Fully automated radiosynthesis of N(1)-[(18)F]fluoroethyl-tryptophan and study of its biological activity as a new potential substrate for indoleamine 2,3-dioxygenase PET imaging.

Introduction: Indoleamine 2,3-dioxygenase (IDO) catalyzes the initial step in the catabolism of l-tryptophan along the kynurenine pathway and exerts immunosuppressive properties in inflammatory and tumor tissues by blocking locally T-lymphocyte proliferation. Recently, 1-(2-[(19)F]fluoroethyl)-dl-tryptophan (1-[(19)F]FE-dl-Trp) was reported as a good and specific substrate of this enzyme. Herein, the radiosynthesis of its radioactive isotopomer (1-[(18)F]FE-dl-Trp, dl-[(18)F]5) is presented along with in vitro enzymatic and cellular uptake studies.

Methods: 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.

Results: The radiolabeling of the tosylate precursor was performed in DMF (in 5min; RCY: 57% (d.c.), n=3). After hydrolysis, HPLC purification and formulation, dl-[(18)F]5 was obtained with a global radiochemical yield of 18±3% (not decay corrected, n=7, in 80min) and a specific activity of 600±180GBq/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-[(18)F]5 is a better substrate than d-[(18)F]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-[(18)F]5, and a negligible uptake of d-[(18)F]5.

Conclusion: In vitro studies show that l-[(18)F]5 is a good and specific substrate of hIDO, while presenting a very low efflux. These results confirm that l-[(18)F]5 could be a very useful PET radiotracer for IDO expressing cells in cancer imaging.