Introduction: Glioblastoma is an aggressive and highly vascular tumor with median survival less than 2 years. Anti-angiogenic agents such as bevacizumab target vascular endothelial growth factor (VEGF). Bevacizumab has been effective in preclinical models though fails to prolong overall survival in phase III trials for newly diagnosed and recurrent glioblastoma. The mechanisms underlying resistance to VEGF inhibition remain unknown. We tested the hypothesis that inducible genetic ablation of proliferating blood vessels would increase the anti-tumor effect of VEGF inhibition and radiotherapy by simultaneously targeting alternative pathways for angiogenesis.

Methods: To inhibit proliferating tumor blood vessels, a transgenic mouse model was used where conditional expression of cytotoxic thymidine kinase (HSV1-TK) was achieved specifically in vascular endothelial cells under control of the Tie2 promoter. Syngenic NRAS/p53 ablated brain tumors were induced in these transgenic mice by transposon mediated integration of plasmid DNA into the subventricular zone. Systemic ganciclovir (GCV) administration kills proliferating TK expressing vascular endothelial cells and slows tumor growth. This model was combined with anti-VEGF antibody administration or radiation therapy to test synergistic treatment efficacy. Survival analysis and immunohistochemistry was performed on post-mortem specimens.

Results: GCV administration reduced tumor growth and vascular density, increased tumor apoptosis, and prolonged survival, as did anti-VEGF therapy or radiation. Paradoxically, adding GCV to radiation, or to anti-VEGF therapy, reduced the individual therapeutic effects of these modalities.

Conclusions: Inhibition of proliferating microvascular endothelium, anti-VEGF therapy, or radiation all individually improve survival. However, elimination of microvascular proliferation in combination decreased the efficacy of anti-VEGF or radiation therapy. These results indicate that the integrity of proliferating vessels may be necessary for the anti-glioma effects of bevacizumab and radiation therapy. This may have clinical significance in patients undergoing combined and overlapping treatments when a regimen includes anti-angiogenics. These findings may also have implications for the development of future anti-angiogenic agents.

Patient Care: This research improves patient care by studying mechanisms of glioma treatment resistance to anti-angiogenic therapy. It takes a novel transgenic approach and reveals unexpected findings about the influence of targeting proliferating vascular endothelium in combination with radiation and bevacizumab. These basic science findings provide important data for future translational investigations and for the development of novel targeted therapeutics.

Learning Objectives: By the conclusion of this session, particpants should be able to: 1) describe the current status of anti-angiogenic therapy for glioma, 2) Discuss, in small groups possible theories why anti-angiogenics have not succeeded in extending overall survival, 3) Identify an effective treatment approach that combines multi-modal targeted strategies.

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