Automated Radiosynthesis of 1-(2-[ 18 F]Fluoroethyl)-L-Tryptophan ([ 18 F]FETrp) for PET Imaging of Cancer in Humans.

The radiotracer 1-(2-[ 18 F]fluoroethyl)-L-tryptophan (L-[ 18 F]FETrp or [ 18 F]FETrp) is a substrate of indoleamine 2,3-dioxygenase (IDO), the initial and key enzyme of the kynurenine pathway associated with tumoral immune resistance. In preclinical PET studies, [ 18 F]FETrp is highly accumulated in a wide range of primary and metastatic cancers, such as lung cancer, prostate cancer, and gliomas. However, the clinical translation of this radiotracer into the first-in-human trial has not been reported, partially due to its racemization during radiofluorination which renders the purification of the final product challenging. However, efficient purification is essential for human studies in order to assure radiochemical and enantiomeric purity. In this work, we report a fully automated radiosynthesis of [ 18 F]FETrp on a Synthra RNPlus research module, including a one-pot two steps radiosynthesis, dual independent chiral and reverse-phase semi-preparative HPLC purifications, and solid-phase extraction (SPE) assisted formulation. The presented approach has led to its Investigational New Drug (IND) application and approval that allows the testing of this tracer in humans.