PACLITAXEL AND IRRADIATION INDUCE APOPTOSIS IN SQUAMOUS-CELL CARCINOMA CELL-LINES IN AN ADDITIVE WAY

Paclitaxel (Taxol(R)) is a new antimicrotubule plant product, which no t only stabilizes the microtubules by inhibiting their disassembly but also promotes the assembly of microtubules. This causes a block in th e G2-M phase of the cell cycle known to be a radiosensitive phase of t he cycle. Previously we demonstrated that 10 nM paclitaxel accumulated the cells of laryngeal carcinoma cell lines in the G2-M phase as meas ured by flow cytometry. Time-lapse videomicroscopy demonstrated that t he cells died morphologically by apoptosis after a premitotic block. A garose gel electrophoresis showed the DNA-laddering typical of apoptos is. To investigate the effects of irradiation and paclitaxel separatel y and concomitantly, we used five newly established laryngeal carcinom a cell lines. The effects were recorded with time-lapse videomicroscop y over 96 hours on controls, cells irradiated with 2 Gy, cells exposed to 1 nM or 5 nM paclitaxel and cells irradiated with 2 Gy after incub ating in 1 nM or 5 nM paclitaxel for 24 hours. Spontaneous apoptoses w ere seen in all cell lines tested. The 2 Gy irradiation dose induced a propagated apoptotic response in two of these cell lines. In all cell lines paclitaxel induced a premitotic block only in some cells at the tested concentrations and these cells died morphologically by apoptos is, whereas irradiation and paclitaxel concomitantly caused an additiv e inhibition in mitotic activity and caused an additive apoptotic resp onse. An additive effect of paclitaxel and radiation was seen with clo ses readily achievable in clinical treatment. This additive effect see ms to be due to other mechanisms of action than the premitotic block.