Synergistic interaction between topotecan and microtubule-interfering agents

Purpose: Topotecan is a topoisomerase I inhibitor with demonstrated antican cer activity in preclinical and clinical studies. The purpose of the presen t study was to evaluate drug-drug interactions in therapeutic regimens that would combine topotecan with microtubule-interfering agents, such as Taxol and vinblastine. Methods: The cytotoxic activities of various drug combina tions and schedules of administration were measured in a colon cancer cell line using the MTT assay. Western blot and flow cytometry were performed to determine the effects of Taxol and vinblastine on topoisomerase I and Bcl- x(L) protein levels and cell cycle distribution. Results: Brief incubation of colon cancer cells with low concentrations of either Taxol or vinblastin e increased the efficacy of a subsequent treatment with topotecan. Preincub ation of cells with vinblastine or Taxol reduced by 10- to 40-fold the conc entration of topotecan necessary to induce a 50% decrease in cell survival. The effects were maximal when the cells were treated for 5 h with microtub ule-interfering agents and then incubated for 19 h in drug-free medium befo re the addition of topotecan. Under these conditions, both Taxol and vinbla stine caused an increase in topoisomerase I protein levels, fraction of S p hase cells, and extent of Bcl-x(L) phosphorylation immediately prior to the addition of topotecan. All these factors may contribute to the increased e fficacy of topotecan observed with sequential therapy. Conclusion: Combinat ions of topotecan and microtubule-interfering agents result in synergistic anticancer activity when the drugs are administered sequentially. The promi sing preclinical data presented here encourage clinical testing of these dr ug combinations using a sequential schedule of administration.