Anti-angiogenic therapy: rationale, challenges and clinical studies.
Physiological angiogenesis occurs during embryogenesis, wound healing and reproductive functions in adults. Abnormal angiogenesis takes place in certain chronic diseases (diabetes, psoriasis, rheumatoid arthritis, etc.) and tumours. Genetic changes and local stresses including hypoxia, glucose deprivation and oxidative stress play a pivotal role in angiogenesis switch, which is necessary for tumour development and is rate-limiting for tumour progression. Angiogenesis is tightly regulated by pro- and anti-angiogenic growth factors with a series of complex and interrelated steps. Activated endothelial cells (ECs) migrate as a solid cord and, subsequently, form lumina; the sprout tips then anastomose to form vessel loops or networks. One of the final events is the laying down of a basement membrane and the structural support of pericytes. The molecular alterations that sustain angiogenesis represent novel targets for rationally designed anti-cancer treatment strategies. Inhibition of angiogenesis presents certain advantages on conventional therapies, such as the direct accessibility from the circulation, and the potential low rate of drug resistance related to the genetic stability of ECs. Certain anti-angiogenic compounds were found to have potent anticancer property in in vivo experimental studies. Nevertheless, in contrast to preclinical studies, the first generation of anti-angiogenic drugs tested in clinical trials have shown a moderate activity in advanced disease partly due to suboptimal schedules of therapy or biases in study design.