Gene Delivery Using Chemical Methods - Challenges in Delivery of Therapeutic Genomics and Proteomics - page 173

artery with cationic polymer-plasmid complexes containing fractured PAMAM den-
drimer of G6 generation was found efficient [302] . PAMAM starburst dendrimer (G9)
was found effective after intravascular and endobronchial administration to murine
lung tissue [293] . PAMAM starburst dendrimer also mediates efficient gene transfer
into murine cardiac grafts [303] . Dermal application of PAMAM starburst dendrimer
in combination with poly(D,L-lactide- co -glycolide) or collagen-based biocompatible
membranes was explored to facilitate transfection of dermal cells [304] .
Various modifications to the basic PAMAM dendrimer structure have been made
to improve the efficiency of gene delivery. Cytotoxicity of PAMAM dendrimers is
reduced by neutralizing the surface of the dendrimer. An internally quaternized
PAMAM dendrimer having a hydroxyl periphery was synthesized [305] . These
dendrimers exhibited significantly reduced cytotoxicity compared to unmodified
PAMAM; however, transfection efficiency was also reduced. Later, the same group
has synthesized triblock copolymer, consisting of a PEG core with two PAMAM
substructures on each side. This dendrimer exhibited high transfection efficiency
and little cytotoxicity as compared to PEI [306] . Starburst PAMAM dendrimers were
conjugated with different cyclodextrins (CDs; , , and ) to improve transfection
efficiency of the dendrimer [307] . In vitro study of -CD conjugate displayed great-
est transfection activity, approximately 100-folds higher than that of the unmodified
dendrimer, and superior to that of Lipofectin TM . In vivo gene transfer was also found
superior than that of unmodified dendrimers [308] .
Arginine-grafted dendrimer, prepared by L-arginine conjugation with primary
amines located on the surface of PAMAM, demonstrated increased gene delivery
compared to native PAMAM dendrimer and L-lysine-grafted PAMAM, when evalu-
ated against 293,HepG2, and Neuro 2A cells and in primary rat vascular smooth mus-
cle cells [309] . Modified PAMAM dendrimer was prepared by conjugating it with
short peptide ligand specific for glucose-regulated protein-78 (GRP-78), a particular
tumor antigen. The peptide-conjugated dendrimers have shown better tumor targeting
than PEI [310] .
4.6.2 Poly(Propylenimine) Dendrimers
The second class of dendrimers represented by commercially available dendri-
poly(propylenimines) (PPI) was first synthesized in the late 1970s [311] . The ini-
tial synthetic route involved repetitive sequence of the Michael addition of amines to
acrylonitrile, followed by the reduction of the nitriles to primary amines with various
reducing agents. This scheme was capable of producing low-molecular-weight den-
drimers (1.5 kDa), but not generation dendrimers. As a result, despite the success-
ful synthesis of this first dendrimer-based structure, industrial application remains
limited. A later modified scheme was proposed, containing a second step of catalytic
hydrogenation of the nitriles, with Rainey cobalt and hydrogen, to primary amines.
The modified synthetic scheme significantly improves the yield and efficiency of the
synthetic process [312,313] .
Much less work has been performed on these polymers related to gene transfec-
tion as compared to PAMAM dendrimers. Diaminobutane (DAB) and diaminoethane