TUMOR REOXYGENATION AS A MECHANISM OF TAXOL-INDUCED ENHANCEMENT OF TUMOR RADIORESPONSE

Paclitaxel is a novel chemotherapeutic agent that arrests cells in the radiosensitive G(2) and M phases of the cell cycle and as such may act as a specific cell cycle radiosensitizer. We recently reported that paclitaxel induces mitotic arrest in the MCA-4 murine mammary carcinoma and enhances radioresponse of this tumor. However, the greatest enhancement was observed not when radiation was given at the time of peak mitotic arrest, which was 9 h after paclitaxel administration, but when it was given 24 h after paclitaxel. This implied the involvement of other mechanisms in radiosensitization; we hypothesized that tumor reoxygenation was a likely mechanism based on the observed massive loss of mitotically arrested cells at 24 h. The present study shows that paclitaxel greatly enhanced MCA-4 tumor radioresponse when radiation was given under air-breathing conditions (DMF = 1.74), but not when it,vas performed under hypoxic conditions, This observation supports the hypothesis of tumor reoxygenation as a mechanism of enhancement of tumor radioresponse. That reoxygenation occurred in tumors treated with paclitaxel 24 h earlier was confirmed by direct measurements of pO(2) values, using the Eppendorf pO(2) histograph, Median pO(2) values increased from 6.2 mmHg in untreated tumors to 10.0 mmHg in tumors treated with paclitaxel, These observations emphasize the importance of timing of paclitaxel administration in relation to radiation treatment.