Extensive exploration of a novel rat model of Parkinson's disease using partial 6-hydroxydopamine lesion of dopaminergic neurons suggests new therapeutic approaches.

Parkinson's disease (PD) is characterized by the degeneration of dopaminergic (DA) neurons constituting the nigrostriatal pathway. Neuroinflammation, related to microglial activation, plays an important role in this process. Exploration of animal models of PD using neuroimaging modalities allows to better understand the pathophysiology of the disease. Here, we fully explored a moderate lesion model in the rat in which 6-hydroxydopamine was unilaterally delivered in three sites along the striatum. The degenerative process was assessed through in vivo Positron Emission Tomography (PET) imaging and in vitro autoradiographic quantitation of the striatal dopamine transporter (DAT) and immunostaining of tyrosine hydroxylase (TH). The microglial activation was studied through in vitro autoradiographic quantitation of the 18 kDa translocator protein (TSPO) in the striatum and CD11b staining in the SN. In addition, a targeted metabolomics exploration was performed in both these structures using mass spectrometry coupled to HPLC. Our results showed a reproducible decrease in the striatal DAT density associated with a reduction in the number of TH-positive cells in the SN and striatum, reflecting a robust moderate degeneration of nigrostriatal DA neurons. In addition, we observed strong microglia activation in both the striatum and SN ipsilateral to the lesion, highlighting that this moderate degeneration of DA neurons was associated with a marked neuroinflammation. Our metabolomics studies revealed alterations of specific metabolites and metabolic pathways such as carnitine, arginine/proline, and histidine metabolisms. These results bring new insights in the PD mechanism knowledge and new potential targets for future therapeutic strategies.

Avidin/Biotin Bioinspired Platform for Dual In Vivo 18F-PET/NIRF Molecular Imaging.

The complementary nature of positron emission tomography (PET) and near-infrared fluorescence (NIRF) imaging makes the development of innovative multimodal PET/NIRF probes a very exciting prospect. Herein, the bioinspired design of novel platform exploiting the strength and specificity of interactions between radioactive and fluorescent biotin derivatives and an avidin core is reported. The combination of an original [18F]fluoropyridinylated-biotin derivative and commercially available fluorescent biotin derivatives (Atto-425 and Atto-680) is investigated. The in vivo distribution of such a customized platform is also reported, for the first time, in healthy rodent using PET and ex vivo fluorescence imaging.

Dynamic Imaging of Individual Remyelination Profiles in Multiple Sclerosis.

BACKGROUND: Quantitative in vivo imaging of myelin loss and repair in patients with multiple sclerosis (MS) is essential to understand the pathogenesis of the disease and to evaluate promyelinating therapies. Selectively binding myelin in the central nervous system white matter, Pittsburgh compound B ([11 C]PiB) can be used as a positron emission tomography (PET) tracer to explore myelin dynamics in MS. METHODS: Patients with active relapsing-remitting MS (n = 20) and healthy controls (n = 8) were included in a longitudinal trial combining PET with [11 C]PiB and magnetic resonance imaging. Voxel-wise maps of [11 C]PiB distribution volume ratio, reflecting myelin content, were derived. Three dynamic indices were calculated for each patient: the global index of myelin content change; the index of demyelination; and the index of remyelination. RESULTS: At baseline, there was a progressive reduction in [11 C]PiB binding from the normal-appearing white matter to MS lesions, reflecting a decline in myelin content. White matter lesions were characterized by a centripetal decrease in the tracer binding at the voxel level. During follow-up, high between-patient variability was found for all indices of myelin content change. Dynamic remyelination was inversely correlated with clinical disability (p = 0.006 and beta-coefficient = -0.67 with the Expanded Disability Status Scale; p = 0.003 and beta-coefficient = -0.68 with the MS Severity Scale), whereas no significant clinical correlation was found for the demyelination index. INTERPRETATION: [11 C]PiB PET allows quantification of myelin dynamics in MS and enables stratification of patients depending on their individual remyelination potential, which significantly correlates with clinical disability. This technique should be considered to assess novel promyelinating drugs. Ann Neurol 2016;79:726-738.

From Structure-Activity Relationships on Thiazole Derivatives to the In Vivo Evaluation of a New Radiotracer for Cannabinoid Subtype 2 PET Imaging.

Upregulation of the cannabinoid type 2 receptors (CB2R) unveils inflammation processes of pathological disorders, such as cancer, pain, or neurodegenerative diseases. Among others, CB2R agonist A-836339 has been labeled with carbon-11 for PET imaging of the CB2R and displayed promising results in a mouse model of Alzheimer's disease. The aim of the present work was to develop fluorinated analogs of A-836339 for labeling with fluorine-18 to design a new PET tracer for CB2R imaging. Seven fluorinated analogs of A-836339 were synthesized in two to three steps and their binding affinities and selectivities for both the human and the mouse CB2R were measured as well as their early ADME profiles. Among them, compound 2f (KihCB2R = 0.1 nM, KihCB1R/KihCB2R = 300) displayed high affinity and selectivity for CB2R but also promising lipophilicity, kinetic solubility, and membrane permeation properties and was further selected for in vitro metabolism studies. Incubation of 2f with human or rat liver microsomes followed by LC/MS analysis revealed the presence of six different metabolites mainly resulting from oxidation reactions. A tosylated precursor of 2f was synthesized in two steps and radiolabeled with fluorine-18 to afford [18F]2f in 15 ± 5% radiochemical yield and a molar activity of 110 ± 30 GBq/µmol. Autoradiographies of rat spleen and biodistribution studies in healthy rats including pretreatments with either CB2R or CB1R-specific compounds suggested that [18F]2f is a specific tracer for the CB2R in vivo. We have therefore demonstrated here that [18F]2f is a promising novel tracer for imaging CB2R in vivo using PET. Further investigation in animal models of inflammation will follow.

(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.

6-hydroxydopamine-induced Parkinson's disease-like degeneration generates acute microgliosis and astrogliosis in the nigrostriatal system but no bioluminescence imaging-detectable alteration in adult neurogenesis.

Parkinson's disease is a slowly progressing neurodegenerative disorder caused by loss of dopaminergic neurons in the substantia nigra (SN), leading to severe impairment in motor and non-motor functions. Endogenous subventricular zone (SVZ) neural stem cells constantly give birth to new cells that might serve as a possible source for regeneration in the adult brain. However, neurodegeneration is accompanied by neuroinflammation and dopamine depletion, potentially compromising regeneration. We therefore employed in vivo imaging methods to study striatal deafferentation (N-ω-fluoropropyl-2ß-carbomethoxy-3ß-(4-[(123) I]iodophenyl)nortropane single photon emission computed tomography, DaTscan(™) ) and neuroinflammation in the SN and striatum (N,N-diethyl-2-(2-(4-(2-[(18) F]fluoroethoxy)phenyl)-5,7-dimethylpyrazolo[1,5-a]pyrimidin-3-yl)acetamide positron emission tomography, [(18) F]DPA-714 PET) in the intranigral 6-hydroxydopamine Parkinson's disease mouse model. Additionally, we transduced cells in the SVZ with a lentivirus encoding firefly luciferase and followed migration of progenitor cells in the SVZ-olfactory bulb axis via bioluminescence imaging under disease and control conditions. We found that activation of microglia in the SN is an acute process accompanying the degeneration of dopaminergic cell bodies in the SN. Dopaminergic deafferentation of the striatum does not influence the generation of doublecortin-positive neuroblasts in the SVZ, but generates chronic astrogliosis in the nigrostriatal system.

The neuronal component of gray matter damage in multiple sclerosis: A [(11) C]flumazenil positron emission tomography study.

OBJECTIVE: Using positron emission tomography (PET) with [(11) C]flumazenil ([(11) C]FMZ), an antagonist of the central benzodiazepine site located within the GABAA receptor, we quantified and mapped neuronal damage in the gray matter (GM) of patients with multiple sclerosis (MS) at distinct disease stages. We investigated the relationship between neuronal damage and white matter (WM) lesions and evaluated the clinical relevance of this neuronal PET metric. METHODS: A cohort of 18 MS patients (9 progressive and 9 relapsing-remitting) was compared to healthy controls and underwent neurological and cognitive evaluations, high-resolution dynamic [(11) C]FMZ PET imaging and brain magnetic resonance imaging. [(11) C]FMZ binding was estimated using the partial saturation protocol providing voxel-wise absolute quantification of GABAA receptor concentration. PET data were evaluated using a region of interest (ROI) approach as well as on a vertex-by-vertex basis. RESULTS: [(11) C]FMZ binding was significantly decreased in the cortical GM of MS patients, compared to controls (-10%). Cortical mapping of benzodiazepine receptor concentration ([(11) C]FMZ Bmax) revealed significant intergroup differences in the bilateral parietal cortices and right frontal areas. ROI analyses taking into account GM volume changes showed extensive decrease in [(11) C]FMZ binding in bilateral parietal, cingulate, and insular cortices as well as in the thalami, amygdalae, and hippocampi. These changes were significant in both progressive and relapsing-remitting forms of the disease and correlated with WM T2-weighted lesion load. [(11) C]FMZ cortical binding correlated with cognitive performance. INTERPRETATION: This pilot study showed that PET with [(11) C]FMZ could be a promising and sensitive quantitative marker to assess and map the neuronal substrate of GM pathology in MS.

Differential influence of propofol and isoflurane anesthesia in a non-human primate on the brain kinetics and binding of [(18)F]DPA-714, a positron emission tomography imaging marker of glial activation.

Translocator protein 18 kDa (TSPO) expression at the mitochondrial membrane of glial cells is related to glial activation. TSPO radioligands such as [(18)F]DPA-714 are useful for the non-invasive study of neuroimmune processes using positron emission tomography (PET). Anesthetic agents were shown to impact mitochondrial function and may influence [(18)F]DPA-714 binding parameters and PET kinetics. [(18) F]DPA-714 PET imaging was performed in Papio anubis baboons anesthetized using either intravenous propofol (n = 3) or inhaled isoflurane (n = 3). Brain kinetics and metabolite-corrected input function were measured to estimate [(18) F]DPA-714 brain distribution (VT). Displacement experiments were performed using PK11195 (1.5 mg/kg). In vitro [(18)F]DPA-714 binding experiments were performed using baboon brain tissue in the absence and presence of tested anesthetics. Brain radioactivity peaked higher in isoflurane-anesthetized animals compared with propofol (SUVmax = 2.7 ± 0.5 vs. 1.3 ± 0.2, respectively) but was not different after 30 min. Brain VT was not different under propofol and isoflurane. Displacement resulted in a 35.8 ± 8.4% decrease of brain radioactivity under propofol but not under isoflurane (0.1 ± 7.0%). In vitro, the presence of propofol increased TSPO density and dramatically reduced its affinity for [(18)F]DPA-714 compared with control. This in vitro effect was not significant with isoflurane. Exposure to propofol and isoflurane differentially influences TSPO interaction with its specific radioligand [(18)F]DPA-714 with subsequent impact on its tissue kinetics and specific binding estimated in vivo using PET. Therefore, the choice of anesthetics and their potential influence on PET data should be considered for the design of imaging studies using TSPO radioligands, especially in a translational research context.

Efficient tritiation of the translocator protein (18 kDa) selective ligand DPA-714.

DPA-714 (N,N-diethyl-2-(2-(4-(2-fluoroethoxy)phenyl)-5,7-dimethylpyrazolo[1,5-a]pyrimidin-3-yl)acetamide) is a recently discovered fluorinated ligand of the translocator protein 18 kDa (TSPO). Labelled with the short-lived positron emitter fluorine-18, this structure is today the radioligand of reference for in vivo imaging of microglia activation and neuroinflammatory processes with positron emission tomography. In the present work, an isotopically tritium-labelled version was developed ([(3) H]DPA-714), in order to access high resolution in vitro and ex vivo microscopic autoradiography studies, repeated and long-lasting receptor binding studies and in vivo pharmacokinetic determination at late time points. Briefly, DPA-714 as reference, and its 3,5-dibrominated derivative as precursor for labelling, were both prepared from DPA-713 in nonoptimized 32% (two steps) and 10% (three steps) yields, respectively. Reductive debromination using deuterium gas and Pd/C as catalyst in methanol, performed at the micromolar scale, confirmed the regioselective introduction of two deuterium atoms at the meta positions of the phenyl ring. Tritiodebromination was analogously performed using no-carrier tritium gas. HPLC purification provided >96% radiochemically pure [(3) H]DPA-714 (7 GBq) with a 2.1 TBq/mmol specific radioactivity. Interestingly, additional hydrogen-for-tritium exchanges were also observed at the 5-methyl and 7-methyl positions of the pyrazolo[1,5-a]pyrimidine, opening novel perspectives in the labelling of compounds featuring this heterocyclic core.

Preparation and evaluation of novel pyrazolo[1,5-a]pyrimidine acetamides, closely related to DPA-714, as potent ligands for imaging the TSPO 18kDa with PET.

A series of four novel analogues of DPA-714, bearing a fluoroalkynyl side chain (with a length ranging from three to six carbon atoms) in replacement of the fluoroethoxy motif, have been synthetized in six steps from commercially available methyl 4-iodobenzoate. The synthetic strategy for the preparation of these N,N-diethyl-2-(2-(4-(ω-fluoroalk-1-ynyl)phenyl)-5,7-dimethylpyrazolo[1,5-a]pyrimidin-3-yl)acetamides (7a-d) consisted in derivatizing a key iodinated building block featuring the pyrazolopyrimidine acetamide backbone of DPA-714, by Sonogashira couplings with various alkynyl reagents. The resulting alkynols were subsequently fluorinated, yielding the expected target derivatives. All four analogues exhibited slightly higher affinity and selectivity towards the TSPO 18kDa (Ki vs [(3)H]PK11195: 0.35-0.79nM; Ki vs [(3)H]flunitrazepam: >1000nM) when compared to DPA-714 (Ki vs [(3)H]PK11195: 0.91nM; Ki vs [(3)H]flunitrazepam: >1000nM). Lipophilicities (HPLC, logD7.4) increased with the chain length (from 3.6 to 4.3) and were significantly higher than the one determined for DPA-714 (2.9). Preliminary in vitro metabolism evaluation using rat microsomal incubations and LC-MS analyses showed, for all four novel analogues, the absence of defluorinated metabolites. Among them, the fluoropentynyl compound, DPA-C5yne (7c), was selected, labelled in one single step with fluorine-18 from the corresponding tosylate and in vivo evaluated with PET on our in-house-developed rat model of acute local neuroinflammation.

[18F]DPA-C5yne, a novel fluorine-18-labelled analogue of DPA-714: radiosynthesis and preliminary evaluation as a radiotracer for imaging neuroinflammation with PET.

DPA-C5yne, the lead compound of a novel series of DPA-714 derivatives in which the fluoroethoxy chain linked to the phenylpyrazolopyrimidine scaffold has been replaced by a fluoroalkyn-1-yl moiety, is a high affinity (Ki : 0.35 nM) and selective ligand targeting the translocator protein 18 kDa. In the present work, DPA-C5yne was labelled with no-carrier-added [(18)F]fluoride based on a one-step tosyloxy-for-fluorine nucleophilic substitution reaction, purified by cartridge and HPLC, and formulated as an i.v. injectable solution using a TRACERLab FX N Pro synthesizer. Typically, 4.3-5.2 GBq of [(18)F]DPA-C5yne, ready-to-use, chemically and radiochemically pure (> 95%), was obtained with specific radioactivities ranging from 55 to 110 GBq/µmol within 50-60 min, starting from a 30 GBq [(18)F]fluoride batch (14-17%). LogP and LogD of [(18)F]DPA-C5yne were measured using the shake-flask method and values of 2.39 and 2.51 were found, respectively. Autoradiography studies performed on slices of ((R,S)-α-amino-3-hydroxy-5-methyl-4-isoxazolopropionique (AMPA)-lesioned rat brains showed a high target-to-background ratio (1.9 ± 0.3). Selectivity and specificity of the binding for the translocator protein was demonstrated using DPA-C5yne (unlabelled), PK11195 and Flumazenil (central benzodiazepine receptor ligand) as competitors. Furthermore, DPA-C5yne proved to be stable in plasma at 37°C for at least 90 min.

Imaging microglial/macrophage activation in spinal cords of experimental autoimmune encephalomyelitis rats by positron emission tomography using the mitochondrial 18 kDa translocator protein radioligand [¹8F]DPA-714.

Multiple sclerosis (MS) is an inflammatory demyelinating disease of the CNS. Activated microglia/macrophages play a key role in the immunopathogenesis of MS and its corresponding animal models, experimental autoimmune encephalomyelitis (EAE). Microglia activation begins at early stages of the disease and is associated with elevated expression of the 18 kDa mitochondrial translocator protein (TSPO). Thus, positron emission tomography (PET) imaging of microglial activation using TSPO-specific radioligands could be valuable for monitoring disease-associated neuroinflammatory processes. EAE was induced in rats using a fragment of myelin basic protein, yielding acute clinical disease that reflects extensive spinal cord inflammation. Enhanced TSPO expression in spinal cords of EAE rats versus those of controls was confirmed by Western blot and immunohistochemistry. Biodistribution studies in control and EAE rats were performed using the TSPO radioligand [¹8F]DPA-714 [N,N-diethyl-2-(2-(4-(2-fluoroethoxy)phenyl)-5,7-dimethylpyrazolo[1,5-a]pyrimidin-3-yl)acetamide]. At 1 h after injection, almost fivefold higher levels of [¹8F]DPA-714 were measured in spinal cords of EAE rats versus controls. The specific binding of [¹8F]DPA-714 to TSPO in spinal cords was confirmed in competition studies, using unlabeled (R,S)-PK11195 [(R,S)-N-methyl-N-(1-methylpropyl)-1-(2-chlorophenyl)isoquinoline-3-carboxamide)] or DPA-714 in excess. MicroPET studies affirm that this differential radioactivity uptake in spinal cords of EAE versus control rats could be detected and quantified. Using [¹8F]DPA-714, neuroinflammation in spinal cords of EAE-induced rats could be visualized by PET, offering a sensitive technique for monitoring neuroinflammatory lesions in the CNS and particularly in the spinal cord. In addition to current MRI protocols, this approach could provide molecular images of neuroinflammation for detection, monitoring, and research in MS.

Reactive astrocytes overexpress TSPO and are detected by TSPO positron emission tomography imaging.

Astrocytes and microglia become reactive under most brain pathological conditions, making this neuroinflammation process a surrogate marker of neuronal dysfunction. Neuroinflammation is associated with increased levels of translocator protein 18 kDa (TSPO) and binding sites for TSPO ligands. Positron emission tomography (PET) imaging of TSPO is thus commonly used to monitor neuroinflammation in preclinical and clinical studies. It is widely considered that TSPO PET signal reveals reactive microglia, although a few studies suggested a potential contribution of reactive astrocytes. Because astrocytes and microglia play very different roles, it is crucial to determine whether reactive astrocytes can also overexpress TSPO and yield to a detectable TSPO PET signal in vivo. We used a model of selective astrocyte activation through lentiviral gene transfer of the cytokine ciliary neurotrophic factor (CNTF) into the rat striatum, in the absence of neurodegeneration. CNTF induced an extensive activation of astrocytes, which overexpressed GFAP and become hypertrophic, whereas microglia displayed minimal increase in reactive markers. Two TSPO radioligands, [(18)F]DPA-714 [N,N-diethyl-2-(2-(4-(2-[(18)F]fluoroethoxy)phenyl)-5,7-dimethylpyrazolo[1,5-a]pyrimidin-3-yl)acetamide] and [(11)C]SSR180575 (7-chloro-N,N-dimethyl-5-[(11)C]methyl-4-oxo-3-phenyl-3,5-dihydro-4H-pyridazino[4,5-b]indole-1-acetamide), showed a significant binding in the lenti-CNTF-injected striatum that was saturated and displaced by PK11195 [N-methyl-N-(1-methylpropyl)-1-(2-chlorophenyl)-isoquinoline-3-carboxamide]. The volume of radioligand binding matched the GFAP immunopositive volume. TSPO mRNA levels were significantly increased, and TSPO protein was overexpressed by CNTF-activated astrocytes. We show that reactive astrocytes overexpress TSPO, yielding to a significant and selective binding of TSPO radioligands. Therefore, caution must be used when interpreting TSPO PET imaging in animals or patients because reactive astrocytes can contribute to the signal in addition to reactive microglia.

Metabolism and quantification of [(18)F]DPA-714, a new TSPO positron emission tomography radioligand.

[(18)F]DPA-714 [N,N-diethyl-2-(2-(4-(2[(18)F]-fluoroethoxy)phenyl)5,7dimethylpyrazolo[1,5a]pyrimidin-3-yl)acetamide] is a new radioligand currently used for imaging the 18-kDa translocator protein in animal models of neuroinflammation and recently in humans. The biodistribution by positron emission tomography (PET) in baboons and the in vitro and in vivo metabolism of [(18)F]DPA-714 were investigated in rats, baboons, and humans. Whole-body PET experiments showed a high uptake of radioactivity in the kidneys, heart, liver, and gallbladder. The liver was a major route of elimination of [(18)F]DPA-714, and urine was a route of excretion for radiometabolites. In rat and baboon plasma, high-performance liquid chromatography (HPLC) metabolic profiles showed three major radiometabolites accounting for 85% and 89% of total radioactivity at 120 minutes after injection, respectively. Rat microsomal incubations and analyses by liquid chromatography-mass spectrometry (LC-MS) identified seven metabolites, characterized as O-deethyl, hydroxyl, and N-deethyl derivatives of nonradioactive DPA-714, two of them having the same retention times than those detected in rat and baboon plasma. The third plasma radiometabolite was suggested to be a carboxylic acid compound that accounted for 15% of the rat brain radioactivity. O-deethylation led to a nonradioactive compound and [(18)F]fluoroacetic acid. Human CYP3A4 and CYP2D6 were shown to be involved in the oxidation of the radioligand. Finally an easy, rapid, and accurate method--indispensable for PET quantitative clinical studies--for quantifying [(18)F]DPA-714 by solid-phase extraction was developed. In vivo, an extensive metabolism of [(18)F]DPA-714 was observed in rats and baboons, identified as [(18)F]deethyl, [(18)F]hydroxyl, and [(18)F]carboxylic acid derivatives of [(18)F]DPA-714. The main route of excretion of the unchanged radioligand in baboons was hepatobiliary while that of radiometabolites was the urinary system.

Synthesis and characterization of an MRI Gd-based probe designed to target the translocator protein.

DPA-713 is the lead compound of a recently reported pyrazolo[1,5-a]pyrimidineacetamide series, targeting the translocator protein (TSPO 18 kDa), and as such, this structure, as well as closely related derivatives, have been already successfully used as positron emission tomography radioligands. On the basis of the pharmacological core of this ligands series, a new magnetic resonance imaging probe, coded DPA-C(6)-(Gd)DOTAMA was designed and successfully synthesized in six steps and 13% overall yield from DPA-713. The Gd-DOTA monoamide cage (DOTA = 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid) represents the magnetic resonance imaging reporter, which is spaced from the phenylpyrazolo[1,5-a]pyrimidineacetamide moiety (DPA-713 motif) by a six carbon-atom chain. DPA-C(6)-(Gd)DOTAMA relaxometric characterization showed the typical behavior of a small-sized molecule (relaxivity value: 6.02 mM(-1) s(-1) at 20 MHz). The good hydrophilicity of the metal chelate makes DPA-C(6)-(Gd)DOTAMA soluble in water, affecting thus its biodistribution with respect to the parent lipophilic DPA-713 molecule. For this reason, it was deemed of interest to load the probe to a large carrier in order to increase its residence lifetime in blood. Whereas DPA-C(6)-(Gd)DOTAMA binds to serum albumin with a low affinity constant, it can be entrapped into liposomes (both in the membrane and in the inner aqueous cavity). The stability of the supramolecular adduct formed by the Gd-complex and liposomes was assessed by a competition test with albumin.

Carbon-11 and fluorine-18 chemistry devoted to molecular probes for imaging the brain with positron emission tomography.

Exploration of the living human brain in real-time and in a noninvasive way was for centuries only a dream, made, however, possible today with the remarkable development during the four last decades of powerful molecular imaging techniques, and especially positron emission tomography (PET). Molecular PET imaging relies, from a chemical point of view, on the use and preparation of a positron-emitting radiolabelled probe or radiotracer, notably compounds incorporating one of two short-lived radionuclides fluorine-18 (T1/2 : 109.8 min) and carbon-11 (T1/2 : 20.38 min). The growing availability and interest for the radiohalogen fluorine-18 in radiopharmaceutical chemistry undoubtedly results from its convenient half-life and the successful use in clinical oncology of 2-[(18) F]fluoro-2-deoxy-d-glucose ([(18) F]FDG). The special interest of carbon-11 is not only that carbon is present in virtually all biomolecules and drugs allowing therefore for isotopic labelling of their chemical structures but also that a given molecule could be radiolabelled at different functions or sites, permitting to explore (or to take advantage of) in vivo metabolic pathways. PET chemistry includes production of these short-lived radioactive isotopes via nuclear transmutation reactions using a cyclotron, and is directed towards the development of rapid synthetic methods, at the trace level, for the introduction of these nuclides into a molecule, as well as the use of fast purification, analysis and formulation techniques. PET chemistry is the driving force in molecular PET imaging, and this special issue of the Journal of Labelled Compounds and Radiopharmaceuticals, which is strongly chemistry and radiochemistry-oriented, aims at illustrating, be it in part only, the state-of-the-art arsenal of reactions currently available and its potential for the research and development of specific molecular probes labelled with the positron emitters carbon-11 and fluorine-18, with optimal imaging properties for PET exploration of the brain.

The potential of carbon-11 and fluorine-18 chemistry: illustration through the development of positron emission tomography radioligands targeting the translocator protein 18 kDa.

The TSPO (translocator protein), also known as the peripheral benzodiazepine receptor, is upregulated in the brain of subjects suffering from neurodegenerative disorders such as Alzheimer's, Parkinson's and Huntington's disease. Moreover, this overexpression has been proved to be linked to microglia activation making thus the TSPO a marker of choice of neuroinflammatory processes and therefore a potential target for the development of radioligands for positron emission tomography imaging. The discovery of selective TSPO ligands and their labelling with the short-lived positron-emitter isotopes carbon-11 and fluorine-18 emerged in the mid-1980s with the preparation of the 3-isoquinolinecarboxamide [(11) C]PK11195. To date, an impressive number of promising compounds-[(11) C]PK11195-challengers-have been developed; some radioligands-for example, [(11) C]PBR28, [(11) C]DPA-713, [(18) F]FEDAA1106 and [(18) F]DPA-714-are currently used in clinical trials. As illustrated in this review, the methodologies applied for the preparation of these compounds remain mainly [(11) C]methylations using [(11) C]MeI or [(11) C]MeOTf and SN 2-type nucleophilic aliphatic [(18) F]fluorinations-two processes illustrating the state-of-the-art arsenal of reactions that involves these two short-lived radioisotopes-but alternative processes, such as [(11) C]carbonylations using [(11) C]CO and [(11) C]COCl2 as well as SN Ar-type nucleophilic [(18) F]fluorinations, have also been reported and as such, reviewed herein.

Imaging central nervous system myelin by positron emission tomography in multiple sclerosis using [methyl-¹¹C]-2-(4'-methylaminophenyl)- 6-hydroxybenzothiazole.

OBJECTIVE: Imaging of myelin tracts in vivo would greatly improve the monitoring of demyelinating diseases such as multiple sclerosis (MS). To date, no imaging technique specifically targets demyelination and remyelination. Recently, amyloid markers related to Congo red have been shown to bind to central nervous system (CNS) myelin. Here we questioned whether the thioflavine-T derivative 2-(4'-methylaminophenyl)-6-hydroxybenzothiazole (PIB), which also binds to amyloid plaques, could serve as a myelin marker. METHODS: PIB fixation to myelin was studied by fluorescence in the normal and dysmyelinating mouse brain, as well as in the postmortem brain of MS patients. Positron emission tomography (PET) experiments were conducted using [¹¹C]PIB in baboons and in a proof of concept clinical study in 2 MS patients. RESULTS: Applied directly on tissue sections or after intraperitoneal injection, PIB stained CNS myelin, and the decrease in the level of fixation paralleled the amount of myelin loss in a dysmyelinating mutant. In normally myelinated areas of postmortem MS brain, demyelinated and remyelinated lesions were clearly distinguishable by the differential intensity of labeling observed with PIB. PET using intravenously injected radiolabeled [¹¹C]PIB imaged CNS myelin in baboons and humans. In MS patients, the dynamic analysis of PET acquisitions allowed quantitative assessment of demyelination. INTERPRETATION: PIB could be used as an imaging marker to quantify myelin loss and repair in demyelinating diseases.

The translocator protein ligand [¹8F]DPA-714 images glioma and activated microglia in vivo.

PURPOSE: In recent years there has been an increase in the development of radioligands targeting the 18-kDa translocator protein (TSPO). TSPO expression is well documented in activated microglia and serves as a biomarker for imaging neuroinflammation. In addition, TSPO has also been reported to be overexpressed in a number of cancer cell lines and human tumours including glioma. Here we investigated the use of [(18)F]DPA-714, a new TSPO positron emission tomography (PET) radioligand to image glioma in vivo. METHODS: We studied the uptake of [(18)F]DPA-714 in three different rat strains implanted with 9L rat glioma cells: Fischer (F), Wistar (W) and Sprague Dawley (SD) rats. Dynamic [(18)F]DPA-714 PET imaging, kinetic modelling of PET data and in vivo displacement studies using unlabelled DPA-714 and PK11195 were performed. Validation of TSPO expression in 9L glioma cell lines and intracranial 9L gliomas were investigated using Western blotting and immunohistochemistry of brain tissue sections. RESULTS: All rats showed significant [(18)F]DPA-714 PET accumulation at the site of 9L tumour implantation compared to the contralateral brain hemisphere with a difference in uptake among the three strains (F > W > SD). The radiotracer showed high specificity for TSPO as demonstrated by the significant reduction of [(18)F]DPA-714 binding in the tumour after administration of unlabelled DPA-714 or PK11195. TSPO expression was confirmed by Western blotting in 9L cells in vitro and by immunohistochemistry ex vivo. CONCLUSION: The TSPO radioligand [(18)F]DPA-714 can be used for PET imaging of intracranial 9L glioma in different rat strains. This preclinical study demonstrates the feasibility of employing [(18)F]DPA-714 as an alternative radiotracer to image human glioma.

Early detection and longitudinal monitoring of experimental primary and disseminated melanoma using [¹°F]ICF01006, a highly promising melanoma PET tracer.

PURPOSE: Here, we report a new and rapid radiosynthesis of (18)F-N-[2-(diethylamino)ethyl]-6-fluoro-pyridine-3-carboxamide ([(18)F]ICF01006), a molecule with a high specificity for melanotic tissue, and its evaluation in a murine model for early specific detection of pigmented primary and disseminated melanoma. METHODS: [(18)F]ICF01006 was synthesized using a new one-step bromine-for-fluorine nucleophilic heteroaromatic substitution. Melanoma models were induced by subcutaneous (primary tumour) or intravenous (lung colonies) injection of B16BL6 melanoma cells in C57BL/6J mice. The relevance and sensitivity of positron emission tomography (PET) imaging using [(18)F]ICF01006 were evaluated at different stages of tumoural growth and compared to (18)F-fluorodeoxyglucose ([(18)F]FDG). RESULTS: The fully automated radiosynthesis of [(18)F]ICF01006 led to a radiochemical yield of 61 % and a radiochemical purity >99 % (specific activity 70-80 GBq/µmol; total synthesis time 42 min). Tumours were visualized before they were palpable as early as 1 h post-injection with [(18)F]ICF01006 tumoural uptake of 1.64 ± 0.57, 3.40 ± 1.47 and 11.44 ± 2.67 percentage of injected dose per gram of tissue (%ID/g) at days 3, 5 and 14, respectively. [(18)F]ICF01006 PET imaging also allowed detection of melanoma pulmonary colonies from day 9 after tumour cell inoculation, with a lung radiotracer accumulation correlated with melanoma invasion. At day 21, radioactivity uptake in lungs reached a value of 5.23 ± 2.08 %ID/g (versus 0.41 ± 0.90 %ID/g in control mice). In the two models, comparison with [(18)F]FDG showed that both radiotracers were able to detect melanoma lesions, but [(18)F]ICF01006 was superior in terms of contrast and specificity. CONCLUSION: Our promising results provide further preclinical data, reinforcing the excellent potential of [(18)F]ICF01006 PET imaging for early specific diagnosis and follow-up of melanin-positive disseminated melanoma.