Tumors require nutrients and oxygen in order to grow, and new blood vessels provide these requirements. GBM cells are characterized by their invasive abilities and striking angiogenic potential. The blood vessels formed by tumor cells are structurally and functionally abnormal: the blood vessels are leaky and dilated, the endothelial cells exhibit aberrant morphology, the pericytes are loosely attached or absent and the basement membrane is incomplete [14
]. These abnormalities lead to an abnormal tumor microenvironment that is characterized by interstitial hypertension, hypoxia and acidosis. The abnormal vasculature represents a barrier to the delivery and efficacy of anticancer therapeutic agents. These observations suggest that if the structure and function of tumor vessels could be “corrected,” then the tumor microenvironment might be normalized, ultimately improving the efficacy of cancer treatments.
As a key mediator of angiogenesis, VEGF and its receptors are targets for anticancer therapies [58
], in addition to conventional therapies. Targeting the cells that support tumor growth, rather than the actual tumor cells, represents a relatively new approach to cancer therapy. This approach is particularly promising because these support cells are genetically stable and therefore less likely to develop mutations that will allow them to develop drug resistance in a rapid manner.
A significant challenge for antiangiogenic therapy is to design combination protocols that can counteract the diverse angiogenic stimuli produced by the tumor and its microenvironment.
VEGF signaling inhibitors have been shown to significantly suppress or delay tumor growth in several animal models [59
] and in clinical trials. The humanized monoclonal anti-VEGF antibody bevacizumab is the first VEGF-targeting drug approved for use in patients with metastatic colorectal cancer [60
], metastatic breast cancer, lung cancer, renal cell carcinoma, and glioblastoma multiforme [61
VEGF expression is regulated by intrinsic and extrinsic factors. Hypoxia and hypoglycemia are major stimulators of VEGF expression [62
]. Factors that can potentiate VEGF production and stimulate angiogenesis include tumor necrosis factor and transforming growth factor.
Several approaches have been used to eliminate the hypoxic cells within tumors [63
3.1. Antiangiogenic Strategies
Angiogenesis inhibitors have been divided into two classes: direct and indirect [64
]. Direct angiogenesis inhibitors, such as endostatin, target the microvascular ECs, preventing their response to various proangiogenic stimuli and thereby enhancing the effects of chemotherapy.
In contrast, indirect angiogenesis inhibitors interfere with the proangiogenic communication between the tumor cells and the endothelial cell compartments. Antiangiogenic therapies act predominantly by blocking the binding of VEGF to its receptor and comprise neutralizing antibodies against the ligand or the receptor, soluble receptors, or small molecule inhibitors directed against the tyrosine kinase activity of the VEGF receptors.
Due to the potential of tumor “escape” when specific, indirect antiangiogenic agents (e.g., anti-VEGF) are delivered individually, appropriate combination protocols employing these agents are required for maximal benefit [65
]. Abdollahi and coworkers show that the treatment of tumor xenografts with a combination of endostatin and with VEGF blockers results in an enhanced therapeutic effect, which may be attributed to the endostatin-mediated downregulation of many regulators of proangiogenic pathways and suppression of alternative angiogenic mechanisms that might be upregulated by VEGF blockade [66
Here, we focus on several molecules that interfere with the VEGF/VEGFR signaling pathway, which have been evaluated in clinical trials for solid tumors. In , we summarize the available treatments and the relative clinical phases and results.
Summary of the available treatments and the relative clinical phase and results.
3.2. Indirect Antiangiogenic Drugs
As mentioned previously, Bevacizumab (Avastin) is a humanized neutralizing monoclonal antibody that blocks the binding of human VEGF to its receptors. A significant tumor response was observed in response to Bevacizumab treatment: the 6-month progression-free survival was 32% in GBM patients [84
]. However, glioblastoma appears to adapt rapidly to anti-VEGF therapy, resulting in rapid tumor progression without improvement in overall survival [85
In recent study demonstrated that anti-VEGF therapies can significantly reduce the vascular supply, as demonstrated by a decrease in intratumoral blood flow and a strong reduction of large- and medium-size blood vessels, however these events were also shown to be accompanied by a strong increase in infiltrating tumor cells in adjacent brain parenchyma [87
]. Finally, a preclinical study [88
] and a clinical trial [89
] suggest that high doses of bevacizumab could directly enhance the invasiveness of human glioblastoma cell lines and that dosages lower than those currently used might improve patient outcome.
In the endothelial cells of normal animals, VEGF-A treatment results in the upregulation of both integrins α
1 and α
1. The functional blocking of these integrins impairs angiogenesis in vitro
and reduces VEGF-A-induced angiogenesis and tumor growth in vivo
3 integrins are highly expressed by proliferating and activated vascular endothelial cells. Therefore, they are a major contributor to the formation of vasculature by supporting the migration and survival of endothelial cells [92
]. The blockade of α
3 integrins inhibits tumor angiogenesis as well as blood vessel formation in in vivo
]. Consequently, α
3 might represent a potential target in antiangiogenic therapy. Antagonizing integrins has generally included the targeting of the receptor binding sites or other nearby sites, although new alternative approaches target downstream signaling proteins.
Cilengitide is a cyclic RGD-peptide inhibitor of α
3 and α
5 integrins. Blocking α
3 integrin inhibits blood vessel formation in vivo
]. In a phase II trial, cilengitide was associated with a median survival of 10 months in recurrent glioma patients [96
]. Cilengitide is currently in clinical phase III studies for the treatment of glioblastomas and is in phase II studies for the treatment of several other tumor types, including breast cancer, squamous cell cancer, nonsmall cell lung cancer, and melanoma [97
Other drugs targeting integrins include the following agents.
Abergrin is a humanized antibody against α
3 integrins. It blocks integrin binding to vitronectin and fibrinogen, preventing cell adhesion, migration, proliferation, and integrin-mediated cell signaling [99
Volociximab is a chimeric human-mouse monoclonal antibody that binds to α
1 integrins. It induces cell death and prevents capillary tube formation in vitro
. In vivo,
volociximab exhibits antitumor and antiangiogenic effects [100
Increased matrix metalloproteinase (MMP) levels are associated with glioma invasion and angiogenesis. Marimastat reduces MMP levels in patients with gliomas [73
]. Phase II clinical trials evaluating the administration of marimastat in combination with temozolomide demonstrated promising results (the progression-free survival after six months was 39%), although further investigation is needed for the associated therapy-induced joint pain [101
Sorafenib (Nexavar) is a multi-kinase inhibitor of VEGFR2-3, PDGFR, Raf kinase, and c-Kit. It is currently approved for the treatment of advanced HCC and renal cell carcinoma. Phase II trials evaluating the efficacy of sorafenib in patients with malignant glioma are currently ongoing [102
]. Hypertension is a specific side effect of sorafenib and of most antiangiogenic agents due to the decreased production of nitric oxide and prostacyclins in vascular endothelial cells [103
Cediranib (Recentin) is a potent inhibitor of both VEGFR-1 and VEGFR-2. It also exhibits activity against c-kit, PDGFR-beta and FLT4. It is well tolerated, and an inverse correlation was found between cediranib dose- and time-dependent treatment and soluble VEGFR-2 [104
Sunitinib (Sutent) is a multi-kinase inhibitor of VEGFR 1-3, RET and PDGFR, approved for treatment of RCC, imatinib-resistant gastrointestinal stromal tumors (GIST) and pancreatic neuroendocrine tumors (pNET) [105
]. A recent preclinical study [108
] shows that after starting sunitinib treatment, there is a period when tumor oxygenation is higher in treated compared to untreated mice. The improved oxygenation suggests that the residual blood vessels had improved function in terms of delivering oxygen and nutrients. A synergistic delay in tumor growth was observed when radiation was applied during the enhanced tumor oxygenation after 4 days of sunitinib administration.
Imatinib is a kinase inhibitor of PDGFR, c-kit, and bcr-abl. Administration of imatinib at low concentrations can act as a cytostatic agent, whereas at high concentrations, it predominantly behaves as a cytotoxic agent [109
]. Imatinib monotherapy has failed due to the limited penetration of the drug across the BBB, and for that reason, the inhibition of PDGFR alone is insufficient to prevent the growth of malignant gliomas [110
Antiangiogenic therapies are integrated into the treatment strategies for many different tumor types. However, not all patients respond to therapy; only a few benefit with progression-free survival. In most tumors, antiangiogenic treatment is combined with chemotherapy. Furthermore, a major problem of this therapy is the development of resistance. Extensive evidence indicates that antiangiogenic therapy might actually enhance tumor progression by promoting an invasive phenotype that allows for tumor cells to escape angiogenic inhibition.
The identification of predictive biological markers of objective response will be critical for the assessment of the response rates correlated with overall survival and of the development of resistance to antiangiogenic drugs. These markers will provide important indices to aid in the improvement of therapeutic efficacy or in the development of alternative antiangiogenic therapies in the event of treatment failure.