ESTRO 2024 - Abstract Book

S5281 ESTRO 2024 Mark R Jackson 1 , Rodrigo Gutierrez-Quintana 1 , Duncan Forster 2 , David J Walker 1 , Karin Williams 1 , Michael K Harte 3 , Kaye J Williams 3 , Anthony J Chalmers 1 1 University of Glasgow, School of Cancer Sciences, Glasgow, United Kingdom. 2 University of Manchester, Manchester Molecular Imaging Centre, Manchester, United Kingdom. 3 University of Manchester, Manchester Cancer Research Centre, Manchester, United Kingdom Radiobiology - Normal tissue radiobiology

Purpose/Objective:

Radiotherapy (RT) is the central component of the treatment of brain tumours including glioblastoma (GBM). Cure of GBM is rarely achieved, leading to median survival of 12-14 months. RT dose escalation is prohibited by irreversible, late-onset adverse effects on the normal brain, including global atrophic changes. As a result, inhibitors of key DNA damage response (DDR) proteins, including the hub kinase ATM, have been developed as potent radiosensitizers of GBM cells. The effect of this radiosensitization strategy on late normal brain radiotoxicity is unknown, and must be understood to ensure a benefit exists in terms of therapeutic index. Intriguingly, preclinical evidence suggests that inhibition of ATM may have differential effects in tumours and normal brain cells. In the non-proliferating normal tissue context, ATM inhibition may decrease pro-inflammatory signalling via disruption of NFkB activation [1]. Since chronic neuroinflammation is thought to be a central mechanism contributing to the neurotoxic effects of RT [2], inhibition of ATM may simultaneously radiosensitize tumour cells while offering protection against neurocognitive decline.

This work aims to explore mechanisms of RT-induced neurotoxicity and define the normal brain effects of combined RT and ATM inhibition, with a view to inform clinical use of this promising therapeutic approach.

Material/Methods:

Non-tumour bearing 6-week-old female C57BL/6 mice were treated with the ATM inhibitor AZD1390 (10 mg/kg) or vehicle for a total of 7 days. Mice were exposed to a single dose of 20 Gy hemibrain X-ray or sham-irradiation 24 hours after the first drug treatment, planned and delivered using a small animal radiation research platform (SARRP, Xstrahl). Mice were sacrificed, and brains harvested and formalin-fixed 24 hours and 50 days post-RT, to represent acute and late events. Brains were coronally sectioned (4 mm thickness), and immunohistochemically stained using antibodies specific for markers of different cell populations (Ki67, Sox2, DCX, TSPO, Iba1). Novel object discrimination behavioural analysis was performed on 6-month-old non-tumour bearing male C57BL/6 mice, treated as above, for up to 6 months post RT.

Results:

Fifty days post-RT, brains of mice exposed to 20 Gy exhibited unilateral marked reduction in proliferating (Ki67+) and precursor (DCX+) cells in the subventricular zone (SVZ) and hippocampal stem cell niches. The proportion of Sox2+ putative neural stem cells in these brain regions was modestly reduced 50 days after RT. Neither effect was modified by combination treatment with AZD1390. When used as a single agent but not in combination with RT, AZD1390 reduced staining for TSPO, a marker of neuroinflammation, in the SVZ. Staining for the microglia/macrophage marker Iba1 revealed that the combination of RT and AZD1390 caused a marked and bilateral reduction in cell density in hippocampus, cortex and subcortical regions 50 days post-RT (Figure 1). Acutely (24 hours post-RT) the RT-AZD1390-associated decrease in Iba1+ cell density was confined to the irradiated hemisphere (Figure 2). Morphological analyses of Iba1+ cells indicated that the remaining microglia/macrophage population exhibited increased ramification, indicative of reduced activation