Evaluation of (188)Re-labeled NGR-VEGI protein for radioimaging and radiotherapy in mice bearing human fibrosarcoma HT-1080 xenografts

In Vivo Imaging of Acute Hindlimb Ischaemia in Rat Model: A Pre-Clinical PET Study

BACKGROUND

To better understand ischaemia-related molecular alterations, temporal changes in angiogenic Aminopeptidase N (APN/CD13) expression and glucose metabolism were assessed with PET using a rat model of peripheral arterial disease (PAD).

METHODS

The mechanical occlusion of the base of the left hindlimb triggered using a tourniquet was applied to establish the ischaemia/reperfusion injury model in Fischer-344 rats. 2-[18F]FDG and [68Ga]Ga-NOTA-c(NGR) PET imaging performed 1, 3, 5, 7, and 10 days post-ischaemia induction was followed by Western blotting and immunohistochemical staining for APN/CD13 in ischaemic and control muscle tissue extracts.

RESULTS

Due to a cellular adaptation to hypoxia, a gradual increase in [68Ga]Ga-NOTA-c(NGR) and 2-[18F]FDG uptake was observed from post-intervention day 1 to 7 in the ischaemic hindlimbs, which was followed by a drop on day 10. Conforming pronounced angiogenic recovery, the NGR accretion of the ischaemic extremities differed significantly from the controls 5, 7, and 10 days after ischaemia induction (p ≤ 0.05), which correlated with the Western blot and immunohistochemical results. No remarkable radioactivity was depicted between the normally perfused hindlimbs of either the ischaemic or the control groups.

CONCLUSIONS

The PET-based longitudinal assessment of angiogenesis-associated APN/CD13 expression and glucose metabolism during ischaemia may continue to broaden our knowledge on the pathophysiology of PAD.

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.

Scandium-44: Diagnostic Feasibility in Tumor-Related Angiogenesis

Angiogenesis-related cell-surface molecules, including integrins, aminopeptidase N, vascular endothelial growth factor, and gastrin-releasing peptide receptor (GRPR), play a crucial role in tumour formation. Radiolabelled imaging probes targeting angiogenic biomarkers serve as valuable vectors in tumour identification. Nowadays, there is a growing interest in novel radionuclides other than gallium-68 (⁶⁸Ga) or copper-64 (⁶⁴Cu) to establish selective radiotracers for the imaging of tumour-associated neo-angiogenesis. Given its ideal decay characteristics (E_β⁺_average: 632 KeV) and a half-life (T_1/2 = 3.97 h) that is well matched to the pharmacokinetic profile of small molecules targeting angiogenesis, scandium-44 (⁴⁴Sc) has gained meaningful attention as a promising radiometal for positron emission tomography (PET) imaging. More recently, intensive research has been centered around the investigation of ⁴⁴Sc-labelled angiogenesis-directed radiopharmaceuticals. Previous studies dealt with the evaluation of ⁴⁴Sc-appended a_vb₃ integrin-affine Arg-Gly-Asp (RGD) tripeptides, GRPR-selective aminobenzoyl-bombesin analogue (AMBA), and hypoxia-associated nitroimidazole derivatives in the identification of various cancers using experimental tumour models. Given the tumour-related hypoxia- and angiogenesis-targeting capability of these PET probes, ⁴⁴Sc seems to be a strong competitor of the currently used positron emitters in radiotracer development. In this review, we summarize the preliminary preclinical achievements with ⁴⁴Sc-labelled angiogenesis-specific molecular probes.

Therapeutic Performance Evaluation of 213Bi-Labelled Aminopeptidase N (APN/CD13)-Affine NGR-Motif ([213Bi]Bi-DOTAGA-cKNGRE) in Experimental Tumour Model: A Treasured Tailor for Oncology

Since NGR-tripeptides (asparagine-glycine-arginine) selectively target neoangiogenesis-associated Aminopeptidase N (APN/CD13) on cancer cells, we aimed to evaluate the in vivo tumour targeting capability of radiolabelled, NGR-containing, ANP/CD13-selective [²¹³Bi]Bi-DOTAGA-cKNGRE in CD13pos. HT1080 fibrosarcoma-bearing severe combined immunodeficient CB17 mice. 10 ± 1 days after cancer cell inoculation, positron emission tomography (PET) was performed applying [⁶⁸Ga]Ga-DOTAGA-cKNGRE for tumour verification. On the 7th, 8th, 10th and 12th days the treated group of tumourous mice were intraperitoneally administered with 4.68 ± 0.10 MBq [²¹³Bi]Bi-DOTAGA-cKNGRE, while the untreated tumour-bearing animals received 150 μL saline solution. In addition to body weight (BW) and tumour volume measurements, ex vivo biodistribution studies were conducted 30 and 90 min postinjection (pi.). The following quantitative standardised uptake values (SUV) confirmed the detectability of the HT1080 tumours: SUV_mean and SUV_max: 0.37 ± 0.09 and 0.86 ± 0.14, respectively. Although no significant difference (p ≤ 0.05) was encountered between the BW of the treated and untreated mice, their tumour volumes measured on the 9th, 10th and 12th days differed significantly (p ≤ 0.01). Relatively higher [²¹³Bi]Bi-DOTAGA-cKNGRE accumulation of the HT1080 neoplasms (%ID/g: 0.80 ± 0.16) compared with the other organs at 90 min time point yields better tumour-to-background ratios. Therefore, the therapeutic application of APN/CD13-affine [²¹³Bi]Bi-DOTAGA- cKNGRE seems to be promising in receptor-positive fibrosarcoma treatment.

Theranostic Agents in Musculoskeletal Disorders

Theranostic-based strategies, combining therapeutic and diagnostic properties of a single agent, have gained enormous attention in the past few years. Today, various multifunctional theranostic modalities have been examined, using different bioactive targeting, for the detection, quantifying, and monitoring of therapy response in different pathologies. Herein we review the newly emerging approaches in theranostic nanomedicine for the detection and therapy for musculoskeletal disorders to provide valuable insights for developing more efficient agents for clinical use. Some potential preclinical applications of radionuclide nanotheranostic agents are described in rheumatoid arthritis, osteoarthrosis, multiple myeloma, and neoplastic diseases.

Research Progress of Radiolabeled Asn-Gly-Arg (NGR) Peptides for Imaging and Therapy

Asn-Gly-Arg (NGR) motifs have vasculature-homing properties via interactions with the aminopeptidase N (CD13) expressed on tumor neovasculature. Numerous NGR peptides with different molecular scaffolds have been exploited for targeted delivery of different compounds for imaging and therapy. When conjugated with NGR, complexes recognize the CD13 receptor expressed on the tumor vasculature, which improves the specificity to tumor and avoids systematic toxic reactions. Both preclinical and clinical studies performed with these products suggest that NGR-mediated vascular targeting is an effective strategy for delivering bioactive amounts of cytokines to tumor endothelial cells. For molecular imaging, radiolabeled peptides have been the most successful approach and have been translated into clinic. This review describes current data on radiolabeled tumor vasculature-homing NGR peptides for imaging and therapy.

Molecular Imaging of Angiogenesis in Oncology: Current Preclinical and Clinical Status

Angiogenesis is an active process, regulating new vessel growth, and is crucial for the survival and growth of tumours next to other complex factors in the tumour microenvironment. We present possible molecular imaging approaches for tumour vascularisation and vitality, focusing on radiopharmaceuticals (tracers). Molecular imaging in general has become an integrated part of cancer therapy, by bringing relevant insights on tumour angiogenic status. After a structured PubMed search, the resulting publication list was screened for oncology related publications in animals and humans, disregarding any cardiovascular findings. The tracers identified can be subdivided into direct targeting of angiogenesis (i.e., vascular endothelial growth factor, laminin, and fibronectin) and indirect targeting (i.e., glucose metabolism, hypoxia, and matrix metallo-proteases, PSMA). Presenting pre-clinical and clinical data of most tracers proposed in the literature, the indirect targeting agents are not 1:1 correlated with angiogenesis factors but do have a strong prognostic power in a clinical setting, while direct targeting agents show most potential and specificity for assessing tumour vascularisation and vitality. Within the direct agents, the combination of multiple targeting tracers into one agent (multimers) seems most promising. This review demonstrates the present clinical applicability of indirect agents, but also the need for more extensive research in the field of direct targeting of angiogenesis in oncology. Although there is currently no direct tracer that can be singled out, the RGD tracer family seems to show the highest potential therefore we expect one of them to enter the clinical routine.

Peptide-Based Strategies for Targeted Tumor Treatment and Imaging

Cancer is one of the leading causes of death worldwide. The development of cancer-specific diagnostic agents and anticancer toxins would improve patient survival. The current and standard types of medical care for cancer patients, including surgery, radiotherapy, and chemotherapy, are not able to treat all cancers. A new treatment strategy utilizing tumor targeting peptides to selectively deliver drugs or applicable active agents to solid tumors is becoming a promising approach. In this review, we discuss the different tumor-homing peptides discovered through combinatorial library screening, as well as native active peptides. The different structure–function relationship data that have been used to improve the peptide’s activity and conjugation strategies are highlighted.

In vivo assessment of aminopeptidase N (APN/CD13) specificity of different 68Ga-labelled NGR derivatives using PET/MRI imaging

Aminopeptidase N (APN/CD13) plays an important role in neoangiogenic process in malignancies. Our previous studies have already shown that ⁶⁸Ga-labelled NOTA conjugated asparagine-glycine-arginine peptide (c[KNGRE]-NH₂) specifically bind to APN/CD13 expressing tumors. The aim of this study was to evaluate and compare the APN/CD13 specificity of newly synthesized ⁶⁸Ga-labelled NGR derivatives in vivo by PET/MRI imaging using hepatocellular carcinoma (He/De) and mesoblastic nephroma (Ne/De) tumor models. PET/MRI and ex vivo biodistribution studies were performed 11 ± 1 days after subcutaneous injection of tumor cells and 90 min after intravenous injection of ⁶⁸Ga-NOTA-c(NGR), ⁶⁸Ga-NODAGA-c(NGR), ⁶⁸Ga-NODAGA-c(NGR) (MG1) or ⁶⁸Ga-NODAGA-c(NGR) (MG2). The APN/CD13 selectivity was confirmed by blocking experiments and the APN/CD13 expression was verified by immunohistochemistry. ⁶⁸Ga-labelled c(NGR) derivatives were produced with high specific activity and radiochemical purity. In control animals, low radiotracer accumulation was found in abdominal and thoracic organs. Using tumor-bearing animals we found that the ⁶⁸Ga-NOTA-c(NGR), ⁶⁸Ga-NODAGA-c(NGR), and ⁶⁸Ga-NODAGA-c(NGR) (MG1) derivatives showed higher uptake in He/De and Ne/De tumors, than that of the accumulation of ⁶⁸Ga-NODAGA-c(NGR) (MG2). APN/CD13 is a very promising target in PET imaging, however, the selection of the appropriate ⁶⁸Ga-labelled NGR-based radiopharmaceutical is critical for the precise detection of tumor neo-angiogenesis and for monitoring the efficacy of anticancer therapy.

Molecular Imaging of Aminopeptidase N in Cancer and Angiogenesis

This review focuses on recent advances in the molecular imaging of aminopeptidase N (APN, also known as CD13), a zinc metalloenzyme that cleaves N-terminal neutral amino acids. It is overexpressed in multiple cancer types and also on the surface of vasculature undergoing angiogenesis, making it a promising target for molecular imaging and targeted therapy. Molecular imaging probes for APN are divided into two large subgroups: reactive and nonreactive. The structures of the reactive probes (substrates) contain a reporter group that is cleaved and released by the APN enzyme. The nonreactive probes are not cleaved by the enzyme and contain an antibody, peptide, or nonpeptide for targeting the enzyme exterior or active site. Multivalent homotopic probes utilize multiple copies of the same targeting unit, whereas multivalent heterotopic molecular probes are equipped with different targeting units for different receptors. Several recent preclinical cancer imaging studies have shown that multivalent APN probes exhibit enhanced tumor specificity and accumulation compared to monovalent analogues. The few studies that have evaluated APN-specific probes for imaging angiogenesis have focused on cardiac regeneration. These promising results suggest that APN imaging can be expanded to detect and monitor other diseases that are associated with angiogenesis.

The uptake exploration of 68Ga-labeled NGR in well-differentiated hepatocellular carcinoma xenografts: Indication for the new clinical translational of a tracer based on NGR

18F-FDG has low uptake and poor diagnostic efficiency in hepatocellular carcinoma (HCC), particularly in well-differentiated HCC. The NGR peptide selectively targets CD13, which is overexpressed in many types of tumor cells as well as neovasculature cells. In the present study, we aimed to evaluate the feasibility of utilizing 68Ga-NGR to image CD13-positive well-differentiated HCC xenografts. The in vitro cellular uptake, in vivo micro-PET/CT imaging and biodistribution studies of 68Ga-NGR and 18F-FDG were quantitatively compared in SMMC-7721-based well‑differentiated HCC xenografts. The human fibrosarcoma (HT-1080) and human colorectal adenocarcinoma (HT-29) xenografts were respectively used as positive and negative reference groups for CD13. The expression of CD13 was qualitatively verified by immunofluorescence staining and immunohistostaining studies. The expression levels of CD13 and glucose-6-phosphatase (G6Pase) were semi-quantitatively analyzed by western blotting. The in vitro SMMC-7721 cellular uptake of 68Ga‑NGR was significantly higher than that of 18F-FDG (1.23±0.11 vs. 0.515±0.14%; P<0.01). The in vivo micro-PET/CT imaging results revealed that the uptake of 68Ga-NGR in SMMC-7721-derived tumors was 2.17±0.21% ID/g (percentage of injected dose per gram of tissue), which was higher compared to that of 18F-FDG (0.73±0.26% ID/g; P<0.01); however, the tumor/liver ratio of 68Ga-NGR was 2-fold higher than that of 18F-FDG. We concluded that the uptake of 68Ga-NGR was significantly higher both in vitro and in vivo than 18F-FDG in the well‑differentiated HCC xenografts and therefore, it is promising for further clinical translation in well-differentiated HCC PET/CT diagnosis.