Radiosynthesis of [18F]PBR111, a selective radioligand for imaging the translocator protein (18 kDa) with PET

PBR111 (2-(6-chloro-2-(4-(3-fluoropropoxy)phenyl)imidazo[1,2-a]pyridin-3-yl)-N,N-diethylacetamide) is a novel, reported, high-affinity and selective ligand for the translocator protein (18 kDa). PBR111 has been labelled with fluorine-18 (half-life: 109.8min) using our Zymate-XP robotic system. The process involves (A) a simple one-step tosyloxy-for-fluorine nucleophilic aliphatic substitution (performed at 165 degrees C for 5 min in DMSO using K[F-18]F-Kryptofix 222 and 6.8-7.6 mu mol of the corresponding tosylate as precursor for labelling) followed by (B) C-18 PrepSep cartridge pre-purification and (C) semi-preparative HPLC purification on a Waters Symmetry C-18. Up to 4.8 GBq (130 mCi) of [F-18]PBR111 could be obtained with specific radioactivities ranging from 74 to 148 GBq/mu mol (2-4 Ci/mu mol) in 75-80 min (HPLC purification and SepPak-based formulation included), starting from a 37.0 GBq (1.0 Ci) [F-18]fluoride batch. Overall non-decay-corrected isolated yields were 8-13% (13-21% decay-corrected).