Synthesis and Biological Evaluation of 18F-Labeled Fluoroethoxy Tryptophan Analogues as Potential PET Tumor Imaging Agents

As a continuation of our research efforts toward the development of tryptophan-based radiotracers for tumor imaging with positron emission tomography (PET), three new fluoroethoxy tryptophan analogues were synthesized and evaluated in vivo. These new tracers (namely, 4-(2-[18F]fluoroethoxy)-dl-tryptophan ([18F]4-FEHTP), 6-(2-[18F]fluoroethoxy)-dl-tryptophan ([18F]6-FEHTP), and 7-(2-[18F]fluoroethoxy)-dl-tryptophan ([18F]7-FEHTP) carry the fluoroethoxy side chain either at positions 4-, 6-, or 7- of the indole core. Reference compounds and precursors were synthesized by multistep approaches. Radiosynthesis was accomplished by no-carrier-added nucleophilic 18F-fluorination following either an indirect approach (O-alkylation of the corresponding hydroxytryptophan with [18F]fluoroethyltosylate) or a direct approach (nucleophilic [18F] fluorination using a protected mesyl precursor). Radiochemical yields (decay corrected) for both methods were in the range of 10–18%. Small animal PET imaging with xenograft-bearing mice revealed the highest tumor/background ratio for [18F]6-FEHTP which, in a direct comparison, outperformed the other two tryptophan tracers and also the well-established tyrosine analogue O-(2-[18F]fluoroethyl)-l-tyrosine ([18F]l-FET). Investigation of the transport mechanism of [18F]6-FEHTP in small cell lung cancer cells (NCI-H69) revealed that it is most probably taken up exclusively via the large neutral amino acid transporter(s) (LAT).