Anti-angiogenic gene therapy of cancer: Current status and future prospects

The discovery of endogenous inhibitors of angiogenesis has made it possible to test the hypothesis that blocking the angiogenic switch may keep tumor growth in check, and has added a new investigational arm to the field of cancer gene therapy. Angiogenesis inhibitors are heterogeneous in origin and potency, and their growing list includes proteolysis products of larger molecules with a different function, such as angiostatin, endostatin and vasostatin, modulators of vascular endothelial growth factor activity, such as sFLT-1, and some cytokines/chemokines with marked anti-endothelial activity, such as IL-12, IFN-α, and CXCL10. Pre-clinical studies have clearly indicated that these factors are essentially cytostatic and that they need long-term administration in order to obtain prolonged anti-tumor effects, representing a rational basis for their delivery by a gene therapy approach. The experimental approaches attempted to date, reviewed herein, indicate overall that anti-angiogenic gene therapy has efficacy mainly as an early intervention strategy and that a better understanding of the biological mechanisms underlying resistance to angiogenesis inhibition, as well as appropriate combined treatments, are required to generate a conceptual advancement which could drive the field towards successful management of established tumors.