Labelling and biological evaluation of [(11)C]methoxy-Sch225336: a radioligand for the cannabinoid-type 2 receptor

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

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

The development status of PET radiotracers for evaluating neuroinflammation

Neuroinflammation is associated with the pathophysiologies of neurodegenerative and psychiatric disorders. Evaluating neuroinflammation using positron emission tomography (PET) plays an important role in the early diagnosis and determination of proper treatment of brain diseases. To quantify neuroinflammatory responses in vivo, many PET tracers have been developed using translocator proteins, imidazole-2 binding site, cyclooxygenase, monoamine oxidase-B, adenosine, cannabinoid, purinergic P2X7, and CSF-1 receptors as biomarkers. In this review, we introduce the latest developments in PET tracers that can image neuroinflammation, focusing on clinical trials, and further consider their current implications.

PET Imaging of Neuroinflammation in Alzheimer's Disease

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

Design and synthesis of fluorescent ligands for the detection of cannabinoid type 2 receptor (CB2R)

The Cannabinoid 2 receptor, CB2R, belonging to the endocannabinoid system, ECS, is involved in the first steps of neurodegeneration and cancer evolution and progression and thus its modulation may be exploited in the therapeutic and diagnostic fields. However, CB2Rs distribution and signaling pathways in physiological and pathological conditions are still controversial mainly because of the lack of reliable diagnostic tools. With the aim to produce green and safe systems to detect CB2R, we designed a series of fluorescent ligands with three different green fluorescent moieties (4-dimethylaminophthalimide, 4-DMAP, 7-nitro-4-yl-aminobenzoxadiazole, NBD, and Fluorescein-thiourea, FTU) linked to the N1-position of the CB2R pharmacophore N-adamantyl-4-oxo-1,4-dihydroquinoline-3-carboxamide through polymethylene chains. Compound 28 emerged for its compromise between good pharmacodynamic properties (CB2R K_i = 130 nM and no affinity vs the other subtype CB1R) and optimal fluorescent spectroscopic properties. Therefore, compound 28 was studied through FACS (saturation and competitive binding studies) and fluorescence microscopy (visualization and competitive binding) in engineered cells (CB2R-HEK293 cells) and in diverse tumour cells. The fluoligand binding assays were successfully set up, and affinity values for the two reference compounds GW405833 and WIN55,212-2, comparable to the values obtained by radioligand binding assays, were obtained. Fluoligand 28 also allowed the detection of the presence and quantification of the CB2R in the same cell lines. The interactions of compound 28 within the CB2R binding site were also investigated by molecular docking simulations, and indications for the improvement of the CB2R affinity of this class of compounds were provided. Overall, the results obtained through these studies propose compound 28 as a safe and green alternative to the commonly used radioligands for in vitro investigations.

Structure-Activity Relationship Studies of Pyridine-Based Ligands and Identification of a Fluorinated Derivative for Positron Emission Tomography Imaging of Cannabinoid Type 2 Receptors

The cannabinoid type 2 (CB2) receptor has emerged as a valuable target for therapy and imaging of immune-mediated pathologies. With the aim to find a suitable radiofluorinated analogue of the previously reported CB2 positron emission tomography (PET) radioligand [11C]RSR-056, 38 fluorinated derivatives were synthesized and tested by in vitro binding assays. With a Ki (hCB2) of 6 nM and a selectivity factor of nearly 700 over cannabinoid type 1 receptors, target compound 3 exhibited optimal in vitro properties and was selected for evaluation as a PET radioligand. [18F]3 was obtained in an average radiochemical yield of 11 ± 4% and molar activities between 33 and 114 GBq/μmol. Specific binding of [18F]3 to CB2 was demonstrated by in vitro autoradiography and in vivo PET experiments using the CB2 ligand GW-405 833. Metabolite analysis revealed only intact [18F]3 in the rat brain. [18F]3 detected CB2 upregulation in human amyotrophic lateral sclerosis spinal cord tissue and may thus become a candidate for diagnostic use in humans.

Positron emission tomography of type 2 cannabinoid receptors for detecting inflammation in the central nervous system

Cannabinoid receptor CB2 (CB2R) is upregulated on activated microglia and astrocytes in the brain under inflammatory conditions and plays important roles in many neurological diseases, such as Alzheimer's disease, amyotrophic lateral sclerosis, and ischemic stroke. The advent of positron emission tomography (PET) using CB2R radiotracers has enabled the visualization of CB2R distribution in vivo in animal models of central nervous system inflammation, however translation to humans has been less successful. Several novel CB2R radiotracers have been developed and evaluated to quantify microglial activation. In this review, we summarize the recent preclinical and clinical imaging results of CB2R PET tracers and discuss the prospects of CB2R imaging using PET.

Preclinical evaluation of [18 F]MA3: a CB2 receptor agonist radiotracer for PET

BACKGROUND AND PURPOSE

Non-invasive in vivo imaging of cannabinoid CB₂ receptors using PET is pursued to study neuroinflammation. The purpose of this study is to evaluate the in vivo binding specificity of [¹⁸ F]MA3, a CB₂ receptor agonist, in a rat model with local overexpression of human (h) CB₂ receptors.

METHODS

[¹⁸ F]MA3 was produced with good radiochemical yield and radiochemical purity. The radiotracer was evaluated in rats with local overexpression of hCB₂ receptors and in a healthy non-human primate using PET.

KEY RESULTS

Ex vivo autoradiography demonstrated CB₂ -specific binding of [¹⁸ F]MA3 in rat hCB₂ receptor vector injected striatum. In a PET study, increased tracer binding in the hCB₂ receptor vector-injected striatum compared to the contralateral control vector-injected striatum was observed. Binding in hCB₂ receptor vector-injected striatum was blocked with a structurally non-related CB₂ receptor inverse agonist, and a displacement study confirmed the reversibility of tracer binding. This study identified the utility of mutated inactive vector model for evaluation of CB₂ receptor agonist PET tracers. [¹⁸ F]MA3 PET scans in the non-human primate showed good uptake and fast washout from brain, but no CB₂ receptor-specific binding was observed.

CONCLUSION AND IMPLICATIONS

Evaluation of [¹⁸ F]MA3 in a rat model with local overexpression of hCB₂ receptors showed CB₂ receptor-specific and reversible tracer binding. [¹⁸ F]MA3 showed good brain uptake and subsequent washout in a healthy non-human primate, but no specific binding was observed. Further clinical evaluation of [¹⁸ F]MA3 in patients with neuroinflammation is warranted.

LINKED ARTICLES

This article is part of a themed section on 8^th European Workshop on Cannabinoid Research. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v176.10/issuetoc.

Evaluation of 4-oxo-quinoline-based CB2 PET radioligands in R6/2 chorea huntington mouse model and human ALS spinal cord tissue

The cannabinoid receptor 2 (CB2) has been implicated in a series of neurodegenerative disorders and has emerged as an interesting biological target for therapeutic as well as diagnostic purposes. In the present work, we describe an improved radiosynthetic approach to obtain the previously reported CB2-specific PET radioligand [18F]RS-126 in higher radiochemical yields and molar activities. Additionally, the study revealed that prolongation of the [18F]RS-126 fluoroalkyl side chain ultimately leads to an improved stability towards mouse liver enzymes but is accompanied by a reduction in selectivity over the cannabinoid receptor 1 (CB1). Huntington-related phenotypic changes as well as striatal D2R downregulation were confirmed for the transgenic R6/2 mouse model. CB2 upregulation in R6/2 Chorea Huntington mice was observed in hippocampus, cortex, striatum and cerebellum by qPCR, however, these results could not be confirmed at the protein level by PET imaging. Furthermore, we evaluated the utility of the newly developed [11C]RS-028, a potent [18F]RS-126 derivative with increased polarity and high selectivity over CB1 in post-mortem human ALS spinal cord and control tissue. Applying in vitro autoradiography, the translational relevance of CB2 imaging was demonstrated by the specific binding of [11C]RS-028 to post-mortem human ALS spinal cord tissue.

Radioligands for positron emission tomography imaging of cannabinoid type 2 receptor

The cannabinoid type 2 (CB2) receptor is an immunomodulatory receptor mainly expressed in peripheral cells and organs of the immune system. The expression level of CB2 in the central nervous system under physiological conditions is negligible, however under neuroinflammatory conditions an upregulation of CB2 protein or mRNA mainly colocalized with activated microglial cells has been reported. Consequently, CB2 agonists have been confirmed to play a role in neuroprotective and anti-inflammatory processes. A suitable positron emission tomography radioligand for imaging CB2 would provide an invaluable research tool to explore the role of CB2 receptor expression in inflammatory disorders. In this review, we provide a summary of so far published CB2 radioligands as well as their in vitro and in vivo binding characteristics.

Synthesis, Biodistribution and In vitro Evaluation of Brain Permeable High Affinity Type 2 Cannabinoid Receptor Agonists [11C]MA2 and [18F]MA3

The type 2 cannabinoid receptor (CB2) is a member of the endocannabinoid system and is known for its important role in (neuro)inflammation. A PET-imaging agent that allows in vivo visualization of CB2 expression may thus allow quantification of neuroinflammation. In this paper, we report the synthesis, radiosynthesis, biodistribution and in vitro evaluation of a carbon-11 ([¹¹C]MA2) and a fluorine-18 ([¹⁸F]MA3) labeled analog of a highly potent N-arylamide oxadiazole CB2 agonist (EC₅₀ = 0.015 nM). MA2 and MA3 behaved as potent CB2 agonist (EC₅₀: 3 nM and 0.1 nM, respectively) and their in vitro binding affinity for hCB2 was found to be 87 nM and 0.8 nM, respectively. Also MA3 (substituted with a fluoro ethyl group) was found to have higher binding affinity and EC₅₀ values when compared to the originally reported trifluoromethyl analog 12. [¹¹C]MA2 and [¹⁸F]MA3 were successfully synthesized with good radiochemical yield, high radiochemical purity and high specific activity. In mice, both tracers were efficiently cleared from blood and all major organs by the hepatobiliary pathway and importantly these compounds showed high brain uptake. In conclusion, [¹¹C]MA2 and [¹⁸F]MA3 are shown to be high potent CB2 agonists with good brain uptake, these favorable characteristics makes them potential PET probes for in vivo imaging of brain CB2 receptors. However, in view of its higher affinity and selectivity, further detailed evaluation of MA3 as a PET tracer for CB2 is warranted.

Discovery of a fluorinated 4-oxo-quinoline derivative as a potential positron emission tomography radiotracer for imaging cannabinoid receptor type 2

The cannabinoid receptor type 2 (CB2) is part of the endocannabinoid system and has gained growing attention in recent years because of its important role in neuroinflammatory/neurodegenerative diseases. Recently, we reported on a carbon-11 labeled 4-oxo-quinoline derivative, designated RS-016, as a promising radiotracer for imaging CB2 using PET. In this study, three novel fluorinated analogs of RS-016 were designed, synthesized, and pharmacologically evaluated. The results of our efforts led to the identification of N-(1-adamantyl)-1-(2-(2-fluoroethoxy)ethyl)-8-methoxy-4-oxo-1,4-dihydroquinoline-3-carboxamide (RS-126) as the most potent candidate for evaluation as a CB2 PET ligand. [(18) F]RS-126 was obtained in ≥ 99% radiochemical purity with an average specific radioactivity of 98 GBq/μmol at the end of the radiosynthesis. [(18) F]RS-126 showed a logD7.4 value of 1.99 and is stable in vitro in rat and human plasma over 120 min, whereas 55% intact parent compound was found in vivo in rat blood plasma at 10 min post injection. In vitro autoradiographic studies with CB2-positive rat spleen tissue revealed high and blockable binding which was confirmed in in vivo displacement experiments with rats by dynamic PET imaging. Ex vivo biodistribution studies confirmed accumulation of [(18) F]RS-126 in rat spleen with a specificity of 79% under blocking conditions. The moderate elevated CB2 levels in LPS-treated mice brain did not permit the detection of CB2 by [(18) F]RS-126 using PET imaging. In summary, [(18) F]RS-126 demonstrated high specificity toward CB2 receptor in vitro and in vivo and is a promising radioligand for imaging CB2 receptor expression. Cannabinoid receptor type 2 (CB2) is an interesting target for PET imaging. Specific binding of [(18) F]RS-126 in CB2-positive spleen tissue (white arrow head) was confirmed in in vivo displacement experiments with rats. Time activity curve of [(18) F]RS-126 in the spleen after the addition of GW405833 (CB2 specific ligand, green) demonstrates faster radiotracer elimination (blue) compared to the tracer only (red).

Synthesis and Biological Evaluation of Thiophene-Based Cannabinoid Receptor Type 2 Radiotracers for PET Imaging

Over the past two decades, our understanding of the endocannabinoid system has greatly improved due to the wealth of results obtained from exploratory studies. Currently, two cannabinoid receptor subtypes have been well-characterized. The cannabinoid receptor type 1 (CB1) is widely expressed in the central nervous system, while the levels of the cannabinoid receptor type 2 (CB2) in the brain and spinal cord of healthy individuals are relatively low. However, recent studies demonstrated a CB2 upregulation on activated microglia upon neuroinflammation, an indicator of neurodegeneration. Our research group aims to develop a suitable positron emission tomography (PET) tracer to visualize the CB2 receptor in patients suffering from neurodegenerative diseases. Herein we report two novel thiophene-based 11C-labeled PET ligands designated [11C]AAT-015 and [11C]AAT-778. The reference compounds were synthesized using Gewald reaction conditions to obtain the aminothiophene intermediates, followed by amide formation. Saponification of the esters provided their corresponding precursors. Binding affinity studies revealed Ki-values of 3.3 ± 0.5 nM (CB2) and 1.0 ± 0.2 μM (CB1) for AAT-015. AAT-778 showed similar Ki-values of 4.3 ± 0.7 nM (CB2) and 1.1 ± 0.1 μM (CB1). Radiosynthesis was carried out under basic conditions using [11C]iodomethane as methylating agent. After semi-preparative HPLC purification both radiolabeled compounds were obtained in 99% radiochemical purity and the radiochemical yields ranged from 12 to 37%. Specific activity was between 96 and 449 GBq/μmol for both tracers. In order to demonstrate CB2 specificity of [11C]AAT-015 and [11C]AAT-778, we carried out autoradiography studies using CB2-positive mouse/rat spleen tissues. The obtained results revealed unspecific binding in spleen tissue that was not blocked by an excess of CB2-specific ligand GW402833. For in vivo analysis, [11C]AAT-015 was administered to healthy rats via tail-vein injection. Evaluation of the CB2-positive spleen, however, showed no accumulation of the radiotracer. Despite the promising in vitro binding affinities, specific binding of [11C]AAT-015, and [11C]AAT-778 could not be demonstrated.

Discovery of a high affinity and selective pyridine analog as a potential positron emission tomography imaging agent for cannabinoid type 2 receptor

As part of our efforts to develop CB2 PET imaging agents, we investigated 2,5,6-substituted pyridines as a novel class of potential CB2 PET ligands. A total of 21 novel compounds were designed, synthesized, and evaluated for their potency and binding properties toward human and rodent CB1 and CB2. The most promising ligand 6a was radiolabeled with carbon-11 to yield 16 ([(11)C]RSR-056). Specific binding of 16 to CB2-positive spleen tissue of rats and mice was demonstrated by in vitro autogadiography and verified in vivo in PET and biodistribution experiments. Furthermore, 16 was evaluated in a lipopolysaccharid (LPS) induced murine model of neuroinflammation. Brain radioactivity was strikingly higher in the LPS-treated mice than the control mice. Compound 16 is a promising radiotracer for imaging CB2 in rodents. It might serve as a tool for the investigation of CB2 receptor expression levels in healthy tissues and different neuroinflammatory disorders in humans.

Design, synthesis and preliminary evaluation of (18)F-labelled 1,8-naphthyridin- and quinolin-2-one-3-carboxamide derivatives for PET imaging of CB2 cannabinoid receptor

In the present work, we report the synthesis of new aryliodonium salts used as precursors of single-stage nucleophilic (18)F radiofluorination. The corresponding unlabelled fluorinated derivatives showed to be CB2 cannabinoid receptor specific ligands, with Ki values in the low nanomolar range and high CB2/CB1 selectivity. The radiolabelled compound [(18)F]CB91, was successfully formulated for in vivo administration, and its preliminary biodistribution was assessed with microPET/CT. This tracer presented a reasonable in vivo stability and a preferential extraction in the tissues that constitutionally express CB2 cannabinoid receptor. The results obtained indicate [(18)F]CB91 as a possible candidate marker of CB2 cannabinoid receptor distribution. This study would open the way to further validation of this tracer for assessing pathologies for which the expression of this receptor is modified.

Synthesis, in vitro and in vivo evaluation of 1,3,5-triazines as cannabinoid CB2 receptor agonists

The cannabinoid receptors type 2 (CBR2) are attractive therapeutic targets of the endocannabinoid signaling system (ECS) as they are not displaying the undesired psychotropic and cardiovascular side-effects seen with cannabinoid receptor type 1 (CB1R) agonists. In continuation of our previous work on 2,4,6-trisubstituted 1,3,5-triazines as potent CB2 agonists, we synthesized an additional series of more polar analogues (1-10), which were found to possess high CB2R agonist activity with enhanced water solubility. The most potent compound in the series was N-(adamantan-1-yl)-4-ethoxy-6-(4-(2-fluoroethyl)piperazin-1-yl)-1,3,5-triazin-2-amine (9) with EC50 value of 0.60nM. To further evaluate the biological effects of the compounds, the selected compounds were tested in vitro against four different cell lines. A human retinal pigment epithelial cell line (ARPE-19) was used to evaluate the cytotoxicity of the compounds whereas an androgen-sensitive human prostate adenocarcinoma cell line (LNCaP), a Jurkat leukemia cell line and a C8161 melanoma cell line were used to assess the antiproliferative activity of the compounds. The most interesting results were obtained for N-(adamantan-1-yl)-4-ethoxy-6-(4-methylpiperazin-1-yl)-1,3,5-triazin-2-amine (6), which induced cell viability decrease in prostate and leukemia cell lines, and diminished proliferation of C8161 melanoma cells. The results could be reversed in leukemia cells with the selective CB2R antagonist AM630, whereas in prostate cells the AM630 induced a significant cell viability decrease with a mechanism probably unlinked to CB2 cannabinoid receptor. The antiproliferative effect of 6 on the melanoma cells seemed not to be mediated via the CB1R or CB2R. No cytotoxicity was detected against ARPE-19 cell line at concentrations of 1 and 10μM for compound 6. However, at 30μM concentration the compound 6 decreased the cell viability. Finally, in order to estimate in vivo behavior of these compounds, (18)F labeled PET ligand, N-cyclopentyl-4-ethoxy-6-(4-(2-fluoro-18-ethyl)piperazin-1-yl)-1,3,5-triazin-2-amine ([(18)F]5), was synthesized and its biodistribution was determined in healthy male Sprague-Dawley rats. As a result, the tracer showed a rapid (<15min) elimination in urine accompanied by a slower excretion via the hepatobiliary route. In conclusion, we further demonstrated that 1,3,5-triazine scaffold serves as a suitable template for the design of highly potent CB2R agonists with reasonable water solubility properties. The compounds may be useful when studying the role of the endocannabinoid system in different diseases. The triazine scaffold is also a promising candidate for the development of new CB2R PET ligands.

Tracking neuroinflammation in Alzheimer's disease: the role of positron emission tomography imaging

Alzheimer’s disease (AD) has been reconceptualized as a dynamic pathophysiological process, where the accumulation of amyloid-beta (Aβ) is thought to trigger a cascade of neurodegenerative events resulting in cognitive impairment and, eventually, dementia. In addition to Aβ pathology, various lines of research have implicated neuroinflammation as an important participant in AD pathophysiology. Currently, neuroinflammation can be measured in vivo using positron emission tomography (PET) with ligands targeting diverse biological processes such as microglial activation, reactive astrocytes and phospholipase A2 activity. In terms of therapeutic strategies, despite a strong rationale and epidemiological studies suggesting that the use of non-steroidal anti-inflammatory drugs (NSAIDs) may reduce the prevalence of AD, clinical trials conducted to date have proven inconclusive. In this respect, it has been hypothesized that NSAIDs may only prove protective if administered early on in the disease course, prior to the accumulation of significant AD pathology. In order to test various hypotheses pertaining to the exact role of neuroinflammation in AD, studies in asymptomatic carriers of mutations deterministic for early-onset familial AD may prove of use. In this respect, PET ligands for neuroinflammation may act as surrogate markers of disease progression, allowing for the development of more integrative models of AD, as well as for the measuring of target engagement in the context of clinical trials using NSAIDs. In this review, we address the biological basis of neuroinflammatory changes in AD, underscore therapeutic strategies using anti-inflammatory compounds, and shed light on the possibility of tracking neuroinflammation in vivo using PET imaging ligands.

Whole-body biodistribution and radiation dosimetry of the cannabinoid type 2 receptor ligand [11C]-NE40 in healthy subjects

PURPOSE

The type 2 cannabinoid receptor (CB2R) is part of the human endocannabinoid system and is involved in central and peripheral inflammatory processes. In vivo imaging of the CB2R would allow study of several (neuro)inflammatory disorders. In this study we have investigated the safety and tolerability of [11C]-NE40, a CB2R positron emission tomography (PET) ligand, in healthy human male subjects and determined its biodistribution and radiation dosimetry.

PROCEDURE

Six healthy male subjects (age 20-65 years) underwent a dynamic series of nine whole-body PET/CT scans for up to 140 min, after injection of an average bolus of 286 MBq of [11C]-NE40. Organ absorbed and total effective doses were calculated through OLINDA.

RESULTS

[11C]-NE40 showed high initial uptake in the spleen and a predominant hepatobiliary excretion. In the brain, rapid uptake and swift washout were seen. Organ absorbed doses were largest for the small intestine and liver, with 15.6 and 11.5 μGy/MBq, respectively. The mean effective dose was 3.64±0.81 μSv/MBq. There were no changes with aging observed. No adverse events were encountered.

CONCLUSIONS

This first-in-man study of [11C]-NE40 showed an expected biodistribution compatible with lymphoid tissue uptake and appropriate fast brain kinetics in the healthy human brain, underscoring the potential of this tracer for further application in central and peripheral inflammation imaging. The effective dose is within the typical expected range for 11C ligands.

In vivo type 2 cannabinoid receptor-targeted tumor optical imaging using a near infrared fluorescent probe

The type 2 cannabinoid receptor (CB2R) plays a vital role in carcinogenesis and progression and is emerging as a therapeutic target for cancers. However, the exact role of CB2R in cancer progression and therapy remains unclear. This has driven the increasing efforts to study CB2R and cancers using molecular imaging tools. In addition, many types of cancers overexpress CB2R, and the expression levels of CB2R appear to be associated with tumor aggressiveness. Such upregulation of the receptor in cancer cells provides opportunities for CB2R-targeted imaging with high contrast and for therapy with low side effects. In the present study, we report the first in vivo tumor-targeted optical imaging using a novel CB2R-targeted near-infrared probe. In vitro cell fluorescent imaging and a competitive binding assay indicated specific binding of NIR760-mbc94 to CB2R in CB2-mid delayed brain tumor (DBT) cells. NIR760-mbc94 also preferentially labeled CB2-mid DBT tumors in vivo, with a 3.7-fold tumor-to-normal contrast enhancement at 72 h postinjection, whereas the fluorescence signal from the tumors of the mice treated with NIR760 free dye was nearly at the background level at the same time point. SR144528, a CB2R competitor, significantly inhibited tumor uptake of NIR760-mbc94, indicating that NIR760-mbc94 binds to CB2R specifically. In summary, NIR760-mbc94 specifically binds to CB2R in vitro and in vivo and appears to be a promising molecular tool that may have great potential for use in diagnostic imaging of CB2R-positive cancers and therapeutic monitoring as well as in elucidating the role of CB2R in cancer progression and therapy.

Radiofluorination and biological evaluation of N-aryl-oxadiazolyl-propionamides as potential radioligands for PET imaging of cannabinoid CB2 receptors

Background

The level of expression of cannabinoid receptor type 2 (CB₂R) in healthy and diseased brain has not been fully elucidated. Therefore, there is a growing interest to assess the regional expression of CB₂R in the brain. Positron emission tomography (PET) is an imaging technique, which allows quantitative monitoring of very low amounts of radiolabelled compounds in living organisms at high temporal and spatial resolution and, thus, has been widely used as a diagnostic tool in nuclear medicine. Here, we report on the radiofluorination of N-aryl-oxadiazolyl-propionamides at two different positions in the lead structure and on the biological evaluation of the potential of the two tracers [¹⁸F]1 and [¹⁸F]2 as CB₂ receptor PET imaging agents.

Results

High binding affinity and specificity towards CB₂ receptors of the lead structure remained unaffected by the structural changes such as the insertion of the aliphatic and aromatic fluorine in the selected labelling sites of 1 and 2. Aliphatic and aromatic radiofluorinations were optimized, and [¹⁸F]1 and [¹⁸F]2 were achieved in radiochemical yields of ≥30% with radiochemical purities of ≥98% and specific activities of 250 to 450 GBq/μmol. Organ distribution studies in female CD1 mice revealed that both radiotracers cross the blood–brain barrier (BBB) but undergo strong peripheral metabolism. At 30 min after injection, unmetabolized [¹⁸F]1 and [¹⁸F]2 accounted for 60% and 2% as well as 68% and 88% of the total activity in the plasma and brain, respectively. The main radiometabolite of [¹⁸F]2 could be identified as the free acid [¹⁸F]10, which has no affinity towards the CB₁ and CB₂ receptors but can cross the BBB.

Conclusions

N-aryl-oxadiazolyl-propionamides can successfully be radiolabelled with ¹⁸F at different positions. Fluorine substitution at these positions did not affect affinity and specificity towards CB₂R. Despite a promising in vitro behavior, a rather rapid peripheral metabolism of [¹⁸F]1 and [¹⁸F]2 in mice and the generation of brain permeable radiometabolites hamper the application of these radiotracers in vivo. However, it is expected that future synthetic modification aiming at a replacement of metabolically susceptible structural elements of [¹⁸F]1 and [¹⁸F]2 will help to elucidate the potential of this class of compounds for CB₂R PET studies.

Synthesis and preliminary evaluation of [18F]-labeled 2-oxoquinoline derivatives for PET imaging of cannabinoid CB2 receptor

INTRODUCTION

The cannabinoid receptor type 2 (CB(2)) is an important target for development of drugs and imaging agents for diseases, such as neuroinflammation, neurodegeneration and cancer. Recently, we reported synthesis and results of in vitro receptor binding of a focused library of fluorinated 2-oxoquinoline derivatives as CB(2) receptor ligands. Some of the compounds demonstrated to be good CB(2)-specific ligands with Ki values in the nanomolar to subnanomolar concentrations; therefore, we pursued the development of their (18)F-labeled analogues that should be useful for positron emission tomography (PET) imaging of CB(2) receptor expression. Here, we report the radiosynthesis of two (18)F-labeled 2-oxoquinoline derivatives and the preliminary in vitro and ex vivo evaluation of one compound as a CB(2)-specific radioligand.

METHODS

4-[(18)F]fluorobenzyl amine [(18)F]-3 was prepared by radiofluorination of 4-cyano-N,N,N-trimethylanilinium triflate salt followed by reduction with LiAlH(4) and then coupled with acid chlorides 11 and 12 to afford [(18)F]-13 and [(18)F]-14. In vitro CB(2) receptor binding assay was performed using U87 cells transduced with CB(2) and CB(1) receptor. Ex vivo autoradiography was performed with [(18)F]-14 on spleen and on CB(2)- and CB(1)-expressing and wild-type U87 subcutaneous tumors grown in mice.

RESULTS

The radiochemical yields of [(18)F]-13 and [(18)F]-14 were 10%-15.0% with an average of 12% (n=10); radiochemical purity was >99% with specific activity 1200 mCi/μmol. The dissociation constant Kd for [(18)F]-14 was 3.4 nM. Ex vivo autoradiography showed accumulation of [(18)F]-14 in the CB(2)-expressing tumor.

CONCLUSION

Two new [(18)F]-labeled CB(2) ligands have been synthesized. Compound [(18)F]-14 appears to be a potential PET imaging agent for the assessment of CB(2) receptor expression; however, poor solubility restrain its use in vivo.

Preclinical evaluation of [11C]NE40, a type 2 cannabinoid receptor PET tracer

INTRODUCTION

Up-regulation of the type 2 cannabinoid receptor (CB(2)R) has been reported in (neuro)inflammatory diseases. In this study, we report the preclinical evaluation of [(11)C]NE40 as positron emission tomography (PET) radioligand for visualization of the CB(2)R.

METHODS

The selectivity of NE40 for CB(2)R and its toxicity and mutagenicity were determined. [(11)C]NE40 was evaluated by biodistribution and autoradiography studies in normal rats and a microPET study in normal mice, rats and a rhesus monkey. Specific in vivo binding of [(11)C]NE40 to human CB(2)R (hCB(2)R) was studied in a rat model with hCB(2)R overexpression.

RESULTS

[(11)C]NE40 shows specific CB(2)R binding in the spleen and blood of normal rats and high brain uptake in rhesus monkey. [(11)C]NE40 showed specific and reversible binding to hCB(2)R in vivo in a rat model with local hCB(2)R overexpression.

CONCLUSIONS

[(11)C]NE40 shows favorable characteristics as radioligand for in vivo visualization of the CB(2)R and is a promising candidate for hCB(2)R PET imaging.

¹¹C-labeled analogs of indomethacin esters and amides for brain cyclooxygenase-2 imaging: radiosynthesis, in vitro evaluation and in vivo characteristics in mice

There is great potential in the use of positron emission tomography (PET) and suitable radiotracers for the study of cyclooxygenase type 2 (COX-2) enzyme in living subjects. In the present study, we prepared and evaluated five ¹¹C-labeled ester and amide analogs derived from indomethacin as potential PET imaging agents for the in vivo visualization of the brain COX-2 enzyme. Five ¹¹C-labeled COX-2 inhibitors, with different lipophilicities and moderate COX-2 inhibitory activity, were prepared by treatment of the corresponding O-desmethyl precursors with [¹¹C]methyl triflate and purified by HPLC (radiochemical yields of 55-71%, radiochemical purity of >93%, and the specific activities of 22-331 GBq/µmol). In mice, radioactivity in the brain for all radiotracers was low, with very low brain-to-blood ratios. A clear inverse relationship was observed between brain uptake at 1 min postinjection and the lipophilicity (experimental log P₇.₄) of the studied ¹¹C-radiotracers. Pretreatment of mice with cyclosporine A to block P-glycoproteins caused a significant increase in brain uptake of radioactivity following injection of the ¹¹C-radiotracer compared to control. HPLC analysis showed that each radiotracer was rapidly metabolized, and a few metabolites, which were more polar than the original radiotracers, were found in both plasma and brain. No specific binding of the tracers towards the COX-2 enzyme in the brain was clearly revealed by in vivo blocking study. Further structural refinement of the tracer agent is necessary for better enhancement of brain uptake and for sufficient metabolic stability.

Synthesis, in vitro and in vivo evaluation of fluorine-18 labelled FE-GW405833 as a PET tracer for type 2 cannabinoid receptor imaging

The type 2 cannabinoid receptor (CB₂R) is part of the endocannabinoid system and is expressed in tissues related to the immune system. As the CB₂R has a very low brain expression in non-pathological conditions, but is upregulated in activated microglia, it is an interesting target for visualization of neuroinflammation using positron emission tomography with a suitable radiolabeled CB₂R ligand. In this study, we radiolabelled a fluoroethyl derivative of GW405833, a well known CB₂R partial agonist, with fluorine-18 (half-life 109.8 min) by alkylation of the phenol precursor with 1-bromo-2-[¹⁸F]fluoroethane. In vitro studies showed that FE-GW405833 behaved as a selective high affinity (27 nM) inverse agonist for hCB₂R. [¹⁸F]FE-GW405833 showed moderate initial brain uptake in mice and rats, but a slow washout from brain and plasma due to retention of a radiometabolite. Specific binding of the tracer to human CB₂R was demonstrated in vivo in a rat model with local CB₂R overexpression in the brain. Optimized derivatives of GW405833 that are less susceptible to metabolism will need to be developed in order to provide a useful tracer for CB₂R quantification with PET.

Synthesis and biodistribution of [11C]A-836339, a new potential radioligand for PET imaging of cannabinoid type 2 receptors (CB2)

Recently, A-836339 [2,2,3,3-tetramethylcyclopropanecarboxylic acid [3-(2-methoxyethyl)-4,5-dimethyl-3H-thiazol-(2Z)-ylidene]amide] (1) was reported to be a selective CB2 agonist with high binding affinity. Here we describe the radiosynthesis of [11C]A-836339 ([11C]1) via its desmethyl precursor as a candidate radioligand for imaging CB2 receptors with positron-emission tomography (PET). Whole body and the regional brain distribution of [11C]1 in control CD1 mice demonstrated that this radioligand exhibits specific uptake in the CB2-rich spleen and little specific in vivo binding in the control mouse brain. However, [11C]1 shows specific cerebral uptake in the lipopolysaccharide (LPS)-induced mouse model of neuroinflammation and in the brain areas with Abeta amyloid plaque deposition in a mouse model of Alzheimer's disease (APPswe/PS1dE9 mice). These data establish a proof of principle that CB2 receptors binding in the neuroinflammation and related disorders can be measured in vivo.

Synthesis and biological evaluation of carbon-11- and fluorine-18-labeled 2-oxoquinoline derivatives for type 2 cannabinoid receptor positron emission tomography imaging

INTRODUCTION

The type 2 cannabinoid (CB(2)) receptor is part of the endocannabinoid system and has been suggested as a mediator of several central and peripheral inflammatory processes. Imaging of the CB(2) receptor has been unsuccessful so far. We synthesized and evaluated a carbon-11- and a fluorine-18-labeled 2-oxoquinoline derivative as new PET tracers with high specificity and affinity for the CB(2) receptor.

METHODS

Two 2-oxoquinoline derivatives were synthesized and radiolabeled with either carbon-11 or fluorine-18. Their affinity and selectivity for the human CB(2) receptor were determined. Biological evaluation was done by biodistribution, radiometabolite and autoradiography studies in mice.

RESULTS

In vitro studies showed that both compounds are high affinity CB(2)-specific inverse agonists. Biodistribution study of the tracers in mice showed a high in vivo initial brain uptake and fast brain washout, in accordance with the low CB(2) receptor expression levels in normal brain. A persistently high in vivo binding to the spleen was observed, which was inhibited by pretreatment with two structurally unrelated CB(2) selective inverse agonists. In vitro autoradiography studies with the radioligands confirmed CB(2)-specific binding to the mouse spleen.

CONCLUSION

We synthesized two novel CB(2) receptor PET tracers that show high affinity/selectivity for CB(2) receptors. Both tracers show favourable characteristics as radioligands for central and peripheral in vivo visualization of the CB(2) receptor and are promising candidates for primate and human CB(2) PET imaging.