Schedule-dependent interaction between paclitaxel and doxorubicin in human cancer cell lines in vitro

The schedule-dependent interaction of paclitaxel and doxorubicin was evaluated in four human cancer cell lines. The cells were exposed simultaneously or sequentially to the two agents for 24 h, and were then incubated in drug-free medium for 4 and 3 days, respectively. The cell growth inhibitions were determined by the MTT assay. The cytotoxic interactions at the IC80 level were evaluated by the isobologram method of Steel and Peckham. In non-small cell lung cancer A549, breast cancer MCF7 and colon cancer WiDr cells, antagonistic effects were observed for the paclitaxel and doxorubicin combination on simultaneous exposure to the two agents and on sequential exposure to doxorubicin followed by paclitaxel, while additive effects were observed for the combination on sequential exposure to paclitaxel followed by doxorubicin. In ovarian cancer PA1 cells, additive effects were observed for all schedules. These findings suggest that sequential administration of paclitaxel followed by doxorubicin may be the most suitable sequence, while the simultaneous administration of the two agents and the sequential administration of doxorubicin followed by paclitaxel may result in less tumour cell kill than anticipated. Further preclinical and clinical studies are required to elucidate the relationship between paclitaxel and doxorubicin with regard to both antitumour activity and toxicity.