Angiogenesis and Its Targeting in Glioblastoma with Focus on Clinical Approaches

Angiogenesis is a characteristic of glioblastoma (GBM), the most fatal and therapeutic-resistant brain tumor. Highly expressed angiogenic cytokines and proliferated microvascular system made anti-angiogenesis treatments a thoroughly plausible approach for GBM treatment. Many trials have proved to be not only as a safe but also as an effective approach in GBM retardation in a certain time window as seen in radiographic response rates; however, they have failed to implement significant improvements in clinical manifestation whether alone or in combination with radio/chemotherapy. Bevasizumab, an anti-vascular endothelial growth factor-A (VEGF-A) antibody, is the only agent that exerts meaningful clinical influence by improving progression-free survival (PFS) and partially alleviate clinical symptoms, nevertheless, it could not prolong the overall survival (OS) in patients with GBM. The data generated from phase II trials clearly revealed a correlation between elevated reperfusion, subsequent to vascular normalization induction, and improved clinical outcomes which explicitly indicates anti-angiogenesis treatments are beneficial. In order to prolong these initial benefits observed in a certain period of time after anti-angiogenesis targeting, some aspects of the therapy should be tackled: recognition of other bypass angiogenesis pathways activated following anti-angiogenesis therapy, identification of probable pathways that induce insensitivity to shortage of blood supply, and classifying the patients by mapping their GBM-related gene profile as biomarkers to predict their responsiveness to therapy. Herein, the molecular basis of brain vasculature development in normal and tumoral conditions is briefly discussed and it is explained how "vascular normalization" concept opened a window to a better comprehension of some adverse effects observed in anti-angiogenesis therapy in clinical condition. Then, the most targeted angiogenesis pathways focused on ligand/receptor interactions in GBM clinical trials are reviewed. Lastly, different targeting strategies applied in anti-angiogenesis treatment are discussed.