Development of 68Ga-labeled mannosylated human serum albumin (MSA) as a lymph node imaging agent for positron emission tomography

Macrophage mannose receptor CD206 targeting of fluoride-18 labeled mannosylated dextran: A validation study in mice

PURPOSE

Aluminum fluoride-18-labeled 1,4,7-triazacyclononane-1,4,7-triacetic acid-conjugated mannosylated dextran derivative (Al[18F]F-NOTA-D10CM) is a new tracer for PET imaging. We report here on in vitro and in vivo validation of the tracer's ability to target the macrophage mannose receptor CD206.

METHODS

First, the uptake of intravenously (i.v.) administered Al[18F]F-NOTA-D10CM was compared between wild-type (WT) and CD206-/- knockout (KO) mice. C57BL/6N mice were injected with complete Freund's adjuvant (CFA) in the left hind leg and the uptake of Al[18F]F-NOTA-D10CM after i.v. or intradermal (i.d.) injection was studied at 5 and 14 days after CFA induction of inflammation. Healthy C57BL/6N mice were studied as controls. Mice underwent PET/CT on consecutive days with [18F]FDG, i.v. Al[18F]F-NOTA-D10CM, and i.d. Al[18F]F-NOTA-D10CM. After the last imaging, Al[18F]F-NOTA-D10CM was i.v. injected for an ex vivo biodistribution study and autoradiography of inflamed tissues. Blood plasma samples were analyzed using high-performance liquid chromatography. To evaluate the specificity of Al[18F]F-NOTA-D10CM binding, an in vitro competitive displacement study was performed on inflamed tissue sections using autoradiography. CD206 expression was assessed by immunohistochemical staining.

RESULTS

Compared with WT mice, the uptake of Al[18F]F-NOTA-D10CM was significantly lower in several CD206-/- KO mice tissues, including liver (SUV 8.21 ± 2.51 vs. 1.06 ± 0.16, P < 0.001) and bone marrow (SUV 1.63 ± 0.37 vs. 0.22 ± 0.05, P < 0.0001). The uptake of i.v. injected Al[18F]F-NOTA-D10CM was significantly higher in inflamed ankle joint (SUV 0.48 ± 0.13 vs. 0.18 ± 0.05, P < 0.0001) and inflamed foot pad skin (SUV 0.41 ± 0.10 vs. 0.04 ± 0.01, P < 0.0001) than in the corresponding tissues in healthy mice. The i.d.-injected Al[18F]F-NOTA-D10CM revealed differences between CFA-induced lymph node activation and lymph nodes in healthy mice. Ex vivo γ-counting, autoradiography, and immunohistochemistry supported the results, and a decrease of ~ 80% in the binding of Al[18F]F-NOTA-D10CM in the displacement study with excess NOTA-D10CM confirmed that tracer binding was specific. At 60 min after i.v. injection, an average 96.70% of plasma radioactivity was derived from intact Al[18F]F-NOTA-D10CM, indicating good in vivo stability. The uptake of Al[18F]F-NOTA-D10CM into inflamed tissues was positively associated with the area percentage of CD206-positive staining.

CONCLUSION

The uptake of mannosylated dextran derivative Al[18F]F-NOTA-D10CM correlated with CD206 expression and the tracer appears promising for inflammation imaging.

Detection of metastatic lymph node and sentinel lymph node mapping ​using mannose receptor targeting in in vivo mouse footpad tumor models and rabbit uterine cancer models

BACKGROUND

This study aimed to evaluate the effectiveness of neo-mannosyl human serum albumin-indocyanine green (MSA-ICG) for detecting metastatic lymph node (LN) and mapping sentinel lymph node (SLN) using mouse footpad uterine tumor models. Additionally, the authors assessed the feasibility of MSA-ICG in SLN mapping in rabbit uterine cancer models.

MATERIALS AND METHODS

The authors compared the LN targeting ability of MSA-ICG with ICG. Six mouse footpad tumor models and two normal mice were each assigned to MSA-ICG and ICG, respectively. After the assigned tracers were injected, fluorescence images were taken, and the authors compared the signal-to-background ratio (SBR) of the tracers. A SLN biopsy was performed to confirm LN metastasis status and CD206 expression level. Finally, an intraoperative SLN biopsy was performed in rabbit uterine cancer models using MSA-ICG.

RESULTS

The authors detected 14 groin LNs out of 16 in the MSA-ICG and ICG groups. The SBR of the MSA-ICG group was significantly higher than that of the ICG group. The metastatic LN subgroup of MSA-ICG showed a significantly higher SBR than that of ICG. CD206 was expressed at a high level in metastatic LN, and the signal intensity difference increased as the CD206 expression level increased. SLN mapping was successfully performed in two of the three rabbit uterine cancer models.

CONCLUSIONS

MSA-ICG was able to distinguish metastatic LN for an extended period due to its specific tumor-associated macrophage-targeting property. Therefore, it may be a more distinguishable tracer for identifying metastatic LNs and SLNs during uterine cancer surgery. Further research is needed to confirm these results.

Aluminum Fluoride-18 Labeled Mannosylated Dextran: Radiosynthesis and Initial Preclinical Positron Emission Tomography Studies

PURPOSE

In addition to being expressed on liver sinusoidal endothelial cells, mannose receptors are also found on antigen-presenting cells, including macrophages, which are mainly involved in the inflammation process. Dextran derivatives of various sizes containing cysteine and mannose moieties have previously been labeled with 99mTc and used for single-photon emission computed tomography imaging of sentinel lymph nodes. In this study, we radiolabeled 21.3-kDa D10CM with positron-emitting 18F for initial positron emission tomography (PET) studies in rats.

PROCEDURES

D10CM was conjugated with 1,4,7-triazacyclononane-1,4,7-triacetic acid (NOTA) chelator and radiolabeled with the aluminum fluoride-18 method. The whole-body distribution kinetics and stability of the intravenously administered tracer were studied in healthy male Sprague-Dawley rats by in vivo PET/CT imaging, ex vivo gamma counting, and high-performance liquid chromatography analysis.

RESULTS

Al[18F]F-NOTA-D10CM was obtained with a radiochemical purity of >99% and molar activity of 9.9 GBq/μmol. At 60 minutes after injection, an average of 84% of the intact tracer was found in the blood, indicating excellent in vivo stability. The highest radioactivity concentration was seen in the liver, spleen, and bone marrow, in which mannose receptors are highly expressed under physiological conditions. The uptake specificity was confirmed with in vivo blocking experiments.

CONCLUSIONS

Our results imply that Al[18F]F-NOTA-D10CM is a suitable tracer for PET imaging. Further studies in disease models with mannose receptor CD206-positive macrophages are warranted to clarify the tracer's potential for imaging of inflammation.

Mannosylated-serum albumin nanoparticle imaging to monitor tumor-associated macrophages under anti-PD1 treatment

BACKGROUND

Immune checkpoint inhibitors such as anti-programmed cell death protein 1 (PD1) block tumor growth by reinvigorating the immune system; however, determining their efficacy only by the changes in tumor size may prove inaccurate. As the immune cells including macrophages in the tumor microenvironment (TME) are associated with the response to anti-PD1 therapy, tumor-associated macrophages (TAMs) imaging using nanoparticles can noninvasively provide the immune enrichment status of TME. Herein, the mannosylated-serum albumin (MSA) nanoparticle was labeled with radioactive isotope ⁶⁸Ga to target the mannose receptors on macrophages for noninvasive monitoring of the TME according to anti-PD1 therapy.

RESULTS

B16F10-Luc and MC38-Luc tumor-bearing mice were treated with anti-PD1, and the response to anti-PD1 was determined by the tumor volume. According to the flow cytometry, the responders to anti-PD1 showed an increased proportion of TAMs, as well as lymphocytes, and the most enriched immune cell population in the TME was also TAMs. For noninvasive imaging of TAMs as a surrogate of immune cell augmentation in the TME via anti-PD1, we acquired [⁶⁸Ga] Ga-MSA positron emission tomography. According to the imaging study, an increased number of TAMs in responders at the early phase of anti-PD1 treatment was observed in both B16F10-Luc and MC38-Luc tumor-bearing mice models.

CONCLUSION

As representative immune cells in the TME, non-invasive imaging of TAMs using MSA nanoparticles can reflect the immune cell enrichment status in the TME closely associated with the response to anti-PD1. As non-invasive imaging using MSA nanoparticles, this approach shows a potential to monitor and evaluate anti-tumor response to immune checkpoint inhibitors.

Novel PET Imaging of Inflammatory Targets and Cells for the Diagnosis and Monitoring of Giant Cell Arteritis and Polymyalgia Rheumatica

Giant cell arteritis (GCA) and polymyalgia rheumatica (PMR) are two interrelated inflammatory diseases affecting patients above 50 years of age. Patients with GCA suffer from granulomatous inflammation of medium- to large-sized arteries. This inflammation can lead to severe ischemic complications (e.g., irreversible vision loss and stroke) and aneurysm-related complications (such as aortic dissection). On the other hand, patients suffering from PMR present with proximal stiffness and pain due to inflammation of the shoulder and pelvic girdles. PMR is observed in 40-60% of patients with GCA, while up to 21% of patients suffering from PMR are also affected by GCA. Due to the risk of ischemic complications, GCA has to be promptly treated upon clinical suspicion. The treatment of both GCA and PMR still heavily relies on glucocorticoids (GCs), although novel targeted therapies are emerging. Imaging has a central position in the diagnosis of GCA and PMR. While [18F]fluorodeoxyglucose (FDG)-positron emission tomography (PET) has proven to be a valuable tool for diagnosis of GCA and PMR, it possesses major drawbacks such as unspecific uptake in cells with high glucose metabolism, high background activity in several non-target organs and a decrease of diagnostic accuracy already after a short course of GC treatment. In recent years, our understanding of the immunopathogenesis of GCA and, to some extent, PMR has advanced. In this review, we summarize the current knowledge on the cellular heterogeneity in the immunopathology of GCA/PMR and discuss how recent advances in specific tissue infiltrating leukocyte and stromal cell profiles may be exploited as a source of novel targets for imaging. Finally, we discuss prospective novel PET radiotracers that may be useful for the diagnosis and treatment monitoring in GCA and PMR.

Nanomaterial Probes for Nuclear Imaging

Nuclear imaging is a powerful non-invasive imaging technique that is rapidly developing in medical theranostics. Nuclear imaging requires radiolabeling isotopes for non-invasive imaging through the radioactive decay emission of the radionuclide. Nuclear imaging probes, commonly known as radiotracers, are radioisotope-labeled small molecules. Nanomaterials have shown potential as nuclear imaging probes for theranostic applications. By modifying the surface of nanomaterials, multifunctional radio-labeled nanomaterials can be obtained for in vivo biodistribution and targeting in initial animal imaging studies. Various surface modification strategies have been developed, and targeting moieties have been attached to the nanomaterials to render biocompatibility and enable specific targeting. Through integration of complementary imaging probes to a single nanoparticulate, multimodal molecular imaging can be performed as images with high sensitivity, resolution, and specificity. In this review, nanomaterial nuclear imaging probes including inorganic nanomaterials such as quantum dots (QDs), organic nanomaterials such as liposomes, and exosomes are summarized. These new developments in nanomaterials are expected to introduce a paradigm shift in nuclear imaging, thereby creating new opportunities for theranostic medical imaging tools.

Artificial Glycoproteins as a Scaffold for Targeted Drug Therapy

Akin to a cellular "fingerprint," the glycocalyx is a glycan-enriched cellular coating that plays a crucial role in mediating cell-to-cell interactions. To gain a better understanding of the factors that govern in vivo recognition, artificial glycoproteins were initially created to probe changes made to the accumulation and biodistribution of specific glycan assemblies through biomimicry. As a result, the organ-specific accumulation for a variety of glycoproteins decorated with simple and/or complex glycans was identified. Additionally, binding trends with regard to cancer cell selectivity were also investigated. To exploit the knowledge gained from these studies, numerous groups thus became engaged in developing targeted drug methodologies based on the use of artificial glycoproteins. This has either been done through adopting the glycoprotein scaffold as a drug carrier, or to directly glycosylate therapeutic proteins/enzymes to localize their biological activity. The principle aim of this Review is to present the foundational research that has driven artificial glycoprotein-based targeting and subsequent adaptations with potential therapeutic applications.

Assessment of Inflammation in Pulmonary Artery Hypertension by 68Ga-Mannosylated Human Serum Albumin

Rationale: Diagnosis and monitoring of patients with pulmonary artery hypertension (PAH) is currently difficult.Objectives: We aimed to develop a noninvasive imaging modality for PAH that tracks the infiltration of macrophages into the pulmonary vasculature, using a positron emission tomography (PET) agent, ⁶⁸Ga-2-(p-isothiocyanatobenzyl)-1,4,7-triazacyclononane-1,4,7-triacetic acid (NOTA) mannosylated human serum albumin (MSA), that targets the mannose receptor (MR).Methods: We induced PAH in rats by monocrotaline injection. Tissue analysis, echocardiography, and ⁶⁸Ga-NOTA-MSA PET were performed weekly in rats after monocrotaline injection and in those treated with either sildenafil or macitentan. The translational potential of ⁶⁸Ga-NOTA-MSA PET was explored in patients with PAH.Measurements and Main Results: Gene sets related to macrophages were significantly enriched on whole transcriptome sequencing of the lung tissue in PAH rats. Serial PET images of PAH rats demonstrated increasing uptake of ⁶⁸Ga-NOTA-MSA in the lung by time that corresponded with the MR-positive macrophage recruitment observed in immunohistochemistry. In sildenafil- or macitentan-treated PAH rats, the infiltration of MR-positive macrophages by histology and the uptake of ⁶⁸Ga-NOTA-MSA on PET was significantly lower than that of the PAH-only group. The pulmonary uptake of ⁶⁸Ga-NOTA-MSA was significantly higher in patients with PAH than normal subjects (P = 0.009) or than those with pulmonary hypertension by left heart disease (P = 0.019) (n = 5 per group).Conclusions: ⁶⁸Ga-NOTA-MSA PET can help diagnose PAH and monitor the inflammatory status by imaging the degree of macrophage infiltration into the lung. These observations suggest that ⁶⁸Ga-NOTA-MSA PET has the potential to be used as a novel noninvasive diagnostic and monitoring tool of PAH.

Naphthol Blue Black and 99mTc-Labeled Mannosylated Human Serum Albumin (99mTc-MSA) Conjugate as a Multimodal Lymph Node Mapping Nanocarrier

^99mTc-labeled mannosylated human serum albumin (MSA) has been reported as a sentinel lymph node (SLN)-imaging agent by binding to macrophages in the LNs. By conjugating it with blue dye, we developed a new multimodal radio-nanocarrier by visual investigation, fluorescence imaging, and single photon emission computed tomography (SPECT)/computed tomography (CT). Binding affinities of seven blue dyes to MSA were tested. According to the spectroscopic study and visual inspection of MSA-bound dyes, naphthol blue black (NBB) was selected as the best candidate of multimodal agent. Thus, ^99mTc-MSA-NBB conjugate was prepared and further investigated using mice. After footpad injection, it showed high popliteal LN accumulation at 1 h. SPECT/CT also showed high popliteal as well as inguinal LN uptakes at 10 min that sustained until 2 h. In conclusion, we prepared a multimodal SLN imaging radio-nanocarrier, ^99mTc-MSA-NBB conjugate, and confirmed its excellency as a multimodal probe for SLN mapping.

Analysis of carbohydrates and glycoconjugates by matrix-assisted laser desorption/ionization mass spectrometry: An update for 2011-2012

This review is the seventh update of the original article published in 1999 on the application of MALDI mass spectrometry to the analysis of carbohydrates and glycoconjugates and brings coverage of the literature to the end of 2012. General aspects such as theory of the MALDI process, matrices, derivatization, MALDI imaging, and fragmentation are covered in the first part of the review and applications to various structural types constitute the remainder. The main groups of compound are oligo- and poly-saccharides, glycoproteins, glycolipids, glycosides, and biopharmaceuticals. Much of this material is presented in tabular form. Also discussed are medical and industrial applications of the technique, studies of enzyme reactions, and applications to chemical synthesis. © 2015 Wiley Periodicals, Inc. Mass Spec Rev 36:255-422, 2017.

Noninvasive Imaging of Myocardial Inflammation in Myocarditis using 68Ga-tagged Mannosylated Human Serum Albumin Positron Emission Tomography

The diagnosis of myocarditis traditionally relies on invasive endomyocardial biopsy but none of the imaging studies so far are specific for infiltration of the inflammatory cells itself. We synthesized ⁶⁸Ga-2-(p-isothiocyanatobenzyl)-1,4,7-triazacyclononane-1,4,7-triacetic acid (NOTA) mannosylated human serum albumin (MSA) by conjugating human serum albumin with mannose, followed by conjugation with NOTA and labeling it with ⁶⁸Ga. The efficacy of ⁶⁸Ga-NOTA-MSA positron emission tomography (PET) for imaging myocardial inflammation was tested in a rat myocarditis model. A significant number of mannose receptor-positive inflammatory cells infiltrated the myocardium in both human and rat myocarditis tissue. ⁶⁸Ga-NOTA-MSA uptake was upregulated in organs of macrophage accumulation, such as liver, spleen, bone marrow and myocardium (0.32 (0.31~0.33) for normal versus 1.02 (0.86~1.06) for myocarditis (median (range), SUV); n=4~6 per group, p-value=0.01). ⁶⁸Ga-NOTA-MSA uptake in the left ventricle was upregulated in myocarditis compared with normal rats (2.29 (1.42~3.40) for normal versus 4.18 (3.43~6.15) for myocarditis (median (range), average standard uptake value ratio against paraspinal muscle); n=6 per group, p-value<0.01), which was downregulated in rats with cyclosporine-A treated myocarditis (3.69 (2.59~3.86) for myocarditis versus 2.28 (1.76~2.60) for cyclosporine-A treated myocarditis; n=6 per group, p-value<0.01). The specificity of the tracer was verified by administration of excess non-labeled MSA. ⁶⁸Ga-NOTA-MSA uptake was significantly enhanced earlier in the evolution of myocarditis before any signs of inflammation could be seen on echocardiography. These results demonstrate the potential utility of visualizing infiltration of mannose receptor-positive macrophages with ⁶⁸Ga-NOTA-MSA PET in the early diagnosis of as well as in the monitoring of treatment response of myocarditis.

Novel PET Imaging of Atherosclerosis with 68Ga-Labeled NOTA-Neomannosylated Human Serum Albumin

Activated macrophages take up ¹⁸F-FDG via glucose transporters, so this compound is useful for atherosclerosis imaging by PET. However, ¹⁸F-FDG application is limited for imaging of the heart and brain, in which glucose uptake is high, and in patients with aberrant glucose metabolism. The aims of this study were to confirm that mannosylated human serum albumin (MSA) specifically binds to the mannose receptor (MR) on macrophages and to test the feasibility of ⁶⁸Ga-labeled NOTA-MSA for PET imaging of atherosclerotic plaques.

METHODS

The peritoneal macrophages of C57/B6 mice were collected, incubated with rhodamine B isothiocyanate-MSA (10 μg/mL), and evaluated by confocal microscopy and flow cytometry. The same evaluations were performed after preincubation of the macrophages with anti-CD206 MR blocking antibodies. NOTA-MSA was synthesized by conjugating 2-(p-isothiocyanatobenzyl)-1,4,7-triazacyclononane-1,4,7-triacetic acid to MSA, followed by labeling with ⁶⁸Ga. Rabbits with atherosclerotic aorta induced by a 3-mo cholesterol diet and chronic inflammation underwent consecutive PET/CT with ¹⁸F-FDG and ⁶⁸Ga-NOTA-MSA at 2-d intervals.

RESULTS

The binding of MSA to MR and its dose-dependent reduction by preincubation with anti-CD206 MR blocking antibody were confirmed. Rhodamine B isothiocyanate and fluorescein isothiocyanate fluorescence colocalized at the atherosclerotic plaque. The ⁶⁸Ga-NOTA-MSA SUVs of the atherosclerotic aorta were significantly higher than those of the healthy arteries and inferior vena cava and were comparable to those obtained with ¹⁸F-FDG.

CONCLUSION

These findings suggest that MR-specific ⁶⁸Ga-NOTA-MSA is effective for detecting atherosclerosis in the aorta and is a promising radiopharmaceutical for imaging atherosclerosis because of the presence of M2 macrophages in atherosclerotic plaques.

Exploring the glycan interaction in vivo: Future prospects of neo-glycoproteins for diagnostics

Herein the biodistributions and in vivo kinetics of chemically prepared neoglycoproteins are reviewed. Chemical methods can be used to conjugate various mono- and oligosaccharides onto a protein surface. The kinetics and organ-specific accumulation profiles of these glycoconjugates, which are introduced through intravenous injections, have been analyzed using conventional dissection studies as well as noninvasive methods such as single photon emission computed tomography, positron emission tomography and fluorescence imaging. These studies suggest that glycan-dependent protein distribution kinetics may be useful for pharmacological and diagnostic applications.

Simple bioconjugate chemistry serves great clinical advances: albumin as a versatile platform for diagnosis and precision therapy

Albumin is the most abundant circulating protein in plasma and has recently emerged as a versatile protein carrier for drug targeting and for improving the pharmacokinetic profile of peptide or protein based drugs. Three drug delivery technologies related to albumin have been developed, which include the coupling of low-molecular weight drugs to exogenous or endogenous albumin, conjugating bioactive proteins by albumin fusion technology (AFT), and encapsulation of drugs into albumin nanoparticles. This review article starts with a brief introduction of human serum albumin (HSA), and then summarizes the mainstream chemical strategies of developing HSA binding molecules for coupling with drug molecules. Moreover, we also concisely condense the recent progress of the most important clinical applications of HSA-binding platforms, and specify the current challenges that need to be met for a bright future of HSA-binding.

Chemically synthesized glycoconjugates on proteins: effects of multivalency and glycoform in vivo

The biodistributions and in vivo kinetics of chemically prepared glycoconjugates on proteins are reviewed.

Development of a multimodal imaging probe by encapsulating iron oxide nanoparticles with functionalized amphiphiles for lymph node imaging

Aim: We tried to develop a multimodal iron oxide nanoparticles (IO NP) imaging probe by an encapsulation method using specific amphiphiles for 68Ga-labeling and lymph node-targeting. Materials & methods: Nanoparticles (NPs) were encapsulated with a solution containing polysorbate 60 and the amphiphiles. The prepared NPs were labeled with 68Ga and tested in vitro and in vivo. Results: Prepared 1,4,7-triazacyclononane-1,4,7-triacetic acid-IO-Mannose (NOTA-IO-Man) showed a narrow size distribution, and no significant aggregation or degradation under harsh conditions. The relaxivity coefficient of 68Ga-NOTA-IO-Man was higher than that of ferumoxide. The accumulation of 68Ga-NOTA-IO-Man in the lymph node after injection into rat's footpad was confirmed by both positron emission tomography and MRI. Conclusion: We successfully developed PET/MRI dual-modality imaging probe targeting lymph nodes by using the facile encapsulation method.

Current innovations in sentinel lymph node mapping for the staging and treatment of resectable lung cancer

Despite surgical resectability, early-stage lung cancer remains a challenge to cure. Survival outcomes are hindered by variable performance of adequate lymphadenectomy and the limitations of current pathologic nodal staging. Sentinel lymph node (SLN) mapping, a mainstay in the management of breast cancer and melanoma, permits targeted nodal sampling for efficient and accurate staging that can influence both intraoperative and adjuvant treatment decisions. Unfortunately, standard SLN identification techniques with blue dye and radiocolloid tracers have not been shown to be reproducible in lung cancer. In more recent years, intraoperative near-infrared image-guided lung SLN mapping has emerged as promising technology for the identification of the tumor-associated lymph nodes most likely to contain metastatic disease. Additionally, the clinical relevance of SLN mapping for lung cancer remains pressing, as the ability to identify micrometastatic disease in SLNs could facilitate trials to assess chemotherapeutic response and the clinical effect of occult nodal disease. This review outlines the status of lung cancer lymphatic mapping and techniques in development that may help close the gap between translational research in this field and routine clinical practice.

68Ga-DOTA and analogs: Current status and future perspectives

The construction of the ⁶⁸Ge/⁶⁸Ga generator has increased application of radiopharmaceuticals labeled with this isotope in medicine. ⁶⁸Ga-PET is widely employed in the management of neuroendocrine tumors but favorable chemistry with tri- and tetraaza-ring molecules has opened wide range of ⁶⁸Ga application in other fields of PET imaging. This review covers the radiopharmaceuticals labeled with gallium in molecular imaging and shows perspectives on the use of gallium-68 as a substitute for technetium-99, fluorine-18 and carbon-11 in some applications.

Prospective of ⁶⁸Ga-radiopharmaceutical development

Positron Emission Tomography (PET) experienced accelerated development and has become an established method for medical research and clinical routine diagnostics on patient individualized basis. Development and availability of new radiopharmaceuticals specific for particular diseases is one of the driving forces of the expansion of clinical PET. The future development of the ⁶⁸Ga-radiopharmaceuticals must be put in the context of several aspects such as role of PET in nuclear medicine, unmet medical needs, identification of new biomarkers, targets and corresponding ligands, production and availability of ⁶⁸Ga, automation of the radiopharmaceutical production, progress of positron emission tomography technologies and image analysis methodologies for improved quantitation accuracy, PET radiopharmaceutical regulations as well as advances in radiopharmaceutical chemistry. The review presents the prospects of the ⁶⁸Ga-based radiopharmaceutical development on the basis of the current status of these aspects as well as wide range and variety of imaging agents.

Thoracoscopic color and fluorescence imaging system for sentinel lymph node mapping in porcine lung using indocyanine green-neomannosyl human serum albumin: intraoperative image-guided sentinel nodes navigation

PURPOSE

This study was performed to validate a newly developed sentinel lymph node (SLN) targeting tracer, indocyanine green-neomannosyl human serum albumin (ICG:MSA), and a thoracoscopic version of the intraoperative color and fluorescence imaging system (ICFIS) for lung cancer SLN mapping.

METHODS

ICG alone or ICG:MSA (5 μg/kg) was injected into the rat thigh, and the results were compared. The fluorescence signal-to-background ratios of SLNs were recorded and evaluated over a 2-h period by using ICFIS. Additionally, a SLN biopsy was performed via video-assisted thoracoscopic surgery with the use of ICG:MSA in porcine lung by using thoracoscopic ICFIS.

RESULTS

The newly developed ICG:MSA showed a significantly improved signal-to-background ratio compared with ICG alone throughout the trials. All SLNs were identified in both rats (ten SLNs in ten rat thighs) and pigs (ten SLNs in ten porcine lungs) under in vivo conditions. All SLNs were dissected successfully by using video-assisted thoracoscopic surgery with the help of thoracoscopic ICFIS.

DISCUSSION

ICG:MSA accumulates in the SLN by uptake and retention through the mannose-specific receptors on macrophages. Thoracoscopic ICFIS successfully assisted SLN mapping despite low near-infrared light transmission in the commercial thoracoscope. On the basis of the results of the thoracoscopic SLN mapping, we anticipate that ICG:MSA and thoracoscopic ICFIS can be translated to clinical trials in the near future.

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.

(68)Ga-labeled radiopharmaceuticals for positron emission tomography

(68)Ga is a promising emerging radionuclide for positron emission tomography (PET). It is produced using a (68)Ge/(68)Ga-generator, and thus, would enable the cyclotron-independent distribution of PET. However, new (68)Ga-labeled radiopharmaceuticals that can replace (18)F-labeled agents like [(18)F]fluorodeoxyglucose (FDG) are needed. Most of the (68)Ga-labeled derivatives currently used are peptide agents, but the developments of other agents, such as amino acid derivatives, nitroimidazole derivatives, and glycosylated human serum albumin, are being actively pursued in many laboratories. Thus, appearance of new (68)Ga-labeled radiopharmaceuticals with high impact are expected in the near future. Here, we present an overview of (68)Ga-labeled agents in terms of their clinical significances and relevances to the management of certain tumors, and pertinent pre-clinical developments.