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BACKGROUND Chronotherapy is the optimally timed administration of treatment based on the circadian clock. In non-CNS cancers, circadian rhythms (CR) drive the expression of mechanisms responsible for radiation sensitivity and associated toxicity. Our previous work has shown that cranial irradiation in GBM patients produces a high incidence of hypersomnolence and susceptibility is linked to circadian clock gene polymorphisms. This study determined if CR can be demonstrated using in vitro GBM and CNS cells and then if sensitivity to radiation is circadian time and tissue dependent. METHODS Rat GBM, fibroblasts, and suprachiasmatic nucleus (SCN) cells were used and compared to human U251 GBM cells. Cells were synchronized using the serum shook method; protein levels for PER2 expression over 24 hr in 4 hr intervals were quantified to confirm CR. Colony formation and 3D invasion assays after radiation (0-8Gy or 8Gy, respectively) 8 and 20 hrs after synchronization were evaluated. RESULTS All cell types demonstrated CR by PER2 expression (trough ~8hr post (CT8); peak ~20hrs post (CT20)). In rat, GBM (LD50 = 4Gy) and SCN cells (LD50 =5 Gy) showed lower sensitivity than fibroblasts (LD50= 2 Gy). Human U251 GBM cells had similar radiation sensitivity to rat GBM. Time-of-day effects were dependent on tissue type with both rat and human GBM cell lines more affected by radiation at CT20, with less invasion (CT20:-35488.5±2622.1 vs CT8:-23160.5±3273.4µm) and survival in the colony assay (F(1,71)=4.105,p&lt; 0.05). Conversely, SCN cells which drive sleep-wake, showed the converse with greater effect at the 8hr compared with 20hr (F(1, 35)=19.93,p&lt; 0.001). CONCLUSIONS These data suggest that timing of radiation could be optimized to improve detrimental effects on healthy tissue, while still providing effective intra-tumor doses. In future studies, we will interrogate the mechanisms driving time-of-day effects in cells and test radiation chronotherapy in our radiation-induced hypersomnolence mouse model.

RDNA-04. CIRCADIAN RHYTHMS AND RADIATION CHRONOTHERAPY IN GLIOBLASTOMA CELL LINES AND CENTRAL NERVOUS SYSTEM CELL CONTROLS