Synthesis of 6-[18 F]fluoro-PBR28, a novel radiotracer for imaging the TSPO 18 kDa with PET

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 fluo- rine-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-[18F]fluoro-PBR28 (>95% radiochemically pure). Non- decay-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 neuroinflamma- tion (unilaterally, AMPA-induced, striatum-lesioned rats) and ex vivo autoradiography on the same model demonstrated the potential of 6-[18F]fluoro-PBR28 to image the TSPO 18 kDa using PET.