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.
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.



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 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.



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.
Cyclotron Research Center, B30, Université de Liège, Sart-Tilman, B-4000, Liège, Belgium; Department of Chemistry, B6, Université de Liège, Sart-Tilman, B-4000, Liège, Belgium. Electronic address: Jean.Henrottin@ulg.ac.be.
Jean
J
Henrottin
Cyclotron Research Center, B30, Université de Liège, Sart-Tilman, B-4000, Liège, Belgium.
Christian
C
Lemaire
PET/Biomedical Cyclotron Unit and Department of Nuclear Medicine, Erasme Hospital, Université Libre de Bruxelles, B-1070, Brussels, Belgium; Center for Microscopy and Molecular Imaging, Rue Adrienne Bolland 8, B-6041, Gosselies, Belgium.
Dominique
D
Egrise
Cyclotron Research Center, B30, Université de Liège, Sart-Tilman, B-4000, Liège, Belgium.
Astrid
A
Zervosen
Ludwig Institute for Cancer Research, Brussels Branch and de Duve Institute, Université catholique de Louvain, B-1200, Brussels, Belgium.
Benoit
B
Van den Eynde
Cyclotron Research Center, B30, Université de Liège, Sart-Tilman, B-4000, Liège, Belgium.
Alain
A
Plenevaux
GE Healthcare, MDx PET Chemistry System, Rue Marie Curie 10/2, B-4431, Loncin (Liège), Belgium.
Xavier
X
Franci
PET/Biomedical Cyclotron Unit and Department of Nuclear Medicine, Erasme Hospital, Université Libre de Bruxelles, B-1070, Brussels, Belgium; Center for Microscopy and Molecular Imaging, Rue Adrienne Bolland 8, B-6041, Gosselies, Belgium.
Serge
S
Goldman
Cyclotron Research Center, B30, Université de Liège, Sart-Tilman, B-4000, Liège, Belgium.
André
A
Luxen
D000818:Animals; D001331:Automation; D001692:Biological Transport; D045744:Cell Line, Tumor; D006801:Humans; D057927:Hydrophobic and Hydrophilic Interactions; D050503:Indoleamine-Pyrrole 2,3,-Dioxygenase / Q000378:metabolism*; D051379:Mice; D049268:Positron-Emission Tomography; D011854:Radiochemistry / Q000379:methods*; D013237:Stereoisomerism; D014364:Tryptophan / Q000031:analogs & derivatives* / Q000138:chemical synthesis* / Q000737:chemistry / Q000378:metabolism*
D050503:Indoleamine-Pyrrole 2,3,-Dioxygenase; C000624535:N(1)-fluoroethyltryptophan; D014364:Tryptophan
1-(2-[(18)F]fluoroethyl)tryptophan; FASTlab; Fully automated radiosynthesis; Indoleamine 2,3-dioxygenase; PET imaging; Tryptophan