Gene Delivery Using Chemical Methods - Challenges in Delivery of Therapeutic Genomics and Proteomics - page 138

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Figure 4.4 Chemical structure of poly(-amino ester).

been utilized as discovery-based platforms for the identification of new polymeric
vectors through high-throughput cell-based screening assays, as well as for the identi-
fication of emerging structure-property relationships for this class of materials. These
strategies provide the identification of new poly(-amino ester)s that exhibit gene
delivery in vitro more efficiently than either PEI or lipid-based vectors ( Fig. 4.4 ).
Poly(-amino ester)s are synthetic biodegradable cationic polymers with tertiary
amines in their backbones. Poly(-amino ester)s were synthesized by addition of either
primary or bis(secondary) aliphatic amines to diacrylate esters [70-72] . Polymerization
occurs by a step-growth mechanism, and the resulting linear polymers contain both
tertiary amines as well as esters in their backbones. To minimize hydrolytic degrada-
tion during synthesis of poly(-amino ester)s, synthesis was performed in either neat
(no solvent) or anhydrous organic solvents [70] . The average molecular weights of
polymers synthesized under these conditions are dependent on monomer structure and
type of solvent but normally lie within the range of 2000-50,000 Da [70,73] .
Poly(-amino ester)s are hydrolytically degradable polymers. The poly(-amino
ester)s were hydrolyzed to produce 1,4-butanediol and -amino acids in an acidic
and alkaline condition [70] . An aqueous solution of polymer hydrolyzes quickly, but
polymer self-assembled pDNA complexes degrade at a very slow rate. Normally,
solid or neat samples of polymer can be stored in dry form and used for several
months, without detectable degradation.
Moreover, pH-dependent solubility of several poly(-amino ester)s favors its for-
mulation in nanoparticle or nanosphere formulations that can prompt the polymer
degradation and release of encapsulated DNA in the acidic pH of endosomal vesicles
[73] . Owing to its cationic nature, they are capable of condensing DNA into com-
plexes of the order of 50-200 nm. At low polymer concentration, DNA is initially
condensed into small, negatively charged particles. However, size increases sharply
with addition of polymer due to aggregation of the particles at neutral charge. Further
addition of polymer decreases size and gives stable dispersions of positively charged
complexes ranging from 100 to 200 nm in diameter. Biodegradable polycations are
normally less toxic than polymers such as PLL and PEI.
Although the results obtained from MTT assay are promising indications of bio-
compatibility of the polymer, additional work must be done to firmly establish the
safety of poly(-amino ester)s as gene delivery agents in vivo . By working on the
border range of polymer, it has been demonstrated that cytotoxicity is a function of
polymer structure. It was observed that self-assembled polymer-DNA complexes
formed from certain polymers may be more cytotoxic than free uncomplexed poly-
mers [73,74] . This raises the need to evaluate new polymers over entire conditions
and forms likely to be exhibited in the context of gene delivery.