The cytotoxic effect of the 9-azaellipticine derivative pazelliptine in combination with γ-ray irradiation was investigated using Chinese hamster V-79 cells in culture. γ-ray irradiation and drug treatment (1-h drug exposure) were applied at 1-h intervals for partial DNA damage recovery in growth medium. Isobologram analysis of the clonogenic potential gave evidence of supraadditive interaction in the radiation → drug sequence with 10% survival as an endpoint. No synergistic potentiation was observed at higher survival or as pazelliptine was applied first. Pazelliptine abolished the low-dose shoulder characteristic of asynchronous cell response to γ-rays. Although rejoining of radiation-induced DNA strand breaks was completed at the time of drug exposure, pazelliptine brought about a larger amount of DNA strand breaks in preirradiated than in nonirradiated cells.

The time and dose dependencies of DNA strand break formation and repair with radiation and/or pazelliptine were analyzed by neutral and alkaline filter elution. Pazelliptine in the micromolar range showed the same pattern of double-stranded cleavable complex formation as expected of a DNA topoisomerase II-targeting agent. At a low concentration of pazelliptine, however, protein-concealed breaks were mostly in the form of single-stranded adducts. Such single-stranded complexes have been reported to occur with some topoisomerase II-targeting drugs; their properties are also reminiscent of those induced by the topoisomerase I poison, camptothecin. It is proposed that topoisomerase poisoning interacts with the repair of radiation-induced lesions.

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This work was supported by a research fellowship from the Centre Hospitalier Universitaire de Montpellier to J. B., by financial aid from the Institut National de la Santé et de la Recherche Médicale, and by a grant (No. 89-21-5) from the Institut Curie.

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