title: Synergistic cell killing by ionizing radiation and topoisomerase I inhibitor topotecan (SK&F 104864).
abstract: Topotecan (SK&F 104864), a water-soluble analogue of the topoisomerase I inhibitor camptothecin, is currently in Phase II clinical trial for solid tumors. We have characterized topotecan in terms of its effect upon gamma-radiation-induced cell killing. In colony formation experiments, subtoxic concentrations of topotecan (2 microM) potentiated radiation-induced killing of exponentially growing Chinese hamster ovary or P388 murine leukemia cultured cells. Survival curve shoulders were reduced; the slopes of the exponential portions of the curves were decreased to a small extent. D37 and D10 (radiation dose resulting in 37 and 10% survival of colony-forming ability) values were reduced by approximately 60 and 50%, respectively, in the case of Chinese hamster ovary cells. In P388 cells, topotecan reduced D37 by 35 to 40% and D10 by 20 to 25%. Potentiation of radiation-induced cell killing by topotecan was absolutely dependent upon the presence of the topoisomerase I inhibitor during the first few (less than 30) min after irradiation. Association of topoisomerase I with this effect was confirmed in studies of Chinese hamster ovary cells previously made resistant to camptothecin (and cross-resistant to topotecan), resulting in decreased cellular content of topoisomerase I. These cells were found to be 2- to 3-fold hypersensitive to gamma-radiation-induced killing. P388 camptothecin-resistant cells were further sensitized to the lethal effects of ionizing radiation by nontoxic treatment with the topoisomerase II inhibitor novobiocin, consistent with increased dependence of topoisomerase I-deficient cells upon topoisomerase II.
authors.affiliation: Department of Biomolecular Discovery, SmithKline Beecham Pharmaceuticals, King of Prussia, Pennsylvania 19406.
authors.forename: M R
authors.initials: MR
authors.lastname: Mattern
authors.forename: G A
authors.initials: GA
authors.lastname: Hofmann
authors.forename: F L
authors.initials: FL
authors.lastname: McCabe
authors.forename: R K
authors.initials: RK
authors.lastname: Johnson
mesh_terms: D000818:Animals; D000970:Antineoplastic Agents / Q000494:pharmacology*; D016466:CHO Cells; D002166:Camptothecin / Q000031:analogs & derivatives* / Q000494:pharmacology; D002470:Cell Survival / Q000187:drug effects / Q000528:radiation effects*; D006224:Cricetinae; D004307:Dose-Response Relationship, Radiation; D004354:Drug Screening Assays, Antitumor; D005720:Gamma Rays; D007941:Leukemia P388; D051379:Mice; D059004:Topoisomerase I Inhibitors; D019772:Topotecan; D014407:Tumor Cells, Cultured
chemical_list: D000970:Antineoplastic Agents; D059004:Topoisomerase I Inhibitors; D019772:Topotecan; D002166:Camptothecin