In vitro and In vivo Radiosensitization of Glioblastoma Cells by the Poly (ADP-Ribose) Polymerase Inhibitor E7016

Purpose: Poly (ADP-ribose) polymerase (PARP) inhibitors are undergoing clinical evaluation for cancer therapy, Because PARP inhibition has been shown to enhance tumor cell sensitivity to radiation, we investigated the in vitro and in vivo effects of the novel PARP inhibitor E7016. Experimental Design: The effect of E7016 on the in vitro radiosensitivity of tumor cell lines was evaluated using clonogenic survival. DNA damage and repair were measured using gamma H2AX foci and neutral comet assay. Mitotic catastrophe was determined by immunostaining. Tumor growth delay was evaluated in mice for the effect of E7016 on in vivo (U251) tumor radiosensitivity. Results: Cell lines exposed to E7016 preirradiation yielded an increase in radiosensitivity with dose enhancement factors at a surviving fraction of 0.1 from 1.4 to 1.7. To assess DNA double-strand breaks repair, gamma H2AX measured at 24 hours postirradiation had significantly more foci per cell in the E7016/irradiation group versus irradiation alone. Neutral comet assay further suggested unrepaired double-strand breaks with significantly greater DNA damage at 6 hours postirradiation in the combination group versus irradiation alone. Mitotic catastrophe staining revealed a significantly greater number of cells staining positive at 24 hours postirradiation in the combination group. In vivo, mice treated with E7016/irradiation/temozolomide had an additional growth delay of six days compared with the combination of temozolomide and irradiation. Conclusions: These results indicate that E7016 can enhance tumor cell radiosensitivity in vitro and in vivo through the inhibition of DNA repair. Moreover, enhanced growth delay with the addition of E7016 to temozolomide and radiotherapy in a glioma mouse model suggests a potential role for this drug in the treatment of glioblastoma multiforme.