Therapeutic potential of paclitaxel-radiation treatment of a murine ovarian carcinoma

Background. Paclitaxel has been shown to radiosensitize tumor cells in culture by arresting them in the most radiosensitive G₂ and M cell cycle phases. In vivo preclinical studies are now necessary to obtain full insight into the radiopotentiating potential of this drug and its ability to increase the therapeutic gain of radiotherapy. We tested its ability to enhance the tumor radioresponse of an ovarian carcinoma and to influence the normal tissue radioresponse of recipient mice. Methods. Mice bearing 8-mm isotransplants of a syngeneic ovarian carcinoma, designated OCA-I, in their legs were treated with 40 mg/kg paclitaxel i.v., 14-60 Gy single-dose local tumor irradiation, or both; radiation was given under ambient conditions 1-96 h after paclitaxel. Tumor growth delay, tumor cure rate (TCD₅₀ assay), and delay in tumor recurrences were measured, Normal tissue radioresponse was determined using jejunal crypt cell survival at 3.5 days after exposure of mice to 9-14 Gy single dose of total body irradiation; the mice were untreated or treated with 40 mg/kg i.v. pachtaxel 4-96 h before irradiation. Results. Paclitaxel alone was effective against OCA-I, but its combination with irradiation produced supra-additive tumor growth delay. It also reduced TCD₅₀ values and delayed tumor recurrences. The enhancement of tumor radioresponse ranged from 1.33 to 1.96; the value increased as the time between paclitaxel administration and tumor irradiation increased up to 48 h, but then decreased again at 96 h. In contrast, paclitaxel protected jejunum against radiation damage by factors of 1.03 to 1.07 when given 24-96 h before irradiation. It showed some potentiation of damage (by a factor of 1.07), but only when given 4 h before irradiation. Conclusions. Paclitaxel potentiated tumor radioresponse if given within 4 days before irradiation, whereas it caused radioprotection of normal tissue (jejunum) at that time. Therefore, paclitaxel significantly increased therapeutic gain and so has potential for use in combination with radiotherapy for pelvic malignancies.