18F-FEDAC as a Targeting Agent for Activated Macrophages in DBA/1 Mice with Collagen-Induced Arthritis: Comparison with 18F-FDG

Detection of Voltage-Gated Potassium Channels 1.3 via Immuno-PET Visualizes Rheumatoid Arthritis

Rheumatoid arthritis (RA) is a chronic inflammatory condition affecting approximately 1% of the global population, yet current diagnostic tools for early-stage RA lack sensitivity and fail to identify optimal treatment candidates. Voltage-gated potassium channel 1.3 (Kv1.3) plays a crucial role in immune cell activation, particularly in effector memory T lymphocytes and macrophages, which drive inflammation in active RA. This study aimed to develop and evaluate a novel Kv1.3-targeted radiopharmaceutical, [68Ga]Ga-NOTA-anti-Kv1.3, for precise monitoring and quantification of joint inflammation in a mouse model of RA. PET imaging revealed significantly higher radioactivity concentration in inflamed joints compared to normal joints at all time points, with peak uptake of 11.73 %ID/g at 4 h postinjection compared to 1.55 %ID/g in noninflamed joints. Histological analysis confirmed the presence of T lymphocytes and macrophages in inflamed tissues and their high Kv1.3 expression. Additionally, tracer uptake correlated strongly with clinical arthritis scores (Pearson r = 0.8789, p < 0.0001), demonstrating the probe's sensitivity in tracking arthritis progression. This study successfully developed a 68Ga-labeled Kv1.3-targeting radiopharmaceutical that enables visualization of active inflammation in arthritic joints, showing high sensitivity for early RA diagnosis and disease monitoring. This approach offers potential for guiding immune-targeted therapies and assessing the treatment efficacy in RA.

Diagnostic utility of plasma translocator protein 18 kDa (TSPO) in sepsis: A case-control study

Translocator protein 18 kDa (TSPO) is a mitochondrial membrane protein that is involved in inflammation, oxidative stress, and steroidogenesis. TSPO may be a marker of inflammatory responses in the brain and other organs, but there have been few studies of the potential clinical significance of measuring the circulating TSPO concentration, especially in patients with sepsis. In this study, we compared the circulating TSPO concentrations of patients with sepsis and healthy controls to investigate the utility of plasma TSPO for the diagnosis of sepsis. Patients with sepsis admitted to the intensive care unit of Hiroshima University Hospital between January 2020 and April 2024 were enrolled. Plasma samples were collected from patients within 24 hours of admission and also from healthy volunteers, and their plasma TSPO concentrations were compared. Receiver operating characteristic analysis was used to evaluate the usefulness of plasma TSPO concentration for the diagnosis of sepsis. We also investigated the relationships of TSPO concentration with the severity of sepsis, complications, and prognosis of the patients. Eighty subjects (52 patients and 28 controls) were included in this study. The plasma TSPO concentrations of the patients with sepsis were significantly lower than those of the healthy controls (0.094 vs 0.25 ng/mL, P < .001), and receiver operating characteristic analysis generated an area under the curve of 0.81 (95% confidence interval: 0.72-0.91). In patients with sepsis, the TSPO concentration was not associated with the severity of sepsis, complications, or prognosis. Plasma TSPO may be a useful biomarker for the diagnosis of sepsis.

TSPO Radioligands for Neuroinflammation: An Overview

The translocator protein (TSPO) is predominately localized on the outer mitochondrial membrane in steroidogenic cells. In the brain, TSPO expression, low under normal conditions, results upregulated in response to glial cell activation, that occurs in neuroinflammation. As a consequence, TSPO has been extensively studied as a biomarker of such conditions by means of TSPO-targeted radiotracers. Although [11C]-PK11195, the prototypical TSPO radioligand, is still widely used for in vivo studies, it is endowed with severe limitations, mainly low sensitivity and poor amenability to quantification. Consequently, several efforts have been focused on the design of new radiotracers for the in vivo imaging of TSPO. The present review will provide an outlook on the latest advances in TSPO radioligands for neuroinflammation imaging. The final goal is to pave the way for (radio)chemists in the future design and development of novel effective and sensitive radiopharmaceuticals targeting TSPO.

A first-in-man study of [18F] FEDAC: a novel PET tracer for the 18-kDa translocator protein

PURPOSE

N-benzyl-N-methyl-2-[7, 8-dihydro-7-(2-[18F] fluoroethyl) -8-oxo-2-phenyl-9H-purin-9-yl] acetamide ([18F] FEDAC) is a novel positron emission tomography (PET) tracer that targets the translocator protein (TSPO; 18 kDa) in the mitochondrial outer membrane, which is known to be upregulated in various diseases such as malignant tumors, neurodegenerative diseases, and neuroinflammation. This study presents the first attempt to use [18F]FEDAC PET/CT and evaluate its biodistribution as well as the systemic radiation exposure to the radiotracer in humans.

MATERIALS AND METHODS

Seventeen whole-body [18F]FEDAC PET/CT (injected dose, 209.1 ± 6.2 MBq) scans with a dynamic scan of the upper abdomen were performed in seven participants. Volumes of interest were assigned to each organ, and a time-activity curve was created to evaluate the biodistribution of the radiotracer. The effective dose was calculated using IDAC-Dose 2.1.

RESULTS

Immediately after the intravenous injection, the radiotracer accumulated significantly in the liver and was subsequently excreted into the gastrointestinal tract through the biliary tract. It also showed high levels of accumulation in the kidneys, but showed minimal migration to the urinary bladder. Thus, the liver was the principal organ that eliminated [18F] FEDAC. Accumulation in the normal brain tissue was minimal. The effective dose estimated from biodistribution in humans was 19.47 ± 1.08 µSv/MBq, and was 3.60 mSV for 185 MBq dose.

CONCLUSION

[18F]FEDAC PET/CT provided adequate image quality at an acceptable effective dose with no adverse effects. Therefore, [18F]FEDAC may be useful in human TSPO-PET imaging.

Integrated bioinformatics analysis and experimental validation reveals hub genes of rheumatoid arthritis

Rheumatoid arthritis (RA) is an autoimmune disease characterized by systemic inflammation, especially synovitis, leading to joint damage. It is important to explore potential biomarkers and therapeutic targets to improve the clinical treatment of RA. However, the potential underlying mechanisms of action of available treatments for RA have not yet been fully elucidated. The present study investigated the potential biomarkers of RA and identified specific targets for therapeutic intervention. A comprehensive analysis was performed using mRNA files downloaded from the Gene Expression Omnibus. Differences in gene expression were analyzed and compared between the normal and RA groups. In addition, Gene Ontology and Kyoto Encyclopedia of Genes and Genomes pathway enrichment analyses were performed on differentially expressed genes (DEGs). A protein-protein interaction network, Molecular Complex Detection and cytoHubba network were evaluated to identify hub genes. Finally, using an experimental RA rat model induced by Freund's complete adjuvant (FCA), the expression of potential biomarkers or target genes in RA were verified through reverse transcription-quantitative PCR. The results of the mRNA dataset processing revealed 195 DEGs in patients with RA when compared with the healthy controls. Moreover, 10 hub genes were identified in patients with RA and four candidate mRNAs were identified, as follows: Discs large homolog-associated protein 5 (DLGAP5), kinesin family member 20A (KIF20A), maternal embryonic leucine zipper kinase (MELK) and nuclear division cycle 80 (NDC80). Finally, the bioinformatics analysis results were validated by quantifying the expression of the DLGAP5, KIF20A, MELK and NDC80 genes in the FCA-induced experimental RA rat model. The findings of the present study suggested that the treatment of RA may be successful through the inhibition of DLGAP5, KIF20A, MELK and NDC80 expression. Therefore, the targeting of these genes may result in more effective treatments for patients with RA.

18F-FDG and 18F-NaF PET/CT Global Assessment of Large Joint Inflammation and Bone Turnover in Rheumatoid Arthritis

Rheumatoid arthritis (RA) involves chronic inflammation of synovial joints, causing pain, stiffness, and limited mobility. ¹⁸F-sodium fluoride (NaF) is a PET tracer whose uptake reflects bone turnover, while ¹⁸F-fludeoxyglucose (FDG) shows glucose metabolism and can serve as a marker for inflammation. The aim of this study is to determine the feasibility of calculating the FDG and NaF mean standardized uptake value (SUVmean) in the knee joint, hip joint, and sacroiliac (SI) joint of RA patients and to determine their association with patient characteristics. Prospective FDG-PET/CT as well as NaF-PET/CT imaging was performed on 18 RA patients. The global SUVmean was calculated on FDG-PET/CT and NaF-PET/CT images using a semiautomated CT-based method of segmentation. FDG and NaF uptake were found to be significantly correlated in the knee (r = 0.77, p < 0.001), but not in the hip and SI joints. In the knee, both NaF SUVmean and FDG SUVmean were significantly correlated with body weight, BMI, leptin, and sclerostin levels (p < 0.05). NaF SUVmean was significantly positively correlated with BMI and leptin for both the hip and SI joints (p < 0.05). No significant correlation was observed between either PET parameter and age, height, erythrocyte sedimentation rate (ESR), and interleukins 1 and 6 (IL-1 and IL-6); however, FDG was correlated with inflammatory markers such as C-reactive protein (CRP) and patient global visual analogue scale (VAS-PtGlobal) in some joints. In this study, both FDG and NaF uptake were quantified in large joints of patients with RA using CT segmentation. NaF and FDG SUVmean were correlated with clinical variables related to body weight and adiposity, suggesting that degenerative joint disease may play a larger role in influencing the uptake of these tracers in large joints than RA disease activity. FDG and its correlation with markers of inflammation such as CRP and VAS-PtGlobal suggests that this tracer may serve as a more specific marker for RA disease activity than NaF. Larger prospective and longitudinal data are necessary to gain a better understanding of the roles of FDG and NaF in evaluating RA joint activity in these joints.

Pathological pathway analysis in an experimental rheumatoid arthritis model and the tissue repair effect of acupuncture at ST36

Rheumatoid arthritis (RA) is an autoimmune disease that generally affects the joints. In the face of inflammation-induced cartilage and bone damage, RA treatment remains insufficient. While research evidence indicates that acupuncture can exert anti-inflammatory and analgesic effects, improve the joint function of RA patients, and delay the disease, data on whether it can promote RA repair are lacking. Findings from the present work demonstrated that both the antigen-induced arthritis (AIA) and collagen-induced arthritis (CIA) models can simulate joint swelling of RA. The AIA model was more stable than the CIA model, with a higher incidence of successful arthritis modeling. Moreover, the AIA mice model could simulate the signal molecules and related pathological processes of the autoimmune response in RA, as well as major pathways related to RA and antigen immune response mechanisms. Manual acupuncture (MA) at Zusanli (ST36) significantly improved paw redness and swelling, pain, and inflammatory cell infiltration in the joints in AIA mice. The therapeutic effect of MA on AIA is achieved primarily through the regulation of steroid hormone biosynthesis, cell metabolism, and tissue repair processes. MA at ST36 can increase the gene contents of tissue repair growth factors, including PEG3, GADD45A, GDF5, FGF5, SOX2, and ATP6V1C2 in the inflammatory side joints of AIA mice, as well as the gene expression of the anti-inflammatory cytokine IL-10. In conclusion, acupuncture may alleviate RA in the joints via modulating the tissue healing process.

Update on Imaging of Inflammatory Arthritis and Related Disorders

Arthritis and other rheumatic disorders are very frequent in the general population and responsible for a huge physical and disability burden to affected patients as well as a major cost to the society. Precise evaluation often relies on clinical data only but additional imaging may be required i) for a more objective assessment of the disease status, such as in rheumatoid arthritis (RA) or ankylosing spondyloarthritis (AS), ii) for providing prognostic information and evaluating response to treatment or iii) for establishing diagnosis, in patients with unclear clinical picture, such as polymyalgia rheumatica (PMR) and large-vessel vasculitis (LVV). Besides radiological techniques (x-rays, ultrasound, and MRI), functional and molecular imaging has emerged as a valid tool for this purpose in several disorders. Bone scanning has long been a method of choice but is now more used as a triage tool in patients with unclear complaints, including degenerative disorders (eg osteoarthritis). ¹⁸F-FDG-PET/CT (FDG) proved efficient in assessing the extent of the disease and response to treatment in RA and related disorders, and to provide accurate diagnosis in some systemic disorders, including PMR and LVV. Based on glucose metabolism, FDG-PET/CT is able to show increased metabolism in peripheral cells involved in inflammation (eg neutrophils, lymphocytes or monocytes/macrophages) but also in fibroblasts that proliferate in the pannus. The lack of specificity of FDG is a limitation and many alternative tracers were developed at the preclinical stage or applied in the clinics, especially within clinical trials. They include imaging of macrophages using translocator protein (TSPO), folate-receptors or other targets on activated cells. These new tools will undoubtedly become more and more available in the everyday clinical workup of patients with rheumatisms. Finally, it should be kept in mind that a very simple tracer, ¹⁸F-fluoride is widely more performant in AS than FDG.

Essential Principles and Recent Progress in the Development of TSPO PET Ligands for Neuroinflammation Imaging

The translocator protein 18kDa (TSPO) is expressed in the outer mitochondrial membrane and is implicated in several functions, including cholesterol transport and steroidogenesis. Under normal physiological conditions, TSPO is present in very low concentrations in the human brain but is markedly upregulated in response to brain injury and inflammation. This upregulation is strongly associated with activated microglia. Therefore, TSPO is particularly suited for assessing active gliosis associated with brain lesions following injury or disease. For over three decades, TSPO has been studied as a biomarker. Numerous radioligands for positron emission tomography (PET) that target TSPO have been developed for imaging inflammatory progression in the brain. Although [11C]PK11195, the prototypical first-generation PET radioligand, is still widely used for in vivo studies, mainly now as its single more potent R-enantiomer, it has severe limitations, including low sensitivity and poor amenability to quantification. Second-generation radioligands are characterized by higher TSPO specific signals but suffer from other drawbacks, such as sensitivity to the TSPO single nucleotide polymorphism (SNP) rs6971. Therefore, their applications in human studies have the burden of needing to genotype subjects. Consequently, recent efforts are focused on developing improved radioligands that combine the optimal features of the second generation with the ability to overcome the differences in binding affinities across the population. This review presents essential principles in the design and development of TSPO PET ligands and discusses prominent examples among the main chemotypes.

Preclinical evaluation and pilot clinical study of [18F]AlF-NOTA-FAPI-04 for PET imaging of rheumatoid arthritis

PURPOSE

Fibroblast-like synoviocytes (FLSs) are key effector cells in the inflamed joints of patients with rheumatoid arthritis (RA). Previous studies have suggested that fibroblast activation protein (FAP) is highly expressed in RA-derived FLSs and is a specific marker of activated RA FLSs. In this study, we developed aluminum-[¹⁸F]-labeled 1,4,7-triazacyclononane-N,N',N″-triacetic acid-conjugated FAP inhibitor 04 ([¹⁸F]AlF-NOTA-FAPI-04) to image RA-FLSs in vitro and arthritic joints in collagen-induced arthritis (CIA) mice and RA patients.

METHODS

RA FLSs and NIH3T3 cells transfected with FAP were used to perform in vitro-binding studies. Biodistribution was conducted in normal DBA1 mice. Collagen-induced arthritis (CIA) models with different arthritis scores were subjected to [¹⁸F]AlF-NOTA-FAPI-04 and ¹⁸F-FDG PET imaging. Histological examinations were performed to evaluate FAP expression and Cy3 dye-labeled FAPI-04(Cy3-FAPI-04) uptake. Blocking studies with excess unlabeled FAPI-04 in CIA mice and NIH3T3 xenografts in immunocompromised mice were used to evaluate the binding specificity of [¹⁸F]AlF-NOTA-FAPI-04. Additionally, [¹⁸F]AlF-NOTA-FAPI-04 PET imaging was performed on two RA patients.

RESULTS

The binding of [¹⁸F]AlF-NOTA-FAPI-04 increased significantly in RA FLSs and NIH3T3 cells overexpressing FAP compared to their parental controls (^FAP-GFP-NIH3T3 vs. ^GFP-NIH3T3, 2.40 ± 0.078 vs. 0.297 ± 0.05% AD/10⁵ cells; RA FLSs vs. OA FLSs, 1.54 ± 0.064 vs. 0.343 ± 0.056% AD/10⁵ cells). Compared to ¹⁸F-FDG imaging, [18F]AlF-NOTA-FAPI-04 showed high uptake in inflamed joints in the early stage of arthritis, which was positively correlated with the arthritic scores (Pearson r=0.834, P<0.001). In addition, the binding of [¹⁸F]AlF-NOTA-FAPI-04 to cells with high FAP expression and the uptake of [¹⁸F]AlF-NOTA-FAPI-04 in arthritic joints both could be blocked by excessive unlabeled FAPI-04. Fluorescent staining showed that the intensity of Cy3-FAPI-04 binding to FAP increased accordingly as the expression of FAP protein increased in cells and tissue sections. Furthermore, the uptake of [¹⁸F]AlF-NOTA-FAPI-04 in ^FAP-GFP-NIH3T3 xenografts was significantly higher than that in ^GFP-NIH3T3 xenograft (35.44 ± 4.27 vs 7.92 ± 1.83% ID/mL). Finally, [¹⁸F]AlF-NOTA-FAPI-04 PET/CT imaging in RA patients revealed nonphysiologically high tracer uptake in the synovium of arthritic joints.

CONCLUSION

[¹⁸F]AlF-NOTA-FAPI-04 is a promising radiotracer for imaging RA FLSs and could potentially complement the current noninvasive diagnostic parameters.

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.

Feasibility of TSPO-Specific Positron Emission Tomography Radiotracer for Evaluating Paracetamol-Induced Liver Injury

Macrophages are activated during the early phase of paracetamol-induced liver injury (PLI). [¹⁸F]GE180 is a radiolabeled ligand that recognizes the macrophage translocator protein (TSPO). In this study, we evaluated the feasibility of a TSPO-specific radiotracer in a rat model of PLI. A rat model of liver injury was induced by intraperitoneal administration of paracetamol. [¹⁸F]GE180 positron emission tomography (PET) images were obtained after 24 h. The maximal and mean standardized uptake values (SUV_max and SUV_av) of the liver and serum biomarker levels were examined. The TSPO expression level was examined using real-time polymerase chain reaction and Western blot analysis. [¹⁸F]GE180 hepatic uptake in the PLI group was significantly higher than that in the control group (SUV_max p = 0.001; SUV_av p = 0.005). Both mRNA and protein TSPO expression levels were higher in the PLI group. The mRNA expression level of TSPO was significantly correlated with [¹⁸F]GE180 hepatic uptake in both groups (SUV_max p = 0.019; SUV_av p = 0.007). [¹⁸F]GE180 hepatic uptake in the PLI group showed a significant positive correlation with ALT₂₄ and ALT₄₈ (ALT₂₄ p = 0.016; ALT₄₈p = 0.002). [¹⁸F]GE180 enabled visualization of PLI through TSPO overexpression. Our results support the potential utility of hepatic uptake by TSPO-PET as a non-invasive imaging biomarker for the early phase of PLI.

EANM recommendations based on systematic analysis of small animal radionuclide imaging in inflammatory musculoskeletal diseases

Inflammatory musculoskeletal diseases represent a group of chronic and disabling conditions that evolve from a complex interplay between genetic and environmental factors that cause perturbations in innate and adaptive immune responses. Understanding the pathogenesis of inflammatory musculoskeletal diseases is, to a large extent, derived from preclinical and basic research experiments. In vivo molecular imaging enables us to study molecular targets and to measure biochemical processes non-invasively and longitudinally, providing information on disease processes and potential therapeutic strategies, e.g. efficacy of novel therapeutic interventions, which is of complementary value next to ex vivo (post mortem) histopathological analysis and molecular assays. Remarkably, the large body of preclinical imaging studies in inflammatory musculoskeletal disease is in contrast with the limited reports on molecular imaging in clinical practice and clinical guidelines. Therefore, in this EANM-endorsed position paper, we performed a systematic review of the preclinical studies in inflammatory musculoskeletal diseases that involve radionuclide imaging, with a detailed description of the animal models used. From these reflections, we provide recommendations on what future studies in this field should encompass to facilitate a greater impact of radionuclide imaging techniques on the translation to clinical settings.

Automated radiosynthesis of two 18F-labeled tracers containing 3-fluoro-2-hydroxypropyl moiety, [18F]FMISO and [18F]PM-PBB3, via [18F]epifluorohydrin

Background

[¹⁸F]Fluoromisonidazole ([¹⁸F]FMISO) and 1-[¹⁸F]fluoro-3-((2-((1E,3E)-4-(6-(methylamino)pyridine-3-yl)buta-1,3-dien-1-yl)benzo[d]thiazol-6-yl)oxy)propan-2-ol ([¹⁸F]PM-PBB3 or [¹⁸F]APN-1607) are clinically used radiotracers for imaging hypoxia and tau pathology, respectively. Both radiotracers were produced by direct ¹⁸F-fluorination using the corresponding tosylate precursors 1 or 2 and [¹⁸F]F⁻, followed by the removal of protecting groups. In this study, we synthesized [¹⁸F]FMISO and [¹⁸F]PM-PBB3 by ¹⁸F-fluoroalkylation using [¹⁸F]epifluorohydrin ([¹⁸F]5) for clinical applications.

Results

First, [¹⁸F]5 was synthesized by the reaction of 1,2-epoxypropyl tosylate (8) with [¹⁸F]F⁻ and was purified by distillation. Subsequently, [¹⁸F]5 was reacted with 2-nitroimidazole (6) or PBB3 (7) as a precursor for ¹⁸F-labeling, and each reaction mixture was purified by preparative high-performance liquid chromatography and formulated to obtain the [¹⁸F]FMISO or [¹⁸F]PM-PBB3 injection. All synthetic sequences were performed using an automated ¹⁸F-labeling synthesizer. The obtained [¹⁸F]FMISO showed sufficient radioactivity (0.83 ± 0.20 GBq at the end of synthesis (EOS); n = 8) with appropriate radiochemical yield based on [¹⁸F]F⁻ (26 ± 7.5 % at EOS, decay-corrected; n = 8). The obtained [¹⁸F]PM-PBB3 also showed sufficient radioactivity (0.79 ± 0.10 GBq at EOS; n = 11) with appropriate radiochemical yield based on [¹⁸F]F⁻ (16 ± 3.2 % at EOS, decay-corrected; n = 11).

Conclusions

Both [¹⁸F]FMISO and [¹⁸F]PM-PBB3 injections were successfully synthesized with sufficient radioactivity by ¹⁸F-fluoroalkylation using [¹⁸F]5.

Supplementary Information

The online version contains supplementary material available at 10.1186/s41181-021-00138-9.

Mapping of Translocator Protein (18 kDa) in Peripheral Sterile Inflammatory Disease and Cancer through PET Imaging

Positron emission tomography (PET) imaging of the translocator 18 kDa protein (TSPO) with radioligands has become an effective means of research in peripheral inflammatory conditions that occur in many diseases and cancers. The peripheral sterile inflammatory diseases (PSIDs) are associated with a diverse group of disorders that comprises numerous enduring insults including the cardiovascular, respiratory, gastrointestinal, or musculoskeletal system. TSPO has recently been introduced as a potential biomarker for peripheral sterile inflammatory diseases (PSIDs). The major critical issue related to PSIDs is its timely characterization and localization of inflammatory foci for proper therapy of patients. As an alternative to metabolic imaging, protein imaging expressed on immune cells after activation is of great importance. The five transmembrane domain translocator protein-18 kDa (TSPO) is upregulated on the mitochondrial cell surface of macrophages during inflammation, serving as a potential ligand for PET tracers. Additionally, the overexpressed TSPO protein has been positively correlated with various tumor malignancies. In view of the association of escalated TSPO expression in both disease conditions, it is an immensely important biomarker for PET imaging in oncology and PSIDs. In this review, we summarize the most outstanding advances on TSPO-targeted PSIDs and cancer in the development of TSPO ligands as a potential diagnostic tool, specifically discussing the last five years.

Novel positron emission tomography tracers for imaging of rheumatoid arthritis

Positron emission tomography (PET) is a nuclear imaging modality that relies on visualization of molecular targets in tissues, which is nowadays combined with a structural imaging modality such as computed tomography (CT) or Magnetic Resonance Imaging (MRI) and referred to as hybrid PET imaging. This technique allows to image specific immunological targets in rheumatoid arthritis (RA). Moreover, quantification of the PET signal enables highly sensitive monitoring of therapeutic effects on the molecular target. PET may also aid in stratification of the immuno-phenotype at baseline in order to develop personalized therapy. In this systematic review we will provide an overview of novel PET tracers, investigated in the context of RA, either pre-clinically, or clinically, that specifically visualize immune cells or stromal cells, as well as other factors and processes that contribute to pathology. The potential of these tracers in RA diagnosis, disease monitoring, and prediction of treatment outcome will be discussed. In addition, novel PET tracers established within the field of oncology that may be of use in RA will also be reviewed in order to expand the future opportunities of PET imaging in RA.

Radiosynthesis of 18F-fluoroethylated tracers via a simplified one-pot 18F-fluoroethylation method using [18F]fluoroethyl tosylate

Recently, a straightforward one-pot method for ¹⁸F-fluoroethylation without azeotropic drying of cyclotron-produced [¹⁸F]F^- was developed. In this study, we have attempted to simplify the automated radiosynthesis of two [¹⁸F]fluoroethylated tracers, [¹⁸F]FEDAC and [¹⁸F]FET, using a desmethyl labeling precursor and [¹⁸F]fluoroethyl tosylate, based on the above-mentioned method. The radiochemical yields of [¹⁸F]FEDAC and [¹⁸F]FET were 26 ± 3.7% (n = 5) and 14 ± 2.2% (n = 4), respectively, based on total [¹⁸F]F^- at the end of irradiation.

64Cu-ATSM and 99mTc(CO)3-DCM20 potential in the early detection of rheumatoid arthritis

OBJECTIVES

Molecular imaging constitutes a promising technique for the early detection of rheumatoid arthritis (RA). Macrophages and hypoxia play significant roles in inflamed synovium. In the present study, we evaluated the efficacy of radiopharmaceuticals that target macrophage mannose receptors (^99mTc-labeled mannosylated dextran or ^99mTc(CO)₃-DCM20) and hypoxia (copper(II) diacetyl-di(N⁴-methylthiosemicarbazone) or Cu-ATSM) for the early detection of RA in collagen-induced arthritis (CIA) mice models.

METHODS

CIA model was developed in DBA/1 mice, and the clinical score for arthritis was visually assessed on a regular basis. Two biodistribution studies were performed in a paired-labeled format using 2-deoxy-2-¹⁸F-fluoro-D-glucose (¹⁸F-FDG) as a reference: (1) ^99mTc(CO)₃-DCM20 with ¹⁸F-FDG and (2) ⁶⁷Cu-ATSM with ¹⁸F-FDG.

RESULTS

The accumulation levels of ^99mTc(CO)₃-DCM20 and ⁶⁷Cu-ATSM in forepaws, hindpaws, and knee joints of CIA mice were significantly higher than that of control mice. In contrast, ¹⁸F-FDG uptake in hindpaws and knee joints showed no significant difference between CIA and control mice. The radioactivity levels of ^99mTc(CO)₃-DCM20 and ⁶⁷Cu-ATSM were significantly correlated with the clinical scores for the paws.

CONCLUSION

These results suggest the potential usefulness of ^99mTc(CO)₃-DCM20 and radiolabeled Cu-ATSM for the imaging and early detection of RA.

Basic Science of PET Imaging for Inflammatory Diseases

FDG-PET/CT has recently emerged as a useful tool for the evaluation of inflammatory diseases too, in addition to that of malignant diseases. The imaging is based on active glucose utilization by inflammatory tissue. Autoradiography studies have demonstrated high FDG uptake in macrophages, granulocytes, fibroblasts, and granulation tissue. Especially, activated macrophages are responsible for the elevated FDG uptake in some types of inflammation. According to one study, after activation by lipopolysaccharide of cultured macrophages, the [¹⁴C]2DG uptake by the cells doubled, reaching the level seen in glioblastoma cells. In activated macrophages, increase in the expression of total GLUT1 and redistributions from the intracellular compartments toward the cell surface have been reported. In one rheumatoid arthritis model, following stimulation by hypoxia or TNF-α, the highest elevation of the [³H]FDG uptake was observed in the fibroblasts, followed by that in macrophages and neutrophils. As the fundamental mechanism of elevated glucose uptake in both cancer cells and inflammatory cells, activation of glucose metabolism as an adaptive response to a hypoxic environment has been reported, with transcription factor HIF-1α playing a key role. Inflammatory cells and cancer cells seem to share the same molecular mechanism of elevated glucose metabolism, lending support to the notion of usefulness of FDGPET/CT for the evaluation of inflammatory diseases, besides cancer.

Comparative Evaluation of 68Ga-Citrate PET/CT and 18F-FDG PET/CT in the Diagnosis of Type II Collagen-Induced Arthritis in Rats

Rheumatoid arthritis (RA) is a chronic autoimmune disease characterized by systemic, symmetrical, and erosive synovitis. RA is one of the most common disabling diseases in the clinic. The main clinical intervention strategies are early diagnosis and early treatment. This study aims to predict the diagnostic value of ⁶⁸Ga-citrate and ¹⁸F-FDG PET/CT in RA by comparing and analyzing the value of ⁶⁸Ga-citrate and ¹⁸F-FDG PET/CT for diagnosing type II collagen-induced arthritis (CIA) in rats. Some CIA models were established. Normal rats were selected as the control group, and 23 days and 40 days were selected as the early and late time points of arthritis, respectively. The semiquantitative analysis of CIA rats was carried out with ⁶⁸Ga-citrate PET/CT and ¹⁸F-FDG PET/CT, and the ratio of the maximum standardized uptake (SUV_max) values in the regions of interest (ROIs) of the hind foot ankle joint and thigh muscle was calculated and statistically analyzed. The distribution of CIA rats in vivo at the ⁶⁸Ga-citrate 90 min time point was studied, and the ankle tissues were evaluated with hematoxylin and eosin (HE) staining. ⁶⁸Ga-citrate PET/CT is obviously superior to ¹⁸F-FDG PET/CT for CIA imaging, and the statistical results show that the difference between the two examination methods is statistically significant (P < 0.001). The uptake of these two radiopharmaceuticals showed the same trend in arthritis rats with different scores. The distribution of ⁶⁸Ga-citrate at 90 min is consistent with the trend shown by ⁶⁸Ga-citrate PET/CT. ⁶⁸Ga-citrate PET/CT can reflect the inflammatory activity of affected joints in CIA rats earlier and more sensitively than ¹⁸F-FDG PET/CT, and this imaging advantage continues until the later stage of inflammation. Therefore, ⁶⁸Ga-citrate PET/CT is worthy of further promotion and application in the clinical diagnosis of RA.

Synthesis of two novel [18F]fluorobenzene-containing radiotracers via spirocyclic iodonium ylides and positron emission tomography imaging of translocator protein (18 kDa) in ischemic brain

Two novel radiotracers, namely, N-(4-[18F]fluorobenzyl)-N-methyl-2-(7-methyl-8-oxo-2-phenyl-7,8-dihydro-9H-purin-9-yl)acetamide ([18F]5) and 2-(5-(4-[18F]fluorophenyl)-2-oxobenzo[d]oxazol-3(2H)-yl)-N-methyl-N-phenylacetamide ([18F]6), were developed for positron emission tomography (PET) imaging of translocator protein (18 kDa) (TSPO) in ischemic brain in this study. The two radiotracers with a [18F]fluorobenzene ring were derived from the corresponding [18F]fluoroethyl tracers [18F]7 and [18F]8 which underwent [18F]defluoroethylation in vivo easily. [18F]5 or [18F]6 was synthesized by the radiofluorination of the spirocyclic iodonium ylide precursor 10 or 17 with [18F]F- in 23 ± 10% (n = 7) or 56 ± 9% (n = 7) radiochemical yields (decay-corrected, based on [18F]F-). [18F]5 and [18F]6 showed high in vitro binding affinities (Ki = 0.70 nM and 5.9 nM) for TSPO and moderate lipophilicities (log D = 2.9 and 3.4). Low uptake of radioactivity for both radiotracers was observed in mouse bones. Metabolite analysis showed that the in vivo stability of [18F]5 and [18F]6 was improved in comparison to the parent radiotracers [18F]7 and [18F]8. In particular, no radiolabelled metabolite of [18F]5 was found in the mouse brains at 60 min after the radiotracer injection. PET studies with [18F]5 on ischemic rat brains revealed a higher binding potential (BPND = 3.42) and maximum uptake ratio (4.49) between the ipsilateral and contralateral sides. Thus, [18F]5 was shown to be a useful PET radiotracer for visualizing TSPO in neuroinflammation models.

Emerging imaging targets for infiltrative cardiomyopathy: Inflammation and fibrosis

Molecular imaging in infiltrative cardiomyopathies is increasingly penetrating the clinical arena. Current approaches target the infiltrate directly, or its metabolic, physiologic, or functional consequences. Inflammation may not just play a role as the infiltrative mechanism itself. It is also thought to play a key role in the development and progression of heart failure in general, because it promotes the development of tissue fibrosis. The cascade leading from tissue damage to inflammation and further to fibrosis and loss of function has emerged as a therapeutic target. This review focuses (1) on novel tracers of inflammation, which are on the brink of clinical applicability and may be more specific than the gross metabolic marker F-18 deoxyglucose; and (2) on novel biologic imaging targets in fibrosis, which may be exploited for interrogation of the crosstalk between inflammation and loss of contractile function. Ultimately, the success of any novel molecular imaging assay will depend on whether it can be used for successful guidance of novel, targeted therapies aiming at tissue regeneration.

Automated Synthesis of (rac)-, (R)-, and (S)-[18 F]Epifluorohydrin and Their Application for Developing PET Radiotracers Containing a 3-[18 F]Fluoro-2-hydroxypropyl Moiety

To introduce the 3-[¹⁸ F]fluoro-2-hydroxypropyl moiety into positron emission tomography (PET) radiotracers, we performed automated synthesis of (rac)-, (R)-, and (S)-[¹⁸ F]epifluorohydrin ([¹⁸ F]1) by nucleophilic displacement of (rac)-, (R)-, or (S)-glycidyl tosylate with ¹⁸ F^- and purification by distillation. The ring-opening reaction of (R)- or (S)-[¹⁸ F]1 with phenol precursors gave enantioenriched [¹⁸ F]fluoroalkylated products without racemisation. We then synthesised (rac)-, (R)-, and (S)- 2-{5-[4-(3-[¹⁸ F]fluoro-2-hydroxypropoxy)phenyl]-2-oxobenzo[d]oxazol-3(2H)-yl}-N-methyl-N-phenylacetamide ([¹⁸ F]6) as novel radiotracers for the PET imaging of translocator protein (18 kDa) and showed that (R)- and (S)-[¹⁸ F]6 had different radioactivity uptake in mouse bone and liver. Thus, (rac)-, (R)-, and (S)-[¹⁸ F]1 are effective radiolabelling reagents and can be used to develop PET radiotracers by examining the effects of chirality on their in vitro binding affinities and in vivo behaviour.