Paclitaxel and radiotherapy: sequence-dependent efficacy--a preclinical model

The optimal sequence of a paclitaxel-radiation combination was investigated in vitro in two human colon adenocarcinoma cell lines, HT29 and LoVo. Three schedules of combined treatment were tested by clonogenic and flow cytometric assays. Paclitaxel was given 24 h prior to a single radiation shot (first schedule) or 24 h (second schedule) or 48 h (third schedule) before 3 days of concomitant radiation. Dose-response data were fit to a linear quadratic model, and mean inactivation dose and sensitizer enhanced ratio were calculated. In HT29 cells, the first and second schedule resulted in an additive effect, whereas a supraadditive interaction was observed with the third combination schedule. This effect was obtained with amounts of paclitaxel lower than IC50, which did not result in cell cycle perturbation, and with low radiation dose (2 Gy) that may be given in a clinical setting. LoVo cells were less sensitive to combined treatment than HT29 cells, switching from infraadditive (first and second schedule) to additive interaction (third schedule). Posttreatment recovery studies of third schedule showed a loss of cell survival in HT29 cells but not in LoVo cells. In contrast to LoVo cells, the third schedule in HT29 cells was able to induce perturbation of cell cycle kinetics, an effective impairment of DNA repair, and apoptotic cell death. HT29 and LoVo cells showed constitutional different characteristics: HT29 cells were more sensitive to paclitaxel exposure, less radiosensitive, and had a different cell cycle redistribution after radiation exposure than LoVo cells; moreover, HT29 cells showed a major propensity to undergo apoptosis. These results suggest that the radiosensitizing effect of paclitaxel was strictly schedule dependent, and the inhibition of DNA repair, cell cycle redistribution, and apoptosis could be the mechanisms for the induction of radiosensitization by paclitaxel.