Construction of Targeting-Peptide-Based Imaging Reagents and Their Application in Bioimaging

Stimuli-Responsive Polymeric Nanoprobes for Bioimaging of Cancer Metastasis

Stimuli-responsive polymeric nanoprobes as a type of nanoscale probe can respond to the tumor microenvironment via specific stimuli inside tumors, such as pH, hypoxia, glutathione (GSH), enzymes, aberrant receptors, and high ATP concentration. The ingenious design of the nanoprobes can improve the specificity and sensitivity to distinguish the slight differences between normal tissues and tumors. Thus, the tiny tumor metastasis can be detected by bioimaging of the stimuli-responsive polymeric nanoprobes. This review summarizes the progress and applications of polymeric nanoprobes in the bioimaging of tumor metastasis. The design strategies for the nanoprobes targeting tumor tissues are discussed according to the stimulus types, including tumor pH, hypoxia, glutathione, enzymes, aberrant receptor, and ATP. Moreover, the challenges currently faced in this field are also discussed. This review will provide valuable insights for the design and optimization of stimuli-responsive polymeric nanoprobes to accelerate the development of bioimaging for tumor metastasis and promote the clinical translation.

In Situ Self-Assembled Probe for Antioxidant and Anti-Inflammatory Therapy of Inflammatory Bowel Disease

Inflammatory bowel disease (IBD) is a chronic and relapsing disease of the gastrointestinal tract. At present, antioxidant therapy is a promising strategy for IBD treatment. Since low-molecular-weight antioxidants (e.g., 2,2,6,6-tetramethylpiperidin-N-oxyl (TEMPO)) have a short in vivo half-life and inadequate cellular uptake, researchers have focused on loading antioxidants into nanostructures for improving their antioxidant and anti-inflammatory activities. As we know, in situ self-assembly with the formation of nanostructures under intracellular specific stimuli is a convenient delivery strategy to enhance the accumulation and retention of molecules at target sites in vivo. Herein, we developed an in situ self-assembled TEMPO probe TPP-FFYp-O to improve the antioxidant and anti-inflammatory effects of TEMPO in IBD. Compared to the control probe TPP-O without a self-assembly moiety, TPP-FFYp-O could successfully self-assemble into nanoparticles (NPs) under alkaline phosphatase (ALP)-guided dephosphorylation, with significantly enhanced antioxidant capacity in vitro. Cell experiments confirmed that intracellular formation of NPs by TPP-FFYp-O could improve the antioxidant and anti-inflammatory abilities of TEMPO and alleviate cellular damage. Moreover, TPP-FFYp-O exhibited good biocompatibility in vivo and significantly relieved pathological injury and inflammatory factors in the colon tissues of an IBD model compared to TPP-O. We envision that the in situ self-assembly platform can be used to load various active molecules for more applications in the future.

Peptide-based fluorescent probes for the diagnosis of tumor and image-guided surgery

Fluorescent contrast agents are instrumental in amplifying signals, thereby enhancing the sensitivity and accuracy of live optical imaging. However, a significant proportion of traditional fluorescent contrast agents exhibit drawbacks such as short half-life, suboptimal biocompatibility, and inadequate tumor targeting, all of which impede effective imaging guidance. Peptides, derived from natural structures, offer a flexible modular design that can be precisely engineered and adjusted using synthetic methods to achieve specific biological activity and pharmacokinetic properties. They bind with designated receptors to exert their effects, demonstrating high specificity. The development of fluorescent probes based on peptides significantly overcomes the limitations of conventional contrast agents, offering superior performance. This article provides a comprehensive review of three strategies for constructing peptide-based fluorescent probes, delving into their distinct design concepts, mechanisms of action, and innovative aspects. It also highlights the potential applications of peptide-based fluorescent probes in tumor diagnosis and image-guided surgery, offering insights into their future clinical transformation.

Self-Assembled Triple-Targeted Radiosensitizer Enhances Hypoxic Tumor Targeting and Radio-Immunotherapy Efficacy

Targeted delivery of radiosensitizers and real-time monitoring of hypoxia are crucial for overcoming radiotherapy resistance in hypoxic tumors. Here, we report A-Cy-Ni-RGD, a triple-targeted nitroimidazole (Ni)-linked radiosensitizer that self-assembles into nanoparticles (A-Cy-Ni-RGD NPs) for bimodal near-infrared fluorescence (NIR FL) and photoacoustic (PA) imaging-guided radio-immunotherapy. A-Cy-Ni-RGD NPs specifically accumulate in αvβ3-positive tumors, where they are hydrolyzed by carboxylesterase to form Cy-Ni-RGD NPs, with enhanced FL at 710 nm and dual PA signals at 680 and 730 nm. Under hypoxic conditions, nitroreductase (NTR) further reduces these NPs, covalently labeling endogenous proteins and increasing NP size. This process partially alleviates aggregation-caused quenching effect, increasing the FL710 signal and decreasing the PA730 signal, enabling real-time tracking of tumor-specific delivery and hypoxia. Following low-dose X-ray irradiation (2 Gy), elevated NTR expression promotes further Cy-Ni-RGD NPs reduction, enhancing proteins labeling and causing DNA damage. Moreover, radiosensitization with A-Cy-Ni-RGD NPs triggers robust immunogenic cell death, stimulating antitumor immunity that inhibits tumor growth and metastasis, significantly prolonging survival in mice with orthotopic 4T1 tumors. This work underscores the potential of self-assembling, triple-targeted radiotheranostic agents for improving tumor targeting, imaging, and radiotherapy efficacy in hypoxic tumors.

Cholesterol-modified peptide nanomicelles as a promising platform for cancer therapy: A review

Drug resistance, systemic toxicity, low solubility, and rapid clearance are common issues with chemotherapy drugs and other molecules used to treat cancer. The development of new therapeutic compounds and nanotherapy offers a solution to these issues. Therapeutic peptides have attracted great interest among these molecules due to their unique advantages, including low immunogenicity, efficient cellular internalization, deep tissue penetration, and low systemic toxicity. They have shown promise in cancer treatment by inducing apoptosis, necrosis, or cell lysis and promoting immunotherapy. In addition, peptides can deliver a range of cargoes, such as drugs, nucleic acids, imaging agents, and nanoparticles, and can specifically target cancer cells. However, problems such as their short half-life and low solubility limit their therapeutic use. Recent developments have addressed these constraints through structural alterations and nanoparticle formulations. In particular, cholesterol modification makes it possible for peptides to self-assemble into nanomicelles, which enhances their stability, half-life, and cell penetration. In this review, therapeutic peptides are presented as a versatile and successful cancer treatment option. The potential of cholesterol-modified peptide micelles as a reliable drug, nucleic acid, and imaging agent delivery system is also examined.

In situ self-assembled peptide nanoparticles improve the anti-hepatic fibrosis effect

Antagonistic peptide Leu-Ser-Lys-Leu (LSKL) is capable of blocking the transforming growth factor-β1 (TGF-β1) signaling pathway and exhibits anti-fibrotic effects. Herein, we constructed LSKL nanoparticles (NPs) in situ based on an alkaline phosphatase (ALP)-instructed self-assembly strategy for improving its specific therapeutic effect against liver fibrosis.

Development of Novel Peptide-Based Radiotracers for Detecting FGL1 Expression in Tumors

A novel immune checkpoint, FGL1, is a potentially viable target for tumor immunotherapy. The development of FGL1-targeted PET probes could provide significant insights into the immune system's status and the evaluation of treatment efficacy. A ClusPro 2.0 server was used to analyze the interaction between FGL1 and LAG3, and the candidate peptides were identified by using the Rosetta peptide derivate protocol. Three candidate peptides targeting FGL1, named FGLP21, FGLP22, and FGLP23, with a simulated affinity of -9.56, -8.55, and -8.71 kcal/mol, respectively, were identified. The peptides were readily conjugated with p-NCS-benzyl-NODA-GA, and the resulting compounds were successfully labeled with 68Ga in approximately 70% yields and radiochemical purity greater than 95%. In vitro competitive cell-binding assay demonstrated that all probes bound to FGL1 with IC50 ranging from 100 nM to 160 nM. Among the probes, PET imaging revealed that 68Ga-NODA-FGLP21 exhibited the best tumor imaging performance in mice bearing FGL1 positive Huh7 tumor. At 60 min p.i., the tumor uptake of 68Ga-NODA-FGLP21 was significantly higher than those of 68Ga-NODA-FGLP22 and 68Ga-NODA-FGLP23, respectively (2.51 ± 0.11% ID/g vs 1.00 ± 0.16% ID/g and 1.49 ± 0.05% ID/g). Simultaneously, the tumor-to-muscle uptake ratios of the former were also higher than those of the latter, respectively (19.40 ± 2.30 vs 9.65 ± 0.62 and 12.45 ± 0.72). In the presence of unlabeled FGLP21, the uptake of 68Ga-NODA-FGLP21 in Huh7 xenograft decreased to 0.81 ± 0.09% ID/g at 60 min p.i., which is similar to that observed in the FGL1 negative U87 MG tumor (0.46 ± 0.03% ID/g). The results were consistent with the immunohistochemical analysis and ex vivo autoradiography. No significant radioactivity was accumulated in normal organs, except for kidneys. In summary, a preclinical study confirmed that the tracer 68Ga-NODA-FGLP21 has the potential to specifically detect FGL1 expression in tumors with good contrast to the background.

Frontiers in fluorescence imaging: tools for the in situ sensing of disease biomarkers

Fluorescence imaging has been recognized as a powerful tool for the real-time detection and specific imaging of biomarkers within living systems, which is crucial for early diagnosis and treatment evaluation of major diseases. Over the years, significant advancements in this field have been achieved, particularly with the development of novel fluorescent probes and advanced imaging technologies such as NIR-II imaging, super-resolution imaging, and 3D imaging. These technologies have enabled deeper tissue penetration, higher image contrast, and more accurate detection of disease-related biomarkers. Despite these advancements, challenges such as improving probe specificity, enhancing imaging depth and resolution, and optimizing signal-to-noise ratios still remain. The emergence of artificial intelligence (AI) has injected new vitality into the designs and performances of fluorescent probes, offering new tools for more precise disease diagnosis. This review will not only discuss chemical modifications of classic fluorophores and in situ visualization of various biomarkers including metal ions, reactive species, and enzymes, but also share some breakthroughs in AI-driven fluorescence imaging, aiming to provide a comprehensive understanding of these advancements. Future prospects of fluorescence imaging for biomarkers including the potential impact of AI in this rapidly evolving field are also highlighted.

PSMA-Targeted Intracellular Self-Assembled Probe for Enhanced PET Imaging

Positron-emission tomography (PET) offers high sensitivity for cancer diagnosis. However, small-molecule-based probes often exhibit insufficient accumulation in tumor sites, while nanoparticle-based agents typically have limited delivery efficiency. To address this challenge, this study proposes a novel PET imaging probe, 68Ga-CBT-PSMA, designed for prostate cancer. This probe integrates an intracellular self-assembly strategy to enhance PET imaging signals and significantly improve the signal-to-noise ratio. The glutamate-urea-based prostate-specific membrane antigen (PSMA)-targeting motif enables specific recognition of prostate cancer cells and enhances cellular uptake; then the self-assembly process induced by glutathione reduction effectively accumulates the probe within tumor cells, thereby amplifying PET imaging signals. This approach not only enhances signal intensity and resolution but also facilitates precise cancer localization and diagnosis, offering new avenues for advancing cancer diagnostic techniques.

Development and preclinical evaluation of a cyclic PET tracer targeting integrin-α6 on colorectal cancer models

Integrin-α6 is an attractive diagnostic and therapeutic biomarker in cancer, because it is highly expressed in several types of malignancies. Based on our previous findings, we designed a cyclic peptide, NOTA-A6P, to enhancing affinity, tumor uptake and serum stability, and then developed a cyclic radiotracer, [18F]AlF-NOTA-A6P, for the specific detection of early colorectal cancer by PET/CT imaging. [18F] AlF-NOTA-A6P was automatically labeled for colorectal cancer imaging in a novel synthesis module. The affinity, stability, radiochemical yield (RCY), radiochemical purity (RCP), molar activity (Am), and octanol-water partition coefficient of [18F]AlF-NOTA-A6P were investigated. Results demonstrated that the tracer exhibited high serum stability, high RCY (58.1 ± 4.1 %) (undecay-corrected, n = 5) and hydrophilicity. In vivo microPET/CT imaging of LS174T and HT29 xenograft tumor models with high integrin-α6 expression indicated that [18F]AlF-NOTA-A6P exhibited higher tumor uptake and tumor-to-muscle ratio than SW620, which has low integrin-α6 expression. Moreover, the specificity of [18F]AlF-NOTA-A6P for integrin-α6 was confirmed by additional methods, including autoradiography, hematoxylin and eosin staining, and immunohistochemical staining. In conclusion, a cyclic peptide NOTA-A6P targeting integrin-α6 was designed and a promising PET tracer [18F]AlF-NOTA-A6P was synthesized in a novel cassette-type synthesis module. The tracer demonstrated a favorable binding affinity with integrin-α6, stability in human serum and specificity for colorectal cancer xenograft mice. These properties render it a promising non-invasive PET radiotracer for the detection of integrin-α6-overexpressing cancers, including colorectal cancer.

π-π Stacking Network-Based Supramolecular Peptide Nanoprobe for Visualization of the ICB-Enhanced Ferroptosis Process

The construction of coassembled peptide nanoprobes based on structural adaptation provides an effective template for stable monitoring of the molecular events in physiological and pathological processes. This also greatly expands their applications in biomedicine, such as multimodal combined diagnosis and treatment. However, the insufficient understanding of the physicochemical properties and structural features of different molecules still makes it difficult to construct the coassembled probes with mutually reinforcing functions, leading to unpredictable effects. Here, we showed how to utilize the π-π stacking network on β-sheets formed by PD-L1-targeting peptides to capture small molecules with ferroptosis functions, thus, coassembling them into a visual probe with synergistic effects. Compared with individual components, the coassembled strategy could significantly improve the stability of the nanoprobe, inducing stronger ferroptosis effects and immune checkpoint blocking effects, and track and reflect the process. This study provides new insights into the design of multicomponent collaborative coassembly systems with biological effects.

Smart Molecular Imaging and Theranostic Probes by Enzymatic Molecular In Situ Self-Assembly

Enzymatic molecular in situ self-assembly (E-MISA) that enables the synthesis of high-order nanostructures from synthetic small molecules inside a living subject has emerged as a promising strategy for molecular imaging and theranostics. This strategy leverages the catalytic activity of an enzyme to trigger probe substrate conversion and assembly in situ, permitting prolonging retention and congregating many molecules of probes in the targeted cells or tissues. Enhanced imaging signals or therapeutic functions can be achieved by responding to a specific enzyme. This E-MISA strategy has been successfully applied for the development of enzyme-activated smart molecular imaging or theranostic probes for in vivo applications. In this Perspective, we discuss the general principle of controlling in situ self-assembly of synthetic small molecules by an enzyme and then discuss the applications for the construction of "smart" imaging and theranostic probes against cancers and bacteria. Finally, we discuss the current challenges and perspectives in utilizing the E-MISA strategy for disease diagnoses and therapies, particularly for clinical translation.

Autocleaving Bonds for Better Drugs

While bond formation has historically been the mainstay of medicinal chemistry, the phenomenon of bond cleavage has received less focus. However, the success of numerous oral medications demonstrates the importance of controlled cleavage in prodrugs to achieve desired therapeutic outcomes. Nevertheless, effective strategies to control this cleavage remain limited. This concept article introduces a novel approach: employing peptides as conjugates to drugs to modulate the hydrolysis of these conjugates and enhance drug efficacy. The article begins by briefly outlining common prodrug strategies, followed by a few representative examples of how peptides can be leveraged to control the autohydrolysis of peptide-conjugated prodrugs for bacterial and cancer cell inhibition. Finally, it provides a brief outlook on the future potential of this promising new research direction in molecular medicine.