Abstract
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Objectives: Selective TSPO-binding PET radioligand can provide pathological information related to several inflammatory brain disorders in a non-invasive way1. Recently, single nucleotide polymorphism in the TSPO gene have been known to affect the binding affinities of reported TSPO ligands, eventually leading to the classification of ligand-binding profiles for each genotype2. Thus, a novel TSPO-binding radioligand, which does not affected by polymorphism on binding affinity to TSPO, is urgently needed. This study aimed to develop and evaluate a novel TSPO-selective PET imaging ligand, [18F]BS224, which is less sensitive to TSPO polymorphism.
Methods: A new aromatic fluorinated imidazopyridine analog, [18F]BS224 were synthesized by two different precursors (diaryliodonium tosylate salt and pinacol boronate ester). Aromatic 18F-fluorination was optimized using various phase transfer catalysts, solvents, temperatures, and catalysts. Finally, [18F]BS224 (Rt = 28 min) was purified and collected by semi-preparative HPLC. We prepared 294FT cell transfected with wild TSPO gene (TSPO wild) or A147T point mutant gene (TSPO mutant). Comparison cellular uptake assay of [3H]PK11195, [18F]PBR28, [18F]GE180, and [18F]BS224 was performed and the radioactivity of the samples was normalized by the protein concentration of each sample. Animal PET imaging studies were accomplished in a lipopolysaccharide (LPS) induced inflammatory and middle cerebral artery occlusion (MCAO) rat models.
Results: BS224 showed a subnanomolar binding affinity (Ki=0.51 nM) to TSPO. Optimal radiochemical yield (RCY) of [18F]BS224 was 39±6.8 % (n=10, decay-corrected) with high radiochemical purity (>99 %) and molar activity (130±19.1 GBq/µmol). Log D and in vitro human serum stability were 2.78 and >99% after 2 h incubation, respectively. Cellular uptake ratios (TSPO Wild/TSPO Mutant) of [3H]PK11195, [18F]PBR28, [18F]GE180, and [18F]BS224 were 1.07, 1.36, 1.48 and 1.07, respectively. In animal PET images, [18F]BS224 provided a clearly visible image of the inflammatory lesion with a high signal-to-noise ratio in both animal models without skull uptake.
Conclusions: Aromatic 18F-fluorination of BS224 was successfully performed by nucleophilic substitution of the corresponding the pinacol boronate ester precursor. [18F]BS224 was shown similar cellular uptake ratio value to [3H]PK11195 which already known as a TSPO ligand with less sensitivity to TSPO polymorphism. Based on these results, [18F]BS224 is a promising TSPO-binding radioligand impervious to TSPO polymorphism. References: [1] G. Kreutzberg, et al. (1997) Journal of neurocytology, 26, 77-82.[2] Owen DR, et al. (2012) J Cereb Blood Flow Metab. 32, 1-5.