Vascular disrupting agents.
It has been well established that a functioning vascular supply is essential for solid tumor growth and metastases. In the absence of a viable vascular network, tumors are unable to grow beyond a few millimeters and therefore remain dormant. Initiation of angiogenesis allows for continued tumor growth and progression. Targeting tumor vasculature, either by inhibiting growth of new tumor blood vessels (antiangiogenic agents) or by destroying the already present tumor vessels (vascular disrupting agents; VDA), is an area of extensive research in the development of new antitumor agents. These two groups differ in their direct physiological target, the type or extent of disease that is likely to be susceptible, and the treatment schedule. VDAs target the established tumor blood vessels, resulting in tumor ischemia and necrosis. These agents show more immediate effects compared to antiangiogenic agents and may have more efficacy against advanced bulky disease. VDAs can be divided into two groups--ligand-bound and small molecule agents. Both VDA groups have demonstrated antitumor effects and tumor core necrosis, but consistently leave a thin rim of viable tumor cells at the periphery of the tumor. More evidence suggests VDAs will have their greatest effect in combination with conventional chemotherapy or other modes of treatment that attack this outer rim of cells.