Docetaxel is a microtubule inhibitor that has actions in the S and G(2)-M phase of the cell cycle. The
pyrimidine trifluorothymidine (TFT) induces DNA damage and an arrest in the G(2)-M phase. TFT, as part of
TAS-102, has been clinically evaluated as an oral chemotherapeutic agent in colon and
gastric cancer. The aim of the present study was to determine the optimal administration sequence of TFT and
docetaxel and to investigate the underlying mechanism of cytotoxicity. Drug interactions were examined by
sulforhodamine B assays and subsequent combination index analyses, and for long-term effects the clonogenic assay was used. A preincubation with
docetaxel was synergistic in
sulforhodamine B (combination index 0.6-0.8) and clonogenic assays, and was accompanied by a time-dependent cell death induction (17-36%), the occurrence of polynucleation (22%), and mitotic spindle inhibition as determined by flow cytometry and immunostaining. Interestingly, administration of TFT followed by the combination displayed strong antagonistic activity, and was accompanied by less polynucleation and cell death induction than the synergistic combinations. Western blotting showed that the G(2)-M-phase arrest (25-50%) was accompanied by phosphorylation of Chk2 and dephosphorylation of cdc25c in the synergistic combinations. Together, this indicates that synergistic activity requires
docetaxel to initiate mitotic failure prior to the activation of TFT damage signaling, whereas antagonism is a result of TFT cell cycle-arrested cells being less susceptible to
docetaxel.
Caspase 3 activation was low after
docetaxel, suggestive of
caspase-independent mechanisms of cell death. Taken together, our models indicate that combination treatment with
docetaxel and TFT displays strong synergy when
docetaxel is given first, thus providing clues for possible clinical studies.