RBIO-05. MITOTIC ENRICHMENT AS AN EFFICIENT STRATEGY TO RADIOSENSITIZE GLIOBLASTOMA

Their location and highly aggressive nature renders glioblastoma (GBM) among the most deadly and devastating of human malignancies. Despite extensive treatment involving surgery and adjuvant chemo-radiotherapy, the prognosis is still dismal and novel treatment strategies are urgently needed. Of all existing adjuvant therapies, radiotherapy contributes the most to extending the median overall survival. Increasing the efficacy of existing radiotherapeutic regimens is therefore a logical avenue to improve the survival of GBM patients. We have developed a novel radiosensitization strategy called ‘induction of mitotic enrichment’. It has long been known that the radiosensitivity of a cell depends on the phase of the cell cycle and that especially mitotic cells are especially vulnerable. Enriching the tumor for mitotic cells by arresting them during division prior to each radiotherapy fraction should therefore render the tumor population more sensitive to irradiation. Ideally, induction of mitotic enrichment should be reversible and non-cytotoxic to prevent healthy tissue toxicity and be compatible with clinically applied hyperfractionated radiotherapy regimens. We have now identified an orally available targeted tubulin polymerization inhibitor that can achieve repeated and reversible mitotic enrichment for up to 10 hours prior to radiotherapy, without causing cytotoxicity in vitro or healthy tissue toxicity in vivo. Most importantly, this tubulin inhibitor efficiently radiosensitizes a range of preclinical GBM models in vitro and in vivo, including GSC models, and significantly improves survival, but only in a mitotic enrichment setup when given 6-8 hours prior to radiotherapy to allow accumulation in mitosis. We are currently expanding our preclinical development of mitotic enrichment as a radiosensitization strategy to other mitotic targets and different intra- and extracranial cancer models representing several diseases for which radiotherapy is a mainstay treatment. In parallel, we are preparing a phase 0 trial to demonstrate induction of mitotic enrichment in human GBM.

EXTH-31. INCREASING TUMOR TREATING FIELDS (TTFIELDS) EFFICACY BY TARGETING THE G2 CELL CYCLE CHECKPOINT WITH WEE1 OR CHK1 INHIBITORS IN GLIOBLASTOMA CELL LINES

Tumor Treating Fields (TTFields) are a novel, noninvasive FDA-approved treatment modality for glioblastoma (GBM) that utilizes alternating electric fields. While the mechanism of action was at first exclusively attributed to the effects of TTFields on cells during mitosis, additional effects during interphase have recently come to light. The aim of our research is to elucidate the effect of TTFields on the cell cycle to advance the understanding of TTFields and to find novel targets for increasing its efficacy. We studied the effect of TTFields on cell cycle distribution by (propidium iodide and phospho-Histone H3 labeling) flow cytometry using double-thymidine block (DTB) synchronized cell populations. Following the release of the DTB, TTFields treatment caused GBM cells to accumulate in G2, which was prevented by concomitant exposure to a Wee1 inhibitor. Next, we compared the efficacy of TTFields with or without Wee1 or Chk1 inhibitors in multiple GBM cell lines (A172, SNB-19, and U251) by colony formation assays and observed a strong and synergistic decrease in colony formation potential in all cell lines. To investigate the underlying mechanism of G2 arrest, we quantified the amount of DNA damage, but found no difference in either γH2AX foci or comet tail moments between control and TTFields treated cells. To follow up on this surprising observation, we are using live cell imaging (inovitro Live™) of PIP-FUCCI-transduced cells. This tool will allow us to track individual cells, to evaluate the time spent in each phase of the cell cycle and to determine the ultimate cell fate in control and treatment conditions. Our finding that Wee1 or Chk1 inhibition dramatically boosts the efficacy of TTFields may have great implications for treatment of patients with TTFields. The in vivo validation of our in vitro findings will be performed in tumor-bearing mice by using the inovivo™ system.

Identification of MEK162 as a Radiosensitizer for the Treatment of Glioblastoma

Glioblastoma (GBM) is a highly aggressive and lethal brain cancer type. PI3K and MAPK inhibitors have been studied preclinically in GBM as monotherapy, but not in combination with radiotherapy, which is a key component of the current standard treatment of GBM. In our study, GBM cell lines and patient representative primary cultures were grown as multicellular spheroids. Spheroids were treated with a panel of small-molecule drugs including MK2206, RAD001, BEZ235, MLN0128, and MEK162, alone and in combination with irradiation. Following treatment, spheroid growth parameters (growth rate, volume reduction, and time to regrow), cell-cycle distribution and expression of key target proteins were evaluated. In vivo, the effect of irradiation (3 × 2 Gy) without or with MEK162 (50 mg/kg) was studied in orthotopic GBM8 brain tumor xenografts with endpoints tumor growth and animal survival. The MAPK-targeting agent MEK162 was found to enhance the effect of irradiation as demonstrated by growth inhibition of spheroids. MEK162 downregulated and dephosphorylated the cell-cycle checkpoint proteins CDK1/CDK2/WEE1 and DNA damage response proteins p-ATM/p-CHK2. When combined with radiation, this led to a prolonged DNA damage signal. In vivo data on tumor-bearing animals demonstrated a significantly reduced growth rate, increased growth delay, and prolonged survival time. In addition, RNA expression of responsive cell cultures correlated to mesenchymal stratification of patient expression data. In conclusion, the MAPK inhibitor MEK162 was identified as a radiosensitizer in GBM spheroids in vitro and in orthotopic GBM xenografts in vivo The data are supportive for implementation of this targeted agent in an early-phase clinical study in GBM patients. Mol Cancer Ther; 17(2); 347-54. ©2017 AACRSee all articles in this MCT Focus section, "Developmental Therapeutics in Radiation Oncology."