Cyclic RGD Peptides Incorporating Cycloalkanes: Synthesis and Evaluation as PET Radiotracers for Tumor Imaging

Development of Structurally Identical Therapeutic and Diagnostic Agents for Image-Guided Boron Neutron Capture Therapy: c(RGD-BPA-K) Peptide with 125I/natI Albumin-Binding Moiety

The development of boron neutron capture therapy (BNCT) agents and structurally similar radiolabeled counterparts for diagnostic imaging is an area of significant interest. In this study, we designed structurally same compounds, c(RGD-BPA-K)(PEG2-(4-iodophenylbutyl)) (compound 1) and its radioiodinated counterpart, c(RGD-BPA-K)(PEG2-(4-[125I]iodophenylbutyl)) ([125I]1), for efficient BNCT and SPECT/CT imaging, respectively. An albumin-binding moiety was introduced into the compound to enhance the blood circulation time and tumor accumulation. We evaluated the efficacy of compound 1 and [125I]1 in U87MG tumor-bearing mice using SPECT/CT imaging, biodistribution analysis, and inductively coupled plasma mass spectrometry. Both compound 1 and [125I]1 displayed similar pharmacokinetics, high blood retention, and substantial accumulation in U87MG tumors. This study highlights the potential of compound 1 and [125I]1 for SPECT/CT-guided BNCT. The structural identity between the therapeutic and diagnostic agents in BNCT can enhance its therapeutic efficacy. Further structural modifications to increase boron concentration in tumors, as well as thermal neutron irradiation studies, may be necessary to fully explore the potential of our novel BNCT agent.

64Cu-Labeled Boron-Containing Cyclic RGD Peptides for BNCT and PET Imaging

The burgeoning interest in developing boron neutron capture therapy (BNCT) tracers and their accompanying diagnostics for the treatment of recalcitrant tumors has prompted this investigation. Our study aims to devise a tumor treatment strategy utilizing BNCT to target the αvβ3 integrin. To this end, we propose a pioneering boron-infused cyclic Arg-Gly-Asp (RGD) peptide, cRGD(d-BPA)K, designed as an efficacious BNCT tracer. Additionally, we introduce its diagnostic complement, DOTA-cRGD(d-BPA)K, tailored for positron emission tomography (PET) to visualize αvβ3 expressed tumors. Radiolabeling [64Cu]Cu-DOTA-cRGD(d-BPA)K (64Cu-1) resulted in a high radiochemical yield and purity. The radiotracer exhibited exceptional in vitro stability and demonstrated significant uptake in U87MG tumors via PET imaging. Biodistribution analysis using compound 2 showed a 7.0 ppm accumulation of boron in the U87MG tumor 1 h post-intravenous injection. Furthermore, compound 2 displayed superior tumor/blood (2.41) and tumor/muscle (2.46) ratios compared to the clinically approved l-BPA-fructose. Both compound 2 and its diagnostic counterpart 64Cu-1 hold potential for BNCT and cancer diagnosis, respectively, via molecular imaging.

Recent Research Progress of RGD Peptide–Modified Nanodrug Delivery Systems in Tumor Therapy

There have been great advancements in targeted nanodrug delivery systems for tumor therapy. Liposomes, polymeric nanoparticles, and inorganic nanoparticles are commonly employed as nanocarriers for drug delivery, and it has been found that arginine glycine aspartic acid (RGD) peptides and their derivatives can be used as ligands of integrin receptors to enhance the direct targeting ability. In this paper, we review the recent applications of RGD-modified liposomes, polymeric nanoparticles, and inorganic nanocarriers in cancer diagnosis and treatment, discuss the current challenges and prospects, and examine the progress made by the latest research on RGD peptide–modified nano delivery systems in cancer therapy. In recent years, RGD peptide–modified nanodrug delivery systems have been proven to have great potential in tumor therapy. Finally, we provide an overview of the current limitations and future directions of RGD peptide–modified nano-drug delivery systems for cancer therapy. This review aims to elucidate the contribution of RGD peptide–modified nanodrug delivery systems in the field of tumor therapy.

Preparation and Preliminary Evaluation of 68Ga-Acridine: An Attempt to Study the Potential of Radiolabeled DNA Intercalator as a PET Radiotracer for Tumor Imaging

INTRODUCTION

Acridine is a well-known DNA intercalator and thereby gets easily inserted within DNA. As uncontrolled rapid cell division is one of the primary characteristics of the tumors, it is expected that acridine or its suitable derivatives will have preferential accumulation in the tumorous lesions. Therefore, an attempt was made to radiolabel an acridine derivative with 68Ga and study the potential of the 68Ga-acridine complex as a PET agent for tumor imaging.

METHODS

9-aminoacridine was coupled with p-NCS-benzyl-DOTA to render it suitable for labeling with 68Ga. The purified acridine-DOTA conjugate was radiolabeled with 68Ga, eluted from a 68Ge/68Ga radionuclide generator. Various radiolabeling parameters were optimized and the stability of the radiolabeled preparation was studied. The biological behavior of the 68Ga-acridine complex was studied both in vitro and in vivo using Raji cell line and fibrosarcoma tumor bearing Swiss mice, respectively.

RESULTS

68Ga-acridine complex was obtained with ~100% radiochemical purity under the optimized reaction conditions involving incubation of 2mg/mL of ligand at 100°C for 30 minutes. The complex maintained a radiochemical purity of >95% in normal saline and >65% in human blood serum at 3h post-incubation. In vitro cellular study showed (3.2±0.1)% uptake of the radiotracer in the Raji cells. Biodistribution study revealed significant tumor accumulation [(11.41±0.41)% injected activity in per gram] of the radiotracer within 1h postadministration along with uptake in other non-target organs such as, blood, liver, GIT kidney etc. Conclusion: The present study indicates the potential of 68Ga-acridine as a PET agent for imaging of tumorous lesions. However, further detailed evaluation of the agent is warranted to explore its actual potential.

Syntheses and Preliminary Evaluation of Dual Target PET Probe [18F]-NOTA-Gly3- E (2PEG4-RGD-WH701) for PET Imaging of Breast Cancer

PURPOSE

Tumor Necrosis Factor Receptor 1 (TNFR1) and integrin αvβ3 receptor are overexpressed in breast cancer. We hypothesized that a peptide ligand recognizing both receptors in a single receptor-binding probe would be advantageous. Here, we developed a novel 18F-labeled fusion peptide probe [18F]-NOTA-Gly3- E(2PEG4-RGD-WH701) targeting dual receptors (TNFR1 and αvβ3) and evaluated the diagnostic efficacy of this radioactive probe in both MDA-MB-231 and MCF-7 xenograft models in mice.

METHODS

The NOTA-conjugated RGD-WH701 analog was radiolabeled with 18F using NOTA-AlF chelation method. We used two PEG4 molecules and Glutamic acid (Glu) to covalently link c(RGDyK) with WH701. Gly3 was also added to further improve the water solubility and pharmacokinetic properties of the probe. The expression of TNFR1 and Integrin αvβ3 in MCF-7 and MDA-MB-231 cells was detected by western blot analysis and immunofluorescence staining. The tumor-targeting characteristics of [18F]-NOTA-Gly3-E(2PEG4-RGDWH701) were assessed in nude mice bearing MDA-MB-231 and MCF-7 xenografts.

RESULTS

HPLC analysis of the product NOTA-G3-E (2P4-RGD-WH701) revealed a purity >95%. The yield after attenuation correction was approximately 33.5%±2.8% (n=5), and the radiochemical purity was above 95%. The MDA-MB-231 tumor uptake of [18F]-NOTA-Gly3-E(2PEG4-RGD-WH701) was 1.14±0.14%ID/g, as measured by PET at 40min postinjection (p.i.). In comparison, the tumor uptake of [18F]-NOTA-RGD and [18F]- NOTA-WH701 in MDA-MB-231 xenografts was 0.96±0.13%ID/g and 0.93±0.28%ID/g, respectively. The MCF-7 tumor uptake of [18F]-NOTA-Gly3-E(2PEG4-RGD-WH701) was 1.22±0.11%ID/g, as measured by PET at 40min postinjection (p.i.). In comparison, the tumor uptake of [18F]-NOTA-RGD and [18F]-NOTA-WH701 in MCF-7 xenografts was 0.99±0.18%ID/g and 0.57±0.08%ID/g, respectively.

CONCLUSION

[18F]AlF-NOTA-Gly3-E(2PEG4-RGD-WH701) was successfully synthesized and labeled with 18F. The results from the microPET/CT and biodistribution studies of [18F]AlF-NOTA-Gly3-E(2PEG4-RGDWH701) showed that the tracer could specifically target TNFR1 and integrin αvβ3 receptors.

Dual-targeting nanovesicles enhance specificity to dynamic tumor cells in vitro and in vivo via manipulation of αvβ3-ligand binding

The dynamic or flowing tumor cells just as leukemia cells and circulating tumor cells face a microenvironment difference from the solid tumors, and the related targeting nanomedicines are rarely reported. The existence of fluidic shear stress in blood circulation seems not favorable for the binding of ligand modified nanodrugs with their target receptor. Namely, the binding feature is very essential in this case. Herein, we utilized HSPC, PEG-DSPE, cholesterol and two αvβ3 ligands (RGDm7 and DT4) with different binding rates to build dual-targeting nanovesicles, in an effort to achieve a “fast-binding/slow-unbinding” function. It was demonstrated that the dual-targeting nanovesicles actualized efficient cellular uptake and antitumor effect in vitro both for static and dynamic tumor cells. Besides, the potency of the dual-targeting vesicles for flowing tumor cells was better than that for static tumor cells. Then, a tumor metastasis mice model and a leukemia mice model were established to detect the killing ability of the drug-loaded dual-targeting vesicles to dynamic tumor cells in vivo. The therapy efficacy of the dual-targeting system was higher than other controls including single-targeting ones. Generally, it seems possible to strengthen drug-targeting to dynamic tumor cells via the control of ligand–receptor interaction.

Bicyclic peptides: types, synthesis and applications

Bicyclic peptides form one of the most promising platforms for drug development owing to their biocompatibility, similarity and chemical diversity to proteins, and they are considered as a possible practical tool in various therapeutic and diagnostic applications. Bicyclic peptides are known to have the capability of being employed as an effective alternative to complex molecules, such as antibodies, or small molecules. This review provides a summary of the recent progress on the types, synthesis and applications of bicyclic peptides. More specifically, natural and synthetic bicyclic peptides are introduced with their different production methods and relevant applications, including drug targeting, imaging and diagnosis. Their uses as antimicrobial agents, as well as the therapeutic functions of different bicyclic peptides, are also discussed.

Bicyclic Peptides as Next-Generation Therapeutics

Bicyclic peptides have greater conformational rigidity and metabolic stability than linear and monocyclic peptides and are capable of binding to challenging drug targets with antibody-like affinity and specificity. Powerful combinatorial library technologies have recently been developed to rapidly synthesize and screen large bicyclic peptide libraries for ligands against enzymes, receptors, and protein-protein interaction targets. Bicyclic peptides have been developed as potential therapeutics against a wide range of diseases, drug targeting agents, imaging/diagnostic probes, and research tools. In this Minireview, we provide a summary of the recent progresses on the synthesis and applications of bicyclic peptides.

Novel Linear Peptides with High Affinity to αvβ3 Integrin for Precise Tumor Identification

Development of alternative linear peptides for targeting αvβ3 integrin has attracted much attention, as the traditional peptide ligand, cyclic RGD, is limited by inferior water-solubility and complex synthesis. Using pharmacophore-based virtual screening and high-throughput molecular docking, we identified two novel linear small peptides RWr and RWrNM with high affinity and specificity to αvβ3 integrin. The competitive binding with cyclic RGD (c(RGDyK)) and cellular uptake related to the integrin expression levels verified their affinity to αvβ3 integrin. The intermolecular interaction measurement and dynamics simulation demonstrated the high binding affinity and stability, especially for RWrNM. In vivo peptide-guided tumor imaging and targeted therapy further confirmed their specificity. Results indicated that the newly identified small linear peptide RWrNM, with high affinity and specificity to αvβ3 integrin, better water-solubility, and simplified synthetic process, could overcome limitations of the current cyclic RGD peptides, paving the way for diverse use in diagnosis and therapy.

Cyclic RGD peptide-modified liposomal drug delivery system for targeted oral apatinib administration: enhanced cellular uptake and improved therapeutic effects

Apatinib is an oral tyrosine kinase inhibitor, which selectively targets vascular endothelial growth factor receptor 2 and has the potential to treat many tumors therapeutically. Cyclic arginylglycylaspartic acid (cRGD)- and polyethylene glycol (PEG)-modified liposomes (cRGD-Lipo-PEG) were constructed to act as a targeted delivery system for the delivery of apatinib to the human colonic cancer cell line, HCT116. These cRGD-modified liposomes specifically recognized integrin α_vβ₃ and exhibited greater uptake efficiency with respect to delivering liposomes into HCT116 cells when compared to nontargeted liposomes (Lipo-PEG), as well as greater death of tumor cells and apoptosis. The mechanism by which cRGD-Lipo-PEG targets cells was elucidated further with competition assays. To determine the anticancer efficacy in vivo, nude mice were implanted with HCT116 xenografts and treated with apatinib-loaded liposomes or free apatinib intravenously or via intragastric administration. The active and passive targeting of cRGD-Lipo-PEG led to significant tumor treatment targeting ability, better inhibition of tumor growth, and less toxicity when compared with treatments using uncombined apatinib. The results presented strongly support the case for cRGD-Lipo-PEG representing a targeted delivery system for apatinib in the treatment of colonic cancer.

Improved Pharmacokinetics Following PEGylation and Dimerization of a c(RGD-ACH-K) Conjugate Used for Tumor Positron Emission Tomography Imaging

Improving the in vivo pharmacokinetics (PK) of positron emission tomography (PET) radiotracers is of critical importance to tumor diagnosis and therapy. In the case of peptide-based radiotracers, the modification and addition of a linker or spacer functional group often offer faster in vivo pharmacokinetic behavior. In this study, the authors introduced two new PEGlyated dimeric c(RGD-ACH-K) conjugates, in which an aminocyclohexane carboxylic acid (ACH) is inserted into the ring chain of the cyclic RGD peptides, with a common bifunctional chelator (DOTA or NOTA) used for labeling with radiometals (including ⁶⁸Ga and ⁶⁴Cu). The addition of polyethylene glycol (PEG) and dimerization of c(RGD-ACH-K) affected the PK of the renal system and the tumor-targeting ability, relative to unmodified molecule. As a result, both ⁶⁴Cu-DOTA-E[c(RGD-ACH-K)]₂ (complex 1) and ⁶⁴Cu-NOTA-E[c(RGD-ACH-K)]₂ (complex 2) exhibited specific tumor-targeting properties relative to tumor-blocking control group, most likely resulting from improved in vivo tumor imaging. The in vivo tumor-to-blood ratio of the ⁶⁴Cu(NOTA) complex shows better PET imaging than that of the ⁶⁴Cu(DOTA) complex, which should lead to improved dosimetry and increased suitability for noninvasive monitoring of tumor growth or tumor-targeted radionuclide therapy.

Combinatorial nanocarrier based drug delivery approach for amalgamation of anti-tumor agents in breast cancer cells: an improved nanomedicine strategy

Combination therapy of multiple drugs through a single system is exhibiting high therapeutic effects. We investigate nanocarrier mediated inhibitory effects of topotecan (TPT) and quercetin (QT) on triple negative breast cancer (TNBC) (MDA-MB-231) and multi drug resistant (MDR) type breast cancer cells (MCF-7) with respect to cellular uptake efficiency and therapeutic mechanisms as in vitro and in vivo. The synthesized mesoporous silica nanoparticle (MSN) pores used for loading TPT; the outer of the nanoparticles was decorated with poly (acrylic acid) (PAA)-Chitosan (CS) as anionic inner-cationic outer layer respectively and conjugated with QT. Subsequently, grafting of arginine-glycine-aspartic acid (cRGD) peptide on the surface of nanocarrier (CPMSN) thwarted the uptake by normal cells, but facilitated their uptake in cancer cells through integrin receptor mediated endocytosis and the dissociation of nanocarriers due to the ability to degrade of CS and PAA in acidic pH, which enhance the intracellular release of drugs. Subsequently, the released drugs induce remarkable molecular activation as well as structural changes in tumor cell endoplasmic reticulum, nucleus and mitochondria that can trigger cell death. The valuable CPMSNs may open up new avenues in developing targeted therapeutic strategies to treat cancer through serving as an effective drug delivery podium.

Preparation and characterization of a dual-receptor mesoporous silica nanoparticle–hyaluronic acid–RGD peptide targeting drug delivery system

Novel mesoporous silica nanoparticles conjugated with hyaluronic acid and RGD peptide were developed for dual-receptor mediated targeting drug delivery.

Synthesis and preclinical evaluation of a novel, selective 111In-labelled aminoproline-RGD-peptide for non-invasive melanoma tumor imaging

An ¹¹¹In-labelled Amp-based RGD-DOTA conjugate was synthesized and evaluated in preclinical models of human melanoma as a novel integrin-targeted SPECT imaging tracer.

Improved tumor-targeting MRI contrast agents: Gd(DOTA) conjugates of a cycloalkane-based RGD peptide

Two new MRI contrast agents, Gd-DOTA-c(RGD-ACP-K) (1) and Gd-DOTA-c(RGD-ACH-K) (2), which were designed by incorporating aminocyclopentane (ACP)- or aminocyclohexane (ACH)-carboxylic acid into Gd-DOTA (gadolinium-tetraazacyclo dodecanetetraacetic acid) and cyclic RGDK peptides, were synthesized and evaluated for tumor-targeting ability in vitro and in vivo. Binding affinity studies showed that both 1 and 2 exhibited higher affinity for integrin receptors than cyclic RGDyK peptides, which were used as a reference. These complexes showed high relaxivity and good stability in human serum and have the potential to improve target-specific signal enhancement in vivo MR images.