**3.3 Influence of physico-chemical parameters on the TE** *in vivo*

The low transfection efficiency *in vivo* is one of factors that hinders the design of an efficient liposomal gene delivery systems. Negatively charged components of serum could interact with cationic liposome and compete with DNA for cationic liposome binding, leading to a decrease in TE. Other destructive effects of serum components attributed to its interaction with lipoplexes and early release of DNA from lipid shielding bilayer that reduces the TE. Moreover, the released nucleic acids could be recognized by Toll-like receptor expressed in B cells and dendritic cells; resulting in toxicity *via* induction of the cytokine production (Tousignant et al., 2000). Lipoplexes were reported to be generally more than 100 nm in diameter, as well as tend to self-aggregate in the blood stream, which resulting in limited passage through the vessel walls (Pouton & Seymour, 2001).

The difference between the optimal transfection parameters of *in vitro* and *in vivo* due to the profound difference of the biochemical characteristics between the cells and the organism, is the most severe problem associated with the use and practical implementation of cationic lipid or liposomes for the treatment of genetic and acquired diseases. In the case of monocationic lipids there is a single reference that the *in vivo* result corresponded to the *in* 

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*vitro* one (Ding et al., 2008) and positively charged lipoplexes are significantly more effective than negatively charged ones. Other investigations revealed that the best TE *in vivo*  corresponds to a low N/P ratio, small size of lipoplexes, and negative ξ-potential (Hattori et al., 2007; Gao & Hui, 2001). It is noteworthy when considering polycationic lipids, that lipoplexes that have a total negative charge and small size (200–300 nm) are optimal for both *in vitro* and *in vivo* transfection. (Stewart et al., 2001).

It was revealed that the efficient gene delivery by polycationic lipids (Cooper et al., 1998) *in vivo* requires the cationic liposome systems, which are able to bind DNA more tightly than for the *in vitro* delivery. In comparison to CDAN(**18d**)/DOPE and DC-Chol(**1a**)/DOPE liposomes, CTAP(**18g**)/DOPE was able to neutralize, condense and encapsulate nucleic acids into lipoplex particles with a high efficiency. SDS stimulated the DNA release from a lipoplex and revealed the structure-activity relationships between the TE and lipid shielding of DNA (Bajaj et al., 2008c). Low shielding could facilitate the release of DNA and its hydrolysis within the cells. Certainly, this characteristic could be expected to be useful *in vivo,* given the greater complexity of the extracellular environment *in vivo* as compared to *in vitro*. Caminiti and co-workers demonstrated that both unstable and lipoplexes that are too stable, result in a strong and poor DNA release respectively, and exhibit a low transfection efficiency (Caracciolo et al., 2007).
