VEGF as an inhibitor of tumor vessel maturation: implications for cancer therapy

Tumor angiogenesis is an important component of cancer biology driven in part by the thesis that inhibition of tumor vessel growth would be expected to starve and thereby disrupt tumor growth. A significant portion of research on tumor angiogenesis has focused on VEGF and its blockade with the expectation that dramatic results would be demonstrated in cancer patients as previously documented in animal models. However, to date, anti-VEGF (bevacizumab, Avastin) therapy alone has demonstrated little if any antitumor activity or survival benefit in humans. Interestingly, bevacizumab in combination with chemotherapeutic agents appears to result in a modest survival benefit in patients with various malignancies. This has prompted the re-evaluation of the biological effects resulting from VEGF blockade. Recent reports indicate that inhibition of VEGF and its receptor can have dramatic and unexpected effects on mural and perivascular cells in the tumor microenvironment, leading to vascular smooth muscle cell/pericyte activation and vessel normalization/maturation. Specifically, when VEGF levels in tumors are high, recent studies suggest that this leads to reduced responsiveness of the mural cell to the related growth factor, platelet-derived growth factor. This raises the possibility that in addition to the demonstrated anti-angiogenic effect of VEGF neutralization, mural cell recruitment and thus vascular maturation might be among the most critical activities of anti-VEGF agents. This review seeks to explore how VEGF blockade impacts tumor vascular maturation and considers its implications for cancer therapy.