Ionizing Radiation and Endostatin Gene Therapy Against Lung Tumor

Although many advances have been made in radiotherapy, tumor radioresistance and normal tissue damage continue to always be important issues for radiation oncologists. Numerous reports have also documented that radiation exposure increases the risk for malignancy and suppresses immune mechanisms. However, increasing evidence has suggested that anti-angiogenic therapy that targets tumor blood supply may alter abnormal tumor vasculature, thus synergizing radiotherapy. The governing hypothesis of the present study was that modification of tumor vasculature by antiangiogenic gene therapy can increase the efficacy of radiotherapy and that radiation can also improve DNA transfection efficiency by increasing expression of the administered gene. The major goal of this study was to determine whether combination treatment of endostatin gene therapy (pXLG-mEndo) and radiotherapy have a significantly greater anti-tumor effect against lung carcinoma than either modality alone. The underlying mechanisms were postulated to include alterations in the balance of cytokines critical for generating tumor vasculature. pXLG-mEndo, the unique plasmid containing a mouse endostatin gene under the control of a cytomegalovirus (CMV) promoter was constructed in our laboratory. The functionality of pXLG-mEndo was tested in vitro using both human umbilical vein endothelial cell (HUVEC) and Lewis lung carcinoma (LLC) cell lines with and without radiation. Combined anti-tumor efficacy of pXLG-mEndo and radiotherapy was tested in the LLC model in C57BL/6 mouse. Different doses of plasmid were injected intratumorally and 10 Gy of 60Co irradiation was applied locally to the tumor 16 hrs thereafter. Multiple injections and irradiations were carried out in different animal experiments. Tumor volumes and mouse weights were characterized; and immunohistochemical analyses were performed on excised tumors to evaluate tumor vasculature densities. In addition, angiogenic and anti-angiogenic factors were tested in tumors and peripheral blood in order to identify changes associated with treatment. In later studies, alterations in cytokine secretion and lymphocyte and other immune cell distributions were also investigated after different combination treatments. Collectively, the data demonstrated that the delivery of endostatin via pXLGmEndo as an adjuvant to radiation can significantly enhance the anti-tumor efficacy of radiation therapy in the LLC mouse tumor model up to 50%. Furthermore, gene therapy and radiotherapy had mutually beneficial effects. The findings also demonstrated for the first time in this particular model that the changes of tumor angiogenic and antiangiogenic factors are correlated to tumor volumes and tumor vasculature densities. Finally, changes in the relative status of immune cell compartments accompanied ionizing radiation and gene therapy. These findings warrant further study and evaluation of anti-angiogenic gene therapies in combination with radiation