COX-2 Inhibition Combined with Radiation Reduces Orthotopic Glioma Outgrowth by Targeting the Tumor Vasculature

Cyclooxygenase 2 (COX-2) inhibitors have been shown to enhance tumor's response to radiation in several animal models. The strong association of COX-2 and angiogenesis suggests that the tumor vasculature may be involved in this process. The current study investigated whether treatment with the COX-2 inhibitor E-6087 could influence response to local radiation in orthotopically growing murine gliomas and aimed to analyze the involvement of the tumor vasculature. GL261 glioma cells were injected into the cerebrum of C57bl/6 mice. From day 7 after tumor cell injection, mice were treated with COX-2 inhibitor at 50 mg/kg i.p. every third day. Radiation consisted of three fractions of 2 Gy given daily from day 9 to day 11. Mice were killed at day 21. The COX-2 inhibitor significantly enhanced the response to radiation, reducing mean volume to 32% of tumors treated with radiation only. The combination treatment neither increased apoptosis of tumor cells or stromal cells nor affected tumor microvascular density. In vitro, E-6087 and its active metabolite did not affect clonogenic survival of GL261 cells or human umbilical vein endothelial cell after radiation. In vivo, however, there was a nonsignificant increase in Angiopoietin (Ang)-1 and Tie-2 mRNA levels and a decrease of Ang-2 mRNA levels after combination treatment. These changes coincided with a significant increase in alpha-smooth muscle actin-positive pericyte coverage of tumor vessels. In conclusion, the antitumor effect of radiation on murine intracranial glioma growth is augmented by combining with COX-2 inhibition. Our findings suggest an involvement of the tumor vasculature in the observed effects.