No carrier added synthesis of O-(2′-[18F]fluoroethyl)-l-tyrosine via a novel type of chiral enantiomerically pure precursor, NiII complex of a (S)-tyrosine Schiff base Original Research Article

O-(2′-[18F]fluoroethyl)-l-tyrosine ([18F]FET) has gained much attention as a promising amino acid radiotracer for tumor imaging with positron emission tomography (PET) due to favorable imaging characteristics and relatively long half-life of 18F (110 min) allowing remote-site application. Here we present a novel type of chiral enantiomerically pure labeling precursor for [18F]FET, based on NiII complex of a Schiff’s base of (S)-[N-2-(N′-benzylprolyl)amino]benzophenone (BPB) with alkylated (S)-tyrosine, Ni-(S)-BPB-(S)-Tyr-OCH2CH2X (X = OTs (3a), OMs (3b) and OTf (3c)). A series of compounds 3a–c was synthesized in three steps from commercially available reagents. Non-radioactive FET as a reference was prepared from 3a in a form of (S)-isomer and (R,S) racemic mixture. Radiosynthesis comprised two steps: (1) n.c.a. nucleophilic fluorination of 3a–c (4.5–5.0 mg) in the presence of either Kryptofix 2.2.2.or tetrabutylammonium carbonate (TBAC) in MeCN at 80 °C for 5 min, followed by (2) removal of protective groups by treating with 0.5 M HCl (120 °C, 5 min). The major advantages of this procedure are retention of enantiomeric purity during the 18F-introduction step and easy simultaneous deprotection of amino and carboxy moieties in 3a–c. Radiochemically pure [18F]FET was isolated by semi-preparative HPLC (C18 μ-Bondapak, Waters) eluent aq 0.01 M CH3COONH4, pH 4/C2H5OH 90/10 (v/v). Overall synthesis time operated by Anatech RB 86 laboratory robot was 55 min. In a series of compounds 3a–c, tosyl derivative 3a provided highest radiochemical yield (40–45%, corrected for radioactive decay). Enantiomeric purity was 94–95% and 96–97%, correspondingly, for Kryptofix and TBAC assisted fluorinations. The suggested procedure involved minimal number of synthesis steps and suits perfectly for automation in the modern synthesis modules for PET radiopharmaceuticals. Preliminary biodistribution study in experimental model of turpentine-induced aseptic abscess and Glioma35 rat’s tumor (homografts) in Wistar rats has demonstrated the enhanced uptake of radiotracer in the tumor area with minimal accumulation in the inflamed tissues.