No abstract

The translocator protein (TSPO) is expressed at low levels in the healthy human brain and is markedly upregulated in response to brain injury and inflammation. This increase in TSPO expression is correlated to the extent of microglial activation, making the measurement of TSPO density a useful indicator of active brain disease. Several classes of TSPO radioligands have therefore been developed and evaluated for use in PET, to track the progression and severity of neuroinflammatory disease. TSPO is also overexpressed in cancer and peripheral inflammation, making TSPO PET ligands possible candidates for the imaging of a multitude of pathologies. However, we currently possess a limited understanding about the molecular structure of TSPO and about the interaction of ligands with the protein. Furthermore, the incomplete characterization of multiple TSPO binding sites and the role of TSPO polymerization suggest that current interpretation of PET data may require further refinement.

The translocator protein (TSPO) is a potential drug target for the treatment of CNS diseases, with TSPO ligands being able to modulate steroidogenesis, apoptosis, and cell proliferation. While there exist multiple TSPO binding sites, the nature of these sites--either overlapping or allosterically linked--remains largely uncharacterized. Furthermore, while evidence suggests that microglial activation and polymerization result in changes to TSPO binding sites, these changes are poorly understood. While current pharmacophoric models can be used to synthesize TSPO ligands with high affinity and selectivity, these models are unable to predict ligands with desirable functional effects. Better characterization of TSPO binding sites in health and disease may provide insight into particular sites which mediate promising therapeutic profiles, thus refining the TSPO pharmacophore.

New 1,2-closo- and 7,8-nido-carboranylpyrazolopyrimidines bind to the translocator protein (TSPO) with high affinity, providing the first evidence of a unique two-site binding profile for the closo-carborane derivative. The boron-rich compounds can also deliver boron to human glioma cells far more effectively than clinical agents used in boron neutron capture therapy (BNCT).

a b s t r a c t 6-Fluoro-PBR28 (N-(6-fluoro-4-phenoxypyridin-3-yl)-N-(2-methoxybenzyl)acetamide), a fluorinated analogue of the recently developed TSPO 18 kDa ligand PBR28, was synthesized and labelled with fluorine-18. 6-Fluoro-PBR28 and its 6-chloro/6-bromo counterparts were synthesized in six chemical steps and obtained in 16%, 10% and 19% overall yields, respectively. Labelling with fluorine-18 was performed in one single step (chlorine/bromine-for-fluorine heteroaromatic substitution) using a Zymate-XP robotic system affording HPLC-purified, ready-to-inject, 6-[ 18 F]fluoro-PBR28 (>95% radiochemically pure). Nondecay-corrected overall yields were 9-10% and specific radioactivities ranged from 74 to 148 GBq/lmol.In vitro binding experiments, dynamic lPET studies performed in a rat model of acute neuroinflammation (unilaterally, AMPA-induced, striatum-lesioned rats) and ex vivo autoradiography on the same model demonstrated the potential of 6-[ 18 F]fluoro-PBR28 to image the TSPO 18 kDa using PET.