Automated synthesis of [18F]NKO-035 as a novel L-type amino acid transporter 1 (LAT1) tracer for human use

1130 Objectives: L-type amino acid transporter 1 (LAT1) is expressed specifically in malignant tumor cells and it is a good target of tumor diagnosis and anti-tumor drug development. Currently, PET tracer targeting LAT1, such as 4-borono-2-[18F]fluoro -L-phenylalanine ([18F]FBPA) and 3-[18F]fluoro-L-alpha-methyltyrosine ([18F]FAMT), have been used for clinical examination. However, these tracers can only obtain a low radioactivity because of using [18F]F2 gas for radiosynthesis. In this study, we focused on NKO-035, a derivative of aromatic amino acid that has high affinity against LAT1, which was developed at Osaka University and aimed to establish an automated synthesis of [18F]NKO-035 using [18F]F- with high radioactivity and the sufficient level of quality for human use. Methods: The automated synthesis of [18F]NKO-035 was performed with a MPS200 system (Sumitomo). [18F]F- from the cyclotron was trapped on an anion-exchange (QMA) cartridge and then it was eluted by K222/K2CO3 solution to reactor. After the reactor was completely dried up with acetonitrile for azeotropic, 1 mL of precursor solution (5 mg/mL in acetonitrile) was added and fluorination was performed at 90 0C for 7 minutes. The de-protection step was used 2 mol/L HCl and carried out at 80 0C for 10 minutes. [18F]NKO-035 was collected by semi-preparative HPLC using 20 % ethanol solution as an eluent. After separation, [18F]NKO-035 solution was diluted with water for injection including ascorbic acid. The final solution of [18F]NKO-035 adjusted to a 6 % ethanol concentration was recovered to production vial through the sterility filter. Results: The radioactivity of [18F]NKO-035 with irradiation condition of 25 µA for 30 minutes was 7,050 ± 547 MBq and the synthesis time was 68 ± 3 minutes. The radiochemical purity was > 99 % at the end of synthesis (EOS) and it was > 98 % even after 4 hours of EOS. Molar activity was > 1,000 GBq/µmol (0.1 µg/mL as a NKO-035 concentration) and chemical purity was < 0.2 µg/mL. Acetonitrile as a residual solvents could not be detected and ethanol concentration was 6.5 ± 0.5 %. Furthermore, the abundance ratio of the 18F optical isomer (L-form : D-form) was 50:50 or 51:49, which was checked by HPLC using a chiral column. In addition, it was confirmed that all other quality tests required for the injection passed. Conclusions: We could establish automated synthesis of [18F]NKO-035 with high radioactivity and the sufficient level of quality for human use by 18F- synthesis method. And then, normal volunteers as a first in human study with [18F]NKO-035 synthesized by this procedure has already been completed, and its safety has been confirmed.