ESTRO 38 Abstract book

S601 ESTRO 38

from HeLa cell line. Apoptosis assay and western blot analysis were examined in 0Gy and 8Gy with or without a candidate compound in HeLa and HeLa-RT cell lines. Sphere formation assay was utilized to detect tumor sphere forming capacity.

aneugenic damages, induced by alteration in the mitotic spindle, as well as clastogenic damages produced by chromosome breaks. In this study, a clonogenic cell survival assay was performed to measure the interaction of a static magnetic field with radiation. Additionally, the mechanism of a possible biological interaction was analyzed using multicolor fluorescence in situ Human glioblastoma cells (LN-18) as well as human peripheral blood T-cell lymphocytes were seeded in petri dishes, which were placed inside a phantom. The irradiation was for cell survival test was performed with a 6 MV linac with doses of 0, 1, 2, 4, 6, and 8 Gy in the presence and absence of a SMF of 1 Tesla produced by a permanent magnet. For the chromosome aberration test, irradiations with 2 and 4 Gy in the presence and absence of a SMF of 1 Tesla were performed. Chromosome aberrations were analyzed in at least 300 metaphases per treatment group using Metafer4 software. For both experiments three technical as well as three biological replicates were performed. Results The survival curves were fitted using the linear quadratic model. The survival fractions in absence of an SMF exceed the ones in presence of an SMF for all dose points between 1 and 8 Gy by 12% to 31%. The statistical analysis showed a significant decline (p≤0.05, ANOVA test) of the overall cell survival when irradiation was combined with a static magnetic field. However, the chromosome aberration test did not show an induction of chromosome aberrations in general or a shift in the complexity or completeness of damages. The number of all chromosome aberrations was 0.71 +/- 0.05 without and 0.73 +/- 0.05 with a SMF during the 2 Gy irradiation Conclusion Up to now, there are only very few studies that have investigated the combinational effect of a static magnetic field and radiation. Most of these studies were in line with our results showing that a static magnetic field increases the efficiency of the radiotherapy. Additionally, our study showed evidence that the altered cell survival is not caused by clastogenic DNA damages, which leads us to the hypothesis that aneugenic effects may cause the change in cell survival. Further experiments, including a micronucleus assay with centromere FISH and staining of the mitotic spindles by immunohistochemistry are proposed. PO-1082 Dihydroouabain is a novel radiosensitizer identified by high throughput screening. Z. Li 1 , K. Tamari 1 , Y. Seo 1 , K. Minami 1 , Y. Takahashi 1 , K. Otani 1 , O. Suzuki 1 , F. Isohashi 1 , K. Ogawa 1 1 Osaka University Graduate School of Medicine, Radiation Oncology, Suita-Osaka, Japan Purpose or Objective Radiosensitizers have had only limited success in clinic. High throughput screening (HTS) is a popular method to identify candidate compounds from a large number of compounds for drug discovery, however it has not been used to established a method to identify new radiosensitizers so far. The aims of this study were identifying potential radiosensitizers by HTS and clarifying the specific mechanisms of new radiosensitizers in combination with radiotherapy (RT). Material and Methods We performed HTS by using 1280 compounds. The parental HeLa human cervical cancer cells were pre-treated with compounds and exposed to gamma-irradiation under normoxic and hypoxic conditions. After incubating the cells for 4 days, cellular viabilities were measured by using MTS assay. Then, the radiosensitizing effect of candidate compounds was validated by clonogenic survival assays. The radioresistant cell line (HeLa-RT) was established hybridization (mFISH). Material and Methods

Results HTS identified 22 compounds as candidates for radiosensitizers. Among them, we selected Dihydroouabain (DHO) which was known as an inhibitor of Na + /K + ATPase and was not reported as a radiosensitizer. Clonogenic survival assays showed significant radiosensitizing effect of DHO in HeLa cells and in HeLa- RT cells (Figure 1). The combination of DHO and RT enhanced radiation-induced apoptosis. Western blot showed that γ-H2AX and pS345-Chk1 significantly increased and prolongated in combination treatment with DHO and RT compared with RT alone or DHO alone group. In addition, DHO treatment inhibited the capability of tumor spheres forming in HeLa and HeLa-RT cells. Taken together, DHO enhanced radiosensitivity through inhibition of DNA repair pathway, and induction of cancer cell apoptosis. Conclusion HTS is an efficient method for identifying new radiosensitizers. We identified DHO as a novel radiosensitizer by HTS. Na + /K + ATPase might be a novel target for radiosensitization. PO-1083 The dual inhibitor BEZ235 radiosensitizes HNSCC cells due to impairment of the DSB repair A. Arenz 1 , V. Balzer 1 , K. Dreffke 1 , T. Rieckmann 2 , S. Köcher 3 , E. Dikomey 1,3 , R. Engenhart-Cabillic 1 , A. Wittig 4 , U. Schötz 1 1 Philipps-University Marburg, Department of Radiotherapy and Radiooncology, Marburg, Germany ; 2 University Medical Center Hamburg Eppendorf, Laboratory for Radiobiology & Experimental Radiooncology and Department of Otolaryngology and Head and Neck Surgery, Hamburg, Germany ; 3 University Medical Center Hamburg Eppendorf, Laboratory for Radiobiology & Experimental Radiooncology, Hamburg, Germany ; 4 Friedrich-Schiller-University Jena, Department of Radiation Oncology, Jena, Germany Purpose or Objective The Phosphatidylinositol-3-kinase (PI3K) pathway is the sixth most frequent mutated oncogenic pathway in head and neck squamous cell carcinoma (HNSCC) presenting in 60% of HPV-positive (HPV pos) and 31% of HPV-negative (HPV neg) tumors. The following study examines the