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.