GMP production of [18F]FE-PE2I on a TRACERLab FX2 N synthesis module, a radiotracer for in vivo PET imaging of the dopamine transport.

BACKGROUND: Parkinson's disease is a neurodegenerative disorder that is characterized by a degeneration of the dopaminergic system. Dopamine transporter (DAT) positron emission tomography (PET) imaging has emerged as a powerful and non-invasive method to quantify dopaminergic function in the living brain. The PET radioligand, [18F]FE-PE2I, a cocaine chemical derivative, has shown promising properties for in vivo PET imaging of DAT, including high affinity and selectivity for DAT, excellent brain permeability, and favorable metabolism. The aim of the current study was to scale up the production of [18F]FE-PE2I to fulfil the increasing clinical demand for this tracer. RESULTS: Thus, a fully automated and GMP-compliant production procedure has been developed using a commercially available radiosynthesis module GE TRACERLab FX2 N. [18F]FE-PE2I was produced with a radiochemical yield of 39 ± 8% (n = 4, relative [18F]F- delivered to the module). The synthesis time was 70 min, and the molar activity was 925.3 ± 763 GBq/µmol (250 ± 20 Ci/µmol). The produced [18F]FE-PE2I was stable over 6 h at room temperature. CONCLUSION: The protocol reliably provides a sterile and pyrogen-free GMP-compliant product.

Microsome Mediated in Vitro Metabolism: A Convenient Method for the Preparation of the PET Radioligand Metabolite [18F]FE-PE2I-OH for Translational Dopamine Transporter Imaging.

Undesired radiometabolites can be detrimental to the development of positron emission tomography (PET) radioligands. Methods for quantifying radioligand metabolites in brain tissue include ex vivo studies in small animals or labeling and imaging of the radiometabolite(s) of interest. The latter is a time- and resource-demanding process, which often includes multistep organic synthesis. We hypothesized that this process could be replaced by making use of liver microsomes, an in vitro system that mimics metabolism. In this study, rat liver microsomes were used to prepare radiometabolites of the dopamine transporter radioligand [18F]FE-PE2I for in vitro imaging using autoradiography and in vivo imaging using PET in rats and nonhuman primates. The primary investigated hydroxy-metabolite [18F]FE-PE2I-OH ([18F]2) was obtained in a 2% radiochemical yield and >99% radiochemical purity. In vitro and in vivo imaging demonstrated that [18F]2 readily crossed the blood-brain barrier and bound specifically and reversibly to the dopamine transporter. In conclusions, the current study demonstrates the potential of liver microsomes in the production of radiometabolites for translational imaging studies and radioligand discovery.

Reactive Palladium-Ligand Complexes for 11C-Carbonylation at Ambient Pressure: A Breakthrough in Carbon-11 Chemistry.

The Pd-Xantphos-mediated 11C-carbonylation protocol (also known as the "Xantphos- method"), due to its simplistic and convenient nature, has facilitated researchers in meeting a longstanding need for preparing 11C-carbonyl-labeled radiopharmaceuticals at ambient pressure for positron emission tomography (PET) imaging and drug discovery. This development could be viewed as a breakthrough in carbon-11 chemistry, as evidenced by the rapid global adoption of the method by the pharmaceutical industry and academic laboratories worldwide. The method has been fully automated for the good manufacturing practice (GMP)-compliant production of novel radiopharmaceuticals for human use, and it has been adapted for "in-loop" reactions and microwave technology; an impressive number of 11C-labeled compounds (>100) have been synthesized. Given the simplicity and efficiency of the method, as well as the abundance of carbonyl groups in bioactive drug molecules, we expect that this methodology will be even more widely adopted in future PET radiopharmaceutical research and drug development.

Good manufacturing procedure production of [18 F]SynVesT-1, a radioligand for in vivo positron emission tomography imaging of synaptic vesicle glycoprotein 2A.

[18 F]SynVesT-1 (also known as [18 F]SDM-8 or [18 F]MNI-1126) is a potent and selective synaptic vesicle glycoprotein 2 (SV2A) positron emission tomography (PET) imaging agent. In order to fulfill the increasing clinical demand of an 18 F-labeled SV2A PET ligand, we have developed a fully automated procedure to provide a sterile and pyrogen-free good manufacturing procedure (GMP)-compliant product of [18 F]SynVesT-1 suitable for clinical studies in humans. [18 F]SynVesT-1 is synthesized via a rapid copper-mediated radiofluorination protocol. The procedure was developed and established on a commercially available module, TracerMaker (ScanSys Laboratorieteknik ApS, Copenhagen, Denmark), a synthesis platform originally developed to conduct carbon-11 radiochemistry. From ~130 GBq (end-of-bombardment), our newly developed procedure enabled us to prepare [18 F]SynVesT-1 in an isolated radioactivity yield of 14,220 ± 800 MBq (n = 3), which corresponds to a radiochemical yield (RCY) of 19.5 ± 0.5%. The radiochemical purity (RCP) and enantiomeric purity of each of the final formulated batches exceeded 98%. The overall synthesis time was 90 min and the molar activity was 330 ± 60 GBq/µmol (8.9 ± 1.6 Ci/µmol). The produced [18 F]SynVesT-1 was stable over 8 h at room temperature and is suitable for in vivo PET imaging studies in human subjects.

Synthesis and Preclinical Evaluation of [11C]AZ11895530 for PET Imaging of the Serotonin 1A Receptor.

The serotonin 1A (5-HT1A) receptor is a G-protein-coupled receptor implicated in the pathophysiology of several neuropsychiatric and neurodegenerative disorders. We here report the preparation of two candidate 5-HT1A radioligands, [11C]AZ11132132 ([11C]3) and [11C]AZ11895530 ([11C]4), and their subsequent evaluation in vitro using autoradiography and in vivo using positron emission tomography (PET). Compounds 3 and 4 were radiolabeled at high radiochemical purity (>99%) and high molar activity (>38 GBq/µmol) by heteroatom methylation with [11C]methyl iodide. Autoradiography on whole hemispheres from post-mortem human brain revealed substantial nonspecific binding of [11C]3, while the binding of [11C]4 to brain tissue was consistent with the distribution of 5-HT1A receptors and sensitive to co-incubation with the reference 5-HT1A antagonist WAY-100635 (10 µM). Following intravenous injection of [11C]4 into a cynomolgus monkey, brain radioactivity concentration (Cmax ∼ 2.2 SUV) was high whereafter it decreased rapidly. The regional binding potential (BPND) values were calculated using the simplified reference tissue model with cerebellum as reference region. The values varied between 0.2 and 1.0 for temporal cortex, raphe nuclei, frontal cortex, and hippocampus which is consistent with the known 5-HT1A expression pattern. After pretreatment with WAY100635 (0.5 mg/kg), a homogeneous distribution of radioactivity was observed in non-human primate (NHP) brain. Although [11C]4 fulfilled important criteria for successful in vivo neuroimaging, including good blood-brain-barrier permeability and high specific binding in vitro to human brain tissue, the regional BPND values for [11C]4 in NHP brain were low when compared to those obtained with existing radioligands and thus do not merit further investigation of [11C]4. Evaluation of structurally related analogues is underway in our laboratory to identify improved candidates for clinical imaging.

The Effect of Eight Weeks of Sling-Based Training with Rotational Core Exercises on Ball Velocity in Female Team Handball Players.

The purpose of this study was to investigate whether sling-based training focused on rotational exercises would improve shooting performance in outfield handball players during the competitive season, and whether changes in performance were related to altered levels of core strength and rotational velocity. Twenty-five female outfield handball players (mean age 19.5 ± 2.0 years, height 1.72 ± 0.06 m, body mass 71.5 ± 8.6 kg, training experience 10.3 ± 2.4 years), performed 7 m shots, with and without a run-up, and jump shots. Maximal ball velocity, peak rotational velocity of the trunk with different loads and 1RM in a core strength test were measured before and after an 8-week training intervention. Players were divided into a sling-based and a plyometric/sprint training (control) group that trained three times per week for 8 weeks. The main findings were that sling-based training increased ball velocity by on average 3.2% across three techniques tested, while shooting performance decreased by 3% in the control group. However, both training groups demonstrated increased peak rotational velocity with different loads, but not the calculated 1RM core strength after the training period. It was concluded that sling-based training with rotational core exercises could improve maximal ball velocity in female handball players during a competitive season by around 3%. However, this increased ball velocity may have been caused by increased angular velocity in the core, rather than absolute maximal core strength. It is suggested that sling-based training has impacted timing variables of the different involved segments, or possibly power transfer between segments, which may explain the enhancement in ball velocity.

Fully automated production of the fibroblast activation protein radiotracer [18 F]FAPI-74.

We report herein an efficient and fully automated protocol for the radiosynthesis of [18 F]FAPI-74, a new positron emission tomography (PET) radiopharmaceutical for in vivo detection of the fibroblast activation protein. [18 F]FAPI-74 was synthesized via a rapid [18 F]aluminum fluoride coordination reaction, which was first developed on the flexible GE TRACERLab FX2N (FXN) platform and later translated to the cassette-based module Trasis AllInOne (AIO). The results obtained with both modules were comparable in terms of yield and reproducibility. Automation of [18 F]FAPI-74 radiosynthesis on the FXN was carried out in 35 min with a radiochemical yield (RCY) of 18.5 ± 2.5% (n = 5, relative to starting [18 F]fluoride). Method transfer to the AIO platform following minor optimizations allowed for the production of [18 F]FAPI-74 in an isolated RCY of 20 ± 2.5% [n = 3] with an overall synthesis time of 40 min. The radiochemical purity was greater than 95% for [18 F]FAPI-74, obtained from both modules. Overall, the protocol reliably provides a sterile and pyrogen-free good manufacturing practice (GMP) compliant product of [18 F]FAPI-74 suitable for clinical PET imaging.

One-Pot Synthesis of 11 C-Labelled Primary Benzamides via Intermediate [11 C]Aroyl Dimethylaminopyridinium Salts.

Electrophilic 11 C-labelled aroyl dimethylaminopyridinium salts, obtained by carbonylative cross-coupling of aryl halides with [11 C]carbon monoxide, were prepared for the first time and shown to be valuable intermediates in the synthesis of primary [11 C]benzamides. The methodology furnished a set of benzamide model compounds, including the two poly (ADP-ribose) polymerase (PARP) inhibitors niraparib and veliparib, in moderate to excellent radiochemical yields. In addition to providing a convenient and practical route to primary [11 C]benzamides, the current method paves the way for future application of [11 C]aroyl dimethylaminopyridinium halide salts in positron emission tomography (PET) tracer synthesis.

Preclinical Evaluation of TSPO and MAO-B PET Radiotracers in an LPS Model of Neuroinflammation.

Discovery of novel PET radiotracers targeting neuroinflammation (microglia and astrocytes) is actively pursued. Employing a lipopolysaccharide (LPS) rat model, this longitudinal study evaluated the translocator protein 18-kDa radiotracer [18F]FEPPA (primarily microglia) and monoamine oxidase B radiotracers [11C]L-deprenyl and [11C]SL25.1188 (astrocytes preferred). Increased [18F]FEPPA binding peaked at 1 week in LPS-injected striatum whereas increased lazabemide-sensitive [11C]L-deprenyl binding developed later. No increase in radiotracer uptake was observed for [11C]SL25.1188. The unilateral intrastriatal LPS rat model may serve as a useful tool for benchmarking PET tracers targeted toward distinct phases of neuroinflammatory reactions involving both microglia and astrocytes.

Radiosynthesis of a Bruton's tyrosine kinase inhibitor, [11 C]Tolebrutinib, via palladium-NiXantphos-mediated carbonylation.

Bruton's tyrosine kinase (BTK) is a key component in the B-cell receptor signaling pathway and is consequently a target for in vivo imaging of B-cell malignancies as well as in multiple sclerosis (MS) with positron emission tomography (PET). A recent Phase 2b study with Sanofi's BTK inhibitor, Tolebrutinib (also known as [a.k.a.] SAR442168, PRN2246, or BTK'168) showed significantly reduced disease activity associated with MS. Herein, we report the radiosynthesis of [11 C]Tolebrutinib ([11 C]5) as a potential PET imaging agent for BTK. The N-[11 C]acrylamide moiety of [11 C]5 was labeled by 11 C-carbonylation starting from [11 C]CO, iodoethylene, and the secondary amine precursor via a novel palladium-NiXantphos-mediated carbonylation protocol, and the synthesis was fully automated using a commercial carbon-11 synthesis platform (TracerMaker™, Scansys Laboratorieteknik). [11 C]5 was obtained in a decay-corrected radiochemical yield of 37 ± 2% (n = 5, relative to starting [11 C]CO activity) in >99% radiochemical purity, with an average molar activity of 45 GBq/µmol (1200 mCi/µmol). We envision that this methodology will be generally applicable for the syntheses of labeled N-acrylamides.

Development of a fully automated low-pressure [11 C]CO carbonylation apparatus.

[11 C]carbon monoxide ([11 C]CO) is a versatile synthon for radiolabeling of drug-like molecules for imaging studies with positron emission tomography (PET). We here report the development of a novel, user-friendly, fully automated, and good manufacturing practice (GMP) compliant low-pressure synthesis module for 11 C-carbonylation reactions using [11 C]CO. In this synthesis module, [11 C]CO was reliably prepared from cyclotron-produced [11 C]carbon dioxide ([11 C]CO2 ) by reduction over heated molybdenum and delivered to the reaction vessel within 7 min after end of bombardment, with an overall radiochemical yield (RCY) of 71%. [11 C]AZ13198083, a histamine type-3 receptor ligand, was used as a model compound to assess the functionality of the radiochemistry module. At full batch production conditions (55 µA, 30 min), our newly developed low-pressure 11 C-carbonylation apparatus enabled us to prepare [11 C]AZ13198083 in an isolated radioactivity of 8540 ± 1400 MBq (n = 3). The radiochemical purity of each of the final formulated batches exceeded 99%, and all other quality control tests results conformed with specifications typically set for carbon-11 labeled radiopharmaceuticals. In conclusion, this novel radiochemistry system offers a convenient GMP compliant production drugs and radioligands for imaging studies in human subjects.

Synthesis, in vitro and in vivo evaluation of 11C-O-methylated arylpiperazines as potential serotonin 1A (5-HT1A) receptor antagonist radiotracers.

BACKGROUND: Serotonin 1A (5-HT1A) receptors are implicated in the pathogenesis of several psychiatric and neurodegenerative disorders motivating the development of suitable radiotracers for in vivo positron emission tomography (PET) neuroimaging. The gold standard PET imaging agent for this target is [carbonyl-11C]WAY-100635, labeled via a technically challenging multi-step reaction that has limited its widespread use. While several antagonist and agonist-based PET radiotracers for 5-HT 1A receptors have been developed, their clinical translation has been hindered by methodological challenges and/or and non-specific binding. As a result, there is continued interest in the development of new and more selective 5-HT1A PET tracers having a relatively easier and reliable radiosynthesis process for routine production and with favorable metabolism to facilitate tracer-kinetic modeling. The purpose of the current study was to develop and characterize a radioligand with suitable characteristics for imaging 5-HT1A receptors in the brain. The current study reports the in vitro characterization and radiosyntheses of three candidate 5-HT1A receptor antagonists, DF-100 (1), DF-300 (2) and DF-400 (3), to explore their suitability as potential PET radiotracers. RESULTS: Syntheses of 1-3 and corresponding precursors for radiolabeling were achieved from isonicotinic, picolinic acid or picolino nitrile. In vitro binding studies demonstrated nanomolar affinity of the compounds for 5-HT1A receptors. Binding of 1-3 for other biogenic amines, neurotransmitter receptors, and transporters was negligible with the exception of moderate affinities for α1-adrenergic receptors (4-6-fold less potent than that for 5-HT1A receptor). Radioligands [11C]1-3 were efficiently prepared by 11C-O-methylation of the corresponding phenolic precursor in non-decay corrected radiochemical yields of 7-11% with > 99% chemical and radiochemical purities. Dynamic PET studies in rats demonstrated negligible brain uptake of [11C]1 and [11C]2. In contrast, significant brain uptake of [11C]3 was observed with an early peak SUV of 4-5. However, [11C]3 displayed significant off-target binding attributed to α1-adrenergic receptors based on regional distribution (thalamus>hippocampus) and blocking studies. CONCLUSION: Despite efficient radiolabeling, results from PET imaging experiments limit the application of [11C]3 for in vivo quantification of 5-HT1A receptors. Nevertheless, derivatives of compound 3 may provide a scaffold for alternative PET radiotracers with improved selectivity for 5-HT 1A receptors or α1-adrenergic receptors.

Copper(I)-Mediated 11C-Carboxylation of (Hetero)arylstannanes.

A novel copper-mediated carboxylation strategy of aryl- and heteroaryl-stannanes is described. The method serves as a mild (i.e., 1 atm) carboxylation method using stable carbon dioxide and is transferable as a radiosynthetic approach for carbon-11-labeled aromatic and heteroaromatic carboxylic acids using sub-stoichiometric quantities of [11C]CO2. The methodology was applied to the radiosynthesis of the retinoid X receptor agonist, [11C]bexarotene, with a decay-corrected radiochemical yield of 32 ± 5% and molar activity of 38 ± 23 GBq/µmol (n = 3).

"In-loop" carbonylation-A simplified method for carbon-11 labelling of drugs and radioligands.

Transition-metal mediated carbonylation with 11 C-labelled carbon monoxide ([11 C]CO) is a versatile method for introducing 11 C (t1/2 = 20.3 min) into drugs and radioligands for subsequent use in positron emission tomography (PET). The aim of the current study was to perform the 11 C-carbonylation reaction on the interior surface of a stainless-steel loop used for high performance liquid chromatography (HPLC). In the experimental setup, cyclotron produced 11 C-labelled carbon dioxide ([11 C]CO2 ) was converted to [11 C]CO by reduction over heated Molybdenum and swept into an HPLC loop pre-charged with the appropriate reaction mixture. Following a 5 min reaction, the radiochemical purity (RCP) and the trapping efficiency (TE) of the reaction mixture was determined. After optimization, [11 C]N-Benzylbenzamide was obtained in quantitative radiochemical yield (RCY) following a 5 min reaction at room temperature. The methodology was further applied to label [11 C]benzoic acid (RCP≥99%, TE>91%), [11 C]methyl benzoate (RCP≥99%, TE>93%) and [11 C]phthalide (RCP≥99%, TE>88%). A set of pharmaceuticals was finally radiolabelled using non-optimized conditions. Excellent yields were obtained for the histamine-3 receptor radioligand [11 C]AZ13198083, the oncology drug [11 C]olaparib and the dopamine D2 receptor radioligand [11 C]raclopride, whereas a moderate yield was observed for the high-affinity dopamine D2 receptor radioligand [11 C]FLB457. The presented "in-loop" process proved efficient for diverse 11 C-carbonylations, providing [11 C]amides, [11 C]esters and [11 C]carboxylic acids in moderate to excellent RCYs. Based on the advantages associated with performing the radiolabelling step as an integrated part of the purification system, this methodology may become a valuable addition to the toolbox of methodologies used for 11 C-carbonylation of drugs and radioligands for PET.

"In-loop" 18 F-fluorination: A proof-of-concept study.

There is a great demand to develop more cost-efficient and robust manufacturing processes for fluorine-18 (18 F) labelled compounds and radiopharmaceuticals. Herein, we present to our knowledge the first radiofluorination "in-loop," where [18 F]triflyl fluoride was used as the labelling agent. Initial development of the "in-loop" [18 F]fluorination method was optimized by reacting [18 F]triflyl fluoride with 1,4-dinitrobenzene to form [18 F]1-fluoro-4-nitrobenzene. This methodology was then applied for the syntheses of two well-known radiopharmaceuticals, namely, [18 F]T807 for imaging of tau protein and [18 F]FEPPA for imaging the translocator protein 18 KDa. Both radiotracers were synthesized and formulated using an automated radiosynthesis module with nondecay corrected radiochemical yields of 27% and 29% (relative [18 F]F- ), respectively. The overall syntheses times for [18 F]T807 and [18 F]FEPPA were 65 and 55 minutes, respectively. In these cases, our "in-loop" radiofluorination methodology enabled us to obtain equal or superior yields compared with conventional reactions in a vial. The radiochemical purities were more than 99%, and the molar activities were more than 350 GBq/µmol at the end-of-synthesis for both radiotracers. This novel method is simple, efficient, and allows for a reliable production of radiofluorinated compounds and radiopharmaceuticals.

Synthesis and preclinical evaluation of [18F]FSL25.1188, a reversible PET radioligand for monoamine oxidase-B.

Carbon-11 labeled SL25.1188 is a promising reversible monoamine oxidase-B (MAO-B) radioligand that was recently translated for human positron emission tomography (PET) imaging. Herein, we report the development of a novel fluorinated derivative, namely, [18F](S)-3-(6-(3-fluoropropoxy)benzo[d]isoxazol-3-yl)-5-(methoxymethyl)oxazolidin-2-one ([18F]FSL25.1188; [18F]6), as a candidate 18F-labeled MAO-B radioligand, and, its subsequent preclinical evaluation in non-human primates (NHP). [18F]6 was produced and isolated (>6 GBq) with high radiochemical purity (>99%), and molar activity (>100 GBq/µmol at time of injection). Autoradiography studies conducted in post-mortem human brain sections revealed [18F]6 binding in MAO-B rich regions. PET imaging study of [18F]6 in NHP showed high brain uptake (SUV > 2.5) as well as a regional brain radioactivity distribution in accordance with MAO-B expression. [18F]6 displayed favorable in vivo kinetics, with an early peak in the time-activity curve followed by progressive wash-out from the NHP brain. Specificity of [18F]6 was investigated in a pre-treatment study with l-deprenyl (1.0 mg/kg) wherein reduced radioligand uptake was observed in all MAO-B rich regions. Results from the current preclinical investigation suggests [18F]6 is a promising MAO-B PET radioligand. Further evaluation of [18F]6 and structurally related 18F-analogs are underway to identify an optimized candidate for clinical research studies.

Design, Synthesis, and Evaluation of Reversible and Irreversible Monoacylglycerol Lipase Positron Emission Tomography (PET) Tracers Using a "Tail Switching" Strategy on a Piperazinyl Azetidine Skeleton.

Monoacylglycerol lipase (MAGL) is a serine hydrolase that degrades 2-arachidonoylglycerol (2-AG) in the endocannabinoid system (eCB). Selective inhibition of MAGL has emerged as a potential therapeutic approach for the treatment of diverse pathological conditions, including chronic pain, inflammation, cancer, and neurodegeneration. Herein, we disclose a novel array of reversible and irreversible MAGL inhibitors by means of "tail switching" on a piperazinyl azetidine scaffold. We developed a lead irreversible-binding MAGL inhibitor 8 and reversible-binding compounds 17 and 37, which are amenable for radiolabeling with 11C or 18F. [11C]8 ([11C]MAGL-2-11) exhibited high brain uptake and excellent binding specificity in the brain toward MAGL. Reversible radioligands [11C]17 ([11C]PAD) and [18F]37 ([18F]MAGL-4-11) also demonstrated excellent in vivo binding specificity toward MAGL in peripheral organs. This work may pave the way for the development of MAGL-targeted positron emission tomography tracers with tunability in reversible and irreversible binding mechanisms.

Recent applications of a single quadrupole mass spectrometer in 11C, 18F and radiometal chemistry.

Mass spectrometry (MS) has longstanding applications in radiochemistry laboratories, stemming from carbon-dating. However, research on the development of radiotracers for molecular imaging with either positron emission tomography (PET) or single photon emission computed tomography has yet to take full advantage of MS. This inertia has been attributed to the relatively low concentrations of radiopharmaceutical formulations and lack of access to the required MS equipment due to the high costs for purchase and maintenance of specialized MS systems. To date, single quadrupole (SQ)-MS coupled to liquid chromatography (LC) systems is the main form of MS that has been used in radiochemistry laboratories. These LC/MS systems are primarily used for assessing the chemical purity of radiolabeling precursor or standard molecules but also have applications in the determination of metabolites. Herein, we highlight personal experiences using a compact SQ-MS in our PET radiochemistry laboratories, to monitor the small amounts of carrier observed in most radiotracer preparations, even at high molar activities. The use of a SQ-MS in the observation of the low mass associated with non-radioactive species which are formed along with the radiotracer from the trace amounts of carrier found is demonstrated. Herein, we describe a pre-concentration system to detect dilute radiopharmaceutical formulations and metabolite analyses by SQ-MS. Selected examples where SQ-MS was critical for optimization of radiochemical reactions and for unequivocal characterization of radiotracers are showcased. We also illustrate examples where SQ-MS can be applied in identification of radiometal complexes and development of a new purification methodology for Pd-catalyzed radiofluorination reactions, shedding light on the identity of metal complexes present in the labelling solution.

Emerging PET Radiotracers and Targets for Imaging of Neuroinflammation in Neurodegenerative Diseases: Outlook Beyond TSPO.

The dynamic and multicellular processes of neuroinflammation are mediated by the nonneuronal cells of the central nervous system, which include astrocytes and the brain's resident macrophages, microglia. Although initiation of an inflammatory response may be beneficial in response to injury of the nervous system, chronic or maladaptive neuroinflammation can have harmful outcomes in many neurological diseases. An acute neuroinflammatory response is protective when activated neuroglia facilitate tissue repair by releasing anti-inflammatory cytokines and neurotrophic factors. On the other hand, chronic neuroglial activation is a major pathological mechanism in neurodegenerative diseases, likely contributing to neuronal dysfunction, injury, and disease progression. Therefore, the development of specific and sensitive probes for positron emission tomography (PET) studies of neuroinflammation is attracting immense scientific and clinical interest. An early phase of this research emphasized PET studies of the prototypical imaging biomarker of glial activation, translocator protein-18 kDa (TSPO), which presents difficulties for quantitation and lacks absolute cellular specificity. Many alternate molecular targets present themselves for PET imaging of neuroinflammation in vivo, including enzymes, intracellular signaling molecules as well as ionotropic, G-protein coupled, and immunoglobulin receptors. We now review the lead structures in radiotracer development for PET studies of neuroinflammation targets for neurodegenerative diseases extending beyond TSPO, including glycogen synthase kinase 3, monoamine oxidase-B, reactive oxygen species, imidazoline-2 binding sites, cyclooxygenase, the phospholipase A2/arachidonic acid pathway, sphingosine-1-phosphate receptor-1, cannabinoid-2 receptor, the chemokine receptor CX3CR1, purinergic receptors: P2X7 and P2Y12, the receptor for advanced glycation end products, Mer tyrosine kinase, and triggering receptor expressed on myeloid cells-1. We provide a brief overview of the cellular expression and function of these targets, noting their selectivity for astrocytes and/or microglia, and highlight the classes of PET radiotracers that have been investigated in early-stage preclinical or clinical research studies of neuroinflammation.