| RNA interference mediated by short interfering RNA (siRNA) can downregulate the expression of any targeted gene and is currently being explored as a therapeutic. As with conventional gene therapy, the need for safe and efficient delivery vectors is a barrier to further development of RNAi technologies. This can be achieved through a better understanding of the design criteria of siRNA vectors and determining both the minimum amount of siRNA needed to achieve high knockdown efficiency and amount of vector allowed to minimize toxicity. Some of these siRNA delivery criteria will be discussed through the use of commercially available and modified polyethylenimine (PEI) derivatives.;The research presented in this thesis addresses the issue of how PEI concentration affects siRNA delivery efficiency. It also examines how PEI derivatives affect the three primary in vitro barriers: (1) cellular uptake, (2) endosomal escape, and (3) unpackaging of PEI/siRNA polyplexes. Chapter 2 will demonstrate that high knockdown efficiency can be achieved when appropriate amount of branched 25-kDa PEI is used despite PEI/siRNA ratio if the minimum amount of siRNA is met. It will also examine how PEI concentration affects the three primary barriers by measuring cellular uptake of polyplexes, effect of acidification in endosomal escape, and unpackaging or dissociation of PEI from siRNA in the presence of a competitor. The results indicate that the total amount of PEI used to form polyplexes with siRNA, and not necessarily the PEI/siRNA ratio, significantly affects siRNA knockdown efficiency and cellular uptake. Chapter 3 examines the ability of acetylated PEI to deliver siRNA. Acetylated PEI have lower buffering capacity, better dissociation, and less toxicity than unmodified 25-kDa PEI. The results indicate that acetylated PEI are less efficient than unmodified PEI. It seems as though buffering capacity significantly affects endosomal escape (second delivery barrier), which contributes to lower transfection efficiency. Finally, Chapter 4 examines the ability of 9.7-kDa biodegradable PEI (dPEI) to deliver siRNA. Biodegradable PEI were synthesized from low molecular weight (LMW, 0.8-kDa) PEI monomers. LMW PEI are inefficient siRNA delivery vectors, but exhibit low toxicity. The results indicate that dPEI are more efficient than LMW PEI, but less efficient than 25-kDa PEI for siRNA delivery. However, the low toxicity level associated with dPEI makes it an attractive vector for in vivo siRNA delivery. |
