RADIOSENSITIZATION OF GLIOMA TUMOR CELLS USING Cl-994 (N-ACETYLDINALINE)

Abstract

Radiation therapy continues to be a primary treatment modality for solid tumors. The development of agents that can enhance tumor cell radiosensitivity may thus provide patients a significant advantage. Because tumor cells often express abnormalities in cellular physiology such as apoptotic pathways, DNA repair pathways or cell cycle checkpoints; the current approach in radiosensitizer research is focused on finding agents that have a molecular target that enhances radiation induced damage to tumor cells while being minimally toxic to normal cells. Because tumor cells have demonstrated aberrant histone acetylation, one potential target is histone deacetylase (HDAC). This study investigates the effects of the HDAC inhibitor CI-994 as a potential radiosensitizer on. three glioma tumor lines: U251, U87 and SF295. The acetylation status of histones H3 and H4 was determined as a function of time after addition and removal of CI-994 from culture medium. Histone acetylation increased by 6 hrs after addition of CI-994 reaching a maximum for all cell lines at 24 hrs. However, when the drug was removed and fresh drug free media was added the acetylation returned to control levels by 6 hrs for all lines tested. Cultures exposed to CI- 994 for 24 hrs, followed by drug removal prior to irradiation, showed no increase in radiation induced cell killing. However, cultures treated with CI-994 continuously, both before and after irradiation, resulted in a significant increase in radiation induced cell killing. In order to determine a mechanism for the observed radiosensitizing effects, several possibilities were investigated including cell cycle phase redistribution, activation of apoptotic pathways, and abrogation of the G2 arrest checkpoint. No cell cycle phase changes were observed after continuous treatment with CI-994 nor was there an increase in the percentage of cells undergoing apoptosis. Additionally no abrogation of the G2 arrest checkpoint was observed after continuous CI-994 treatment. Results from the clonogenic survival assays indicate that CI-994 can enhance radiosensitivity however the mechanism remains to be determined.