20.1 Structure-mechanism relationship of lipoplex

Lipoplexes are liquid crystalline globules as studied by fluorescence microscopy. Lipoplex after entering the cell by endocytosis cause DNA to remain trap in the endosome. Charge of cationic liposome vector σ M governs the transfection efficiency. As σ M decreases, the transfection efficiency increases. When the σ M is low, then the lipoplex containing DNA remains intact. Whereas when σ M is high, the DNA is released in the cell. When liposomes contain DOPE, the transfection efficiency is considerably increased. Mixed phospholipids have anionic lipids and cholesterol along with DOPE to promote H II phase organization.

20.2 Formation, structure and stability of polyplexes

20.2.1 Structure of cationic polymers

Cationic polymers like histones are naturally present in the biological system. Polyethylenimine (PEI), cationic dendrimers, 2-dimethylaminoethyl methacrylate (pDMAEM), and chitosan are some of the synthetic cationic polymers. Most widely used polymers for gene delivery includes poly L-lysine (PLL) and PEI, wherein PEI efficiently delivers the transgene and leads to a permanent expression specifically only in the target region. Cationic polymers form a complex with DNA and condense it to a small size, which is an important parameter for in vivo gene transfer.

Cationic polymers unlike cationic lipids lack hydrophobic domain so cannot fuse with endosomal membrane. Polylysine and polyarginine are first generation cationic polymers. They possess a weak endosomal escape and transfection capability, their transfection requires simultaneous co-transfection of endosome lytic agents such as inactivated adenovirus. Second generation polymers include PEI and polyamidoamine dendrimers (PMAM), which acts as proton sponge causing endosome disruption.

The PLLs are biodegradable in nature which enables its use for in vivo application such as gene transfer. However it binds to plasma proteins and is rapidly removed from circulation. To avoid, PLLs are usually coated with PEG which enhance their transfection efficiency and half life. Other modifications which can be made to improve the transfection efficiency include a targeting ligand coupled with PEG and introduction of histidine residues to create proton sponge effect. The chitosan, carbohydrate based polymer, can also condense DNA into small size to form a stable particles which can be used as a transfection reagent. Chitosan polyplexes are more effective for long term expression. Starburst polyamidoamine dendrimers are spherically branch shaped structures used for gene transfer, its efficiency depends on shape, size, primary amine groups, and cell type. Fractured dendrimers are formed by heat degradation and are more flexible then dendrimers. Fractured dendrimers have better transfection ability.

Different approaches for designing and optimizing chemical structures are being formulated. First one is based on the biological barriers for gene transfer that includes rational designing of chemical structure. Second approach is its structure activity relationship by modifying its chemical structure. Molecular weight, branching, and surface charges are considered for designing of polymer.