Cationic Liposomes

Since the pioneering work of Felgner and coworkers [44], cationic lipid/DNA lipoplexes have been used to deliver a variety of genes to the lungs of animals and humans [4,36-38,45-48,50-59]. Lipoplexes are made by the interaction of negatively charged DNA with cationic lipids or liposomes. Two common cationic lipids are dioleoyltrimethylammonium propane (DOTAP) and dioleoxipropyltrimethylammonium (DOTMA), which are both two-chain amphiphiles whose acyl chains are linked to a propyl ammonium group through an ester or ether bond, respectively. Lipids with the more stable ether linkage have exhibited higher toxicity than those with a labile ester linkage [15,61] but DOTMA has been shown to have ten-fold higher transfection efficiency than DOTAP [115]. Marshall et al. found that gene expression in the lung after intratracheal (i.t.) administration of cationic lipid:DNA lipoplexes in mice was dependent on the structure of the lipid [55]. They synthesized a series of lipids in which they varied the cationic headgroup, the hydrophobic anchor, and the linker between the two and found that the lipids whose structure resembled a "T-shape" had higher efficiency than linear lipids. Lipids used in gene carries differ in hydrophobicity, degradability of their linker group, and the positive valence of the polar headgroup, all of which translate into variable transfection efficiencies of the vector [43,50].

Cationic lipids are often mixed with neutral lipids, such as cholesterol, dioleoylphosphatidylethanolamine (DOPE), and dioleoylphosphatidylcholine

(DOPC). Neutral lipids are used to facilitate the formation of liposomes and to ease disassembly of the lipoplexes after internalization [62]. The choice of neutral lipid is also important; for example, cationic liposomes made with DOTAP:DOPE have shown higher transfection efficiency in mammalian cell culture than those made with DOTAP:DOPC [63]. Although in vivo work has shown lower transfection efficiency of DOTAP:DOPE than DOTAP alone following i.v. administration in mice [115].

DNA association with lipids may increase its permeability through the cell membrane due to membrane fusion [15,44] or endocytosis involving clathrin-and nonclathrin-coated vesicles [49,64,65]. Lipids also may protect DNA from degradation [62,66,67]. However, high doses of cationic lipoplexes can trigger an inflammatory response, which may be increased by the inherent immunogenicity of the condensed bacteria-derived pDNA [54]. In addition, lipoplexes are relatively unstable in many physiological environments [68], which may limit their efficacy in vivo compared to in vitro cell culture. Sanders and coworkers showed that DOTAP:DNA lipoplexes released pDNA upon contact with mucus components at concentrations comparable to those of cystic fibrosis (CF) patients [69]. In another study, cationic liposome-mediated gene delivery was prevented by coating cell monolayers with sputum from patients with cystic fibrosis before transfection [48]. Meyer and colleagues found that neither DOTMA-DOPE nor DOTAP liposomes enhanced gene expression over naked DNA in the murine airways [56]. Another study showed similar levels of transfection between DOTAP:DNA and naked DNA in the mouse airways following i.t. administration, but there were higher levels of transfection with lipoplexes made from DORI analogues [50]. Finally, similar to viral vectors, cationic lipids exhibit reduced transfection efficiency following nebulization [35-38]. A review of cationic lipid complexation with DNA can be found in Refs. 43, 60, and 70; a review of lipoplexes for CF can be found in Ref. 71.

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