title: The effect of single versus double-strand substitution on halogenated pyrimidine-induced radiosensitization and DNA strand breakage in human tumor cells.
abstract: To better understand the mechanism underlying halogenated pyrimidine-mediated cytotoxicity and radiosensitization in human tumor cells, a study was undertaken to determine the influence of unifilar (one DNA strand) versus bifilar (both DNA strands) substitution of thymidine by the halogenated bases 5-iodo-2'-deoxyuridine (IdUrd) and 5-bromo-2'-deoxyuridine (BrdUrd) in HT29 human colon cancer cells. Unifilar labeling was obtained by incubating cells with IdUrd or BrdUrd for one doubling time. Cells were incubated for at least three doublings to approximate bifilar substitution. Only IdUrd caused significant cytotoxicity, which correlated with incorporation into DNA. Both BrdUrd and IdUrd were potent radiosensitizers. Radiosensitization was linearly correlated with incorporation of both bases regardless of the number of strands in which thymidine was substituted. In contrast, the relationship between radiosensitization and DNA double-strand breakage was critically dependent in the case of IdUrd, but not for BrdUrd, on whether substitution was unifilar or bifilar. These findings suggest that incorporation is the best predictor of radiation sensitivity, and that the induction of DNA double-strand breaks alone does not account for radiosensitization mediated by halogenated pyrimidines in these human tumor cells.
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
authors.forename: P L
authors.initials: PL
authors.lastname: Stetson
authors.forename: W D
authors.initials: WD
authors.lastname: Ensminger
mesh_terms: D001973:Bromodeoxyuridine / Q000493:pharmacokinetics / Q000494:pharmacology*; D002460:Cell Line; D002470:Cell Survival / Q000187:drug effects / Q000528:radiation effects; D003110:Colonic Neoplasms / Q000473:pathology; D004247:DNA / Q000187:drug effects* / Q000378:metabolism / Q000528:radiation effects; D004249:DNA Damage; D004273:DNA, Neoplasm / Q000187:drug effects* / Q000378:metabolism / Q000528:radiation effects; D004277:DNA, Single-Stranded / Q000187:drug effects* / Q000378:metabolism / Q000528:radiation effects; D004305:Dose-Response Relationship, Drug; D004307:Dose-Response Relationship, Radiation; D006801:Humans; D007065:Idoxuridine / Q000493:pharmacokinetics / Q000494:pharmacology*; D066298:In Vitro Techniques; D011838:Radiation-Sensitizing Agents / Q000493:pharmacokinetics / Q000494:pharmacology*
chemical_list: D004273:DNA, Neoplasm; D004277:DNA, Single-Stranded; D011838:Radiation-Sensitizing Agents; D004247:DNA; D001973:Bromodeoxyuridine; D007065:Idoxuridine