Non-viral Gene Therapy - Intracellular Delivery: Fundamentals and Applications - page 620

HeLa, and HepG2 cells of PSI-PEI/DNA complexes was rather lower than that
based on PEI25, especially at high N/P ratios. The most efficient gene transfection
of PSI-PEI/DNA complexes was even slightly higher than that of PEI25 in the same
cell lines. Decreasing in the transfection efficiency was observed as the molecular
weight of parent PEI increased from 600 to 1,200. PSI-PEI analogues synthesized
using linear LMW-PEI of 423 Da has also been demonstrated to be effective for
in vivo gene delivery and expression(Yu et al. 2009b ). Wen et al. conjugated 423 Da
PEI to a poly(glutamic acid) derivative by aminolysis (Wen et al. 2009 ). In vitro
studies revealed that the newly synthesized polymers displayed markedly higher
transfection efficiency than PEI25 in Hela, HepG2, Bel 7,402, and 293 cell lines.
Importantly, polyplexes based on these polycations were more tolerable to serum
compared with those originated from PEI25 and Lipofectamine 2000. LMW-PEIs
were also covalently coupled to chitosan, and the synthesized polymers with low
cytotoxicity showed significantly higher transfection efficiency in both in vitro and
in vivo tests (Wong et al. 2006 ; Jiang et al. 2007 ; Lu et al. 2008 ). Through a hex-
amethylenediisocyanate spacer, LMW-PEI (800 Da) could be grafted onto dextran
to give cationic polymers with effective vectors. In vivo studies, however, have not
been reported so far to these carriers, although some promising in vitro results
achieved (Sun et al. 2008a, b ). For these LMW-PEI grafted cationic polymers, the
main chain structure should have profound effects on the transfection behaviors of
resulting polyplexes.
In general, cross-linking of LMW-PEI can produce pseudo-HMW-PEI with great
structural diversity and multiple functionalities. For example, polymers with linear,
graft or dendrimer-like architecture can be obtained, using linear or branched LMW-
PEIs as initial materials. All the cross-linked products showed improved character-
istics in terms of transfection efficiency and toxicity, at least to some extent, when
compared to their precursor PEIs or PEI25. Additional functions, such as pH and
chemical (such as intracellular glutathione) responsiveness can be integrated to the
resulting polymers to offer better performances for gene delivery. However, it
remains a challenge currently for delicate structure control over the cross-linked
PEIs, and batch to batch variation is frequently observed. This makes it difficult to
establish a useful structure-function relationship for designing and developing much
more effective and safe PEI-based vectors for clinical applications.

3.2.3 Ligand-PEI Conjugates

To replace the non-specific electrostatic interactions between polyplexes and target
cells with a cell-specific interaction, different types of ligands such as sugars, pep-
tides, proteins and antibodies have been used for modifying PEI. Galactose residues
were conjugated to PEI25, to target hepatocytes which express asialoglycoprotein
receptor. Polyplexes of galactosylated PEI (gal-PEI) showed comparable transfec-
tion efficiencies at low substitution degrees (3.5%), an enhancement in efficiency
with a higher conjugation of 5% (Kunath et al. 2003a ; Zanta et al. 1997 ). When the
amount of galactose was further increased to 31%, however, a decrease in transfection