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

BACKGROUND

(S)-2-amino-3-[3-(2-F-fluoroethoxy)-4-iodophenyl]-2-methylpropanoic acid (F-FIMP) as a promising PET probe for imaging the tumor-specific L-type amino acid transporter (LAT) 1. Our previous study revealed that F-FIMP had a higher affinity for LAT1 than for LAT2 abundantly expressed even in normal cells. F-FIMP showed high accumulation in LAT1-positive tumor tissues and low accumulation in inflamed lesions in tumor-bearing mice. However, the affinity of F-FIMP for other amino acid transporters was not determined yet. Here, we aimed to determine whether F-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), alanine serine cysteine transporter 2 (ASCT2), and cystine/glutamate transporter (xCT).

PROCEDURES

Cells overexpressing LAT1, ATB, ASCT2, or xCT were established by the transfection of expression vectors for LAT1, ATB, 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-FIMP and C-labeled amino acids as substrates.

RESULTS

Intense 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-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-FIMP uptake values were significantly higher in the LAT1- and ATB-overexpressing cells than in the corresponding mock cells, but no such increase was seen in the ASCT2- or xCT-overexpressing cells. These F-FIMP uptake values were significantly decreased by the specific inhibitors for LAT1- and ATB.

CONCLUSIONS

We demonstrated that F-FIMP has affinity not only for LAT1, but also for ATB. Our results may be helpful for understanding the mechanisms of the whole-body distribution and tumor accumulation of F-FIMP.