Angiogenesis in glioblastoma multiforme: navigating the maze

Blood vessel formation is a fundamental process that occurs during both normal and pathologic periods of tissue growth. In aggressive malignancies such as glioblastoma multiforme (GBM), vascularization is often excessive and facilitates tumor progression. In an attempt to maintain tumors in a state of quiescence, multiple anti-angiogenic agents have been developed. Although several angiogenesis inhibitors have produced enhanced clinical benefits in GBM, many of these pharmacologic agents result in transitory initial response phases followed by evasive tumor resistance. Thus, a significant need exists for the discovery of novel and effective anti-angiogenic therapies. The development of new molecular-targeted therapeutic strategies is often complicated by the complexity of angiogenic signal transduction. Due to the labyrinthine nature of these signaling pathways, increased production of other angiogenic factors may compensate for the inhibition of key vascular targets like vascular endothelial growth factor (VEGF). Such compensatory mechanisms facilitate vascularization and allow tumor growth to proceed even in the presence of anti-angiogenic agents. This review presents the challenges of targeting the intricate vascular network of GBM and discusses the clinical implications for recent advancements in targeted anti-angiogenic drug therapy.