Lipid-based vectors

Since Feigner et al. first reported in 1987 on the in-vitro transfection of eukaryotic cells with cationic lipid, cationic liposomes have been used extensively for gene delivery [56]. To date, numerous cationic lipids have been synthesized and tested for gene delivery. In general, cationic lipids are composed of three parts (Fig. 5.4) a positively charged head group a hydrophobic tail group and a linker bond between the two. 

In addition, combinations of these structures were found [59]. 

DMRIE l,2-Dimyristyloxypropyl-3-dimethyl-hydroxyethyl ammonium bromid 

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Liposome-polycation-DNA (LPD) nanoparticles (1) are formed by spontaneous rearrangement of a lipid shell around a polycation-condensed bacterial plasmid DNA core to form a virus-like structure (2). The LPD complexes consist of liposomes that are either made of cationic (LPDI) or anionic (LPDII) lipids and are sometimes referred to as lipopolyplexes, a broader category that also includes other lipid-based vectors (2). 

Cardoso AL, Simoes S, de Almeida LP, Pelisek J, Culmsee C, Wagner E, et al. siRNA delivery by a transferrin-associated lipid-based vector a non-viral strategy to mediate gene silencing. J Gene Med 2007 9 170-183. 

The two classes of non-viral vector that are being investigated most actively are the cationic lipid-based vectors and the cationic polymer-based vectors. The cationic polymer-based vectors represent the newest class of non-viral delivery system to be developed and there are a number of groups working with these vectors. As this is such a new area of research there has yet to be any standardised definition of the terms involved in this field. Complexes formed between cationic polymers and DNA have been referred to as interpolyelectrolj1 e complexes, molecular conjugates, polylysine-DNA complexes, DNA-polylysine complexes, polyplexes and so on. In this review complexes formed between poly-(L-lysine) (pLL) and DNA are referred to as polylysine/DNA complexes or abbreviated to pLL/DNA complexes. Where polymers other dian poly(L-lysine) have been used the term polymer/DNA complex is used. The forward slash (/) is used to denote a non-covalent interaction, while a hyphen (-) is used to denote a covalent link. 

Viruses are infectious particles formed by nucleic acid, proteins, and in some cases lipids. As viruses (for example, retro- and adenoviruses) transfer viral genes into cells with high efficiency, modified forms are sometimes used as vectors for gene transfer. However, procedures using virus-based vectors are often significantly more complicated and time-consuming than other transfection methods. In addition, viral vectors are potentially hazardous, and biological safety issues need to be considered carefully. Therefore, techniques that combine... 

Traditionally, prokaryotic expression, especially employment of E. coli-based vectors, has been the system of choice. However, bacteria are unable to provide many vital components required for post-translational modifications including various forms of glycosyla-tion or lipid attachment and protein processing, all of which can also be important for proper protein folding. For this reason, it is not surprising that much time and effort has been dedicated to the development of alternative systems, summarized in Tab. 1.2. 

Duan Y, Zhang S, Wang B, Yang B, Zhi D (2009) The biological routes of gene delivery mediated by lipid-based non-viral vectors. Expert Opin Drug Deliv 6 1351-1361... 

The first reports of using cationic polymers for DNA delivery can be traced back to 1973, before the use of cationic lipids. However, until recently, research in the area of polymer-mediated gene delivery lagged behind that of lipid-based delivery vectors [26]. Recent advances in the development of polymers have regenerated the interest in polymer-mediated gene delivery. Polycationic carriers are either naturally occurring or chemically synthesized compounds. Examples include his-... 

The alternative approach is to use non-viral vectors, such lipid-based, peptide-based and polymer-based delivery systems, as described in detail in Chapter 14. Liposomes are relatively easy to manufacture, are generally non-toxic and are devoid of the capability to cause an infection (see Section 5.3.1). However, a number of limitations are associated with their use. For example, it is difficult to direct liposomes to a particular type of cell. Liposome/DNA complexes which may be internalized by the target cells are... 

To develop an efficient gene delivery system, it seems necessary to understand the extra- and intracellular processes involved in the overall transfection mechanism. This will lead to understanding the mechanism, which is necessary for developing novel lipid-based non-viral vectors. For this purpose, cationic liposomes and pDNA are used widely to understand the cellular mechanism involved in the transfection (Fig. 13.3). 

Lipid-based carriers, polycationic lipids, polylysine, polyornithine, histones and other chromosomal proteins, hydrogel polymers, all of which can ionically condense DNA and bind to the cell surface are found to be ideal candidates for these vector type. But in the use of different types of cationic liposomes, cationic polymers and dendrimers as non-viral vectors for delivery of genes, it has been observed that in addition to cytotoxicity, these carriers do not lead to satisfactory amount of gene expression in the cells. The reasons are many, particularly, low endosomal escape, no protection of DNA from nuclease... 

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