title: Thymidine selectively enhances growth suppressive effects of camptothecin/irinotecan in MSI+ cells and tumors containing a mutation of MRE11.
abstract: OBJECTIVE
  DNA synthesis inhibitors and damaging agents are widely used in cancer therapy; however, sensitivity of tumors to such agents is highly variable. The response of tumor cells in culture to these agents is strongly influenced by the status of DNA damage response pathways. Here, we attempt to exploit the altered response of mismatch repair (MMR)-deficient colon cancer cells and tumors to camptothecin or irinotecan and thymidine by combining them to improve therapeutic response.
  
  
  METHODS
  A panel of colon cancer cell lines was assayed for response to camptothecin-thymidine combinations by measuring colony formation, cell cycle distribution, and senescence. Cell strains defective in p53, p21, or Mre11 were used in these assays to investigate the role of these cell cycle regulators. The in vivo antitumor response of xenografts to irinotecan and thymidine combinations was assessed in nude mice.
  
  
  RESULTS
  Camptothecin-thymidine combinations suppress colony formation of MMR-deficient tumor cells 10- to 3,000-fold relative to that obtained with camptothecin alone and significantly reduce the concentrations of the agents required to induce late S/G(2) arrest and senescence. Sensitivity is not a direct result of MMR, p53, or p21 status. However MMR-deficient cell lines containing an intronic frameshift mutation of MRE11 show greatest sensitivity to these agents. Increased sensitivity to this combination is also evident in vivo as thymidine enhances irinotecan-induced growth suppression of MMR-deficient tumors carrying the MRE11 mutation in mouse xenografts.
  
  
  CONCLUSIONS
  Irinotecan-thymidine combinations may be particularly effective when targeted to MSI+ tumors containing this readily detectable MRE11 mutation.
authors.affiliation: Institute for Cancer Studies, University of Sheffield, School of Medicine and Biomedical Sciences, Sheffield, United Kingdom.
authors.forename: Rene
authors.initials: R
authors.lastname: Rodriguez
authors.forename: Lasse Tengbjerg
authors.initials: LT
authors.lastname: Hansen
authors.forename: Geraldine
authors.initials: G
authors.lastname: Phear
authors.forename: Jennifer
authors.initials: J
authors.lastname: Scorah
authors.forename: Mogens
authors.initials: M
authors.lastname: Spang-Thomsen
authors.forename: Angela
authors.initials: A
authors.lastname: Cox
authors.forename: Thomas
authors.initials: T
authors.lastname: Helleday
authors.forename: Mark
authors.initials: M
authors.lastname: Meuth