68Ga-DOTA-Siglec-9--a new imaging tool to detect synovitis

Vascular-adhesion protein 1 in giant cell arteritis and polymyalgia rheumatica

Vascular adhesion protein-1 (VAP-1) is a type 2 transmembrane sialoglycoprotein with oxidative deamination functionality, encoded by the amine oxidase copper-containing 3 (AOC3) gene. VAP-1 is widely expressed across various tissues, particularly in highly vascularized tissues and organs essential for lymphocyte circulation. In the vascular system, VAP-1 is predominantly found in vascular smooth muscle cells and endothelial cells, with higher expression levels in vascular smooth muscle cells. Under inflammatory conditions, VAP-1 rapidly translocates to the endothelial cell surface, facilitating leukocyte adhesion and migration through interactions with specific ligands, such as sialic acid-binding immunoglobulin-type lectins (Siglec)-9 on neutrophils and monocytes, and Siglec-10 on B cells, monocytes, and eosinophils. This interaction is crucial for leukocyte transmigration into inflamed tissues. Furthermore, VAP-1's enzymatic activity generates hydrogen peroxide and advanced glycation end-products, contributing to cytotoxic damage and vascular inflammation. In this context, the soluble form of VAP-1 (sVAP-1), produced by matrix metalloproteinase cleavage from its membrane-bound counterpart, also significantly influences leukocyte migration. This review aims to elucidate the multifaceted pathophysiological roles of VAP-1 in vascular inflammation, particularly in giant cell arteritis (GCA) and associated polymyalgia rheumatica (PMR). By exploring its involvement in immune cell adhesion, migration, and its enzymatic contributions to oxidative stress and tissue damage, we investigate the importance of VAP-1 in GCA. Additionally, we discuss recent advancements in imaging techniques targeting VAP-1, such as [68Ga]Ga-DOTA-Siglec-9 PET/CT, which have provided new insights into VAP-1's role in GCA and PMR. Overall, understanding VAP-1's comprehensive roles could pave the way for improved strategies in managing these conditions.

Detection of Intestinal Inflammation by Vascular Adhesion Protein-1-Targeted [68Ga]Ga-DOTA-Siglec-9 Positron Emission Tomography in Murine Models of Inflammatory Bowel Disease

PURPOSE

Inflammatory bowel disease (IBD) can be imaged with positron emission tomography (PET), but existing PET radiopharmaceuticals have limited diagnostic accuracy. Vascular adhesion protein-1 (VAP-1) is an endothelial cell surface molecule that controls leukocyte extravasation into sites of inflammation. However, the role of inflammation-induced VAP-1 expression in IBD is still unclear. Therefore, this study investigated the utility of VAP-1-targeted [68Ga]Ga-DOTA-Siglec-9 positron emission tomography/computed tomography (PET/CT) for assessing inflammation in two mouse models of IBD.

PROCEDURES

Studies were performed using K8-/- mice that develop a chronic colitis-phenotype and C57Bl/6NCrl mice with acute intestinal inflammation chemically-induced using 2.5% dextran sodium sulfate (DSS) in drinking water. In both diseased and control mice, uptake of the VAP-1-targeting peptide [68Ga]Ga-DOTA-Siglec-9 was assessed in intestinal regions of interest using in vivo PET/CT, after which ex vivo gamma counting, digital autoradiography, and histopathological analyses were performed. Immunofluorescence staining was performed to determine VAP-1-expression in the intestine, including in samples from patients with ulcerative colitis.

RESULTS

Intestinal inflammation could be visualized by [68Ga]Ga-DOTA-Siglec-9 PET/CT in two murine models of IBD. In both models, the in vivo PET/CT and ex vivo studies of [68Ga]Ga-DOTA-Siglec-9 uptake were significantly higher than in control mice. The in vivo uptake was increased on average 1.4-fold in the DSS model and 2.0-fold in the K8-/- model. Immunofluorescence staining revealed strong expression of VAP-1 in the inflamed intestines of both mice and patients.

CONCLUSIONS

This study suggests that the VAP-1-targeting [68Ga]Ga-DOTA-Siglec-9 PET tracer is a promising tool for non-invasive imaging of intestinal inflammation. Future studies in patients with IBD and evaluation of the potential value of [68Ga]Ga-DOTA-Siglec-9 in diagnosis and monitoring of the disease are warranted.

Vascular adhesion protein-1-targeted PET imaging in autoimmune myocarditis

BACKGROUND

Vascular adhesion protein-1 (VAP-1) is an adhesion molecule and primary amine oxidase, and Gallium-68-labeled 1,4,7,10-tetraazacyclododecane-N,N',N″,N‴-tetra-acetic acid conjugated sialic acid-binding immunoglobulin-like lectin 9 motif containing peptide ([68Ga]Ga-DOTA-Siglec-9) is a positron emission tomography (PET) tracer targeting VAP-1. We evaluated the feasibility of PET imaging with [68Ga]Ga-DOTA-Siglec-9 for the detection of myocardial lesions in rats with autoimmune myocarditis.

METHODS

Rats (n = 9) were immunized twice with porcine cardiac myosin in complete Freund's adjuvant. Control rats (n = 6) were injected with Freund's adjuvant alone. On day 21, in vivo PET/computed tomography (CT) imaging with [68Ga]Ga-DOTA-Siglec-9 was performed, followed by ex vivo autoradiography, histology, and immunohistochemistry of tissue sections. In addition, myocardial samples from three patients with cardiac sarcoidosis were studied.

RESULTS

[68Ga]Ga-DOTA-Siglec-9 PET/CT images of immunized rats showed higher uptake in myocardial lesions than in myocardium outside lesions (SUVmean, 0.5 ± 0.1 vs 0.3 ± 0.1; P = .003) or control rats (SUVmean, 0.2 ± 0.03; P < .0001), which was confirmed by ex vivo autoradiography of tissue sections. Immunohistochemistry showed VAP-1-positive staining in lesions of rats with myocarditis and in patients with cardiac sarcoidosis.

CONCLUSION

VAP-1-targeted [68Ga]Ga-DOTA-Siglec-9 PET is a potential novel technique for the detection of myocardial lesions.

Roles of the Siglec family in bone and bone homeostasis

Tremendous progress has been seen in the study of the role of sialic acid binding im-munoglobulin type lectins (Siglecs) in osteoimmunology in the past two decades. Interest in Siglecs as immune checkpoints has grown from the recognition that Siglecs have relevance to human disease. Siglecs play important roles in inflammation and cancer, and play key roles in immune cell signaling. By recognizing common sialic acid containing glycans on glycoproteins and glycolipids as regulatory receptors for immune cell signals, Siglecs are expressed on most immune cells and play important roles in normal homeostasis and self-tolerance. In this review, we describe the role that the siglec family plays in bone and bone homeostasis, including the regulation of osteoclast differentiation as well as recent advances in inflammation, cancer and osteoporosis. Particular emphasis is placed on the relevant functions of Siglecs in self-tolerance and as pattern recognition receptors in immune responses, thereby potentially providing emerging strategies for the treatment of bone related diseases.

Gallium-68 Labelled Radiopharmaceuticals for Imaging Inflammatory Disorders

Inflammation is an important component of several chronic and debilitating diseases that result in significant morbidity and mortality. This is best evidenced within the cardiovascular system where it may manifest as atherosclerosis or myocarditis, and at the extreme end of the spectrum as myocardial infarction, ventricular remodeling, or cardiac failure. Early non-invasive detection and monitoring of inflammation in these and other settings may better guide patient management with resultant improved outcomes. Key role players in inflammation pathophysiology include chemokines, macrophages, neutrophils, fibroblasts, integrins, and reactive oxygen species, amongst others. Examples of receptor expression and over-expression include somatostatin receptors, CXCR4-, folate-, mannose-, TSPO- receptors and secretion of various vascular adhesion molecules (such as VCAM and ICAM). Gallium-68-based PET offers imaging possibilities for nearly all the major pathophysiological role players in inflammation, with mounting recent interest in macrophage differentiation, various forms of receptor expression and secretion of chemokines and vascular adhesion molecules. The advantages in terms of logistics and costs of having generator-produced PET probes available is well known, and a ⁶⁸Ga-based tracer provides easily translatable theranostic possibilities to especially Lu-177. Some of the more versatile and better validated Ga-68-based inflammation probes include ⁶⁸Ga-DOTA-TATE/NOC/TOC, ⁶⁸Ga-NOTA-RGD, ⁶⁸Ga-CXCR4, ⁶⁸Ga-citrate and ⁶⁸Ga-FAPI.

Vascular adhesion protein-1 (VAP-1) in vascular inflammatory diseases

Summary: Vascular adhesion protein-1 (VAP-1) also known as amino oxidase copper containing 3 (AOC3) is a pro-inflammatory and versatile molecule with adhesive and enzymatic properties. VAP-1 is a primary amine oxidase belonging to the semicarbazide-sensitive amine oxidase (SSAO) family, which catalyzes the oxidation of primary amines leading to the production of ammonium, formaldehyde, methylglyoxal, and hydrogen peroxide. VAP-1 is mainly expressed by endothelial cells, smooth muscle cells, adipocytes and pericytes. It is involved in a repertoire of biological functions, e.g., immune cell extravasation, angiogenesis, and vascularization. Research into VAP-1 has intensified within the last decade on its role as a novel clinical biomarker and as a potential therapeutic target of vascular inflammatory disorders such as atherosclerosis, stroke, diabetes, neurovascular disorders (e.g., Alzheimer’s Disease), hepatic disease (e.g., non-alcoholic steatohepatitis), and skin conditions (e.g., psoriasis). This is the most up-to-date and comprehensive review on VAP-1 focusing on the translational aspects of VAP-1. Compared to recent reviews, our review provides novel insights on VAP-1 and heart failure, stroke and frailty, diabetes, endometriosis, osteoarthritis, COVID-19, conjunctivitis associated systemic lupus erythematosus, hematopoietic stem cells, gliomas, treatment of colorectal cancer with a novel VAP-1 inhibitor (U-V269), promoting recovery of motor functions and habit learning with a novel VAP-1 inhibitor (PXS-4681A), and 68Ga-DOTA-Siglec-9, a labelled peptide of Siglec-9 (a VAP-1 ligand), which appears to be a safe PET tracer for inflammation in rheumatoid arthritis. Finally, we present the emerging role of VAP-1 in pregnancy as a gatekeeper of immune cells, which are critical for spiral arterial remodeling, the deficiency of which could lead to vascular disorders of pregnancy such as preeclampsia. Future research should prioritize clinical trials on VAP-1 small-molecule inhibitors and monoclonal antibodies, thus, maximizing the potential of VAP-1 targeted therapy as well as research into sVAP-1 as a clinical biomarker of diseases and its prognosis.

[68Ga]Ga-DOTA-Siglec-9 Detects Pharmacodynamic Changes of FAP-Targeted IL2 Variant Immunotherapy in B16-FAP Melanoma Mice

Vascular adhesion protein-1 (VAP-1) is an inflammation-inducible adhesion molecule, which supports contact between leukocytes and inflamed endothelium. There is evidence that VAP-1 is involved in the recruitment of leukocytes to melanoma tumors. Interleukin-2 (IL-2)-based immunotherapy is an efficient therapy that promotes immune system activity against cancers but is associated with toxicity. In the present study, we evaluated the feasibility of PET/CT imaging using the radiotracer [⁶⁸Ga]Ga-DOTA-Siglec-9, which is targeted to VAP-1, to monitor pharmacodynamic effects of a novel FAP-IL2v immunocytokine (a genetically engineered variant of IL-2 fused with fibroblast activation protein) in the B16-FAP melanoma model. At 9 days after the inoculation of B16-FAP melanoma cells, mice were studied with [⁶⁸Ga]Ga-DOTA-Siglec-9 PET/CT as a baseline measurement. Immediately after baseline imaging, mice were treated with FAP-IL2v or vehicle, and treatment was repeated 3 days later. Subsequent PET/CT imaging was performed 3, 5, and 7 days after baseline imaging. In addition to in vivo PET imaging, ex vivo autoradiography, histology, and immunofluorescence staining were performed on excised tumors. B16-FAP tumors were clearly detected with [⁶⁸Ga]Ga-DOTA-Siglec-9 PET/CT during the follow-up period, without differences in tumor volume between FAP-IL2v-treated and vehicle-treated groups. Tumor-to-muscle uptake of [⁶⁸Ga]Ga-DOTA-Siglec-9 was significantly higher in the FAP-IL2v-treated group than in the vehicle-treated group 7 days after baseline imaging, and this was confirmed by tumor autoradiography analysis. FAP-IL2v treatment did not affect VAP-1 expression on the tumor vasculature. However, FAP-IL2v treatment increased the number of CD8⁺ T cells and natural killer cells in tumors. The present study showed that [⁶⁸Ga]Ga-DOTA-Siglec-9 can detect B16-FAP tumors and allows monitoring of FAP-IL2v treatment.

Systematic Review: Targeted Molecular Imaging of Angiogenesis and Its Mediators in Rheumatoid Arthritis

Extensive angiogenesis is a characteristic feature in the synovial tissue of rheumatoid arthritis (RA) from a very early stage of the disease onward and constitutes a crucial event for the development of the proliferative synovium. This process is markedly intensified in patients with prolonged disease duration, high disease activity, disease severity, and significant inflammatory cell infiltration. Angiogenesis is therefore an interesting target for the development of new therapeutic approaches as well as disease monitoring strategies in RA. To this end, nuclear imaging modalities represent valuable non-invasive tools that can selectively target molecular markers of angiogenesis and accurately and quantitatively track molecular changes in multiple joints simultaneously. This systematic review summarizes the imaging markers used for single photon emission computed tomography (SPECT) and/or positron emission tomography (PET) approaches, targeting pathways and mediators involved in synovial neo-angiogenesis in RA.

Therapeutic targets, novel drugs, and delivery systems for diabetes associated NAFLD and liver fibrosis

Type 2 diabetes mellitus (T2DM) associated non-alcoholic fatty liver disease (NAFLD) is the fourth-leading cause of death. Hyperglycemia induces various complications, including nephropathy, cirrhosis and eventually hepatocellular carcinoma (HCC). There are several etiological factors leading to liver disease development, which involve insulin resistance and oxidative stress. Free fatty acid (FFA) accumulation in the liver exerts oxidative and endoplasmic reticulum (ER) stresses. Hepatocyte injury induces release of inflammatory cytokines from Kupffer cells (KCs), which are responsible for activating hepatic stellate cells (HSCs). In this review, we will discuss various molecular targets for treating chronic liver diseases, including homeostasis of FFA, lipid metabolism, and decrease in hepatocyte apoptosis, role of growth factors, and regulation of epithelial-to-mesenchymal transition (EMT) and HSC activation. This review will also critically assess different strategies to enhance drug delivery to different cell types. Targeting nanocarriers to specific liver cell types have the potential to increase efficacy and suppress off-target effects.

First-in-Humans Study of 68Ga-DOTA-Siglec-9, a PET Ligand Targeting Vascular Adhesion Protein 1

Sialic acid–binding immunoglubulinlike lectin 9 (Siglec-9) is a ligand of vascular adhesion protein 1. A ⁶⁸Ga-labeled peptide of Siglec-9, ⁶⁸Ga-DOTA-Siglec-9, holds promise as a novel PET tracer for imaging of inflammation. This first-in-humans study investigated the safety, tolerability, biodistribution, and radiation dosimetry of this radiopharmaceutical. Methods: Six healthy men underwent dynamic whole-body PET/CT. Serial venous blood samples were drawn from 1 to 240 min after intravenous injection of 162 ± 4 MBq of ⁶⁸Ga-DOTA-Siglec-9. In addition to γ-counting, the plasma samples were analyzed by high-performance liquid chromatography to detect intact tracer and radioactive metabolites. Radiation doses were calculated using the OLINDA/EXM software, version 2.2. In addition, a patient with early rheumatoid arthritis was studied with both ⁶⁸Ga-DOTA-Siglec-9 and ¹⁸F-FDG PET/CT to determine the ability of the new tracer to detect arthritis. Results:⁶⁸Ga-DOTA-Siglec-9 was well tolerated by all subjects. ⁶⁸Ga-DOTA-Siglec-9 was rapidly cleared from the blood circulation, and several radioactive metabolites were detected. The organs with the highest absorbed doses were the urinary bladder wall (0.38 mSv/MBq) and kidneys (0.054 mSv/MBq). The mean effective dose was 0.022 mSv/MBq (range, 0.020–0.024 mSv/MBq). Most importantly, however, ⁶⁸Ga-DOTA-Siglec-9 was comparable to ¹⁸F-FDG in detecting arthritis. Conclusion: Intravenous injection of ⁶⁸Ga-DOTA-Siglec-9 was safe and biodistribution was favorable for testing of the tracer in larger group of patients with rheumatoid arthritis, as is planned for the next phase of clinical trials. The effective radiation dose of ⁶⁸Ga-DOTA-Siglec-9 was within the same range as the effective radiation doses of other ⁶⁸Ga-labeled tracers. Injection of 150 MBq of ⁶⁸Ga-DOTA-Siglec-9 would expose a subject to 3.3 mSv. These findings support the possible repeated clinical use of ⁶⁸Ga-DOTA-Siglec-9, such as in trials to elucidate the treatment efficacy of novel drug candidates.

Human Copper-Containing Amine Oxidases in Drug Design and Development

Two members of the copper-containing amine oxidase family are physiologically important proteins: (1) Diamine oxidase (hDAO; AOC1) with a preference for diamines is involved in degradation of histamine and (2) Vascular adhesion protein-1 (hVAP-1; AOC3) with a preference for monoamines is a multifunctional cell-surface receptor and an enzyme. hVAP-1-targeted inhibitors are designed to treat inflammatory diseases and cancer, whereas the off-target binding of the designed inhibitors to hDAO might result in adverse drug reactions. The X-ray structures for both human enzymes are solved and provide the basis for computer-aided inhibitor design, which has been reported by several research groups. Although the putative off-target effect of hDAO is less studied, computational methods could be easily utilized to avoid the binding of VAP-1-targeted inhibitors to hDAO. The choice of the model organism for preclinical testing of hVAP-1 inhibitors is not either trivial due to species-specific binding properties of designed inhibitors and different repertoire of copper-containing amine oxidase family members in mammalian species. Thus, the facts that should be considered in hVAP-1-targeted inhibitor design are discussed in light of the applied structural bioinformatics and structural biology approaches.

Clinical Nononcologic Applications of PET/CT and PET/MRI in Musculoskeletal, Orthopedic, and Rheumatologic Imaging

OBJECTIVE

With improvements in PET/CT and PET/MRI over the last decade, as well as increased understanding of the pathophysiology of musculoskeletal diseases, there is an emerging potential for PET as a primary or complementary modality in the management of rheumatologic and orthopedic conditions.

CONCLUSION

We discuss the role of PET/CT and PET/MRI in nononcologic musculoskeletal disorders, including inflammatory and infectious conditions and postoperative complications. There is great potential for an increased role for PET to serve as a primary or complementary modality in the management of orthopedic and rheumatologic disorders.

Safety Study of Single-Dose Intravenously Administered DOTA-Siglec-9 Peptide in Sprague Dawley Rats

The peptide-based radioactive compound [68Ga]Ga-DOTA-Siglec-9 is a novel agent for imaging of inflammation with positron emission tomography. The drug target of [68Ga]Ga-DOTA-Siglec-9 is vascular adhesion protein 1. Previous studies have obtained promising results with [68Ga]Ga-DOTA-Siglec-9 in experimental animals. However, before taking this novel imaging agent into clinical trials, safety and toxicological studies need to be performed with the nonradioactive precursor compound DOTA-Siglec-9. This extended single-dose toxicity study was designed to provide information on the major toxic effects of DOTA-Siglec-9 and to indicate possible target organs after a single intravenous (iv) injection in rats. The study was performed using 60 adult Hsd: Sprague Dawley rats and included a control group and a treatment group to investigate the toxicity of DOTA-Siglec-9 solution at a final concentration of 0.2 mg/mL after a single iv injection of 582 µg/kg. The maximum dose tested was 1,000-fold the clinical dose on a mg/kg basis as indicated in European Medicines Agency International Council for Harmonisation of Technical Requirements for Pharmaceuticals for Human Use guideline M3(R2). The planned human clinical dose is approximately 0.582 µg of DOTA-Siglec-9 per kg of body mass. This study demonstrates that iv administration of DOTA-Siglec-9 at a dose of 582 µg/kg was well tolerated in rats and did not produce toxicologically significant adverse effects.

Mapping the interaction site and effect of the Siglec-9 inflammatory biomarker on human primary amine oxidase

Human primary amine oxidase (hAOC3), also known as vascular adhesion protein 1, mediates leukocyte rolling and trafficking to sites of inflammation by a multistep adhesion cascade. hAOC3 is absent on the endothelium of normal tissues and is kept upregulated during inflammatory conditions, which is an applicable advantage for imaging inflammatory diseases. Sialic acid binding immunoglobulin like-lectin 9 (Siglec-9) is a leukocyte ligand for hAOC3. The peptide (CARLSLSWRGLTLCPSK) based on the region of Siglec-9 that interacts with hAOC3, can be used as a specific tracer for hAOC3-targeted imaging of inflammation using Positron Emission Tomography (PET). In the present study, we show that the Siglec-9 peptide binds to hAOC3 and triggers its amine oxidase activity towards benzylamine. Furthermore, the hAOC3 inhibitors semicarbazide and imidazole reduce the binding of wild type and Arg/Ala mutated Siglec-9 peptides to hAOC3. Molecular docking of the Siglec-9 peptide is in accordance with the experimental results and predicts that the R3 residue in the peptide interacts in the catalytic site of hAOC3 when the topaquinone cofactor is in the non-catalytic on-copper conformation. The predicted binding mode of Siglec-9 peptide to hAOC3 is supported by the PET studies using rodent, rabbit and pig AOC3 proteins.

Exploring Alternative Radiolabeling Strategies for Sialic Acid-Binding Immunoglobulin-Like Lectin 9 Peptide: [68Ga]Ga- and [18F]AlF-NOTA-Siglec-9

Amino acid residues 283–297 from sialic acid-binding immunoglobulin-like lectin 9 (Siglec-9) form a cyclic peptide ligand targeting vascular adhesion protein-1 (VAP-1). VAP-1 is associated with the transfer of leukocytes from blood to tissues upon inflammation. Therefore, analogs of Siglec-9 peptide are good candidates for visualizing inflammation non-invasively using positron emission tomography (PET). Gallium-68-labeled 1,4,7,10-tetraazacyclododecane-N,N′,N″,N‴-tetraacetic acid (DOTA)-conjugated Siglec-9 has been evaluated extensively for this purpose. Here, we explored two alternative strategies for radiolabeling Siglec-9 peptide using a 1,4,7-triazacyclononane-triacetic acid (NOTA)-chelator to bind [⁶⁸Ga]Ga or [¹⁸F]AlF. The radioligands were evaluated by in vivo PET imaging and ex vivo γ-counting of turpentine-induced sterile skin/muscle inflammation in Sprague-Dawley rats. Both tracers showed clear accumulation in the inflamed tissues. The whole-body biodistribution patterns of the tracers were similar.

Prospective of 68Ga Radionuclide Contribution to the Development of Imaging Agents for Infection and Inflammation

During the last decade, the utilization of ⁶⁸Ga for the development of imaging agents has increased considerably with the leading position in the oncology. The imaging of infection and inflammation is lagging despite strong unmet medical needs. This review presents the potential routes for the development of ⁶⁸Ga-based agents for the imaging and quantification of infection and inflammation in various diseases and connection of the diagnosis to the treatment for the individualized patient management.

Adventures in radiosynthesis of clinical grade [68Ga]Ga-DOTA-Siglec-9

[⁶⁸Ga]Ga-DOTA-Siglec-9 is the first vascular adhesion protein-1 targeting radiopharmaceutical for positron emission tomography imaging of inflammation, and here we present its long-awaited clinical grade radiosynthesis.

Comparison of 68Ga-DOTA-Siglec-9 and 18F-Fluorodeoxyribose-Siglec-9: Inflammation Imaging and Radiation Dosimetry

Sialic acid-binding immunoglobulin-like lectin 9 (Siglec-9) is a ligand of inflammation-inducible vascular adhesion protein-1 (VAP-1). We compared ⁶⁸Ga-DOTA- and ¹⁸F-fluorodeoxyribose- (FDR-) labeled Siglec-9 motif peptides for PET imaging of inflammation. Methods. Firstly, we examined ⁶⁸Ga-DOTA-Siglec-9 and ¹⁸F-FDR-Siglec-9 in rats with skin/muscle inflammation. We then studied ¹⁸F-FDR-Siglec-9 for the detection of inflamed atherosclerotic plaques in mice and compared it with previous ⁶⁸Ga-DOTA-Siglec-9 results. Lastly, we estimated human radiation dosimetry from the rat data. Results. In rats, ⁶⁸Ga-DOTA-Siglec-9 (SUV, 0.88 ± 0.087) and ¹⁸F-FDR-Siglec-9 (SUV, 0.77 ± 0.22) showed comparable (P = 0.29) imaging of inflammation. In atherosclerotic mice, ¹⁸F-FDR-Siglec-9 detected inflamed plaques with a target-to-background ratio (1.6 ± 0.078) similar to previously tested ⁶⁸Ga-DOTA-Siglec-9 (P = 0.35). Human effective dose estimates for ⁶⁸Ga-DOTA-Siglec-9 and ¹⁸F-FDR-Siglec-9 were 0.024 and 0.022 mSv/MBq, respectively. Conclusion. Both tracers are suitable for PET imaging of inflammation. The easier production and lower cost of ⁶⁸Ga-DOTA-Siglec-9 present advantages over ¹⁸F-FDR-Siglec-9, indicating it as a primary choice for clinical studies.

Targeting of vascular adhesion protein-1 by positron emission tomography visualizes sites of inflammation in Borrelia burgdorferi-infected mice

Background

In the present study, we sought to evaluate the feasibility of targeting vascular adhesion protein-1 (VAP-1) by positron emission tomography (PET) for the longitudinal quantitative assessment of Borrelia burgdorferi infection-induced inflammation in mice.

Methods

Mice with B. burgdorferi infection-induced arthritis were studied. During a 7-week follow-up period, the progression of arthritis was monitored weekly with ⁶⁸Ga-DOTA-Siglec-9 PET/computed tomography (CT) and measurement of tibiotarsal joint swellings. A subgroup of infected mice was treated with ceftriaxone. Finally, histopathological assessment of joint inflammation was performed and VAP-1 expression in joints were determined.

Results

Explicit joint swelling and ⁶⁸Ga-DOTA-Siglec-9 uptake could be demonstrated in the affected joints from B. burgdorferi-infected mice. By contrast, no obvious accumulation of ⁶⁸Ga-DOTA-Siglec-9 was detected in joints of uninfected mice. The maximum swelling and highest uptake in the affected joints were observed 4 weeks after the infection. ⁶⁸Ga-DOTA-Siglec-9 uptake in joints correlated with joint swelling (P < 0.0001) and histopathological scoring of inflammation (P = 0.020). Despite short-term antibiotic treatment, the arthritis persisted, and the PET signal remained as high as in nontreated mice. Immunohistochemistry revealed strong-to-moderate expression of VAP-1 in the synovium of B. burgdorferi-infected mice, while only weak expression of VAP-1 was detected in uninfected mice.

Conclusions

The present study showed that ⁶⁸Ga-DOTA-Siglec-9 can detect B. burgdorferi infection-induced arthritis in mice. Furthermore, longitudinal PET/CT imaging allowed monitoring of arthritis development over time.

Non-oncologic Applications of PET/CT and PET/MR in Musculoskeletal, Orthopedic, and Rheumatologic Imaging: General Considerations, Techniques, and Radiopharmaceuticals

Positron Emission Tomography (PET) is often underutilized in the field of musculoskeletal imaging, with key reasons including the excellent performance of conventional musculoskeletal MRI, the limited spatial resolution of PET, and the lack of reimbursement for PET for non-oncologic musculoskeletal indications. However, with improvements in PET/CT and PET/MR imaging over the last decade as well as an increased understanding of the pathophysiology of musculoskeletal diseases, there is an emerging potential for PET as a primary or complementary modality in the management of rheumatologic and orthopedic patients. Specific advantages of PET include the convenience of whole body imaging in a single session, the relative resilience of the modality in the imaging of metallic implants compared to CT and MRI, the ability to evaluate deep joints not amenable to palpation, and the potential for improved specificity of diagnosis with novel radiopharmaceuticals. In this review, we discuss multiple radiopharmaceuticals and technical consideration of PET/CT and PET/MRI that can be employed in imaging of non-tumoral bone and soft tissue disorders. Both PET/CT and PET/MR hold significant promise in the field of musculoskeletal imaging, and with further radiopharmaceutical development and clinical research, these hybrid modalities can potentially transform the current management of patients with orthopedic and rheumatologic disease.

In vivo imaging of Lyme arthritis in mice by [18F]fluorodeoxyglucose positron emission tomography/computed tomography

OBJECTIVE

Lyme borreliosis (LB) is a tick-borne infectious disease caused by Borrelia burgdorferi spirochaetes, which are able to disseminate from the tick-bite site to distant organs. Mouse models are widely used to study LB and especially Lyme arthritis (LA), but only a few whole-animal in vivo imaging studies on the pathogenesis of B. burgdorferi infection in mice have been published so far. The existing imaging techniques have their drawbacks and, therefore, novel tools to complement the array of available LB imaging methodologies are needed.

METHOD

The applicability of positron emission tomography combined with computed tomography (PET/CT) imaging was evaluated as a method to monitor LB and especially LA in the C3H/HeN mouse model infected with wild-type B. burgdorferi N40 bacteria. The imaging results were compared with the traditional LA analysis methods, such as tibiotarsal joint swelling and histopathological assessment of joint inflammation.

RESULTS

PET/CT imaging provided high-resolution images with quantitative information on the spatial and temporal distribution of the [¹⁸F]fluorodeoxyglucose ([¹⁸F]FDG) tracer in B. burgdorferi-infected mice. The [¹⁸F]FDG accumulated in the affected joints and activated lymph nodes of infected mice, while the tracer signal could not be visualized in these organs in uninfected control animals. Importantly, in vivo PET/CT imaging data were in agreement with the histopathological scoring of inflammation of mouse joints.

CONCLUSION

PET/CT imaging with [¹⁸F]FDG is a reliable method to longitudinally monitor the development and progression of B. burgdorferi infection-induced inflammation in vivo in mouse joints.

Exploring the radiosynthesis and in vitro characteristics of [68 Ga]Ga-DOTA-Siglec-9

Vascular adhesion protein 1 is a leukocyte homing-associated glycoprotein, which upon inflammation rapidly translocates from intracellular sources to the endothelial cell surface. It has been discovered that the cyclic peptide residues 283-297 of sialic acid-binding IgG-like lectin 9 (Siglec-9) "CARLSLSWRGLTLCPSK" bind to vascular adhesion protein 1 and hence makes the radioactive analogues of this compound ([⁶⁸ Ga]Ga-DOTA-Siglec-9) interesting as a noninvasive visualizing marker of inflammation. Three different approaches to the radiosynthesis of [⁶⁸ Ga]Ga-DOTA-Siglec-9 are presented and compared with previously published methods. A simple, robust radiosynthesis of [⁶⁸ Ga]Ga-DOTA-Siglec-9 with a yield of 62% (non decay-corrected) was identified, and it had a radiochemical purity >98% and a specific radioactivity of 35 MBq/nmol. Furthermore, the protein binding and stability of [⁶⁸ Ga]Ga-DOTA-Siglec-9 were analyzed in vitro in mouse, rat, rabbit, pig, and human plasma and compared with in vivo pig results. The plasma in vitro protein binding of [⁶⁸ Ga]Ga-DOTA-Siglec-9 was the lowest in the pig followed by rabbit, human, rat, and mouse. It was considerably higher in the in vivo pig experiments. The in vivo stability in pigs was lower than the in vitro stability. Despite considerable species differences, the observed characteristics of [⁶⁸ Ga]Ga-DOTA-Siglec-9 are suitable as a positron emission tomography tracer.

Gallium-68 PET: A Powerful Generator-based Alternative to Infection and Inflammation Imaging

The process of inflammation (with or without infection) forms part of essentially every major debilitating disease. Early detection and accurate distinction of inflammation from infection are important to optimize and individualize therapy. Nuclear medicine is ideally suited for the detection of pathologic changes early on and is able to target a magnitude of role players involved in the aforementioned processes. Hybrid modalities such as PET/CT and PET/MRI offer high spatial resolution that combines morphologic and pathophysiological changes and add various quantification possibilities that are preferable in these settings. It follows then that the development of PET radiopharmaceuticals is imperative to make use of these latest advances. Gallium-68 (Ga-68)-based tracers are exceptionally well suited to these indications, considering the year-round availability from a single generator, the relative cost-effectiveness, and relative ease of labeling. Over the past few years, the development of Ga-68-based tracers has understandably exploded with a recent growing interest in infection and inflammation imaging. This review aims to highlight some of the most important and interesting advances made with Ga-68-based PET/CT in the field of infection and inflammation imaging.

Diagnosis and management of rheumatoid arthritis; What is the current role of established and new imaging techniques in clinical practice?

Ultrasound and magnetic resonance imaging (MRI) have become established imaging techniques for the management of rheumatoid arthritis. Several publications have pointed out the advantages of these techniques for a more complete evaluation of the inflammation and structural damage at joint level. Recently new imaging techniques as the positron emission tomography (PET) associated with computed tomography (CT) or MRI scan, and the optical imaging have been introduced in the panorama. This article presents the advantages and limitations of each imaging techniques in light with the recent publications.

Leukocyte trafficking-associated vascular adhesion protein 1 is expressed and functionally active in atherosclerotic plaques

Given the important role of inflammation and the potential association of the leukocyte trafficking-associated adhesion molecule vascular adhesion protein 1 (VAP-1) with atherosclerosis, this study examined whether functional VAP-1 is expressed in atherosclerotic lesions and, if so, whether it could be targeted by positron emission tomography (PET). First, immunohistochemistry revealed that VAP-1 localized to endothelial cells of intra-plaque neovessels in human carotid endarterectomy samples from patients with recent ischemic symptoms. In low-density lipoprotein receptor-deficient mice expressing only apolipoprotein B100 (LDLR^-/-ApoB^100/100), VAP-1 was expressed on endothelial cells lining inflamed atherosclerotic lesions; normal vessel walls in aortas of C57BL/6N control mice were VAP-1-negative. Second, we discovered that the focal uptake of VAP-1 targeting sialic acid-binding immunoglobulin-like lectin 9 based PET tracer [⁶⁸Ga]DOTA-Siglec-9 in atherosclerotic plaques was associated with the density of activated macrophages (r = 0.58, P = 0.022). As a final point, we found that the inhibition of VAP-1 activity with small molecule LJP1586 decreased the density of macrophages in inflamed atherosclerotic plaques in mice. Our results suggest for the first time VAP-1 as a potential imaging target for inflamed atherosclerotic plaques, and corroborate VAP-1 inhibition as a therapeutic approach in the treatment of atherosclerosis.

Feasibility of (68)Ga-labeled Siglec-9 peptide for the imaging of acute lung inflammation: a pilot study in a porcine model of acute respiratory distress syndrome

There is an unmet need for noninvasive, specific and quantitative imaging of inherent inflammatory activity. Vascular adhesion protein-1 (VAP-1) translocates to the luminal surface of endothelial cells upon inflammatory challenge. We hypothesized that in a porcine model of acute respiratory distress syndrome (ARDS), positron emission tomography (PET) with sialic acid-binding immunoglobulin-like lectin 9 (Siglec-9) based imaging agent targeting VAP-1 would allow quantification of regional pulmonary inflammation. ARDS was induced by lung lavages and injurious mechanical ventilation. Hemodynamics, respiratory system compliance (Crs) and blood gases were monitored. Dynamic examination using [(15)O]water PET-CT (10 min) was followed by dynamic (90 min) and whole-body examination using VAP-1 targeting (68)Ga-labeled 1,4,7,10-tetraaza cyclododecane-1,4,7-tris-acetic acid-10-ethylene glycol-conjugated Siglec-9 motif peptide ([(68)Ga]Ga-DOTA-Siglec-9). The animals received an anti-VAP-1 antibody for post-mortem immunohistochemistry assay of VAP-1 receptors. Tissue samples were collected post-mortem for the radioactivity uptake, histology and immunohistochemistry assessment. Marked reduction of oxygenation and Crs, and higher degree of inflammation were observed in ARDS animals. [(68)Ga]Ga-DOTA-Siglec-9 PET showed significant uptake in lungs, kidneys and urinary bladder. Normalization of the net uptake rate (Ki) for the tissue perfusion resulted in 4-fold higher uptake rate of [(68)Ga]Ga-DOTA-Siglec-9 in the ARDS lungs. Immunohistochemistry showed positive VAP-1 signal in the injured lungs. Detection of pulmonary inflammation associated with a porcine model of ARDS was possible with [(68)Ga]Ga-DOTA-Siglec-9 PET when using kinetic modeling and normalization for tissue perfusion.