title: Transport of 3-fluoro-L-α-methyl-tyrosine by tumor-upregulated L-type amino acid transporter 1: a cause of the tumor uptake in PET.
abstract: l-3-(18)F-α-methyl tyrosine ((18)F-FAMT) has been developed as a PET radiotracer for tumor imaging. Clinical studies have demonstrated the usefulness of (18)F-FAMT PET for the prediction of prognosis and the differentiation of malignant tumors and benign lesions. (18)F-FAMT exhibits higher cancer specificity in peripheral organs than other amino acid PET tracers and (18)F-FDG. The accumulation of (18)F-FAMT is strongly correlated with the expression of L-type amino acid transporter 1 (LAT1), an isoform of system L highly upregulated in cancers. In this study, we examined the interaction of 3-fluoro-l-α-methyl-tyrosine (FAMT) with amino acid transporters to assess the mechanisms of (18)F-FAMT uptake in PET.
  
  
  METHODS
  We applied in vitro assays using established mammalian cell lines stably expressing LAT1 or a non-cancer-type system L isoform LAT2. The inhibitory effect on l-(14)C-leucine uptake and the induction effect on efflux of preloaded l-(14)C-leucine were examined for FAMT and other amino acid tracers. FAMT transport was compared among cell lines with varied LAT1 expression level.
  
  
  RESULTS
  FAMT prominently inhibited LAT1-mediated l-(14)C-leucine uptake in a competitive manner but had less of an effect on LAT2. In the efflux experiments, FAMT induced the efflux of preloaded l-(14)C-leucine through LAT1, indicating that FAMT is transported by LAT1 and not by LAT2. Among amino acid-related compounds examined in this study, including those used for PET tracers, the compounds with an α-methyl group such as FAMT, 2-fluoro-l-α-methyl-tyrosine, 3-iodo-l-α-methyl-tyrosine, and l-α-methyl-tyrosine were well transported by LAT1 but not by LAT2. However, l-methionine, l-tyrosine, 3-fluoro-l-tyrosine, 2-fluoro-l-tyrosine, and O-(2-fluoroethyl)-l-tyrosine were transported by both LAT1 and LAT2, suggesting that the α-methyl moiety is responsible for the LAT1 selectivity of FAMT. FAMT transport rate and LAT1 protein level were well correlated, supporting the importance of LAT1 for the cellular uptake of FAMT.
  
  
  CONCLUSIONS
  Distinct from other amino acid PET tracers, because of its α-methyl moiety, FAMT is selective to LAT1 and not transported by LAT2. This property of FAMT is proposed to contribute to highly tumor-specific accumulation of (18)F-FAMT in PET.
authors: Division of Bio-system Pharmacology, Department of Pharmacology, Graduate School of Medicine, Osaka University, Suita, Osaka, Japan.
authors: Pattama
authors: P
authors: Wiriyasermkul
authors: Shushi
authors: S
authors: Nagamori
authors: Hideyuki
authors: H
authors: Tominaga
authors: Noboru
authors: N
authors: Oriuchi
authors: Kyoichi
authors: K
authors: Kaira
authors: Hidekazu
authors: H
authors: Nakao
authors: Takeru
authors: T
authors: Kitashoji
authors: Ryuichi
authors: R
authors: Ohgaki
authors: Hidekazu
authors: H
authors: Tanaka
authors: Hitoshi
authors: H
authors: Endou
authors: Keigo
authors: K
authors: Endo
authors: Hiroyuki
authors: H
authors: Sakurai
authors: Yoshikatsu
authors: Y
authors: Kanai
mesh_terms: D048868:Adaptor Proteins, Signal Transducing / Q000378:metabolism; D000818:Animals; D001692:Biological Transport; D002460:Cell Line; D005462:Fluorine Radioisotopes; D015972:Gene Expression Regulation, Neoplastic; D006801:Humans; D027283:Large Neutral Amino Acid-Transporter 1 / Q000378:metabolism*; D007930:Leucine / Q000378:metabolism; D051379:Mice; D009369:Neoplasms / Q000000981:diagnostic imaging / Q000378:metabolism / Q000473:pathology*; D049268:Positron-Emission Tomography; D014443:Tyrosine / Q000031:analogs & derivatives* / Q000737:chemistry / Q000378:metabolism; D015854:Up-Regulation
chemical_list: D048868:Adaptor Proteins, Signal Transducing; D005462:Fluorine Radioisotopes; C419196:LAT2 protein, human; D027283:Large Neutral Amino Acid-Transporter 1; C107830:3-fluoro-alpha-fluoromethyltyrosine; D014443:Tyrosine; D007930:Leucine