Radiolabeled NGR-Based Heterodimers for Angiogenesis Imaging: A Review of Preclinical Studies

Since angiogenesis/neoangiogenesis has a major role in tumor development, progression and metastatic spread, the establishment of angiogenesis-targeting imaging and therapeutic vectors is of utmost significance. Aminopeptidase N (APN/CD13) is a pivotal biomarker of angiogenic processes abundantly expressed on the cell surface of active vascular endothelial and various neoplastic cells, constituting a valuable target for cancer diagnostics and therapy. Since the asparagine-glycine-arginine (NGR) sequence has been shown to colocalize with APN/CD13, the research interest in NGR-peptide-mediated vascular targeting is steadily growing. Earlier preclinical experiments have already demonstrated the imaging and therapeutic feasibility of NGR-based probes labeled with different positron emission tomography (PET) and single-photon emission computed tomography (SPECT) radionuclides, including Gallium-68 (68Ga), Copper-64 (64Cu), Technetium-99m (99mTc), Lutetium-177 (177Lu), Rhenium-188 (188Re) or Bismuth-213 (213Bi). To improve the tumor binding affinity and the retention time of single-receptor targeting peptides, NGR motifs containing heterodimers have been introduced to identify multi-receptor overexpressing malignancies. Preclinical studies with various tumor-bearing experimental animals provide useful tools for the investigation of the in vivo imaging behavior of NGR-based heterobivalent ligands. Herein, we review the reported preclinical achievements on NGR heterodimers that could be highly relevant for the development of further target-specific multivalent compounds in diagnostic and therapeutic settings.

NGR-Based Radiopharmaceuticals for Angiogenesis Imaging: A Preclinical Review

Angiogenesis plays a crucial role in tumour progression and metastatic spread; therefore, the development of specific vectors targeting angiogenesis has attracted the attention of several researchers. Since angiogenesis-associated aminopeptidase N (APN/CD13) is highly expressed on the surface of activated endothelial cells of new blood vessels and a wide range of tumour cells, it holds great promise for imaging and therapy in the field of cancer medicine. The selective binding capability of asparagine-glycine-arginine (NGR) motif containing molecules to APN/CD13 makes radiolabelled NGR peptides promising radiopharmaceuticals for the non-invasive, real-time imaging of APN/CD13 overexpressing malignancies at the molecular level. Preclinical small animal model systems are major keystones for the evaluation of the in vivo imaging behaviour of radiolabelled NGR derivatives. Based on existing literature data, several positron emission tomography (PET) and single-photon emission computed tomography (SPECT) radioisotopes have been applied so far for the labelling of tumour vasculature homing NGR sequences such as Gallium-68 (68Ga), Copper-64 (64Cu), Technetium-99m (99mTc), Lutetium-177 (177Lu), Rhenium-188 (188Re), or Bismuth-213 (213Bi). Herein, a comprehensive overview is provided of the recent preclinical experiences with radiolabelled imaging probes targeting angiogenesis.

Novel fusion protein NGR-sIL-24 for targetedly suppressing cancer cell growth via apoptosis

Pro-apoptotic peptides have attracted much attention as promising anticancer agents due to their high activity. However, poor cellular uptake of the peptides is often associated with limited therapeutic application. Cell-penetrating homing peptides (CPHPs) were found to increase cell internalization as well as anticancer efficacy of the peptide conjugates. In this study, we developed a novel recombinant fusion protein composed of sIL-24 peptide as a pro-apoptotic moiety and asparagine-glycine-arginine (NGR) motif as a CD13-targeting CPHP component. In silico analysis demonstrated that flexible GGGGS linker provided the best structure and stability for our designed fusion protein. Cell adhesion experiments showed a significant binding affinity toward high CD13-expressing cells (U937 and A549) for NGR-sIL-24. Moreover, confocal microscopy revealed that NGR strongly facilitated the binding and cellular uptake of sIL-24 in U937 and A549 cancer cells. NGR-sIL-24 treatment markedly inhibited the growth of U937 and A549 cancer cells in a dose and time-dependent manner, without affecting the normal cell line MRC-5. Flow cytometric analysis and Hoechst 33342 staining exhibited potent apoptosis induction in U937 and A549 cells treated with NGR-sIL-24. Further mechanism elucidation uncovered that apoptotic death promoted by NGR-sIL-24 was attributed to upregulation of BiP/GRP78, Bax/Bcl-2, GADD34, cytochrome c release, and cleavage of caspase-3, suggesting NGR-sIL-24 penetration into cancerous cells and subsequent apoptosis induction, mainly through endoplasmic reticulum (ER) stress-dependent and mitochondria-dependent signaling pathways. Our results indicate that the designed recombinant fusion protein NGR-sIL-24 may serve as a potential targeted therapy agent for cancers with high expression of CD13.

Peptide ligand-modified nanomedicines for targeting cells at the tumor microenvironment

Since their initial discovery more than 30years ago, tumor-homing peptides have become an increasingly useful tool for targeted delivery of therapeutic and diagnostic agents into tumors. Today, it is well accepted that cells at the tumor microenvironment (TME) contribute in many ways to cancer development and progression. Tumor-homing peptide-decorated nanomedicines can interact specifically with surface receptors expressed on cells in the TME, improve cellular uptake of nanomedicines by target cells, and impair tumor growth and progression. Moreover, peptide ligand-modified nanomedicines can potentially accumulate in the target tissue at higher concentrations than would small conjugates, thus increasing overall target tissue exposure to the therapeutic agent, enhance therapeutic efficacy and reduce side effects. This review describes the most studied peptide ligands aimed at targeting cells in the TME, discusses major obstacles and principles in the design of ligands for drug targeting and provides an overview of homing peptides in ligand-targeted nanomedicines that are currently in development for cancer therapy and diagnosis.

Angiogenesis regulation by nanocarriers bearing RNA interference

Since the approval of bevacizumab as anti-angiogenic therapy in 2004 by the FDA, an array of angiogenesis inhibitors have been developed and approved. However, results were disappointing with regard to their therapeutic efficacy. RNA interference approaches offer the possibility of rational design with high specificity, lacking in many current drug treatments for various diseases including cancer. However, in vivo delivery issues still represent a significant obstacle for widespread clinical applications. In the current review, we summarize the advances in the last decade in the field of angiogenesis-targeted RNA interference approaches, with special emphasis on oncology applications. We present pro-angiogenic and anti-angiogenic factors as potential targets, experimental evidence and clinical trials data on angiogenesis regulation by RNA interference. Consequent challenges and opportunities are discussed.

Two birds, one stone: dual targeting of the cancer cell surface and subcellular mitochondria by the galectin-3-binding peptide G3-C12

Active tumor-targeting approaches using specific ligands have drawn considerable attention over the years. However, a single ligand often fails to simultaneously target the cancer cell surface and subcellular organelles, which limits the maximum therapeutic efficacy of delivered drugs. We describe a polymeric delivery system modified with the G3-C12 peptide for sequential dual targeting. In this study, galectin-3-targeted G3-C12 peptide was conjugated onto the N-(2-hydroxypropyl) methacrylamide (HPMA) copolymer for the delivery of _D(KLAKLAK)₂ (KLA) peptide. G3-C12-HPMA-KLA exhibited increased receptor-mediated internalization into galectin-3-overexpressing PC-3 cells. Furthermore, G3-C12 peptide also directed HPMA-KLA conjugates to mitochondria. This occurred because the apoptosis signal triggered the accumulation of galectin-3 in mitochondria, and the G3-C12 peptide that specifically bound to galectin-3 was trafficked along with its receptor intracellularly. As a result, G3-C12-HPMA-KLA disrupted the mitochondrial membrane, increased the generation of reactive oxygen species (ROS) and induced cytochrome c release, which ultimately resulted in enhanced cytotoxicity. An in vivo study revealed that the G3-C12 peptide significantly enhanced the tumor accumulation of the KLA conjugate. In addition, G3-C12-HPMA-KLA exhibited the best therapeutic efficacy and greatly improved the animal survival rate. Our work demonstrates that G3-C12 is a promising ligand with dual-targeting functionality.

Synthesis and evaluation of multivalent M2pep peptides for targeting alternatively activated M2 macrophages

The tumor microenvironment in the majority of cancers is known to favor polarization of tumor-associated macrophages (TAMs) to alternatively activated M2 phenotype, promoting disease progression and reducing patient survival. Effective therapy targeting this M2 macrophage population is thus a promising adjuvant to approved cancer therapies. One of the challenges in targeting M2-like TAMs is a lack of high affinity targeting ligand with good selectivity over anti-tumor M1-like TAMs. We have previously identified an M2 macrophage-targeting peptide (M2pep) that binds preferentially to murine M2 macrophages and M2-like TAMs. A fusion peptide of M2pep with pro-apoptotic peptide KLA (M2pepKLA) was further used to reduce TAM population in vivo but high concentrations and frequent dosing were required due to low binding affinity of M2pep for M2 macrophage. The goal of this study was to develop more potent TAM depletion constructs by increasing the valency of both the M2pep targeting and KLA drug domains. Divalent and tetravalent displays of M2pep ([M2pep]2-Biotin and [M2pep]4-Biotin) were synthesized and evaluated for improvement in binding avidity to the murine macrophages. High avidity and selective binding of [M2pep]2-Biotin to M2 macrophages were achieved with at least 10-fold lower concentration than required for monovalent M2pep activity. Increasing M2pep valency to four, however, resulted in a reduction in both binding activity and selectivity. Surprisingly, both divalent and tetravalent M2pep, without conjugation of any cytotoxic drug cargo, exhibited M2 macrophage-selective toxicity not observed in monovalent M2pep treatment. We next synthesized divalent M2pep with monovalent and divalent KLA ([M2pep]2-[KLA] and [M2pep]2-[KLA]2) to evaluate its enhanced potency compared to M2pepKLA. While both constructs were significantly more toxic than M2pepKLA to primary, bone marrow-derived M2 macrophage, desired selectivity was retained only with [M2pep]2-[KLA]. Finally, we evaluated all multivalent M2pep and M2pepKLA analogs using a syngeneic CT-26 tumor cell suspension. In this setting, [M2pep]4-Biotin and [M2pep]2-[KLA]2 exhibited selective toxicity to both M2-like TAMs and malignant cells but not to M1-like TAMs. Therefore, these constructs are promising anti-cancer constructs with dual-modality mechanisms: malignant cell killing and TAM-based immunomodulation.

Enhanced in vivo tumour imaging by EDTA-bis-GNGR functionalized core shell CdSe:ZnS quantum dot: synergistic effect of active passive targeting

Use of QD-homodimeric system enhances tumour targeting due to the synergistic effect of active passive targeting.

Multivalent display of pendant pro-apoptotic peptides increases cytotoxic activity

Several cationic antimicrobial peptides have been investigated as potential anti-cancer drugs due to their demonstrated selective toxicity towards cancer cells relative to normal cells. For example, intracellular delivery of KLA, a pro-apoptotic peptide, results in toxicity against a variety of cancer cell lines; however, the relatively low activity and small size lead to rapid renal excretion when applied in vivo, limiting its therapeutic potential. In this work, apoptotic peptide-polymer hybrid materials were developed to increase apoptotic peptide activity via multivalent display. Multivalent peptide materials were prepared with comb-like structure by RAFT copolymerization of peptide macromonomers with N-(2-hydroxypropyl) methacrylamide (HPMA). Polymers displayed a GKRK peptide sequence for targeting p32, a protein often overexpressed on the surface of cancer cells, either fused with or as a comonomer to a KLA macromonomer. In three tested cancer cell lines, apoptotic polymers were significantly more cytotoxic than free peptides as evidenced by an order of magnitude decrease in IC50 values for the polymers compared to free peptide. The uptake efficiency and intracellular trafficking of one polymer construct was determined by radiolabeling and subcellular fractionation. Despite their more potent cytotoxic profile, polymeric KLA constructs have poor cellular uptake efficiency (<1%). A significant fraction (20%) of internalized constructs localize with intact mitochondrial fractions. In an effort to increase cellular uptake, polymer amines were converted to guanidines by reaction with O-methylisourea. Guanidinylated polymers disrupted function of isolated mitochondria more than their lysine-based analogs, but overall toxicity was decreased, likely due to inefficient mitochondrial trafficking. Thus, while multivalent KLA polymers are more potent than KLA peptides, these materials can be substantially improved by designing next generation materials with improved cellular internalization and mitochondrial targeting efficiency.

Self-assembly of cytotoxic peptide conjugated poly(β-amino ester)s for synergistic cancer chemotherapy

DOX-encapsulated self-assembled micelle formation by cytotoxic peptide conjugated poly(β-amino ester)s for synergistic cancer chemotherapy.

Biodistribution and SPECT imaging study of (99m)Tc labeling NGR peptide in nude mice bearing human HepG2 hepatoma

A peptide containing Asn-Gly-Arg(NGR) sequence was synthesized and directly labeled with ^99mTc. Its radiochemical characteristics, biodistribution, and SPECT imaging were evaluated in nude mice bearing human HepG2 hepatoma. Nude mice bearing HepG2 were randomly divided into 5 groups with 5 mice in each group and injected with ~7.4 MBq ^99mTc-NGR. The SPECT images were acquired in 1, 4, 8, and 12 h postinjection via caudal vein. The metabolism of tracers was determined in major organs at different time points, which demonstrated rapid, significant tumor uptake and slow tumor washout. The control group mice were blocked by coinjecting unlabelled NGR (20 mg/kg). Tumor uptake was (2.52 ± 0.83%) ID/g at 1 h, with the highest uptake of (3.26 ± 0.63%) ID/g at 8 h. In comparison, the uptake of the blocked control group was (1.65 ± 0.61%) ID/g at 1 h after injection. The SPECT static images and the tumor/muscle (T/NT) value were obtained. The highest T/NT value was 7.58 ± 1.92 at 8 h. The xenografted tumor became visible at 1 h and the clearest image of the tumor was observed at 8 h. In conclusion, ^99mTc-NGR can be efficiently prepared and it exhibited good properties for the potential SPECT imaging agent of tumor.

Novel polymeric nanoparticles for intracellular delivery of peptide Cargos: antitumor efficacy of the BCL-2 conversion peptide NuBCP-9

The preclinical development of peptidyl drugs for cancer treatment is hampered by their poor pharmacologic properties and cell penetrative capabilities in vivo. In this study, we report a nanoparticle-based formulation that overcomes these limitations, illustrating their utility in studies of the anticancer peptide NuBCP-9, which converts BCL-2 from a cell protector to a cell killer. NuBCP-9 was encapsulated in polymeric nanoparticles composed of a polyethylene glycol (PEG)-modified polylactic acid (PLA) diblock copolymer (NuBCP-9/PLA-PEG) or PEG-polypropylene glycol-PEG-modified PLA-tetrablock copolymer (NuBCP-9/PLA-PEG-PPG-PEG). We found that peptide encapsulation was enhanced by increasing the PEG chain length in the block copolymers. NuBCP-9 release from the nanoparticles was controlled by both PEG chain length and the PLA molecular weight, permitting time-release over sustained periods. Treatment of human cancer cells with these nanoparticles in vitro triggered apoptosis by NuBCP-9-mediated mechanism, with a potency similar to NuBCP-9 linked to a cell-penetrating poly-Arg peptide. Strikingly, in vivo administration of NuBCP-9/nanoparticles triggered complete regressions in the Ehrlich syngeneic mouse model of solid tumor. Our results illustrate an effective method for sustained delivery of anticancer peptides, highlighting the superior qualities of the novel PLA-PEG-PPG-PEG tetrablock copolymer formulation as a tool to target intracellular proteins.

(99m)Tc-labeled monomeric and dimeric NGR peptides for SPECT imaging of CD13 receptor in tumor-bearing mice

CD13 receptor plays a critical role in tumor angiogenesis and metastasis. We therefore aimed to develop (99m)Tc-labeled monomeric and dimeric NGR-containing peptides, namely, NGR1 and NGR2, for SPECT imaging of CD13 expression in HepG2 hepatoma xenografts. Both NGR-containing monomer and dimer were synthesized and labeled with (99m)Tc. In vivo receptor specificity was demonstrated by successful blocking of tumor uptake of (99m)Tc-NGR dimer in the presence of 20 mg/kg NGR2 peptide. Western blot and immunofluorescence staining confirmed the CD13 expression in HepG2 cells. The NGR dimer showed higher binding affinity and cell uptake in vitro than the NGR-containing monomer, presumably due to a multivalency effect. (99m)Tc-Labeled monomeric and dimeric NGR-containing peptides were subjected to SPECT imaging and biodistribution studies. SPECT scans were performed in HepG2 tumor-bearing mice at 1, 4, 12, and 24 h post-injection of ~7.4 MBq tracers. The metabolism of tracers was determined in major organs at different time points after injection which demonstrated rapid, significant tumor uptake and slow tumor washout for both traces. Predominant clearance from renal and hepatic system was also observed in (99m)Tc-NGR1 and (99m)Tc-NGR2. In conclusion, monomeric and dimeric NGR peptide were developed and labeled with (99m)Tc successfully, while the high integrin avidity and long retention in tumor make (99m)Tc-NGR dimer a promising agent for tumor angiogenesis imaging.