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Synergistic Cell Killing by Ionizing Radiation and Topoisomerase I Inhibitor Topotecan (SK&F 104864)¹

Department of Biomolecular Discovery, SmithKline Beecham Pharmaceuticals, King of Prussia, Pennsylvania 19406

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 -radiation-induced cell killing. In colony formation experiments, subtoxic concentrations of topotecan (2 µM) 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 (<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 -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.

¹ Supported in part by Grant CA40884 from the National Cancer Institute.

² To whom requests for reprints should be addressed.

Received 2/21/91. Accepted 8/26/91.

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