Gene Delivery Using Chemical Methods - Challenges in Delivery of Therapeutic Genomics and Proteomics - page 127

Carriers for Gene Delivery: Nonviral Vectors
Nonviral vectors are relatively cheap, easy to formulate and scale up, nonimmuno-
genic, and relatively safe for human administration [7] . The chemical structure of these
vectors determines efficacy of transgene expression. Thus characteristics of the nonvi-
ral vectors could modify the efficacy of some processes, such as interaction with cell
surfaces, internalization in cells, and transgene expression. The chemical approaches
use synthetic or naturally occurring compounds as vectors to deliver the transgene into
cells. Four decades back, DADE-Dextran was the first compound used as a synthetic
nonviral vector [8] . Since then, continuous attempts to produce an efficient and safe
nonviral vector led to the evolution of several nonviral vectors with different character-
istics or properties. Each of these types of vector has its own advantages and disadvan-
tages. The ideal characteristics of a vector for nonviral gene therapy include:

l

The vector must condense or encapsulate DNA into a smaller size able to enter into the
cell.

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The vector must be capable of protecting DNA from the external environment.

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The vector should maintain integrity and stability after in vivo administration.

l

The vector must possess some functional moieties to facilitate its escape from the endo-
somes into the cytoplasm.

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The vector should release DNA from the complex inside the cell in its active form.

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The vector should have some targeting moiety to direct the delivery to a particular type of
cell for gene transfection and nuclear localization.

For efficient gene transfection, the DNA has to reach the nucleus of the cell by
passing several barriers such as unfavorable physiological conditions and cellular bar-
riers. The cellular barriers are the cell membrane, endosomal membrane, and nucleus
membrane. Basic requirements for an efficient vector are for it to be able to protect the
DNA from degradation and to condense or encapsulate the DNA into small particles
for easy internalization. By virtue of its excess cationic charge, vectors disguise the
negative charge of the DNA, thus providing a net positively charged stable complex.
Another requirement is that the complex formed be able to withstand harsh enzymatic
conditions present in the lysosome of the cell. The mechanism of nonviral gene trans-
fer in cell cultures is basically pinocytosis facilitated by electrostatic or hydrophobic
interactions between the gene vector and the cell surface composed of proteoglycans
or sialylated glycoproteins.

4.2 Polymeric Vectors

The polymer plays a vital role in the delivery of DNA to the cell. In contrast to con-
ventional polymeric formulation, where the drug is either simply diffuse or released
by hydrolysis or esterification of polymer, the DNA cannot diffuse easily because of
its high molecular weight. Thus, polymeric vectors continue to play a key role in the
interaction with cell membranes, in intracellular trafficking, and in transcription of
the transgene in the nucleus. The efficient vector polymer delivers the transgene to
the nucleus after bypassing several barriers. Although viral vectors are still widely