Challenges and Opportunities in Gene Delivery - Challenges in Delivery of Therapeutic Genomics and Proteomics - page 53

Apart from hepatic degradation, the blood cells, proteins, and enzymes present in
the blood circulation bind to DNA or the carriers designed to transport DNA, leading
to decreased stability and increased particle size or charge, and premature metabo-
lism [57] . Some of these cells also take up and degrade plasmid DNA, resulting in
little transgene expression following a simple intravenous injection of naked plasmid
DNA [58] .
2.3.2 Extracellular Barriers for Vector-Mediated DNA Delivery
For tissues and cells that are not amenable to systemic and local delivery of naked
DNA, vector-mediated delivery incorporating the DNA is used. These vectors are
used to facilitate the DNA uptake and transgene expression in the desired target cells
and tissues after systemic administration, and they protect the DNA against extracel-
lular and intracellular enzymes. Ideally the vector system should be nontoxic; should
not self-aggregate; should evade the adaptive immune system and RES uptake;
should minimize interactions with plasma proteins, extracellular matrices, and non-
targeted cell surfaces; and should protect and deliver the DNA at the desired site of
action for effective transgene expression [1,15,17,21] .
2.3.2.1 Partial Degradation of DNA after Complexation with Polycation
Naked DNA delivery after systemic administration is significantly inhibited by the
barriers of size, shape, and polyanionic charge of DNA, thus inhibiting the cell per-
meability of DNA and DNA susceptibility against serum nuclease. However, these
barriers have been partially resolved by mixing the DNA with a polycationic lipid,
polymer, or inorganic material capable of complexing with or entrapping the DNA by
ionic interaction. Interaction of cationic lipid and polymers with negatively charged
DNA forms lipoplexes and polyplexes, respectively, generally with a net cationic
charge. These positively charged complexes enhance the interaction with cell surface
and cell uptake by associating with the negatively charged cellular membrane [59] .
These complexes are formed by condensing of negatively (-) charged supercoiled,
open circular and linear DNA on the positively () charged nitrogen atom of the cat-
ionic lipid and polymer, thus forming a condensed layer of DNA on the polycation, to
form a complex. The condensing results in neutralization of the polyanionic charge
of DNA to form a cationic complex, and it also protects the DNA from degradation
against serum nuclease, probably by blocking the enzyme interaction sites [34,36,48] .
However, protection to the DNA has been found to be partial, with DNA-polycation
complexes formed near charge neutrality showing partial degradation in vitro and
in vivo . By increasing the ratio of positively charged lipid to DNA, the degradation
barrier may be resolved by tighter condensation of DNA over the lipid and polymer
layer [44,60] . The DNA degradation by the serum nuclease and its protection after
complexation with cationic lipid or polymer is shown in Fig. 2.2 .
However, after complexation with DNA, stability of these cationic systems in the
extracellular milieu, such as intercellular or intravascular spaces, has been in question;
in addition, various physical barriers encountered during the extracellular compartment