Delivery systems for siRNA: towards targeted inhibition of tumor angiogenesis

Since 40 years, angiogenesis has been recognized to be a prominent factor in the development of solid tumors and has been regarded as an attractive target for cancer therapy. A promising strategy to interfere with diseases where (over)expression of specific genes contributes to the pathological process, such as angiogenesis, is gene silencing by RNA interference (RNAi). So far, clinical success of RNAi-based strategies to inhibit tumor angiogenesis have been limited due to (1) scarcity of clinically promising pharmacological targets and/or (2) inefficiency or toxicity of the siRNA carrier system. In this thesis, we attempted to improve both siRNA target and carrier. Therefore, we studied a novel small Rho GTPase protein, Rac1, as a potential target for anti-angiogenic therapy. Our results indicate that Rac1 is an important regulator of VEGF-mediated angiogenesis and that knockdown of Rac1 using siRNA may represent an attractive approach to inhibit tumor angiogenesis and growth. At the same time, we tried to adapt and improve a novel class of biodegradable poly(amido amine)s, with built-in characteristics for intracellular release of its siRNA payload, for drug delivery purposes. By titrating the positive charges, these polymers were optimized for siRNA delivery. PEGylation of the polymers resulted in increased stability of siRNA polyplexes in salt and serum and further decreased their toxicity. Additionally, we modified a previously described lipid-based carrier for siRNA to target angiogenic endothelial cells. We show that these systems ensure cellular uptake and gene silencing in both murine and human endothelial cells. Further research should evaluate the use of these carriers for gene silencing in vivo. This thesis provides starting points for future research on the optimization of polymeric and lipidic carrier systems for silencing of genes involved in angiogenesis. This may ultimately lead to the development of safe delivery systems that enable therapeutic RNA interference in cancer therapy.