**10. Angiogenesis: Vascular Endothelial Growth Factor (VEGF) and Glioblastoma stem-like cells (GSC)**

As alluded to earlier in the chapter, angiogenesis is an essential part of the pathogenesis of malignant gliomas which, as a rule, are among the most vascularized of tumors [40, 41]. This is due to: (1) upregulation of genes encoding proangiogenic factors, which include VEGF, fibroblast growth factor (FGF), IL-8 and-6, hypoxia-inducible factor 1 alpha (HIF-1alpha) and angiopoietins, and (2) downregulation of angiogenesis inhibitors, including thrombospodins, angiostatin, endostatin and interferons. Indeed, the lynchpin that begets the transformation from low grade to high grade gliomas is induction of expression of the above-cited proangio‐ genic factors [40]. The most prominent and well-characterized of the proangiogenic factors is VEGF-A, commonly denoted simply as VEGF, which is directly secreted by tumor cells. VEGF exerts its function by binding the receptor VEGFR2 on endothelial cells nearby the tumor. This action initiates a paracrine signaling loop that results in the proliferation of endothelial cells and, as a result, neo-vasculature. Interestingly, the level of VEGF produced by a tumor is proportional to the degree of malignancy, the aggressiveness and poor outcome; high-grade tumors are found to have orders of magnitude more VEGF than low grade tumors [13, 42]. There has thusly been a significant amount of clinical research focus on anti-angiogenic therapy for malignant gliomas. Anti-angiogenic therapy has multiple hypothesized mechanisms of action for the treatment of malignant gliomas. The primary mechanism of action is direct cytotoxicity to endothelial cells, inducing apoptosis. This, by merit of the resultant attenuated blood supply, decreases oxygen and nutrient delivery to tumor cells which preempts further growth for a short period of time. A second hypothesized mechanism of action based upon the results of select clinical studies is that, when used alongside cytotoxic chemotherapeutic agents, anti-VEGF agents are thought to synergistically sensitize endothelial cells to penetra‐ tion by these cytotoxic agents. Intriguingly, it is also very likely that anti-VEGF agents work to counteract an upsurge of VEGF expression and endothelial cell recruitment observed with the tumoral insult caused by chemotherapy and radiation. Another hypothesis is that, during a discrete window of time after administration, anti-VEGF agents elicit a phenomenon known as "vascular normalization" during which there is reduced vessel diameter/permeability, improved vessel perfusion, a reduction in tumor interstitial pressure, and improved tumor oxygenation. Summarily, these changes all translate into an observed improvement in the delivery and efficacy of cytotoxic chemotherapy. A newer and exceedingly compelling hypothesis is that antiangiogenic agents appear to exercise antagonism to Glioblastoma stemlike cells (GSCs). GSCs play an inextricable part in the angiogenic potentiation of malignant gliomas. They appear to contribute to the resistance that glioblastoma is known to have to cytotoxic chemotherapy treatment by augmenting the repair of DNA damaged by cytotoxic agents and activating the DNA damage checkpoint response system. Antiangiogenic therapy appears to antagonize the functionality of GSCs by merit of GSCs embodying a categorically structural and functional vascular niche in the tumoral micro-milieu. GSCs have been found to upregulate VEGF expression, instigate formation of very angiogenic tumors in animal models, and bear a predilection for stem cell hot beds in areas around endothelial cells. In the self-same animal models, antiangiogenic agents appear to fundamentally disrupt the struc‐ tural framework of the hot beds in which GSCs reside and resultantly provoke GSC death.

The canonical agent that has garnered the most investigation and use is the humanized anti-VEGF monoclonal antibody bevacizumab, originally used for treating colorectal cancer and also used routinely for metastatic lung adenocarcinoma. Its use for CNS tumors, recurrent gliomas in particular, was conceived of after improved outcomes were noted when it was used in conjunction with chemotherapy for colorectal and lung cancers. Recurrent gliomas have historically had a low radiographic response rate after re-exposure to temozolomide after failing initial therapy, ranging from only 5-8%. However, in the very first published study of the use of bevacizumab with irinotecan, a radiographic response rate of 66% (19 of 29 patients) was found [40]. This was followed by a number of retrospective studies on recurrent gliomas which showed progression free survival at 6 months (PF6) of 32-64% with bevacizumab vs. 21% PF6 rate for temozolomide [13]. The aggregate of these very positive results prompted two phase 2 trials designed for the purposes of fast track FDA approval of bevacizumab for the indication of recurrent gliomas. These two trials corroborated the prior results--a signifi‐ cant radiographic response rate and increase in PF6. The largest bevacizumab trial to date, designated the BRAIN trial and conducted by Freidman et al in 2009, randomized patientswith glioblastoma either after first or second recurrence to be treated with bevacizumab alone(n=85) or bevacizumab plus irinotecan (n=82). Both response rates (using MacDonald response criteria) and six month progression free survival were markedly higher when compared to historical controls in both groups (higher response rate in bevacizumab plus irinotecan group). However, overall survival was not statistically significant between the two groups at 9.2 months for bevacizumab alone and 8.7 months for the combination regimen. Bevacizumab was well tolerated. The most common or significant adverse events included thromboembo‐ licevents, hypertension and proteinuria.

is for this reason that many patients with brain tumors are maintained over long periods of time on high doses of corticosteroids, not without insubstantial side effects. Both clinical and radiographic studies have found reductions in peritumoral edema with the administration of anti-VEGF agents. This in turn allowed for reduction in corticosteroid use by as much as 50%, a significant benefit for many of the patients experiencing the multifarious ill effects of chronic

High Grade Glioma — Standard Approach, Obstacles and Future Directions

http://dx.doi.org/10.5772/58548

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Despite the many promising aspects of antiangiogenic therapy outlined above, there have recently come to the fore many challenges that remain to the use of antiangiogenic agents for the treatment of malignant gliomas. One challenge that has become evident is based upon, in a matter of speaking, "tipping the balance" too far towards antiangiogenesis by significant use of antiangiogenesis agents. Animal and human biopsy results have showed that when used overzealously, antiangiogenic therapy actually works to promote a hypoxic environment within the tumor bed. Tumor hypoxia has been well established as a formidable means of resistance to chemotherapy and radiation [38]. Pre-clinical data obtained from xenograft models suggests that hypoxia and the hypoxia-inducible factors (HIFs) play a central role in maintaining the stem-like fraction in gliomas. This is achieved by providing the essential cellular interactions and signals needed to arrest differentiation of these stem-like cells. These interactions lead to stem-like cell survival and self-propagation. Upregulation of HIF-1alpha

**11. Hypoxia as means of resistance and poor outcome**

corticosteroid use [41].

It is important to remind the reader here of the interdependence between angiogenesis and the propagation of peritumoral edema in malignant gliomas. In fact, the original name for VEGF was vascular permeability factor due to its increasing the permeability of tumor vessels, which leads to the phenomenon of vasogenic brain edema. Vasogenic brain edema is a telltale hallmark of malignant gliomas and qualifies much of the morbidity associated with them. It

is for this reason that many patients with brain tumors are maintained over long periods of time on high doses of corticosteroids, not without insubstantial side effects. Both clinical and radiographic studies have found reductions in peritumoral edema with the administration of anti-VEGF agents. This in turn allowed for reduction in corticosteroid use by as much as 50%, a significant benefit for many of the patients experiencing the multifarious ill effects of chronic corticosteroid use [41].
