title: Dependence of 5-fluorouracil-mediated radiosensitization on DNA-directed effects.
abstract: OBJECTIVE
  Although 5-fluorouracil (FUra) has been demonstrated to be a radiation sensitizer both in the laboratory and the clinic, it is not known whether radiosensitization results primarily from FUra's DNA or RNA-directed effects.
  
  
  METHODS
  We studied the radiosensitizing effects of FUra +/- thymidine (dThd)) on HT29 human colon cancer cells, which are relatively sensitive to the DNA-directed action of FUra, in comparison to SW620 and HuTu80 human colon cancer cells, which are relatively resistant to FUra's DNA-directed effects. We hypothesized that if FUra were acting chiefly through DNA dependent mechanisms, HT29 cells would (a) show greater radiosensitization than SW620 and HuTu80 cells under the same conditions of exposure; and (b) demonstrate selective reversal of radiation sensitivity (compared to cytotoxicity) in the presence of FUra + dThd, compared to FUra alone.
  
  
  RESULTS
  We found that the enhancement ratio produced by a 24 h exposure to 10 microM FUra was significantly greater in HT29 cells compared to SW620 and HuTu80 cells (enhancement ratios of 2.1 +/- 0.1; 1.1 +/- 0.1, and 1.3 +/- 0.1, respectively). Furthermore, in HT29 cells, dThd blocked FUra-mediated radiosensitization to a greater extent than FUra-mediated cytotoxicity. Thus, our hypotheses were confirmed.
  
  
  CONCLUSIONS
  These findings support the concept that the manipulation of FUra's DNA-dependent actions, for example, through modulators of thymidylate synthase (TS) activity, may increase radiosensitization in clinical trials in the treatment of gastrointestinal cancers. However, since resistance to the DNA-directed effects of fluoropyrimidines can result from mechanisms unrelated to TS inhibition, additional strategies will be required to potentiate fluoropyrimidine-mediated radiosensitization.
authors.affiliation: Department of Radiation Oncology, University of Michigan Medical Center, Ann Arbor 48109.
authors.forename: T S
authors.initials: TS
authors.lastname: Lawrence
authors.forename: M A
authors.initials: MA
authors.lastname: Davis
authors.forename: J
authors.initials: J
authors.lastname: Maybaum
mesh_terms: D002453:Cell Cycle / Q000187:drug effects; D003110:Colonic Neoplasms / Q000473:pathology; D004247:DNA / Q000187:drug effects*; D005472:Fluorouracil / Q000494:pharmacology*; D006801:Humans; D011838:Radiation-Sensitizing Agents / Q000494:pharmacology*; D013936:Thymidine / Q000494:pharmacology; D013940:Thymidylate Synthase / Q000037:antagonists & inhibitors; D014407:Tumor Cells, Cultured
chemical_list: D011838:Radiation-Sensitizing Agents; D004247:DNA; D013940:Thymidylate Synthase; D005472:Fluorouracil; D013936:Thymidine