Purpose: Paclitaxel is a potentially useful drug for augmenting the cytotoxic action of radiotherapy because it has independent cytotoxic activity against certain cancers and blocks cells in the radiosensitive mitotic phase of the cell cycle. However, all rapidly proliferating tissues, both normal and neoplastic, may be affected by this therapeutic strategy. The aim of this study was to define the in vivo response of rapidly dividing cells of the small bowel mucosa to paclitaxel given alone and in combination with radiation.
Methods and materials: Mice were given single IV doses of 10 or 40 mg/kg paclitaxel or four doses of 10 mg/kg paclitaxel at 6, 12, or 24 h intervals. The kinetics of mitotic arrest and apoptosis in jejunal crypts of mice at 1-24 h after treatment were defined histologically. An in vivo stem cell microcolony assay was used to assess the radiosensitizing potential of paclitaxel when radiation was delivered at the peak of mitosis and at 24 h after drug treatment.
Results: Paclitaxel blocked jejunal crypt cells in mitosis and induced apoptosis in a dose-dependent manner. Fractionating the paclitaxel dose over 1-4 days did not result in any greater accumulation of mitotically blocked cells than did a single dose. Mitosis peaked 2-4 h after paclitaxel and returned to near normal by 24 h. Apoptosis lagged several hours behind mitosis and peaked about 6 h later than mitosis. Despite these kinetic perturbations, there was little or no enhancement of radiation effect when single doses were delivered 2-4 h after paclitaxel administration. The maximum sensitizer enhancement ratio of 1.07 observed after a single paclitaxel dose of 40 mg/kg is consistent with independent crypt cell killing. Conversely, when radiation was given 24 h after paclitaxel, a significant protective effect of the drug (SER 0.89-0.92), most probably due to a regenerative overshoot induced by paclitaxel, was observed.
Conclusion: Stem cells of the jejunal mucosa determining radiation response were not radiosensitized by paclitaxel with the drug concentrations and dose delivery schedules used, although additive cytotoxicity was observed with the highest drug dose. A radioprotective effect was observed when radiation was given 24 h after paclitaxel administration.