To search for possible synergy between topoisomerase (topo) II-directed chemotherapeutic agents and topo I-directed agents, HL-60 human progranulocytic leukemia cells were incubated with etoposide in the absence or presence of camptothecin (CPT). Treatment of HL-60 cells for 1 h with 15–20 µm etoposide resulted in the death of 99–99.9% of the cells as assessed by colony formation in soft agar. Unexpectedly, simultaneous incubation with 1 µm CPT increased the survival of etoposide-treated cells as much as 30-fold. Inhibition of etoposide cytotoxicity was observed at CPT concentrations as low as 0.01 µm and was one-half maximal at 0.1 µm. CPT also antagonized the cytotoxicity of 4′-(9-acridinylamino)methanesulfon-m-anisidide and daunorubicin, two structurally unrelated topo II-directed agents. Topotecan, a CPT analogue currently undergoing Phase I clinical trials, had a similar effect. Studies using an alkaline unwinding assay (to measure DNA strand breaks) and Western blotting (to assess formation of covalent adducts involving topo II) revealed that CPT did not alter the ability of etoposide to stabilize topo II-DNA adducts. CPT is a potent inhibitor of both DNA and RNA synthesis. To further assess the mechanism by which CPT diminished the cytotoxicity of topo II-directed agents, inhibitors of DNA synthesis or RNA synthesis were substituted for CPT. Aphidicolin, an inhibitor of replicative DNA polymerases, enhanced the survival of etoposide-treated HL-60 cells <3-fold. In contrast, inhibitors of RNA synthesis (cordycepin or 5,6-dichloro-1-β-d-ribofuranosylbenzimidazole) enhanced the survival of etoposide-treated HL-60 cells as much as 20-fold. The potential biological and therapeutic implications of these results are discussed.

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Supported in part by Grant CA50435 from the NIH and a grant from the American Cancer Society.

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