Antagonistic effect of the combination gemcitabine/topotecan in ovarian cancer cells

The in vitro interaction between the new antimetabolite gemcitabine (GEM) a nd topotecan (TPT) was analyzed in A2780 ovarian cancer cells. The growth i nhibitory effect was assessed after 3 days of drug exposure. GEM and TPT ob tained in vitro IC50 values of 2.1 +/- 0.9 and 33.7 +/- 10.2 nM, respective ly. The interaction between GEM and TPT was evaluated by exposing cancer ce lls at increasing doses of GEM (0.1, 1, and 10 nM) and TPT (1, 10, 100, and 1000 nM). Analysis of data about the interaction between GEM nd TPT was pe rformed by applying the isobole method. An antagonistic effect was noticed when GEM was combined with TPT in the tested concentration range. DNA analy sis was also performed and showed an augmentation of cells blocked in the G (2)/M phase during TPT exposure, while an increase of blocked cells in the G(0/1) phase was observed after GEM treatment. This latter effect was predo minant when the two drugs were used in combination. We also investigated th e effect of sequential exposure to drugs, pretreating A2780 cells for 24 h with TPT and then for 48 h with GEM, and, conversely, pretreating A2780 cel ls with GEM for 24 h and thereafter with TPT for 48 h. Both these combined sequential treatments showed an antagonist effect of the drugs' combination . Long-term growth inhibition effect was established by clonogenic assay pe rformed after 10 days of culture after drug treatment. Also these data conf irmed the antagonistic effect between GEM and TPT in A2780 ovarian cancer c ells.