**5. Cationic lipids**

into the cell wall is based on the interaction between the negatively charged ions of the cell membrane and positive charge of poly-L-lysine. Simple electrostatic mixing of DNA and poly-L-lysine produces DNA particles with various structures. The mode of binding between the poly-L-lysine and DNA is cooperative and non-cooperative binding. Condensation between the DNA with the PLA depends upon the PLL chain length. Increase in the length of the PLL

Polyethylenimine (PEI), as given in **Figures 8** and **9**, is water-soluble, linear or branched

weakly basic aliphatic polymer and polycationic one due to primary, secondary and tertiary amino groups. PEIs are available in different molecular masses and forms. Various forms of PEIs are shown in **Figure 7**–9. Linear polyethylenimines contain all secondary amines, whereas branched PEIs contain primary, secondary and tertiary amino groups. Due to their high cationic charge density at physiological pH, PEIs are able to form non-covalent complexes with DNA, siRNA and antisense oligodeoxynucleotide, and then brought into the cell via endocytosis. Once inside the cell, protonation of the amines results in an influx of counter-ions and a lowering of the osmotic potential, leading to bursts in the vesicle releasing the polymer-DNA complex (polyplex) into the cytoplasm. If the polyplex unpacks, then the DNA is free to diffuse to the nucleus; however, the long PEI chains have higher effi-

CH2

spacer. It is a

polymers composed of the amine group and two carbon aliphatic CH2

ciency in gene transfection, and are more cytotoxic [69–93].

chain increases the condensation [61–68].

144 Advanced Technology for Delivering Therapeutics

**4.3. Polyethylenimine**

**Figure 8.** Structure of branched PEI.

**Figure 7.** Structure of linear PEI.

The four constituents are given as follows:


## **5.1. Monovalent cationic lipids**

## *5.1.1. DOTMA*

Chemically, it is N-[1-(2,3-dioleyloxy) propyl]-N,N,N-trimethylammonium chloride, as given in **Figure 10**, that consists of four different moieties: (1) a quaternary ammonium head group as the cationic head group, (2) a glycerol-based backbone, (3) two linkage bonds and (4) two hydrocarbon chains. Alternations can be made in the above moieties to reduce the toxicity and increase the gene transfection efficiencies. Replacement of a methyl group on the quaternary amine of DOTMA with a hydroxyl improves protein expression after gene transfection due to the replaced hydroxyl group in contact with the aqueous layer surrounding the liposome. Increase in the length of the aliphatic chain decreases the gene transfection and vice versa [94–98].

## *5.1.2. DOTAP*

DOTAP, [1,2-bis(oleoyloxy)-3-(trimethylammonio) propane], as given in **Figure 11**, consists of a quaternary amine head group coupled to a glycerol backbone with two oleoyl chains.

**Figure 11.** Structure of DOTAP.

The only differences between this molecule and DOTMA are that ester bonds link the chains to the backbone rather than ether bonds. The ester bonds present in the backbone are hydrolysable and lead to render the lipid biodegradable and reduce cytotoxicity. DOTAP cannot be used alone as a cationic liquid for gene delivery due to its dense positive charge, thereby preventing the ion exchange. Its gene-delivery efficiency can be changed by combining with other helper liquids [94, 99–103].

## *5.1.3. DC-Chol*

3β[*N*-(*N*′,*N*′-dimethylaminoethane)-carbamoyl] cholesterol, as given in **Figure 12**, contains a cholesterol moiety attached by an ester bond to a hydrolysable dimethylethylenediamine. Due to the presence of cholesterol moiety, it is biocompatible and has good stability. The combination of DC-Chol and dioleoylphosphatidylethanolamine (DOPE) in the ratio 1:1

**Figure 12.** Structure of DC-Col.

**Figure 13.** Structure of DOSPA.

reduces the lipoplex aggregation; it assists the DNA dissociation during gene delivery [94, 99, 100, 103, 104].

## **5.2. Multivalent cationic lipids**

## *5.2.1. DOSPA*

The only differences between this molecule and DOTMA are that ester bonds link the chains to the backbone rather than ether bonds. The ester bonds present in the backbone are hydrolysable and lead to render the lipid biodegradable and reduce cytotoxicity. DOTAP cannot be used alone as a cationic liquid for gene delivery due to its dense positive charge, thereby preventing the ion exchange. Its gene-delivery efficiency can be changed by combining with

3β[*N*-(*N*′,*N*′-dimethylaminoethane)-carbamoyl] cholesterol, as given in **Figure 12**, contains a cholesterol moiety attached by an ester bond to a hydrolysable dimethylethylenediamine. Due to the presence of cholesterol moiety, it is biocompatible and has good stability. The combination of DC-Chol and dioleoylphosphatidylethanolamine (DOPE) in the ratio 1:1

other helper liquids [94, 99–103].

**Figure 11.** Structure of DOTAP.

**Figure 10.** Structure of DOTMA.

146 Advanced Technology for Delivering Therapeutics

*5.1.3. DC-Chol*

DOSPA is a derivative of DOTMA. Chemically, it is 2,3-dioleyloxy-N-[2(sperminecarboxamido) ethyl]-N,N-dimethyl-l-propanaminium trifluoroacetate, which is given in **Figure 13**. The difference between DOSPA and DOTMA is a spermine group, which is bound through a peptide bond to the hydrophobic chains. Spermine group allows more efficient packing of DNA due to its hydrogen bond interaction with the DNA [43, 94].

## *5.2.2. DOGS*

DOGS, chemically it is di-octadecyl-amido-glycyl-spermine, structure of the DOGS is similar to DOSPA, as given in **Figure 14**. The molecular structures of both DOGS and DOSPA consist of a multivalent spermine head group and two 18-carbon alkyl chains. The saturated chains

**Figure 14.** Structure of DOGS.

in DOGS are linked to the head group through a peptide bond. The packing ability of DNA by DOGS is due to its large head group molecule and the length of long unsaturated carbon chains. DOGS have efficient packing of DNA, due to its spermine head group. The presence of spermine head group in DOGS leads to efficient packing of DNA [94, 105–107].
