Radiotracers for different angiogenesis receptors in a melanoma model

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.

Radionuclide imaging and therapy directed towards the tumor microenvironment: a multi-cancer approach for personalized medicine

Targeted radionuclide theranostics is becoming more and more prominent in clinical oncology. Currently, most nuclear medicine compounds researched for cancer theranostics are directed towards targets expressed in only a small subset of cancer types, limiting clinical applicability. The identification of cancer-specific targets that are (more) universally expressed will allow more cancer patients to benefit from these personalized nuclear medicine–based interventions. A tumor is not merely a collection of cancer cells, it also comprises supporting stromal cells embedded in an altered extracellular matrix (ECM), together forming the tumor microenvironment (TME). Since the TME is less genetically unstable than cancer cells, and TME phenotypes can be shared between cancer types, it offers targets that are more universally expressed. The TME is characterized by the presence of altered processes such as hypoxia, acidity, and increased metabolism. Next to the ECM, the TME consists of cancer-associated fibroblasts (CAFs), macrophages, endothelial cells forming the neo-vasculature, immune cells, and cancer-associated adipocytes (CAAs). Radioligands directed at the altered processes, the ECM, and the cellular components of the TME have been developed and evaluated in preclinical and clinical studies for targeted radionuclide imaging and/or therapy. In this review, we provide an overview of the TME targets and their corresponding radioligands. In addition, we discuss what developments are needed to further explore the TME as a target for radionuclide theranostics, with the hopes of stimulating the development of novel TME radioligands with multi-cancer, or in some cases even pan-cancer, application.

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.

The Influence of a Polyethylene Glycol Linker on the Metabolism and Pharmacokinetics of a 89Zr-Radiolabeled Antibody

Most experimental work in the space of bioconjugation chemistry focuses on using new methods to construct covalent bonds between a cargo molecule and a protein of interest such as a monoclonal antibody (mAb). Bond formation is important for generating new diagnostic tools, yet when these compounds advance to preclinical in vitro and in vivo studies, and later for translation to the clinic, understanding the fate of potential metabolites that arise from chemical or enzymatic degradation of the construct is important to obtain a full picture of the pharmacokinetic performance of a new compound. In the context of designing new bioconjugate methods for labeling antibodies with the positron-emitting radionuclide ⁸⁹Zr, we previously developed a photochemical process for making ⁸⁹Zr-mAbs. Experimental studies on [⁸⁹Zr]ZrDFO-PEG₃-azepin-mAb constructs revealed that incorporation of the tris-polyethylene glycol (PEG₃) linker improved the aqueous phase solubility and radiochemical conversion. However, the use of a PEG₃ linker also has an impact on the whole-body residence time of the construct, leading to a more rapid excretion of the ⁸⁹Zr activity when compared with radiotracers that lack the PEG₃ chain. In this work, we investigated the metabolic fate of eight possible metabolites that arise from the logical disconnection of [⁸⁹Zr]ZrDFO-PEG₃-azepin-mAb at bonds which are susceptible to chemical or enzymatic cleavage. Synthesis combined with ⁸⁹Zr-radiolabeling, small-animal positron emission tomography imaging at multiple time points from 0 to 20 h, and measurements of the effective half-life for whole-body excretion are reported. The conclusions are that the use of a PEG₃ linker is non-innocent in terms of its impact on enhancing the metabolism of [⁸⁹Zr]ZrDFO-PEG₃-azepin-mAbs. In most cases, degradation can produce metabolites that are rapidly eliminated from the body, thereby enhancing image contrast by reducing nonspecific accumulation and retention of ⁸⁹Zr in background organs such as the liver, spleen, kidney, and bone.

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.

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.

Functional investigation of bone implant viability using radiotracers in a new model of osteonecrosis

OBJECTIVES:

Conventional imaging methods are excellent for the morphological characterization of the consequences of osteonecrosis; however, only specialized techniques have been considered useful for obtaining functional information. To explore the affinity of radiotracers for severely devascularized bone, a new mouse model of isolated femur implanted in a subcutaneous abdominal pocket was devised. To maintain animal mobility and longevity, the femur was harvested from syngeneic donors. Two technetium-99m-labeled tracers targeting angiogenesis and bone matrix were selected.

METHODS:

Medronic acid and a homodimer peptide conjugated with RGDfK were radiolabeled with technetium-99m, and biodistribution was evaluated in Swiss mice. The grafted and control femurs were evaluated after 15, 30 and 60 days, including computed tomography (CT) and histological analysis.

RESULTS:

Radiolabeling achieved high (>95%) radiochemical purity. The biodistribution confirmed good blood clearance 1 hour after administration. For 99mTc-hydrazinonicotinic acid (HYNIC)-E-[c(RGDfK)2, remarkable renal excretion was observed compared to 99mTc-methylene diphosphonate (MDP), but the latter, as expected, revealed higher bone uptake. The results obtained in the control femur were equal at all time points. In the implanted femur, 99mTc-HYNIC-E-[c(RGDfK)2 uptake was highest after 15 days, consistent with early angiogenesis. Regarding 99mTc-MDP in the implant, similar uptake was documented at all time points, consistent with sustained bone viability; however, the uptake was lower than that detected in the control femur, as confirmed by histology.

CONCLUSIONS:

1) Graft viability was successfully diagnosed using radiotracers in severely ischemic bone at all time points. 2) Analogously, indirect information about angiogenesis could be gathered using 999mTc-HYNIC-E-[c(RGDfK)2. 3) These techniques appear promising and warrant further studies to determine their potential clinical applications.

Synthesis and evaluation of novel Tc-99m labeled NGR-containing hexapeptides as tumor imaging agents

Asparagine-glycine-arginine (NGR)-containing peptides targeting aminopeptidase N (APN)/CD13 can be an excellent candidate for targeting ligands in molecular tumor imaging. In this study, we developed two NGR-containing hexapeptides, and evaluated the diagnostic performance of Tc-99m labeled hexapeptides as molecular imaging agents in an HT-1080 fibrosarcoma-bearing murine model. Peptides were synthesized using Fmoc solid-phase peptide synthesis. Radiochemical purity of Tc-99m was evaluated using instant thin-layer chromatography. The uptake of two NGR-containing hexapeptides within HT-1080 cells was evaluated in vitro. In HT-1080 fibrosarcoma tumor-bearing mice, gamma images were acquired. A biodistribution study was performed to calculate percentage of the injected dose per gram of tissue (%ID/g). Two hexapeptides, glutamic acid-cysteine-glycine (ECG)-NGR and NGR-ECG were successfully synthesized. After radiolabeling procedures with Tc-99m, the complexes Tc-99m hexapeptides were prepared in high yield. The uptake of Tc-99m ECG-NGR within the tumor cells had been assured by in vitro studies. The gamma camera imaging in the murine model showed that Tc-99m ECG-NGR was accumulated substantially in the subcutaneously engrafted tumor. However, Tc-99m NGR-ECG was accumulated minimally in the tumor. Two NGR-containing hexapeptides, ECG-NGR and NGR-ECG were developed as molecular imaging agents to target APN/CD13 in HT-1080 fibrosarcoma. Tc-99m ECG-NGR showed a significant uptake in the tumor, and it is a good candidate for tumor imaging.

Evaluation of ανβ3-mediated tumor expression with a 99mTc-labeled ornithine-modified RGD derivative during glioblastoma growth in vivo

In this study, a novel way of distinguishing the intrinsic relationship between ανβ3 integrin targeting and detection of tumor growth by using a radiolabeled tracer based on a cyclic Arg-Gly-Asp (RGD) peptide was provided. The potential of the in vivo scintigraphic imaging of the developing vasculature from the early stage of tumor growth was evaluated. Alongside with the scintigraphic images, biodistribution studies were performed at distinct time points to validate this noninvasive imaging approach. The ability to noninvasively assess the tumor growth of ανβ3 integrin-positive glioblastoma tumors provides a method to better understand tumor angiogenesis in vivo and allows for a direct assessment of anti-integrin treatment efficacy.

Radiolabeling and biological evaluation of the GX1 and RGD-GX1 peptide sequence for angiogenesis targeting

INTRODUCTION

Aiming to develop a novel (99m)Tc-labeled imaging agent, for angiogenesis and tumor receptors, two peptides obtained from phage display library, namely GX1 and the heterodimer RGD-GX1, were synthesized in a cyclic conformation. They were radiolabeled with (99m)Tc, employing the HYNIC chelator, for radiochemical evaluation and biological properties.

METHODS

Radiolabeling, radiochemical control, plasma protein binding, and partition coefficient were assessed for both radioconjugates. Biodistribution in healthy Balb/c mice was carried out, in order to evaluate the biological behaviour of the radiocomplexes.

RESULTS

The conjugates displayed a rather similar pharmacokinetic profile. They were prepared with high radiochemical purity (>96%), and both were hydrophilic (log P of -2.25 and -2.51 respectively). Preferential renal excretion was observed. Kidney uptake (42.31±5.35 %ID/g) for (99m)Tc-HYNIC-E-[c(RGDfk)-c(GX1)], 1h post-injection was about three times higher than the uptake of (99m)Tc-HYNIC-PEG4-c(GX1) (11.92±4.77%ID/g). Total blood, bone and muscle values revealed a slightly slower clearance for the RGD-GX1 radiocomplex.

CONCLUSION

The high radiochemical purity achieved, and the similar in vivo profile observed for both radioconjugates, make them potential candidates for radiopharmaceuticals for tumor imaging. Further investigations of binding affinity, and uptake of GX1 and RGD-GX1 peptides in tumor models, are warranted.

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.

Radiolabeled NGR phage display peptide sequence for tumor targeting

The asparagine-glycine-arginine (NGR) peptide sequence found by phage display, was radiolabeled with technetium-99m and tested in different tumor models. Similar uptake occurred with ovarian and lung tumor cells. Biodistribution of the radiotracer revealed predominant renal excretion with more substantial uptake in animals bearing ovarian tumor cells. In contrast imaging studies indicated better visualization for lung tumor. NGR peptide was characterized as a promising diagnostic candidate, particularly for lung cancer. Improvements are envisaged using NGR combined with RGD as a heterodimer molecule.