**4.1. Chitosan**

Chitosan is a polysaccharide copolymer composed of randomly distributed β-(1-4)-linked d-glucosamines and *N*-acetyl-d-glucosamines, obtained by partial alkaline deacetylation of chitin [46], with different molecular weights (50–200 kDa), degrees of deacetylation (40–98%) and viscosities [47]. Chitosan is a natural polymer, **Figure 5**, with linear polyamine, having reactive amino and hydroxyl groups, biodegradable to normal body constituent, safe and non-toxic, and binds to mammalian and microbial cells. The main commercial sources of chitosan are the crustacean shell wastes of crabs, shrimps and

**Figure 5.** Structure of chitosan.

lobsters [48]. Chitosan is soluble in aqueous solutions of some acids and some selective N-alkylidination. Its solubility, biodegradability, reactivity and adsorptivity of many substrates depend on the amount of protonation of the –NH2 function on the C-2 position of the D-glucosamine unit, whereby the polysaccharide is converted to a polyelectrolyte in acidic media. Chitosan is considered one of the most valuable polymers for biomedical and pharmaceutical applications due to its biodegradability, biocompatibility, antimicrobial, non-toxicity and anti-tumour properties.

Chitosan effectively condenses DNA and protects it from nuclease degradation. Various conjugates such as thiolation, glycolation and folate chitosan are available. Chitosan is biodegradable, biocompatible, low immunogenicity and non-toxic at low molecular weights (10–50 kDa). It has been suggested that the toxicity of chitosan is perhaps due to impurities in the chitosan polymers [49–60].

## **4.2. Poly-L-lysine**

to effectively retain hydrophobic drug molecules in the voids of their branching architecture,

Asymmetric dendrimers are synthesized by coupling dendrons of different generations to a linear core, which yields a branched dendrimer with a nonuniform orthogonal

DNA, when combined with sufficient amounts of cationic polymers, will condense into discrete entities which are called as polyplexes [43]. The polyplexes are compact nanoparticles formed through electrostatic interactions between the positive charges of amines and the negative charges of DNA phosphates. The strength of DNA binding to the polymers is related to

The most common cationic polymers used as nonviral gene-delivery vectors include chitosan, PLL, polyethylenimine (PEI), poly(amido amine) (PAMAM) dendrimers and select polypep-

Chitosan is a polysaccharide copolymer composed of randomly distributed β-(1-4)-linked d-glucosamines and *N*-acetyl-d-glucosamines, obtained by partial alkaline deacetylation of chitin [46], with different molecular weights (50–200 kDa), degrees of deacetylation (40–98%) and viscosities [47]. Chitosan is a natural polymer, **Figure 5**, with linear polyamine, having reactive amino and hydroxyl groups, biodegradable to normal body constituent, safe and non-toxic, and binds to mammalian and microbial cells. The main commercial sources of chitosan are the crustacean shell wastes of crabs, shrimps and

**1. Segment-block dendrimers—**segmented with segments of different constitution.

**2. Layer-block dendrimers—**concentric spheres of differing chemistry [25–42].

mimicking amphiphilic polymer micelles.

142 Advanced Technology for Delivering Therapeutics

There are two different types of dendrimeric copolymers:

*3.1.4. Asymmetric dendrimers*

**4. Cationic polymers**

architecture.

the N:P ratio.

tides [24, 44, 45].

**Figure 5.** Structure of chitosan.

**4.1. Chitosan**

Poly-L-lysine (ε-poly-L-lysine), as given in **Figure 6**, is a small natural homopolymer of the essential amino acid L-lysine that is produced by bacterial fermentation. Poly-L-lysine is a positively charged amino acid polymer with approximately one HBr per lysine residue. The hydrobromide allows the poly-L-lysine to be in a crystalline form soluble in water. Adhesion

**Figure 6.** Structure of poly-L-lysine.

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 chain increases the condensation [61–68].

## **4.3. Polyethylenimine**

Polyethylenimine (PEI), as given in **Figures 8** and **9**, is water-soluble, linear or branched polymers composed of the amine group and two carbon aliphatic CH2 CH2 spacer. It is a 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 efficiency in gene transfection, and are more cytotoxic [69–93].

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

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

**Figure 9.** Structure of dendrimer PEI.
