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.
Early trials are under way.
See list of Angiogenesis Inhibitors in Clinical Trials.