Enhancement of tumor radioresponse by docetaxel: Involvement of immune system

Docetaxel, a potent chemotherapeutic drug and a strong enhancer of tumor radioresponse, possesses immunomodulating properties. We previously reported that 40% of murine tumors responding to docetaxel by growth delay showed heavy infiltration with macrophages or lymphocytes. The present study explored the effect of whole body irradiation on antitumor action of docetaxel alone or docetaxel plus tumor irradiation. Mice bearing 8-mm MCa-K mammary carcinoma in the leg received 33 mg/kg docetaxel i.v., 5 to 65 Gy tumor irradiation, or both (radiation given 24 h after docetaxel). Docetaxel delayed tumor growth and enhanced the efficacy of radiation: it dramatically reduced TCD50 (radiation dose yielding 50% tumor cure) from the control value of 38.6 Gy to 11.8 Gy, for an enhancement factor of 3.27. In addition to enhancing tumor radioresponse, docetaxel decreased the lung metastatic rate in mice with local tumor control from 26% in mice receiving radiation alone to 11% in mice receiving docetaxel plus radiation. Docetaxel induced heavy infiltration of tumors with lymphocytes, determined 2-4 days after treatment: the percentage of lymphocytes increased from the control value of <2% to 27% in mice that received docetaxel 3 days earlier. This increase was due to the influx of helper/inducer T-lymphocytes and natural killer cells. Immunosuppression of tumor-bearing mice with 6 Gy whole-body irradiation prior to tumor isotransplantation reduced docetaxel-induced lymphocyte infiltration of tumors, antitumor and anti-metastatic action of docetaxel, and docetaxel-induced enhancement of tumor radioresponse. Thus, our results showed that docetaxel stimulates tumor infiltration with immune cells, which then participate in antitumor action of docetaxel alone or when combined with radiotherapy.