Differential remodeling of the oxylipin pool following FLASH vs. conventional dose-rate irradiation in vitro and in vivo.

Ultrahigh performance liquid chromatography coupled to tandem mass spectrometry was used to quantify the expression of 37 oxylipins derived from eicosatetraenoic, eicosapentaenoic and docosahexaenoic acid in mouse lung and in normal or cancer cells exposed to ultrahigh (FLASH) vs. conventional dose-rate irradiation (CONV) under precise monitoring of the temperature and oxygenation. Among the 37 isomers assayed, 14-16 were present in high enough amount to enable quantitative analysis. The endpoints were the expression of oxylipins as a function of the dose of radiation, hypoxia, temperature and post-irradiation time. In normal, normoxic cells at 37°C radiation elicited destruction and neosynthesis of oxylipins acting antagonistically on a background subject to rapid remodeling by the oxygenases involved in the metabolism of polyunsaturated fatty acids. Neosynthesis occurred in the CONV mode only, in such a way that the level of oxylipins at 5 min after FLASH irradiation was 20-50% lower than in non-irradiated and CONV-irradiated cells. This “FLASH effect” was abolished by lowering the temperature to 20°C and altered by hypoxia in some oxylipins. These patterns were not reproduced in cancer cells, and temperature effects evoke a major difference between normal and tumor cells with regard to the formation and decay of free radical intermediates in phospholipid membranes. Down-regulation of specific oxylipins following FLASH irradiation was also observed in mouse lung at 5 min following irradiation, with near complete recovery in 24 hours and further remodeling at one week and two months post-irradiation.