A direct comparison of tumor angiogenesis with ⁶⁸Ga-labeled NGR and RGD peptides in HT-1080 tumor xenografts using microPET imaging

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.

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.

Synthesis of Novel Carborane-Containing Derivatives of RGD Peptide

Short peptides containing the Arg-Gly-Asp (RGD) fragment can selectively bind to integrins on the surface of tumor cells and are attractive transport molecules for the targeted delivery of therapeutic and diagnostic agents to tumors (for example, glioblastoma). We have demonstrated the possibility of obtaining the N- and C-protected RGD peptide containing 3-amino-closo-carborane and a glutaric acid residue as a linker fragment. The resulting carboranyl derivatives of the protected RGD peptide are of interest as starting compounds in the synthesis of unprotected or selectively protected peptides, as well as building blocks for preparation of boron-containing derivatives of the RGD peptide of a more complex structure.

Structure-guided design enables development of a hyperpolarized molecular probe for the detection of aminopeptidase N activity in vivo

Dynamic nuclear polarization (DNP) is a cutting-edge technique that markedly enhances the detection sensitivity of molecules using nuclear magnetic resonance (NMR)/magnetic resonance imaging (MRI). This methodology enables real-time imaging of dynamic metabolic status in vivo using MRI. To expand the targetable metabolic reactions, there is a demand for developing exogenous, i.e., artificially designed, DNP-NMR molecular probes; however, complying with the requirements of practical DNP-NMR molecular probes is challenging because of the lack of established design guidelines. Here, we report Ala-[1-¹³C]Gly-d₂-NMe₂ as a DNP-NMR molecular probe for in vivo detection of aminopeptidase N activity. We developed this probe rationally through precise structural investigation, calculation, biochemical assessment, and advanced molecular design to achieve rapid and detectable responses to enzyme activity in vivo. With the fabricated probe, we successfully detected enzymatic activity in vivo. This report presents a comprehensive approach for the development of artificially derived, practical DNP-NMR molecular probes through structure-guided molecular design.

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.

PET Diagnostic Molecules Utilizing Multimeric Cyclic RGD Peptide Analogs for Imaging Integrin αvβ3 Receptors

Multimeric ligands consisting of multiple pharmacophores connected to a single backbone have been widely investigated for diagnostic and therapeutic applications. In this review, we summarize recent developments regarding multimeric radioligands targeting integrin α_vβ₃ receptors on cancer cells for molecular imaging and diagnostic applications using positron emission tomography (PET). Integrin α_vβ₃ receptors are glycoproteins expressed on the cell surface, which have a significant role in tumor angiogenesis. They act as receptors for several extracellular matrix proteins exposing the tripeptide sequence arginine-glycine-aspartic (RGD). Cyclic RDG peptidic ligands c(RGD) have been developed for integrin α_vβ₃ tumor-targeting positron emission tomography (PET) diagnosis. Several c(RGD) pharmacophores, connected with the linker and conjugated to a chelator or precursor for radiolabeling with different PET radionuclides (¹⁸F, ⁶⁴Cu, and ⁶⁸Ga), have resulted in multimeric ligands superior to c(RGD) monomers. The binding avidity, pharmacodynamic, and PET imaging properties of these multimeric c(RGD) radioligands, in relation to their structural characteristics are analyzed and discussed. Furthermore, specific examples from preclinical studies and clinical investigations are included.

Design of Nuclear Magnetic Resonance Molecular Probes for Hyperpolarized Bioimaging

Nuclear hyperpolarization has emerged as a method to dramatically enhance the sensitivity of NMR spectroscopy. By application of this powerful tool, small molecules with stable isotopes have been used for highly sensitive biomedical molecular imaging. The recent development of molecular probes for hyperpolarized in vivo analysis has demonstrated the ability of this technique to provide unique metabolic and physiological information. This review presents a brief introduction of hyperpolarization technology, approaches to the rational design of molecular probes for hyperpolarized analysis, and examples of molecules that have met with success in vitro or in vivo.

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.

Aminopeptidase N (CD13): Expression, Prognostic Impact, and Use as Therapeutic Target for Tissue Factor Induced Tumor Vascular Infarction in Soft Tissue Sarcoma

Aminopeptidase N (CD13) is expressed on tumor vasculature and tumor cells. It represents a candidate for targeted therapy, e.g., by truncated tissue factor (tTF)-NGR, binding to CD13, and causing tumor vascular thrombosis. We analyzed CD13 expression by immunohistochemistry in 97 patients with STS who were treated by wide resection and uniform chemo-radio-chemotherapy. Using a semiquantitative score with four intensity levels, CD13 was expressed by tumor vasculature, or tumor cells, or both (composite value, intensity scores 1-3) in 93.9% of the STS. In 49.5% tumor cells, in 48.5% vascular/perivascular cells, and in 58.8%, composite value showed strong intensity score 3 staining. Leiomyosarcoma and synovial sarcoma showed low expression; fibrosarcoma and undifferentiated pleomorphic sarcoma showed high expression. We found a significant prognostic impact of CD13, as high expression in tumor cells or vascular/perivascular cells correlated with better relapse-free survival and overall survival. CD13 retained prognostic significance in multivariable analyses. Systemic tTF-NGR resulted in significant growth reduction of CD13-positive human HT1080 sarcoma cell line xenografts. Our results recommend further investigation of tTF-NGR in STS patients. CD13 might be a suitable predictive biomarker for patient selection.

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.

NIRF Optical/PET Dual-Modal Imaging of Hepatocellular Carcinoma Using Heptamethine Carbocyanine Dye

Combining near-infrared fluorescence (NIRF) and nuclear imaging techniques provides a novel approach for hepatocellular carcinoma (HCC) diagnosis. Here, we report the synthesis and characteristics of a dual-modality NIRF optical/positron emission tomography (PET) imaging probe using heptamethine carbocyanine dye and verify its feasibility in both nude mice and rabbits with orthotopic xenograft liver cancer. This dye, MHI-148, is an effective cancer-specific NIRF imaging agent and shows preferential uptake and retention in liver cancer. The corresponding NIRF imaging intensity reaches 10⁹/cm² tumor area at 24 h after injection in mice with HCC subcutaneous tumors. The dye can be further conjugated with radionuclide ⁶⁸Ga (⁶⁸Ga-MHI-148) for PET tracing. We applied the dual-modality methodology toward the detection of HCC in both patient-derived orthotopic xenograft (PDX) models and rabbit orthotopic transplantation models. NIRF/PET images showed clear tumor delineation after probe injection (MHI-148 and ⁶⁸Ga-MHI-148). The tumor-to-muscle (T/M) standardized uptake value (SUV) ratios were obtained from PET at 1 h after injection of ⁶⁸Ga-MHI-148, which was helpful for effectively capturing small tumors in mice (0.5 cm × 0.3 cm) and rabbits (1.2 cm × 1.8 cm). This cancer-targeting NIRF/PET dual-modality imaging probe provides a proof of principle for noninvasive detection of deep-tissue tumors in mouse and rabbit and is a promising technique for more accurate and early detection of HCC.

Combinatorial effects of doxorubicin and retargeted tissue factor by intratumoral entrapment of doxorubicin and proapoptotic increase of tumor vascular infarction

Truncated tissue factor (tTF), retargeted to tumor vasculature by GNGRAHA peptide (tTF-NGR), and doxorubicin have therapeutic activity against a variety of tumors. We report on combination experiments of both drugs using different schedules. We have tested fluorescence- and HPLC-based intratumoral pharmacokinetics of doxorubicin, flow cytometry for cellular phosphatidylserine (PS) expression, and tumor xenograft studies for showing in vivo apoptosis, proliferation decrease, and tumor shrinkage upon combination therapy with doxorubicin and induced tumor vascular infarction. tTF-NGR given before doxorubicin inhibits the uptake of the drug into human fibrosarcoma xenografts in vivo. Reverse sequence does not influence the uptake of doxorubicin into tumor, but significantly inhibits the late wash-out phase, thus entrapping doxorubicin in tumor tissue by vascular occlusion. Incubation of endothelial and tumor cells with doxorubicin in vitro increases PS concentrations in the outer layer of the cell membrane as a sign of early apoptosis. Cells expressing increased PS concentrations show comparatively higher procoagulatory efficacy on the basis of equimolar tTF-NGR present in the Factor X assay. Experiments using human M21 melanoma and HT1080 fibrosarcoma xenografts in athymic nude mice indeed show a combinatorial tumor growth inhibition applying doxorubicin and tTF-NGR in sequence over single drug treatment. Combination of cytotoxic drugs such as doxorubicin with tTF-NGR-induced tumor vessel infarction can improve pharmacodynamics of the drugs by new mechanisms, entrapping a cytotoxic molecule inside tumor tissue and reciprocally improving procoagulatory activity of tTF-NGR in the tumor vasculature via apoptosis induction in tumor endothelial and tumor cells.

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.

Angiogenesis Imaging Using (68)Ga-RGD PET/CT: Therapeutic Implications

Angiogenesis imaging is important for diagnostic and therapeutic treatment of various malignant and nonmalignant diseases. The Arg-Gly-Asp (RGD) sequence has been known to bind with the αvβ3 integrin that is expressed on the surface of angiogenic blood vessels or tumor cells. Thus, various radiolabeled derivatives of RGD peptides have been developed for angiogenesis imaging. Among the various radionuclides, (68)Ga was the most widely studied for RGD peptide imaging because of its excellent nuclear physical properties, easy-to-label chemical properties, and cost-effectiveness owing to the availability of a (68)Ge-(68)Ga generator. Thus, various (68)Ga-labeled RGD derivatives have been developed and applied for preclinical and clinical studies. Clinical trials were performed for both malignant and nonmalignant diseases. Breast cancer, glioma, and lung cancer were malignant, and myocardial infarction, atherosclerosis, and moyamoya disease were nonmalignant among the investigated diseases. Further, these (68)Ga-labeled RGD derivatives could be applied to assess the effects of antiangiogenic treatment or theragnosis or both, of cancers. In conclusion, the angiogenesis imaging technology using (68)Ga-labeled RGD derivatives might be useful for the development of new therapeutic assessments, and for diagnostic and theragnostic applications.

Triple-phase 99mTc-3P-RGD2 imaging of peripheral primitive neuroectodermal tumor in the hip muscle group with bone metastasis

Peripheral primitive neuroectodermal tumors (pPNETs) are a group of aggressive neoplasms that are most commonly encountered in pediatric patients and may be located in the abdomen, pelvis, thoracopulmonary region and, rarely, in the head and neck region. pPNETs in adults are extremely rare. The present study reports a case of pPNET located in the hip muscles with bone metastasis. The patient was a 44-year-old woman who complained of progressive pain and swelling with a mass near the left hip. Computed tomography (CT) and enhanced CT revealed a soft tissue mass lesion in the hip muscle group measuring 4.3×4.3×4.4 cm. The lesion was ill-defined, heterogeneous, exhibiting mild post-contrast enhancement. There was a large number of bent neovessels and several branches from the left internal iliac artery and deep femoral artery on enhanced CT scan. Triple-phase dynamic imaging with integrin α_vβ₃-targeted ^99mTc-3P-RGD₂ as the radiotracer revealed increased blood perfusion and radiotracer aggregation in the large, ill-defined, heterogeneous, hypodense mass and adjacent bone. The patient was suspected of having pPNET with bone metastasis, which was confirmed by histological examination of a sample obtained by needle aspiration. Due to the high blood perfusion of primary pPNETs and high RGD uptake by the primary and metastatic lesions, chemoembolization and anti-angiogenic therapy were considered to be the optimal therapeutic choice. This also suggested that ¹⁷⁷Lu-labeled RGD has great potential for the targeted treatment of pPNETs with multiple metastases.

Evaluation of Tc-99 m Labeled Dimeric GX1 Peptides for Imaging of Colorectal Cancer Vasculature

PURPOSE

This study aimed to evaluate the potential of PEGylated dimeric GX1 peptide as a radiotracer for imaging of colorectal cancer vasculature in a LoVo tumor xenografted mouse model.

PROCEDURES

The [(99m)Tc]PEG-(GX1)2 peptide was synthesized and identified. Confocal immunofluorescence analysis, receptor binding assay, and competitive inhibition assay were performed to evaluate the binding specificity and the receptor binding affinity of PEG-(GX1)2 to Co-human umbilical vein endothelial cells (HUVECs). Single photon emission computed tomography imaging and biodistribution were performed to evaluate the targeting ability of PEG-(GX1)2 to colorectal cancer.

RESULTS

The studies in vitro suggested that PEG-(GX1)2 co-localized with Factor VIII in the perinuclear cytoplasm of Co-HUVECs and bound specifically to Co-HUVECs with a high affinity. The studies in vivo demonstrated that the targeting efficacy of PEG-(GX1)2 was superior to GX1.

CONCLUSIONS

PEGylation improved the affinity and the targeting ability of the GX1 peptide. PEG-(GX1)2 is a more promising probe for imaging of colorectal vasculature than GX1.

Evaluation of 68Ga-labeled iNGR peptide with tumor-penetrating motif for microPET imaging of CD13-positive tumor xenografts

The aim of the study is to evaluate the efficacy of ⁶⁸Ga-labeled iNGR, containing Asn-Gly-Arg (NGR) homing sequence and CendR (R/KXXR/K) penetrating motif, as a new molecular probe for microPET imaging of CD13-positive xenografts. The synthesized iNGR and NGR peptides were conjugated with DOTA and then labeled with ⁶⁸Ga. ⁶⁸Ga-iNGR and ⁶⁸Ga-NGR were compared in the performance of the in vitro stability, partition coefficient, binding affinity, cell uptake analysis, in vivo microPET imaging, and biodistribution studies in CD13-positive HT-1080 and CD13-negative HT-29 cell lines. The in vitro results revealed that both probes exhibited high radiochemical purity and stability, and no significant difference between two probes was observed in terms of the binding affinity to CD13. In vivo microPET/CT imaging showed that the uptake of ⁶⁸Ga-iNGR in HT-1080 tumor was significantly higher than that of ⁶⁸Ga-NGR. Moreover, tumor ⁶⁸Ga-iNGR uptake could be completely blocked by cold NGR and partially blocked by neutralizing NRP-1 antibody. We concluded that ⁶⁸Ga-iNGR has a higher tumor uptake and better tumor retention than ⁶⁸Ga-NGR through NRP-1, indicating that CendR motif modification is a promising method for improving NGR peptide performance.

Small Molecules for Active Targeting in Cancer

For the purpose of this review, active targeting in cancer research encompasses strategies wherein a ligand for a cell surface receptor expressed on tumor cells is used to deliver a cytotoxic or imaging cargo. This area of research is more than two decades old, but in those 20 and more years, how many receptors have been studied extensively? What kinds of the ligands are used for active targeting? Are they mostly naturally occurring molecules such as folic acid, or synthetic substances developed in campaigns for medicinal chemistry efforts? This review outlines the most important receptor or ligand combinations that have been used in active targeting to answer these questions, and therefore to address the most important one of all: is research in active targeting affording diminishing returns, or is this an area for which the potential far exceeds progress made so far?

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

Vascular endothelial growth inhibitor (VEGI) is an anti-angiogenic protein, which includes three isoforms: VEGI-174, VEGI-192, and VEGI-251. The NGR (asparagine-glycine-arginine)-containing peptides can specifically bind to CD13 (Aminopeptidase N) receptor which is overexpressed in angiogenic blood vessels and tumor cells. In this study, a novel NGR-VEGI fusion protein was prepared and labeled with (188)Re for radioimaging and radiotherapy in mice bearing human fibrosarcoma HT-1080 xenografts. Single photon emission computerized tomography (SPECT) imaging results revealed that (188)Re-NGR-VEGI exhibits good tumor-to-background contrast in CD13-positive HT-1080 tumor xenografts. The CD13 specificity of (188)Re-NGR-VEGI was further verified by significant reduction of tumor uptake in HT-1080 tumor xenografts with co-injection of the non-radiolabeled NGR-VEGI protein. The biodistribution results demonstrated good tumor-to-muscle ratio (4.98 ± 0.25) of (188)Re-NGR-VEGI at 24 h, which is consistent with the results from SPECT imaging. For radiotherapy, 18.5 MBq of (188)Re-NGR-VEGI showed excellent tumor inhibition effect in HT-1080 tumor xenografts with no observable toxicity, which was confirmed by the tumor size change and hematoxylin and eosin (H&E) staining of major mouse organs. In conclusion, these data demonstrated that (188)Re-NGR-VEGI has the potential as a theranostic agent for CD13-targeted tumor imaging and therapy.

Design of a Hyperpolarized Molecular Probe for Detection of Aminopeptidase N Activity

Aminopeptidase N (APN) is an important enzyme that is involved in tumor angiogenesis. Detection of APN activity can thus lead to early diagnosis and elucidation of tumor development. Although some molecular probes for APN have been developed, the detection of APN activity in opaque biological samples remains a challenge. To this end, we designed a hyperpolarized NMR probe [1-(13) C]Ala-NH2 which satisfies the prerequisites for APN detection, namely, sufficient retention of the hyperpolarized state, a high reactivity to APN, and an APN-induced chemical shift change. The [1-(13) C]Ala-NH2 probe allowed sensitive detection of APN activity using (13) C NMR spectroscopy.

Tumor Growth Inhibition via Occlusion of Tumor Vasculature Induced by N-Terminally PEGylated Retargeted Tissue Factor tTF-NGR

tTF-NGR retargets the extracellular domain of tissue factor via a C-terminal peptide GNGRAHA, a ligand of the surface protein aminopeptidase N (CD13) and upon deamidation of integrin αvβ3, to tumor vasculature. tTF-NGR induces tumor vascular infarction with consecutive antitumor activity against xenografts and selectively inhibits tumor blood flow in cancer patients. Since random PEGylation resulted in favorable pharmacodynamics of tTF-NGR, we performed site-directed PEGylation of PEG units to the N-terminus of tTF-NGR to further improve the antitumor profile of the molecule. Mono-PEGylation to the N-terminus did not change the procoagulatory activity of the tTF-NGR molecule as measured by Factor X activation. Experiments to characterize pharmacokinetics in mice showed a more than 1 log step higher mean area under the curve of PEG20k-tTF-NGR over tTF-NGR. Acute (24 h) tolerability upon intravenous application for the mono-PEGylated versus non-PEGylated tTF-NGR compounds was comparable. PEG20k-tTF-NGR showed clear antitumor efficacy in vivo against human tumor xenografts when systemically applied. However, site-directed mono-PEGylation to the N-terminus does not unequivocally improve the therapeutic profile of tTF-NGR.