Putative Transport Mechanism and Intracellular Fate of Trans-1-Amino-3-18F-Fluorocyclobutanecarboxylic Acid in Human Prostate Cancer

Trans-1-amino-3-18F-fluorocyclobutanecarboxylic acid (anti–18F-FACBC) is an amino acid PET tracer that has shown promise for visualizing prostate cancer. Therefore, we aimed to clarify the anti–18F-FACBC transport mechanism in prostate cancer cells. We also studied the fate of anti–18F-FACBC after it is transported into cells. Methods: For convenience, because of their longer half-lives, 14C compounds were used instead of 18F-labeled tracers. Trans-1-amino-3-fluoro-1-14C-cyclobutanecarboxylic acid (14C-FACBC) uptake was examined in human prostate cancer DU145 cells with the following substrates of amino acid transporters: α-(methylamino) isobutyric acid (a system A–specific substrate) and 2-amino-2-norbornanecarboxylic acid (a system L–specific substrate). The messenger RNA expression of amino acid transporters in human prostate cancer specimens was analyzed by complementary DNA microarray and quantitative real-time reverse transcription polymerase chain reaction (qRT-PCR). Gene expression in DU145 cells was analyzed by qRT-PCR. We also examined the knockdown effect of the amino acid transporters system ASC transporter 2 (ASCT2) and sodium-coupled neutral amino acid transporter 2 (SNAT2) on 14C-FACBC uptake. In addition, the possibility of 14C-FACBC incorporation into proteins was examined. Results: 14C-FACBC uptake by DU145 cells was markedly decreased to approximately 20% in the absence of Na+, compared with that in its presence, indicating that Na+-dependent transporters are mainly responsible for the uptake of this tracer. Moreover, 2-amino-2-norbornanecarboxylic acid inhibited the transport of 14C-FACBC to the basal level in Na+-free buffer. In contrast, α-(methylamino) isobutyric acid did not inhibit 14C-FACBC accumulation in DU145 cells. Human prostate tumor specimens and DU145 cells had similar messenger RNA expression patterns of amino acid transporter genes. Although SNAT2 and ASCT2 are 2 major amino acid transporters expressed in prostate tumor tissues and DU145 cells, ASCT2 knockdown using small interfering RNA was more effective in lowering 14C-FACBC transport than SNAT2. Almost all intracellular 14C-FACBC was recovered from the nonprotein fraction. Conclusion: ASCT2, which is a Na+-dependent amino acid transporter, and to a lesser extent Na+-independent transporters play a role in the uptake of 14C-FACBC by DU145 cells. Among the Na+-independent transporters, system L transporters are also involved in the transport of 14C-FACBC. Moreover, 14C-FACBC is not incorporated into proteins in cells. These findings suggest a possible mechanism of anti–18F-FACBC PET for prostate cancer.