title: Potentiation of cell killing by low-dose-rate radiation by camptothecin is related to an increase in the level of DNA double-strand breaks.
abstract: We investigated the ability of camptothecin to potentiate cell killing by low-dose-rate irradiation and whether this potentiation was associated with an increase in the level of residual DNA double-strand breaks (DSBs). Human melanoma (Sk-Mel-3) cells, grown to the confluent phase, were treated with low-dose-rate radiation (0.88 cGy/min) alone, camptothecin alone, or concurrent camptothecin and low-dose-rate radiation. Cell survival was determined using a clonogenic assay. The interactions between camptothecin and low-dose-rate radiation were analyzed further using isobolograms. DNA DSBs were determined using the neutral comet assay. We found that 10 and 25 microM camptothecin, but not 1 microM, camptothecin potentiated cell killing significantly relative to that seen with low-dose-rate radiation alone. Unexpectedly, the potentiation of the effects of low-dose-rate radiation by camptothecin was accompanied by large increases in the alpha parameter of the linear-quadratic fit rather than in the beta parameter. This suggests a modification of intrinsic radiosensitivity rather than of repair of sublethal damage. From isobologram analysis, low-dose-rate radiation interacted either additively or supra-additively with 25 or 10 microM camptothecin. Conversely, the interaction of low-dose-rate radiation with 1 microM camptothecin was subadditive. Finally, there were strong correlations (correlation coefficients >0.9) between surviving fraction and either comet tail length or comet tail moment after concurrent treatment with 25 microM camptothecin and low-dose-rate radiation. This suggests that the level of residual DNA DSBs was a good indicator of cell killing after treatment with low-dose-rate radiation plus 25 microM camptothecin.
authors.affiliation: Ottawa Regional Cancer Centre, 503 Smyth Road, Ottawa, Ontario, K1H 1C4, Canada.
authors.forename: Daron G
authors.initials: DG
authors.lastname: Owen
authors.forename: James P
authors.initials: JP
authors.lastname: McNamee
authors.forename: G Peter
authors.initials: GP
authors.lastname: Raaphorst
authors.forename: Cheng E
authors.initials: CE
authors.lastname: Ng
mesh_terms: D002166:Camptothecin / Q000494:pharmacology*; D002470:Cell Survival / Q000528:radiation effects; D020552:Comet Assay; D004247:DNA / Q000528:radiation effects*; D004249:DNA Damage / Q000187:drug effects / Q000528:radiation effects*; D004273:DNA, Neoplasm / Q000187:drug effects / Q000528:radiation effects; D004307:Dose-Response Relationship, Radiation; D005720:Gamma Rays / Q000009:adverse effects; D006801:Humans; D008545:Melanoma / Q000473:pathology; D011836:Radiation Tolerance; D011838:Radiation-Sensitizing Agents / Q000494:pharmacology*; D014407:Tumor Cells, Cultured / Q000187:drug effects / Q000528:radiation effects; D014410:Tumor Stem Cell Assay
chemical_list: D004273:DNA, Neoplasm; D011838:Radiation-Sensitizing Agents; D004247:DNA; D002166:Camptothecin