ESTRO 37 Abstract book

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ESTRO 37

rates. Hypoxic stress leads to the activation of several adaptive pathways including the unfolded protein response (UPR) and autophagy. Autophagy is a pro- survival mechanism that degrades cellular components and provides nutrients to cells exposed to stressful environments. Hypoxia and the UPR promote autophagy activation through increased expression of the autophagy initiator kinase ULK1. The importance of autophagy activation through ULK1 and its contribution to hypoxia tolerance and the hypoxic tumour microenvironment in pancreatic cancer is unclear. The purpose of this study was to identify the underlying mechanisms through which ULK1 may promote hypoxia tolerance and consequently influence the tumour microenvironment. We used pancreatic cancer cell lines and patient-derived organoid models to investigate this question in vitro and in vivo. ULK1 was repressed using inducible lentiviral-shRNAs or the small molecule ULK1 kinase inhibitor 6965. First, we found that ULK1 is particularly important for the survival of pancreatic cancer cell lines and patient-derived organoids, and that its depletion led to increased endoplasmic reticulum (ER) mass, ER stress, and UPR activation. Inhibition of ULK1 in patient-derived pancreatic tumour organoids, using genetic and pharmacologic means, significantly decreased the organoid formation efficiency in hypoxia. Second, we uncovered a mechanism through which ULK1 can modify the oxygenation levels of a whole tumour by regulating the mitochondrial content and subsequent oxygen consumption. We found that loss or inhibition of ULK1 leads to dramatic increases in both mitochondrial content and oxygen consumption. In vivo immunohistochemical analysis demonstrated that this resulted in steeper oxygen gradients and more rapid development of tumour hypoxia. However, ULK1 loss also sensitized hypoxic cells to ER stress and cell death, resulting in increased areas of necrosis. The dual effects of ULK1 inhibition on the mitochondria and the ER resulting in more hypoxia, but decreased hypoxia tolerance resulted in dramatic tumour regression of pancreatic cancer xenografts. Together, our data suggest that ULK1 and autophagy promote hypoxia tolerance through two distinct mechanisms. Upon ULK1 knockdown, loss of mitochondrial homeostasis leads to increased oxygen consumption, and enhanced severity of hypoxia. At the same time, loss of ER homeostasis exacerbates hypoxia induced ER stress, reducing the tolerance of cells to hypoxia and causing a substantial tumour growth delay. This new mechanistic understanding of the importance of autophagy provides new opportunities for development of hypoxia directed therapies. SP-0338 Targeting oxygen consumption G. Higgins 1 1 Higgins Geoff, Department of Radiation Oncology, Oxford, United Kingdom Abstract text Previous attempts to improve clinical outcomes of patients treated with radiotherapy by overcoming tumour hypoxia have met with limited success. To date, no such treatment is in widespread clinical use. Most hypoxia modification strategies such as the use of oxygen mimetics, hyperbaric oxygen, and carbogen have sought to increase oxygen ‘supply’ to tumours. Pre- clinical and mathematical modelling data suggests that decreasing oxygen ‘demand’ may be a more effective way to reduce tumour hypoxia. Drugs that reduce mitochondrial oxygen consumption at clinically relevant concentrations might therefore be able to significantly reduce tumour hypoxia, leading to increased tumour radiosensitivity. I will discuss results from two drugs that reduce OCR: 1) We have identified that atovaquone, a safe, and widely used antimalarial drug reduces the oxygen

consumption rate (OCR) of tumour cells via inhibition of complex III of the electron transport chain. Atovaquone causes rapid reduction of hypoxia in both 3D spheroid, and subcutaneous xenograft models; and shows marked radiosensitisation in tumour regrowth delay experiments. This work has led to the establishment of a proof-of- principle, window of opportunity clinical trial in non- small cell lung cancer patients. This study will utilise functional imaging data, along with putative circulating and tissue markers of hypoxia to assess whether standard clinical doses of atovaquone are able to reduce tumour hypoxia. 2) Buparlisib (BKM120), a highly specific pan class-1 PI3K inhibitor, has been shown to reduce tumour hypoxia in vivo, due to a reduction in OCR, as well as by ‘vascular normalisation’. A window-of-opportunity study combining Buparlisib with palliative thoracic radiotherapy has recently been completed. 18 F-Misonidazole PET-CT scans were obtained pre- and post Buparlisib treatment to assess whether this drug is able to reduce hypoxia in non- small cell lung cancer patients. An outline of this imaging data will be presented. SP-0339 Targeting hypoxia with DNA repair inhibitors R. Syljuasen 1 1 Norwegian Radium Hospital/ Oslo University Hospital, Department of Radiation Biology- Institute for Cancer ResearchSyljuåsen, Oslo, Norway Abstract text Tumor hypoxia represents a major obstacle to radiotherapy due to the oxygen effect. On the other hand, hypoxia is a tumor specific feature that may be exploited to achieve tumor selective treatment. Here, I will review how inhibitors of DNA repair and DNA damage signaling may selectively kill hypoxic cells, sparing the surrounding normal tissue. For example, hypoxia-induced replication stress or hypoxia-mediated downregulation of DNA repair proteins can cause increased effects of such inhibitors. Secondly, I will focus on our previous and new results with inhibitors of the ATR, Chk1 and Wee1 checkpoint kinases (VE822, AZD6738, MK1775, AZD7762, UCN01). These results show either similar or increased effects of these inhibitors in cancer cells grown at hypoxic compared to normoxic conditions. SP-0340 Model-based clinical validation of proton therapy in head and neck cancer. A. Lin 1 , J.C. Rwigema 2 , J. Langendijk 3 , J. Lukens 1 , S. Swisher-McClure 1 , J. Langendijk 3 1 University of Pennsylvania, Radiation Oncology, Philadelphia, USA 2 Mayo Clinic- Scottsdale, Radiation Oncology, Scottsdale, USA 3 University Medical Centre- Groningen, Radiation Oncology, Groningen, The Netherlands Many patients with head and neck cancer are cured after undergoing definitive multimodality therapy. Despite technological advances and improvements in radiotherapy delivery, many of these patients experience severe or permanent toxicity that negatively impacts quality of life. Single-institution data have suggested advantages of proton beam therapy (PBT) over intensity- modulated radiotherapy (IMRT), and randomized trials are ongoing to provide level I evidence establishing the clinical benefit of PBT. However, performing comparative randomized trials for new technology is increasingly Abstract text Purpose Symposium: Proton therapy in head and neck cancer: patient selection, validation and future directions