Assessment of radioligands for PET imaging of cyclooxygenase-2 in an ischemic neuronal injury model

PET imaging of neuroinflammation: any credible alternatives to TSPO yet?

Over the last decades, the role of neuroinflammation in neuropsychiatric conditions has attracted an exponentially growing interest. A key driver for this trend was the ability to image brain inflammation in vivo using PET radioligands targeting the Translocator Protein 18 kDa (TSPO), which is known to be expressed in activated microglia and astrocytes upon inflammatory events as well as constitutively in endothelial cells. TSPO is a mitochondrial protein that is expressed mostly by microglial cells upon activation but is also expressed by astrocytes in some conditions and constitutively by endothelial cells. Therefore, our current understanding of neuroinflammation dynamics is hampered by the lack of alternative targets available for PET imaging. We performed a systematic search and review on radiotracers developed for neuroinflammation PET imaging apart from TSPO. The following targets of interest were identified through literature screening (including previous narrative reviews): P2Y12R, P2X7R, CSF1R, COX (microglial targets), MAO-B, I2BS (astrocytic targets), CB2R & S1PRs (not specific of a single cell type). We determined the level of development and provided a scoping review for each target. Strikingly, astrocytic biomarker MAO-B has progressed in clinical investigations the furthest, while few radiotracers (notably targeting S1P1Rs, CSF1R) are being implemented in clinical investigations. Other targets such as CB2R and P2X7R have proven disappointing in clinical studies (e.g. poor signal, lack of changes in disease conditions, etc.). While astrocytic targets are promising, development of new biomarkers and tracers specific for microglial activation has proven challenging.

Novel brain PET imaging agents: Strategies for imaging neuroinflammation in Alzheimer’s disease and mild cognitive impairment

Alzheimer’s disease (AD) is a devastating neurodegenerative disease with a concealed onset and continuous deterioration. Mild cognitive impairment (MCI) is the prodromal stage of AD. Molecule-based imaging with positron emission tomography (PET) is critical in tracking pathophysiological changes among AD and MCI patients. PET with novel targets is a promising approach for diagnostic imaging, particularly in AD patients. Our present review overviews the current status and applications of in vivo molecular imaging toward neuroinflammation. Although radiotracers can remarkably diagnose AD and MCI patients, a variety of limitations prevent the recommendation of a single technique. Recent studies examining neuroinflammation PET imaging suggest an alternative approach to evaluate disease progression. This review concludes that PET imaging towards neuroinflammation is considered a promising approach to deciphering the enigma of the pathophysiological process of AD and MCI.

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).

The Repertoire of Small-Molecule PET Probes for Neuroinflammation Imaging: Challenges and Opportunities beyond TSPO

Neuroinflammation is an adaptive response of the central nervous system to diverse potentially injurious stimuli, which is closely associated with neurodegeneration and typically characterized by activation of microglia and astrocytes. As a noninvasive and translational molecular imaging tool, positron emission tomography (PET) could provide a better understanding of neuroinflammation and its role in neurodegenerative diseases. Ligands to translator protein (TSPO), a putative marker of neuroinflammation, have been the most commonly studied in this context, but they suffer from serious limitations. Herein we present a repertoire of different structural chemotypes and novel PET ligand design for classical and emerging neuroinflammatory targets beyond TSPO. We believe that this Perspective will support multidisciplinary collaborations in academic and industrial institutions working on neuroinflammation and facilitate the progress of neuroinflammation PET probe development for clinical use.

PET Imaging of Neuroinflammation in Alzheimer's Disease

Neuroinflammation play an important role in Alzheimer's disease pathogenesis. Advances in molecular imaging using positron emission tomography have provided insights into the time course of neuroinflammation and its relation with Alzheimer's disease central pathologies in patients and in animal disease models. Recent single-cell sequencing and transcriptomics indicate dynamic disease-associated microglia and astrocyte profiles in Alzheimer's disease. Mitochondrial 18-kDa translocator protein is the most widely investigated target for neuroinflammation imaging. New generation of translocator protein tracers with improved performance have been developed and evaluated along with tau and amyloid imaging for assessing the disease progression in Alzheimer's disease continuum. Given that translocator protein is not exclusively expressed in glia, alternative targets are under rapid development, such as monoamine oxidase B, matrix metalloproteinases, colony-stimulating factor 1 receptor, imidazoline-2 binding sites, cyclooxygenase, cannabinoid-2 receptor, purinergic P2X7 receptor, P2Y12 receptor, the fractalkine receptor, triggering receptor expressed on myeloid cells 2, and receptor for advanced glycation end products. Promising targets should demonstrate a higher specificity for cellular locations with exclusive expression in microglia or astrocyte and activation status (pro- or anti-inflammatory) with highly specific ligand to enable in vivo brain imaging. In this review, we summarised recent advances in the development of neuroinflammation imaging tracers and provided an outlook for promising targets in the future.

Progress in PET Imaging of Neuroinflammation Targeting COX-2 Enzyme

Neuroinflammation and cyclooxygenase-2 (COX-2) upregulation are associated with the pathogenesis of degenerative brain diseases such as Alzheimer’s disease (AD), Parkinson’s disease (PD), amyotrophic lateral sclerosis (ALS), epilepsy, and a response to traumatic brain injury (TBI) or stroke. COX-2 is also induced in acute pain, depression, schizophrenia, various cancers, arthritis and in acute allograft rejection. Positron emission tomography (PET) imaging allows for the direct measurement of in vivo COX-2 upregulation and thereby enables disease staging, therapy evaluation and aid quantifying target occupancy of novel nonsteroidal anti-inflammatory drugs or NSAIDs. Thus far, no clinically useful radioligand is established for monitoring COX-2 induction in brain diseases due to the delay in identifying qualified COX-2-selective inhibitors entering the brain. This review examines radiolabeled COX-2 inhibitors reported in the past decade and identifies the most promising radioligands for development as clinically useful PET radioligands. Among the radioligands reported so far, the three tracers that show potential for clinical translation are, [11CTMI], [11C]MC1 and [18F]MTP. These radioligands demonstrated BBB permeablity and in vivo binding to constitutive COX-2 in the brain or induced COX-2 during neuroinflammation.

Neuroinflammation in psychiatric disorders: PET imaging and promising new targets

Neuroinflammation is a multifaceted physiological and pathophysiological response of the brain to injury and disease. Given imaging findings of 18 kDa translocator protein (TSPO) and the development of radioligands for other inflammatory targets, PET imaging of neuroinflammation is at a particularly promising stage. This Review critically evaluates PET imaging results of inflammation in psychiatric disorders, including major depressive disorder, schizophrenia and psychosis disorders, substance use, and obsessive-compulsive disorder. We also consider promising new targets that can be measured in the brain, such as monoamine oxidase B, cyclooxygenase-1 and cyclooxygenase-2, colony stimulating factor 1 receptor, and the purinergic P2X7 receptor. Thus far, the most compelling TSPO imaging results have arguably been found in major depressive disorder, for which consistent increases have been observed, and in schizophrenia and psychosis, for which patients show reduced TSPO levels. This pattern highlights the importance of validating brain biomarkers of neuroinflammation for each condition separately before moving on to patient stratification and treatment monitoring trials.

Radiosynthesis and evaluation of [18F]FMTP, a COX-2 PET ligand

BACKGROUND

The upregulation of cyclooxygenase-2 (COX-2) is involved in neuroinflammation associated with many neurological diseases as well as cancers of the brain. Outside the brain, inflammation and COX-2 induction contribute to the pathogenesis of pain, arthritis, acute allograft rejection, and in response to infections, tumors, autoimmune disorders, and injuries. Herein, we report the radiochemical synthesis and evaluation of [¹⁸F]6-fluoro-2-(4-(methylsulfonyl)phenyl)-N-(thiophen-2-ylmethyl)pyrimidin-4-amine ([¹⁸F]FMTP), a high-affinity COX-2 inhibitor, by cell uptake and PET imaging studies.

METHODS

The radiochemical synthesis of [¹⁸F]FMTP was optimized using chlorine to fluorine displacement method, by reacting [¹⁸F]fluoride/K222/K₂CO₃ with the precursor molecule. Cellular uptake studies of [¹⁸F]FMTP was performed in COX-2 positive BxPC3 and COX-2 negative PANC-1 cell lines with unlabeled FMTP as well as celecoxib to define specific binding agents. Dynamic microPET image acquisitionwas performed in anesthetized nude mice (n = 3), lipopolysaccharide (LPS) induced neuroinflammation mice (n = 4), and phosphate-buffered saline (PBS) administered control mice (n = 4) using a Trifoil microPET/CT for a scan period of 60 min.

RESULTS

A twofold higher binding of [¹⁸F]FMTP was found in COX-2 positive BxPC3 cells compared with COX-2 negative PANC-1 cells. The radioligand did not show specific binding to COX-2 negative PANC-1 cells. MicroPET imaging in wild-type mice indicated blood-brain barrier (BBB) penetration and fast washout of [¹⁸F]FMTP in the brain, likely due to the low constitutive COX-2 expression in the normal brain. In contrast, a ~ twofold higher uptake of the radioligand was found in LPS-induced mice brain than PBS treated control mice.

CONCLUSIONS

Specific binding to COX-2 in BxPC3 cell lines, BBB permeability, and increased brain uptake in neuroinflammation mice qualifies [¹⁸F]FMTP as a potential PET tracer for studying inflammation.

2-(4-Methylsulfonylphenyl)pyrimidines as Prospective Radioligands for Imaging Cyclooxygenase-2 with PET-Synthesis, Triage, and Radiolabeling

Cyclooxygenase 2 (COX-2) is an inducible enzyme responsible for the conversion of arachidonic acid into the prostaglandins, PGG2 and PGH2. Expression of this enzyme increases in inflammation. Therefore, the development of probes for imaging COX-2 with positron emission tomography (PET) has gained interest because they could be useful for the study of inflammation in vivo, and for aiding anti-inflammatory drug development targeting COX-2. Nonetheless, effective PET radioligands are still lacking. We synthesized eleven COX-2 inhibitors based on a 2(4-methylsulfonylphenyl)pyrimidine core from which we selected three as prospective PET radioligands based on desirable factors, such as high inhibitory potency for COX-2, very low inhibitory potency for COX-1, moderate lipophilicity, and amenability to labeling with a positron-emitter. These inhibitors, namely 6-methoxy-2-(4-(methylsulfonyl)phenyl-N-(thiophen-2ylmethyl)pyrimidin-4-amine (17), the 6-fluoromethyl analogue (20), and the 6-(2-fluoroethoxy) analogue (27), were labeled in useful yields and with high molar activities by treating the 6-hydroxy analogue (26) with [¹¹C]iodomethane, [¹⁸F]2-fluorobromoethane, and [d₂-¹⁸F]fluorobromomethane, respectively. [¹¹C]17, [¹⁸F]20, and [d₂-¹⁸F]27 were readily purified with HPLC and formulated for intravenous injection. These methods allow these radioligands to be produced for comparative evaluation as PET radioligands for measuring COX-2 in healthy rhesus monkey and for assessing their abilities to detect inflammation.

Positron emission tomography imaging in evaluation of MS pathology in vivo

Positron emission tomography (PET) gives an opportunity to quantitate the expression of specific molecular targets in vivo and longitudinally in brain and thus enhances our possibilities to understand and follow up multiple sclerosis (MS)-related pathology. For successful PET imaging, one needs a relevant target molecule within the brain, to which a blood–brain barrier–penetrating specific radioligand will bind. 18-kDa translocator protein (TSPO)-binding radioligands have been used to detect activated microglial cells at different stages of MS, and remyelination has been measured using amyloid PET. Several PET ligands for the detection of other inflammatory targets, besides TSPO, have been developed but not yet been used for imaging MS patients. Finally, synaptic density evaluation has been successfully tested in human subjects and gives opportunities for the evaluation of the development of cortical and deep gray matter pathology in MS. This review will discuss PET imaging modalities relevant for MS today.

Evaluation of Two Potent and Selective PET Radioligands to Image COX-1 and COX-2 in Rhesus Monkeys

This study assessed whether the newly developed PET radioligands ¹¹C-PS13 and ¹¹C-MC1 could image constitutive levels of cyclooxygenase (COX)-1 and COX-2, respectively, in rhesus monkeys. Methods: After intravenous injection of either radioligand, 24 whole-body PET scans were performed. To measure enzyme-specific uptake, scans of the 2 radioligands were also performed after administration of a nonradioactive drug preferential for either COX-1 or COX-2. Concurrent venous samples were obtained to measure parent radioligand concentrations. SUVs were calculated from 10 to 90 min. Results: ¹¹C-PS13 showed specific uptake in most organs, including spleen, gastrointestinal tract, kidneys, and brain, which was blocked by COX-1, but not COX-2, preferential inhibitors. Specific uptake of ¹¹C-MC1 was not observed in any organ except the ovaries and possibly kidneys. Conclusion: The findings suggest that ¹¹C-PS13 has adequate signal in monkeys to justify its extension to human subjects. In contrast, ¹¹C-MC1 is unlikely to show significant signal in healthy humans, though it may be able to do so in inflammatory conditions.

Radiosynthesis and in vivo evaluation of [11C]MOV as a PET imaging agent for COX-2

Radiosynthesis and in vivo evaluation of [¹¹C]4-[5-(4-methylphenyl)-3-(trifluoromethyl)-1H-pyrazol-1-yl]benzenesulfonamide (methoxy analogue of valdecoxib, [¹¹C]MOV), a COX-2 inhibitor, was conducted in rat and baboon. Synthesis of the reference standard MOV (3), and its desmethyl precursor 2 for radiolabeling were performed using 1,2-diphenylethan-1-one as the starting material in five steps with 15% overall yield. Radiosynthesis of [¹¹C]MOV was accomplished in 40 ± 10% yield and >99% radiochemical purity by reacting the precursor 2 in dimethyl formamide (DMF) with [¹¹C]CH₃I followed by removal of the dimethoxytrityl (DMT) protective group using trifluroacetic acid. PET studies in anesthetized baboon showed very low uptake and homogeneous distribution of [¹¹C]MOV in brain. The radioligand underwent rapid metabolism in baboon plasma. MicroPET studies in male Sprague Dawley rats revealed [¹¹C]MOV binding in lower thorax. The tracer binding in rats was partially blocked in heart and duodenum by the administration of 1 mg/kg oral dose of COX-2 inhibitor valdecoxib.

Imaging of microglial activation in MS using PET: Research use and potential future clinical application

Multiple sclerosis (MS) is a complex disease, where several processes can be selected as a target for positron emission topography (PET) imaging. Unlike magnetic resonance imaging (MRI), PET provides specific and quantitative information, and unlike neuropathology, it can be non-invasively applied to living patients, which enables longitudinal follow-up of the MS pathology. In the study of MS, PET can be useful for in vivo evaluation of specific pathological characteristics at various stages of the disease. Increased understanding of the progressive MS pathology will enhance the treatment options of this undertreated condition. The ultimate goal of developing and expanding PET in the study of MS is to have clinical non-invasive in vivo imaging biomarkers of neuroinflammation that will help to establish prognosis and accurately measure response to therapeutics. This topical review provides an overview of the promises and challenges of the use of PET in MS.

Precision Medicine in Multiple Sclerosis: Future of PET Imaging of Inflammation and Reactive Astrocytes

Non-invasive molecular imaging techniques can enhance diagnosis to achieve successful treatment, as well as reveal underlying pathogenic mechanisms in disorders such as multiple sclerosis (MS). The cooperation of advanced multimodal imaging techniques and increased knowledge of the MS disease mechanism allows both monitoring of neuronal network and therapeutic outcome as well as the tools to discover novel therapeutic targets. Diverse imaging modalities provide reliable diagnostic and prognostic platforms to better achieve precision medicine. Traditionally, magnetic resonance imaging (MRI) has been considered the golden standard in MS research and diagnosis. However, positron emission tomography (PET) imaging can provide functional information of molecular biology in detail even prior to anatomic changes, allowing close follow up of disease progression and treatment response. The recent findings support three major neuroinflammation components in MS: astrogliosis, cytokine elevation, and significant changes in specific proteins, which offer a great variety of specific targets for imaging purposes. Regardless of the fact that imaging of astrocyte function is still a young field and in need for development of suitable imaging ligands, recent studies have shown that inflammation and astrocyte activation are related to progression of MS. MS is a complex disease, which requires understanding of disease mechanisms for successful treatment. PET is a precise non-invasive imaging method for biochemical functions and has potential to enhance early and accurate diagnosis for precision therapy of MS. In this review we focus on modulation of different receptor systems and inflammatory aspect of MS, especially on activation of glial cells, and summarize the recent findings of PET imaging in MS and present the most potent targets for new biomarkers with the main focus on experimental MS research.

Molecular Imaging of Inflammation: Current Status

The ability to image inflammation in vivo can improve our understanding of the pathophysiology underlying various disease etiologies, including cancer, atherosclerosis, and neurodegeneration. A great wealth of preclinical and translational research has been and is currently being developed to decipher the involvement of the immune system in disease pathophysiology, quantify the course of a disease, and visualize the potential detrimental effects of excessive inflammation. Down the road, the ultimate goal is to have clinical noninvasive in vivo imaging biomarkers of inflammation that will help diagnose disease, establish prognosis, and gauge response to preventative and therapeutic strategies.

Imaging of neuroinflammation in Alzheimer's disease, multiple sclerosis and stroke: Recent developments in positron emission tomography

Neuroinflammation is thought to play a pivotal role in many diseases affecting the brain, including Alzheimer's disease, multiple sclerosis and stroke. Neuroinflammation is characterised predominantly by microglial activation, which can be visualised using positron emission tomography (PET). Traditionally, translocator protein 18kDa (TSPO) is the target for imaging of neuroinflammation using PET. In this review, recent preclinical and clinical research using PET in Alzheimer's disease, multiple sclerosis and stroke is summarised. In addition, new molecular targets for imaging of neuroinflammation, such as monoamine oxidases, adenosine receptors and cannabinoid receptor type 2, are discussed. This article is part of a Special Issue entitled: Neuro Inflammation edited by Helga E. de Vries and Markus Schwaninger.

Roles of lipid-modulating enzymes diacylglycerol kinase and cyclooxygenase under pathophysiological conditions

Lipid not only represents a constituent of the plasma membrane, but also plays a pivotal role in intracellular signaling. Lipid-mediated signaling system is strictly regulated by several enzymes, which act at various steps of the lipid metabolism. Under pathological conditions, prolonged or insufficient activation of this system results in dysregulated signaling, leading to diseases such as cancer or metabolic syndrome. Of the lipid-modulating enzymes, diacylglycerol kinase (DGK) and cyclooxygenase (COX) are intimately involved in the signaling system. DGK consists of a family of enzymes that phosphorylate a second messenger diacylglycerol (DG) to produce phosphatidic acid (PA). Both DG and PA are known to activate signaling molecules such as protein kinase C. COX catalyzes the committed step in prostanoid biosynthesis, which involves the metabolism of arachidonic acid to produce prostaglandins. Previous studies have shown that the DGK and COX are engaged in a number of pathological conditions. This review summarizes the functional implications of these two enzymes in ischemia, liver regeneration, vascular events, diabetes, cancer and inflammation.