Abstract Book

S1274

ESTRO 37

Results In all four colorectal cancer cell lines, 5 µM Fatostatin enhanced the response to radiotherapy in terms of relative survival. In absence of radiotherapy, 5 µM Fatostatin only displayed significant toxicity in HCA7 and CaCo2 cells. When using the higher dose of 10 µM Fatostatin, little to no additional benefit of the combination treatment was observed in HCT116 and HCA7 cells as the drug itself already resulted in significant cytotoxicity. In HT29 and CaCo2 cells however, in analogy with 5 µM Fatostatin, 10 µM Fatostatin decreased relative survival when combined with increasing doses of ionising radiation.

Conclusion In four colorectal cancer cell lines with different molecular characteristics and varying levels of sensitivity to radiotherapy, we demonstrated a radiosensitising effect of the SREBP-1 inhibitor Fatostatin. Further research is currently being performed to unravel the underlying working mechanisms of Fatostatin as a radiosensitising agent and its potential in the multimodality treatment for rectal cancer. EP-2309 Irinotecan radiosensitizes p53-mutant colon cancer cells through G2/M arrest followed by apoptosis Y. Wang 1 , J. Zhang 1 , L. Yang 1 , L. Liang 1 , H. Zhang 1 , L. Shen 1 , R. Hu 1 , W. Yang 1 , Z. Zhang 1 1 Fudan University Shanghai Cancer Center, Department of Radiation Oncology, Shanghai, China Purpose or Objective Irinotecan, an analog of camptothecin, the inhibitor of Topoisomerase I, is currently used to treat metastatic colorectal cancer. Camptothecin derivatives were demonstrated to have radiosensitization effects on several kinds of cancer cells. However, few studies have been published in terms of colorectal cancer cell lines. In this study, we try to examine the radiosentization effects of irinotecan on p53 mutant colon cancer lines HT29, SW620 and explore the potential mechanisms. Material and Methods HT29 (radiation-resistant) cells and SW620 (radiation- sensitive) cells were chosen for the following experiments. First, 24h IC50s of irinotecan as a single agent were obtained through drug cytotoxicity tests. Concentrations below 10% IC50 were considered to be combined with radiation. Second, cells were treated with irinotecan (24h) followed by radiation at a dosage gradient, and radiosentization effects of irinotecan were examined through clonogenic assays. Next, cells were divided into four arms: control arm, irinotecan arm, radiation arm and the combined arm. Cell growth curves were obtained through CCK-8 assays. Foci of γH2AX were illustrated through immunofluorescence straining. Cell cycle and apoptosis were examined using Flow Cytometry. Expression of proteins related to DNA damage and repair system and cell cycle were examined through Western Blot. Results The 24h IC50s of irinotecan were 39.84µg/ml (HT29 95% CI 38.27-41.48) and 96.86µg/ml (SW620 95%CI 89.04- 105.4), thus concentrations below 4µg/ml were chosen for the experiments. Clonogenic assays showed that irinotecan had radiosentization effects on HT29 and SW620 cells, and the SERs increased with the increase of concentration (SER= 1.41 for HT29 cells, SER=1.75 for SW620 cells, with irinotecan at 2µg/ml, Figure 1a, b). Compared with the control arm, irinotecan arm and radiation arm, the combined arm had the slowest cell growing rate (Figure 1c, d) and the most obvious foci of γH2AX (not shown). The combined treatment could result in the most significant G2/M phase arrest and last for about 48h, followed by the most significant increase of apoptosis (Figure 1e-h). And results of Western Blot indicated that the expression of proteins related to DNA damage and repair system (p-ATM ， p-Chk1 ， p- Chk2 ， γH2AX) ， cell cycle (p-Cdc2 ， Cyclin B1) increased the most in the combined arm (Figure 2). In addition, we found that the expression of p-Cdc25c also increased the most in the combined arm, which indicating that irinotecan radiosentized the p53-mutant HT29 and SW620 cells probably through ATM/Chk/Cdc25c/Cdc2 pathway.