In vitro evaluation of (S)-2-amino-3-[3-(2-18F-fluoroethoxy)-4-iodophenyl]-2-methylpropanoic acid (18F-FIMP) as a positron emission tomography probe for imaging amino acid transporters

Background(S)-2-amino-3-[3-(2-F-18-fluoroethoxy)-4-iodophenyl]-2-methylpropanoic acid (F-18-FIMP) as a promising PET probe for imaging the tumor-specific L-type amino acid transporter (LAT) 1. Our previous study revealed that F-18-FIMP had a higher affinity for LAT1 than for LAT2 abundantly expressed even in normal cells. F-18-FIMP showed high accumulation in LAT1-positive tumor tissues and low accumulation in inflamed lesions in tumor-bearing mice. However, the affinity of F-18-FIMP for other amino acid transporters was not determined yet. Here, we aimed to determine whether F-18-FIMP has affinity for other tumor-related amino acid transporters, such as sodium- and chloride-dependent neutral and basic amino acid transporter B(0 +) (ATB(0,+)), alanine serine cysteine transporter 2 (ASCT2), and cystine/glutamate transporter (xCT). ProceduresCells overexpressing LAT1, ATB(0,+), ASCT2, or xCT were established by the transfection of expression vectors for LAT1, ATB(0,+), ASCT2, or xCT. Protein expression levels were determined by western blot and immunofluorescent analyses. Transport function was evaluated by a cell-based uptake assay using F-18-FIMP and C-14-labeled amino acids as substrates.ResultsIntense signals were observed only for expression vector-transfected cells on western blot and immunofluorescent analyses. These signals were strongly reduced by gene-specific small interfering ribonucleic acid treatment. The uptake values for each C-14-labeled substrate were significantly higher in the transfected cells than in the mock-transfected cells and were significantly inhibited by the corresponding specific inhibitors. The F-18-FIMP uptake values were significantly higher in the LAT1- and ATB(0,+)-overexpressing cells than in the corresponding mock cells, but no such increase was seen in the ASCT2- or xCT-overexpressing cells. These F-18-FIMP uptake values were significantly decreased by the specific inhibitors for LAT1- and ATB(0,+).ConclusionsWe demonstrated that F-18-FIMP has affinity not only for LAT1, but also for ATB(0,+). Our results may be helpful for understanding the mechanisms of the whole-body distribution and tumor accumulation of F-18-FIMP.

Automated summary

F-18-FIMP, a promising PET probe, has affinity not only for LAT1 but also for ATB(0,+). This study provides insights into the whole-body distribution and tumor accumulation mechanisms of F-18-FIMP.