Nucleic-acid- and cell-based therapeutics

CH14 NUCLEIC-ACID- AND CELL-BASED THERAPEUTICS... 

Peptoids have also shown great utility in their ability to complex with and deliver nucleic acids to cells, a critical step toward the development of antisense drugs, DNA vaccines, or gene-based therapeutics. Most non-viral nucleic acid delivery systems are based on cationic molecules that can form complexes with the polyan-... 

Other systems like electroporation have no lipids that might help in membrane sealing or fusion for direct transfer of the nucleic acid across membranes they have to generate transient pores, a process where efficiency is usually directly correlated with membrane destruction and cytotoxicity. Alternatively, like for the majority of polymer-based polyplexes, cellular uptake proceeds by clathrin- or caveolin-dependent and related endocytic pathways [152-156]. The polyplexes end up inside endosomes, and the membrane disruption happens in intracellular vesicles. It is noteworthy that several observed uptake processes may not be functional in delivery of bioactive material. Subsequent intracellular obstacles may render a specific pathway into a dead end [151, 154, 156]. With time, endosomal vesicles become slightly acidic (pH 5-6) and finally fuse with and mature into lysosomes. Therefore, polyplexes have to escape into the cytosol to avoid the nucleic acid-degrading lysosomal environment, and to deliver the therapeutic nucleic acid to the active site. Either the carrier polymer or a conjugated endosomolytic domain has to mediate this process [157], which involves local lipid membrane perturbation. Such a lipid membrane interaction could be a toxic event if occurring at the cell surface or mitochondrial membrane. Thus, polymers that show an endosome-specific membrane activity are favorable. 

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. 

According to a review by Walsh [10], of 165 biopharmaceuhcal products approved in the United States and Europe by 2006, only two are nucleic acid-based drugs, whereas nine of the 31 therapeutic proteins approved since 2003 are produced in E, coli, and 17 are produced by mammalian cell lines. In 2004 market distribution and manufacture of therapeutic proteins, non-glycosylated (non-antibody) proteins constitutes 40% of the total market, with 12% armual growth rate, and are produced in E. coli or the yeast Saccharomyces cerevisiae glycoproteins (primarily mAbs) constitute 60% of the total market, with 26% armual growth rate, and are produced by mammalian cell culture (mostly with cells from Chinese Hamster Ovary, or CHO). 

In nucleic acid-based therapy, oligonucleic acids are used as a drug to treat genetic and acquired diseases. The stability or half-lives of such oligonucleic acids in cells are likely to be affected by cellular enzymes. One group of enzymes, which would interact with the therapeutic oligonucleic acids, are those involved in the process of DNA repair. 

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