Hypoxia Tumor reoxygenation following administration of Mitogen-Activated Protein Kinase inhibitors: A rationale for combination with radiation therapy Oussama Karroum, Julie Kengen, Pierre Danhier, Julie Magat, Lionel Mignion, Caroline Bouzin, Julien Verrax, Nicolas Charette, Peter Starkel, Pedro Buc Calderon, Pierre Sonveaux, Oliver Feron, Vincent Grégoire, Bernard Gallez, Bénédicte F. Jordan ⇑ Université Catholique de Louvain, Brussels, Belgium article info Article history: Received 13 December 2011 Received in revised form 12 April 2012 Accepted 14 May 2012 Available online 8 June 2012 Keywords: MAPK Tumor Oxygenation Radiosensitivity O 2 consumption abstract Background and purpose: The relevance of Mitogen Activated Protein Kinase (MAPK) inhibitors as co- treatments for radiation therapy is investigated, with special focus on a potential link between the MAPK pathway and tumor hypoxia, which is a critical determinant for response to therapy. Materials and methods: The effects of two MAPK inhibitors, Sorafenib and PD0325901, were monitored daily using in vivo EPR (Electron Paramagnetic Resonance) oximetry in FSaII and TLT tumor models in order to identify a window of reoxygenation, during which tumor blood flow, oxygen consumption and radiation sensitivity were assessed. Results: Reoxygenation was shown after two days of treatments with Sorafenib or PD0325901 in two tumor models, which was further successfully exploited with Sorafenib for improving the radiation response of FSaII tumors by a factor of 1.5. The increase in tumor oxygenation was shown to be the result of two major factors: (i) an increase in blood flow for Sorafenib, that might be linked to its anti-angiogenic effect (vascular normalization), and (ii) a decrease in oxygen consumption for Sorafenib and PD0325901, due to an alteration of the mitochondrial activity. Conclusion: We evidenced tumor reoxygenation in vivo following MAPK inhibition and suggest a ratio- nale for the combination of radiation therapy with Sorafenib. Ó 2012 Elsevier Ireland Ltd. All rights reserved. Radiotherapy and Oncology 105 (2012) 64–71 Hypoxia has emerged as an important factor in tumor biology and in the response to cancer treatment since it has been corre- lated with angiogenesis, tumor aggressiveness, local recurrence, and metastasis [1–3]. Because of the so-called ‘‘oxygen enhance- ment effect’’, the radiation dose required to achieve the same bio- logical effect is about three times higher in the absence of oxygen than in the presence of normal levels of oxygen [4,5]. Hypoxic tu- mor cells, which are therefore more resistant to radiotherapy than well oxygenated ones, remain clonogenic and contribute to the therapeutic outcome of fractionated radiotherapy [6]. Direct evi- dence of hypoxia in human cancer has been largely demonstrated in the past [2,7]. A particular area of interest is therefore to com- bine radiotherapy with co-treatments that are able to transiently increase tumor pO 2 at the time of irradiation [8–10]. Farnesyl-transferase inhibitors (FTI), which are described as RAS inhibitor have been shown to be able to reduce tumor hypoxia in experimental models using the hypoxic cell marker pentafluorinat- ed 2-nitroimidazole [11]. FTI have also been shown to potentiate the effect of radiation therapy in independent studies [12]. Activa- tion of RAS leads to the activation of several effector pathways, the best characterized of which being the RAF/MEK/ERK pathway or ‘‘classical Mitogen-Activated Protein Kinase’’ pathway [13]. This pathway, generally expressed in all cell types, transduces extracel- lular stimulations, e.g. growth factors, cytokines and stress signals, into intracellular responses [14]. MAPK have a central role in inte- grating extracellular stimulations into various cellular activities, such as gene expression, cell proliferation, survival and migration [15]. Phosphorylation and consecutive activation of ERK regulates the transcription of target genes that promote cell cycle progres- sion and cell survival [13]. MAPK inhibitors have subsequently been developed and are currently used in the clinics in different types of cancers [13–15]. Until now, no study focused on the link between a potential tumor reoxygenation following the administration of MAPK inhib- itors and tumor radiosensitization. This was the aim of the current work which was designed to study the effect of MAPK inhibitors in terms of tumor oxygenation and radiation response. 0167-8140/$ - see front matter Ó 2012 Elsevier Ireland Ltd. All rights reserved. http://dx.doi.org/10.1016/j.radonc.2012.05.005 ⇑ Corresponding author. Address: Biomedical Magnetic Resonance Group, Lou- vain Drug Research Institute, Université Catholique de Louvain, Avenue Mounier 73.08, 1200 Brussels, Belgium. E-mail address: Benedicte.jordan@uclouvain.be (B.F. Jordan). Radiotherapy and Oncology 105 (2012) 64–71 Contents lists available at SciVerse ScienceDirect Radiotherapy and Oncology journal homepage: www.thegreenjournal.com