title: Preclinical characterization of 18F-D-FPHCys, a new amino acid-based PET tracer.
abstract: OBJECTIVE
  The imaging potential of a new (18)F-labelled methionine derivative, S-(3-[(18)F]fluoropropyl)-D-homocysteine ((18)F-D-FPHCys), and its selectivity for amino acid transporter subtypes were investigated in vitro and by imaging of human tumour xenografts.
  
  
  METHODS
  Expression of members of the system L (LAT isoforms 1-4 and 4F2hc) and ASCT (ASCT isoforms 1 and 2) amino acid transporter subclasses were assessed by quantitative real-time PCR in four human tumour models, including A431 squamous cell carcinoma, PC3 prostate cancer, and Colo 205 and HT-29 colorectal cancer lines. The first investigations for the characterization of (18)F-D-FPHCys were in vitro uptake studies by comparing it with [1-(14)C]-L-methionine ((14)C-MET) and in vivo by PET imaging. In addition, the specific involvement of LAT1 transporters in (18)F-D-FPHCys accumulation was tested by silencing LAT1 mRNA transcription with siRNAs. To determine the proliferative activity in tumour xenografts ex vivo, Ki-67 staining was used as a biomarker.
  
  
  RESULTS
  A431 cells showed the highest (18)F-D-FPHCys uptake in vitro and in vivo followed by Colo 205, PC3 and HT-29. A similar pattern of retention was observed with (14)C-MET. (18)F-D-FPHCys retention was strongly correlated with LAT1 expression both in vitro (R(2) = 0.85) and in vivo (R(2) = 0.99). Downregulation of LAT1 by siRNA inhibited (18)F-D-FPHCys uptake, demonstrating a clear dependence on this transporter for tumour uptake. Furthermore, (18)F-D-FPHCys accumulation mirrored cellular proliferation.
  
  
  CONCLUSIONS
  The favourable properties of (18)F-D-FPHCys make this tracer a promising imaging probe for detection of tumours as well as for the noninvasive evaluation and monitoring of tumour growth.
authors.affiliation: Molecular Imaging and Targeted Therapeutics Laboratory, Peter MacCallum Cancer Centre, East Melbourne, Victoria, Australia. Delphine.Denoyer@petermac.org
authors.forename: Delphine
authors.initials: D
authors.lastname: Denoyer
authors.forename: Laura
authors.initials: L
authors.lastname: Kirby
authors.forename: Kelly
authors.initials: K
authors.lastname: Waldeck
authors.forename: Peter
authors.initials: P
authors.lastname: Roselt
authors.forename: Oliver C
authors.initials: OC
authors.lastname: Neels
authors.forename: Thomas
authors.initials: T
authors.lastname: Bourdier
authors.forename: Rachael
authors.initials: R
authors.lastname: Shepherd
authors.forename: Andrew
authors.initials: A
authors.lastname: Katsifis
authors.forename: Rodney J
authors.initials: RJ
authors.lastname: Hicks
mesh_terms: D000818:Animals; D001692:Biological Transport; D045744:Cell Line, Tumor; D049109:Cell Proliferation; D002471:Cell Transformation, Neoplastic; D005786:Gene Expression Regulation; D006710:Homocysteine / Q000031:analogs & derivatives* / Q000378:metabolism; D006801:Humans; D027283:Large Neutral Amino Acid-Transporter 1 / Q000378:metabolism; D051379:Mice; D049268:Positron-Emission Tomography / Q000379:methods*; D011849:Radioactive Tracers
chemical_list: D027283:Large Neutral Amino Acid-Transporter 1; D011849:Radioactive Tracers; C558745:S-(3-fluoropropyl)homocysteine; D006710:Homocysteine