MOVING BEYOND VEGF: INHIBING GLIOMA ANGIOGENESIS BY TARGETING THE TIE2/ANGIOPOIETIN SIGNALING PATHWAY

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Abstract

BACKGROUND: Angiogenesis inhibitors have evolved in the past decade as one of the most promising biology based therapeutic strategies. Promising pre-clinical studies (published in 1993 and 1994) provided proof of principle that blocking of VEGF dramatically inhibits tumor growth. These observations led to the successful development of inhibitors for VEGF and VEGF receptors by various pharmaceutical companies. Bevacizumab, a monoclonal antibody neutralizing VEGF, was the first-in class drug to achieve FDA approval for the treatment of colorectal carcinoma in 2004 and later on in other cancer types, including glioblastoma. Anti-VEGF therapy led to an increase in progression free survival in recurrent and primary GBM, but its role in first-line treatment is less clear. Importantly, several preclinical studies suggested that resistance to anti-angiogenic therapy might evolve in the course of the treatment due to an infiltration of specially polarized myeloid cells. METHODS: In order to define new options for anti-angiogenic therapy we investigated the Tie2/Angiopoietin (Angpt) signaling pathway in human and murine glioblastomas by gene expression analysis. Functional studies were performed in GFAP-tTA/Tetos-Angpt1 and Tie1-tTA/Tetos-Angpt2 transgenic mice. In addition, Tie2/Angpt signaling was modulated by pegulated Angpt1 mimetics and peptide based Angpt2 blockers. Finally, anti-Angpt therapy was combined with anti-VEGF therapy. RESULTS: In more than 200 human brain tumor specimens investigated, Angpt2 protein expression was found to be up-regulated in tumor endothelial cells whereas it was absent in the normal brain. Further, Angpt2 expression levels correlated with WHO grade as well as the number of infiltrating monocytes/macrophages. Patients with GBM displayed increased pre-op serum levels of Angpt2. Notably, transgenic mice with endothelial cell-specific expression of Angpt2 showed a significant higher number of infiltrating myeloid cells, thus corroborating our findings in human GBM. These findings suggest, that Angpt signaling links inflammation and angiogenesis in GBM and may thus provide a novel therapeutic target.In a GL261 orthotopic syngeneic glioma model, interference with Angpt signaling led to a significant increase in survival, increased pericyte coverage and decreased myeloid cell infiltration. Combining anti-Angpt with anti-VEGF based therapies showed synergistic effects. CONCLUSIONS: These findings strongly argue for targeting the Tie2/Angpt signaling pathways in human GBM, either alone or in combination with anti-VEGF therapies. SECONDARY CATEGORY: Preclinical Experimental Therapeutics.

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