Non-viral Gene Therapy - Intracellular Delivery: Fundamentals and Applications - page 623

3.2.5 Other Hydrophilic Modifications on PEI

PEI derivatives have been synthesized by conjugating with hydrophilic polymers
other than PEG, in an attempt to develop PEI-based polyplex formulations with
increased solubility, sufficient stabilization and prolonged in vivo circulation as well
as enhanced endosomolytic activity and improved nuclear transport. Table 5 sum-
marizes the selected examples about the polymers employed for PEI decoration.
A multivalent poly[N-(2-hydroxypropyl)methacrylamide] (PHPMA) bearing
reactive ester groups has been used for the surface modification of preformed PEI/
DNA polyplexes (Oupicky et al. 2002 ). The aqueous solubility and systemic circu-
lation times of the laterally stabilized complexes were enhanced compared with
those without PHPMA modification.
Tseng and coworkers systemically investigated the effect of dextran modification
of bPEI on the transfection mechanisms (Tseng and Jong 2003 ; Tseng et al. 2004,
2005 ). Polyplexes prepared using the modified polycations showed significantly
diminished cytotoxicity and improved stability against bovine serum albumin (BSA).
Unfortunately, transfection using green fluorescent protein (GFP) indicated that the
modified polymers were less efficient than the unmodified PEI. The impaired conden-
sation capability, buffering capacity, cellular entry and the integrity of DNA-polymer
polyplexes were proposed to be responsible for the decreased transgene expression.
Poly(N-isopropylacrylamide) (PNIPAm) is a water-soluble polymer that exhibits a
lower critical solution temperature (LCST) at about 32°C. Generally, copolymers con-
taining PNIPAm segment are also thermosensitive. To achieve a temperature-regulated
gene transfection, studies have been conducted to modify PEI using PNIPAm (Twaites
et al. 2004, 2005 ; Zintchenko et al. 2006 ; Lavigne et al. 2007 ). Temperature-dependent
transfection activities were observed for polyplexes based on PNIPAm/PEI copolymers.
These results indicate that changes in molecular structures triggered by carefully
designed stimuli during intracellular trafficking may be used to enhance gene delivery.
Pluronic, also termed 'Poloxamer' is triblock copolymers of poly(ethylene oxide)-
b -poly(propylene oxide)- b -poly(ethylene oxide) (PEO-PPO-PEO). Despite their
amphiphilicity, they are essentially water soluble polymers. The hydrophobic PPO
block in Pluronic is considered to be able to stabilize the polyplexes of polycation and
DNA, and enhance the intracellular translocation of Pluronic molecules. PEI conju-
gates with Pluronic 85 could target the payload oligonucleotide (ODN) to the hepa-
tocytes of the liver, different from the kidney accumulation mediated by PEGylated
PEI (Ochietti et al. 2002 ). In addition, Pluronic (P85) modified PEI (PEI-P85) could
improve the solubility of PEI/DNA complexes (Belenkov et al. 2004 ). Polyplexes
formed by PEI-P85 and Ku86 antisense oligonucleotide were highly stable in aque-
ous dispersions. In vivo therapeutic effect was implemented using this new formation
in athymic nude mice bearing subcutaneous human HT29 colon adenocarcinoma
xenografts. The applications of Pluronic copolymers for gene delivery can refer to
three reviews(Lemieux et al. 2000 ; Kabanov et al. 2002 ; Bromberg et al. 2006 ).
CDs, especially b-CD, have been used for modifying PEIs (PEI-CDs) to lower
their cytotoxicity (Pun et al. 2004a ; Forrest et al. 2005 ). PEI-CDs showed signifi-
cantly enhanced in vitro transfection levels for several cell lines compared with
PEI25. After they were further stabilized using a 1-adamantanemethylamine (Ada)