title: Rationale of 5-(125)I-iodo-4'-thio-2'-deoxyuridine as a potential iodinated proliferation marker.
abstract: The aim of this investigation was to synthesize and test a novel metabolically stable iodinated nucleoside as a proliferation-imaging agent for SPECT.
  
  
  METHODS
  5-Iodo-4'-thio-2'-deoxyuridine (ITdU) and 5-iodo-1-(4-thio-beta-D-arabinofuranosyl)uracil (ITAU) were tested. The radiolabeling of ITdU and ITAU with (125)I was achieved by a destannylation reaction of the trimethylstannyl precursor of each nucleoside. The products were isolated in high yields and with >99% radiochemical purities.
  
  
  RESULTS
  (125)I-ITdU was effectively phosphorylated by cytosolic nucleoside kinases and specifically incorporated into a thymidine kinase-expressing L-M cell rather than a thymidine kinase-deficient mutant L-M (TK(-)) cell. In addition, an in vitro cell metabolism study of (125)I-ITdU clarified that (125)I-ITdU was effectively and specifically incorporated into a DNA fraction (>90% at 60 min). Therefore, (125)I-ITdU was proven to be an ideal DNA synthesis marker such as 5-(125)I-iodo-2'-deoxyuridine (IUdR). In contrast, (125)I-ITAU was neither remarkably phosphorylated by cytosolic nucleoside kinases nor notably incorporated into an L-M cell rather than an L-M (TK(-)) cell. (125)I-ITdU and (125)I-ITAU showed a higher resistance to phosphorolytic cleavage by recombinant thymidine phosphorylase than did (125)I-IUdR. Furthermore, biodistribution of (125)I-ITdU and (125)I-ITAU revealed better in vivo stability of radioiodination than that of (125)I-IUdR. (125)I-ITdU also displayed a significantly higher uptake in proliferating organs (thymus, spleen, small intestine, and bone) than in nonproliferating organs (brain, muscle, liver, and lung), as did (125)I-IUdR, at 18 h after injection. As indicated by the in vitro study, (125)I-ITAU did not show any significant uptake in proliferating organs.
  
  
  CONCLUSIONS
  Radioiodine-labeled ITdU is potentially useful as a proliferation-imaging agent, and further studies should clarify the usefulness of this compound as a tumor-imaging agent.
authors.affiliation: Research and Development Division, Research Center, Nihon Medi-Physics Company, Limited, Chiba, Japan.
authors.forename: Jun
authors.initials: J
authors.lastname: Toyohara
authors.forename: Akio
authors.initials: A
authors.lastname: Hayashi
authors.forename: Mikiko
authors.initials: M
authors.lastname: Sato
authors.forename: Hiromichi
authors.initials: H
authors.lastname: Tanaka
authors.forename: Kazuhiro
authors.initials: K
authors.lastname: Haraguchi
authors.forename: Yuichi
authors.initials: Y
authors.lastname: Yoshimura
authors.forename: Yoshiharu
authors.initials: Y
authors.lastname: Yonekura
authors.forename: Yasuhisa
authors.initials: Y
authors.lastname: Fujibayashi
mesh_terms: D000818:Animals; D001086:Arabinofuranosyluracil; D002455:Cell Division; D003857:Deoxyuridine / Q000031:analogs & derivatives; D006801:Humans; D007457:Iodine Radioisotopes; D007553:Isotope Labeling; D051379:Mice; D008810:Mice, Inbred C57BL; D009005:Monosaccharides; D014018:Tissue Distribution; D015899:Tomography, Emission-Computed, Single-Photon; D014498:Uracil / Q000031:analogs & derivatives; D014529:Uridine
chemical_list: C465668:5-iodo-1-(4-thio-arabinofuranosyl)uracil; C465667:5-iodo-4'-thio-2'-deoxyuridine; D007457:Iodine Radioisotopes; D009005:Monosaccharides; D001086:Arabinofuranosyluracil; D014498:Uracil; D003857:Deoxyuridine; D014529:Uridine