2503 - Identification of Triapine as a Novel Radiosensitizer in Glioblastoma

T. Cui¹, S. Corrales-Guerrero², V. Castro-Aceituno¹, L. Yang¹, S. Nair¹, H. Feng¹, M. Venere³, S. Yoon¹, T. A. DeWees⁴, C. Shen¹, and T. M. Williams¹; ¹Department of Radiation Oncology, City of Hope National Medical Center, Duarte, CA, ²Biomedical Sciences Graduate Program, The Ohio State University College of Medicine, Columbus, OH, ³Department of Radiation Oncology, The Ohio State University Wexner Medical Center, Columbus, CA, ⁴Department of Computational and Quantitative Medicine, City of Hope National Medical Center, Duarte, CA

Purpose/Objective(s): Despite aggressive multimodality treatments, the median survival of glioblastoma (GBM) patients remains within the range of 12-15 months after diagnosis with standard-of-care; therefore, new therapeutic strategies are critically needed. Radiation is a common and effective therapeutic option for GBM. However, some GBMs are relatively radioresistant. Radiation-induced DNA damage repair requires a constant and balanced supply of dNTPs, which is catalyzed by ribonucleotide reductase (RNR). Therefore, we hypothesized that the inhibition of RRM2 by triapine sensitizes GBM to radiation. Materials/

Methods: We analyzed RRM2 expression in GBM at the mRNA level and assessed its correlation with clinical outcomes in GBM patients using different public datasets. We examined the effect of RRM2 inhibition, either by small interfering RNA (siRNA) or RRM2 inhibitor (3-aminopyridine-2-carboxaldehyde thiosemicarbazone; triapine), on radiosensitization using clonogenic assays in both established and GBM patient-derived primary cells, as well as mouse GBM cells in vitro. Radiation-induced DNA damage was evaluated via ?-H2AX foci assays, and the underlying mechanism was studied using western blotting. To explore the radiosensitizing effect of triapine in vivo, we used both heterotopic and orthotopic syngeneic mouse models.
Results: RRM2 expression was significantly higher in GBM tissues than in non-tumor tissues, and higher RRM2 expression was significantly associated with higher tumor grade and worse overall survival. Interestingly, RRM2 inhibition sensitized GBM cells to radiation in vitro, led to reductions in tumor growth, and significantly increased mouse survival in vivo, with acceptable levels of toxicity. Mechanistically, RRM2 inhibition-mediated radiosensitization was associated with higher ?-H2AX foci counts and expression, indicating higher levels of unresolved DNA damage after radiation. Moreover, we confirmed that triapine leads to replication stress, which in turn upregulates RRM2 via a CHK1 mediated mechanism, as inhibition of CHK1 abrogated the triapine-induced induction of RRM2.
Conclusion: Our preclinical study suggests that RRM2 is a promising therapeutic target for GBM, and triapine inhibition of RRM2 sensitizes GBM cells to radiation treatment in vitro and in vivo. Our findings suggest that combining triapine with radiation may improve therapeutic outcomes in GBM, warranting additional preclinical studies and providing a justification for a phase I clinical trial potentially in the recurrent setting.