ESTRO 36 Abstract Book

S535 ESTRO 36 2017 _______________________________________________________________________________________________

cm) with 10Gy single fraction prescribed to the 70% isodose line (Dmax 14 Gy). No patient got chemotherapy/immune therapy.

'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.

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

Results 10Gy- in vitro induced abscopal effect in hypoxic conditions was very effective in inducing growth delay of both, unirradiated normoxic and hypoxic lung cancer cells (Table 1), so we moved forward with clinical application of bystander/abscopal effect. In all the treated patients, a significant bystander effect after mean time of 3 weeks and in 1 of the patients significant abscopal effect was also observed (Figure 2). Overall response rates for symptom relief and mass response were 100% (1 complete and 4 good partial response).No patient experienced acute or late toxicity of any grade.

Conclusion Considering the clinical benefit/toxicity ratio, the clinical exploitation of biological properties of bystander/abscopal effect induced by partial irradiation of large tumor masses, and almost any dose distribution to the normal tissue outside the irradiated tumor, could make bystander/abscopal effect at least more effective than conventional radiation therapy for treatment of advanced cancers and the perfect treatment option for symptomatic patient. Further, by inducing the distal responses, like in the case of one of the patients,