In vivo imaging of neuroinflammation in the rodent brain with [11C]SSR180575, a novel indoleacetamide radioligand of the translocator protein (18 kDa).

PURPOSE: Neuroinflammation is involved in neurological disorders through the activation of microglial cells. Imaging of neuroinflammation with radioligands for the translocator protein (18 kDa) (TSPO) could prove to be an attractive biomarker for disease diagnosis and therapeutic evaluation. The indoleacetamide-derived 7-chloro-N,N,5-trimethyl-4-oxo-3-phenyl-3,5-dihydro-4H-pyridazino[4,5-b]indole-1-acetamide, SSR180575, is a selective high-affinity TSPO ligand in human and rodents with neuroprotective effects. METHODS: Here we report the radiolabelling of SSR180575 with (11)C and in vitro and in vivo imaging in an acute model of neuroinflammation in rats. RESULTS: The image contrast and the binding of [(11)C]SSR180575 are higher than that obtained with the isoquinoline-based TSPO radioligand, [(11)C]PK11195. Competition studies demonstrate that [(11)C]SSR180575 has high specific binding for the TSPO. CONCLUSION: [(11)C]SSR180575 is the first PET radioligand for the TSPO based on an indoleacetamide scaffold designed for imaging neuroinflammation in animal models and in the clinic.

11C-DPA-713: a novel peripheral benzodiazepine receptor PET ligand for in vivo imaging of neuroinflammation.

UNLABELLED: The induction of neuroinflammatory processes, characterized by upregulation of the peripheral benzodiazepine receptor (PBR) expressed by microglial cells, is well correlated with neurodegenerative diseases and with acute neuronal loss. The continually increasing incidence of neurodegenerative diseases in developed countries has become a major health problem, for which the development of diagnostic and follow-up tools is required. Here we investigated a new PBR ligand suitable for PET to monitor neuroinflammatory processes as an indirect hallmark of neurodegeneration. METHODS: We compared PK11195, the reference compound for PBR binding sites, with the new ligand DPA-713 (N,N-diethyl-2-[2-(4-methoxyphenyl)-5,7-dimethylpyrazolo[1,5-a]pyrimidin-3-yl]acetamide), using a small-animal dedicated PET camera in a model of neuroinflammation in rats. Seven days after intrastriatal injection of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA), a PET scan was performed using (11)C-PK11195 or (11)C-DPA-713. Immunohistochemistry for neuronal (NeuN), astrocyte (glial fibrillary acidic protein), and microglial (CD11) specific markers as well as (3)H-PK11195 autoradiographic studies were then correlated with the imaging data. RESULTS: Seven days after a unilateral injection of AMPA in the striatum, (11)C-DPA-713 exhibits a better contrast between healthy and damaged brain parenchyma than (11)C-PK11195 (2.5-fold +/- 0.14 increase vs. 1.6-fold +/- 0.05 increase, respectively). (11)C-DPA-713 and (11)C-PK11195 exhibit similar brain uptake in the ipsilateral side, whereas, in the contralateral side, (11)C-DPA-713 uptake was significantly lower than (11)C-PK11195. Modeling of the data using the simplified reference tissue model shows that the binding potential was significantly higher for (11)C-DPA-713 than for (11)C-PK11195. CONCLUSION: (11)C-DPA-713 displays a higher signal-to-noise ratio than (11)C-PK11195 because of a lower level of unspecific binding that is likely related to the lower lipophilicity of (11)C-DPA-713. Although further studies in humans are required, (11)C-DPA-713 represents a suitable alternative to (11)C-PK11195 for PET of PBR as a tracer of neuroinflammatory processes induced by neuronal stress.

Radiosynthesis of (E)-N-(2-[11C]methoxybenzyl)-3-phenyl-acrylamidine, a novel subnanomolar NR2B subtype-selective NMDA receptor antagonist.

Recently, a novel series of amidines has been described, exhibiting high NR2B-subtype selective N-methyl-D-aspartate (NMDA) antagonist activity with nanomolar or subnanomolar affinity. Within the styrylamidine subclass, (E)-N-(2-methoxybenzyl)-3-phenyl-acrylamidine (1), displayed the highest affinity (Ki=0.7 nM versus [(3)H]ifenprodil) and was considered an appropriate candidate for isotopic labelling with carbon-11 (T(1/2): 20.38 min) at its methoxy group for imaging of NMDA receptors with PET. Derivative 1 has been labelled from the corresponding nor-analogue using [(11)C]methyl triflate and the following experimental conditions : (1) trapping at -10 degrees C of [(11)C]methyl triflate in 300 microL of acetone containing 0.6-0.8 mg of precursor 5 (2.4-3.2 micromol) and 5 microL of a 3M solution of NaOH in water (about 5 eq.); (2) concentration to dryness of the reaction mixture (at 110 degrees C, using a helium stream for 1-2 min); (3) taking up the residue with 0.5 mL of the HPLC mobile phase and (4) purification using semi-preparative HPLC (SymmetryPrep) C-18, Waters, 300 x 7.8 mm). Typically, starting from a 1.5 Ci (55.5 GBq) [(11)C]CO(2) production batch, 120-240 m Ci (4.44-8.88 GBq) of [(11)C]-1 (20-40% decay-corrected radiochemical yield, n=5) was obtained within a total synthesis time of 25-30 min. Specific radioactivities ranged from 0.8 to 1.2 Ci/micromol (29.6-44.4 GBq/micromol) at the end of radiosynthesis. No attempts were made to further optimise these reactions, as sufficient material was obtained to allow for preliminary pharmacological characterisation.

In vivo imaging of brain lesions with [(11)C]CLINME, a new PET radioligand of peripheral benzodiazepine receptors.

The peripheral benzodiazepine receptor (PBR) is expressed by microglial cells in many neuropathologies involving neuroinflammation. PK11195, the reference compound for PBR, is used for positron emission tomography (PET) imaging but has a limited capacity to quantify PBR expression. Here we describe the new PBR ligand CLINME as an alternative to PK11195. In vitro and in vivo imaging properties of [(11)C]CLINME were studied in a rat model of local acute neuroinflammation, and compared with the reference compound [(11)C]PK11195, using autoradiography and PET imaging. Immunohistochemistry study was performed to validate the imaging data. [(11)C]CLINME exhibited a higher contrast between the PBR-expressing lesion site and the intact side of the same rat brain than [(11)C]PK11195 (2.14 +/- 0.09 vs. 1.62 +/- 0.05 fold increase, respectively). The difference was due to a lower uptake for [(11)C]CLINME than for [(11)C]PK11195 in the non-inflammatory part of the brain in which PBR was not expressed, while uptake levels in the lesion were similar for both tracers. Tracer localization correlated well with that of activated microglial cells, demonstrated by immunohistochemistry and PBR expression detected by autoradiography. Modeling using the simplified tissue reference model showed that R(1) was similar for both ligands (R(1) approximately 1), with [(11)C]CLINME exhibiting a higher binding potential than [(11)C]PK11195 (1.07 +/- 0.30 vs. 0.66 +/- 0.15). The results show that [(11)C]CLINME performs better than [(11)C]PK11195 in this model. Further studies of this new compound should be carried out to better define its capacity to overcome the limitations of [(11)C]PK11195 for PBR PET imaging.