What is angiogenesis?
Angiogenesis is the formation of new blood vessels. This process involves the migration, growth, and differentiation of endothelial cells, which line the inside wall of blood vessels.
The process of angiogenesis is controlled by chemical signals in the body. These signals can stimulate both the repair of damaged blood vessels and the formation of new blood vessels. Other chemical signals, called angiogenesis inhibitors, interfere with blood vessel formation. Normally, the stimulating and inhibiting effects of these chemical signals are balanced so that blood vessels form only when and where they are needed.
Why is angiogenesis important in cancer?
Angiogenesis plays a critical role in the growth and spread of cancer. A blood supply is necessary for tumors to grow beyond a few millimeters in size. Tumors can cause this blood supply to form by giving off chemical signals that stimulate angiogenesis. Tumors can also stimulate nearby normal cells to produce angiogenesis signaling molecules. The resulting new blood vessels “feed” growing tumors with oxygen and nutrients, allowing the cancer cells to invade nearby tissue, to move throughout the body, and to form new colonies of cancer cells, called metastases.
Because tumors cannot grow beyond a certain size or spread without a blood supply, scientists are trying to find ways to block tumor angiogenesis. They are studying natural and synthetic angiogenesis inhibitors, also called antiangiogenic agents, with the idea that these molecules will prevent or slow the growth of cancer.
How do angiogenesis inhibitors work?
Angiogenesis requires the binding of signaling molecules, such as vascular endothelial growth factor (VEGF), to receptors on the surface of normal endothelial cells. When VEGF and other endothelial growth factors bind to their receptors on endothelial cells, signals within these cells are initiated that promote the growth and survival of new blood vessels.
Angiogenesis inhibitors interfere with various steps in this process. For example, bevacizumab (Avastin®) is a monoclonal antibody that specifically recognizes and binds to VEGF (1). When VEGF is attached to bevacizumab, it is unable to activate the VEGF receptor. Other angiogenesis inhibitors, including sorafenib and sunitinib, bind to receptors on the surface of endothelial cells or to other proteins in the downstream signaling pathways, blocking their activities (2).
Are any angiogenesis inhibitors currently being used to treat cancer in humans?
Yes. The U.S. Food and Drug Administration (FDA) has approved bevacizumab to be used alone for glioblastoma that has not improved with other treatments and to be used in combination with other drugs to treat metastatic colorectal cancer, some non-small cell lung cancers, and metastatic renal cell cancer. Bevacizumab was the first angiogenesis inhibitor that was shown to slow tumor growth and, more important, to extend the lives of patients with some cancers.
The FDA has approved other drugs that have antiangiogenic activity, including sorafenib (Nexavar®), sunitinib (Sutent®), pazopanib (Votrient®), and everolimus (Afinitor®). Sorafenib is approved for hepatocellular carcinoma and kidney cancer, sunitinib and everolimus for both kidney cancer and neuroendocrine tumors, and pazopanib for kidney cancer. Researchers are exploring the use of angiogenesis inhibitors to treat other types of cancer (see Question 7). In addition, angiogenesis inhibitors are being used to treat some diseases that involve the development of abnormal blood vessel growth in noncancer conditions, such as macular degeneration.
How are angiogenesis inhibitors different from conventional anticancer drugs?
Angiogenesis inhibitors are unique cancer-fighting agents because they tend to inhibit the growth of blood vessels rather than tumor cells. In some cancers, angiogenesis inhibitors are most effective when combined with additional therapies, especially chemotherapy. It has been hypothesized that these drugs help normalize the blood vessels that supply the tumor, facilitating the delivery of other anticancer agents, but this possibility is still being investigated.
Angiogenesis inhibitor therapy does not necessarily kill tumors but instead may prevent tumors from growing. Therefore, this type of therapy may need to be administered over a long period.