A Novel PET Imaging Probe for the Detection and Monitoring of Translocator Protein 18 kDa Expression in Pathological Disorders

A new fluorine-substituted ligand, compound 1 (CB251), with a very high affinity (Ki = 0.27 ± 0.09 nM) and selectivity for the 18-kDa translocator protein (TSPO), is presented as an attractive biomarker for the diagnosis of neuroinflammation, neurodegeneration and tumour progression. To test compound 1 as a TSPO PET imaging agent in vivo , 2-(2-(4-(2-[ 18 F]fluoroethoxy)phenyl)-6,8-dichloroimidazo[1,2-a]pyridin-3-yl)- N,N -dipropylacetamide ([ 18 F] 1 ; [ 18 F]CB251) was synthesized by nucleophilic aliphatic substitution in a single-step radiolabelling procedure with a 11.1 ± 3.5% ( n  = 14, decay corrected) radiochemical yield and over 99% radiochemical purity. In animal PET imaging studies, [ 18 F]CB251 provided a clearly visible image of the inflammatory lesion with the binding potential of the specifically bound radioligand relative to the non-displaceable radioligand in tissue (BP ND 1.83 ± 0.18), in a neuroinflammation rat model based on the unilateral stereotaxic injection of lipopolysaccharide (LPS), comparable to that of [ 11 C]PBR28 (BP ND 1.55 ± 0.41). [ 18 F]CB251 showed moderate tumour uptake (1.96 ± 0.11%ID/g at 1 h post injection) in human glioblastoma U87-MG xenografts. These results suggest that [ 18 F]CB251 is a promising TSPO PET imaging agent for neuroinflammation and TSPO-rich cancers.