ESTRO 2021 Abstract Book

S191

ESTRO 2021

Conclusion Hypoxia quantification in HCT116 xenografts demonstrated significant reoxygenation following H 2 O 2 injection. 3D spheroid data are in keeping with clinical reports demonstrating enhancement of radiation response by H 2 O 2 . Recapitulation of the clinical effect in vitro allows for further investigation of mechanisms of tumour response. IT H 2 O 2 represents a novel method of oxygen delivery and alleviating tumour hypoxia, which has implications for scheduling of H 2 O 2 relative to RT in the clinic. If efficacy of IT H 2 O 2 + RT is confirmed in the randomised phase II trial in breast cancer, it could improve treatment of several other tumour types where hypoxia is known to affect outcomes following RT. OC-0282 Inhibition of oxidative phosphorylation alters the tumor microenvironment in syngeneic mouse models D. Boreel 1 , P. Span 1 , A. Kip 2 , H. Peters 1 , R. van den Bijgaart 1 , S. Heskamp 3 , G. Adema 1 , J. Bussink 1 1 RadboudUMC, Radiation Oncology, Nijmegen, The Netherlands; 2 RadboudUMC, Medical Imaging, RadboudUMC, The Netherlands; 3 RadboudUMC, Medical Imaging, Nijmegen, The Netherlands Purpose or Objective When tumors outgrow their chaotic vasculature, limited diffusion of oxygen into remote tumor areas combined with increased oxygen consumption leads to hypoxia in most solid tumors. This scarcity of oxygen is known to induce radioresistance, but can also have a disrupting effect on the anti-tumor immune response. Hypoxia inhibits immune effector cell function, while immune cells with a more suppressive phenotype become more active. As opposed to the longstanding notion that tumors are primarily glycolytic, even under aerobic conditions as postulated by Warburg, it has recently been described that also oxidative phosphorylation (OXPHOS) remains active in many cancers. Using the novel mitochondrial complex I inhibitor IACS-010759, we aim to pharmacologically attenuate OXPHOS and subsequently decrease tumor oxygen consumption, hereby relieving hypoxia and activating anti-tumor immunity. Materials and Methods A panel of syngeneic murine cell lines and tumor models on a C57Bl/6 background was used (B16ova, MOC1, MC38 and GL261). In vitro oxygen consumption of these tumor cells was measured using the Agilent XF Seahorse Analyzer and cell growth was measured using the Incucyte ZOOM Live Cell Imaging system. Hypoxia and Immune marker expression by these cells in vitro and in vivo in the tumor microenvironment (TME) were determined using flow cytometry and immunohistochemistry. Results In vitro , IACS-010759 treatment potently inhibits oxygen consumption in a dose dependent manner (Fig. 1), while also inducing a 2-fold increase in phosphorylation of AMPK, an intracellular sensor of energetic stress. Under physiological glucose concentrations IACS-010759 reduces cell growth at least 2-fold more potently than under high glucose culture conditions, suggesting OXPHOS indeed is an important metabolic pathway in such an environment. In MOC1 tumors, hypoxic tumor areas showed less infiltration of F4/80+ and MHCII+ immune cells (Fig 2). Daily IACS-010759 treatment (10mg/kg) of mice bearing B16ova tumors for 4 days alters the immune composition of the TME in vivo . Preliminary data revealed an increase in Ly6C high MHCII high cells from 23.8 ± 9.5% to 49.9 ± 11.6% (p=0.02) in the myeloid cell population. These cells are thought to be the more classical macrophages that stimulate anti-tumor immunity. On the other hand, a reduction from 37.9 ± 9.5% to 8.9 ± 6.4% (p=0.006) was seen in Ly6G high neutrophils, cells that can attribute to an immunosuppressive phenotype.