Synthesis and evaluation of 64Cu-labeled monomeric and dimeric NGR peptides for MicroPET imaging of CD13 receptor expression

Anti-cancer properties of Sansalvamide A, its derivatives, and analogs: an updated review

As peptide-based therapies gain recognition for their potential anti-cancer activity, cyclic peptides like Sansalvamide A, a marine-derived cyclic depsipeptide, have emerged as a potential anti-cancer agent due to their potent activity against various cancer types in preclinical studies. This review offers a comprehensive overview of Sansalvamide A, including its sources, structure-activity relationship, and semi-synthetic derivatives. The review also aims to outline the mechanisms through which Sansalvamide A and its analogs exert their anti-proliferative effects and to discuss the need for enhancements in pharmacokinetic profiles for better clinical utility. An extensive literature search was conducted, focusing on studies that detailed the anti-cancer activity of Sansalvamide A, its pharmacokinetics, and mechanistic pathways. Data from both in vitro and in vivo studies were collated and analyzed. Sansalvamide A and its analogs demonstrated significant anti-cancer activity across various cancer models, mediated through Hsp 90 inhibition, Topoisomerase inhibition, and G0/G1 cell cycle arrest. However, their pharmacokinetic properties were identified as a significant limitation, requiring improvement for effective clinical translation. Despite its notable anti-cancer effects, the utility of Sansalvamide A is currently limited by its pharmacokinetic characteristics. Therefore, while Sansalvamide A exhibits promise as an anti-cancer agent, there is a compelling need for further clinical and toxicological studies and optimization of its pharmacokinetic profile to fully exploit its therapeutic potential alongside modern cancer therapies.

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.

Design and synthesis of novel N-terminal peptides of integrin and aminopeptidase are new finding for anticancer activity

The short peptides, containing the amino acid sequence asparagine-glycine-arginine (NGR) and arginine-glycine-aspartic acid (RGD), possess the strong binding ability to N (APN/CD13) aminopeptidase receptor and integrin proteins involved in antitumor properties are overexpressed. A novel short N-terminal modified hexapeptides P1 and P2 was designed and synthesized using the Fmoc-chemistry solid phase peptide synthesis protocol. Notably, the cytotoxicity of the MTT assay demonstrated the viability of normal and cancer cells up to lower peptide concentrations. Interestingly, both peptides show good anticancer activities against the four cancer cells and normal cells namely, Hep-2, HepG2, MCF-7, A375, and Vero and compared with standard drugs, doxorubicin and paclitaxel. Additionally, in silico studies were applied to predict the binding sites and binding orientation of the peptides for potential anticancer targets. Steady-state fluorescence measurements showed that peptide P1 exhibits preferential interactions with POPC/POPG anionic bilayers rather than the zwitterionic POPC lipid bilayers and peptide P2, did not show any preferential interaction with lipids bilayers. But impressively, peptide P2 shows anticancer activity due to the NGR/RGD motif. Circular dichroism studies demonstrated that the peptide's secondary structure changes only minimally upon binding to the anionic lipid bilayers.

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.

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.

Synthesis of 68Ga-Labeled cNGR-Based Glycopeptides and In Vivo Evaluation by PET Imaging

Tumor hypoxia induces angiogenesis, which is required for tumor cell survival. The aminopeptidase N receptor (APN/CD13) is an excellent marker of angiogenesis since it is overexpressed in angiogenic blood vessels and in tumor cells. Asparagine-glycine-arginine (NGR) peptide analogs bind selectively to the APN/CD13 recepto, therefore, they are important vector molecules in the development of a PET radiotracer which is capable of detecting APN-rich tumors. To investigate the effect of glycosylation and pegylation on in-vivo efficacy of an NGR-based radiotracer, two ⁶⁸Ga-labeled radioglycopeptides were synthesized. A lactosamine derivative was applied to glycosylation of the NGR derivative and PEG₄ moiety was used for pegylation. The receptor targeting potential and biodistribution of the radiopeptides were evaluated with in vivo PET imaging studies and ex vivo tissue distribution studies using B16-F10 melanoma tumor-bearing mice. According to these studies, all synthesized radiopeptides were capable of detecting APN expression in B16-F10 melanoma tumor. In addition, lower hepatic uptake, higher tumor-to background (T/M) ratio and prolonged circulation time were observed for the novel [⁶⁸Ga]-10 radiotracer due to pegylation and glycosylation, resulting in more contrasting PET imaging. These in vivo PET imaging results correlated well with the ex vivo tissue distribution data.

A comparison of the monomeric [68Ga]NODAGA-NGR and dimeric [68Ga]NOTA-(NGR)2 as aminopeptidase N ligand for positron emission tomography imaging in tumor-bearing mice

The aminopeptidase N (APN/CD13) is a key protein specifically expressed on activated endothelial cells and by various tumors, representing a promising target for molecular imaging and therapy of malignant diseases. It is known that the tripeptide NGR is a specific ligand for CD13, therefore radiolabeled NGR peptides are auspicious radiotracers for non-invasive imaging of CD13-positive tumors. From previous studies, it is known that the target affinity could be improved by molecules with multiple ligand sequences. Therefore, the aim of this study was to compare two NGR radioligands [⁶⁸Ga]NODAGA-NGR (NGR monomer) and [⁶⁸Ga]NOTA-(NGR)₂ (NGR dimer), the latter with two NGR ligand motifs, in vitro and in vivo. CD13 expression was determined by FACS in the human tumor cells A549, SKHep-1, and MDA-MB-231, followed by the investigation of the cell uptake of [⁶⁸Ga]NODAGA-NGR and [⁶⁸Ga]NOTA-(NGR)₂. For in vivo evaluation of [⁶⁸Ga]NODAGA-NGR and [⁶⁸Ga]NOTA-(NGR)₂, microPET and biodistribution were carried out in A549- and SKHep-1-bearing mice. After the final examination, tumors were cryo-conserved, cut, and stained against CD13 and CD31. A549 and SKHep-1 cells were identified as CD13 positive, whereas no CD13 expression was detected in MDA-MB-231 cells. The cell uptake study showed relatively low accumulation of both the NGR monomer and dimer in all tumor cell lines examined, with consistently higher cell uptake observed for the dimer than for the monomer. In vivo, [⁶⁸Ga]NODAGA-NGR and [⁶⁸Ga]NOTA-(NGR)₂ accumulated in the tumors, with slightly higher tumor-to-muscle ratio for the NGR dimer in A549 and SKHep-1. The tumor-to-liver ratio of the NGR dimer was diminished in comparison to the NGR monomer. This finding was confirmed by biodistribution, which revealed higher accumulation in liver and spleen for the NGR dimer. Immunohistochemical staining confirmed the CD13 expression in the tumors and tumor-associated vessels. In conclusion, both the [⁶⁸Ga]NODAGA-NGR and the [⁶⁸Ga]NOTA-(NGR)₂ were found to be suitable for PET imaging of CD13-positive tumors. Despite slight differences in tumor-to-background ratio and organ accumulation, both radiotracers can be considered comparable.

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.

Multimodality labeling of NGR-functionalized hyaluronan for tumor targeting and radiotherapy

Hyaluronan (HA) is a negatively charged linear polysaccharide that can interact with cluster determinant 44 (CD44) overexpressed cancers. However, HA can also bind to excess substrates in the human body leading to the lower specificity of tumor targeting. Conjugation of other targeting group to HA could enhance the uptake by cancer cell comparing to that of native HA. In this study, we develop the multi-functionalized HA (¹⁷⁷Lu-DOTA/Alexa647-HA100-N) for malignant tumor targeting. An asparagine-glycine-arginine (NGR) based peptide was selected for HA functionalization. The peptide is known to target CD13 receptor that is overexpressed in malignant tumors with abundant blood vessels, such as lung cancer. Furthermore, the fluorescent probe Alexa Fluor 647 for ex vivo/in vivo tracking and the radionuclide ¹⁷⁷Lu for radioactive therapy were both labeled on the material. The functionalized HA could be bound by lung cancer cells and breast cancer cells. In vivo fluorescent imaging showed that the material could accumulate in the tumor site for more than 96 h. The ¹⁷⁷Lu labeling of functionalized HA was stable for more 48 h at physiological conditions. The accumulation of ¹⁷⁷Lu-DOTA/Alexa647-HA100-N in the tumor of lung cancer (NCI-H292) bearing mice was 1.91±0.97%ID/g, and it was about 17 times higher than the value in blood. Conclusion: The multimodality labeled functional HA was successfully prepared and could be fluorescent trackable ex vivo and in vivo. It showed high potential to be used for malignant cancer radiotherapy for its specific targeting property to tumors and radiotoxicity from the labeled ¹⁷⁷Lu radionuclide.

In vivo preclinical assessment of novel 68Ga-labelled peptides for imaging of tumor associated angiogenesis using positron emission tomography imaging

Formation and growth of metastases require a new vascular network. Angiogenesis plays an essential role in the expansion and progression of most malignancies. A high number of molecular pathways regulate angiogenesis, including vascular endothelial growth factor (VEGF), α_vβ₃ integrin, matrix metalloproteinases (MMPs), or aminopeptidase N. The aim of this study is to involve new, easily accessible peptide sequences into the of neo-angiogenesis in malignant processes. Labelling of these peptide ligands with ⁶⁸Ga enable PET imaging of neo-vascularization.

Synthesis and evaluation of 68Ga-labeled dimeric cNGR peptide for PET imaging of CD13 expression with ovarian cancer xenograft

Introduction: Previous studies have shown that peptides containing the asparagine-glycine-arginine (NGR) sequence can specifically bind to CD13 (aminopeptidase N) receptor, a tumor neovascular biomarker that is over-expressed on the surface of angiogenic blood vessels and various tumor cells, and it plays an important role in angiogenesis and tumor progression. In the present study, we aimed to evaluate the efficacy of a gallium-68 (⁶⁸Ga)-labeled dimeric cyclic NGR (cNGR) peptide as a new molecular probe that binds to CD13 in vitro and in vivo.

Materials and Methods: A dimeric cNGR peptide conjugated with 1,4,7,10-tetraazacyclododecane-N,N',N'',N'''-tetraacetic acid (DOTA) was synthesized and labeled with ⁶⁸Ga. In vitro uptake and binding analyses of the ⁶⁸Ga- DOTA-c(NGR)₂ were performed in two ovarian tumor cell lines, ES2 and SKOV3, which had different CD13 expression patterns. An in vivo biodistribution study was performed in normal mice, and micro positron emission tomography (PET) imaging was conducted in nude mice bearing ES2 and SKOV3 tumors.

Results:⁶⁸Ga-DOTA-c(NGR)₂ was prepared with high radiochemical purity (>95%), and it was stable both in saline at room temperature and in bovine serum at 37°C for 3 h. In vitro studies showed that the uptake of ⁶⁸Ga-DOTA-c(NGR)₂ in ES2 cells was higher compared with SKOV3 cells, and such uptake could be blocked by the cold DOTA-c(NGR)₂. Biodistribution studies demonstrated that ⁶⁸Ga-DOTA-c(NGR)₂ was rapidly cleared from blood and mainly excreted from the kidney. MicroPET imaging of ES2 tumor xenografts showed the focal uptake of ⁶⁸Ga-DOTA-c(NGR)₂ in tumors from 1 to 1.5 h post-injection. The high-contrast tumor visualization occurred at 1 h, corresponding to the highest tumor/background ratio of 10.30±0.26. The CD13-specific tumor targeting of the ⁶⁸Ga-DOTA-c(NGR)₂ was further supported by the reduced uptake of the probe in ES2 tumors by co-injection of the unlabeled cold peptide. In SKOV3 tumor models, the tumor was not obviously visible under the same imaging conditions.

Conclusions:⁶⁸Ga-DOTA-c(NGR)₂ was easily synthesized, and it showed favorable CD13-specific targeting ability by in vitro data and microPET imaging with ovarian cancer xenografts. Collectively, ⁶⁸Ga-DOTA-c(NGR)₂ might be a potential PET imaging probe for non-invasive evaluation of the CD13 receptor expression in tumors.

Review: PET imaging with macro- and middle-sized molecular probes

Recent progress in radiolabeling of macro- and middle-sized molecular probes has been extending possibilities to use PET molecular imaging for dynamic application to drug development and therapeutic evaluation. Theranostics concept also accelerated the use of macro- and middle-sized molecular probes for sharpening the contrast of proper target recognition even the cellular types/subtypes and proper selection of the patients who should be treated by the same molecules recognition. Here, brief summary of the present status of immuno-PET, and then further development of advanced technologies related to immuno-PET, peptidic PET probes, and nucleic acids PET probes are described.

Current Concepts in Multi-Modality Imaging of Solid Tumor Angiogenesis

Simple Summary

The recent increase in the use of targeted molecular therapy including anti-angiogenetic agents in cancer treatment necessitate the use of robust tools to assess and guide treatment. Angiogenesis, the formation of new disorganized blood vessels, is used by tumor cells to grow and spread using different mechanisms that could be targeted by anti-angiogenetic agents. In this review, we discuss the biological principles of tumor angiogenesis and the imaging modalities that could provide information beyond gross tumor size and morphology to capture the efficacy of anti-angiogenetic therapeutic response.

Abstract

There have been rapid advancements in cancer treatment in recent years, including targeted molecular therapy and the emergence of anti-angiogenic agents, which necessitate the need to quickly and accurately assess treatment response. The ideal tool is robust and non-invasive so that the treatment can be rapidly adjusted or discontinued based on efficacy. Since targeted therapies primarily affect tumor angiogenesis, morphological assessment based on tumor size alone may be insufficient, and other imaging modalities and features may be more helpful in assessing response. This review aims to discuss the biological principles of tumor angiogenesis and the multi-modality imaging evaluation of anti-angiogenic therapeutic responses.

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.

Radiofluorinated GPC3-Binding Peptides for PET Imaging of Hepatocellular Carcinoma

PURPOSE

Hepatocellular carcinoma (HCC) remains one of the most challenging diseases worldwide. Glypican-3 (GPC-3) is a cell surface proteoglycan that is overexpressed on the membrane of HCC cells. The purpose of this study was to develop a target-specific radiofluorinated peptide for positron emission tomography (PET) imaging of GPC3 expression in hepatocellular carcinoma.

PROCEDURES

New GPC3-binding peptides (GP2076 and GP2633) were radiolabeled with F-18 using Al[¹⁸F]F labeling approach, and the resulting PET probes were subsequently subject to biological evaluations. A highly hydrophilic linker was incorporated into GP2633 with an aim of reducing the probe uptake in liver and increasing tumor-to-liver (T/L) contrast. Both GP2076 and GP2633 were radiolabeled using Al[¹⁸F]F chelation approach. The binding affinity, octanol/water partition coefficient, cellular uptake and efflux, and stability of both F-18 labeled peptides were tested. Tumor targeting efficacy and biodistribution of Al[¹⁸F]F-GP2076 and Al[¹⁸F]F-GP2633 were determined by PET imaging in HCC-bearing mice. Immunohistochemistry analyses were performed to compare the findings from PET scans.

RESULTS

Al[¹⁸F]F-GP2076 and Al[¹⁸F]F-GP2633 were rapidly radiosynthesized within 20 min in excellent radiochemical purity (> 97 %). Al[¹⁸F]F-GP2633 was determined to be more hydrophilic than Al[¹⁸F]F-GP2076 in terms of octanol/water partition coefficient. Both Al[¹⁸F]F-GP2076 and Al[¹⁸F]F-GP2633 demonstrated good in vitro and in vivo stability and binding specificity to GPC3-positive HepG2 cells. For PET imaging, Al[¹⁸F]F-GP2633 exhibited enhanced uptake in HepG2 tumor (%ID/g 3.37 ± 0.35 vs. 2.13 ± 0.55, P = 0.031) and reduced accumulation in liver (%ID/g 1.70 ± 0.26 vs. 3.70 ± 0.98, P = 0.027) at 60 min post-injection (pi) as compared to Al[¹⁸F]F-GP2076, resulting in significantly improved tumor-to-liver (T/L) contrast (ratio 2.00 ± 0.18 vs. 0.59 ± 0.14, P = 0.0004). Higher uptake of Al[¹⁸F]F-GP2633 in GPC3-positive HepG2 tumor was observed as compared to GPC3-negative McA-RH7777 tumor (%ID/g 3.37 ± 0.35 vs. 1.64 ± 0.03, P = 0.001) at 60 min pi, confirming GPC3-specific accumulation of Al[¹⁸F]F-GP2633 in HepG2 tumor.

CONCLUSION

The results demonstrated that Al[¹⁸F]F-GP2633 is a promising probe for PET imaging of GPC3 expression in HCC. Convenient preparation, excellent GPC3 specificity in HCC, and favorable excretion profile of Al[¹⁸F]F-GP2633 warrant further investigation for clinical translation. PET imaging with a GPC3-specific probe would provide clinicians with vital diagnostic information that could have a significant impact on the management of HCC patients.

Design, synthesis, and comparative evaluation of 99m Tc(CO)3 -labeled N-terminal and C-terminal modified asparagine-glycine-arginine peptide constructs

The present study describes modification of asparagine-glycine-arginine (NGR) peptide at N-terminally and C-terminally by introduction of a tridentate chelating scaffold via click chemistry reaction. The N-terminal and C-terminal modified peptides were radiometalated with [^99m Tc(CO)₃ ]⁺ precursor. The influence of these moieties at the two termini on the targeting properties of NGR peptide was determined by in vitro cell uptake studies and in vivo biodistribution studies. The two radiolabeled constructs did not exhibit any significant variation in uptake in murine melanoma B16F10 cells during in vitro studies. In vivo studies revealed nearly similar tumor uptake of N-terminally modified peptide construct 5 and C-terminally construct 6 at 2 h p.i. (1.9 ± 0.1 vs 2.4 ± 0.2% ID/g, respectively). The tumor-to-blood (T/B) and tumor-to-liver (T/L) ratios of the two radiometalated peptides were also quite similar. The two constructs cleared from all the major organs (heart, lungs, spleen, stomach, and blood) at 4 h p.i. (<1% ID/g). Blocking studies carried out by coinjection of cCNGRC peptide led to approximately 50% reduction in the tumor uptake at 2 h p.i. This work thus illustrates the possibility of convenient modification/radiometalation of NGR peptide at either N- or C-terminus without hampering tumor targeting and pharmacokinetics.

Peptide-Conjugated Nanoparticles as Targeted Anti-angiogenesis Therapeutic and Diagnostic in Cancer

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Targeting angiogenesis in the microenvironment of a tumor can enable suppression of tumor angiogenesis and delivery of anticancer drugs into the tumor. Anti-angiogenesis targeted delivery systems utilizing passive targeting such as Enhanced Permeability and Retention (EPR) and specific receptor-mediated targeting (active targeting) should result in tumor-specific targeting. One targeted anti-angiogenesis approach uses peptides conjugated to nanoparticles, which can be loaded with anticancer agents. Anti-angiogenesis agents can suppress tumor angiogenesis and thereby affect tumor growth progression (tumor growth arrest), which may be further reduced with the targetdelivered anticancer agent. This review provides an update of tumor vascular targeting for therapeutic and diagnostic applications, with conventional or long-circulating nanoparticles decorated with peptides that target neovascularization (anti-angiogenesis) in the tumor microenvironment.

Multifunctional Carbon-Based Nanomaterials: Applications in Biomolecular Imaging and Therapy

Molecular imaging has been widely used not only as an important detection technology in the field of medical imaging for cancer diagnosis but also as a theranostic approach for cancer in recent years. Multifunctional carbon-based nanomaterials (MCBNs), characterized by unparalleled optical, electronic, and thermal properties, have attracted increasing interest and demonstrably hold the greatest promise in biomolecular imaging and therapy. As such, it should come as no surprise that MCBNs have already revealed a great deal of potential applications in biomedical areas, such as bioimaging, drug delivery, and tumor therapy. Carbon nanomaterials can be categorized as graphene, single-walled carbon nanotubes, mesoporous carbon, nanodiamonds, fullerenes, or carbon dots on the basis of their morphologies. In this article, reports of the use of MCBNs in various chemical conjugation/functionalization strategies, focusing on their applications in cancer molecular imaging and imaging-guided therapy, will be comprehensively summarized. MCBNs show the possibility to serve as optimal candidates for precise cancer biotheranostics.

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.

PET imaging of Hsp90 expression in pancreatic cancer using a new 64Cu-labeled dimeric Sansalvamide A decapeptide

Heat shock protein 90 (Hsp90) plays a vital role in the progress of malignant disease and elevated Hsp90 expression has been reported in pancreatic cancer. In this study, we radiolabeled a dimeric Sansalvamide A derivative (Di-San A1) with ⁶⁴Cu, and evaluated the feasibility of using ⁶⁴Cu-Di-San A1 for PET imaging of Hsp90 expression in a mouse model of pancreatic cancer. A macrocyclic chelator NOTA (1,4,7-triazacyclononane-1,4,7-trisacetic acid) was conjugated to Di-San A1. ⁶⁴Cu-Di-San A1 was successfully prepared in a radiochemical yield > 97% with a radiochemical purity > 98%. ⁶⁴Cu-Di-San A1 is stable in PBS and mouse serum with > 92% of parent probe intact after 4 h incubation. The cell binding and uptake revealed that ⁶⁴Cu-Di-San A1 binds to Hsp90-positive PL45 pancreatic cancer cells, and the binding can be effectively blocked by an Hsp90 inhibitor (17AAG). For microPET study, ⁶⁴Cu-Di-San A1 shows good in vivo performance in terms of tumor uptake in nude mice bearing PL45 tumors. The Hsp90-specific tumor activity accumulation of ⁶⁴Cu-Di-San A1 was further demonstrated by significant reduction of PL45 tumor uptake with a pre-injected blocking dose of 17AAG. The ex vivo PET imaging and biodistribution results were consistent with the quantitative analysis of PET imaging, demonstrating good tumor-to-muscle ratio (5.35 ± 0.46) of ⁶⁴Cu-Di-San A1 at 4 h post-injection in PL45 tumor mouse xenografts. ⁶⁴Cu-Di-San A1 allows PET imaging of Hsp90 expression in PL45 tumors, which may provide a non-invasive method to quantitatively characterize Hsp90 expression in pancreatic cancer.

Evaluation of a dimeric-cRGD peptide for targeted PET-CT imaging of peripheral angiogenesis in diabetic mice

The α V  β3 integrin plays an important role in many physiological functions and pathological disorders. α V  β3 is minimally expressed in normal quiescent endothelial cells, but significantly upregulated during neovascularization. In this study, we evaluated a 64Cu-labeled dimeric cRGD tracer targeted at α V  β3 integrin and report its applicability to assess peripheral angiogenesis in diabetes mellitus (DM). We established a murine model of type-1 DM characterized by elevated glucose, glycated serum protein (GSP), and glycated hemoglobin A1c (HbA1c). We demonstrated that our imaging probe is specific to α V  β3 integrin under both normo- and hyperglycemic conditions. We found that the analysis of in vivo PET-CT images correlated well with gamma well counting (GWC). Both GWC and PET-CT imaging demonstrated increased uptake of 64Cu-NOTA-PEG4-cRGD2 in the ischemic hindlimb in contrast to non-ischemic control. GWC of the distal ischemic tissue from DM mice showed significantly lower probe accumulation than in non-DM mice. The immunofluorescence staining of the ischemic tissues showed a 3-fold reduction in CD31 and 4-fold reduction in the α V  β3 expression in DM vs. non-DM animals. In conclusion, we successfully demonstrated that diabetes-associated reductions in peripheral angiogenesis can be non-invasively detected with PET-CT imaging using targeted dimeric-cRGD probe.

64Cu-Labeled multifunctional dendrimers for targeted tumor PET imaging

We report the use of multifunctional folic acid (FA)-modified dendrimers as a platform to radiolabel with 64Cu for PET imaging of folate receptor (FR)-expressing tumors. In this study, amine-terminated generation 5 (G5) poly(amidoamine) dendrimers were sequentially modified with fluorescein isothiocyanate (FI), FA, and 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA), followed by acetylation of the remaining dendrimer terminal amines. The as-formed multifunctional DOTA-FA-FI-G5·NHAc dendrimers were then radiolabeled with 64Cu via the DOTA chelation. We show that the FA modification renders the dendrimers with targeting specificity to cancer cells overexpressing FR in vitro. Importantly, the radiolabeled 64Cu-DOTA-FA-FI-G5·NHAc dendrimers can be used as a nanoprobe for specific targeting of FR-overexpressing cancer cells in vitro and targeted microPET imaging of the FR-expressing xenografted tumor model in vivo. The developed 64Cu-labeled multifunctional dendrimeric nanoprobe may hold great promise to be used for targeted PET imaging of different types of FR-expressing cancer.

Preparation and comparative evaluation of 99m Tc-HYNIC-cNGR and 99m Tc-HYNIC-PEG2 -cNGR as tumor-targeting molecular imaging probes

The tripeptide sequence asparagine-glycine-arginine (NGR) specifically recognizes aminopeptidase N (APN or CD13) receptors highly expressed on tumor cells and vasculature. Thus, NGR peptides can precisely deliver therapeutic and diagnostic compounds to CD13 expressing cancer sites. In this regard, 2 NGR peptide ligands, HYNIC-c(NGR) and HYNIC-PEG₂ -c(NGR), were synthesized, radiolabeled with ^99m Tc, and evaluated in CD13-positive human fibrosarcoma HT-1080 tumor xenografts. The radiotracers, ^99m Tc-HYNIC-c(NGR) and ^99m Tc-HYNIC-PEG₂ -c(NGR), could be prepared in approximately 95% radiochemical purity and exhibited excellent in vitro and in vivo stability. The radiotracers were hydrophilic in nature with log P values being -2.33 ± 0.05 and -2.61 ± 0.08. The uptake of 2 radiotracers ^99m Tc-HYNIC-c(NGR) and ^99m Tc-HYNIC-PEG₂ -c(NGR) was similar in nude mice bearing human fibrosarcoma HT-1080 tumor xenografts, which was significantly reduced (P < .05) during blocking studies. The 2 radiotracers being hydrophilic cleared rapidly from blood, liver, and intestine and were excreted through renal pathway. The pharmacokinetics of ^99m Tc-labeled HYNIC peptide could not be modulated through introduction of PEG₂ unit, thus posing a challenge for studies with other linkers towards enhanced tumor uptake and retention.

177Lu-labeled cyclic Asn-Gly-Arg peptide tagged carbon nanospheres as tumor targeting radio-nanoprobes

This study explores the potential of ¹⁷⁷Lu-labeled carbon nanospheres as radio-nanoprobes for molecular imaging and therapy. The carboxyl functionalized surface of carbon nanospheres (CNS) was conjugated with [Gly-Gly-Gly-c(Asn-Gly-Arg)], G3-cNGR peptide through amide bond for targeting tumor vasculature and with [2-(4-Aminobenzyl)-1,4,7,10-tetraazacyclododecane tetraacetic acid], p-NH₂-Bz-DOTA for chelation with ¹⁷⁷Lu. The nanosphere-peptide conjugate, DOTA-CNS-cNGR, was characterized by dynamic light scattering and zeta potential measurements, IR and UV experiments and did not show any in vitro cytotoxicity. The pharmacokinetics and biodistribution of ¹⁷⁷Lu-labeled nanosphere-peptide conjugate, ¹⁷⁷Lu-DOTA-CNS-cNGR was compared with ¹⁷⁷Lu-DOTA-CNS (without the peptide) as well as with ¹⁷⁷Lu-DOTA-cNGR (without carbon nanospheres). The radiolabeled nanosphere-peptide conjugate exhibited higher tumor accumulation than nanosphere-free radiolabeled peptide. The accumulation of the two radiolabeled probes in the tumor reduced to half during blocking studies with unlabeled G3-cNGR peptide. ¹⁷⁷Lu-DOTA-CNS exhibited higher tumor uptake than ¹⁷⁷Lu-DOTA-CNS-cNGR but rapid clearance of the latter nanoprobe from non-target organs resulted in significantly higher (p < 0.05) tumor-to-blood and tumor-to-muscle ratios at 24 and 48 h p.i. It is evident from this study that carbon nanospheres conjugated to specific vectors shall form an important part of targeted radionanomedicine in future.

Comparative evaluation of 68 Ga-labeled NODAGA, DOTAGA, and HBED-CC-conjugated cNGR peptide chelates as tumor-targeted molecular imaging probes

The biological behavior of ⁶⁸ Ga-based radiopharmaceuticals can be significantly affected by the chelators' attributes (size, charge, lipophilicity). Thus, this study aimed at examining the influence of three different chelators, DOTAGA, NODAGA, and HBED-CC on the distribution pattern of ⁶⁸ Ga-labeled NGR peptides targeting CD13 receptors. ⁶⁸ Ga-DOTAGA-c(NGR), ⁶⁸ Ga-NODAGA-c(NGR), and ⁶⁸ Ga-HBED-CC-c(NGR) were observed to be hydrophilic with respective log p values being -3.5 ± 0.2, -3.3 ± 0.08, and -2.8 ± 0.14. The three radiotracers exhibited nearly similar uptake in human fibrosarcoma HT-1080 tumor cells with 86%, 63%, and 33% reduction during blocking studies with unlabeled cNGR peptide for ⁶⁸ Ga-DOTAGA-c(NGR), ⁶⁸ Ga-NODAGA-c(NGR), and ⁶⁸ Ga-HBED-CC-c(NGR), respectively, indicating higher receptor specificity of the first two radiotracers. The neutral radiotracer ⁶⁸ Ga-NODAGA-c(NGR) demonstrated better target-to-non-target ratios during in vivo studies compared to its negatively charged counterparts, ⁶⁸ Ga-DOTAGA-c(NGR) and ⁶⁸ Ga-HBED-CC-c(NGR). The three radiotracers had similar HT-1080 tumor uptake and being hydrophilic exhibited renal excretion with minimal uptake in non-target organs. Significant reduction (p < .005) in HT-1080 tumor uptake of the radiotracers was observed during blocking studies. It may be inferred from these studies that the three radiotracers are promising probes for in vivo imaging of CD13 receptor expressing cancer sites; however, ⁶⁸ Ga-NODAGA-c(NGR) is a better candidate.

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.

Synthesis, Chemical Characterization and Multiscale Biological Evaluation of a Dimeric-cRGD Peptide for Targeted Imaging of α V β 3 Integrin Activity

Cyclic peptides containing the Arg-Gly-Asp (RGD) sequence have been shown to specifically bind the angiogenesis biomarker α_Vβ₃ integrin. We report the synthesis, chemical characterization, and biological evaluation of two novel dimeric cyclic RGD-based molecular probes for the targeted imaging of α_Vβ₃ activity (a radiolabeled version, ⁶⁴Cu-NOTA-PEG₄-cRGD₂, for PET imaging, and a fluorescent version, FITC-PEG₄-cRGD₂, for in vitro work). We investigated the performance of this probe at the receptor, cell, organ, and whole-body levels, including its use to detect diabetes associated impairment of ischemia-induced myocardial angiogenesis. Both versions of the probe were found to be stable, demonstrated fast receptor association constants, and showed high specificity for α_Vβ₃ in HUVECs (K_d ~ 35 nM). Dynamic PET-CT imaging indicated rapid blood clearance via kidney filtration, and accumulation within α_Vβ₃-positive infarcted myocardium. ⁶⁴Cu-NOTA-PEG₄-cRGD₂ demonstrated a favorable biodistribution, slow washout, and excellent performance with respect to the quality of the PET-CT images obtained. Importantly, the ratio of probe uptake in infarcted heart tissue compared to normal tissue was significantly higher in non-diabetic rats than in diabetic ones. Overall, our probes are promising agents for non-invasive quantitative imaging of α_Vβ₃ expression, both in vitro and in vivo.

NGR-peptide-drug conjugates with dual targeting properties

Peptides containing the asparagine-glycine-arginine (NGR) motif are recognized by CD13/aminopeptidase N (APN) receptor isoforms that are selectively overexpressed in tumor neovasculature. Spontaneous decomposition of NGR peptides can result in isoAsp derivatives, which are recognized by RGD-binding integrins that are essential for tumor metastasis. Peptides binding to CD13 and RGD-binding integrins provide tumor-homing, which can be exploited for dual targeted delivery of anticancer drugs. We synthesized small cyclic NGR peptide-daunomycin conjugates using NGR peptides of varying stability (c[KNGRE]-NH2, Ac-c[CNGRC]-NH2 and the thioether bond containing c[CH2-CO-NGRC]-NH2, c[CH2-CO-KNGRC]-NH2). The cytotoxic effect of the novel cyclic NGR peptide-Dau conjugates were examined in vitro on CD13 positive HT-1080 (human fibrosarcoma) and CD13 negative HT-29 (human colon adenocarcinoma) cell lines. Our results confirm the influence of structure on the antitumor activity and dual acting properties of the conjugates. Attachment of the drug through an enzyme-labile spacer to the C-terminus of cyclic NGR peptide resulted in higher antitumor activity on both CD13 positive and negative cells as compared to the branching versions.

A Modular Dual-Labeling Scaffold That Retains Agonistic Properties for Somatostatin Receptor Targeting

Fluorescence-guided surgery is an emerging imaging technique that can enhance the ability of surgeons to detect tumors when compared with visual observation. To facilitate characterization, fluorescently labeled probes have been dual-labeled with a radionuclide to enable cross-validation with nuclear imaging. In this study, we selected the somatostatin receptor imaging agent DOTATOC as the foundation for developing a dual-labeled analog. We hypothesized that a customized dual-labeling approach with a multimodality chelation (MMC) scaffold would minimize steric effects of dye conjugation and retain agonist properties. Methods: An MMC conjugate (MMC-TOC) was synthesized on solid-phase and compared with an analog prepared using conventional methods (DA-TOC). Both analogs were conjugated to IRDye 800 using copper-free click chemistry. The resulting compounds, MMC(IR800)-TOC and DA(IR800)-TOC, were labeled with Cu and ⁶⁴Cu and tested in vitro in somatostatin receptor subtype 2-overexpressing HEK-293 cells to assess agonist properties, and in AR42J rat pancreatic cancer cells to determine receptor binding characteristics. Multimodality imaging was performed in AR42J xenografts. Results: Cu-MMC(IR800)-TOC demonstrated higher potency for cyclic adenosine monophosphate inhibition (half maximal effective concentration [EC₅₀]: 0.21 ± 0.18 vs. 1.38 ± 0.54 nM) and receptor internalization (EC₅₀: 41.9 ± 29.8 vs. 455 ± 299 nM) than Cu-DA(IR800)-TOC. Radioactive uptake studies showed that blocking with octreotide caused a dose-dependent reduction in ⁶⁴Cu-MMC(IR800)-TOC uptake whereas ⁶⁴Cu-DA(IR800)-TOC was not affected. In vivo studies revealed higher tumor uptake for ⁶⁴Cu-MMC(IR800)-TOC than ⁶⁴Cu-DA(IR800)-TOC (5.2 ± 0.2 vs. 3.6 ± 0.4 percentage injected dose per gram). In vivo blocking studies with octreotide reduced tumor uptake of ⁶⁴Cu-MMC(IR800)-TOC by 66%. Excretion of ⁶⁴Cu-MMC(IR800)-TOC was primarily through the liver and spleen whereas ⁶⁴Cu-DA(IR800)-TOC was cleared through the kidneys. Ex vivo analysis at 24 h confirmed PET/CT data by showing near-infrared fluorescence signal in tumors and a tumor-to-muscle ratio of 5.3 ± 0.8 as determined by γ-counting. Conclusion: The findings demonstrate that drug design affected receptor pharmacology and suggest that the MMC scaffold is a useful tool for the development of dual-labeled imaging agents.

68Ga-Chelation and comparative evaluation of N,N'-bis-[2-hydroxy-5-(carboxyethyl)benzyl]ethylenediamine-N,N'-diacetic acid (HBED-CC) conjugated NGR and RGD peptides as tumor targeted molecular imaging probes

Peptides containing RGD and NGR motifs display high affinity towards tumor vasculature molecular markers, integrin α_vβ₃ and CD13 receptors, respectively. In the present study, RGD and NGR peptides were conjugated with the novel acyclic chelator N,N'-bis-[2-hydroxy-5-(carboxyethyl)benzyl]ethylenediamine-N,N'-diacetic acid (HBED-CC) for radiolabeling with ⁶⁸Ga. The radiotracers [⁶⁸Ga-HBED-CC-c(NGR)] and [⁶⁸Ga-HBED-CC-c(RGD)] were quite hydrophilic with respective log P values being -2.8 ± 0.14 and -2.1 ± 0.17. ⁶⁸Ga-HBED-CC-c(RGD) displayed a significantly higher (p < 0.05) uptake in murine melanoma B16F10 tumors as compared to ⁶⁸Ga-HBED-CC-c(NGR) indicating its higher specificity towards integrin α_vβ₃-positive tumors. The two radiotracers showed similar uptake in CD13-positive human fibrosarcoma HT-1080 tumor xenografts (∼1.5 ± 0.2% ID g^-1). The tumor uptake of the two radiotracers was significantly reduced (p < 0.05) in both animal models during blocking studies. The tumor-to-blood ratio was observed to be ∼2-2.5 for the two radiotracers, whereas the tumor-to-muscle ratio was significantly higher (p < 0.005) for ⁶⁸Ga-HBED-CC-c(RGD) in the two animal models. The two radiotracers ⁶⁸Ga-HBED-CC-c(NGR) and ⁶⁸Ga-HBED-CC-c(RGD) exhibited renal excretion with rapid clearance from blood and other non-target organs. Thus, ⁶⁸Ga-chelated HBED-CC conjugated NGR and RGD peptides expressed features conducive towards development as tumor targeted molecular imaging probes. This study further opens avenues for the successful conjugation of different peptides with the acyclic chelator HBED-CC and expansion of ⁶⁸Ga-based radiopharmaceuticals.

In Vivo Tumor Angiogenesis Imaging Using Peptide-Based Near-Infrared Fluorescent Probes

Near-infrared fluorescence (NIRF) imaging is an emerging imaging technique for studying diseases at the molecular level. Optical imaging with a near-infrared emitting fluorophore for targeting tumor angiogenesis offers a noninvasive method for early tumor detection and efficient monitoring of tumor response to anti-angiogenesis therapy. CD13 receptor, a zinc-dependent membrane-bound ectopeptidase, plays important roles in regulating tumor angiogenesis and the growth of new blood vessels. In this chapter, we use CD13 receptor as an example to demonstrate how to construct CD13-specific NGR-containing peptides via bioorthogonal click chemistry for visualizing and quantifying the CD13 receptor expression in vivo by means of NIRF optical imaging.

Direct flow separation strategy, to isolate no-carrier-added 90Nb from irradiated Mo or Zr targets

90Nb has an intermediate half-life of 14.6 h, a high positron branching of 53% and optimal β + emission energy of only E mean 0.35 MeV per decay. These favorable characteristics suggest it may be a potential candidate for application in immuno-PET. Our recent aim was to conduct studies on distribution coefficients for ZrIV and NbV in mixtures of HCl/H2O2 and HCl/oxalic acid for anion exchange resin (AG 1 × 8) and UTEVA resin to develop a “direct flow” separation strategy for 90Nb. The direct flow concept refers to a separation accomplished using a single eluent on multiple columns, effectively streamlining the separation process and increasing the time efficiency. Finally, we also demonstrated that this separation strategy is applicable to the production of the positron emitter 90Nb via the irradiation of molybdenum targets and isolation of 90Nb from the irradiated molybdenum target.

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.

NGR Tumor-Homing Peptides: Structural Requirements for Effective APN (CD13) Targeting

Cyclic CNGRC (cCNGRC) peptides are very important targeting ligands for Aminopeptidase N (APN or CD13), which is overexpressed on the surface of many cancer cells. In this work we have (1) developed an efficient solid-phase synthesis and (2) tested on purified porcine APN and APN-expressing human cells two different classes of cCNGRC peptides: the first carrying a biotin affinity tag or a fluorescent tag attached to the carboxyl Arg-Cys-COOH terminus and the second with the tags attached to the amino H2N-Cys-Asn terminus. Carboxyl-terminus functionalized cCNGRC peptides 3, 6, and 8 showed good affinity for porcine APN and very good capacity to target and be internalized into APN-expressing cells. In contrast, amino-terminus functionalized cCNGRC peptides 4, 5, and 7 displayed significantly decreased affinity and targeting capacity. These results, which are in agreement with the recently reported X-ray structure of a cCNGRC peptide bound to APN showing important stabilizing interactions between the unprotected cCNGRC amino terminus and the APN active site, indicate that the carboxyl and not the amino-terminus of cCNGRC peptides should be used as a "handle" for the attachment of toxic payloads for therapy or isotopically labeled functions for imaging and nuclear medicine.

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.

Development of cyclic NGR peptides with thioether linkage: structure and dynamics determining deamidation and bioactivity

NGR peptides that recognize CD13 receptors in tumor neovasculature are of high interest, in particular due to their potential applications in drug targeting. Here we report the synthesis and structural analysis of novel thioether bond-linked cyclic NGR peptides. Our results show that their chemostability (resistance against spontaneous decomposition forming isoAsp and Asp derivatives) strongly depends on both sample handling conditions and structural properties. A significant correlation was found between chemostability and structural measures, such as NH(Gly)-CO(Asn-sc) distances. The side-chain orientation of Asn is a key determining factor; if it is turned away from HN(Gly), the chemostability increases. Structure stabilizing factors (e.g., H-bonds) lower their internal dynamics, and thus biomolecules become even more resistant against spontaneous decomposition. The effect of cyclic NGR peptides on cell adhesion was examined in A2058 melanoma cell lines. It was found that some of the investigated peptides gradually increased cell adhesion with long-term characteristics, indicating time-dependent formation of integrin binding isoAsp derivatives that are responsible for the adhesion-inducing effect.

A novel multivalent (99m)Tc-labeled EG2-C4bpα antibody for targeting the epidermal growth factor receptor in tumor xenografts

INTRODUCTION

The C4b binding protein (C4bp) α/β-chain C-terminal effectively induces polymerization during protein synthesis. Using this fragment and the single-domain antibody EG2, which targets the epidermal growth factor receptor (EGFR), we generated the novel multimeric antibody EG2-C4bpα. We radiolabeled EG2-C4bpα with (99m)Tc and evaluated its targeting efficiency and pharmacokinetics in tumor xenografts.

METHODS

EGFR expression and EGFR-EG2-C4bpα binding was evaluated in A431 and OCM-1 cells by Western blotting and flow cytometry, respectively. EG2-C4bpα was radiolabeled with [(99m)Tc(CO)3(OH2)3](+) using a tricarbonyl vial followed by purification on a PD-10 column. In vitro studies with (99m)Tc-EG2-C4bpα were performed in A431 and/or OCM-1 cells. Single photon emission computed tomography (SPECT) imaging and biodistribution studies were carried out in (99m)Tc-EG2-C4bpα-injected mice bearing A431- and OCM-1-derived tumors. EGFR immunofluorescent staining in A431 and OCM-1 tumors was performed.

RESULTS

A431 cells showed higher EGFR expression levels than OCM-1 cells, and flow cytometry confirmed EG2-C4bpα bound more A431 cells than OCM-1 cells. (99m)Tc-EG2-C4bpα was successfully prepared with radiochemical yields of 30.3-50.4%. The binding affinity of (99m)Tc-EG2-C4bpα to A431 cells was approximately 20 nM. (99m)Tc-EG2-C4bpα specifically bound A431 cells and this binding was blocked by 41% in the presence of 50 nM excess unlabeled EG2-C4bpα. In vivo radioactivity uptake in A431 tumors was detected 2h after (99m)Tc-EG2-C4bpα administration and sustained up to 18h. The highest ratio of A431 tumor-to-muscle and tumor-to-blood was 3.69 ± 0.48 at 10h and 0.77 ± 0.14 at 20 h, respectively. Excess unlabeled EG2-C4bpα blocked radioactivity uptake in A431 tumors by 55% at 10h. (99m)Tc-EG2-C4bpα was barely detectable in OCM-1 tumors, and biodistribution analysis confirmed that radioactivity uptake was significantly lower than in A431 tumors.

CONCLUSIONS

(99m)Tc-EG2-C4bpα specifically and efficiently targets EGFR over-expressing tumors suggesting that EG2-C4bpα may be a promising antibody alternative for future diagnostic application and potential radioimmunotherapy. However, the high activity in the blood and liver, and the relative low ratio of tumor-to-blood should be noticed and improved.

In vivo imaging of Aminopeptidase N (CD13) receptors in experimental renal tumors using the novel radiotracer (68)Ga-NOTA-c(NGR)

PURPOSE

Aminopeptidase N (APN/CD13) plays an important role in tumor neoangiogenic process and the development of metastases. Furthermore, it may serve as a potential target for cancer diagnosis and therapy. Previous studies have already shown that asparagine-glycine-arginine (NGR) peptides specifically bind to APN/CD13. The aim of the study was to synthesize and investigate the APN/CD13 specificity of a novel (68)Ga-labeled NOTA-c(NGR) molecule in vivo using miniPET.

METHODS

c[KNGRE]-NH2 peptide was conjugated with p-SCN-Bn-NOTA and was labeled with Ga-68 ((68)Ga-NOTA-c(NGR)). Orthotopic and heterotopic transplanted mesoblastic nephroma (NeDe) bearing Fischer-344 rats were prepared, on which biodistribution studies and miniPET scans were performed for both (68)Ga-NOTA-c(NGR) and ανβ3 integrin selective (68)Ga-NODAGA-[c(RGD)]2 tracers. APN/CD13 receptor expression of NeDe tumors and metastases was analyzed by western blot.

RESULTS

(68)Ga-NOTA-c(NGR) was produced with high specific activity (5.13-5.92GBq/μmol) and with excellent radiochemical purity (95%<), at all cases. Biodistribution studies in normal rats showed that uptake of the (68)Ga-NOTA-c(NGR) was significantly (p⩽0.05) lower in abdominal organs in comparison with (68)Ga-NODAGA-[c(RGD)]2. Both radiotracers were mainly excreted from the kidney. In NeDe tumor bearing rats higher (68)Ga-NOTA-c(NGR) accumulation was found in the tumors than that of the (68)Ga-NODAGA-[c(RGD)]2. Using orthotopic transplantation, metastases were developed which showed specific (68)Ga-NOTA-c(NGR) uptake. Western blot analysis confirmed the presence of APN/CD13 expression in NeDe tumors and metastases.

CONCLUSION

Our novel radiotracer (68)Ga-NOTA-c(NGR) showed specific binding to the APN/CD13 expressed ortho- and heterotopic transplanted NeDe tumors. Therefore, (68)Ga-NOTA-c(NGR) is a suitable tracer for the detection of APN/CD13 positive tumors and metastases in vivo.

In vivo NIRF imaging-guided delivery of a novel NGR-VEGI fusion protein for targeting tumor vasculature

Pathological angiogenesis is crucial in tumor growth, invasion and metastasis. Previous studies demonstrated that the vascular endothelial growth inhibitor (VEGI), a member of the tumor necrosis factor superfamily, can be used as a potent endogenous inhibitor of tumor angiogenesis. Molecular probes containing the asparagine-glycine-arginine (NGR) sequence can specifically bind to CD13 receptor which is overexpressed on neovasculature and several tumor cells. Near-infrared fluorescence (NIRF) optical imaging for targeting tumor vasculature offers a noninvasive method for early detection of tumor angiogenesis and efficient monitoring of response to anti-tumor vasculature therapy. The aim of this study was to develop a new NIRF imaging probe on the basis of an NGR-VEGI protein for the visualization of tumor vasculature. The NGR-VEGI fusion protein was prepared from prokaryotic expression, and its function was characterized in vitro. The NGR-VEGI protein was then labeled with a Cy5.5 fluorophore to afford Cy5.5-NGR-VEGI probe. Using the NIRF imaging technique, we visualized and quantified the specific delivery of Cy5.5-NGR-VEGI protein to subcutaneous HT-1080 fibrosarcoma tumors in mouse xenografts. The Cy5.5-NGR-VEGI probe exhibited rapid HT-1080 tumor targeting, and highest tumor-to-background contrast at 8 h post-injection (pi). Tumor specificity of Cy5.5-NGR-VEGI was confirmed by effective blocking of tumor uptake in the presence of unlabeled NGR-VEGI (20 mg/kg). Ex vivo NIRF imaging further confirmed in vivo imaging findings, demonstrating that Cy5.5-NGR-VEGI displayed an excellent tumor-to-muscle ratio (18.93 ± 2.88) at 8 h pi for the non-blocking group and significantly reduced ratio (4.92 ± 0.75) for the blocking group. In conclusion, Cy5.5-NGR-VEGI provided highly sensitive, target-specific, and longitudinal imaging of HT-1080 tumors. As a novel theranostic protein, Cy5.5-NGR-VEGI has the potential to improve cancer treatment by targeting tumor vasculature.

68Ga-labeled cyclic NGR peptide for microPET imaging of CD13 receptor expression

Peptides containing the asparagines-glycine-arginine (NGR) motif have been identified as specific ligands binding to CD13/aminopeptidase N (APN) receptor, a tumor neovascular biomarker. In this study, we synthesized a novel NGR-containing peptide (NOTA-G₃-NGR), and labeled NOTA-G₃-NGR with ⁶⁸Ga (t_1/2 = 67.7 min). The resulting ⁶⁸Ga-NOTA-G₃-NGR peptide was subject to in vitro and in vivo characterization. The microPET imaging results revealed that the ⁶⁸Ga-NOTA-G₃-NGR peptide exhibits rapid and specific tumor uptake, and high tumor-to-background contrast in a subcutaneous HT-1080 fibrosarcoma mouse model. We concluded that the ⁶⁸Ga-NOTA-G₃-NGR peptide has potential in the diagnosis of CD13-targeted tumor angiogenesis.

Evaluation of (99m)Tc-probestin SPECT as a novel technique for noninvasive imaging of kidney aminopeptidase N expression

Aminopeptidase N (APN; CD13; EC 3.4.11.2) is a zinc-dependent membrane-bound exopeptidase that catalyzes the removal of N-terminal amino acids from peptides. APN is known to be highly expressed on renal cortical proximal tubules. APN expression levels are markedly decreased under the influence of nephrotoxins and in the tumor regions of renal cancers. Thus, molecular imaging of kidney APN expression could provide pathophysiological information about kidneys noninvasively. Probestin is a potent APN inhibitor and binds to APN. Abdominal SPECT imaging was conducted at 1 h postinjection of (99m)Tc-probestin in a group of 12 UPII-SV40T transgenic and wild-type mice. UPII-SV40T mice spontaneously develop urothelial carcinoma in situ and invasive transitional cell carcinoma (TCC) that invade kidneys. Histopathology and immunohistochemistry analysis were used to confirm the presence of tumor and to evaluate APN expression in kidney. Radioactivity in normal tissue regions of renal cortex was clearly visible in SPECT images, whereas tumor regions of renal cortex displayed significantly lower or no radioactivity uptake. Histopathological analysis of kidney sections showed normal morphology for both renal pelvic and cortical regions in wild-type mice and abnormal morphology in some transgenic mice. Proliferating cell nuclear antigen staining confirmed the presence of tumor in those abnormal regions. Immunohistochemical analysis of kidney sections using anti-CD13 antibody showed significantly lower APN expression in tumor regions compared to normal regions. Results obtained in this study demonstrate the potential use of (99m)Tc-probestin SPECT as a novel technique for noninvasive imaging of kidney APN expression.

MicroPET imaging of CD13 expression using a (64)Cu-labeled dimeric NGR peptide based on sarcophagine cage

CD13 receptor as a tumor vasculature biomarker has attracted great attention in cancer research. Through phage display screening, NGR-containing peptides have been characterized as specific ligands binding to CD13 receptor. In this study, a (64)Cu-labeled dimeric NGR peptide based on sarcophagine cage was synthesized and evaluated for micropositron emission tomography (PET) imaging of CD13 expression in vivo. Macrocyclic chelating agent (sarcophagine cage) was conjugated with two azide moieties, followed by mixing with an alkyne-containing NGR peptide to rapidly provide the Sar-NGR2 peptide by click chemistry. Radiolabeling of Sar-NGR2 with (64)Cu was achieved in >90% decay-corrected yield with radiochemical purity of >99%. The cell uptake study showed that (64)Cu-Sar-NGR2 binds to CD13-positive HT-1080 cells, but not to CD13-negative MCF-7 cells. MicroPET imaging results revealed that (64)Cu-Sar-NGR2 exhibits good tumor uptake in CD13-positive HT-1080 xenografts and significantly lower tumor uptake in CD13-negative MCF-7 xenografts. The CD13-specific binding of (64)Cu-Sar-NGR2 was further verified by significant reduction of tumor uptake in HT-1080 tumor xenografts with coinjection of a nonradiolabeled NGR peptide. The biodistribution results demonstrated good tumor/muscle ratio (8.28 ± 0.37) of (64)Cu-Sar-NGR2 at 24 h pi in HT-1080 tumor xenografts, which is in agreement with the quantitative analysis of microPET imaging. In conclusion, sarcophagine cage has been successfully applied to the construction of a (64)Cu-labeled dimeric NGR-containing peptide. In vitro and in vivo studies demonstrated that (64)Cu-Sar-NGR2 is a promising PET probe for imaging CD13 expression in living mice.

PET imaging of cardiac wound healing using a novel [68Ga]-labeled NGR probe in rat myocardial infarction

PURPOSE

Peptides containing the asparagine-glycine-arginine (NGR) motif bind to aminopeptidase N (CD13), which is expressed on inflammatory cells, endothelial cells, and fibroblasts. It is unclear whether radiolabeled NGR-containing tracers could be used for in vivo imaging of the early wound-healing phase after myocardial infarction (MI) using positron emission tomography (PET).

PROCEDURES

Uptake of novel tracer [(68)Ga]NGR was assessed together with [(68)Ga]arginine-glycine-aspartic acid ([(68)Ga]RGD) and 2-deoxy-2-[(18) F]fluoro-D-glucose after myocardial ischemia/reperfusion (MI/R) injury using μ-PET and autoradiography, and relative expressions of CD13 and integrin β3 were assessed in fibroblasts, inflammatory cells, and endothelial cells by immunohistochemistry.

RESULTS

In the infarcted myocardium, uptake of [(68)Ga]NGR was maximal from days 3 to 7 after MI/R, and correlated with fibroblast and inflammatory cell infiltration as well as [(68)Ga]RGD uptake.

CONCLUSIONS

[(68)Ga]NGR allows noninvasive and sequential determination of CD13 expression in fibroblasts and inflammatory cells by PET. This will facilitate monitoring of CD13 in the individual wound healing processes, allowing patient-specific therapies to improve outcome after MI.

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

Peptides containing asparagine-glycine-arginine (NGR) and arginine-glycine-aspartic acid (RGD) sequence are being developed for tumor angiogenesis-targeted imaging and therapy. The aim of this study was to compare the efficacy of NGR- and RGD-based probes for imaging tumor angiogenesis in HT-1080 tumor xenografts. Two PET probes, (68)Ga-NOTA-G₃-NGR2 and ⁶⁸Ga-NOTA-G₃-RGD2, were successfully prepared. In vitro stability, partition coefficient, tumor cell binding, as well as in vivo biodistribution properties were also analyzed for both PET probes. The results revealed that the two probes were both hydrophilic and stable in vitro and in vivo, and they were excreted predominately and rapidly through the kidneys. For both probes, the higher tumor uptake and lower accumulation in vital organs were determined. No significant difference between two probes was observed in terms of tumor uptake and the in vivo biodistribution properties. We concluded that these two probes are promising in tumor angiogenesis imaging. ⁶⁸Ga-NOTA-G₃-NGR2 has the potential as an alternative for PET imaging in patients with fibrosarcoma, and it may offer an opportunity to noninvasively monitor CD13-targeted therapy.