Antiangiogenic therapy.

Angiogenesis, the formation of blood vessels from preexisting ones, plays a crucial role in tumor progression. Activation of an "angiogenic switch" allows tumor cells to invade and metastasize. The growing interest in the use of antiangiogenic agents in the treatment and prevention of cancer lies in the theoretical advantages of this molecularly targeted modality of chemotherapy. Delivery of antiangiogenic agents are not complicated by having to penetrate large bulky masses but, instead, have easy access to tumoral endothelial cells. Antiangiogenic drugs may not cause cytopenias and thus will avoid many of the unwarranted toxicities of standard chemotherapeutic agents. Because they act directly on nascent endothelial cells, antiangiogenic agents may avoid tumor resistance mechanisms. If antiangiogenic agents are successful, they might be applicable to many tumor types and not be dependent on cell type or growth fraction of cells within a tumor. However, several important obstacles remain with regards to using antiangiogenic drugs in clinical trials with which we must contend in order to determine accurately the efficacy of these agents. In this article, we review the different classes of antiangiogenic agents available, ongoing clinical trials, as well as potential pitfalls and future directions in this exciting field.