Synthesis of [ 18 F] 4‐amino‐ N ‐(3‐chloro‐4‐fluorophenyl)‐ N′ ‐hydroxy‐1,2,5‐oxadiazole‐3‐carboximidamide (IDO5L): a novel potential PET probe for imaging of IDO1 expression

To synthesize 18 F‐labeled positron emission tomography (PET) ligands, reliable labeling techniques inserting 18 F into a target molecule are necessary. The 18 F‐fluorobenzene moiety has been widely utilized in the synthesis of 18 F‐labeled compounds. The present study utilized [ 18 F]‐labeled aniline as intermediate in [ 18 F]‐radiolabeling chemistry for the facile radiosynthesis of 4‐amino‐ N ‐(3‐chloro‐4‐fluorophenyl)‐ N′ ‐hydroxy‐1,2,5‐oxadiazole‐3‐carboximidamide ([ 18 F]IDO5L) as indoleamine 2,3‐dioxygenase 1 (IDO1) targeted tracer. IDO5L is a highly potent inhibitor of IDO1 with low nanomolar IC 50 . [ 18 F]IDO5L was synthesized via coupling [ 18 F]3‐chloro‐4‐fluoroaniline with carboximidamidoyl chloride as a potential PET probe for imaging IDO1 expression. Under the optimized labeling conditions, chemically and radiochemically pure (>98%) [ 18 F]IDO5L was obtained with specific radioactivity ranging from 11 to 15 GBq/µmol at the end of synthesis within ~90 min, and the decay‐corrected radiochemical yield was 18.2 ± 2.1% ( n  = 4).