Radiosynthesis of [11C]SL25.1188 via [11C]CO2 fixation for imaging monoamine oxidase B

Synthesis and evaluation of a targeted PET radioligand [18F]FCOB02 for monoamine oxidase B

Monoamine oxidase B (MAO-B) is a membrane-bound flavinase that plays an important role in the regulation of monoamine neurotransmission. Positron emission tomography (PET) provides a way to study the molecular mechanisms of MAO-B-related diseases and to evaluate the effects of drugs. In this study, we designed and synthesized [18F]FCOB02, a 4-methylcoumarin-like targeting probe.[18F]FCOB02 is straightforward to synthesize and has a high affinity for MAO-B with an IC50 = 10.68 ± 3.25 nM. Successful radiolabeling with fluorine-18 was achieved, resulting in a labeling rate of 35 % along with favorable lipid solubility (log D7.4 = 2.4). Automated radiolabelling was achieved after optimization of the conditions. The radiochemical yield was 9.6 % ± 1.2 %(n = 3) with good radiochemical purity (>98 %), good stability in saline for 4 h and high specific activity (105.08 ± 19GBq/μmol，n = 3). Biodistribution studies conducted in mice revealed significant initial brain uptake of 8.22 ± 0.86 % ID/g at 2 min post-injection, followed by rapid metabolism primarily via the liver and kidneys. Brain uptake was comparable to the same type of probe [18F]SMBT-1 (brain 2min = 7.85 % ID/g). PET-CT images of [18F]FCOB02 in SD rats showed significant differences in brain uptake before and after inhibition by the inhibitor L-deprenyl. Whole brain uptake was reduced by 20 % after inhibition, indicating specific uptake of the probe in the brain, with a 40-min brain clearance rate of 81 %. The potential utility of [18F]FCOB02 for achieving specific MAO-B imaging as well as quantitative analysis in vivo warrants further investigation regarding its clinical translational value.

Astrogliosis Marker [11C]SL25.1188 After COVID-19 With Ongoing Depressive and Cognitive Symptoms

BACKGROUND

After acute COVID-19, 5% of people experience persistent depressive symptoms and reduced cognitive function (COVID-DC). Theoretical models propose that astrogliosis is important in long COVID, but measures primarily indicative of astrogliosis have not been studied in the brain of long COVID or COVID-DC. The objective of the current study was to measure [11C]SL25.1188 total distribution volume ([11C]SL25.1188 VT), an index of monoamine oxidase B density and a marker of astrogliosis, with positron emission tomography in participants with COVID-DC and compare with healthy control participants.

METHODS

In 21 COVID-DC cases and 21 healthy control participants, [11C]SL25.1188 VT was measured in the prefrontal cortex, anterior cingulate cortex, hippocampus, dorsal putamen, and ventral striatum. Depressive symptoms were measured with the Beck Depression Inventory-II, and cognitive symptoms were measured with neuropsychological tests.

RESULTS

[11C]SL25.1188 VT was higher in participants with COVID-DC in the prefrontal cortex, anterior cingulate cortex, hippocampus, dorsal putamen, and ventral striatum than in healthy control participants. Depressive symptom severity negatively correlated with [11C]SL25.1188 VT across prioritized brain regions. More recent acute COVID-19 positively correlated with [11C]SL25.1188 VT, reflecting higher values since predominance of the Omicron variant. Exploratory analyses found greater [11C]SL25.1188 VT in the hippocampus, dorsal putamen, and ventral striatum of COVID-DC participants than control participants with a major depressive episode with no history of COVID-19, and there was no relationship to cognitive testing in prioritized regions.

CONCLUSIONS

Results strongly support the presence of monoamine oxidase B-labeled astrogliosis in COVID-DC throughout the regions assessed, although the association of greater astrogliosis with fewer symptoms raises the possibility of a protective role. The magnitude of astrogliosis in COVID-DC is greater since the emergence of the Omicron variant.

Ionic Liquid Aided [11C]CO Fixation for Synthesis of 11C-carbonyls

Tributyl(ethyl)phosphonium oxopentenolate ([P4442][Pen]) is an ionic liquid developed to capture CO and has shown ability to catalyze carbonylation reactions in organic chemistry. Carbon-11 (11C, t1/2=20.4 min) labeled CO is a highly versatile building block for the synthesis of positron emission tomography (PET) radiotracers that are applied for medical imaging. The use of [11C]CO is limited by its low solubility in organic solvents. Herein, we report a proof-of-concept study evaluating a new method to prepare 11C-labeled amides, ureas and carbamates via reaction of [11C]CO in [P4442][Pen] and applied for fully automated radiosyntheses of Bruton's tyrosine kinase inhibitors, [11C]evobrutinib and [11C]ibrutinib.

11C-Fixation Techniques

This protocol describes the application of cyclotron-generated [11C]CO2 fixation reactions for direct 11C-carboxylation reactions and [11C]CO for 11C-carbonylations. Herein we describe one-pot methods wherein the radioactive gas is first trapped in a reaction mixture at room temperature and atmospheric pressure prior to the radiolabeling reactions. Such procedures are widely applicable to numerous small molecules to form 11C-labeled carboxylic acids, amides, esters, ketones, oxazolidinones, carbamates, and ureas. The steps for 11C-fixation techniques described herein are tailored for a commercial automated synthesis unit and are readily adapted for routine radiopharmaceutical production.

Astrogliosis marker 11C-SL25.1188 PET in traumatic brain injury with persistent symptoms

Traumatic brain injury (TBI) is common but little is known why up to a third of patients have persisting symptoms. Astrogliosis, a pathophysiological response to brain injury, may be a potential therapeutic target, but demonstration of astrogliosis in the brain of humans with TBI and persistent symptoms is lacking. Astroglial marker monoamine oxidase B (MAO-B) total distribution volume (11C-SL25.1188 VT), an index of MAO-B density, was measured in 29 TBI and 29 similarly aged healthy control cases with 11C-SL25.1188 PET, prioritizing prefrontal cortex (PFC) and cortex proximal to cortical convexity. Correlations of PFC 11C-SL25.1188 VT with psychomotor and processing speed; and serum blood measures implicated in astrogliosis were determined. 11C-SL25.1188 VT was greater in TBI in PFC (P = 0.00064) and cortex (P = 0.00038). PFC 11C-SL25.1188 VT inversely correlated with Comprehensive Trail Making Test psychomotor and processing speed (r = -0.48, P = 0.01). In participants scanned within 2 years of last TBI, PFC 11C-SL25.1188 VT correlated with serum glial fibrillary acid protein (r = 0.51, P = 0.037) and total tau (r = 0.74, P = 0.001). Elevated 11C-SL25.1188 VT argues strongly for astrogliosis and therapeutics modifying astrogliosis towards curative phenotypes should be tested in TBI with persistent symptoms. Given substantive effect size, astrogliosis PET markers should be applied to stratify cases and/or assess target engagement for putative therapeutics targeting astrogliosis.

Ring-opening of non-activated aziridines with [11C]CO2 via novel ionic liquids

Novel ionic liquids based on DBU and DBN halide salts were developed as a catalytic system for ring-opening of non-activated aziridines with [¹¹C]CO₂. The ability of ionic liquids to activate aziridines represents a simple methodology for the synthesis of ¹¹C-carbamates and can be extended for CO₂-fixation in organic and radiochemistry.

Novel ionic liquids based on DBU and DBN halide salts were developed as a catalytic system for ring-opening of non-activated aziridines with [¹¹C]CO₂.

Development and Clinical Application of Positron Emission Tomography Imaging Agents for Monoamine Oxidase B

Monoamine oxidase B (MAO-B) is a high-density protein in the brain mainly found on outer mitochondrial membranes, primarily in astroglia, but additionally in serotonergic neurons and in the substantia nigra in the midbrain. It is an enzyme that participates in the oxidative metabolism of important monoamines including dopamine, norepinephrine, benzylamine, and phenylethylamine. Elevated MAO-B density may be associated with astrogliosis and inhibiting MAO-B may reduce astrogliosis. MAO-B density is elevated in postmortem sampling of pathology for many neuropsychiatric diseases including Alzheimer’s disease, Parkinson’s disease, Huntington’s disease, amyotrophic lateral sclerosis, and alcohol use disorder. Initial development of positron emission tomography (PET) imaging agents focused on analogs of [¹¹C]L-deprenyl, with the most commonly applied being the deuterium substituted [¹¹C]L-deprenyl-D2. This latter radiotracer was modeled with an irreversible trapping compartment reflecting its irreversible binding to MAO-B. Subsequently, [¹¹C]SL25.1188, a reversible binding MAO-B radioligand with outstanding properties including high specific binding and excellent reversibility was developed. [¹¹C]SL25.1188 PET was applied to discover a substantive elevation of MAO-B binding in the prefrontal cortex in major depressive disorder (MDD) with an effect size of more than 1.5. Longer duration of MDD was associated with greater MAO-B binding throughout most gray matter regions in the brain, suggesting progressive astrogliosis. Important applications of [¹¹C]L-deprenyl-D2 PET are detecting a 40% loss in radiotracer accumulation in cigarette smokers, and substantial occupancy of novel therapeutics like EVT301 and sembragiline. Given the number of diseases with elevations of MAO-B density and astrogliosis, and the advance of [¹¹C]SL25.1188, clinical applications of MAO-B imaging are still at an early stage.

Characterization of neuroinflammatory positron emission tomography biomarkers in chronic traumatic encephalopathy

Chronic traumatic encephalopathy is a neurological disorder associated with head trauma and is confirmed upon autopsy. PET imaging of chronic traumatic encephalopathy may provide a means to move towards ante-mortem diagnosis and therapeutic intervention following brain injuries. Characterization of the neuroinflammatory PET biomarkers, 18 kDa translocator protein and monoamine oxidase-B was conducted using [3H]PBR-28 and [3H]L-deprenyl, respectively, in post-mortem chronic traumatic encephalopathy brain tissue. [3H]PBR-28 displayed high specific binding in both chronic traumatic encephalopathy (95.40 ± 1.87%; n = 11 cases) and healthy controls (89.89 ± 8.52%, n = 3 cases). Cell-type expression of the 18 kDa translocator protein was confirmed by immunofluorescence to microglia, astrocyte and macrophage markers. [3H]L-deprenyl also displayed high specific binding in chronic traumatic encephalopathy (96.95 ± 1.43%; n = 12 cases) and healthy controls (93.24 ± 0.43%; n = 2 cases), with the distribution co-localized to astrocytes by immunofluorescence. Saturation analysis was performed to quantify the target density of the 18 kDa translocator protein and monoamine oxidase-B in both chronic traumatic encephalopathy and healthy control tissue. Using [3H]PBR-28, the target density of the 18 kDa translocator protein in healthy controls was 177.91 ± 56.96 nM (n = 7 cases; mean ± standard deviation); however, a highly variable target density (345.84 ± 372.42 nM; n = 11 cases; mean ± standard deviation) was measured in chronic traumatic encephalopathy. [3H]L-deprenyl quantified a monoamine oxidase-B target density of 304.23 ± 115.93 nM (n = 8 cases; mean ± standard deviation) in healthy control tissue and is similar to the target density in chronic traumatic encephalopathy tissues (365.80 ± 128.55 nM; n = 12 cases; mean ± standard deviation). A two-sample t-test determined no significant difference in the target density values of the 18 kDa translocator protein and monoamine oxidase-B between healthy controls and chronic traumatic encephalopathy (P > 0.05), albeit a trend towards increased expression of both targets was observed in chronic traumatic encephalopathy. To our knowledge, this work represents the first in vitro characterization of 18 kDa translocator protein and monoamine oxidase-B in chronic traumatic encephalopathy and reveals the variability in neuroinflammatory pathology following brain injuries. These preliminary findings will be considered when designing PET imaging studies after brain injury and for the ultimate goal of imaging chronic traumatic encephalopathy in vivo.

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

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

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.

Recent developments on PET radiotracers for TSPO and their applications in neuroimaging

The 18 kDa translocator protein (TSPO), previously known as the peripheral benzodiazepine receptor, is predominately localized to the outer mitochondrial membrane in steroidogenic cells. Brain TSPO expression is relatively low under physiological conditions, but is upregulated in response to glial cell activation. As the primary index of neuroinflammation, TSPO is implicated in the pathogenesis and progression of numerous neuropsychiatric disorders and neurodegenerative diseases, including Alzheimer's disease (AD), amyotrophic lateral sclerosis (ALS), Parkinson's disease (PD), multiple sclerosis (MS), major depressive disorder (MDD) and obsessive compulsive disorder (OCD). In this context, numerous TSPO-targeted positron emission tomography (PET) tracers have been developed. Among them, several radioligands have advanced to clinical research studies. In this review, we will overview the recent development of TSPO PET tracers, focusing on the radioligand design, radioisotope labeling, pharmacokinetics, and PET imaging evaluation. Additionally, we will consider current limitations, as well as translational potential for future application of TSPO radiopharmaceuticals. This review aims to not only present the challenges in current TSPO PET imaging, but to also provide a new perspective on TSPO targeted PET tracer discovery efforts. Addressing these challenges will facilitate the translation of TSPO in clinical studies of neuroinflammation associated with central nervous system diseases.

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).

Carbon isotope labeling of carbamates by late-stage [11C], [13C] and [14C]carbon dioxide incorporation

A procedure which allows labelling cyclic carbamates with all carbon isotopes has been developed. This protocol valorizes carbon dioxide, the universal building block for radiolabeling. A series of pharmaceuticals were obtained and a disconnection/reconnection strategy was implemented.

"In-loop" [11 C]CO2 fixation: Prototype and proof of concept

Carbon-11-labeled carbon dioxide is the most common feedstock for the synthesis of positron emission tomography radiotracers and can be directly used for 11 C-carbonylation. Herein, we report the development of an apparatus that takes advantage of "in-loop" technologies to facilitate robust and reproducible syntheses of 11 C-carbonyl-based radiotracers by [11 C]CO2 -fixation. Our "in-loop" [11 C]CO2 -fixation method is simple, efficient, and proceeds smoothly at ambient pressure and temperature. We selected model 11 C-carbonyl-labeled carbamates as well as symmetrical and unsymmetrical ureas based on their widespread use in radiotracer design and our clinical research interests for proof of concept. Utility of this method is demonstrated by the synthesis of a reversible radiopharmaceutical for monoamine oxidase B, [11 C]SL25.1188, and 2 novel fatty acid amide hydrolase inhibitors. These radiotracers were isolated and formulated (>3.5 GBq; 100 mCi) with radiochemical purities (>99%) and molar radioactivity (≥80 GBq/μmol; ≥2162 mCi/μmol).

In-loop flow [11 C]CO2 fixation and radiosynthesis of N,N'-[11 C]dibenzylurea

Cyclotron-produced carbon-11 is a highly valuable radionuclide for the production of positron emission tomography (PET) radiotracers. It is typically produced as relatively unreactive carbon-11 carbon dioxide ([11 C]CO2 ), which is most commonly converted into a more reactive precursor for synthesis of PET radiotracers. The development of [11 C]CO2 fixation methods has more recently enabled the direct radiolabelling of a diverse array of structures directly from [11 C]CO2 , and the advantages afforded by the use of a loop-based system used in 11 C-methylation and 11 C-carboxylation reactions inspired us to apply the [11 C]CO2 fixation "in-loop." In this work, we developed and investigated a new ethylene tetrafluoroethylene (ETFE) loop-based [11 C]CO2 fixation method, enabling the fast and efficient, direct-from-cyclotron, in-loop trapping of [11 C]CO2 using mixed DBU/amine solutions. An optimised protocol was integrated into a proof-of-concept in-loop flow radiosynthesis of N,N'-[11 C]dibenzylurea. This reaction exhibited an average 78% trapping efficiency and a crude radiochemical purity of 83% (determined by radio-HPLC), giving an overall nonisolated radiochemical yield of 72% (decay-corrected) within just 3 minutes from end of bombardment. This proof-of-concept reaction has demonstrated that efficient [11 C]CO2 fixation can be achieved in a low-volume (150 μL) ETFE loop and that this can be easily integrated into a rapid in-loop flow radiosynthesis of carbon-11-labelled products. This new in-loop methodology will allow fast radiolabelling reactions to be performed using cheap/disposable ETFE tubing setup (ideal for good manufacturing practice production) thereby contributing to the widespread usage of [11 C]CO2 trapping/fixation reactions for the production of PET radiotracers.

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.

Use of carbon-11 labelled tool compounds in support of drug development

The pharmaceutical industry is facing key challenges to improve return on R&D investment. Positron emission tomography (PET), by itself or in combination with complementary technologies such as magnetic resonance imaging (MRI), provides a unique opportunity to confirm a candidate's ability to meet the so-called 'three pillars' of drug development. Positive confirmation provides confidence for go/no-go decision making at an early stage of the development process and enables informed clinical progression. Whereas fluorine-18 has probably gained wider use in the community, there are benefits to using carbon-11 given the greater flexibility the use of this isotope permits in adaptive clinical study design. This review explores the scope of available carbon-11 chemistries and provides clinical examples to highlight its value in PET studies in support of drug development.

New methodologies for the preparation of carbon-11 labeled radiopharmaceuticals

PURPOSE

This short review aims to cover the more recent and promising developments of carbon-11 (11C) labeling radiochemistry and its utility in the production of novel radiopharmaceuticals, with special emphasis on methods that have the greatest potential to be translated for clinical positron emission tomography (PET) imaging.

METHODS

A survey of the literature was undertaken to identify articles focusing on methodological development in 11C chemistry and their use within novel radiopharmaceutical preparation. However, since 11C-labeling chemistry is such a narrow field of research, no systematic literature search was therefore feasible. The survey was further restricted to a specific timeframe (2000-2016) and articles in English.

RESULTS

From the literature, it is clear that the majority of 11C-labeled radiopharmaceuticals prepared for clinical PET studies have been radiolabeled using the standard heteroatom methylation reaction. However, a number of methodologies have been developed in recent years, both from a technical and chemical point of view. Amongst these, two protocols may have the greatest potential to be widely adapted for the preparation of 11C-radiopharmaceuticals in a clinical setting. First, a novel method for the direct formation of 11C-labeled carbonyl groups, where organic bases are utilized as [11C]carbon dioxide-fixation agents. The second method of clinical importance is a low-pressure 11C-carbonylation technique that utilizes solvable xenon gas to effectively transfer and react [11C]carbon monoxide in a sealed reaction vessel. Both methods appear to be general and provide simple paths to 11C-labeled products.

CONCLUSION

Radiochemistry is the foundation of PET imaging which relies on the administration of a radiopharmaceutical. The demand for new radiopharmaceuticals for clinical PET imaging is increasing, and 11C-radiopharmaceuticals are especially important within clinical research and drug development. This review gives a comprehensive overview of the most noteworthy 11C-labeling methods with clinical relevance to the field of PET radiochemistry.

(11)C[double bond, length as m-dash]O bonds made easily for positron emission tomography radiopharmaceuticals

The positron-emitting radionuclide carbon-11 ((11)C, t1/2 = 20.3 min) possesses the unique potential for radiolabeling of any biological, naturally occurring, or synthetic organic molecule for in vivo positron emission tomography (PET) imaging. Carbon-11 is most often incorporated into small molecules by methylation of alcohol, thiol, amine or carboxylic acid precursors using [(11)C]methyl iodide or [(11)C]methyl triflate (generated from [(11)C]carbon dioxide or [(11)C]methane). Consequently, small molecules that lack an easily substituted (11)C-methyl group are often considered to have non-obvious strategies for radiolabeling and require a more customized approach. [(11)C]Carbon dioxide itself, [(11)C]carbon monoxide, [(11)C]cyanide, and [(11)C]phosgene represent alternative reactants to enable (11)C-carbonylation. Methodologies developed for preparation of (11)C-carbonyl groups have had a tremendous impact on the development of novel PET tracers and provided key tools for clinical research. (11)C-Carbonyl radiopharmaceuticals based on labeled carboxylic acids, amides, carbamates and ureas now account for a substantial number of important imaging agents that have seen translation to higher species and clinical research of previously inaccessible targets, which is a testament to the creativity, utility and practicality of the underlying radiochemistry.

Synthesis of Diverse (11)C-Labeled PET Radiotracers via Direct Incorporation of [(11)C]CO2

Three new positron emission tomography (PET) radiotracers of interest to our functional neuroimaging and translational oncology programs have been prepared through new developments in [(11)C]CO2 fixation chemistry. [(11)C]QZ (glutaminyl cyclase) was prepared via a tandem trapping of [(11)C]CO2/intramolecular cyclization; [(11)C]tideglusib (glycogen synthase kinase-3) was synthesized through a tandem trapping of [(11)C]CO2 followed by an intermolecular cycloaddition between a [(11)C]isocyanate and an isothiocyanate to form the 1,2,4-thiadiazolidine-3,5-dione core; [(11)C]ibrutinib (Bruton's tyrosine kinase) was synthesized through a HATU peptide coupling of an amino precursor with [(11)C]acrylic acid (generated from [(11)C]CO2 fixation with vinylmagnesium bromide). All radiochemical syntheses are fully automated on commercial radiochemical synthesis modules and provide radiotracers in 1-5% radiochemical yield (noncorrected, based upon [(11)C]CO2). All three radiotracers have advanced to rodent imaging studies and preliminary PET imaging results are also reported.

Total Radiosynthesis: Thinking outside "the box"

The logic of total synthesis transformed a stagnant state of medicinal and synthetic organic chemistry when there was a paucity of methods and reagents to synthesize drug molecules and/or natural products. Molecular imaging by positron emission tomography (PET) is now experiencing a renaissance in the way radiopharmaceuticals for molecular imaging are synthesized, however, a paradigm shift is desperately needed in the discovery pipeline to accelerate in vivo imaging studies. A significant challenge in radiochemistry is the limited choice of labeled reagents (or building blocks) available for the synthesis of novel radiopharmaceuticals with the most commonly used short-lived radionuclides carbon-11 (11C; half-life ~20 minutes) and fluorine-18 (18F; half-life ~2 hours). In fact, most drugs cannot be labeled with 11C or 18F due to a lack of efficient and diverse radiosynthetic methods. In general, routine radiopharmaceutical production relies on the incorporation of the isotope at the last or penultimate step of synthesis, ideally within one half-life of the radionuclide, to maximize radiochemical yields and specific activities thereby reducing losses due to radioactive decay. Reliance on radiochemistry conducted within the constraints of an automated synthesis unit ("box") has stifled the exploration of multi-step reactions with short-lived radionuclides. Radiopharmaceutical synthesis can be transformed by considering logic of total synthesis to develop novel approaches for 11C- and 18F-radiolabeling complex molecules via retrosynthetic analysis and multi-step reactions. As a result of such exploration, new methods, reagents and radiopharmaceuticals for in vivo imaging studies are discovered. A new avenue to develop radiotracers that were previously unattainable due to the lack of efficient radiosynthetic methods is necessary to work towards our ultimate, albeit impossible goal - the concept we term total radiosynthesis - to radiolabel virtually any molecule. As with the vast majority of drugs, most radiotracers also fail, therefore expeditious evaluation of tracers in preclinical models prior to optimization or derivatization of the lead molecules/drugs is necessary. Furthermore the exact position of the 11C and 18F radionuclide in tracers is often critical for metabolic considerations, and flexible methodologies to introduce the radiolabel are needed. Using the principles of total synthesis our laboratory and others have shown that multi-step radiochemical reactions are indeed suitable for preclinical and even clinical use. As the goal of total synthesis is to be concise, we have also simplified the syntheses of radiopharmaceuticals. We are presently developing new strategies via [11C]CO2 fixation which has enabled library radiosynthesis as well as labeling non-activated arenes using [18F]fluoride via iodonium ylides. Both of which have proven to be suitable for human PET imaging. We concurrently utilize state-of-the-art automation technologies including microfluidic flow chemistry and rapid purification strategies for radiopharmaceutical production. In this account we highlight how total radiosynthesis has impacted our radiochemistry program, with prominent examples from others, focusing on its impact towards preclinical and clinical research studies.

Monoamine oxidase: radiotracer chemistry and human studies

Monoamine oxidase (MAO) oxidizes amines from both endogenous and exogenous sources thereby regulating the concentration of neurotransmitter amines such as serotonin, norepinephrine, and dopamine as well as many xenobiotics. MAO inhibitor drugs are used in the treatment of Parkinson's disease and in depression stimulating the development of radiotracer tools to probe the role of MAO in normal human biology and in disease. Over the past 30 years since the first radiotracers were developed and the first positron emission tomography (PET) images of MAO in humans were carried out, PET studies of brain MAO in healthy volunteers and in patients have identified different variables that have contributed to different MAO levels in brain and in peripheral organs. MAO radiotracers and PET have also been used to study the current and developing MAO inhibitor drugs including the selection of doses for clinical trials. In this article, we describe the following: (1) the development of MAO radiotracers; (2) human studies including the relationship of brain MAO levels to genotype, personality, neurological, and psychiatric disorders; and (3) examples of the use of MAO radiotracers in drug research and development. We will conclude with outstanding needs to improve the radiotracers that are currently used and possible new applications.

Radiosynthesis and ex vivo evaluation of [(11)C-carbonyl]carbamate- and urea-based monoacylglycerol lipase inhibitors

INTRODUCTION

Monoacylglycerol lipase (MAGL) and fatty acid amide hydrolase (FAAH) are the two primary enzymes that regulate the tone of endocannabinoid signaling. Although new PET radiotracers have been discovered for imaging FAAH in vivo, no such radiotracer exists for imaging MAGL. Here we report the radiosynthesis of five candidate MAGL radiotracers and their ex vivo evaluations in mice and rats.

METHODS

Candidate carbamate and urea MAGL inhibitors were radiolabeled at the carbonyl position by [(11)C]CO2 fixation. Radiotracers were administered (tail-vein injection) to rodents and brain uptake of radioactivity measured at early and late time points ex vivo. Specificity of uptake was explored by pretreatment with unlabeled inhibitors (2 mg/kg, ip) 30 min prior to radiotracer administration.

RESULTS

All five candidate MAGL radiotracers were prepared in high specific activity (>65 GBq/μmol) and radiochemical purity (>98%). Moderate brain uptake (0.2-0.8 SUV) was observed for each candidate while pretreatment did not reduce uptake for four of the five tested. For two candidates ([(11)C]12 and [(11)C]14), high retention of radioactivity was observed in the blood (ca. 10 and 4 SUV at 40 min) which was blocked by pretreatment with unlabeled inhibitors. The most promising candidate, [(11)C]18, demonstrated moderate brain uptake (ca. 0.8 SUV) which showed circa 50% blockade by pretreatment with unlabeled 18.

CONCLUSION

One putative and four reported potent and selective MAGL inhibitors have been radiolabeled via [(11)C]CO2 fixation as radiotracers for this enzyme. Despite the promising in vitro pharmacological profile, none of the five candidate radiotracers exhibited in vivo behavior suitable for PET neuroimaging.

Kinetic modeling of the monoamine oxidase B radioligand [¹¹C]SL25.1188 in human brain with high-resolution positron emission tomography

This article describes the kinetic modeling of [(11)C]SL25.1188 ([(S)-5-methoxymethyl-3-[6-(4,4,4-trifluorobutoxy)-benzo[d]isoxazol-3-yl]-oxazolidin-2-[(11)C]one]) binding to monoamine oxidase B (MAO-B) in the human brain using high-resolution positron emission tomography (PET). Seven healthy subjects underwent two separate 90- minute PET scans after an intravenous injection of [(11)C]SL25.1188. Complementary arterial blood sampling was acquired. Radioactivity was quickly eliminated from plasma with 80% of parent compound remaining at 90 minutes. Metabolites were more polar than the parent compound. Time-activity curves showed high brain uptake, early peak and washout rate consistent with known regional MAO-B concentration. A two-tissue compartment model (2-TCM) provided better fits to the data than a 1-TCM. Measurement of total distribution volume (VT) showed very good identifiability (based on coefficient of variation (COV)) for all regions of interest (ROIs) (COV(VT)<8%), low between-subject variability (∼20%), and quick temporal convergence (within 5% of final value at 45 minutes). Logan graphical method produces very good estimation of VT. Regional VT highly correlated with previous postmortem report of MAO-B level (r(2)= ≥ 0.9). Specific binding would account from 70% to 90% of VT. Hence, VT measurement of [(11)C]SL25.1(1)88 PET is an excellent estimation of MAO-B concentration.

Monoamine oxidase A and B substrates: probing the pathway for drug development

Drug-discovery and -development efforts focused on the MAOs have increased at an accelerated rate over the past decade. Since the first crystal structure of human MAO-B was solved in 2002, over 40 additional structures have been reported and have helped define new, or confirm speculative, binding modes of inhibitors. The detailed mechanism of the MAO-catalyzed oxidation of amine substrates has not been fully elucidated, but its significance is central in the development of new mechanism-based inactivators. Novel fungal MAO-N variants derived from directed evolution strategies are enabling the production of new chiral amine products. Robust assays have been established for measuring MAO status in tissue and cells, while improved MAO radioligands are being deployed for PET imaging studies. This review will attempt to highlight the more recent and salient aspects of MAO research in drug discovery and development, with emphasis on substrates 'probing the pathway'.

Alternative approaches for PET radiotracer development in Alzheimer's disease: imaging beyond plaque

Alzheimer's disease (AD) and related dementias show increasing clinical prevalence, yet our understanding of the etiology and pathobiology of disease-related neurodegeneration remains limited. In this regard, noninvasive imaging with radiotracers for positron emission tomography (PET) presents a unique tool for quantifying spatial and temporal changes in characteristic biological markers of brain disease and for assessing potential drug efficacy. PET radiotracers targeting different protein markers are being developed to address questions pertaining to the molecular and/or genetic heterogeneity of AD and related dementias. For example, radiotracers including [(11) C]-PiB and [(18) F]-AV-45 (Florbetapir) are being used to measure the density of Aβ-plaques in AD patients and to interrogate the biological mechanisms of disease initiation and progression. Our focus is on the development of novel PET imaging agents, targeting proteins beyond Aβ-plaques, which can be used to investigate the broader mechanism of AD pathogenesis. Here, we present the chemical basis of various radiotracers which show promise in preclinical or clinical studies for use in evaluating the phenotypic or biochemical characteristics of AD. Radiotracers for PET imaging neuroinflammation, metal ion association with Aβ-plaques, tau protein, cholinergic and cannabinoid receptors, and enzymes including glycogen-synthase kinase-3β and monoamine oxidase B amongst others, and their connection to AD are highlighted.

Easy upgrade of the TRACERLab FX C Pro for [¹¹C]carboxylation reactions: application to the routine production of [1-¹¹C]acetate

Carbon-11-labeled acetate ([1-(11)C]acetate) is a radiopharmaceutical of importance in clinical practice as well as in preclinical research in cardiology and oncology. Its preparation is based on the [(11)C]carboxylation reaction of a Grignard reagent with [(11)C]CO2. Most of the commercially available synthesizers are only dedicated to the preparation of [(11)C]methyl iodide (or [(11)C]methyl triflate) for the radiomethylation of an appropriate precursor but not for the direct use of cyclotron-produced [(11)C]CO2. Based on the classical [(11)C]carboxylation reaction and SPE purification, we propose in this technical note a detailed, simple, easy-to-handle and fully reversible modification of the TRACERLab FX C Pro to operate, on demand, [(11)C]carboxylation reactions, exemplified herein by the production of [1-(11)C]acetate, or [(11)C]radiomethylation reactions. This also opens new prospects to other type of radiochemical reactions involving [(11)C]CO2.

11CO2 fixation: a renaissance in PET radiochemistry

Carbon-11 labelled carbon dioxide is the cyclotron-generated feedstock reagent for most positron emission tomography (PET) tracers using this radionuclide. Most carbon-11 labels, however, are installed using derivative reagents generated from [(11)C]CO2. In recent years, [(11)C]CO2 has seen a revival in applications for the direct incorporation of carbon-11 into functional groups such as ureas, carbamates, oxazolidinones, carboxylic acids, esters, and amides. This review summarizes classical [(11)C]CO2 fixation strategies using organometallic reagents and then focuses on newly developed methods that employ strong organic bases to reversibly capture [(11)C]CO2 into solution, thereby enabling highly functionalized labelled compounds to be prepared. Labelled compounds and radiopharmaceuticals that have been translated to the clinic are highlighted.

Synthesis and preclinical evaluation of [11C-carbonyl]PF-04457845 for neuroimaging of fatty acid amide hydrolase

INTRODUCTION

Fatty acid amide hydrolase (FAAH) has a significant role in regulating endocannabinoid signaling in the central nervous system. As such, FAAH inhibitors are being actively sought for pain, addiction, and other indications. This has led to the recent pursuit of positron emission tomography (PET) radiotracers targeting FAAH. We report herein the preparation and preclinical evaluation of [(11)C-carbonyl]PF-04457845, an isotopologue of the potent irreversible FAAH inhibitor.

METHODS

PF-04457845 was radiolabeled at the carbonyl position via automated [(11)C]CO(2)-fixation. Ex vivo brain biodistribution of [(11)C-carbonyl]PF-04457845 was carried out in conscious rats. Specificity was determined by pre-administration of PF-04457845 or URB597 prior to [(11)C-carbonyl]PF-04457845. In a separate experiment, rats injected with the title radiotracer had whole brains excised, homogenized and extracted to examine irreversible binding to brain parenchyma.

RESULTS

The title compound was prepared in 5 ± 1% (n = 4) isolated radiochemical yield based on starting [(11)C]CO(2) (decay uncorrected) within 25 min from end-of-bombardment in >98% radiochemical purity and a specific activity of 73.5 ± 8.2 GBq/μmol at end-of-synthesis. Uptake of [(11)C-carbonyl]PF-04457845 into the rat brain was high (range of 1.2-4.4 SUV), heterogeneous, and in accordance with reported FAAH distribution. Saturable binding was demonstrated by a dose-dependent reduction in brain radioactivity uptake following pre-treatment with PF-04457845. Pre-treatment with the prototypical FAAH inhibitor, URB597, reduced the brain radiotracer uptake in all regions by 71-81%, demonstrating specificity for FAAH. The binding of [(11)C-carbonyl]PF-04457845 to FAAH at 40 min post injection was irreversible as 98% of the radioactivity in the brain could not be extracted.

CONCLUSIONS

[(11)C-carbonyl]PF-04457845 was rapidly synthesized via an automated radiosynthesis. Ex vivo biodistribution studies in conscious rodents demonstrate that [11C PF-04457845 is a promising candidate radiotracer for imaging FAAH in the brain with PET. These results coupled with the known pharmacology and toxicology of PF-04457845 should facilitate clinical translation of this radiotracer.