Objective: To determine the mechanism of action of paclitaxel (Taxol) and carboplatin in cell lines of head and neck squamous cell carcinoma (HNSCC).
Design: Four HNSCC cell lines were treated with paclitaxel and carboplatin, alone or in combination, and evaluated for cell-cycle position by means of flow cytometry, for molecular determinants of cell cycle by means of Western blotting and kinase analysis, and for anchorage-independent growth by means of soft-agar assays.
Results: Paclitaxel was more effective at inducing apoptosis and inhibiting anchorage-independent cell growth, compared with carboplatin. The activity of paclitaxel was correlated with an elevation of cyclin B1/CDC2 activity, prolonged mitotic arrest, and Bcl-2 phosphorylation. In contrast, carboplatin arrested cells before mitosis. Combination treatment with both agents, simultaneously or sequentially, was more effective at inhibiting cell growth than either single agent. Cellular outcome was the same regardless of which drug was used first. The order of addition of these 2 drugs differentially affected cell-cycle position. Paclitaxel pretreatment arrested cells in mitosis, whereas carboplatin pretreatment or cotreatment resulted in premitotic arrest.
Conclusions: To our knowledge, this study is the first to explore how paclitaxel and carboplatin, alone or in combination, differentially affect cell-cycle checkpoint response and HNSCC cell growth. These results provide molecular validation for the current clinical use of both drugs in combination and set the stage for analyses of patient tumor specimens.