Gene Delivery Using Chemical Methods - Challenges in Delivery of Therapeutic Genomics and Proteomics - page 176

rior transfection efficiency as compared to conventional PLL dendrimers, which was
attributed to their more globular nature [333] . The application of PLL dendrimers as
vectors for ODN and siRNA was also explored [334-336] .
4.6.4 Phosphorus-Containing Dendrimers
Using synthetic strategies, phosphorus-containing dendrimers have generated mac-
romolecular structures with a considerably higher generational number than that of
either PAMAM or PPI. A synthetic scheme was initiated by reaction between hexa-
chlorocyclotriphosphazene and 4-hydroxybenzaldehyde. Furthermore, reaction
with methylhydrazine followed by the addition of diphenylchlorophosphine con-
fers a dendron structure that can undergo a Staudinger-type reaction with an azide
group to give a first-generation dendrimer structure. A Staudinger-type reaction can
be repeated to give higher generational dendrimer structure [337,338] . This amine-
terminated polyaminophosphine dendrimer demonstrated considerable gene trans-
fer activity. Also, it displayed reduced aggregation behavior, which imparts serum
stability [339,340] . Incorporation of anionic oligomers into the pDNA solution prior
to complexation with the dendrimer leads to the formation of less-condensed, phos-
phorus-based dendriplexes, which exhibited significantly increased gene transfection
efficiency [341] .
4.7 Biopolymers
4.7.1 Chitosan
Chitosan is biodegradable, biocompatible, polycationic, naturally derived polymer
with low immunogenicity ( Fig. 4.19 ). It is produced by alkaline deacetylation of chi-
tin, one of the most abundant natural polymers as a component of the exoskeletons
of crustaceans, to form a polymer composed of two subunits, D-glucosamine and
N -acetyl-D-glucosamine, linked by 1,4-glycosidic bonds [342] . The glucosamine back-
bone of chitosan contains a high density of amino groups that can be protonated, and
thus offer the opportunity to complex chitosans with negatively charged DNA mol-
ecules via electrostatic interactions. The high-molecular-weight chitosan (more than
100 kDa) is soluble only in dilute acid, whereas the low-molecular-weight chitosan
(22 kDa) is highly soluble in physiological buffer solutions [343] .
Cho et al. established that chitosan polyplexes adsorb to the cell surface through
electrostatic interactions, followed by internalization into the cells by endocytosis
[344] . TEM and confocal laser-scanning microscopy (CLMS) studies have shown
the delivery of chitosan polyplexes in endosomes [345,346] . Chitosan can effectively
bind DNA and protect it from nuclease degradation. The molecular weight, degree
of deacetylation N-P charge ratio, and pH are the major factors that govern gene
transfer efficiency of chitosan. Although high-molecular-weight chitosan increases
the size of the polyplex formed [347] , they form more stable complexes with DNA
because of a chain entanglement effect [348] . High-molecular-weight chitosan is