Gene Delivery Using Chemical Methods - Challenges in Delivery of Therapeutic Genomics and Proteomics - page 131

blood. However, PLL 211 and naked DNA do not aggregate. Despite encouraging
results, there are still problems that prevent PLL entering into clinical studies. Efforts
have been made to beat these challenges and improve the characteristics of PLL.
PLL lacks tissue specificity; therefore, to improve the specificity and transfection
efficiency, targeting moieties have been conjugated to PLL.
4.2.1.1.1 PLL Conjugates
Poly(ethylene glycol) (PEG) is conjugated to prevent the interparticular aggrega-
tion of the complexes and to increase complex stability in the presence of negatively
charged serum proteins. To bestow tissue specificity, specific targeting moieties, such
as peptides, sugars, antibodies, and folate (FOL), are conjugated to the complexes.
Introduction of biodegradable bonds or moieties to PLL also reduces the cytotox-
icity. Poly(-[4-aminobutyl]-L-glycolic acid) (PAGA) is a biodegradable analog of
PLL, in which the peptide bond is substituted by a degradable ester bond.
4.2.1.1.1.1 PLL-PEG Copolymers PEGylation of cationic polymers reduces cyto-
toxicity, improves protection from nucleases, and enhances solubility of polyplexes,
making it suitable for in vivo applications [21-23] . Besides, the presence of PEG
prevents interactions with serum proteins and imparts a long circulation time to poly-
plexes [24] . An A-B type PLL-PEG block copolymer, synthesized by ring-opening
polymerization, of size over 100 nm, showed a significant reduction in surface zeta
potential and cell cytotoxicity [23] . In another study, the PEG-PLL-DNA complex had
a diameter of about 48.5 nm, with a small fraction of secondary aggregates. It was con-
cluded that the size of the complex increases with an increase in the length of the PEG-
PLL chain [22] . PEG can also stabilize the complex sterically. The protective effect of
PEG-PLL-DNA complex demonstrated 1.5% degradation as compared to naked DNA,
which increased with increasing molecular weight of PLL [21] . The transfection effi-
ciency of PEG-PLL-DNA complex was increased by six fold to 30%. This may be
due to membrane activity or the dehydrating fusogenic effect of PEG when applied at
a high local concentration.
Comb-shaped PEG-grafted PLL (PEG- g -PLL) was first synthesized by Choi et al.
[16] . The PEG- g -PLL-DNA complex demonstrated enhanced transfection efficiency
and reduced cytotoxicity in HepG2 cells compared to PLL. When PEG- g -PLL-DNA
complexes were prepared using different grafting ratios (10, 15, and 20 mol%),
10 mol% PEG- g -PLL showed the highest transfection efficiency and most efficient
protection of DNA from enzymatic degradation [25] . The delivery of 10 mol% PEG-
g -PLL in combination with a plasmid-encoding antisense mRNA reduced expression
of glutamic acid decarboxylase (GAD) in mouse insulinoma cells in vitro . The sys-
temic administration of the PEG- g -PLL-antisense plasmid complex into mice could
deliver the antisense plasmid to the target organ, the pancreas, although PEG- g -PLL
is devoid of any targeting moiety. This is indicative of the possible role of PEG- g -
PLL with a specific targeting moiety in enhancement of the pancreatic delivery of
therapeutic plasmids [25] .
4.2.1.1.1.2 Antibody-Conjugated PLL Although antibody-antigen interaction is
one of the most specific interactions in biological systems, it should be confirmed