This is the first study to examine the preclinical efficacy of pharmacologic inhibition of VEGF in an orthotopic model of glioblastoma. We demonstrate for the first time that systemic anti-VEGF therapy has the potential to significantly prolong survival in human glioblastoma. Our data suggest that systemic anti-angiogenesis therapy is not limited by the constraints of the blood-brain barrier; indeed, the very target for angiogenesis inhibition — the tumor neovasculature — is defective and leaky, lacking tight junctions and other specializations of the blood-brain barrier [22
]. In addition, our data support the contention that VEGF production is critical in glioblastoma angiogenesis.
Our results also highlight several potential problems in the application of anti-angiogenesis therapy to brain tumors. To our knowledge, we have made the first observation that tumors may adapt to anti-angiogenesis therapy by increased invasiveness and cooption of host vessels. The formation of satellite tumors less than 0.5 mm in diameter, which appear to utilize existing blood vessels, may represent a general mechanism for resistance to anti-angiogenesis therapy. As we detected no TUNEL-positive cells in the satellite tumors, it appears that this is a successful adaptation.
While glioblastoma virtually never metastasizes out of the brain, it is one of the most infiltrative of all tumors, uniformly defying the most radical surgical resections. Our glioma model reflects this capacity and suggests that this invasiveness may be modulated in response to the tumor microenvironment. Because of the vascularity of the normal brain, and the well-developed vasculature of advanced glioblastomas, it is likely that the application of anti-angiogenesis strategies alone will not be successful in this disease. Our data suggest that anti-angiogenesis therapy for glioblastoma is most likely to be successful when initiated early after maximal tumor resection. Treatment of these tumors with a combination of anti-angiogenesis agents and anti-invasive strategies to prevent tumor infiltration of the brain and association with its rich vasculature may prove to be a successful approach. As anti-angiogenesis therapies for a variety of types of tumors proceed in clinical trials, it may be valuable to determine whether increased invasiveness is a general mechanism by which tumors overcome strategies which target only the neovasculature.
It is worthwhile to note the similarities and differences between our results and those of other investigators. Chang et al.
] demonstrated that the ex vivo
anti-sense inhibition of VEGF production by U87 human glioblastoma cells causes a significant reduction in the size and vascularity of intracranial U87 tumors. The investigators compared tumor sizes at a similar time point post-implantation but did not compare the outcomes with respect to survival or the pattern of growth in the two cohorts.
Also, Holash et al.
] reported a study on intracranial brain tumor angiogenesis using the rat RT-2 glioma cell line. In the rat glioma model system, angiogenesis occurs relatively late, at 28 days post-implantation, and is preceded by a period in which tumor cells appear to survive by association with the host vasculature, a process termed vessel cooption. Ultimately, the investigators detected apoptosis and regression of the host endothelium, resulting, first, in tumor necrosis and second, in a VEGF-associated angiogenic response. In our study using human glioblastoma cells, VEGF-associated angiogenesis begins much earlier, within 7 days post-implantation. Twelve-day-old tumors already exhibited a rich neovasculature. Increased vessel “cooption” appears to occur as a consequence of increased tumor invasion of normal brain. The latter is a consequence of angiogenesis inhibition. In our study, we did not detect apoptosis or regression of the vessels associated with satellite tumors. This process may occur more slowly in our model and therefore would not be detectable in a time frame that is limited by the progressive growth of the tumors.
It is tempting to speculate on the mechanism of satellite tumor formation. A variety of evidence suggests that these tumors exhibit increased invasive activity compared to the primary tumor. One possibility is that a threshold level of hypoxia induces the infiltrative phenotype, perhaps by increasing the expression of proteases such as the urokinase type plasminogen activator, as described [24
]. Another possibility is that cells in tumor satellites exhibit a chemotactic response to factors in the extratumoral environment, such as gradients of oxygen, glucose, or growth factors produced by brain endothelia. Whatever the mechanism, this phenomenon must be taken into account when designing future therapeutic strategies.