In vivo characterization of 68Ga-NOTA-VEGF 121 for the imaging of VEGF receptor expression in U87MG tumor xenograft models

Radiopharmaceuticals and their applications in medicine

Radiopharmaceuticals involve the local delivery of radionuclides to targeted lesions for the diagnosis and treatment of multiple diseases. Radiopharmaceutical therapy, which directly causes systematic and irreparable damage to targeted cells, has attracted increasing attention in the treatment of refractory diseases that are not sensitive to current therapies. As the Food and Drug Administration (FDA) approvals of [177Lu]Lu-DOTA-TATE, [177Lu]Lu-PSMA-617 and their complementary diagnostic agents, namely, [68Ga]Ga-DOTA-TATE and [68Ga]Ga-PSMA-11, targeted radiopharmaceutical-based theranostics (radiotheranostics) are being increasingly implemented in clinical practice in oncology, which lead to a new era of radiopharmaceuticals. The new generation of radiopharmaceuticals utilizes a targeting vector to achieve the accurate delivery of radionuclides to lesions and avoid off-target deposition, making it possible to improve the efficiency and biosafety of tumour diagnosis and therapy. Numerous studies have focused on developing novel radiopharmaceuticals targeting a broader range of disease targets, demonstrating remarkable in vivo performance. These include high tumor uptake, prolonged retention time, and favorable pharmacokinetic properties that align with clinical standards. While radiotheranostics have been widely applied in tumor diagnosis and therapy, their applications are now expanding to neurodegenerative diseases, cardiovascular diseases, and inflammation. Furthermore, radiotheranostic-empowered precision medicine is revolutionizing the cancer treatment paradigm. Diagnostic radiopharmaceuticals play a pivotal role in patient stratification and treatment planning, leading to improved therapeutic outcomes in targeted radionuclide therapy. This review offers a comprehensive overview of the evolution of radiopharmaceuticals, including both FDA-approved and clinically investigated agents, and explores the mechanisms of cell death induced by radiopharmaceuticals. It emphasizes the significance and future prospects of theranostic-based radiopharmaceuticals in advancing precision medicine.

Preparation, In Vitro Affinity, and In Vivo Biodistribution of Receptor-Specific 68Ga-Labeled Peptides Targeting Vascular Endothelial Growth Factor Receptors

As angiogenesis plays a key role in tumor growth and metastasis, the angiogenic process has attracted scientific interest as a target for diagnostic and therapeutic agents. Factors influencing angiogenesis include the vascular endothelial growth factor (VEGF) family and the two associated receptor types (VEGFR-1 and VEGFR-2). VEGFR-1/-2 detection and quantification in cancer lesions are essential for tumor process management. As a result of the advantageous pharmacokinetics and image contrast, peptides radiolabeled with PET emitters have become interesting tools for the visualization of VEGFR-1/-2-positive tumors. In this study, we prepared ⁶⁸Ga-labeled peptides containing 15 (peptide 1) and 23 (peptide 2) amino acids as new PET tracers for tumor angiogenic process imaging.

METHODS

The peptides were conjugated with NODAGA-tris(t-Bu ester) and subsequently radiolabeled with [⁶⁸Ga]Ga-chloride. The prepared [⁶⁸Ga]Ga-NODAGA-peptide 1 and [⁶⁸Ga]Ga-NODAGA-peptide 2 were tested for radiochemical purity and saline/plasma stability. Consequently, the binding affinity toward VEGFRs was assessed in vitro on human glioblastoma and kidney carcinoma cells. The found peptide receptor affinity was compared with the calculated values in the PROtein binDIng enerGY prediction (PRODIGY) server. Finally, the biodistribution study was performed on BALB/c female mice to reveal the basic pharmacokinetic behavior of radiopeptides.

RESULTS

The in vitro affinity testing of [⁶⁸Ga]Ga-NODAGA-peptides 1 and 2 showed retained receptor binding as characterized by equilibrium dissociation constant (K_D) values in the range of 0.5-1.2 μM and inhibitory concentration 50% (IC₅₀) values in the range of 3.0-5.6 μM. Better binding properties of peptide 2 to VEGFR-1/-2 were found in the PRODIGY server. The biodistribution study on mice showed remarkable accumulation of both peptides in the kidneys and urinary bladder with a short half-life after intravenous application. The in vitro plasma stability of [⁶⁸Ga]Ga-NODAGA-peptide 2 was superior to that of [⁶⁸Ga]Ga-NODAGA-peptide 1.

CONCLUSIONS

The obtained results demonstrated a high radiolabeling yield with no need for purification and preserved binding potency of ⁶⁸Ga-labeled peptides 1 and 2 toward VEGFRs in cancer cells. The peptide-receptor protein interaction assessed in protein-peptide docking determined the strongest interaction of peptide 2 with domain 2 of VEGFR-2 in addition to a more acceptable plasma stability (t_1/2 = 120 min) than that for peptide 1. We found both radiolabeled peptides very potent in their receptor binding, which makes them suitable imaging agents. The rapid transition of the radiopeptides into the urinary tract indicates suitable pharmacokinetic characteristics.

Synthetic Pept-Ins as a Generic Amyloid-Like Aggregation-Based Platform for In Vivo PET Imaging of Intracellular Targets

Amyloid-like aggregation of proteins is induced by short amyloidogenic sequence segments within a specific protein sequence resulting in self-assembly into β-sheets. We recently validated a technology platform in which synthetic amyloid peptides ("Pept-ins") containing a specific aggregation-prone region (APR) are used to induce specific functional knockdown of the target protein from which the APR was derived, including bacterial, viral, and mammalian cell proteins. In this work, we investigated if Pept-ins can be used as vector probes for in vivo Positron Emission Tomography (PET) imaging of intracellular targets. The radiolabeled Pept-ins [68Ga]Ga-NODAGA-PEG4-vascin (targeting VEGFR2) and [68Ga]Ga-NODAGA-PEG2-P2 (targeting E. coli) were evaluated as PET probes. The Pept-in based radiotracers were cross-validated in a murine tumor and muscle infection model, respectively, and were found to combine target specificity with favorable in vivo pharmacokinetics. When the amyloidogenicity of the interacting region of the peptide is suppressed by mutation, cellular uptake and in vivo accumulation are abolished, highlighting the importance of the specific design of synthetic Pept-ins. The ubiquity of target-specific amyloidogenic sequence segments in natural proteins, the straightforward sequence-based design of the Pept-in probes, and their spontaneous internalization by cells suggest that Pept-ins may constitute a generic platform for in vivo PET imaging of intracellular targets.

The Role of VEGF Receptors as Molecular Target in Nuclear Medicine for Cancer Diagnosis and Combination Therapy

Simple Summary

The rapid development of diagnostic and therapeutic methods of the cancer treatment causes that these diseases are becoming better known and the fight against them is more and more effective. Substantial contribution in this development has nuclear medicine that enables very early cancer diagnosis and early start of the so-called targeted therapy. This therapeutic concept compared to the currently used chemotherapy, causes much fewer undesirable side effects, due to targeting a specific lesion in the body. This review article discusses the possible applications of radionuclide-labelled tracers (peptides, antibodies or synthetic organic molecules) that can visualise cancer cells through pathological blood vessel system in close tumour microenvironment. Hence, at a very early step of oncological disease, targeted therapy can involve in tumour formation and growth.

Abstract

One approach to anticancer treatment is targeted anti-angiogenic therapy (AAT) based on prevention of blood vessel formation around the developing cancer cells. It is known that vascular endothelial growth factor (VEGF) and vascular endothelial growth factor receptors (VEGFRs) play a pivotal role in angiogenesis process; hence, application of angiogenesis inhibitors can be an effective approach in anticancer combination therapeutic strategies. Currently, several types of molecules have been utilised in targeted VEGF/VEGFR anticancer therapy, including human VEGF ligands themselves and their derivatives, anti-VEGF or anti-VEGFR monoclonal antibodies, VEGF binding peptides and small molecular inhibitors of VEGFR tyrosine kinases. These molecules labelled with diagnostic or therapeutic radionuclides can become, respectively, diagnostic or therapeutic receptor radiopharmaceuticals. In targeted anti-angiogenic therapy, diagnostic radioagents play a unique role, allowing the determination of the emerging tumour, to monitor the course of treatment, to predict the treatment outcomes and, first of all, to refer patients for AAT. This review provides an overview of design, synthesis and study of radiolabelled VEGF/VEGFR targeting and imaging agents to date. Additionally, we will briefly discuss their physicochemical properties and possible application in combination targeted radionuclide tumour therapy.

PET Imaging of VEGFR with a Novel 64Cu-Labeled Peptide

Vascular endothelial growth factor receptors (VEGFRs) are well recognized as significant biomarkers of tumor angiogenesis. Herein, we have developed a first-of-its-kind peptide-based VEGFR positron emission tomography (PET) tracer. The novel [⁶⁴Cu]VEGF_125-136 peptide possessed satisfactory radio-characteristics and showed good specificity for the visualization of VEGFR in various mouse models, in which the tumor-specific radioactivity uptake was highly correlated to the VEGFR expression level. Moreover, the tracer showed high tumor uptake (ca. 5.89 %ID/g at 20 min postinjection in B16F10 mice) and excellent pharmacokinetics, achieving the maximum imaging quality within 1 h after injection. These features convey [⁶⁴Cu]VEGF_125-136 as a promising, clinically translatable PET tracer for the imaging of tumor angiogenesis.

Gallium-68: methodology and novel radiotracers for positron emission tomography (2012–2017)

Commercial 68Ge/68Ga generators provide a means to produce positron emission tomography agents on site without use of a cyclotron. This development has led to a rapid growth of academic literature and patents ongallium-68 (68Ga). As 68Ga positron emission tomography agents usually involve a targeting moiety attached to a metal chelator, the development lends itself to the investigation of theragnostic applications; the 68Ga-based diagnostic is utilized to determine if the biological target is present and, if so, a therapeutic isotope (e.g., 177Lu, 225Ac) can be complexed with the same scaffold to generate a corresponding radiotherapeutic. This review considers patents issued between 2012 and 2017 that contain a 68Ga-labeled molecule indexed by Chemical Abstract Services (a division of the American Chemical Society).

Radionuclide imaging of VEGFR2 in glioma vasculature using biparatopic affibody conjugate: proof-of-principle in a murine model

Vascular endothelial growth factor receptor-2 (VEGFR2) is a key mediator of angiogenesis and therefore a promising therapeutic target in malignancies including glioblastoma multiforme (GBM). Molecular imaging of VEGFR2 expression may enable patient stratification for antiangiogenic therapy. The goal of the current study was to evaluate the capacity of the novel anti-VEGFR2 biparatopic affibody conjugate (Z_VEGFR2-Bp₂) for in vivo visualization of VEGFR2 expression in GBM. Methods: Z_VEGFR2-Bp₂ coupled to a NODAGA chelator was generated and radiolabeled with indium-111. The VEGFR2-expressing murine endothelial cell line MS1 was used to evaluate in vitro binding specificity and affinity, cellular processing and targeting specificity in mice. Further tumor targeting was studied in vivo in GL261 glioblastoma orthotopic tumors. Experimental imaging was performed. Results: [¹¹¹In]In-NODAGA-Z_VEGFR2-Bp₂ bound specifically to VEGFR2 (K_D=33±18 pM). VEGFR2-mediated accumulation was observed in liver, spleen and lungs. The tumor-to-organ ratios 2 h post injection for mice bearing MS1 tumors were approximately 11 for blood, 15 for muscles and 78 for brain. Intracranial GL261 glioblastoma was visualized using SPECT/CT. The activity uptake in tumors was significantly higher than in normal brain tissue. The tumor-to-cerebellum ratios after injection of 4 µg [¹¹¹In]In-NODAGA-Z_VEGFR2-Bp₂ were significantly higher than the ratios observed for the 40 µg injected dose and for the non-VEGFR2 binding size-matched conjugate, demonstrating target specificity. Microautoradiography of cryosectioned CNS tissue was in good agreement with the SPECT/CT images. Conclusion: The anti-VEGFR2 affibody conjugate [¹¹¹In]In-NODAGA-Z_VEGFR2-Bp₂ specifically targeted VEGFR2 in vivo and visualized its expression in a murine GBM orthotopic model. Tumor-to-blood ratios for [¹¹¹In]In-NODAGA-Z_VEGFR2-Bp₂ were higher compared to other VEGFR2 imaging probes. [¹¹¹In]In-NODAGA-Z_VEGFR2-Bp₂ appears to be a promising probe for in vivo noninvasive visualization of tumor angiogenesis in glioblastoma.

Ultrasound Molecular Imaging of Angiogenesis Using Vascular Endothelial Growth Factor-Conjugated Microbubbles

Imaging of angiogenesis receptors could provide a sensitive and clinically useful method for detecting neovascularization such as occurs in malignant tumors, and responses to antiangiogenic therapies for such tumors. We tested the hypothesis that microbubbles (MB) tagged with human VEGF₁₂₁ (MB_VEGF) bind to the kinase insert domain receptor (KDR) in vitro and angiogenic endothelium in vivo, and that this specific binding can be imaged on a clinical ultrasound system. In this work, targeted adhesion of MB_VEGF was evaluated in vitro using a parallel plate flow system containing adsorbed recombinant human KDR. There was more adhesion of MB_VEGF to KDR-coated plates when the amount of VEGF₁₂₁ on each MB or KDR density on the plate was increased. MB_VEGF adhesion to KDR-coated plates decreased with increasing wall shear rate. On intravital microscopic imaging of bFGF-stimulated rat cremaster muscle, there was greater microvascular adhesion of MB_VEGF compared to that of isotype IgG-conjugated control MB (MB_CTL). To determine if MB_VEGF could be used to ultrasonically image angiogenesis, ultrasound imaging was performed in mice bearing squamous cell carcinoma after intravenous injection of MB_VEGF. Ultrasound videointensity enhancement in tumor was significantly higher for MB_VEGF (17.3 ± 9.7 dB) compared to MB_CTL (3.8 ± 4.4 dB, n = 6, p < 0.05). This work demonstrates the feasibility of targeted ultrasound imaging of an angiogenic marker using MB_VEGF. This approach offers a noninvasive bedside method for detecting tumor angiogenesis and could be extended to other applications such as molecular monitoring of therapeutic angiogenesis or antiangiogenic therapies in cardiovascular disease or cancer.

Evaluation of 68Ga-labeled MG7 antibody: a targeted probe for PET/CT imaging of gastric cancer

MG7-Ag, a specific gastric cancer-associated antigen, can be used to non-invasively monitor gastric cancer by molecular imaging with positron emission tomography/computed tomography (PET/CT). In this study, we prepared and evaluated a ⁶⁸Ga-labeled MG7 antibody as a molecular probe for nanoPET/CT imaging of gastric cancer in a BGC-823 tumor xenografted mouse model. Macrocyclic chelator 1,4,7-triazacyclononane-N,N0,N00-triacetic acid (NOTA)-conjugated MG7 antibody was synthesized and radiolabeled with ⁶⁸Ga (t_1/2 = 67.71 min). Then, ⁶⁸Ga-NOTA-MG7 was tested using in vitro cytological studies, in vivo nanoPET/CT and Cerenkov imaging studies as well as ex vivo biodistribution and histology studies. The in vitro experiments demonstrated that ⁶⁸Ga-NOTA-MG7 has an excellent radiolabeling efficiency of approximately 99% without purification, and it is stable in serum after 120 min of incubation. Cell uptake and retention studies confirmed that ⁶⁸Ga-NOTA-MG7 has good binding affinity and tumor cell retention. For the nanoPET imaging study, the predominant uptake of ⁶⁸Ga-NOTA-MG7 was visualized in tumor, liver and kidneys. The tumor uptake reached at its peak (2.53 ± 0.28%ID/g) at 60 min pi. Cherenkov imaging also confirmed the specificity of tumor uptake. Moreover, the biodistribution results were consistent with the quantification data of nanoPET/CT imaging. Histologic analysis also demonstrated specific staining of BGC-823 tumor cell lines.

In-vivo detection of the erythropoietin receptor in tumours using positron emission tomography

OBJECTIVE

Recombinant human erythropoietin (rhuEpo) is used clinically to treat anaemia. However, rhuEpo-treated cancer patients show decreased survival rates and erythropoietin receptor (EpoR) expression has been found in patient tumour tissue. Thus, rhuEpo application might promote EpoR(+) tumour progression. We therefore developed the positron emission tomography (PET)-probe (68)Ga-DOTA-rhuEpo and evaluated its performance in EpoR(+) A549 non-small-cell lung cancer (NSCLC) xenografts.

METHODS

(68)Ga-DOTA-rhuEpo was generated by coupling DOTA-hydrazide to carbohydrate side-chains of rhuEpo. Biodistribution was determined in tumour-bearing mice 0.5, 3, 6, and 9 h after probe injection. Competition experiments were performed by co-injecting (68)Ga-DOTA-rhuEpo and rhuEpo in five-fold excess. Probe specificity was further evaluated histologically using Epo-Cy5.5 stainings.

RESULTS

The blood half-life of (68)Ga-DOTA-rhuEpo was 2.6 h and the unbound fraction was cleared by the liver and kidney. After 6 h, the highest tumour to muscle ratio was reached. The highest (68)Ga-DOTA-rhuEpo accumulation was found in liver (10.06 ± 6.26%ID/ml), followed by bone marrow (1.87 ± 0.53%ID/ml), kidney (1.58 ± 0.39%ID/ml), and tumour (0.99 ± 0.16%ID/ml). EpoR presence in these organs was histologically confirmed. Competition experiments showed significantly (p < 0.05) lower PET-signals in tumour and bone marrow at 3 and 6 h.

CONCLUSION

(68)Ga-DOTA-rhuEpo shows favourable pharmacokinetic properties and detects EpoR specifically. Therefore, it might become a valuable radiotracer to monitor EpoR status in tumours and support decision-making in anaemia therapy.

KEY POINTS

• PET-probe (68) Ga-DOTA-rhuEpo was administered to assess the EpoR status in vivo • (68) Ga-DOTA-rhuEpo binds specifically to EpoR positive organs in vivo • Tumour EpoR status determination might enable decision-making in anaemia therapy with rhuEpo.

Alternative Non-Antibody Protein Scaffolds for Molecular Imaging of Cancer

The development of improved methods for early detection and characterization of cancer presents a major clinical challenge. One approach that has shown excellent potential in preclinical and clinical evaluation is molecular imaging with small-scaffold, non-antibody based, engineered proteins. These novel diagnostic agents produce high contrast images due to their fast clearance from the bloodstream and healthy tissues, can be evolved to bind a multitude of cancer biomarkers, and are easily functionalized by site-specific bioconjugation methods. Several small protein scaffolds have been verified for in vivo molecular imaging including affibodies and their two-helix variants, knottins, fibronectins, DARPins, and several natural ligands. Further, the biodistribution of these engineered ligands can be optimized through rational mutation of the conserved regions, careful selection and placement of chelator, and modification of molecular size.

⁶⁸Ga-NODAGA-VEGF₁₂₁ for in vivo imaging of VEGF receptor expression

PURPOSE

Vascular endothelial growth factor (VEGF) is a crucial regulator of angiogenesis. In this study, we labeled VEGF₁₂₁ with (68)Ga using a hydrophilic chelating agent, NODAGA and evaluated the resulting (68)Ga-NODAGA-VEGF₁₂₁ for in vivo imaging of VEGF receptor (VEGFR) expression.

METHODS

NODAGA-VEGF₁₂₁ was prepared and its binding affinity for VEGFR2 was measured using (125)I-VEGF₁₂₁. (68)Ga-NODAGA-VEGF₁₂₁ was prepared by labeling NODAGA-VEGF₁₂₁ with (68)GaCl3 followed by purification using a PD-10 column. Human aortic endothelial cell (HAEC) binding studies of (68)Ga-NODAGA-VEGF₁₂₁ were performed at 37°C for 4 h. MicroPET imaging followed by biodistribution studies were performed in U87MG tumor-bearing mice injected with (68)Ga-NODAGA-VEGF₁₂₁. Immunofluorescence staining of the tumor tissues was performed to verify VEGFR2 expression.

RESULTS

Binding affinity of NODAGA-VEGF₁₂₁ for VEGFR2 was found to be comparable to that of VEGF₁₂₁. (68)Ga-NODAGA-VEGF₁₂₁ was prepared in 47.8% yield with specific activity of 3.4 GBq/mg. (68)Ga-NODAGA-VEGF₁₂₁ was avidly taken up by HAECs with a time-dependent increase from 9.88 %ID at 1 h to 20.86 %ID at 4h. MicroPET imaging of mice demonstrated high liver and spleen uptake with clear visualization of tumor at 1h after injection. ROI analysis of tumors revealed 2.53 ± 0.11 %ID/g at 4 h after injection. In the blocking study, tumor uptake was inhibited by 29% at 4 h. Subsequent biodistribution studies demonstrated tumor uptake of 2.38 ± 0.15 %ID/g. Immunofluorescence staining of the tumor tissues displayed high level of VEGFR2 expression.

CONCLUSIONS

These results demonstrate that (68)Ga-NODAGA-VEGF₁₂₁ led to VEGFR-specific distribution in U87MG tumor-bearing mice. This study also suggests that altered physicochemical properties of VEGF₁₂₁ after radiolabeling may affect biodistribution of the radiolabeled VEGF₁₂₁.

Mammary cancer bone metastasis follow-up using multimodal small-animal MR and PET imaging

Despite tremendous progress in the management of breast cancer, the survival rate of this disease is still correlated with the development of metastases-most notably, those of the bone. Diagnosis of bone metastasis requires a combination of multiple imaging modalities. MR imaging remains the best modality for soft-tissue visualization, allowing for the distinction between benign and malignant lesions in many cases. On the other hand, PET imaging is frequently more specific at detecting bone metastasis by measuring the accumulation of radiotracers, such as (18)F-sodium fluoride ((18)F-NaF) and (18)F-FDG. Thus, the main purpose of this study was to longitudinally monitor bone tumor progression using PET/MR image coregistration to improve noninvasive imaging-assisted diagnoses.

METHODS

After surgical implantation of mammary MRMT-1 cells in a rat femur, we performed minimally invasive imaging procedures at different time points throughout tumor development. The procedure consisted of sequential coregistered MR and PET image acquisition, using gadolinium-diethylenetriaminepentaacetic acid (DTPA) as a contrast agent for MR imaging and (18)F-FDG, (11)C-methionine, and (18)F-NaF as molecular tracers for PET imaging. The animals were then euthanized, and complementary radiologic (micro-CT scans) and histologic analyses were performed.

RESULTS

In this preclinical study, we demonstrated that coregistered MR and PET images provide helpful information in a rat mammary-derived bone cancer model. First, MR imaging provided a high-definition anatomic resolution that made the localization of bone resorption and tumor extension detectable between days 9 and 18 after the injection of cancer cells in the medullary channel of the femur. Indeed, the calculation of mean standardized uptake value (SUVmean) and maximal SUV (SUVmax) in bone and soft-tissue regions, as defined from the gadolinium-DTPA contrast-enhanced MR images, showed (18)F-NaF uptake modifications and increased (18)F-FDG or (11)C-methionine uptake in the bone and surrounding soft tissues. (18)F-FDG and (11)C-methionine were compared in terms of the magnitude of change in their uptake and variability. We observed that (11)C-methionine SUVmean variations in the tumor were more important than those of (18)F-FDG. We also found fewer interindividual variations using SUVmean as a quantitative parameter than SUVmax.

CONCLUSION

This preclinical evaluation demonstrated that a PET/MR image coregistration protocol provided a powerful tool to evaluate bone tumor progression in a rat model of bone metastasis and that this protocol could be translated to improve the clinical outcome for metastatic breast cancer management.