Fluorine-18 Labelled Radioligands for PET Imaging of Cyclooxygenase-2

Molecular imaging probes enable the early and accurate detection of disease-specific biomarkers and facilitate personalized treatment of many chronic diseases, including cancer. Among current clinically used functional imaging modalities, positron emission tomography (PET) plays a significant role in cancer detection and in monitoring the response to therapeutic interventions. Several preclinical and clinical studies have demonstrated the crucial involvement of cyclooxygenase-2 (COX-2) isozyme in cancer development and progression, making COX-2 a promising cancer biomarker. A variety of COX-2-targeting PET radioligands has been developed based on anti-inflammatory drugs and selective COX-2 inhibitors. However, many of those suffer from non-specific binding and insufficient metabolic stability. This article highlights examples of COX-2-targeting PET radioligands labelled with the short-lived positron emitter ¹⁸F, including radiosynthesis and PET imaging studies published in the last decade (2012–2021).     

PET Imaging of Fructose Metabolism in a Rodent Model of Neuroinflammation with 6-[18F]fluoro-6-deoxy-D-fructose

Fluorine-18 labeled 6-fluoro-6-deoxy-D-fructose (6-[¹⁸F]FDF) targets the fructose-preferred facilitative hexose transporter GLUT5, which is expressed predominantly in brain microglia and activated in response to inflammatory stimuli. We hypothesize that 6-[¹⁸F]FDF will specifically image microglia following neuroinflammatory insult. 6-[¹⁸F]FDF and, for comparison, [¹⁸F]FDG were evaluated in unilateral intra-striatal lipopolysaccharide (LPS)-injected male and female rats (50 µg/animal) by longitudinal dynamic PET imaging in vivo. In LPS-injected rats, increased accumulation of 6-[¹⁸F]FDF was observed at 48 h post-LPS injection, with plateaued uptake (60–120 min) that was significantly higher in the ipsilateral vs. contralateral striatum (0.985 ± 0.047 and 0.819 ± 0.033 SUV, respectively; p = 0.002, n = 4M/3F). The ipsilateral–contralateral difference in striatal 6-[¹⁸F]FDF uptake expressed as binding potential (BP_SRTM) peaked at 48 h (0.19 ± 0.11) and was significantly decreased at one and two weeks. In contrast, increased [¹⁸F]FDG uptake in the ipsilateral striatum was highest at one week post-LPS injection (BP_SRTM = 0.25 ± 0.06, n = 4M). Iba-1 and GFAP immunohistochemistry confirmed LPS-induced activation of microglia and astrocytes, respectively, in ipsilateral striatum. This proof-of-concept study revealed an early response of 6-[¹⁸F]FDF to neuroinflammatory stimuli in rat brain. 6-[¹⁸F]FDF represents a potential PET radiotracer for imaging microglial GLUT5 density in brain with applications in neuroinflammatory and neurodegenerative diseases.     

Multimodality Imaging Agents with PET as the Fundamental Pillar

Positron emission tomography (PET) provides quantitative information in vivo with ultra‐high sensitivity but is limited by its relatively low spatial resolution. Therefore, PET has been combined with other imaging modalities, and commercial systems such as PET/computed tomography (CT) and PET/magnetic resonance (MR) have become available. Inspired by the emerging field of nanomedicine, many PET‐based multimodality nanoparticle imaging agents have been developed in recent years. This Minireview highlights recent progress in the design of PET‐based multimodality imaging nanoprobes with an aim to overview the major advances and key challenges in this field and substantially improve our knowledge of this fertile research area.     

Preliminary Assessment of the Anti-inflammatory Activity of New Structural Honokiol Analogs with a 4′-O-(2-Fluoroethyl) Moiety and the Potential of Their 18F-Labeled Derivatives for Neuroinflammation Imaging

Neolignans honokiol and 4′-O-methylhonokiol (MH) and their derivatives have pronounced anti-inflammatory activity, as evidenced by numerous pharmacological studies. Literature data suggested that cyclooxygenase type 2 (COX-2) may be a target for these compounds in vitro and in vivo. Recent studies of [¹¹C]MPbP (4′-[¹¹C]methoxy-5-propyl-1,1′-biphenyl-2-ol) biodistribution in LPS (lipopolysaccharide)-treated rats have confirmed the high potential of MH derivatives for imaging neuroinflammation. Here, we report the synthesis of four structural analogs of honokiol, of which 4′-(2-fluoroethoxy)-2-hydroxy-5-propyl-1, 1′-biphenyl (F-IV) was selected for labeling with fluorine-18 (T_1/2 = 109.8 min) due to its high anti-inflammatory activity confirmed by enzyme immunoassays (EIA) and neuromorphological studies. The high inhibitory potency of F-IV to COX-2 and its moderate lipophilicity and chemical stability are favorable factors for the preliminary evaluation of the radioligand [¹⁸F]F-IV in a rodent model of neuroinflammation. [¹⁸F]F-IV was prepared with good radiochemical yield and high molar activity and radiochemical purity by ¹⁸F-fluoroethylation of the precursor with Boc-protecting group (15) with [¹⁸F]2-fluoro-1-bromoethane ([¹⁸F]FEB). Ex vivo biodistribution studies revealed a small to moderate increase in radioligand uptake in the brain and peripheral organs of LPS-induced rats compared to control animals. Pretreatment with celecoxib resulted in significant blocking of radioactivity uptake in the brain (pons and medulla), heart, lungs, and kidneys, indicating that [¹⁸F]F-IV is likely to specifically bind to COX-2 in a rat model of neuroinflammation. However, in comparison with [¹¹C]MPbP, the new radioligand showed decreased brain uptake in LPS rats and high retention in the blood pool, which apparently could be explained by its high plasma protein binding. We believe that the structure of [¹⁸F]F-IV can be optimized by replacing the substituents in the biphenyl core to eliminate these disadvantages and develop new radioligands for imaging activated microglia.     

Practical Synthesis of FEt-penta-cyclofenil and Its Derivatives for Potential PET Imaging

Generally, FEt-penta-cyclofenil and its derivatives have greater relative binding affinity to estradiol receptors than estradiol. (4-Fluoroethoxyphenyl)–(4′-hydroxyphenyl) methylenecyclopentane and its derivatives were synthesized for potential radioactive image agents, and their structures were characterized by ultraviolet, infrared, ¹H NMR, ¹⁹F NMR, and high-resolution mass spectrometry.     

2型环氧酶抑制剂赛来昔布的合成

以对甲苯乙酮和三氟醋酸乙酯为原料，经缩合、环合两步反应得到2型环氧酶抑制剂赛来昔布，总收率为67％。     

PET Diagnostic Molecules Utilizing Multimeric Cyclic RGD Peptide Analogs for Imaging Integrin αvβ3 Receptors

Multimeric ligands consisting of multiple pharmacophores connected to a single backbone have been widely investigated for diagnostic and therapeutic applications. In this review, we summarize recent developments regarding multimeric radioligands targeting integrin α_vβ₃ receptors on cancer cells for molecular imaging and diagnostic applications using positron emission tomography (PET). Integrin α_vβ₃ receptors are glycoproteins expressed on the cell surface, which have a significant role in tumor angiogenesis. They act as receptors for several extracellular matrix proteins exposing the tripeptide sequence arginine-glycine-aspartic (RGD). Cyclic RDG peptidic ligands c(RGD) have been developed for integrin α_vβ₃ tumor-targeting positron emission tomography (PET) diagnosis. Several c(RGD) pharmacophores, connected with the linker and conjugated to a chelator or precursor for radiolabeling with different PET radionuclides (¹⁸F, ⁶⁴Cu, and ⁶⁸Ga), have resulted in multimeric ligands superior to c(RGD) monomers. The binding avidity, pharmacodynamic, and PET imaging properties of these multimeric c(RGD) radioligands, in relation to their structural characteristics are analyzed and discussed. Furthermore, specific examples from preclinical studies and clinical investigations are included.     

Short Synthesis of COX-2 Inhibitor Inotilone

An efficient three-step synthesis of COX-2 inhibitor inotilone from acetaldoxime is described. The structure of inotilone was elucidated via an aldol reaction between 5-methyl-3(2H)-furanone and 3,4-dihydroxybenzaldehyde. This approach describes a convenient pathway to 5-alkyl-3-furanones through isoxazole chemistry.     

Fully Automated Synthesis of Novel TSPO PET Imaging Ligand [18F]Fluoroethyltemazepam

Introduction: Benzodiazepines, including temazepam are described as TSPO antagonists. In fact, TSPO was initially described as a peripheral benzodiazepine receptor (PBR) with a secondary binding site for diazepam. TSPO is a potential imaging target of neuroinflammation because there is an amplification of the expression of this receptor. Objectives: Herein, we developed a novel fluorinated benzodiazepine ligand, [¹⁸F]Fluoroethyltemazepam ([¹⁸F]F-FETEM), for positron emission tomography (PET) imaging of translocator protein (18 kDa). Methods: [¹⁸F]F-FETEM was radiolabelled with an automated synthesizer via a one-pot procedure. We conducted a [¹⁸F]F-aliphatic nucleophilic substitution of a tosylated precursor followed by purification on C18 and Alumina N SPE cartridges. Quality control tests was also carried out. Results: We obtained 2.0–3.0% decay-uncorrected radiochemical activity yield (3.7% decay-corrected) within the whole synthesis time about 33 min. The radiochemical purity of [¹⁸F]F-FETEM was over 90% by TLC analysis. Conclusions: This automated procedure may be used as basis for future production of [¹⁸F]F-FETEM for preclinical PET imaging studies.     

新型PET核素68Ga标记D-脱氧葡萄糖的合成及生物学评价

以2-氨基-2-脱氧-D-葡萄糖为原料,通过微波加热法合成得到可用于核素标记的葡萄糖胺-1,4,7,10-四氮杂环十二烷-1,4,7,10-四乙酸(DOTA)前体化合物.产物通过IR,1H NMR,HPLC-MS,ESI-HRMS表征.通过标记条件的优化设计,在微波作用下,与新型正电子显像核素Ga-68进行放射性标记,高产率得到68Ga-DOTA-DG.而后对该标记化合物进行了K-562,Hela,A431等肿瘤细胞摄取实验,测定了对肿瘤的特异性.68Ga-DOTA-DG的合成,拓宽了以18F-FDG为首的肿瘤代谢类显像剂的新应用,为PET(Positron emission tomography)显像肿瘤代谢情况提供了一种新的途径.     

Mitigating Effect of Lindera obtusiloba Blume Extract on Neuroinflammation in Microglial Cells and Scopolamine-Induced Amnesia in Mice

Lindera obtusiloba Blume (family, Lauraceae), native to Northeast Asia, has been used traditionally in the treatment of trauma and neuralgia. In this study, we investigated the neuroinflammatory effect of methanol extract of L. obtusiloba stem (LOS-ME) in a scopolamine-induced amnesia model and lipopolysaccharide (LPS)-stimulated BV2 microglia cells. LOS-ME downregulated the expression of inducible nitric oxide synthase (iNOS), cyclooxygenase (COX)-2, inflammatory cytokines, and inhibited the phosphorylation of nuclear factor kappa-B (NF-ĸB) and extracellular signal-regulated kinase (ERK) in LPS-stimulated BV2 cells. Male C57/BL6 mice were orally administered 20 and 200 mg/kg of LOS-ME for one week, and 2 mg/kg of scopolamine was administered intraperitoneally on the 8th day. In vivo behavioral experiments (Y-maze and Morris water maze test) confirmed that LOS-ME alleviated cognitive impairments induced by scopolamine and the amount of iNOS expression decreased in the hippocampus of the mouse brain. Microglial hyper-activation was also reduced by LOS-ME pretreatment. These findings suggest that LOS-ME might have potential in the treatment for cognitive improvement by regulating neuroinflammation.     

Synthesis and Biological Evaluation of Quinoxaline Derivatives for PET Imaging of the NMDA Receptor

Due to the biological complexity of the N‐methyl‐d ‐aspartate receptor (NMDAR ), the development of a positron emission tomography radiotracer for the imaging of NMDAR has met with limited success. Recent studies have established the presence of GluN2A subunit of the NMDAR in the heart and its role in the regulation of intracellular calcium levels. In our efforts to develop an imaging agent for the GluN2A subunit, we designed three new compounds based on a quinoxaline scaffold. The synthesis of the analogues was based on a two‐step Kabachnik–Fields reaction in sequence with Suzuki cross‐coupling and acid hydrolysis. They exhibited comparable high binding affinity values below 5 nm . A two‐step radiolabeling procedure was successfully developed for the synthesis of [¹⁸F] 1 . [¹⁸F] 1 was obtained in a modest overall radiochemical yield of 5.5 ± 4.2%, a good specific radioactivity of 254 ± 158 GBq/μmol, and a radiochemical purity > 99%. While compounds 2 and 3 showed comparable binding affinity towards NMDAR , sluggish radiolabeling, prevented their further evaluation. For [¹⁸F] 1 , in vitro autoradiography on rat heart slices demonstrated heterogeneous but unspecific accumulation, whereas for the brain a high in vitro specificity towards NMDAR , could be demonstrated.     

Chemiluminescent Imaging Assay of SARS-CoV-2 Protein with Target-Induced Enzyme Activity Regulation

Simple but robust testing assays are essential for screening and diagnosis of individuals infected with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) in COVID-19 pandemic. Here, we described a chemiluminescent imaging assay (CLIA) for sensitive and convenient detection of SARS-CoV-2 nucleocapsid protein (NP) by a target-induced enzyme activity regulation (T-EAR) strategy. The T-EAR used a pair of antibody-DNA probes to recognize SARS-CoV-2 NP and proximity-induce rolling circle amplification for mass-production of pyrophosphate to coordinate with Cu²⁺, which prevented the reduction of Cu²⁺ to Cu⁺ by sodium ascorbate as well as the Cu⁺-caused inactivation of horseradish peroxidase (HRP). The activity retention of HRP produced strong CL signal for the detection of SARS-CoV-2 NP by catalyzing the oxidation of luminol by H₂O₂. The T-EAR based CLIA showed a wide detection range from 1 pg/mL to 100 ng/mL (13 fM to 1.3 nM) with the requirement of only 0.75 μL of sample. This CLIA had advantages of good sensitivity, simple wash-free operation, acceptable accuracy, and high-throughput imaging detection, displaying potential applicability in screening assay of COVID-19 infection.     

A new synthesis of 3-arylthioindoles as selective COX-2 inhibitors using PIFA

The direct 3-arylthiolation of 2-substituted indoles using phenyliodine(III)bis trifluoroacetate (PIFA) in (CF₃)₂CHOH with a wide variety of benzenethiols has been accomplished. In particular, indoles bearing a 6-MeSO₂ and either a 2-methyl or 2-carboxymethyl substituent could be 3-arylthiolated in good to excellent yields to afford the corresponding 3-arylthioindoles as selective COX-2 inhibitors. In a study varying the electronic nature of the 5-substituent of 2-CO₂Et indoles, it was discovered that the yield of the reaction improved as the substituent became more electron withdrawing. This result was consistent with a proposed mechanism involving benzenethiol displacement of an intermediate 3-IPh indole complex.     

Design,Synthesis, and Validation of a Novel [11C]Promethazine PET Probe for Imaging Abeta Using Autoradiography

Promethazine, an antihistamine drug used in the clinical treatment of nausea, has been demonstrated the ability to bind Abeta in a transgenic mouse model of Alzheimer’s disease. However, so far, all of the studies were performed in vitro using extracted tissues. In this work, we report the design and synthesis of a novel [¹¹C]promethazine PET radioligand for future in vivo studies. The [¹¹C]promethazine was isolated by RP-HPLC with radiochemical purity >95% and molar activity of 48 TBq/mmol. The specificity of the probe was demonstrated using human hippocampal tissues via autoradiography.     

Preparation of 3-arylmethylindoles as selective COX-2 inhibitors

The 3-arylmethylation of indoles using TMSOTf/Et₃SiH with a wide variety of substituted benzaldehydes has been accomplished. Under these mild Lewis acid mediated reductive conditions, it was demonstrated that indoles bearing both 6-MeSO₂ and 2-methyl substituents could be 3-arylmethylated in good to excellent yields to afford the corresponding 3-arylmethyl indoles, effective as selective COX-2 inhibitors. In addition, the viability of this method for the reductive alkylation of indoles by ketones was demonstrated and shown to be C-3 regioselective. For indoles bearing both a 6-MeSO₂ and 2-cyano substituent where this indole reductive alkylation methodology was unsuccessful, an unprecedented Pd(0) mediated arylorganozinc coupling with the requisite substituted 3-methylcarbonatomethylindole proved successful in affording the desired 2-cyano-6-MeSO₂-3-arylmethylindoles effective as selective COX-2 inhibitors.     

Antiinflammation Derived Suzuki-Coupled Fenbufens as COX-2 Inhibitors: Minilibrary Construction and Bioassay

A small fenbufen library comprising 18 compounds was prepared via Suzuki Miyara coupling. The five-step preparations deliver 9–17% biphenyl compounds in total yield. These fenbufen analogs exert insignificant activity against the IL-1 release as well as inhibiting cyclooxygenase 2 considerably. Both the para-amino and para-hydroxy mono substituents display the most substantial COX-2 inhibition, particularly the latter one showing a comparable activity as celecoxib. The most COX-2 selective and bioactive disubstituted compound encompasses one electron-withdrawing methyl and one electron-donating fluoro groups in one arene. COX-2 is selective but not COX-2 to bioactive compounds that contain both two electron-withdrawing groups; disubstituted analogs with both resonance-formable electron-donating dihydroxy groups display high COX-2 activity but inferior COX-2 selectivity. In silico simulation and modeling for three COX-2 active—p-fluoro, p-hydroxy and p-amino—fenbufens show a preferable docking to COX-2 than COX-1. The most stabilization by the p-hydroxy fenbufen with COX-2 predicted by theoretical simulation is consistent with its prominent COX-2 inhibition resulting from experiments.     

Furostanol saponins with inhibitory action against COX-2 production from Tupistra chinensis rhizomes

Two furostanol saponins were obtained from the n-butanol fraction of methanol extract from Tupistra chinensis rhizomes,a folk medicine of Shennongjia Forest District of Hubei Province.Their structures were determined as (25S)-26-O-(β-D-glucopyranosyl)- furost-1β,3β,22α,26-tetrol-3-O-β-D-glucopyranosyl-(1→4)-β-D-glucopyranosyl-(1→2)-β-D-glucopyranoside (1) and (25R)- 26-O-(β-D-glucopyranosyl)-furost-1β,3β22α,26-tetrol 3-O-β-D-glucopyranosyl-(1→4)-β-D-glucopyranosyl-(1→2)-β-D-glu- copyranoside (2),on basis of chemical and spectroscopic evidences.1 and 2 displayed marked inhibitory action towards COX-2 production in macrophages of the rat abdomen induced by LPS at 20μg/mL.