title: Thymidine phosphorylase influences [(18)F]fluorothymidine uptake in cancer cells and patients with non-small cell lung cancer.
abstract: OBJECTIVE
  Thymidine phosphorylase (TP), a key enzyme in the pyrimidine nucleoside salvage pathway, catalyses the reversible phosphorylation of thymidine, thereby generating thymine and 2-deoxy-D-ribose-1-phosphate. By regulating the levels of endogenous thymidine, TP may influence [(18)F]fluorothymidine ([(18)F]FLT) uptake. We investigated the effect of TP activity on [(18)F]FLT uptake by tumours.
  
  
  METHODS
  Uptake of [(3)H]FLT and [(3)H]thymidine ([(3)H]Thd) and the activities of TP, thymidine kinase 1 (TK1), and equilibrative nucleoside transporter 1 (ENT1) were determined in exponentially growing A431, A549, HT29, HOP92, ACHN, and SKOV3 cells in the presence or absence of tipiracil hydrochloride, a TP inhibitor. Eighty-five non-small cell lung cancer tissues from a patient cohort that was previously studied with [(18)F]FLT positron emission tomography (PET) were retrieved and subjected to immunohistochemical analysis of TP expression. Factors that affected the maximum standardised uptake value (SUVmax) of [(18)F]FLT-PET were identified by multiple linear regression analysis.
  
  
  RESULTS
  A431 cells had the highest TP activity; A549 and HT29 cells had moderate TP activity; and ACHN, SKOV3, and HOP92 cells had little detectable TP activity. Cell lines with high TP activity took up more [(3)H]FLT than [(3)H]Thd, whereas cells with little TP activity took up more [(3)H]Thd than [(3)H]FLT. In cells with high TP activity, TP inhibition decreased [(3)H]FLT uptake and increased [(3)H]Thd uptake. However, TP inhibition had no effect on ACHN, SKOV3, and HOP92 cells. TP inhibition did not change TK1 or ENT1 activity, but did increase the intracellular level of thymidine. The SUVmax of [(18)F]FLT was affected by three independent factors: Ki-67 expression (P < 0.001), immunohistochemical TP score (P < 0.001), and tumour size (P = 0.015).
  
  
  CONCLUSIONS
  TP activity influences [(18)F]FLT uptake, and may explain preferential uptake of [(18)F]FLT over [(3)H]Thd. These results provide important insights into the biology of [(18)F]FLT as a proliferation marker.
authors.affiliation: College of Pharmacy, Gachon University, Incheon, Republic of Korea.
authors.forename: Seung Jin
authors.initials: SJ
authors.lastname: Lee
authors.forename: Jeong Seok
authors.initials: JS
authors.lastname: Yeo
authors.forename: Haeng Jung
authors.initials: HJ
authors.lastname: Lee
authors.forename: Eun Jung
authors.initials: EJ
authors.lastname: Lee
authors.forename: Seog Young
authors.initials: SY
authors.lastname: Kim
authors.forename: Se Jin
authors.initials: SJ
authors.lastname: Jang
authors.forename: Jong Jin
authors.initials: JJ
authors.lastname: Lee
authors.forename: Jin-Sook
authors.initials: JS
authors.lastname: Ryu
authors.forename: Dae Hyuk
authors.initials: DH
authors.lastname: Moon
mesh_terms: D000328:Adult; D000368:Aged; D001692:Biological Transport / Q000187:drug effects; D002289:Carcinoma, Non-Small-Cell Lung / Q000201:enzymology / Q000378:metabolism* / Q000473:pathology*; D045744:Cell Line, Tumor; D015224:Dideoxynucleosides / Q000378:metabolism*; D004305:Dose-Response Relationship, Drug; D004791:Enzyme Inhibitors / Q000494:pharmacology; D005260:Female; D015972:Gene Expression Regulation, Neoplastic / Q000187:drug effects; D006801:Humans; D042541:Intracellular Space / Q000187:drug effects / Q000378:metabolism; D008175:Lung Neoplasms / Q000201:enzymology / Q000378:metabolism* / Q000473:pathology*; D008297:Male; D008875:Middle Aged; D011759:Pyrrolidines / Q000378:metabolism; D013936:Thymidine / Q000378:metabolism; D013939:Thymidine Phosphorylase / Q000037:antagonists & inhibitors / Q000378:metabolism*; D013941:Thymine / Q000378:metabolism; D013997:Time Factors
chemical_list: D015224:Dideoxynucleosides; D004791:Enzyme Inhibitors; D011759:Pyrrolidines; D013939:Thymidine Phosphorylase; C000613754:tipiracil; C002854:alovudine; D013941:Thymine; D013936:Thymidine