ESTRO 36 Abstract Book

S540 ESTRO 36 _______________________________________________________________________________________________

'radiation/hypoxia induced abscopal effects” offer one more possibility for the oligometastatic population also to get cured. We continue to investigate this hypothesis in the laboratory and clinical setting.

autophagy protein LC3-II and decreased levels of p62.

Poster: Radiobiology track: Radiobiology of colorectal cancer

PO-0976 Mechanisms of normal tissue t oxicity from SAHA, an HDAC inhibitor and radiosens itizer I.S. Barua 1,2 , A.H. Ree 1,2 , L. Sønstevold 1 , K.R . Redalen 1 , E. Kala nxhi 1 1 Akershus University Hospital- Norway, Dep artment of Oncology, Oslo, Norway 2 Institute of Clinical Medicine- University of Oslo, Campus AHUS, Oslo, Norway Purpose or Objective Histone deacetylase inhibitors (HDACi) are therapeutic agents, which through epigenetic alterations can cause tumor cell death and have shown radiosensitizing properties in preclinical models. HDACi have been largely regarded as tumor-specific, while their effects on normal tissues remain poorly investigated. The latter is important as an increase in therapeutic efficacy resulting from combining such agents with radiotherapy may come at the expense of patient tolerance, undesired treatment interruptions and dose limitations. In the phase I Pelvic Radiation and Vorinostat (PRAVO) study, we investigated mechanisms of adverse effects to the HDACi vorinostat (suberoylanilide hydroxamic acid; SAHA) when given as potential radiosensitizer. Vorinostat-induced transcriptional responses in patients’ peripheral blood mononuclear cells implicated cell death pathways as a possible mechanism of toxicity. In experimental models we showed that apoptosis in epithelial cells of the intestinal mucosa may account for the gastrointestinal- related adverse effects commonly associated with the use of HDACi (Kalanxhi E, et al. Cancer Res Treat, 2016). In the current work we further investigate HDACi-induced apoptosis and the possible interplay with autophagy in experimental normal and colorectal cancer (CRC) models. Material and Methods Two normal cell lines (rat IEC-6 intestinal epithelial cells, human BJ fibroblasts) and two CRC cell lines (HCT116, HT29) were exposed to a therapeutically relevant concentration of SAHA alone, or in combination with the caspase inhibitor ZVAD-fmk and the autophagy inhibitor bafilomycin A1 for 24 hours. Induction of apoptosis and autophagy were analyzed with flow cytometry (Annexin V/PI staining) and western blot analysis (LC3 I/II and p62 expression). Results SAHA induced apoptosis in the CRC cell lines with 42% and 26% of the HCT116 and HT29 cell populations respectively, showing Annexin V/PI staining indicative of early and late phases of apoptosis (Figure 1). Normal BJ fibroblasts remained unaffected, whereas intriguingly, intestinal epithelial IEC-6 cells responded similarly as the cancer cells, although apoptosis was induced at a lesser extent (18% of cells). Addition of ZVAD-fmk halved the number of apoptotic cells in CRC cells, whereas the same number of IEC-6 cells (16%) displayed apoptotic phenotypes. We further looked into induction of autophagy and found that SAHA induced autophagy both in the CRC cell lines and the IEC-6 cells, as reflected by increased levels of the

Conclusion Treatment with the HDACi SAHA resulted in induction of apoptosis and autophagy in both CRC cells and at a lesser extent in a relevant normal tissue model. Firstly, our results may contribute to explain adverse effects of SAHA on normal intestinal epithelial cells, and secondly, identify a therapeutic window where tumor radiosensitization can be achieved by SAHA. PO-0977 Plasma lipidomics for predictive biomarker analysis in rectal cancer. P. Bulens 1,2 , A. Debucquoy 2 , K. Bloch 2 , S. Fieuws 2 , J. Swinnen 2 , K. Haustermans 1,2 1 University Hospital Leuven, Radiation Oncology, Leuven, Belgium 2 KU Leuven - University of Leuven, Oncology, Leuven, Belgium Purpose or Objective Selection of patients with locally advanced rectal cancer, eligible for individualized treatment strategies, is hampered by the lack of reliable predictors of response. Plasma markers based on liquid biopsies would allow minimally invasive patient stratification. Most liquid biopsy approaches are based on the detection of free circulating DNA or tumor cells. Since the development and progression of cancer is associated with dramatic changes in lipid metabolism, we propose a radically different approach based on alterations in circulating lipids. Material and Methods From prospectively collected plasma samples of 85 rectal cancer patients at 3 time points (before chemoradiation (CRT), 2 weeks into CRT, end of CRT), lipids were extracted using a modified Bligh-Dyer protocol. Samples were subjected to mass spectrometry-based lipid profiling on a fully operational lipidomics platform. This approach allowed us to assess the concentration of approximately 200 different lipid species including phosphatidylcholine (PC), phosphatidylethanolamines (PE), phosphatidylinositol (PI), phosphatidylserine (PS) and ceramides (Cer). Based on the assessment of these species, discriminative lipid profiles of patients achieving a pathologic complete response (pCR) and patients lacking such response will be delineated using biostatistical approaches including PCA analysis followed by LDA and correction for false discovery due to multiple testing. Results 13 out of 85 patients achieved a pCR (15,3%). Preliminary analyses showed slightly less lipogenic profiles for patients