Non-viral Gene Therapy - Intracellular Delivery: Fundamentals and Applications - page 648

(Høgset et al. 2000 ). The efficacy of PEI-based polyplexes can also be enhanced by
PCI mediated transfection. Successful PCI-mediated gene delivery in vivo has been
achieved by Kataoka and coworkers. To this purpose, they developed a ternary
complex composed of a core polyplex containing DNA condensed with cationic
peptides and enveloped in the anionic dendrimer phthalocyanine that served as
photosensitizer. This ternary complex showed more than 100-fold photochemical
increase in transgene expression in vitro with reduced photocytotoxicity. In a rat
model, subconjuctival injection of the ternary complex followed by laser irradiation
led to transgene expression only in the laser-irradiated site.
In addition to PCI mediated transfection, photoresponsive gene delivery can also
be implemented by directly incorporating photosensitive unit into the non-viral
vector. Nagasaki et al. synthesized a novel cationic lysine-modified polyazoben-
zene dendrimer (Lys-G2) (Nagasaki et al. 2000 ). Polyplexes formed by Lys-G2/
pDNA showed a 50% increase in the transfection efficiency in COS1 cells upon UV
irradiation, as compared with the case without UV irradiation. This was attributed
to the decreased cationic density on the surface of the complex to suppress the
cationic repulsion, resulting from trans -to- cis photo-isomerization.

3.9.5 Protein Kinase Responsive Vectors

Katayama et al. have developed a novel gene regulation system responding to
specifically and abnormally activated intracellular enzymes in diseased cells
(Katayama et al. 2002 ; Kawamura et al. 2005 ; Oishi et al. 2006a ). For this purpose,
a cationic polymer grafted with an oligopeptide that is a substrate for cyclic AMP-
dependent protein kinase (PKA) and could regulate gene-expression in a cell-free
system. PAK/DNA polyplexes showed no expression of the loaded gene in the
unstimulated NIH 3T3 cells. This suggested that PAK formed a stable complex with
DNA in the normal cells to totally suppress gene transfection. In contrast, significant
transgene expression was observed when the PAK/DNA polyplexes were incubated
with forskolin-treated cells. In this case, the activated PKA resulted in the disasso-
ciation of the polyplexes even in living cells, and therefore an efficient gene expres-
sion could be achieved.

4 Lipids as Gene Vectors

Since the landmark report by Felgner and coworkers (Felgner et al. 1987 ), cationic
lipids or lipid-like molecules have been extensively studied for gene transfection in
cultured cells, in animals and in patients enrolled in clinical trials (Dass 2004 ).
Most of the commonly used cationic lipids are commercially available, technically
simple and easy to formulate. Compared with viral vectors, lipids based vectors
are less biologically hazardous. Table 8 shows the structures of cationic lipids
frequently used for gene delivery.