of USIONPs were related to their sizes; 15.4 nm of S2 could be quickly and more internalized and excreted in comparison to S1 (7.8 nm) and S3 (30.7 nm). In RAW264.7 cells, USIONPs were observed in endosomes, lysosomes, the Golgi apparatus, and autophagosomes via a transmission electron microscope. Based on flux analysis of intracellular transport pathways of USIONPs, it was found that 43% of S1, 40% of S2, and 44% of S3 were individually transported extracellularly through the Golgi apparatus-involved middle-fast pathway, while 24% of S1, 23% of S2, and 26% of S3 were transported through the fast recycling endosomal pathway, and the residues were transported through the slower speed lysosomal pathway. USIONPs might be transported via size-related endocytosis and exocytosis pathways. The pathway flux could be calculated on the basis of disturbance analysis of special transporters as well as their coding genes. Because there were rate differences among these transport pathways, this pathway flux could anticipate the intracellular remaining time and distribution of different-sized nanoparticles, the function exertion, and side effects of nanomaterials. The size of the nanomaterials could be optimized for improving functions and safety.

An Activatable Phototheranostic Probe for Anti-hypoxic Type I Photodynamic- and Immuno-Therapy of Cancer

Photodynamic therapy (PDT), which utilizes type I photoreactions, has great potential as an effective cancer treatment because of its hypoxia-tolerant superiority over the commonly used type II pathway. A few type I photosensitizers are exploited; however, they majorly induce cytotoxicity and possess poor tumor specificity and low-efficient theranostics. To resolve this issue, herein an aminopeptidase N (APN)-activated type I phototheranostic probe (CyA) is reported for anti-hypoxic PDT in conjunction with immunotherapy for effective cancer treatment. CyA can specifically activate near-infrared fluorescence, photoacoustic signals, and phototoxicity following APN-induced substrate cleavage and the subsequent generation of active phototheranostic molecules (such as CyBr). CyA endows specific imaging capabilities and effective phototoxicity toward tumor cells overexpressing APN under both normoxia and hypoxia. In addition, the locally activatable PDT induces systemic antitumor immune responses. More importantly, the integration of localized activated PDT and systemic immunotherapy evokes enhanced therapeutic effects with improved tumor inhibition efficiency in live mice compared with individual treatments. This study aims to present an activatable phototheranostic probe for effective hypoxia-tolerant PDT and combination therapy.

Caspase-3-Triggered Intracellular Gadolinium Nanoparticle Formation for T1-Weighted Magnetic Resonance Imaging of Apoptosis In Vivo

Apoptosis, with a hallmark of upregulated protease Caspase-3, has been frequently imaged with various probes to reveal the therapeutic efficiencies of different drugs. However, activatable molecular probes with programmable self-assembling behaviors that enable enhanced T₁-weighted magnetic resonance imaging (MRI) of apoptosis remain scarce. Herein, taking advantage of a CBT-Cys click reaction, we rationally designed a Caspase-3-activatable self-assembling probe Ac-Asp-Glu-Val-Asp-Cys(StBu)-Lys(DOTA(Gd))-CBT (DEVDC_S-Gd-CBT) for apoptosis imaging in vivo. After Caspase-3 cleavage in apoptotic cells, DEVDC_S-Gd-CBT underwent CBT-Cys click reaction to form a cyclic dimer, which self-assembled into Gd nanoparticles. With this probe, enhanced T₁-weighted MR images of apoptosis were achieved at low magnetic fields in vitro, in cis-dichlorodiamineplatinum-induced apoptotic cells and in tail-amputation-simulated apoptotic zebrafish. We anticipate that the smart probe DEVDC_S-Gd-CBT could be applied for T₁-weighted MRI of apoptosis-related diseases in the clinic in the future.

Single-Atom Gd Nanoprobes for Self-Confirmative MRI with Robust Stability

Gadolinium (Gd)-based complexes are extensively utilized as contrast agents (CAs) in magnetic resonance imaging (MRI), yet, suffer from potential safety concerns and poor tumor targeting. Herein, as a mimic of Gd complex, single-atom Gd nanoprobes with r₁ and r₂ values of 34.2 and 80.1 mM^-1 s^-1 (far higher than that of commercial Gd CAs) at 3 T are constructed, which possessed T₁ /T₂ dual-mode MRI with excellent stability and good tumor targeting ability. Specifically, single-atom Gd is anchored on nitrogen-doped carbon matrix (Gd-N_x C) through spatial-confinement method, which is further subjected to controllable chemical etching to afford fully etched bowl-shape Gd-N_x C (feGd-N_x C) with hydrophilic properties and defined coordination structure, similar to commercial Gd complex. Such nanostructures not only maximized the Gd³⁺ site exposure, but also are suitable for self-confirmative diagnosis through one probe with dual-mode MRI. Moreover, the strong electron localization and interaction between Gd and N atoms afforded feGd-N_x C excellent kinetic inertness and thermal stability (no significant Gd³⁺ leaching is observed even incubated with Cu²⁺ and Zn²⁺ for two months), providing a creative design protocol for MRI CAs.

Dynamic-Reversible MRI Nanoprobe for Continuous Imaging Redox Homeostasis in Hepatic Ischemia-Reperfusion Injury

Hepatic ischemia-reperfusion (I/R) injury accompanied by oxidative stress is responsible for postoperative liver dysfunction and failure of liver surgery. However, the dynamic non-invasive mapping of redox homeostasis in deep-seated liver during hepatic I/R injury remains a great challenge. Herein, inspired by the intrinsic reversibility of disulfide bond in proteins, a kind of reversible redox-responsive magnetic nanoparticles (RRMNs) is designed for reversible imaging of both oxidant and antioxidant levels (ONOO^-/GSH), based on sulfhydryl coupling/cleaving reaction. We develop a facile strategy to prepare such reversible MRI nanoprobe via one-step surface modification. Owing to the significant change in size during the reversible response, the imaging sensitivity of RRMNs is greatly improved, which enables RRMNs to monitor the tiny change of oxidative stress in liver injury. Notably, such reversible MRI nanoprobe can non-invasively visualize the deep-seated liver tissue slice by slice in living mice. Moreover, this MRI nanoprobe can not only report molecular information about the degree of liver injury but also provide anatomical information about where the pathology occurred. The reversible MRI probe is promising for accurately and facilely monitoring I/R process, accessing injury degree and developing powerful strategy for precise treatment.

Tumor microenvironment responsive T1-T2 dual-mode contrast agent Fe3O4@ZIF-8-Zn-Mn NPs for in vivo magnetic resonance imaging

Activated T₁-T₂ contrast agents can effectively improve the sensitivity and diagnosis accuracy of magnetic resonance imaging (MRI), but the construction of such contrast agents still remains a great challenge. In this work, a pH- and glutathione (GSH)-responsive T₁-T₂ dual-mode contrast agent, Fe₃O₄@ZIF-8-Zn-Mn nanoparticles (NPs), with simple components was constructed via simply assembly of paramagnetic Mn²⁺ ions (as T₁ contrast agent) and Fe₃O₄ NPs (as T₂ contrast agent) into a pH- and GSH-sensitive Zn-zeolitic imidazole framework (ZIF-8) matrix. Under neutral conditions, Fe₃O₄@ZIF-8-Zn-Mn NPs show good stability and weak T₁-T₂ dual-mode MRI contrast effect (r₁ = 0.82 mM^-1 s^-1, r₂ = 21.28 mM^-1 s^-1) due to the magnetic interference between Fe₃O₄ NPs and paramagnetic Mn²⁺ ions. In contrast, under acidic environment (pH = 6.5-5.5) and in the present GSH (0-4 mM), Fe₃O₄@ZIF-8-Zn-Mn NPs can be disassembled and release Fe₃O₄ NPs and paramagnetic Mn²⁺ ions, which causes simultaneous recovery of T₁ and T₂ imaging performances with enhanced r₁ and r₂ relaxation values up to 6.9 and 9.9 times, respectively. Moreover, in vivo MRI experiments showed that after the intravenous injection of Fe₃O₄@ZIF-8-Zn-Mn NPs for about one hour, the T₁-weighted imaging of the tumor site becomes brighter with T₁ signal enhanced by about 31%, while the T₂-weighted imaging of the tumor site becomes darker with T₂ signal enhanced by nearly 30%, suggesting the great potential of Fe₃O₄@ZIF-8-Zn-Mn NPs to be used as a tumor microenvironment-responsive T₁-T₂ dual-mode contrast agent for sensitive tumor imaging.

A γ-Glutamyl Transpeptidase (GGT)-Triggered Charge Reversal Drug-Delivery System for Cervical Cancer Treatment: In Vitro and In Vivo Investigation

Neutral/negatively charged nanoparticles are beneficial to reduce plasma protein adsorption and prolong their blood circulation time, while positively charged nanoparticles easily transverse the blood vessel endothelium into a tumor and easily penetrate the depth of the tumor via transcytosis. Γ-Glutamyl transpeptidase (GGT) is overexpressed on the external surface of endothelial cells of tumor blood vessels and metabolically active tumor cells. Nanocarriers modified by molecules containing γ-glutamyl moieties (such as glutathione, G-SH) can maintain a neutral/negative charge in the blood, as well as can be easily hydrolyzed by the GGT enzymes to expose the cationic surface at the tumor site, thus achieving good tumor accumulation via charge reversal. In this study, DSPE-PEG2000-GSH (DPG) was synthesized and used as a stabilizer to generate paclitaxel (PTX) nanosuspensions for the treatment of Hela cervical cancer (GGT-positive). The obtained drug-delivery system (PTX-DPG nanoparticles) was 164.6 ± 3.1 nm in diameter with a zeta potential of -9.85 ± 1.03 mV and a high drug-loaded content of 41.45 ± 0.7%. PTX-DPG NPs maintained their negative surface charge in a low concentration of GGT enzyme (0.05 U/mL), whereas they showed a significant charge-reversal property in the high-concentration solution of GGT enzyme (10 U/mL). After intravenous administration, PTX-DPG NPs mainly accumulated more in the tumor than in the liver, achieved good tumor-targetability, and significantly improved anti-tumor efficacy (68.48% vs. 24.07%, tumor inhibition rate, p < 0.05 in contrast to free PTX). This kind of GGT-triggered charge-reversal nanoparticle is promising to be a novel anti-tumor agent for the effective treatment of such GGT-positive cancers as cervical cancer.

Redox ferrocenylseleno compounds modulate longitudinal and transverse relaxation times of FNPs-Gd MRI contrast agents for multimodal imaging and photo-Fenton therapy

Developing a feasible way to feature longitudinal (T₁) and transverse (T₂) relaxation performance of contrast agents for magnetic resonance imaging (MRI) is important in cancer diagnosis and therapy. Improved accessibility to water molecule is essential for accelerating the relaxation rate of water protons around the contrast agents. Ferrocenyl compounds have reversible redox property for modulating the hydrophobicity/hydrophilicity of assemblies. Thus, they could be the candidates that can change water accessibility to the contrast agent surface. Herein, we incorporated ferrocenylseleno compound (FcSe) with Gd³⁺-based paramagnetic UCNPs, to obtain FNPs-Gd nanocomposites using T₁-T₂ MR/UCL trimodal imaging and simultaneous photo-Fenton therapy. When the surface of NaGdF₄:Yb,Tm UNCPs was ligated by FcSe, the hydrogen bonding between hydrophilic selenium and surrounding water molecules accelerated their proton exchange to initially endow FNPs-Gd with high r₁ relaxivity. Then, hydrogen nuclei from FcSe disrupted the homogeneity of the magnetic field around the water molecules. This facilitated T₂ relaxation and resulted in enhanced r₂ relaxivity. Notably, upon the near-infrared light-promoted Fenton-like reaction in the tumor microenvironment, hydrophobic ferrocene(II) of FcSe was oxidized into hydrophilic ferrocenium(III), which further increased the relaxation rate of water protons to obtain r₁ = 1.90±0.12 mM^-1 s^-1 and r₂ = 12.80±0.60 mM^-1 s^-1. With an ideal relaxivity ratio (r₂/r₁) of 6.74, FNPs-Gd exhibited high contrast potential of T₁-T₂ dual-mode MRI in vitro and in vivo. This work confirms that ferrocene and selenium are effective boosters that enhance the T₁-T₂ relaxivities of MRI contrast agents, which could provide a new strategy for multimodal imaging-guided photo-Fenton therapy of tumors. STATEMENT OF SIGNIFICANCE: T₁-T₂ dual-mode MRI nanoplatform with tumor-microenvironment-responsive features has been an attractive prospect. Herein, we designed redox ferrocenylseleno compound (FcSe) modified paramagnetic Gd³⁺-based UCNPs, to modulate T₁-T₂ relaxation time for multimodal imaging and H₂O₂-responsive photo-Fenton therapy. Selenium-hydrogen bond of FcSe with surrounding water molecules facilitated water accessibility for fast T₁ relaxation. Hydrogen nucleus in FcSe perturbed the phase coherence of water molecules in an inhomogeneous magnetic field and thus accelerated T₂ relaxation. In tumor microenvironment, FcSe was oxidized into hydrophilic ferrocenium via NIR light-promoted Fenton-like reaction which further increased both T₁ and T₂ relaxation rates; Meanwhile, the released toxic •OH performed on-demand cancer therapy. This work confirms that FcSe is an effective redox mediate for multimodal imaging-guided cancer therapy.

Intracellular Construction of Cathepsin B-Guided Gadolinium Nanoparticles for Enhanced T2 -Weighted MR Tumor Imaging

Magnetic resonance imaging (MRI) is a superior and noninvasive imaging technique with unlimited tissue penetration depth and superb spatiotemporal resolution, however, using intracellular self-assembly of Gd-containing nanoparticles to enhance the T₂ -weighted MR contrast of cancer cells in vivo for precise tumor MRI is rarely reported. The lysosomal cysteine protease cathepsin B (CTSB) is regarded as an attractive biomarker for the early diagnosis of cancers and metastasis. Herein, taking advantage of a biocompatible condensation reaction, a "smart" Gd-based CTSB-responsive small molecular contrast agent VC-Gd-CBT is developed, which can self-assemble into large intracellular Gd-containing nanoparticles by glutathione reduction and CTSB cleavage to enhance the T₂ -weighted MR contrast of CTSB-overexpressing MDA-MB-231 cells at 9.4 T. In vivo T₂ -weighted MRI studies using MDA-MB-231 murine xenografts show that the T₂ -weighted MR contrast change of tumors in VC-Gd-CBT-injected mice is distinctly larger than the mice injected with the commercial agent gadopentetate dimeglumine, or co-injected with CTSB inhibitor and VC-Gd-CBT, indicating that the accumulation of self-assembled Gd-containing nanoparticles at tumor sites effectively enhances the T₂ -weighted MR tumor imaging. Hence, this CTSB-targeted small molecule VC-Gd-CBT has the potential to be employed as a T₂ contrast agent for the clinical diagnosis of cancers at an early stage.

A Telomerase-Activated Magnetic Resonance Imaging Probe for Consecutively Monitoring Tumor Growth Kinetics and In Situ Screening Inhibitors

Telomerase has long been considered as a biomarker for cancer diagnosis and a therapeutic target for drug discovery. Detecting telomerase activity in vivo could provide more direct information of tumor progression and response to drug treatment, which, however, is hampered by the lack of an effective probe that can generate an output signal without a tissue penetration depth limit. In this study, using the principle of distance-dependent magnetic resonance tuning, we constructed a telomerase-activated magnetic resonance imaging probe (TAMP) by connecting superparamagnetic ferroferric oxide nanoparticles (SPFONs) and paramagnetic Gd-DOTA (Gd(III) 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid) complexes via telomerase-responsive DNA motifs. Upon telomerase-catalyzed extension of the primer in TAMP, Gd-DOTA-conjugated oligonucleotides can be liberated from the surface of SPFONs through a DNA strand displacement reaction, restoring the T₁ signal of the Gd-DOTA for a direct readout of the telomerase activity. Here we show that, by tracking telomerase activity, this probe provides consistent monitoring of tumor growth kinetics during progression and in response to drug treatment and enables in situ screening of telomerase inhibitors in whole-animal models. This study provides an alternative toolkit for cancer diagnosis, treatment response assessment, and anticancer drug screening.

Hofmeister Effect-Based T1-T2 Dual-Mode MRI and Enhanced Synergistic Therapy of Tumor

The imaging resolution of magnetic resonance imaging (MRI) is influenced by many factors. The development of more effective MRI contrast agents (CAs) is significant for early tumor detection and radical treatment, albeit challenging. In this work, the Hofmeister effect of Fe2O3 nanoparticles within the tumor microenvironment was confirmed for the first time. Based on this discovery, we designed a nanocomposite (FePN) by loading Fe2O3 nanoparticles on black phosphorus nanosheets. After reacting with glutathione, the FePN will undergo two stages in the tumor microenvironment, resulting in the robust enhancement of r1 and r2 based on the Hofmeister effect in the commonly used magnetic field (3.0 T). The glutathione-activated MRI signal of FePN was higher than most of the activatable MRI CAs, enabling a more robust visualization of tumors. Furthermore, benefiting from the long circulation time of FePN in the blood and retention time in tumors, the synergistic therapy of FePN exhibited an outstanding inhibition toward tumors. The FePN with good biosafety and biocompatibility will not only pave a new way for designing a common magnetic field-tailored T1-T2 dual-mode MRI CA but also offer a novel pattern for the accurate clinical diagnosis and therapy of tumors.

Supramolecular Self-Assembly in Living Cells

Supramolecular interactions rely on non-covalent forces, such as hydrophobic effects, hydrogen-bonding, and electrostatic interactions, which govern many intracellular biological pathways. In cellulo supramolecular self-assembly is mainly based on host-guest interactions, changes in pH, enzymes, and polymerization-induced self-assembly to accurately induce various unnatural reactions without disturbing natural biological processes. This process can produce synthetic biocompatible macromolecules to control cell properties and regulate biological functions, such as cell proliferation and differentiation. This Minireview focuses on the latest reports in the field of in cellulo supramolecular self-assembly and anticipates future advances regarding its activation in response to internal and external stimuli, such as pH changes, reactive oxygen species, and enzymes, as well as external light illumination.

Stimuli-Responsive Macrocyclization Scaffold Allows In Situ Self-Assembly of Radioactive Tracers for Positron Emission Tomography Imaging of Enzyme Activity

Target-enabled bioorthogonal reaction and self-assembly of a small-molecule probe into supramolecules have shown promise for molecular imaging. In this paper, we report a new stimuli-responsive bioorthogonal reaction scaffold (SF) for controlling in situ self-assembly by engineering the condensation reaction between 2-cyanobenzothiazole and cysteine. For probes with the SF scaffold, intramolecular cyclization took place soon after activation, which could efficiently outcompete free cysteine even at a low concentration and result in efficient aggregation in the target. Through integration with different enzyme-responsive substrates and an ammoniomethyl-trifluoroborate moiety (AmBF₃), two radioactive positron emission tomography (PET) tracers, [¹⁸F]SF-DEVD and [¹⁸F]SF-Glu, were designed, which showed high stability under physiological conditions and could produce clear PET signal in tumors to detect enzyme activity (e.g., caspase-3, γ-glutamyltranspeptidase) timely and accurately. Our results demonstrated that the scaffold SF could serve as a general molecular scaffold in the development of smart PET tracers for noninvasive imaging of enzyme activity, which could contribute to tumor detection and treatment efficacy evaluation.

Designing bioresponsive nanomaterials for intracellular self-assembly

Supramolecular assemblies are essential components of living organisms. Cellular scaffolds, such as the cytoskeleton or the cell membrane, are formed via secondary interactions between proteins or lipids and direct biological processes such as metabolism, proliferation and transport. Inspired by nature’s evolution of function through structure formation, a range of synthetic nanomaterials has been developed in the past decade, with the goal of creating non-natural supramolecular assemblies inside living mammalian cells. Given the intricacy of biological pathways and the compartmentalization of the cell, different strategies can be employed to control the assembly formation within the highly crowded, dynamic cellular environment. In this Review, we highlight emerging molecular design concepts aimed at creating precursors that respond to endogenous stimuli to build nanostructures within the cell. We describe the underlying reaction mechanisms that can provide spatial and temporal control over the subcellular formation of synthetic nanostructures. Showcasing recent advances in the development of bioresponsive nanomaterials for intracellular self-assembly, we also discuss their impact on cellular function and the challenges associated with establishing structure–bioactivity relationships, as well as their relevance for the discovery of novel drugs and imaging agents, to address the shortfall of current solutions to pressing health issues.

Creating artificial nanostructures inside living cells requires the careful design of molecules that can transform into active monomers within a complex cellular environment. This Review explores the recent development of bioresponsive precursors for the controlled formation of intracellular supramolecular assemblies.

When imaging meets size-transformable nanosystems

Imaging techniques, including magnetic, optical, acoustic and nuclear imaging, are gaining popularity as a research tool and clinical diagnostics. The advent of imaging agents-incorporated nanosystems (NSs), with sufficient contrast and high resolution, facilitates better monitoring of disease progression, targeted delivery and therapeutic process. Of note, the size of NSs remarkably affects imaging performance, while both large and small NSs enjoy respective features and superiority for imaging aspect, including penetration depth, signal-to-background ratio and spatiotemporal resolution. In this review, after a systematic summary of the basic knowledge of imaging techniques and its relation with size-tunable strategies, we further provide insights into the opportunities and challenges facing size-transformable NSs of the future for bio-imaging application and clinical translation.

High relaxivity Gd3+-based organic nanoparticles for efficient magnetic resonance angiography

Contrast-enhanced MR angiography (MRA) is a critical technique for vascular imaging. Nevertheless, the efficacy of MRA is often limited by the low rate of relaxation, short blood-circulation time, and metal ion-released potential long-term toxicity of clinical available Gd-based contrast agents. In this work, we report a facile and efficient strategy to achieve Gd-chelated organic nanoparticles with high relaxivity for T₁-weighted MRA imaging. The Gd-chelated PEG-TCPP nanoparticles (GPT NPs) have been engineered composite structured consisting of Gd-chelated TCPP and PEG. The spherical structure of TCPP offers more chemical sites for Gd³⁺ coordination to improve the relaxivity and avoid leakage of the Gd³⁺ ions. The synthesized GPT NPs exhibit a high relaxation rate of 35.76 mM^− 1 s^− 1 at 3.0 T, which is higher than the rates for most reported MR contrast agents. Therefore, GPT NPs can be used for MRA with much stronger vascular signals, longer circulation time, and high-resolution arterial vascular visualization than those using clinical MR contrast agents at the same dose. This work may make the T₁ MRI contrast agents for high-resolution angiography possible and offer a new candidate for preclinical and clinical applications of MR vascular imaging and vascular disease diagnosis.

A tumor microenvironment dual responsive contrast agent for contrary contrast-magnetic resonance imaging and specific chemotherapy of tumors

Development of magnetic resonance imaging (MRI) contrast agents (CAs) is still one of the research hotspots due to the inherent limitations of T₁- or T₂-weighted CAs and T₁/T₂ dual-mode CAs. To dramatically enhance the MRI contrast between tumors and normal tissues, we propose a new concept of contrary contrast-MRI (CC-MRI), whose specific definition is that CC-MRI CAs present a positive or negative signal at normal tissues, but show contrary signals at diseased tissues. To realize CC-MRI of tumors, we designed and developed a tumor microenvironment (TME) dual responsive CA (i.e., SA-FeGdNP-DOX@mPEG), which is almost not responsive under normal physiological conditions, but highly responsive to the acidic and reductive TME. Our SA-FeGdNP-DOX@mPEG shows a negative MRI signal under normal physiological conditions due to the high r₂ value (336.9 mM^-1 s^-1) and high r₂/r₁ ratio (18.4), but switches to a positive MRI signal in the TME because of the high r₁ value (20.32 mM^-1 s^-1) and low r₂/r₁ ratio (7.2). Our TME dual responsive SA-FeGdNP-DOX@mPEG significantly enhanced the contrast of MR images between tumors and livers, and the ΔSNR difference reached 501%. In addition, our SA-FeGdNP-DOX2@mPEG2 with tumor targetability and controlled DOX release responding to the TME was also used for tumor-specific chemotherapy with reduced side effects.

Metal-free bioorthogonal click chemistry in cancer theranostics

Bioorthogonal chemistry is a powerful tool to site-specifically activate drugs in living systems. Bioorthogonal reactions between a pair of biologically reactive groups can rapidly and specifically take place in a mild physiological milieu without perturbing inherent biochemical processes. Attributed to their high selectivity and efficiency, bioorthogonal reactions can significantly decrease background signals in bioimaging. Compared with metal-catalyzed bioorthogonal click reactions, metal-free click reactions are more biocompatible without the metal catalyst-induced cytotoxicity. Although a great number of bioorthogonal chemistry-based strategies have been reported for cancer theranostics, a comprehensive review is scarce to highlight the advantages of these strategies. In this review, recent progress in cancer theranostics guided by metal-free bioorthogonal click chemistry will be depicted in detail. The elaborate design as well as the advantages of bioorthogonal chemistry in tumor theranostics are summarized and future prospects in this emerging field are emphasized.

A tumor microenvironment responsive nanoplatform with oxidative stress amplification for effective MRI-based visual tumor ferroptosis

As a promising new form of non-apoptotic regulated cell death, ferroptosis has potential as an effective supplement to apoptosis-based cancer treatments. However, high intracellular glutathione (GSH) levels and insufficient hydrogen peroxide (H₂O₂) in the tumor limit the efficacy of ferroptosis. Here, we designed a theranostic nanoplatform, named FCS/GCS, by incorporating amphiphilic polymer skeletal (P-SS-D), cinnamaldehyde prodrug (CA-OH) and iron ions (Fe³⁺)/gadolinium ions (Gd³⁺) via chelation reactions between Fe³⁺/Gd³⁺ and polyphenols. When delivered in the tumor microenvironment with high GSH level, the nanoparticles are depolymerized by the poly(disulfide) backbone of P-SS-D. The activated CA consumes the GSH and elevates intracellular H₂O₂, followed by a high level of Fenton reaction to generate abundant •OH levels. The generation of reactive oxygen species (ROS) further accelerates CA activation. The GSH consumption by disulfide, CA and Fe³⁺, downregulates GPX4 and generates •OH, which accelerate lipid peroxides (LPO) accumulation and consequently enhances ferroptosis. Additionally, the released Gd³⁺ may serve as a contrast agent for tumor-specific T₁-weighted magnetic resonance imaging (MRI). Thus, the rationally designed FCS/GCS system is a promising strategy for effective MRI-based visual ferroptosis therapy. STATEMENT OF SIGNIFICANCE: Ferroptosis is a new form of non-apoptotic regulated cell death and has potential as an effective supplement to apoptosis-based cancer treatment. However, the efficiency of ferroptosis is limited by excessive glutathione level and insufficient hydrogen peroxide level in tumor site. In this study, we fabricate a theranostic nanoplatform (FCS/GCS) to amplify oxidation stress in tumor site for effective ferroptosis-based cancer treatment, and tumor specific magnetic resonance imaging is introduced for supervision. Our nanoplatform may provide a promising strategy for MRI-based visual ferroptosis therapy with high specificity and efficiency.

Nature-inspired dynamic gene-loaded nanoassemblies for the treatment of brain diseases

Gene therapy has great potential to treat brain diseases. However, genetic drugs need to overcome a cascade of barriers for their full potential. The conventional delivery systems often struggle to meet expectations. Natural biological particles that are highly optimized for specific functions in body, can inspire optimization of dynamic gene-loaded nanoassemblies (DGN). The DGN refer to gene loaded nanoassemblies whose functions and structures are changeable in response to the biological microenvironments or can dynamically interact with tissues or cells. The nature-inspired DGN can meet the needs in brain diseases treatment, including i) Non-elimination in blood (N), ii) Across the blood-brain barrier (A), iii) Targeting cells (T), iv) Efficient uptake (U), v) Controllable release (R), vi) Eyeable (E)-abbreviated as the "NATURE". In this Review, from nature to "NATURE", we mainly summarize the specific application of nature-inspired DGN in the "NATURE" cascade process. Furthermore, the Review provides an outlook for this field.

Simultaneous enhancement of T1 and T2 magnetic resonance imaging of liver tumor at respective low and high magnetic fields

Background: Nowadays, magnetic resonance imaging (MRI) is routinely applied in clinical diagnosis. However, using one contrast agent (CA) to simultaneously enhance the T1 and T2 MR contrast at low and high magnetic fields respectively has not been reported. Methods: Herein, we investigated the MR property of a γ-glutamyl transpeptidase (GGT)-instructed, intracellular formed gadolinium nanoparticle (DOTA-Gd-CBT-NP) at low and high magnetic fields. Results: Experimental results showed that DOTA-Gd-CBT-NP possesses a low r2/r1 ratio 0.91 which enables it to enhance T1 MR imaging of liver tumor at 1.0 T, and a high r2/r1 ratio 11.8 which renders the nanoparticle to largely enhance T2 MR imaging of liver tumor at 9.4 T. Conclusion: We expect that our GGT-responsive Gd-nanoparticle could be applied for simultaneous T1 and T2 MRI diagnosis of early liver cancer in clinic at respective low and high magnetic fields when the 9.4 T MR machine is clinically available in the future.

Optimized molecular design of PET probe for the visualization of γ-glutamyltranspeptidase activity in tumors

A fluorine-18 labeled probe [18F]JM-10 was rationally designed and optimized, which can self-assemble to generate more rigid and hydrophobic dimers upon response to γ-glutamyltranspeptidase (GGT) and glutathione (GSH) in tumor cells.

Revisiting the factors influencing the magnetic resonance contrast of Gd2O3 nanoparticles

Gadolinium oxide nanoparticles (GONs) have the potential to be one of the best candidates for the contrast agents of magnetic resonance imaging. Even though the influence of parameters on the relaxation has been substantially demonstrated, the variation of the r ₁ of GONs with a similar structure and surface chemistry implied our limited understanding. We herein synthesized GONs with adjustable size, shape, and crystallinity, modified them with a series of molecules with different acidities, and recorded their r ₁ values and imaging contrast. Our results showed that the isoelectric point could be regarded as an indicator of the relaxation covering the influence of both surface modification and size, which highlighted the impact of protons dissociated from the contrast agents. We further showed that the nanoparticles with lower crystallinity possess higher relaxivity, and this phenomenon manifested significantly under a low field. Our work clarified that the longitudinal relaxivity of Gd₂O₃ nanoparticles is sensitively dependent on the numbers of H⁺ generated from the surface and in the environment, which may shed light on developing high-performance nanoparticulate T ₁ contrast agents.

Cathespin B-Initiated Cypate Nanoparticle Formation for Tumor Photoacoustic Imaging

Cathepsin B (CTSB) is a lysosomal protease that is overexpressed in the early stage of many cancer types. Precise evaluation of CTSB expression in vivo may provide a promising method for the early diagnosis of cancers. By virtue of the high-resolution PA imaging modality, a "smart" photoacoustic (PA) probe Cypate-CBT, which can self-assemble to cypate-containing nanoparticles in response to abundant GSH and CTSB inside tumor cells, was developed for the sensitive and specific detection of CTSB activity. Compared with unmodified Cypate, our probe Cypate-CBT exhibited a 4.9-fold or 4.7-fold PA signal enhancement in CTSB-overexpressing MDA-MB-231 cancer cells or tumors, respectively, revealing intracellular accumulation of the probe after CTSB-initiated self-assembly. We expect Cypate-CBT to be employed as an effective PA imaging agent for clinical diagnosis of cancer at early stages.

Tumor Microenvironment Responsive "Head-to-Foot" Self-Assembly Nanoplatform for Positron Emission Tomography Imaging in Living Subjects

Sensitivity and specificity of molecular probes are two important factors in determining the accuracy of cancer diagnosis or the efficacy of cancer treatment. However, the development of probes with high sensitivity and strong specificity still poses many challenges. Herein, we report an ¹⁸F-labeled smart tracer ([¹⁸F]1) targeting cancer-associated biotin receptor (BR) and self-assembling into nanoparticles in response to intracellular glutathione. The tracer [¹⁸F]1 selectively targeted BR-positive cancer cells A549 and Hela and formed nanoparticles through self-assembly with an average diameter of 138.2 ± 16.3 nm. The character of self-assembly into nanoparticles enhanced the uptake and extended the retention of probe [¹⁸F]1 in the target tissue and hence improved the quality of positron emission tomography (PET) images. Thus, [¹⁸F]1 is a promising PET tracer for accurately detecting BR-positive cancers. Moreover, the tumor microenvironment responsive "head-to-foot" self-assembly nanoplatform is particularly attractive for development of other smart molecular probes.

Synthesis and Photothermal Effects of Intracellular Aggregating Nanodrugs Targeting Nasopharyngeal Carcinoma

Chemotherapy for the treatment of nasopharyngeal carcinoma (NPC) is usually associated with many side effects; therefore, its treatment options have not yet been completely resolved. Improving distribution to the targeted tumor region and enhancing the cellular uptake of drugs can efficiently alleviate the above adverse medical effects. Near-infrared (NIR) laser light-mediated photothermal therapy (PTT) and photodynamic therapy (PDT) are promising strategies for cancer treatment. In the present study, we developed an efficient multifunctional nanocluster with enhanced targeting and aggregation efficiency for PTT and PDT that is composed of a biocompatible folic acid (FA), indocyanine green (ICG) and 2-cyanobenzothiazole (CBT)-functionalized peptide labeled with an aldehyde sodium alginate-modified magnetic iron oxide nanoparticle (ASA-MNP)-based nanocarrier. FA can bind to folate receptors on cancer cell membranes to enhance nanocluster uptake. CBT-modified peptide can react with glutathione (GSH), which is typically present at higher levels in cancer cells, to form intracellular aggregates and increase the local concentration of the nanodrug. In in vitro studies, these nanodrugs displayed the desired uptake capacity by NPC cells and the ability to suppress the growth of cancer cells under laser irradiation. Animal studies validated that these nanodrugs are safe and nontoxic, efficiently accumulate in NPC tumor sites following injection via the caudal vein, and shows superior inhibition of tumor growth in a tumor-bearing mouse model upon near-infrared laser irradiation. The results indicate the potential application of the multifunctional nanoparticles (NPs), which can be used as a new method for the treatment of folate receptor-positive NPC.

Intramitochondrial Disulfide Polymerization Controls Cancer Cell Fate

Recent advances in supramolecular chemistry research have led to the development of artificial chemical systems that can form self-assembled structures that imitate proteins involved in the regulation of cellular function. However, intracellular polymerization systems that operate inside living cells have been seldom reported. In this study, we developed an intramitochondrial polymerization-induced self-assembly system for regulating the cellular fate of cancer cells. It showed that polymeric disulfide formation inside cells occurred due to the high reactive oxygen species (ROS) concentration of cancer mitochondria. This polymerization barely occurs elsewhere in the cell owing to the reductive intracellular environment. The polymerization of the thiol-containing monomers further increases the ROS level inside the mitochondria, thereby autocatalyzing the polymerization process and creating fibrous polymeric structures. This process induces dysfunction of the mitochondria, which in turn activates cell necroptosis. Thus, this in situ polymerization system shows great potential for cancer treatment, including that of drug-resistant cancers.

A fluorine-18 labeled radiotracer for PET imaging of γ-glutamyltranspeptidase in living subjects

The expression level of γ-glutamyltranspeptidase (GGT) in some malignant tumors is often abnormally high, while its expression is low in normal tissues. Therefore, GGT is considered as a key biomarker for cancer diagnosis. Several GGT-targeting fluorescence probes have been designed and prepared, but their clinical applications are limited due to their shallow tissue penetration. Considering the advantages of positron emission tomography (PET) such as high sensitivity and deep tissue penetration, we designed a novel PET imaging probe for targeted monitoring of the expression of GGT in living subjects, ([¹⁸F]γ-Glu-Cys-PPG(CBT)-AmBF₃)₂, hereinafter referred to as ([¹⁸F]GCPA)₂. The non-radioactive probe (GCPA)₂ was synthesized successfully and [¹⁸F]fluorinated rapidly via the isotope exchange method. The radiotracer ([¹⁸F]GCPA)₂ could be obtained within 0.5 h with the radiochemical purity over 98% and the molar activity of 10.64 ± 0.89 GBq μmol⁻¹. It showed significant difference in cellular uptake between GGT-positive HCT116 cells and GGT-negative L929 cells (2.90 ± 0.12% vs. 1.44 ± 0.15% at 4 h, respectively). In vivo PET imaging showed that ([¹⁸F]GCPA)₂ could quickly reach the maximum uptake in tumor (4.66 ± 0.79% ID g⁻¹) within 5 min and the tumor-to-muscle uptake ratio was higher than 2.25 ± 0.08 within 30 min. Moreover, the maximum tumor uptake of the control group co-injected with the non-radioactive probe (GCPA)₂ or pre-treated with the inhibitor GGsTop decreased to 3.29 ± 0.24% ID g⁻¹ and 2.78 ± 0.32% ID g⁻¹ at 10 min, respectively. In vitro and in vivo results demonstrate that ([¹⁸F]GCPA)₂ is a potential PET probe for sensitively and specifically detecting the expression level of GGT.

A radiotracer ([¹⁸F]GCPA)₂ for sensitively and specifically detecting the expression level of γ-glutamyltranspeptidase in living subjects was reported.

Dynamic Continuum of Nanoscale Peptide Assemblies Facilitates Endocytosis and Endosomal Escape

Alkaline phosphatase (ALP) enables intracellular targeting by peptide assemblies, but how the ALP substrates enter cells remains elusive. Here we show that nanoscale phosphopeptide assemblies cluster ALP to enable caveolae-mediated endocytosis (CME) and endosomal escape. Specifically, fluorescent phosphopeptides undergo enzyme-catalyzed self-assembly to form nanofibers. Live cell imaging unveils that phosphopeptides nanoparticles, coincubated with HEK293 cells overexpressing red fluorescent protein-tagged tissue-nonspecific ALP (TNAP-RFP), cluster TNAP-RFP in lipid rafts to enable CME. Further dephosphorylation of the phosphopeptides produces peptidic nanofibers for endosomal escape. Inhibiting TNAP, cleaving the membrane anchored TNAP, or disrupting lipid rafts abolishes the endocytosis. Decreasing the transformation to nanofibers prevents the endosomal escape. As the first study establishing a dynamic continuum of nanoscale assemblies for cellular uptake, this work illustrates an effective design for enzyme-responsive supramolecular therapeutics and provides mechanism insights for understanding the dynamics of cellular uptake of proteins or exogenous peptide aggregates.

Biological functions of supramolecular assemblies of small molecules in cellular environment

Like biomacromolecules, certain small molecules (e.g., aggregators) are able to self-assemble in aqueous phase to form nanoscale aggregates. Though it is well-established that the aggregates may interact with enzymes in vitro, the study of the biological activities of the assemblies of small molecules in cellular environment is only at its beginning. This review summarizes the recent progresses in exploring the biological functions of supramolecular assemblies of small molecules (SASMs). We first discuss the use of SASMs to inhibit pathogenic cells, such as cancer cells and bacteria. The use of SASMs to target different parts of cancer cells, such as pericellular space, cytosol, and subcellular organelles, and to combine with other bioactive entities (e.g., proteins and clinically used drugs), is particularly promising for addressing the challenge of acquired multidrug resistance in cancer therapy. Then, we describe the use of SASMs to sustain physiological functions of normal cells, that is, promoting cells proliferation and differentiation for tissue regeneration. After that, we show the use of SASMs as a basic tool to research cell behaviors, for instance, identifying the specific cells, improving enzyme probes, revealing membrane dynamics, enhancing molecular imaging, and mimicking context-dependent signaling. Finally, we give the outlook of the research of SASMs. We expect that this review, by highlighting the biological functions of SASMs, provides a starting point to explore the chemical biology of SASMs.

Scratching the Surface of Unventured Possibilities with In Situ Self-Assembly: Protease-Activated Developments for Imaging and Therapy

In situ self-assembly has attracted increasing research interest for applications in imaging and therapy in recent years. Particularly for protease-activated developments, inspiration is drawn from the innate specificity of their catalytic activities, rapid discovery of the various roles they play in the proliferation of certain diseases, and inherent susceptibility of small molecule peptide conjugates to proteolytic digestion in vivo. The overexpression of a disease-related protease of interest can be exploited as an endogenous stimulus for site-specific self-assembly to largely amplify a molecular event happening at the cellular level. This holds great potential for applications in early stage disease detection, long-term disease monitoring, and sustained therapeutic effects. This review summarizes the recent developments in protease-activated self-assemblies for imaging and therapeutic applications toward the manifestation of tumors, bacterial infections, neurodegenerative disorders, and wound recovery.

Pathological environment directed in situ peptidic supramolecular assemblies for nanomedicines

Peptidic self-assembly provides a powerful method to build biomedical materials with integrated functions. In particular, pathological environment instructed peptidic supramolecular have gained great progress in treating various diseases. Typically, certain pathology related factors convert hydrophilic precursors to corresponding more hydrophobic motifs to assemble into supramolecular structures. Herein, we would like to review the recent progress of nanomedicines based on the development of instructed self-assembly against several specific disease models. Firstly we introduce the cancer instructed self-assembly. These assemblies have exhibited great inhibition efficacy, as well as enhanced imaging contrast, against cancer models both in vitro and in vivo. Then we discuss the infection instructed peptidic self-assembly. A number of different molecular designs have demonstrated the potential antibacterial application with satisfied efficiency for peptidic supramolecular assemblies. Further, we discuss the application of instructed peptidic self-assembly for other diseases including neurodegenerative disease and vaccine. The assemblies have succeeded in down-regulating abnormal Aβ aggregates and immunotherapy. In summary, the self-assembly precursors are typical two-component molecules with (1) a self-assembling motif and (2) a cleavable trigger responsive to the pathological environment. Upon cleavage, the self-assembly occurs selectively in pathological loci whose targeting capability is independent from active targeting. Bearing the novel targeting regime, we envision that the pathological conditions instructed peptidic self-assembly will lead a paradigm shift on biomedical materials.

Naphthalene-facilitated self-assembly of a Gd-chelate as a novel T2 MRI contrast agent for visualization of stem cell transplants

Naphthalene is coupled with DOTA via a peptide sequence to yield an amphipathic MRI probe Nap-CFGKTG-DOTA-Gd (Nap-Gd) that can self-assemble into nanofibers. Incubation of NSCs, hMSCs and L929 cells in the presence of Nap-Gd in the μM level can introduce a significant amount of Nap-Gd into the cells as nanoclusters or nanofibers. The resultant intracellular Gd content is 10-60 times that achieved by incubation with Dotarem at the same concentration. The labelled cells exhibit a significant hyperintensive effect under T1-weighted MRI and a significant hypointensive effect under T2-weighted MRI. The hypointensive effect is more persistent than the hyperintensive effect, which allows in vivo tracking of labelled hMSCs for over 12 days under T2-weighted MRI. A comprehensive interpretation of the MRI signal intensity and the associated relaxation times reveals the structure-function relationship between the binding status of Nap-Gd in cells (structure) and the magnetic relaxation processes (function) toward a full understanding of the observed hyperintensive and hypointensive effects.

A Simple Aggregation-Induced Emission Nanoprobe with Deep Tumor Penetration for Hypoxia Detection and Imaging-Guided Surgery in Vivo

The pan-cancer detection and precise visualization of tiny tumors in surgery still face great challenges. As tumors grow aggressively, hypoxia is a common feature of solid tumors and has supplied a general way for detecting tumors. Herein, we report a simple aggregation-induced emission nanoprobe-TPE-4NE-O that can specifically switch on their fluorescence in the presence of cytochrome P450 reductase, a reductase which is overexpressed under hypoxia conditions. The probe can selectively light up the hypoxia cells and has shown enhanced deep tumor penetration via charge conversion both in vitro and in vivo. After being modified with FA-DSPE-PEG, higher tumor uptake can be seen and FA-DSPE/TPE-4NE-O showed specific visualization to the hypoxia cancer cells. Excitingly, much brighter fluorescence was accumulated at the tumors in the FA-DSPE/TPE-4NE-O group, even though the tumor was as small as 2.66 mm. The excellent performance of FA-DSPE/TPE-4NE-O in detecting tiny tumors has made it possible for imaging-guided tumor resection. More importantly, the probe exhibited good biocompatibility with negligible organ damage and eliminated a hemolysis risk. The simple but promising probe has supplied a new strategy for pan-cancer detection and tiny tumor visualization, which have shown great potential in clinical translation.

pH/Reduction Dual Stimuli-Triggered Self-Assembly of NIR Theranostic Probes for Enhanced Dual-Modal Imaging and Photothermal Therapy of Tumors

Tumor microenvironment plays a pivotal role in the growth and metastasis of tumors, and has become a promising target for precise diagnosis and treatment of tumors. Herein, a novel smart NIR theranostic probe Cy-1 that can simultaneously respond to low intracellular pH and reductive glutathione (GSH) is reported. This probe has demonstrated to be able to intermolecularly undergo a biologically compatible CBT-Cys condensation reaction to selectively form large nanoaggregates in the tumor microenvironment resulting in its enhanced accumulation and retention in the tumor, which as a consequence significantly improves the sensitivity of NIR/photoacoustic imaging and photothermal therapeutic efficacy of tumors in living mice. We thus believe that this dual stimuli-mediated self-assembly strategy may offer a promising and universal platform for cancer theranostics.

Self-Assembled Hybrid Nanocomposites for Multimodal Imaging-Guided Photothermal Therapy of Lymph Node Metastasis

Multimodal imaging-guided therapy holds great potential for precise theranostics of cancer metastasis. However, imaging agents enabling the convergence of complementary modalities with therapeutic functions to achieve perfect theranostics have been less exploited. This study reports the construction of a multifunctional nanoagent (FIP-^99mTc) that comprises Fe₃O₄ for magnetic resonance imaging, radioactive ^99mTc for single-photon-emission computed tomography, and IR-1061 to serve for the second near-infrared fluorescence imaging, photoacoustic imaging, and photothermal therapy treatment of cancer metastasis. The nanoagent possessed superior multimodal imaging capability with high sensitivity and resolution attributing to the complement of all the imaging modalities. Moreover, the nanoagent showed ideal photothermal conversion ability to effectively kill tumor cells at low concentration and power laser irradiation. In the in vivo study, FIP-^99mTc confirmed the fast accumulation and clear delineation of metastatic lymph nodes within 1 h after administration. Attributing to the efficient uptake and photothermal conversion, FIP-^99mTc could raise the temperature of metastatic lymph nodes to 54 °C within 10 min laser irradiation, so as to facilitate tumor cell ablation. More importantly, FIP-^99mTc not only played an active role in suppressing cancer growth in metastatic lymph nodes with high efficiency but also could effectively prevent further lung metastasis after resection of the primary tumor. This study proposes a simple but effective theranostic approach toward lymph node metastasis.

Biomaterials based on noncovalent interactions of small molecules

Unlike conventional materials that covalent bonds connecting atoms as the major force to hold the materials together, supramolecular biomaterials rely on noncovalent intermolecular interactions to assemble. The reversibility and biocompatibility of supramolecular biomaterials render them with diverse range of functions and lead to rapid development in the past two decades. This review focuses on the noncovalent and enzymatic control of supramolecular biomaterials, with the introduction to various triggering mechanism to initiate self-assembly. Representative applications of supramolecular biomaterials are highlighted in four categories: tissue engineering, cancer therapy, drug delivery, and molecular imaging. By introducing various applications, we intend to show enzymatic control and noncovalent interactions as a powerful tool for achieving spatiotemporal control of biomaterials both invitro and in vivo for biomedicine.

Enzymatic Noncovalent Synthesis

Enzymatic reactions and noncovalent (i.e., supramolecular) interactions are two fundamental nongenetic attributes of life. Enzymatic noncovalent synthesis (ENS) refers to a process where enzymatic reactions control intermolecular noncovalent interactions for spatial organization of higher-order molecular assemblies that exhibit emergent properties and functions. Like enzymatic covalent synthesis (ECS), in which an enzyme catalyzes the formation of covalent bonds to generate individual molecules, ENS is a unifying theme for understanding the functions, morphologies, and locations of molecular ensembles in cellular environments. This review intends to provide a summary of the works of ENS within the past decade and emphasize ENS for functions. After comparing ECS and ENS, we describe a few representative examples where nature uses ENS, as a rule of life, to create the ensembles of biomacromolecules for emergent properties/functions in a myriad of cellular processes. Then, we focus on ENS of man-made (synthetic) molecules in cell-free conditions, classified by the types of enzymes. After that, we introduce the exploration of ENS of man-made molecules in the context of cells by discussing intercellular, peri/intracellular, and subcellular ENS for cell morphogenesis, molecular imaging, cancer therapy, and other applications. Finally, we provide a perspective on the promises of ENS for developing molecular assemblies/processes for functions. This review aims to be an updated introduction for researchers who are interested in exploring noncovalent synthesis for developing molecular science and technologies to address societal needs.

Reduction Triggered In Situ Polymerization in Living Mice

"Smart" biomaterials that are responsive to physiological or biochemical stimuli have found many biomedical applications for tissue engineering, therapeutics, and molecular imaging. In this work, we describe in situ polymerization of activatable biorthogonal small molecules in response to a reducing environment change in vivo. We designed a carbohydrate linker- and cyanobenzothiazole-cysteine condensation reaction-based small molecule scaffold that can undergo rapid condensation reaction upon physiochemical changes (such as a reducing environment) to form polymers (pseudopolysaccharide). The fluorescent and photoacoustic properties of a fluorophore-tagged condensation scaffold before and after the transformation have been examined with a dual-modality optical imaging method. These results confirmed the in situ polymerization of this probe after both local and systemic administration in living mice.