Challenges and Opportunities in Gene Delivery - Challenges in Delivery of Therapeutic Genomics and Proteomics - page 55

followed by immobilization of the complex. The increased ionic strength weakens
the protective electrostatic double layer around the complex and decreases the elec-
trostatic interactions between polycation and DNA; it also shields the interparticulate
electrostatic repulsive forces, resulting in aggregation of the complexes.
Several methods have been used to stabilize the DNA-polycation delivery systems
against the effect of increased ionic strength. Steric stabilization has been a popu-
lar method to achieve enhanced colloidal stability and can be achieved by creating a
“brush” layer of hydrophilic polymer on the surface of lipoplexes or polyplexes, thereby
decreasing self and nonself, nonspecific interactions. The length and density of the
PEG chain can have a significant influence on its stabilization and protective proper-
ties, and chain length is typically optimized for each individual delivery system [66,67] .
Steric stabilization of the polycations by using long-chain polyethylene glycol has been
found to avoid the aggregation in both the cationic lipids [67,68] and polymers [66,69] .
Conjugation using HPMA (poly[ N -(2-hydroxypropyl)methacrylamide]) [70] and PEG
linking to polyplexes containing cyclodextrins [71] have also shown to enhance the col-
loidal stability. The steric stabilization of these systems can be accomplished without
alteration of polyplex morphology or, obviously, disruption of the polyplex. Reversible
crosslinking of cationic polymers complexed with DNA has also been a successful
approach in enhancing the colloidal stability at increased ionic strength [72,73] .
Another alternative method used to increase the stability of DNA in the extracel-
lular milieu has been the use of hydrophilic, nonionic polymers, such as polyvinylpyr-
rolidone or polyvinyl alcohol, which weakly interact with DNA to form a reversible
neutral or anionic complex and prevent its degradation by nucleases [74,75] . These
systems avoid the problems related to the electrostatic charge-based interactions of
DNA-polycation complexes, resulting in enhanced colloidal stability of particles with
improved distribution in tissues after administration.
Steric stabilization of lipoplexes and polyplexes should allow specific cell types
to be targeted by utilizing the interactions between surface receptors and ligand-
containing nonviral gene delivery systems. The ligands attached to the lipoplexes and
polyplexes help to target the specific cells. However, these targeted lipoplexes or poly-
plexes should be sterically stabilized to minimize nonspecific interactions, and there
is a requirement to place the targeting ligand several nanometers away from the sur-
face of the particle to provide for effective binding to cell surface receptors. Several
investigators have projected the targeting ligand away from polyplexes through PEG
linkers, where the ligand is attached at the distal end of the PEG [10,56,71,76] . This
configuration has also been successfully exploited with lipoplexes.
2.3.2.3 RES Uptake
After intravenous administration of the DNA-polycation complex, it is necessary to
prevent unwanted interactions between these complexes and the dynamic environ-
ment of the blood circulation. The positively charged DNA-polycation complexes
have also shown instability because of their interaction with endogenous negatively
charged molecules, such as glycosaminoglycans, serum albumin, and other extracel-
lular proteins. This complexation facilitates the aggregation of the delivery systems