Abstract: A solid tumor is like an aberrant organ - comprised of cancer cells and a variety of host cells embedded in an extra-cellular matrix - nourished by blood vessels and drained by lymphatic vessels. Using an array of imaging technologies and animal models, we have shown that blood and lymphatic vessels as well as matrix associated with tumors are abnormal and these abnormalities can create a hostile tumor microenvironment (e.g., hypoxia, high interstitial fluid pressure). These abnormalities can fuel tumor progression as well as prevent treatments from reaching and attacking tumor cells (Nature 2011). In my presentation, I will discuss two emerging strategies to “normalize” the tumor microenvironment to enhance the outcome of radiation, chemo- and immuno-therapies. Vascular normalization hypothesis: After realizing that the abnormal structure and function of tumor vessels is a result of the imbalance between endogenous pro- and anti-angiogenic molecules, we proposed a novel hypothesis: Anti-angiogenic therapy can transiently “normalize” the abnormal tumor vasculature, resulting in more efficient delivery of drugs and oxygen to cancer cells. We also hypothesized that chemo-, immuno- and/or radiation therapy given during this “window of normalization” is likely to yield the best outcome for combination therapy (Nature Medicine 2001; Science 2005). This hypothesis offered a potential explanation for why drugs, such as bevacizumab (whose goal is to destroy tumor vessels) improve the outcome of chemotherapeutics (that require blood vessels for drug delivery), and more importantly, offered guidelines to improve such combination therapies. We first tested this hypothesis in a variety of pre-clinical models (PNAS 1996; 1998; Nature 2002; Cancer Research, 2004; Cancer Cell, 2004). Our work demonstrated that blockade of VEGF signaling or upregulation of thrombospondin transiently prunes the immature and leaky vessels of tumors in mice and actively remodels the remaining vasculature so that it more closely resembles the normal vasculature. This “normalized” vasculature is characterized by less leaky, less dilated, and less tortuous vessels, with a more normal basement membrane and greater vessel coverage by pericytes. These morphological changes are accompanied by functional changes: decreased interstitial fluid pressure, decreased tumor hypoxia, and improved penetration of drugs in these tumors. The outcome of combination therapy was found to be synergistic when the cytotoxic therapy was given during the normalization window (Cancer Cell 2004). We then dissected the molecular and cellular mechanisms of vascular normalization (Cancer Cell, 2004). We discovered that an increase in angiopoietin 1 expression contributes to the increased pericyte coverage and an increase in MMP activity contributes to the basement membrane normalization. We further showed that the kinetics of vascular normalization determines the outcome of combined antiangiogenic and radiation therapy (Cancer Cell, 2004). After careful and rigorous characterization of tumor vasculature in pre-clinical models, in collaboration with medical, surgical and radiation oncologists, we evaluated the molecular, structural and functional changes in the vasculature of rectal carcinomas in patients receiving bevacizumab with radiation and chemotherapy. This study provided the first glimpse of how anti-angiogenic therapy might work in patients (Nature Medicine, 2004), and supported our pre-clinical findings on vascular normalization. In collaboration with neuro-oncologists and neuro-radiologists, we also demonstrated vascular normalization in recurrent glioblastoma in patients receiving an oral pan-VEGF receptors tyrosine kinase inhibitor - cediranib - and discovered that the extent of normalization correlates with both progression-free and overall survival (Cancer Cell 2007; Cancer Research 2009). More crucially, patients whose tumor blood flow went up - and not down - after cediranib treatment survived longer (Cancer Research 2012). Our pre-clinical and clinical findings on normalization have been confirmed by a number of laboratories worldwide, and represent a paradigm shift for the field of anti-angiogenesis therapy (Physiological Reviews 2011). Vascular normalization offers unprecedented opportunities to improve treatment of various vascular disorders - besides cancer - that afflict millions of people worldwide, including wet age-related macular degeneration - a leading cause of blindness. Indeed, our team has demonstrated that vascular normalization using bevacizumab can reverse chronic progressive hearing loss in patients with schwannomas (neurofibromatosis type 2) (New England Journal of Medicine, 2009). Matrix normalization hypothesis: In parallel, our laboratory discovered that cancer cells co-opt the host stromal cells into producing pro- and antiangiogenic cytokines and extra-cellular matrix. By revealing that stromal cells are active participants rather than passive bystanders in tumor progression and therapeutic response (Cell 1998; Nature Medicine 1999, 2001; Nature 2002), we proposed these cells as a novel target for cancer therapy (Nature Medicine 2003). We also discovered that both cancer cells and stromal cells compress tumor vessels, and depleting these cells can open blood vessels (Nature 2004). By imaging collagen in vivo (Nature Medicine 2003) and directly measuring movement of drugs in tumors in vivo (Nature Medicine 2004), we discovered that collagen matrix produced by stromal cells can compress blood vessels and impede drug delivery. More importantly, the hormone relaxin and the FDA-approved anti-hypertensive drugs that target stromal cells can “normalize” the tumor microenvironment by reorganizing the collagen matrix (PNAS 2011). These findings offer new hope for improving delivery and efficacy of therapeutics in highly fibrotic tumors (e.g., desmoplastic breast cancer, pancreatic cancer). Citation Format: {Authors}. {Abstract title} [abstract] . In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr PL02-01. doi:1538-7445.AM2012-PL02-01