Challenges and Opportunities in Gene Delivery - Challenges in Delivery of Therapeutic Genomics and Proteomics - page 54

Figure 2.2 DNA degradation by serum nuclease and its prevention after complexation with
cationic lipid or polymer.
are responsible for rapidly clearing these complexes from the systemic circulation,
thus limiting their in vivo activity.
2.3.2.2 Colloidal Instability
Lipoplexes and polyplexes are colloidal suspensions or complexes of DNA condensed
with cationic lipids and polymers, respectively, which have shown significant instabil-
ity in the extracellular compartment of the body. For most systems employing cat-
ionic lipids or polymers to deliver DNA, aggregation of the complexes is frequently
observed, with complexes prepared near charge neutrality on standing [60] . In the
presence of excess positive charge, the DNA-polycation complex often shows reduced
colloidal stability in the presence of serum and during in vivo condition, because of
interaction with negatively charged serum proteins. It has been observed that cationic
polymers with high charge densities, such as polyethyleneimine, are most resistant
to particle disintegration when treated with the polyanions [61] rather than cationic
lipids. In the case of cationic lipids, incorporation of the helper lipids DOPE or cho-
lesterol can significantly enhance the resistance of these complexes to polyanion-
mediated disassembly [61-64] . Overall, cholesterol has been found to increase the
stability of cationic lipid-DNA complexes in serum better than 1,2-dioleoylphos-
phatidylethanolamine (DOPE) and hence may show better in vivo stability [62-64] .
Cholesterol also stabilizes complexes against fusion with red blood cells when admin-
istered intravenously [64] . It has been observed that cationic lipid-based delivery sys-
tems show a significant effect of charge density of lipid bilayer on colloidal stability
of the lipoplexes, with mole fractions of helper lipids (e.g., DOPE) of 0.4 producing
considerable instability at elevated ionic strength [65] .
The increased ionic strength of the biological milieu also significantly reduces
the physical stability of the DNA-polycation complex with a tendency to aggregate,