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(ALDH1A1+) and ALDH1A1 knockdown (ALDH1A1-) cells. To determine the radiosensitivity of ALDH1A1+ and ALDH1A1- cells, cells were irradiated using a X-ray irradiation device (RS225, Gulmay Medical, Surrey, UK) and a clonogenic assay was carried out. Results Western Blot analysis confirmed a stable knockdown of ALDH1A1 in LN18 glioblastoma cells by lentiviral transduction. In the IBIDI TM migration assay ALDH1A1 expressing LN18 cells (ALDH1A1+) revealed a higher migratory capacity than ALDH1A1 knockdown cells (ALDH1A1-). And, most importantly, ALDH1A1 knockdown significantly increased the radiosensitivity of LN18 cells. Conclusion Our data confirm the hypothesis, that ALDH1A1 expression affects malignancy and radioresistance of cancer cells, especially glioblastoma cells, and therefore influences the success of radiotherapy, a cornerstone in GBM therapy. Therefore, ALDH1A1 could constitute a novel prognostic biomarker in GBM. Furthermore, ALDH1A1 could be a new therapeutic target for GBM patients. EP-2327 Hypoxic cell killing by CP-506, a novel hypoxia-activated prodrug R. Niemans 1 , A. Yaromina 1 , J. Theys 1 , D. Marcus 1 , A. Ashoorzadeh 2 , M. Abbattista 2 , A. Mowday 2 , R. Biemans 1 , N. Lieuwes 1 , S. Deschoemaeker 3 , A. Heyerick 3 , C. Guise 2 , J.B. Smaill 2 , A.V. Patterson 2 , L. Dubois 1 , P. Lambin 1 1 Department of Radiotherapy, GROW - School for Oncology and Developmental Biology- Maastricht Comprehensive Cancer Centre- Maastricht University Medical Centre, Maastricht, The Netherlands 2 Auckland Cancer Society Research Centre, University of Auckland, Auckland, New Zealand 3 Convert Pharmaceuticals, Convert Pharmaceuticals, Liège, Belgium Purpose or Objective Hypoxia is a common feature of solid tumors and causes resistance to conventional treatments such as chemo- and radiotherapy (RT). Hypoxia-activated prodrugs (HAPs) are specifically activated under hypoxic conditions to directly target these as well as adjacent more oxygenated tumor cells via their bystander effect. CP-506 is a newly developed nitroaromatic HAP with highly favorable properties: activation under hypoxia, high bystander effect, and no off-mechanism activation by human aerobic reductases. Here, we tested the cytotoxic effects of CP-506 in vitro and in vivo . Material and Methods IC 50 viability ratios were assessed in 2D cell cultures of a panel of human tumor cell lines exposed to normoxic or anoxic (≤0.02% O 2 ) conditions. H460 and HCT116 spheroids were incubated with CP-506, single cell suspensions were made and cells were plated for clonogenic survival. Mice bearing xenografts generated from a range of tumor cell lines received one i.p. dose of CP-506 per day on 5 consecutive days (QD5) when tumors reached a volume of approximately 200 mm 3 . Tumor growth was monitored until reaching 4× start treatment volume (T4×SV). Additional mice were treated with CP- 506 and injected with the hypoxia markers CCI-103F and pimonidazole 2 h before or 24 h after the last CP-506 injection, respectively. Tumors were excised and sections were stained to determine the hypoxic fraction (HF). In CT26 xenografts, RT (5 Gy) was directly followed by CP- 506 (800 mg/kg QD5).

Biomarker response was measure via immuno-blotting, immuno-fluorescence and immuno-histochemistry. In vivo experiments utilised a intracranial xenograft model using a primary GBM cell line. Mice were irradiated using a Small Animal Radiation Research Platform (SARRP, Xstrahl). Results GBM cells classically adopt a mesenchymal mode of cell invasion, characterised by an elongated cell body and actin-rich protrusions. Mesenchymal migration requires the activation of the actin-myosin regulatory kinase, MRCK, and consequent phosphorylation of the downstream effectors MLC2 and MYPT1. By measuring relative phosphorylation levels of the MRCK biomarkers, pMLC2 and pMYPT1, we demonstrate that MRCK activity is enhanced at the invasive edges of GBM tumours and is further enhanced by irradiation both in vitro and in vivo. We demonstrate that this response is essential to the phenomenon of radiation induced migration of GBM cells in vitro through MRCK targeted RNAi. Crucially, we were also able to fully abrogate radiation driven invasion in vivo by using a novel small molecule inhibitor, BDP-9066, to specifically target MRCK in a clinically relevant intracranial mouse model of GBM. These data confirm the pivotal role of MRCK in radiation driven infiltration. We are currently undertaking an experiment to investigate whether the anti-invasive effects of BDP-9066 translate into increased survival in our intracranial GBM model, and therefore further validate MRCK as a potential anti- invasive therapeutic target in GBM. Conclusion In conclusion, we have identified a novel and highly promising anti-invasive therapeutic target in GBM, MRCK, which in combination with existing standard of care for glioblastoma, has the potential to improve outcomes for this cancer of unmet need. EP-2326 Downregulation of ALDH1A1 reduces radioresistance and migration of glioblastoma cells F. Martin 1 , M. Wank 2 , F. Lämmer 1,3 , J. Schlegel 3 , D. Schilling 1 , T.E. Schmid 1,2 , S.E. Combs 1,2 1 Klinikum rechts der Isar- Technische Universität München TUM, Department of Radiation Oncology, Munich, Germany 2 Institute of Innovative Radiotherapy iRT- Helmholtz Zentrum München, Department of Radiation Sciences DRS, Munich, Germany 3 Institute of Pathology- Technische Universität München, Division of Neuropathology, Munich, Germany Purpose or Objective Glioblastoma multiforme (GBM) is the most common primary brain tumor in adults with a very bad prognosis. The median survival of GBM patients is still less then 15 months, despite extensive research and multimodal therapy concepts, including surgery, chemotherapy and radiotherapy. It was shown that the cytosolic enzyme Aldehyde dehydrogenase 1A1 (ALDH1A1) is overexpressed in cancer stem cells of several tumor entities, including GBM. These studies suggest that ADLH1A1 expression might be associated with a poor prognosis. Therefore, we analyzed whether ALDH1A1 expression affects radioresistance and migration of glioblastoma cells. Material and Methods Using lentiviral transduction, a stable ALDH1A1 knockdown in the GBM cell line LN18 was generated. The stable knockdown was proved by Western Blot analysis. IBIDI TM wound healing assay was performed to analyze and compare the migratory capacity of ALDH1A1 expressing