The one-pot n.c.a. radiosynthesis of this novel potential PET imaging tracer, including HPLC purification and formulation, has been fully automated on a FASTlab™ synthesizer. Chiral separation of both isomers and their formulation were implemented on a second cassette. In vitro enzymatic and cellular uptake studies were then conducted with the d-, l- and dl-radiotracers.
The radiolabeling of the tosylate precursor was performed in DMF (in 5 min; RCY: 57% (d.c.), n = 3). After hydrolysis, HPLC purification and formulation, dl-[18F]5 was obtained with a global radiochemical yield of 18 ± 3% (not decay corrected, n = 7, in 80 min) and a specific activity of 600 ± 180 GBq/μmol (n = 5). The subsequent separation of l- and d-enantiomers was performed by chiral HPLC and both were obtained after formulation with an RCY (d.c.) of 6.1% and 5.8%, respectively. In vitro enzymatic assays reveal that l-[18F]5 is a better substrate than d-[18F]5 for human IDO. In vitro cellular assays show an IDO-specific uptake of the racemate varying from 30% to 50% of that of l-[18F]5, and a negligible uptake of d-[18F]5.
In vitro studies show that l-[18F]5 is a good and specific substrate of hIDO, while presenting a very low efflux. These results confirm that l-[18F]5 could be a very useful PET radiotracer for IDO expressing cells in cancer imaging.