Gene Delivery Using Chemical Methods - Challenges in Delivery of Therapeutic Genomics and Proteomics - page 140

[NVP] or ethoxytriethylene glycol methacrylate [triEGMA]) revealed comparable
cytotoxicity and transfection efficiency to the pDMAEMA homopolymer of compa-
rable molecular weight (around 100,000 Da) [80] . The 48 mol% content of triEGMA
copolymer showed both reduced transfection efficiency and reduced cytotoxicity,
whereas 54 mol% of NVP copolymer demonstrated enhanced transfection efficiency
and decreased cytotoxicity compared to the pDMAEMA homopolymer. This dem-
onstrated role of charge density of pDMAEMA polymers as an important parameter
for optimizing the efficacy-toxicity ratio. Nevertheless, a high-molecular-weight
pDMAEMA (mol. wt 10,000 Da) is still better in mediating transfection.
Poly[2-(trimethylamino)ethyl methacrylate] (pTMAEMA), a quaternary ammonium
analogue of p(DMAEMA), did not possess the inherent property of endosomal escape,
in contrast to the buffering effect for endosomal escape possessed by pDMAEMA [81] .
However, pTMAEMA exhibited higher affinity for DNA compared to pDMAEMA, as
observed in the complex dissociation studies using poly(aspartic acid) [82] .
In vitro transfection efficiency for pDMAEMA-DNA complexes was not affected
by the presence of 2% serum. However, the particle size of pDMAEMA-DNA com-
plexes increased to greater than 600 nm [76] . The particle formation of 600 nm size
is a possible indication of aggregation with negatively charged serum proteins. The
DNA complexed to pDMAEMA may be displaced by serum proteins. Also, systemic
administration of pDMAEMA polyplexes demonstrated aggregate formation of posi-
tively charged polyplexes with blood components.
Cell trafficking experiments demonstrated that endosomal escape is a limiting
bottleneck for pDMAEMA copolymers. To overcome the problem of endosomal
escape, new polymer pDAMA, with two tertiary amine groups in each monomeric
unit, poly(2-methyl-acrylic acid 2-[(2-(dimethylamino)-ethyl)-methyl-amino]-ethyl
ester) was synthesized [83] . pDAMA has two p K a; one p K a of 5 provides endosomal
buffering and the other p K a of 9 is responsible for cationic charge. pDAMA has a
low toxicity but also very low transfection activity.
The addition of functional group poly- N -(2-hydroxypropyl)methacrylamide
(pHPMA) to cationic-hydrophilic block copolymers was believed to prolong circulation
time by providing steric stabilization and minimizing interactions with cells and pro-
teins [23] . pHPMA was copolymerized with poly(trimethylammonioethyl methacrylate
chloride) (pTMAEM). Although pHPMA-pTMAEM-DNA complexes give monodis-
perse particles of 70 nm size, the complexes failed in the heparin challenge test. These
results were indicative of lack of stability in circulation when exposed to blood proteins.
Biodistribution studies after IV administration of positively charged pDMAEMA-
[ 32 P]-DNA polyplexes showed that it was primarily distributed to the lungs, and 80%
of the injected IV dose was recovered from the lungs within minutes. Although these
present pDMAEMA systems demonstrated positive result in some studies, the stabil-
ity and toxicity issues are of major concern and must be addressed effectively before
establishing pDMAEMA as an efficient vector for gene delivery.
4.2.1.5 Polyesters
Polyesters are water-soluble, readily degradable cationic polymers. Poly(4-hydroxy-
L-proline ester) (PHP) was the first polymer of this class used as a gene vector, as