Sequence dependence of paclitaxel (Taxol) combined with cisplatin or alkylators in human cancer cells.

: Clinical trials combining paclitaxel with other active chemotherapy agents are currently underway. In vitro preclinical studies may assist in the selection of appropriate drug combinations or sequences for clinical investigation. We have used clonogenic cell survival assays and DNA flow cytometry to examine the effect of paclitaxel combined with melphalan, thiotepa, or cisplatin on the survival and cell-cycle parameters of human lung A549 and breast MCF-7 adenocarcinoma cells. A549 and MCF-7 cells were incubated with paclitaxel for 24 h, followed by a 1 h exposure to cisplatin, melphalan, or thiotepa. Both cell types were also incubated with cisplatin or an alkylator for 1 h followed by a 24 h paclitaxel exposure. When the paclitaxel exposure preceded either melphalan, thiotepa, or cisplatin, the cytotoxicity was additive for both cell lines tested. When cisplatin or alkylator exposure was given prior to the paclitaxel, cytotoxicity was also additive in MCF-7 cells. However, cisplatin or alkylators were antagonistic to paclitaxel cytotoxicity when they preceded the paclitaxel exposure in A549 cells (e.g., 2% survival with 100 nM paclitaxel vs. 7% survival with cisplatin and paclitaxel). Cell-cycle analysis revealed that exposure to 100 nM paclitaxel for 24 h blocked a majority of the cells into the G2/M phase (> or = 80% for A549 cells, 60-65% for MCF-7 cells). However, exposure to alkylators before incubation in paclitaxel reduced the proportion of A549 but not MCF-7 cells in G2/M--e.g., exposure to 30 micrograms/ml cisplatin prior to paclitaxel exposure caused only 25% of A549 and 55% of MCF-7 cells to block in G2/M.(ABSTRACT TRUNCATED AT 250 WORDS)