Gene Delivery Using Chemical Methods - Challenges in Delivery of Therapeutic Genomics and Proteomics - page 174

(DAE) cores are most frequently used for PPI dendrimer synthesis, due to their abil-
ity to form dendrimers up to G5.
PPI dendrimers are characterized by hyperbranching and radial symmetry. As
observed with PAMAM dendrimers, size of PPI dendrimers is directly related to
generational number. The outward growth of radial branching would suggest genera-
tional number limits on growth due to steric hindrance at the molecular surface.
The small-angle neutron-scattering measurements and molecular dynamics of
PPI dendrimers also support limitation on generational number [314] . Based on
the structure of PPI, it has acidic (p K a 5-8) internal ammonium functionalities and
basic amine peripheral groups (p K a 9-11). It was apparent that an atom of basic pro-
tonable nitrogen readily interacts with negatively charged DNA. PPI binds to DNA
via electrostatic interactions with only the peripheral amine moieties. Studies were
performed using Astramol™, commercially available PPI G5 DAB core dendrimers.
Tertiary amine at the interior of the structure mediates the proton sponge effect to
execute endosomal lysis [315] . The same group has reported in another study that
all generation PPI are capable of condensing DNA into compact particles and higher
generation structures condense to form water-soluble complexes [316] .
The first study performed using Astramol™ demonstrated very low transfection
efficiency in COS-7 cells as measured by luciferase expression [317] . In another
study conducted with human epidermoid carcinoma cells (A431) with a -galactosi-
dase, the reporter gene demonstrated that generations two and three show equivalent
gene expression as compared to positive control cationic liposomal transfecting agent
DOTAP. However, generations G4 and G5 displayed significantly lower gene expres-
sion as compared to DOTAP [318] .
PPI dendriplexes conjugated with TNF- expression plasmid showed that trans-
gene expression was efficient to achieve 100% survival of studied mice [319] .
N-quaternized PPI (G2) dendrimers displayed significantly reduced toxicity and liver-
targeted gene expression in contrast to Exgen 500, which accumulates in the capil-
laries of the lungs [320] . Surprisingly, PPI dendrimers (G2, G3) displayed alter the
expression of endogenous genes in both A431 and A549 cell lines and hence gene
transfer activity cannot be correlated [321] . Arginine was attached onto the periphery
of PPI dendrimers (G2). These arginine-conjugated dendrimers have shown signifi-
cantly higher transfection efficiency and reduced toxicity compared to both 25 kDa
PEI and unmodified PPI dendrimers (G2) [322] .
4.6.3 PLL Dendrimers
PLL-based dendrimers have been synthesized to minimize the cytotoxicity of lin-
ear PLL. Denkewalter et al. proposed synthesis of PLL-based dendrimers for the
first time, which followed a divergent route from a two-directional asymmetric core
derived from L-lysine and benzhydrylamine. The reaction proceeds as coupling
between the core and a Boc-protected lysine derivative activated by a p -nitrophenyl-
ester followed by an acid deprotection [323] . Aharoni et al. introduced synthesis of
dendritic PLL using Boc-L-Lys(Boc)-OH as a branch unit [324] . There is another