Serum nucleases

Whereas lipoplexes/polyplexes generally protect the plasmid from serum nucleases, the overall positive charge characteristic of these structures leads to their non-specific interactions with cells... 

RNAi technology has obvious therapeutic potential as an antisense agent, and initial therapeutic targets of RNAi include viral infection, neurological diseases and cancer therapy. The synthesis of dsRNA displaying the desired nucleotide sequence is straightforward. However, as in the case of additional nucleic-acid-based therapeutic approaches, major technical hurdles remain to be overcome before RNAi becomes a therapeutic reality. Naked unmodified siRNAs for example display a serum half-life of less than 1 min, due to serum nuclease degradation. Approaches to improve the RNAi pharmacokinetic profile include chemical modification of the nucleotide backbone, to render it nuclease resistant, and the use of viral or non-viral vectors, to achieve safe product delivery to cells. As such, the jury remains out in terms of the development and approval of RNAi-based medicines, in the short to medium term at least. 

Aptamers appear to display low immunogenicity but, when administered systemically, they are quickly excreted via size-mediated renal clearance. In order to prevent renal removal, such aptamers are usually conjugated to PEG. PEG may also help further protect the aptamers from degradation by serum nucleases native aptamers are prone to nuclease attack, but their half-lives can most effectively be extended via chemical modification, as discussed earlier in the context of antisense agents. 

Calcium phosphate co-precipitation, first developed by Graham and van der Eb (9), is popularly used for both stable and transient transfections in many different cell lines due to easy availability of component materials and their low cost. DNA when mixed with calcium chloride and added in a controlled manner to a buffered saline-phosphate solution at room temperature (RT) results in the formation of a precipitate that is dispersed on the attached cells (10). The precipitate is presumably taken up by the cells by endocytosis. Calcium phosphate, like several other nucleic acid delivery reagents, protects the nucleic acid in the complex from the action of intracellular and serum nucleases. 

Lipoplexes are prepared by the interaction of anionic nucleic acids with the surface of cationic lipid to afford multilamellar lipid-nucleic acid complexes. Cationic liposomes can protect nucleic acids from serum nucleases and facilitate the cellular uptake and release of nucleic acids into the cytosol [218]. Pedroso et al. [219] have extensively discussed the structure-activity relationships of cationic lipo-some/DNA complexes and the key formulation parameters influencing the properties of lipoplexes. In addition, optimization of the cationic liposomal complexes for in vivo application has been reviewed by Smyth [220]. 

Bioavailability of DNA can be improved because of protection from serum nuclease degradation by the polymer matrix. 

Currently, transport of exogenous DNA to cells can be achieved using viral and nonviral vectors or as naked DNA. The simplest nonviral gene delivery system simply uses naked DNA. The overall level of expression is much lower with naked DNA than with either viral or liposomal vectors. Naked DNA is also unsuitable for systemic administration due to the presence of serum nucleases. 

Boado, R.). and Pardridge, W.M., Complete protection of antisense oHgonucleotides against serum nuclease degradation by an avidin-biotin system, Bioconjug. Chem., 3,519,1992. 

Degradation of DNA by serum nucleases Targeting of DNA to particular tissue types DNA uptake by cells... 

The physicochemical properties of particles and macromolecules have a major effect on their biodistribution and blood clearance as these properties govern how the particles will interact with serum proteins (including serum nucleases), endothelium and the reticuloendothelial system. Some of these systems are described below. 

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