Glioblastoma multiforme is the most common and treatment-resistant human brain tumor in adults. 5-Iodo-2’-deoxyuridine (IUdR) as a clinical radiosensitizer can only enhance the survival to 18 month. In this study, we are developing a new chemo-radiotherapy strategy of the combined treatment of IUdR and histone deacetylase inhibitor, in an attempt to further enhance IUdR-mediated radiosensitization for these tumors. Three histone deacetylase inhibitors (HDACi) of different classes were tested in this study including MS-275, Trichostatin A (TSA) and Valproic acid (VPA). The U251 cells were co-treated with IUdR and HDACi before ionizing radiation (IR) and were then grown in the medium with HDACi after IR. The preliminary results showed that compared with IUdR or HDACi treatment alone the co-treatment of IUdR and HDACi before IR significantly sensitized U251 cells to radiation. The interaction between IUdR and HDACi before IR was examined. HDACi slightly enhanced IUdR DNA incorporation. IUdR slightly reduced HDACi-induced acetylated-H3 and -H4, but enhanced HDACi-induced p21. IUdR also slightly reduced HDACi-induced G2 population before IR. DNA damage signaling responses were examined early after IR. The levels of gammaH2AX (as a marker of DNA double strand breaks) was found to be the lowest while pChk1 (S317) and pChk2 (T68) were the highest in the IUdR/HDACi pretreated cells at 30 min following IR. Cell cycle analysis showed that IUdR/HDACi pretreated cells had a sustained larger G2/M arrest population following IR. When the cell death pattern was studied, the IUdR/HDACi pretreated cells showed apoptotic cell death between 24 to 72h following IR as measured by sub-G1 population. However, a significant amount of the cells appeared to be alive with an enlarged or elongated morphology before eventual death. We hypothesize that HDACi enhances IUdR radiosensitization through chromatin modification interference. HDACi changes the pattern of histone acetylation, which may change the location of H2AX in the chromatin and further may affect phosphorylation of H2AX in response to DNA damage. Insufficient formation of gammaH2AX may compromise the repair of IUdR-enhanced IR-induced DNA double strand breaks due to the important role of gammaH2AX in the assembly of DNA repairosome. This novel two-agent chemo-radiotherapy approach may have a potential as a treatment strategy for glioblastoma. Supported by NIH grant CA112963 to TJK.
98th AACR Annual Meeting-- Apr 14-18, 2007; Los Angeles, CA