Synthesis, Radiolabeling, and Biological Evaluation of 5-Hydroxy-2-[(18)F]fluoroalkyl-tryptophan Analogues as Potential PET Radiotracers for Tumor Imaging

Aiming at developing mechanism-based amino acid (18)F-PET tracers for tumor imaging, we synthesized two (18)F-labeled analogues of 5-hydroxy-l-[β-(11)C]tryptophan ([(11)C]5HTP) whose excellent in vivo performance in neuroendocrine tumors is mainly attributed to its decarboxylation by aromatic amino acid decarboxylase (AADC), an enzyme overexpressed in these malignancies. Reference compounds and precursors were synthesized following multistep synthetic approaches. Radiosynthesis of tracers was accomplished in good radiochemical yields (15-39%), high specific activities (45-95 GBq/μmol), and excellent radiochemical purities. In vitro cell uptake was sodium-independent and was inhibited ≥95% by 2-amino-2-norbornanecarboxylic acid (BCH) and ∼30% by arginine. PET imaging in mice revealed distinctly high tumor/background ratios for both tracers, outperforming the well-established O-(2-[(18)F]fluoroethyl)tyrosine ([(18)F]FET) tracer in a head-to-head comparison. Biological evaluation revealed that the in vivo performance is most probably independent of any interaction with AADC. Nevertheless, the excellent tumor visualization qualifies the new tracers as interesting probes for tumor imaging worthy for further investigation.