Human EGFR-2, EGFR and HDAC triple-inhibitor CUDC-101 enhances radiosensitivity of GBM cells

Radiotherapy remains the standard treatment for glioblastoma multiforme (GBM) following surgical resection. Given the aberrant expression of human epidermal growth factor receptor 2 (HER2) and epidermal growth factor receptor (EGFR) which may play a role in therapeutic resistance to receptor tyrosine kinase inhibitors, and the emerging use of histone deacetylase (HDAC) inhibitors as radiosensitizers, we defined the effects of CUDC-101, a triple inhibitor of HER2, EGFR and HDAC on the radiosensitivity of GBM cells. Clonogenic survival was used to determine the in vitro radiosensitizing potential of CUDC-101 on GBM, breast cancer, and normal fibroblast cell lines. Inhibitory activity was defined using immunoblots and DNA double strand breaks were evaluated using yH2AX foci. Effects of CUDC-101 on cell cycle and radiation-induced cell kill were determined using flow cytometry and fluorescent microscopy. CUDC-101 inhibited HER2, EGFR and HDAC and enhanced in vitro radiosensitivity of both GBM and breast cancer cell lines, with no effect on normal fibroblasts. Retention of yH2AX foci was increased by CUDC-101 alone and in combination with irradiation for 24 h. Treatment with CUDC-101 increased the number of cells in G2 and M phase, with only increase in M phase statistically significant. An increase in mitotic catastrophe was seen in a time-dependent fashion with combination treatment. The results indicate the tumor specific CUDC-101 enhanced radiosensitization in GBM, and suggest that the effect involves inhibition of DNA repair.