Dual inhibition of the PI3K/mTOR pathway increases tumor radiosensitivity by normalizing tumor vasculature Full author names

Running Title Vascular normalization by PI3K/mTOR inhibitors Author manuscripts have been peer reviewed and accepted for publication but have not yet been edited. ABSTRACT The aberrant vascular architecture of solid tumors results in hypoxia that limits the efficacy of radiotherapy. Vascular normalization using anti-angiogenic agents has been proposed as a means to improve radiation therapy by enhancing tumor oxygenation, but only short-lived effects for this strategy have reported so far. Here we show that NVP-BEZ235, a dual inhibitor of phosphoinoside-3-kinase (PI3K) and mTOR, can improve tumor oxygenation and vascular structure over a prolonged period that achieves the aim of effective vascular normalization. Because PI3K inhibition can radiosensitize tumor cells themselves, our experimental design explicitly distinguished effects on the blood vasculature versus tumor cells. Drug administration coincident with radiation enhanced the delay in tumor growth without changing tumor oxygenation, establishing that radiosensitiztion is a component of the response. However, the enhanced growth delay was substantially greater after induction of vascular normalization, meaning that this treatment enhanced the tumoral radioresponse. Importantly, changes in vascular morphology persisted throughout the entire course of the experiment. Our findings indicate that targeting the PI3K/mTOR pathway can modulate the tumor microenvironment to induce a prolonged normalization of blood vessels. The substantial therapeutic gain observed after combination of NVP-BEZ235 with irradiation has conceptual implications for cancer therapy and could be of broad translational importance.