5-(2-¹⁸F-Fluoroethoxy)-l-Tryptophan as a Substrate of System L Transport for Tumor Imaging by PET

Abstract

Large neutral L-amino acids are substrates of system L amino acid transporters. The level of one of these, LAT1, is increased in many tumors. Aromatic L-amino acids may also be substrates of aromatic L-amino acid decarboxylase (AADC), the level of which is enhanced in endocrine tumors. Increased amino acid uptake and subsequent decarboxylation result in the intracellular accumulation of the amino acid and its decarboxylation product. 18 F-and 11 C-labeled neutral aromatic amino acids, such as L-3,4-dihydroxy-6-18 F-fluorophenylalanine ( 18 F-FDOPA) and 5-hydroxy-L-[b-11 C]tryptophan, are thus successfully used in PET to image endocrine tumors. However, 5-hydroxy-L-[b-11 C]tryptophan has a relatively short physical half-life (20 min). In this work, we evaluated the in vitro and in vivo characteristics of the 18 F-labeled tryptophan analog 5-(2-18 F-fluoroethoxy)-L-tryptophan ( 18 F-L-FEHTP) as a PET probe for tumor imaging. Methods: 18 F-L-FEHTP was synthesized by no-carrier-added 18 F fluorination of 5-hydroxy-L-tryptophan. In vitro cell uptake and efflux of 18 F-L-FEHTP and 18 F-FDOPA were studied with NCI-H69 endocrine small cell lung cancer cells, PC-3 pseudoendocrine prostate cancer cells, and MDA-MB-231 exocrine breast cancer cells. Small-animal PET was performed with the respective xenograft-bearing mice. Tissues were analyzed for potential metabolites. Results: 18 F-L-FEHTP specific activity and radiochemical purity were 50-150 GBq/mmol and greater than 95%, respectively. In vitro cell uptake of 18 F-L-FEHTP was between 48% and 113% of added radioactivity per milligram of protein within 60 min at 37°C and was blocked by greater than 95% in all tested cell lines by the LAT1/2 inhibitor 2-amino-2-norboranecarboxylic acid. 18 F-FDOPA uptake ranged from 26% to 53%/mg. PET studies revealed similar xenograft-to-reference tissue ratios for 18 F-L-FEHTP and 18 F-FDOPA at 30-45 min after injection. In contrast to the 18 F-FDOPA PET results, pretreatment with the AADC inhibitor S-carbidopa did not affect the 18 F-L-FEHTP PET results. No decarboxylation products of 18 F-L-FEHTP were detected in the xenograft homogenates. Conclusion: 18 F-L-FEHTP accumulates in endocrine and nonendocrine tumor models via LAT1 transport but is not decarboxylated by AADC. 18 F-L-FEHTP may thus serve as a PET probe for tumor imaging and quantification of tumor LAT1 activity. These findings are of interest in view of the ongoing evaluation of LAT1 substrates and inhibitors for cancer therapy.

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

“…SUV in PC3 xenografts was 0.6 without S-carbidopa pre-administration. 19 The ratios close to unity would be in agreement with equilibration across the blood-brain barrier by LAT1 43,44 and are a promising feature towards the development of the tracers for the imaging of brain cancer.…”

Serum Concentration of Free Amino Acids in Dogs Suffering From Perianal Tumours

“…Increased uptake of aromatic amino acids and their subsequent decarboxylation results in intracellular accumulation of the amino acids and their decarboxylation products in neoplastic cells, which leads to significantly changed levels of aromatic amino acids in serum. Considering the variable concentration of aromatic AAs observed in the study between dogs with non-malignant and malignant changes, it may be hypothesised that aromatic AA metabolism increases in accordance to tumour malignancy stage (16). In case of alkaline amino acids, lack of significant changes in serum concentrations of dogs with non-malignant or malignant tumours may limit their diagnostic value related to perianal tumour type differentiation and further tumour growth prognosis.…”

Assessment of Tryptophan Uptake and Kinetics Using 1-(2-18F-Fluoroethyl)-l-Tryptophan and α-11C-Methyl-l-Tryptophan PET Imaging in Mice Implanted with Patient-Derived Brain Tumor Xenografts

“…Several novel 18 F-labeled tryptophan derivatives have been developed and tested as potential PET tracers for tumor imaging (27)(28)(29)(30)(31)(32). Although most compounds showed tumor uptake, they were not designed to target IDO1, IDO12, or TDO2 activity.…”