A philosophy for CNS radiotracer design

[18F]RS-127445 radiosynthesis and evaluation as a 5-HT2B receptor PET radiotracer in rat brain

Serotonin (5-hydroxytryptamine, 5-HT) is a neurotransmitter involved in many physiological and pathological mechanisms through its numerous receptors. Among these, the 5-HT2B receptor is known to play a key role in multiple brain disorders but remains poorly understood. Positron emission tomography (PET) can contribute to a better understanding of pathophysiological mechanisms regulated by the 5-HT2B receptor. To develop the first PET radiotracer for the 5-HT2B receptor, RS-127445, a well-known 5-HT2B receptor antagonist, was labeled with fluorine-18. [18F]RS-127445 was synthesized in a high radiochemical purity and with a good molar activity and radiochemical yield. Preliminary PET scans in rats showed good brain penetration of [18F]RS-127445. However, competition experiments and in vitro autoradiography showed high non-specific binding, especially to brain white matter.

DESI-TQ-MS imaging for ex vivo brain biodistribution assessment: evaluation of LBT-999, a ligand of the dopamine transporter (DAT)

BACKGROUND

Selection of the most promising radiotracer candidates for radiolabeling is a difficult step in the development of radiotracer pharmaceuticals, especially for the brain. Mass spectrometry (MS) is an alternative to study ex vivo the characteristics of candidates, but most MS studies are complicated by the pharmacologic doses injected and the dissection of regions to study candidate biodistribution. In this study, we tested the ability of a triple quadrupole analyzer (TQ LC-MS/MS) to quantify low concentrations of a validated precursor of a radiotracer targeting the DAT (LBT-999) in dissected regions. We also investigated its biodistribution on brain slices using MS imaging with desorption electrospray ionization (DESI) coupled to time-of-flight (TOF) vs. TQ mass analyzers.

RESULTS

TQ LC-MS/MS enabled quantification of LBT-999 injected at sub-tracer doses in dissected striata. DESI-MS imaging (DESI-MSI) with both analyzers provided images of LBT-999 biodistribution on sagittal slices that were consistent with positron emission tomography (PET). However, the TOF analyzer only obtained biodistribution images at a high injected dose of LBT-999, while the TQ analyzer provided biodistribution images at lower injected doses of LBT-999 with a better signal-to-noise ratio. It also allowed simultaneous visualization of endogenous metabolites such as dopamine.

CONCLUSIONS

Our results show that LC-TQ MS/MS in combination with DESI-MSI can provide important information (biodistribution, specific and selective binding) that can facilitate the selection of the most promising candidates for radiolabeling and support the development of radiotracers.

Emerging targets for positron emission tomography imaging in proteinopathies

Positron emission tomography (PET) imaging of neurodegenerative disease has historically focused on a small number of established targets. The development of selective PET radiotracers for novel biological targets enables new ways to interrogate the neuropathology of proteinopathies and will advance our understanding of neurodegeneration. This perspective aims to highlight recent PET radiotracers developed for five emerging targets in proteinopathies (i.e., mHTT, BACE1, TDP-43, OGA, and CH24H).

Advances and Insights in Positron Emission Tomography Tracers for Metabotropic Glutamate Receptor 4 Imaging

Positron emission tomography (PET) imaging employs positron-emitting radioisotopes to visualize biological processes in living subjects with high sensitivity and quantitative accuracy. As the most translational molecular imaging modality, PET can detect and image a wide range of radiotracers with minimal or no modification to parent drugs or targeting molecules. This Perspective provides a comprehensive analysis of developing PET radioligands using allosteric modulators for the metabotropic glutamate receptor subtype 4 (mGluR4) as a therapeutic target for neurological disorders. We focus on the selection of lead compounds from various chemotypes of mGluR4 positive allosteric modulators (PAMs) and discuss the challenges and systematic characterization required in developing brain-penetrant PET tracers specific for mGluR4. Through this analysis, we offer insights into the development and evaluation of PET ligands. Our review concludes that further research and development in this field hold great promise for discovering effective treatments for neurological disorders.

Investigation of imaging the somatostatin receptor by opening the blood-brain barrier with melittin - A feasibility study using positron emission tomography and [64Cu]Cu-DOTATATE

DOTATATE is a somatostatin peptide analog used in the clinic to detect somatostatin receptors which are highly expressed on neuroendocrine tumors. Somatostatin receptors are found naturally in the intestines, pancreas, lungs, and brain (mainly cortex). In vivo measurement of the somatostatin receptors in the cortex has been challenging because available tracers cannot cross the blood-brain barrier (BBB) due to their intrinsic polarity. A peptide called melittin, a main component of honeybee venom, has been shown to disrupt plasma membranes and increase the permeability of biological membranes. In this study, we assessed the feasibility of using melittin to facilitate the passage of [64Cu]Cu-DOTATATE through the BBB and its binding to somatostatin receptors in the cortex. Evaluation included in vitro autoradiography on Long Evans rat brains to estimate the binding affinity of [64Cu]Cu-DOTATATE to the somatostatin receptors in the cortex and an in vivo evaluation of [64Cu]Cu-DOTATATE binding in NMRI mice after injection of melittin. This study found an in vitro Bmax = 89 ± 4 nM and KD = 4.5 ± 0.6 nM in the cortex, resulting in a theoretical binding potential (BP) calculated as Bmax/KD ≈ 20, which is believed suitable for in vivo brain PET imaging. However, the in vivo results showed no significant difference between the control and melittin injected mice, indicating that the honeybee venom failed to open the BBB. Additional experiments, potentially involving faster injection rates are required to verify that melittin can increase brain uptake of non-BBB permeable PET tracers. Furthermore, an evaluation of whether a venom with a narrow therapeutic range can be used for clinical purposes needs to be considered.

Molecular Imaging of Alzheimer's Disease-Related Sigma-1 Receptor in the Brain a Novel Ru-Mediated Aromatic 18F-deoxyfluorination Probe

Sigma-1 receptor (σ1R) is an intracellular protein implicated in a spectrum of neurodegenerative conditions, notably Alzheimer's disease (AD). Positron emission tomography (PET) imaging of brain σ1R could provide a powerful tool for better understanding the underlying pathomechanism of σ1R in AD. In this study, we successfully developed a 18F-labeled σ1R radiotracer [18F]CNY-05 via an innovative ruthenium (Ru)-mediated 18F-deoxyfluorination method. [18F]CNY-05 exhibited preferable brain uptake, high specific binding, and slightly reversible pharmacokinetics within the PET scanning time window. PET imaging of [18F]CNY-05 in nonhuman primates (NHP) indicated brain permeability, metabolic stability, and safety. Moreover, autoradiography and PET studies of [18F]CNY-05 in the AD mouse model found a significantly decreased brain uptake compared to that in wild-type mice. Collectively, we have provided a novel 18F-radiolabeled σ1R PET probe, which enables visualizing brain σ1R in health and neurological diseases.

Insights into the development of 99mTc-radioligands for serotonergic receptors imaging: Synthesis, labeling, In vitro, and In vivo studies

Serotonergic (5-hydroxytryptamine; 5-HT) receptors play critical roles in neurological and psychological disorders such as schizophrenia, anxiety, depression, and Alzheimer's diseases. Therefore, it is particularly important to develop novel radioligands or modify the existing ones to identify the serotonergic receptors involved in psychiatric disorders. Among the 16 subtypes of serotonergic systems, only technetium-99m based radiopharmaceuticals have been evaluated for serotonin-1A (5-HT1A), serotonin-2A (5-HT2A), 5-HT1A/7 heterodimers and serotonin receptor neurotransmitter (SERT). This review focuses on recent efforts in the design, synthesis and evaluation of 99mTc-radioligands used for single photon emission computerized tomography (SPECT) imaging of serotonergic (5-HT) receptors. Additionally, the discussion will cover aspects such as chemical structure, in vitro/vivo stability, affinity toward serotonin receptors, blood-brain barrier permeation (BBB), and biodistribution study.

Development and Characterization of a Novel Carbon-11 Labeled Positron Emission Tomography Radiotracer for Neuroimaging of Sirtuin 1 with Benzoxazine-Based Compounds

Introduction

Sirtuins (SIRTs) comprise a group of histone deacetylase enzymes crucial for regulating metabolic pathways and contributing significantly to various disease mechanisms. Sirtuin 1 (SIRT1), among the seven known mammalian homologs, is extensively investigated and understood, playing a key role in neurodegenerative disorders and cancer. This study focuses on potential as a therapeutic target for conditions such as Parkinson's disease (PD), Huntington's disease (HD), and Alzheimer's disease (AD).

Methods

Utilizing positron emission tomography (PET) as a noninvasive molecular imaging modality, we aimed to expedite the validation of a promising sirtuin 1 inhibitor for clinical trials. However, the absence of a validated sirtuin 1 PET radiotracer impedes clinical translation. We present the development of [11C]1, and 11C-labeled benzoxazine-based derivative, as a lead imaging probe. The radiosynthesis of [11C]1 resulted in a radiochemical yield of 31 ± 4%.

Results

Baseline studies demonstrated that [11C]1 exhibited excellent blood-brain barrier (BBB) penetration capability, with uniform accumulation throughout various brain regions. Self-blocking studies revealed that introducing an unlabeled compound 1, effectively blocking sirtuin 1, led to a substantial reduction in whole-brain uptake, emphasizing the in vivo specificity of [11C]1 for sirtuin 1.

Discussion

The development of [11C]1 provides a valuable tool for noninvasive imaging investigations in rodent models with aberrant sirtuin 1 expression. This novel radiotracer holds promise for advancing our understanding of sirtuin 1's role in disease mechanisms and may facilitate the validation of sirtuin 1 inhibitors in clinical trials.

Radiosynthesis and In Vitro Evaluation of [11C]tozadenant as Adenosine A2A Receptor Radioligand

Tozadenant (4-hydroxy-N-(4-methoxy-7-morpholinobenzo[d]thiazol-2-yl)-4-methylpiperidine-1-carboxamide) is a highly selective adenosine A2A receptor (A2AR) antagonist and a promising lead structure for the development of A2AR-selective positron emission tomography (PET) probes. Although several 18F-labelled tozadenant derivatives showed favorable in vitro properties, recent in vivo PET studies observed poor brain penetration and lower specific binding than anticipated from the in vitro data. While these findings might be attributable to the structural modification associated with 18F-labelling, they could also reflect inherent properties of the parent compound. However, PET studies with radioisotopologues of tozadenant to evaluate its cerebral pharmacokinetics and brain distribution are still lacking. In the present work, we applied N-Boc-O-desmethyltozadenant as a suitable precursor for the preparation of [O-methyl-11C]tozadenant ([11C]tozadenant) by O-methylation with [11C]methyl iodide followed by acidic deprotection. This approach afforded [11C]tozadenant in radiochemical yields of 18 ± 2%, with molar activities of 50-60 GBq/µmol (1300-1600 mCi/µmol) and radiochemical purities of 95 ± 3%. In addition, in vitro autoradiography in pig and rat brain slices demonstrated the expected striatal accumulation pattern and confirmed the A2AR specificity of the radioligand, making it a promising tool for in vivo PET studies on the cerebral pharmacokinetics and brain distribution of tozadenant.

Identification of (R)-[18F]YH134 for Monoacylglycerol Lipase Neuroimaging and Exploration of Its Use for Central Nervous System and Peripheral Drug Development

This study aimed to evaluate (R)-[18F]YH134 as a novel PET tracer for imaging monoacylglycerol lipase (MAGL). Considering the ubiquitous expression of MAGL throughout the whole body, the impact of various MAGL inhibitors on (R)-[18F]YH134 brain uptake and its application in brain-periphery crosstalk were explored. Methods: MAGL knockout and wild-type mice were used to evaluate (R)-[18F]YH134 in in vitro autoradiography and PET experiments. To explore the impact of peripheral MAGL occupancy on (R)-[18F]YH134 brain uptake, PET kinetics with an arterial input function were studied in male Wistar rats under baseline and blocking conditions. Results: In in vitro autoradiography, (R)-[18F]YH134 revealed a heterogeneous distribution pattern with high binding to MAGL-rich brain regions in wild-type mouse brain slices, whereas the radioactive signal was negligible in MAGL knockout mouse brain slices. The in vivo brain PET images of (R)-[18F]YH134 in wild-type and MAGL knockout mice demonstrated its high specificity and selectivity in mouse brain. A Logan plot with plasma input function was applied to estimate the distribution volume (V T) of (R)-[18F]YH134. V T was significantly reduced by a brain-penetrant MAGL inhibitor but was unchanged by a peripherally restricted MAGL inhibitor. The MAGL target occupancy in the periphery was estimated using (R)-[18F]YH134 PET imaging data from the brain. Conclusion: (R)-[18F]YH134 is a highly specific and selective PET tracer with favorable kinetic properties for imaging MAGL in rodent brain. Our results showed that blocking of the peripheral target influences brain uptake but not the V T of (R)-[18F]YH134. (R)-[18F]YH134 can be used for estimating the dose of MAGL inhibitor at half-maximal peripheral target occupancy.

Design and Discovery of Novel NLRP3 Inhibitors and PET Imaging Radiotracers Based on a 1,2,3-Triazole-Bearing Scaffold

The NOD-like receptor (NLR) family pyrin-domain-containing 3 (NLRP3) inflammasome, an essential component of the innate immune system, has been emerging as a viable drug target and a potential biomarker for human diseases. In our efforts to develop novel small molecule NLRP3 inhibitors, a 1-(5-chloro-2-methoxybenzyl)-4-phenyl-1H-1,2,3-triazole scaffold was designed via a rational approach based on our previous leads. Structure-activity relationship studies and biophysical studies identified a new lead compound 8 as a potent (IC50: 0.55 ± 0.16 μM), selective, and direct NLRP3 inhibitor. Positron emission tomography (PET) imaging studies of [11C]8 demonstrated its rapid and high brain uptake as well as fast washout in mice and rhesus macaque. Notably, plasma kinetic analysis of this radiotracer from the PET/magnetic resonance imaging studies in rhesus macaque suggested radiometabolic stability. Collectively, our data not only encourage further studies of this lead compound but also warrant further optimization to generate additional novel NLRP3 inhibitors and suitable central nervous system PET radioligands with translational promise.

Combined PET/MR: Where Anatomical Imaging Meets Cellular Function

Recent technological advances in medical imaging have allowed for both sequential and simultaneous acquisition of magnetic resonance imaging (MRI) and positron emission tomography (PET) data. Simultaneous PET/MRI offers distinct advantages by efficiently capturing functional and metabolic processes with co-localized, high-resolution anatomical images while minimizing time and movement. We will describe some of the technical and logistic requirements for optimizing sequential and simultaneous PET/MRI in the preclinical research setting.

PET Imaging of Innate Immune Activation Using 11C Radiotracers Targeting GPR84

Chronic innate immune activation is a key hallmark of many neurological diseases and is known to result in the upregulation of GPR84 in myeloid cells (macrophages, microglia, and monocytes). As such, GPR84 can potentially serve as a sensor of proinflammatory innate immune responses. To assess the utility of GPR84 as an imaging biomarker, we synthesized 11C-MGX-10S and 11C-MGX-11Svia carbon-11 alkylation for use as positron emission tomography (PET) tracers targeting this receptor. In vitro experiments demonstrated significantly higher binding of both radiotracers to hGPR84-HEK293 cells than that of parental control HEK293 cells. Co-incubation with the GPR84 antagonist GLPG1205 reduced the binding of both radiotracers by >90%, demonstrating their high specificity for GPR84 in vitro. In vivo assessment of each radiotracer via PET imaging of healthy mice illustrated the superior brain uptake and pharmacokinetics of 11C-MGX-10S compared to 11C-MGX-11S. Subsequent use of 11C-MGX-10S to image a well-established mouse model of systemic and neuro-inflammation revealed a high PET signal in affected tissues, including the brain, liver, lung, and spleen. In vivo specificity of 11C-MGX-10S for GPR84 was confirmed by the administration of GLPG1205 followed by radiotracer injection. When compared with 11C-DPA-713-an existing radiotracer used to image innate immune activation in clinical research studies-11C-MGX-10S has multiple advantages, including its higher binding signal in inflamed tissues in the CNS and periphery and low background signal in healthy saline-treated subjects. The pronounced uptake of 11C-MGX-10S during inflammation, its high specificity for GPR84, and suitable pharmacokinetics strongly support further investigation of 11C-MGX-10S for imaging GPR84-positive myeloid cells associated with innate immune activation in animal models of inflammatory diseases and human neuropathology.

Design, synthesis and preclinical evaluation of muscarine receptor antagonists via a scaffold-hopping approach

Our research group recently identified a rearrangement product of pirenzepine as starting point for a comprehensive rational drug design approach towards orthosteric muscarinic acetylcholine receptor ligands. Chemical reduction and bioscaffold hop lead to the development of sixteen promising compounds featuring either a benzimidazole or carbamate moiety, all exhibiting comparable pharmacophoric characteristics. The synthesized compounds were characterized by NMR, HR-MS, and RP-HPLC techniques. Subsequent evaluation encompassed binding affinity assessment on CHO-hM1-5 cells, mode of action determination, and analysis of physico-chemical parameters. The CNS MPO score indicated favorable drug-like attributes and potential CNS activity for the antagonistic ligands. The most promising compounds displayed Ki-values within a desirable low nanomolar range, and their structural features allow for potential carbon-11 radiolabeling. Our optimization efforts resulted in compounds with a remarkable 138-fold increase in binding affinity compared to the previously mentioned rearrangement product towards human M5, suggesting their prospective utility in positron emission tomography applications.

Site-specifically radiolabeled nanobodies for imaging blood-brain barrier penetration and targeting in the brain

Nanobodies (Nbs) hold significant potential in molecular imaging due to their unique characteristics. However, there are challenges to overcome when it comes to brain imaging. To address these obstacles, collaborative efforts and interdisciplinary research are needed. This article aims to raise awareness and encourage collaboration among researchers from various fields to find solutions for effective brain imaging using Nbs. By fostering cooperation and knowledge sharing, we can make progress in overcoming the existing limitations and pave the way for improved molecular imaging techniques in the future.

Unlocking the Hidden Depths: Multi-Modal Integration of Imaging Mass Spectrometry-Based and Molecular Imaging Techniques

Multimodal imaging (MMI) has emerged as a powerful tool in clinical research, combining different imaging modes to acquire comprehensive information and enabling scientists and surgeons to study tissue identification, localization, metabolic activity, and molecular discovery, thus aiding in disease progression analysis. While multimodal instruments are gaining popularity, challenges such as non-standardized characteristics, custom software, inadequate commercial support, and integration issues with other instruments need to be addressed. The field of multimodal imaging or multiplexed imaging allows for simultaneous signal reproduction from multiple imaging strategies. Intraoperatively, MMI can be integrated into frameless stereotactic surgery. Recent developments in medical imaging modalities such as magnetic resonance imaging (MRI), and Positron Emission Topography (PET) have brought new perspectives to multimodal imaging, enabling early cancer detection, molecular tracking, and real-time progression monitoring. Despite the evidence supporting the role of MMI in surgical decision-making, there is a need for comprehensive studies to validate and perform integration at the intersection of multiple imaging technologies. They were integrating mass spectrometry-based technologies (e.g., imaging mass spectrometry (IMS), imaging mass cytometry (IMC), and Ion mobility mass spectrometry ((IM-IM) with medical imaging modalities, offering promising avenues for molecular discovery and clinical applications. This review emphasizes the potential of multi-omics approaches in tissue mapping using MMI integrated into desorption electrospray ionization (DESI) and matrix-assisted laser desorption ionization (MALDI), allowing for sequential analyses of the same section. By addressing existing knowledge gaps, this review encourages future research endeavors toward multi-omics approaches, providing a roadmap for future research and enhancing the value of MMI in molecular pathology for diagnosis.

Metabotropic Glutamate Receptor Subtype 5 Positron-Emission-Tomography Radioligands as a Tool for Central Nervous System Drug Development: Between Progress and Setbacks

The metabotropic glutamate receptor subtype 5 (mGluR5) is a class C G-protein-coupled receptor (GPCR) that has been implicated in various neuronal processes and, consequently, in several neuropsychiatric or neurodevelopmental disorders. Over the past few decades, mGluR5 has become a major focus for pharmaceutical companies, as an attractive target for drug development, particularly through the therapeutic potential of its modulators. In particular, allosteric binding sites have been targeted for better specificity and efficacy. In this context, Positron Emission Tomography (PET) appears as a useful tool for making decisions along a drug candidate's development process, saving time and money. Thus, PET provides quantitative information about a potential drug candidate and its target at the molecular level. However, in this area, particular attention has to be given to the interpretation of the PET signal and its conclusions. Indeed, the complex pharmacology of both mGluR5 and radioligands, allosterism, the influence of endogenous glutamate and the choice of pharmacokinetic model are all factors that may influence the PET signal. This review focuses on mGluR5 PET radioligands used at several stages of central nervous system drug development, highlighting advances and setbacks related to the complex pharmacology of these radiotracers.

In vivo mapping of pharmacologically induced functional reorganization onto the human brain's neurotransmitter landscape

To understand how pharmacological interventions can exert their powerful effects on brain function, we need to understand how they engage the brain's rich neurotransmitter landscape. Here, we bridge microscale molecular chemoarchitecture and pharmacologically induced macroscale functional reorganization, by relating the regional distribution of 19 neurotransmitter receptors and transporters obtained from positron emission tomography, and the regional changes in functional magnetic resonance imaging connectivity induced by 10 different mind-altering drugs: propofol, sevoflurane, ketamine, lysergic acid diethylamide (LSD), psilocybin, N,N-Dimethyltryptamine (DMT), ayahuasca, 3,4-methylenedioxymethamphetamine (MDMA), modafinil, and methylphenidate. Our results reveal a many-to-many mapping between psychoactive drugs' effects on brain function and multiple neurotransmitter systems. The effects of both anesthetics and psychedelics on brain function are organized along hierarchical gradients of brain structure and function. Last, we show that regional co-susceptibility to pharmacological interventions recapitulates co-susceptibility to disorder-induced structural alterations. Collectively, these results highlight rich statistical patterns relating molecular chemoarchitecture and drug-induced reorganization of the brain's functional architecture.

High-Contrast PET Imaging with [18F]NT160, a Class-IIa Histone Deacetylase Probe for In Vivo Imaging of Epigenetic Machinery in the Central Nervous System

We utilized positron emission tomography (PET) imaging in vivo to map the spatiotemporal biodistribution/expression of class-IIa histone deacetylases (class-IIa HDACs) in the central nervous system (CNS). Herein we report an improved radiosynthesis of [18F]NT160 using 4-hydroxy-TEMPO which led to a significant improvement in radiochemical yield and molar activity. PET imaging with [18F]NT160, a highly potent class-IIa HDAC inhibitor, led to high-quality and high-contrast images of the brain. [18F]NT160 displayed excellent pharmacokinetic and imaging characteristics: brain uptake is high in gray matter regions, tissue kinetics are appropriate for a 18F-tracer, and specific binding for class-IIa HDACs is demonstrated by self-blockade. Higher uptake with [18F]NT160 was observed in the hippocampus, thalamus, and cortex while the uptake in the cerebellum was relatively low. Overall, our current studies with [18F]NT160 will likely facilitate the development and clinical translation of PET tracers for imaging of class-IIa HDACs biodistribution/expression in cancer and the CNS.

Mutated Isocitrate Dehydrogenase (mIDH) as Target for PET Imaging in Gliomas

Gliomas are the most common primary brain tumors in adults. A diffuse infiltrative growth pattern and high resistance to therapy make them largely incurable, but there are significant differences in the prognosis of patients with different subtypes of glioma. Mutations in isocitrate dehydrogenase (IDH) have been recognized as an important biomarker for glioma classification and a potential therapeutic target. However, current clinical methods for detecting mutated IDH (mIDH) require invasive tissue sampling and cannot be used for follow-up examinations or longitudinal studies. PET imaging could be a promising approach for non-invasive assessment of the IDH status in gliomas, owing to the availability of various mIDH-selective inhibitors as potential leads for the development of PET tracers. In the present review, we summarize the rationale for the development of mIDH-selective PET probes, describe their potential applications beyond the assessment of the IDH status and highlight potential challenges that may complicate tracer development. In addition, we compile the major chemical classes of mIDH-selective inhibitors that have been described to date and briefly consider possible strategies for radiolabeling of the most promising candidates. Where available, we also summarize previous studies with radiolabeled analogs of mIDH inhibitors and assess their suitability for PET imaging in gliomas.

Synthesis and Evaluation of [11C]MCC950 for Imaging NLRP3-Mediated Inflammation in Atherosclerosis

Overexpression of the NLRP3 inflammasome has been attributed to the progressive worsening of a multitude of cardiovascular inflammatory diseases such as myocardial infarction, pulmonary arterial hypertension, and atherosclerosis. The recently discovered potent and selective NLRP3 inhibitor MCC950 has shown promise in hindering disease progression, but NLRP3-selective cardiovascular positron emission tomography (PET) imaging has not yet been demonstrated. We synthesized [¹¹C]MCC950 with no-carrier-added [¹¹C]CO₂ fixation chemistry using an iminophosphorane precursor (RCY 45 ± 4%, >99% RCP, 27 ± 2 GBq/μmol, 23 ± 3 min, n = 6) and determined its distribution both in vivo and ex vivo in C57BL/6 and atherogenic ApoE^-/- mice. Small animal PET imaging was performed in both strains following intravenous administration via the lateral tail vein and revealed considerable uptake in the liver that stabilized by 20 min (7-8.5 SUV), coincident with secondary renal excretion. Plasma metabolite analysis uncovered excellent in vivo stability of [¹¹C]MCC950 (94% intact). Ex vivo autoradiography performed on excised aortas revealed heterogeneous uptake in atherosclerotic plaques of ApoE^-/- mice in comparison to C57BL/6 controls (48 ± 17 %ID/m² vs 18 ± 8 %ID/m², p = 0.002, n = 4-5). Treatment of ApoE^-/- mice with nonradioactive MCC950 (5 mg/kg, iv) 10 min prior to radiotracer administration increased uptake in the intestine (5.3 ± 1.8 %ID/g vs 11.0 ± 3.7 %ID/g, p = 0.04, n = 4-6) and in aortic lesions (48 ± 17 %ID/m² vs 104 ± 15 %ID/m², p = 0.0002, n = 5) by 108% and 117%, respectively, without significantly increasing plasma free fraction (f_p, 1.3 ± 0.4% vs 1.7 ± 0.8%, n = 2). These results suggest that [¹¹C]MCC950 uptake demonstrates specific binding and may prove useful for in vivo NLRP3 imaging in atherosclerosis.

Evaluation of advanced, pathophysiologic new targets for imaging of CNS

The inadequate information about the in vivo pathological, physiological, and neurological impairments, as well as the absence of in vivo tools for assessing brain penetrance and the efficiency of newly designed drugs, has hampered the development of new techniques for the treatment for variety of new central nervous system (CNS) diseases. The searching sites such as Science Direct and PubMed were used to find out the numerous distinct tracers across 16 CNS targets including tau, synaptic vesicle glycoprotein, the adenosine 2A receptor, the phosphodiesterase enzyme PDE10A, and the purinoceptor, among others. Among the most encouraging are [¹⁸ F]FIMX for mGluR imaging, [¹¹ C]Martinostat for Histone deacetylase, [¹⁸ F]MNI-444 for adenosine 2A imaging, [¹¹ C]ER176 for translocator protein, and [¹⁸ F]MK-6240 for tau imaging. We also reviewed the findings for each tracer's features and potential for application in CNS pathophysiology and therapeutic evaluation investigations, including target specificity, binding efficacy, and pharmacokinetic factors. This review aims to present a current evaluation of modern positron emission tomography tracers for CNS targets, with a focus on recent advances for targets that have newly emerged for imaging in humans.

Identification and Prioritization of PET Neuroimaging Targets for Microglial Phenotypes Associated with Microglial Activity in Alzheimer's Disease

Activation of microglial cells accompanies the progression of many neurodegenerative disorders, including Alzheimer's disease (AD). Development of molecular imaging tools specific to microglia can help elucidate the mechanism through which microglia contribute to the pathogenesis and progression of neurodegenerative disorders. Through analysis of published genetic, transcriptomic, and proteomic data sets, we identified 19 genes with microglia-specific expression that we then ranked based on association with the AD characteristics, change in expression, and potential druggability of the target. We believe that the process we used to identify and rank microglia-specific genes is broadly applicable to the identification and evaluation of targets in other disease areas and for applications beyond molecular imaging.

A positron emission tomography imaging probe selectively targeting the BD1 bromodomain and extra-terminal domain

The two tandem bromodomains of BET (bromodomain and extra-terminal domain) proteins (BD1 and BD2) may play distinct and critical roles in neurological diseases. To better understand the underlying mechanisms of the BD1 bromodomain and facilitate brain permeable domain-selective inhibitor development, we describe here the development of the first BET BD1 positron emission tomography (PET) radioligand [¹¹C]1a. Compound 1a was tested to possess potent binding affinities and good selectivity (>20-fold over BD2) for BD1 bromodomains of BRD2 (K_d = 25 nM), BRD3 (K_d = 24 nM), and BRD4 (K_d = 19 nM). Physicochemical characterization of 1a indicated the brain permeability and specific binding. [¹¹C]1a was radiosynthesized in a good radiochemical yield (RCY: 25-30%) and molar activity (258 GBq μmol^-1). The PET imaging studies of [¹¹C]1a in mice showed moderate brain uptake (with peak SUV = 0.7) and binding specificity. Furthermore, [¹¹C]1a demonstrated translational potential in the non-human primate (NHP) PET imaging study, which sets the stage for clinical translation.

Design and synthesis of aryl-piperidine derivatives as potent and selective PET tracers for cholesterol 24-hydroxylase (CH24H)

Cholesterol 24-hydroxylase (CH24H, CYP46A1) is a cytochrome P450 family enzyme that maintains the homeostasis of brain cholesterol. Soticlestat, a potent and selective CH24H inhibitor, is in development as a therapeutic agent for Dravet syndrome and Lennox-Gastaut syndrome. Herein, we report the discovery of aryl-piperidine derivatives as potent and selective CH24H positron emission tomography (PET) tracers which can be used for dose guidance of a clinical CH24H inhibitor and as a diagnostic tool for CH24H-related pathology. Starting from compound 1 (IC₅₀ = 16 nM, logD = 1.7), which was reported as a CH24H inhibitor with lower lipophilicity, a¹⁸F-labeling site (3-fluoroazetidine) was incorporated by structure-based drug design (SBDD) utilizing the co-crystal structure of a compound 1 analog. Subsequent optimization to adjust key parameters for PET tracers, such as potency, lipophilicity, brain penetration, and unbound plasma protein binding, enabled compounds 3f (IC₅₀ = 8.8 nM) and 3g (IC₅₀ = 8.7 nM) as PET imaging candidates. Selectivity of these compounds for CH24H was validated by a brain distribution study using CH24H-WT and KO mice. In non-human primate PET imaging, [¹⁸F]3f and [¹⁸F]3g showed similar regional uptake in the brain, indicating that these tracers were specific to the CH24H-expressed regions and validated the expression of CH24H in the living brain by different tracers.

In Silico Approaches for Addressing Challenges in CNS Radiopharmaceutical Design

Positron emission tomography (PET) is a highly sensitive and versatile molecular imaging modality that leverages radiolabeled molecules, known as radiotracers, to interrogate biochemical processes such as metabolism, enzymatic activity, and receptor expression. The ability to probe specific molecular and cellular events longitudinally in a noninvasive manner makes PET imaging a particularly powerful technique for studying the central nervous system (CNS) in both health and disease. Unfortunately, developing and translating a single CNS PET tracer for clinical use is typically an extremely resource-intensive endeavor, often requiring synthesis and evaluation of numerous candidate molecules. While existing in vitro methods are beginning to address the challenge of derisking molecules prior to costly in vivo PET studies, most require a significant investment of resources and possess substantial limitations. In the context of CNS drug development, significant time and resources have been invested into the development and optimization of computational methods, particularly involving machine learning, to streamline the design of better CNS therapeutics. However, analogous efforts developed and validated for CNS radiotracer design are conspicuously limited. In this Perspective, we overview the requirements and challenges of CNS PET tracer design, survey the most promising computational methods for in silico CNS drug design, and bridge these two areas by discussing the potential applications and impact of computational design tools in CNS radiotracer design.

Synthesis and characterization of a new Positron emission tomography probe for orexin 2 receptors neuroimaging

The orexin receptors (OXRs) have been involved in multiple physiological and neuropsychiatric functions. Identification of PET imaging probes specifically targeting OXRs enables us to better understand the OX system. Seltorexant (JNJ-42847922) is a potent OX2R antagonist with the potential to be an OX2R PET imaging probe. Here, we describe the synthesis and characterization of [18F]Seltorexant as an OX2R PET probe. The ex vivo autoradiography studies indicated the good binding specificity of [18F]Seltorexant. In vivo PET imaging of [18F]Seltorexant in rodents showed suitable BBB penetration with the highest brain uptake of %ID/cc = 3.4 at 2 min post-injection in mice. The regional brain biodistribution analysis and blocking studies showed that [18F]Seltorexant had good binding selectivity and specificity. However, pretreatment with unlabelled Seltorexant and P-gp competitor CsA observed significantly increased brain uptake of [18F]Seltorexant, indicating [18F]Seltorexant could interact P-gp at the blood-brain barrier. Our findings demonstrated that [18F]Seltorexant is a potential brain OX2R PET imaging probe, which paves the way for new OX2R PET probes development and OX system investigation.

Application of Advanced Imaging Modalities in Veterinary Medicine: A Review

Veterinary anatomy has traditionally relied on detailed dissections to produce anatomical illustrations, but modern imaging modalities, now represent an enormous resource that allows for fast non-invasive visualizations in living animals for clinical and research purposes. In this review, advanced anatomical imaging modalities and their applications, safety issues, challenges, and future prospects of the techniques commonly employed for animal imaging would be highlighted. The quality of diagnostic imaging equipment in veterinary practice has greatly improved. Recent advances made in veterinary advanced imaging specifically about cross-sectional modalities (CT and MRI), nuclear medicine (PET, SPECT), and dual imaging modalities (PET/CT, PET/MR, and SPECT/CT) have become widely available, leading to greater demands and expectations from veterinary clients. These modalities allow for the creation of three-dimensional representations that can be of considerable value in the dissemination of clinical diagnosis and anatomical studies. Despite, the modern imaging modalities well established in developed countries across the globe, it is yet to remain in its infancy stage in veterinary practice in developing countries due to heavy initial investment and maintenance costs, lack of expert interpretation, a requirement of specialized technical staff and need of adjustable machines to accommodate the different range of animal sizes. Therefore, veterinarians should take advantage of these imaging techniques in designing future experiments by considering the availability of these varied imaging modalities and the creation of three-dimensional graphical representations of internal structures.

The Application of in silico Methods for Prediction of Blood-Brain Barrier Permeability of Small Molecule PET Tracers

Designing positron emission tomography (PET) tracers for targets in the central nervous system (CNS) is challenging. Besides showing high affinity and high selectivity for their intended target, these tracers have to be able to cross the blood-brain barrier (BBB). Since only a small fraction of small molecules is estimated to be able to cross the BBB, tools that can predict permeability at an early stage during the development are of great importance. One such tool is in silico models for predicting BBB-permeability. Thus far, such models have been built based on CNS drugs, with one exception. Herein, we sought to discuss and analyze if in silico predictions that have been built based on CNS drugs can be applied for CNS PET tracers as well, or if dedicated models are needed for the latter. Depending on what is taken into account in the prediction, i.e., passive diffusion or also active influx/efflux, there may be a need for a model build on CNS PET tracers. Following a brief introduction, an overview of a few selected in silico BBB-permeability predictions is provided along with a short historical background to the topic. In addition, a combination of previously reported CNS PET tracer datasets were assessed in a couple of selected models and guidelines for predicting BBB-permeability. The selected models were either predicting only passive diffusion or also the influence of ADME (absorption, distribution, metabolism and excretion) parameters. To conclude, we discuss the potential need of a prediction model dedicated for CNS PET tracers and present the key issues in respect to setting up a such a model.

Studies of the hydrophobic interaction between a pyrene - containing dye and a tetra-aza macrocyclic gadolinium complex

An in vivo and in vitro investigation of the hydrophobic interaction between HPTS and gadolinium(III)-complex of tetra-aza macrocyclic ligand HP-DO3A‡ (Gd(HP-DO3A)) is reported. UV-spectra at variable pH showed that the...

Positron emission tomography in multiple sclerosis - straight to the target

Following the impressive progress in the treatment of relapsing-remitting multiple sclerosis (MS), the major challenge ahead is the development of treatments to prevent or delay the irreversible accumulation of clinical disability in progressive forms of the disease. The substrate of clinical progression is neuro-axonal degeneration, and a deep understanding of the mechanisms that underlie this process is a precondition for the development of therapies for progressive MS. PET imaging involves the use of radiolabelled compounds that bind to specific cellular and metabolic targets, thereby enabling direct in vivo measurement of several pathological processes. This approach can provide key insights into the clinical relevance of these processes and their chronological sequence during the disease course. In this Review, we focus on the contribution that PET is making to our understanding of extraneuronal and intraneuronal mechanisms that are involved in the pathogenesis of irreversible neuro-axonal damage in MS. We consider the major challenges with the use of PET in MS and the steps necessary to realize clinical benefits of the technique. In addition, we discuss the potential of emerging PET tracers and future applications of existing compounds to facilitate the identification of effective neuroprotective treatments for patients with MS.

Design, Synthesis, and Evaluation of Thienodiazepine Derivatives as Positron Emission Tomography Imaging Probes for Bromodomain and Extra-Terminal Domain Family Proteins

To better understand the role of bromodomain and extra-terminal domain (BET) proteins in epigenetic mechanisms, we developed a series of thienodiazepine-based derivatives and identified two compounds, 3a and 6a, as potent BET inhibitors. Further in vivo pharmacokinetic studies and analysis of in vitro metabolic stability of 6a revealed excellent brain penetration and reasonable metabolic stability. Compounds 3a and 6a were radiolabeled with fluorine-18 in two steps and utilized in positron emission tomography (PET) imaging studies in mice. Preliminary PET imaging results demonstrated that [¹⁸F]3a and [¹⁸F]6a have good brain uptake (with maximum SUV = 1.7 and 2, respectively) and binding specificity in mice brains. These results show that [¹⁸F]6a is a potential PET radiotracer that could be applied to imaging BET proteins in the brain. Further optimization and improvement of the metabolic stability of [¹⁸F]6a are still needed in order to create optimal PET imaging probes of BET family members.

Hydrofluoromethylation of alkenes with fluoroiodomethane and beyond

A process for the direct hydrofluoromethylation of alkenes is reported for the first time. This straighforward silyl radical-mediated reaction utilises CH₂FI as a non-ozone depleting reagent, traditionally used in electrophilic, nucleophilic and carbene-type chemistry, but not as a CH₂F radical source. By circumventing the challenges associated with the high reduction potential of CH₂FI being closer to CH₃I than CF₃I, and harnessing instead the favourable bond dissociation energy of the C–I bond, we demonstrate that feedstock electron-deficient alkenes are converted into products resulting from net hydrofluoromethylation with the intervention of (Me₃Si)₃SiH under blue LED activation. This deceptively simple yet powerful methodology was extended to a range of (halo)methyl radical precursors including ICH₂I, ICH₂Br, ICH₂Cl, and CHBr₂F, as well as CH₃I itself; this latter reagent therefore enables direct hydromethylation. This versatile chemistry was applied to ¹⁸F-, ¹³C-, and D-labelled reagents as well as complex biologically relevant alkenes, providing facile access to more than fifty products for applications in medicinal chemistry and positron emission tomography.

Herein, we report the direct hydro(halo)methylation of alkenes from a variety of (halo)methyl iodides (including F-18, C-13, D-2 isotopologues), enabling the incorporation of a plethora of C-1 fragments into complex biologically active molecules.

Discovery of Highly Potent Adenosine A1 Receptor Agonists: Targeting Positron Emission Tomography Probes

Adenosine receptor (AR) radiotracers for positron emission tomography (PET) have provided knowledge on the in vivo biodistribution of ARs in the central nervous system (CNS), which is of therapeutic interest for various neuropsychiatric disorders. Additionally, radioligands that can image changes in endogenous adenosine levels in different physiological and pathological conditions are still lacking. The binding of known antagonist adenosine A₁ receptor (A₁R) radiotracer, [¹¹C]MDPX, failed to be inhibited by elevated endogenous adenosine in a rodent PET study. Since most of the known AR PET radiotracers were antagonists, we propose that an A₁R agonist radioligand may possess higher sensitivity to measure changes in endogenous adenosine concentration. Herein, we report our latest findings toward the development of a full agonist adenosine A₁ radioligand for PET. Based on a 3,5-dicyanopyridine template, 16 new derivatives were designed and synthesized to optimize both binding affinity and functional activity, resulting in two full agonists (compounds 27 and 29) with single-digit nanomolar affinities and good subtype selectivity (A₁/A_2A selectivity of ∼1000-fold for compound 27 and 29-fold for compound 29). Rapid O-[¹¹C]methylation provided [¹¹C]27 and [¹¹C]29 in high radiochemical yields and radiochemical purity. However, subsequent brain PET imaging in rodents showed poor brain permeability for both radioligands. An in vivo PET study using knockout mice for MDR 1a/a, BCRP, and MRP1 indicated that these compounds might be substrates for brain efflux pumps. In addition, in silico evaluation using multiparameter optimization identified high molecular weight and high polar surface area as the main molecular descriptors responsible for low brain penetration. These results will provide further insight toward development of full agonist adenosine A₁ radioligands and also highly potent CNS A₁AR drugs.

Profiling Transcription Initiation in Peripheral Leukocytes Reveals Severity-Associated Cis-Regulatory Elements in Critical COVID-19

The contribution of transcription factors (TFs) and gene regulatory programs in the immune response to COVID-19 and their relationship to disease outcome is not fully understood. Analysis of genome-wide changes in transcription at both promoter-proximal and distal cis-regulatory DNA elements, collectively termed the 'active cistrome,' offers an unbiased assessment of TF activity identifying key pathways regulated in homeostasis or disease. Here, we profiled the active cistrome from peripheral leukocytes of critically ill COVID-19 patients to identify major regulatory programs and their dynamics during SARS-CoV-2 associated acute respiratory distress syndrome (ARDS). We identified TF motifs that track the severity of COVID- 19 lung injury, disease resolution, and outcome. We used unbiased clustering to reveal distinct cistrome subsets delineating the regulation of pathways, cell types, and the combinatorial activity of TFs. We found critical roles for regulatory networks driven by stimulus and lineage determining TFs, showing that STAT and E2F/MYB regulatory programs targeting myeloid cells are activated in patients with poor disease outcomes and associated with single nucleotide genetic variants implicated in COVID-19 susceptibility. Integration with single-cell RNA-seq found that STAT and E2F/MYB activation converged in specific neutrophils subset found in patients with severe disease. Collectively we demonstrate that cistrome analysis facilitates insight into disease mechanisms and provides an unbiased approach to evaluate global changes in transcription factor activity and stratify patient disease severity.

Exploring Translocator Protein (TSPO) Medicinal Chemistry: An Approach for Targeting Radionuclides and Boron Atoms to Mitochondria

Translocator protein 18 kDa [TSPO or peripheral-type benzodiazepine receptor (PBR)] was identified in the search of binding sites for benzodiazepine anxiolytic drugs in peripheral regions. In these areas, binding sites for TSPO ligands were recognized in steroid-producing tissues. TSPO plays an important role in many cellular functions, and its coding sequence is highly conserved across species. TSPO is located predominantly on the membrane of mitochondria and is overexpressed in several solid cancers. TSPO basal expression in the CNS is low, but it becomes high in neurodegenerative conditions. Thus, TSPO constitutes not only as an outstanding drug target but also as a valuable marker for the diagnosis of a number of diseases. The aim of the present article is to show the lesson we have learned from our activity in TSPO medicinal chemistry and in approaching the targeted delivery to mitochondria by means of TSPO ligands.

Radionuclide Imaging for Neuroscience: Current Opinion and Future Directions

This meeting report summarizes a Consultants Meeting that was held at International Atomic Energy Agency headquarters in Vienna to provide an update on radionuclide imaging for neuroscience applications.

Longitudinal evaluation of a novel BChE PET tracer as an early in vivo biomarker in the brain of a mouse model for Alzheimer disease

Purpose: The increase in butyrylcholinesterase (BChE) activity in the brain of Alzheimer disease (AD) patients and animal models of AD position this enzyme as a potential biomarker of the disease. However, the information on the ability of BChE to serve as AD biomarker is contradicting, also due to scarce longitudinal studies of BChE activity abundance. Here, we report ¹¹C-labeling, in vivo stability, biodistribution, and longitudinal study on BChE abundance in the brains of control and 5xFAD (AD model) animals, using a potent BChE selective inhibitor, [¹¹C]4, and positron emission tomography (PET) in combination with computerised tomography (CT). We correlate the results with in vivo amyloid beta (Aβ) deposition, longitudinally assessed by [¹⁸F]florbetaben-PET imaging.

Methods: [¹¹C]4 was radiolabelled through ¹¹C-methylation. Metabolism studies were performed on blood and brain samples of female wild type (WT) mice. Biodistribution studies were performed in female WT mice using dynamic PET-CT imaging. Specific binding was demonstrated by ex vivo and in vivo PET imaging blocking studies in female WT and 5xFAD mice at the age of 7 months. Longitudinal PET imaging of BChE was conducted in female 5xFAD mice at 4, 6, 8, 10 and 12 months of age and compared to age-matched control animals. Additionally, Aβ plaque distribution was assessed in the same mice using [¹⁸F]florbetaben at the ages of 2, 5, 7 and 11 months. The results were validated by ex vivo staining of BChE at 4, 8, and 12 months and Aβ at 12 months on brain samples.

Results: [¹¹C]4 was produced in sufficient radiochemical yield and molar activity for the use in PET imaging. Metabolism and biodistribution studies confirmed sufficient stability in vivo, the ability of [¹¹C]4 to cross the blood brain barrier (BBB) and rapid washout from the brain. Blocking studies confirmed specificity of the binding. Longitudinal PET studies showed increased levels of BChE in the cerebral cortex, hippocampus, striatum, thalamus, cerebellum and brain stem in aged AD mice compared to WT littermates. [¹⁸F]Florbetaben-PET imaging showed similar trend of Aβ plaques accumulation in the cerebral cortex and the hippocampus of AD animals as the one observed for BChE at ages 4 to 8 months. Contrarily to the results obtained by ex vivo staining, lower abundance of BChE was observed in vivo at 10 and 12 months than at 8 months of age.

Conclusions: The BChE inhibitor [¹¹C]4 crosses the BBB and is quickly washed out of the brain of WT mice. Comparison between AD and WT mice shows accumulation of the radiotracer in the AD-affected areas of the brain over time during the early disease progression. The results correspond well with Aβ accumulation, suggesting that BChE is a promising early biomarker for incipient AD.

Tauvid™: The First FDA-Approved PET Tracer for Imaging Tau Pathology in Alzheimer's Disease

Tauvid has been approved by the U.S. Food and Drug Administration (FDA) in 2020 for positron emission tomography (PET) imaging of adult patients with cognitive impairments undergoing evaluation for Alzheimer’s disease (AD) based on tau pathology. Abnormal aggregation of tau proteins is one of the main pathologies present in AD and is receiving increasing attention as a diagnostic and therapeutic target. In this review, we summarised the production and quality control of Tauvid, its clinical application, pharmacology and pharmacokinetics, as well as its limitation due to off-target binding. Moreover, a brief overview on the second-generation of Tau PET tracers is provided. The approval of Tauvid marks a step forward in the field of AD research and opens up opportunities for second-generation tau tracers to advance tau PET imaging in the clinic.

Synthesis and Biological Evaluation of a Novel 18F-Labeled Radiotracer for PET Imaging of the Adenosine A2A Receptor

The adenosine A_2A receptor (A_2AR) has emerged as a potential non-dopaminergic target for the treatment of Parkinson’s disease and, thus, the non-invasive imaging with positron emission tomography (PET) is of utmost importance to monitor the receptor expression and occupancy during an A_2AR-tailored therapy. Aiming at the development of a PET radiotracer, we herein report the design of a series of novel fluorinated analogs (TOZ1-TOZ7) based on the structure of the A_2AR antagonist tozadenant, and the preclinical evaluation of [¹⁸F]TOZ1. Autoradiography proved A_2AR-specific in vitro binding of [¹⁸F]TOZ1 to striatum of mouse and pig brain. Investigations of the metabolic stability in mice revealed parent fractions of more than 76% and 92% of total activity in plasma and brain samples, respectively. Dynamic PET/magnetic resonance imaging (MRI) studies in mice revealed a brain uptake but no A_2AR-specific in vivo binding.

Development and biological evaluation of[18F]FMN3PA & [18F]FMN3PU for leucine-rich repeat kinase 2 (LRRK2) in vivo PET imaging

PURPOSE

Among all genetic mutations of LRRK2, the G2019S mutation is the most commonly associated with the late-onset of Parkinson's disease (PD). Hence, one potential therapeutic approach is to block the hyperactivity of mutated LRRK2 induced by kinase inhibition. To date, only a few LRRK2 kinase inhibitors have been tested for in vivo quantification of target engagement by positron emission tomography (PET). In this study, we performed biological evaluations of two radiolabeled kinase inhibitors i.e. [¹⁸F]FMN3PA (14) and [¹⁸F]FMN3PU for LRRK2 (15).

PROCEDURES

Radiosyntheses of [¹⁸F]FMN3PA (14) and [¹⁸F]FMN3PU (15) were performed using K[¹⁸F]-F-K222 complex in a TRACERlab FXN module and purification was carried out via C18 plus (Sep-Pak) cartridges. In vitro specific binding assays were performed in rat brain striatum and kidney tissues using GNE-0877 as a blocking agent (K_i = 0.7 nM). For in vivo blocking, 3 mg/kg of GNE-0877 was injected 30 min before radiotracer injection via tail vein in wild-type (WT) mice (n = 4). Dynamic scans by PET/CT (Siemens Inveon) were performed in WT mice (n = 3).

RESULTS

Radiofluorinations resulted in radiochemical yields (RCYs) of 25 ± 1.3% (n = 6) ([¹⁸F]FMN3PU, 15) and 37 ± 1.6% (n = 6) ([¹⁸F]FMN3PA, 14) with ≥96% radiochemical purity (RCP) and a molar activity (MA) of 3.55 ± 1.6 Ci/μmol (131 ± 56 GBq/μmol) for [¹⁸F]FMN3PU (15) and 4.57 ± 1.7 Ci/μmol (169 ± 63 GBq/μmol) for [¹⁸F]FMN3PA (14), respectively. Saturation assays showed high specific binding for rat brain striatum with K_d 20 ± 1.3 nM ([¹⁸F]FMN3PA, 14) and 23.6 ± 4.0 nM ([¹⁸F]FMN3PU, 15). In vivo blocking data for [¹⁸F]FMN3PA (14) was significant for brain (p < 0.0001, 77% blocking) and kidney (p = 0.0041, 65% blocking). PET images showed uptake in mouse brain striatum.

CONCLUSION

In the presence of GNE-0877 as a blocking agent, the specific binding of [¹⁸F]FMN3PA (14) and [¹⁸F]FMN3PU (15) was significant in vitro. [¹⁸F]FMN3PA (14) showed good brain uptake in vivo, though fast clearance from brain was observed (within 10-15 min).

Rapidly (and Successfully) Translating Novel Brain Radiotracers From Animal Research Into Clinical Use

The advent of preclinical research scanners for in vivo imaging of small animals has added confidence into the multi-step decision-making process of radiotracer discovery and development. Furthermore, it has expanded the utility of imaging techniques available to dissect clinical questions, fostering a cyclic interaction between the clinical and the preclinical worlds. Significant efforts from medicinal chemistry have also made available several high-affinity and selective compounds amenable for radiolabeling, that target different receptors, transporters and enzymes in vivo. This substantially increased the range of applications of molecular imaging using positron emission tomography (PET) or single photon emission computed tomography (SPECT). However, the process of developing novel radiotracers for in vivo imaging of the human brain is a multi-step process that has several inherent pitfalls and technical difficulties, which often hampers the successful translation of novel imaging agents from preclinical research into clinical use. In this paper, the process of radiotracer development and its relevance in brain research is discussed; as well as, its pitfalls, technical challenges and future promises. Examples of successful and unsuccessful translation of brain radiotracers will be presented.

Dealing with PET radiometabolites

Abstract

Positron emission tomography (PET) offers the study of biochemical, physiological, and pharmacological functions at a cellular and molecular level. The performance of a PET study mostly depends on the used radiotracer of interest. However, the development of a novel PET tracer is very difficult, as it is required to fulfill a lot of important criteria. PET radiotracers usually encounter different chemical modifications including redox reaction, hydrolysis, decarboxylation, and various conjugation processes within living organisms. Due to this biotransformation, different chemical entities are produced, and the amount of the parent radiotracer is declined. Consequently, the signal measured by the PET scanner indicates the entire amount of radioactivity deposited in the tissue; however, it does not offer any indication about the chemical disposition of the parent radiotracer itself. From a radiopharmaceutical perspective, it is necessary to quantify the parent radiotracer’s fraction present in the tissue. Hence, the identification of radiometabolites of the radiotracers is vital for PET imaging. There are mainly two reasons for the chemical identification of PET radiometabolites: firstly, to determine the amount of parent radiotracers in plasma, and secondly, to rule out (if a radiometabolite enters the brain) or correct any radiometabolite accumulation in peripheral tissue. Besides, radiometabolite formations of the tracer might be of concern for the PET study, as the radiometabolic products may display considerably contrasting distribution patterns inside the body when compared with the radiotracer itself. Therefore, necessary information is needed about these biochemical transformations to understand the distribution of radioactivity throughout the body. Various published review articles on PET radiometabolites mainly focus on the sample preparation techniques and recently available technology to improve the radiometabolite analysis process. This article essentially summarizes the chemical and structural identity of the radiometabolites of various radiotracers including [¹¹C]PBB3, [¹¹C]flumazenil, [¹⁸F]FEPE2I, [¹¹C]PBR28, [¹¹C]MADAM, and (+)[¹⁸F]flubatine. Besides, the importance of radiometabolite analysis in PET imaging is also briefly summarized. Moreover, this review also highlights how a slight chemical modification could reduce the formation of radiometabolites, which could interfere with the results of PET imaging.

Graphical abstract

Enhanced arecoline derivatives as muscarinic acetylcholine receptor M1 ligands for potential application as PET radiotracers

Supported by their involvement in many neurodegenerative disorders, muscarinic acetylcholine receptors (mAChRs) are an interesting target for PET imaging. Nevertheless, no radiotracer is established in clinical routine. Within this work we aim to develop novel PET tracers based on the structure of arecoline. Fifteen novel arecoline derivatives were synthesized, characterized and tested for their affinity to the mAChRs M1-M5 and the conceivable off-target acetylcholinesterase. Five arecoline derivatives and arecoline were labeled with carbon-11 in good yields. Arecaidine diphenylmethyl ester (3b), arecaidine bis(4-fluorophenyl)methyl ester (3c) and arecaidine (4-bromophenyl)(4-fluorophenyl)methyl ester (3e) showed a tremendous gain in mAChR affinity compared to arecoline and a pronounced subtype selectivity for M1. Metabolic stability and serum protein binding of [¹¹C]3b and [¹¹C]3c were in line with properties of established brain tracers. Nonspecific binding of [¹¹C]3c was prevalent in kinetic and endpoint experiment on living cells as well as in autoradiography on native mouse brain sections, which motivates us to decrease the lipophilicity of this substance class prior to in vivo experiments.

Human Positron Emission Tomography Neuroimaging

Neuroimaging with positron emission tomography (PET) is the most powerful tool for understanding pharmacology, neurochemistry, and pathology in the living human brain. This technology combines high-resolution scanners to measure radioactivity throughout the human body with specific, targeted radioactive molecules, which allow measurements of a myriad of biological processes in vivo. While PET brain imaging has been active for almost 40 years, the pace of development for neuroimaging tools, known as radiotracers, and for quantitative analytical techniques has increased dramatically over the past decade. Accordingly, the fundamental questions that can be addressed with PET have expanded in basic neurobiology, psychiatry, neurology, and related therapeutic development. In this review, we introduce the field of human PET neuroimaging, some of its conceptual underpinnings, and motivating questions. We highlight some of the more recent advances in radiotracer development, quantitative modeling, and applications of PET to the study of the human brain.

Methods to Enhance the Metabolic Stability of Peptide-Based PET Radiopharmaceuticals

The high affinity and specificity of peptides towards biological targets, in addition to their favorable pharmacological properties, has encouraged the development of many peptide-based pharmaceuticals, including peptide-based positron emission tomography (PET) radiopharmaceuticals. However, the poor in vivo stability of unmodified peptides against proteolysis is a major challenge that must be overcome, as it can result in an impractically short in vivo biological half-life and a subsequently poor bioavailability when used in imaging and therapeutic applications. Consequently, many biologically and pharmacologically interesting peptide-based drugs may never see application. A potential way to overcome this is using peptide analogues designed to mimic the pharmacophore of a native peptide while also containing unnatural modifications that act to maintain or improve the pharmacological properties. This review explores strategies that have been developed to increase the metabolic stability of peptide-based pharmaceuticals. It includes modifications of the C- and/or N-termini, introduction of d- or other unnatural amino acids, backbone modification, PEGylation and alkyl chain incorporation, cyclization and peptide bond substitution, and where those strategies have been, or could be, applied to PET peptide-based radiopharmaceuticals.

In Vitro and In Vivo Characterization of Dibenzothiophene Derivatives [125I]Iodo-ASEM and [18F]ASEM as Radiotracers of Homo- and Heteromeric α7 Nicotinic Acetylcholine Receptors

The α7 nicotinic acetylcholine receptor (α7 nAChR) is involved in several cognitive and physiologic processes; its expression levels and patterns change in neurologic and psychiatric diseases, such as schizophrenia and Alzheimer’s disease, which makes it a relevant drug target. Development of selective radioligands is important for defining binding properties and occupancy of novel molecules targeting the receptor. We tested the in vitro binding properties of [¹²⁵I]Iodo-ASEM [(3-(1,4-diazabycyclo[3.2.2]nonan-4-yl)-6-(¹²⁵I-iododibenzo[b,d]thiopentene 5,5-dioxide)] in the mouse, rat and pig brain using autoradiography. The in vivo binding properties of [¹⁸F]ASEM were investigated using positron emission tomography (PET) in the pig brain. [¹²⁵I]Iodo-ASEM showed specific and displaceable high affinity (~1 nM) binding in mouse, rat, and pig brain. Binding pattern overlapped with [¹²⁵I]α-bungarotoxin, specific binding was absent in α7 nAChR gene-deficient mice and binding was blocked by a range of α7 nAChR orthosteric modulators in an affinity-dependent order in the pig brain. Interestingly, relative to the wild-type, binding in β2 nAChR gene-deficient mice was lower for [¹²⁵I]Iodo-ASEM (58% ± 2.7%) than [¹²⁵I]α-bungarotoxin (23% ± 0.2%), potentially indicating different binding properties to heteromeric α7β2 nAChR. [¹⁸F]ASEM PET in the pig showed high brain uptake and reversible tracer kinetics with a similar spatial distribution as previously reported for α7 nAChR. Blocking with SSR-180,711 resulted in a significant decrease in [¹⁸F]ASEM binding. Our findings indicate that [¹²⁵I]Iodo-ASEM allows sensitive and selective imaging of α7 nAChR in vitro, with better signal-to-noise ratio than previous tracers. Preliminary data of [¹⁸F]ASEM in the pig brain demonstrated principal suitable kinetic properties for in vivo quantification of α7 nAChR, comparable to previously published data.

A New Positron Emission Tomography Probe for Orexin Receptors Neuroimaging

The orexin receptor (OX) is critically involved in motivation and sleep−wake regulation and holds promising therapeutic potential in various mood disorders. To further investigate the role of orexin receptors (OXRs) in the living human brain and to evaluate the treatment potential of orexin-targeting therapeutics, we herein report a novel PET probe ([¹¹C]CW24) for OXRs in the brain. CW24 has moderate binding affinity for OXRs (IC₅₀ = 0.253 μM and 1.406 μM for OX₁R and OX₂R, respectively) and shows good selectivity to OXRs over 40 other central nervous system (CNS) targets. [¹¹C]CW24 has high brain uptake in rodents and nonhuman primates, suitable metabolic stability, and appropriate distribution and pharmacokinetics for brain positron emission tomography (PET) imaging. [¹¹C]CW24 warrants further evaluation as a PET imaging probe of OXRs in the brain.