It is generally true that tumors with higher densities of blood vessels are more likely to metastasize and are correlated with poorer clinical outcomes. Also, the shedding of cells from the primary tumor begins only after the tumor has a full network of blood vessels. Strategies Of the anti-angiogenesis drugs now in clinical trials, some were designed to target specific molecules involved in new blood vessel formation. Several differences between standard chemotherapy and anti-angiogenesis therapy result from the fact that angiogenesis inhibitors target dividing endothelial cells rather than tumor cells. Also, since anti-angiogenic drugs may not necessarily kill tumors, but rather hold them in check indefinitely, the endpoint of early clinical trials may be different than for standard therapies.
Drug resistance is a major problem with chemotherapy agents.
This is because most cancer cells are genetically unstable, are more prone to mutations and are therefore likely to produce drug resistant cells. Since angiogenic drugs target normal endothelial cells which are not genetically unstable, drug resistance may not develop. So far, resistance has not been a major problem in long-term animal studies or in clinical trials. Finally, anti-angiogenic therapy may prove useful in combination with therapy directly aimed at tumor cells. Because each therapy is aimed at a different cellular target, the hope is that the combination will prove more effective.