Synthesis of a new fluorine-18-labeled bexarotene analogue for PET imaging of retinoid X receptor

The reference standard 2-fluoro-4-(1-(3,5,5,8,8-pentamethyl-5,6,7,8-tetrahydronaphthalen-2-yl)vinyl)benzoic acid was synthesized from 2,5-dimethyl-2,5-hexanediol and 2-fluoro-4-methylbenzoic acid in 10 steps with 3% overall chemical yield. The precursor 2-nitro-4-(1-(3,5,5,8,8-pentamethyl-5,6,7,8-tetrahydronaphthalen-2-yl)vinyl)benzoic acid was synthesized from 2,5-dimethyl-2,5-hexanediol and dimethyl-2-nitroterephthalate in seven steps with 2% overall chemical yield. The target tracer 2-[18F]fluoro-4-(1-(3,5,5,8,8-pentamethyl-5,6,7,8-tetrahydronaphthalen-2-yl)vinyl)benzoic acid was synthesized from its nitro-precursor by the nucleophilic substitution with K[18F]F/Kryptofix 2.2.2 and isolated by HPLC combined with solid-phase extraction (SPE) purification in 20-30% radiochemical yield with 37-370 GBq/μmol specific activity at end of bombardment (EOB).