Gene Delivery Using Chemical Methods - Challenges in Delivery of Therapeutic Genomics and Proteomics - page 134

N N

H H

l m

N N NH

H

H NH 2

O

l

m

NH

NH 2

O

n

O

n

O

: RGD
Antibody
Folate

N

H

O

n

N

n

NH 2

Linear PEI Branched PEI PEI- g -PEG PEI- g -PEG-ligand

Figure 4.3 Chemical structures of PEI-based gene vectors.

synthesized by acid-catalyzed aziridine (ethyleneimine) ring opening or by hydroly-
sis of poly(2-ethyl-2-oxazolium) oxazoline, leading to branched or linear polymeric
backbones, respectively. Molecular weight varies depending upon chain length. The
desirable properties that promote its use in gene delivery are low cost, ease of prepa-
ration or modification, and safety.
Unlike PLL, PEI shows efficient gene transfer without the need of any agents facili-
tating receptor-mediated uptake as it promotes endosomal escape from intracellular
vesicles. This attribute is based on the “proton sponge” effect [43] . PEI has a very high
charge density because every third atom in the backbone is amino nitrogen capable of
being protonated over almost the entire pH range, thus offering a considerable buffering
capacity [44,45] . PEI is able to change its protonation with respect to surrounding pH.
Electrostatic interaction between these positively charged amino groups in PEI
and the negatively charged phosphate groups in DNA molecules permits condensa-
tion of DNA, thereby the formation of polyplexes. This process alleviates intramolec-
ular repulsions and releases many sodium and chloride ions. A kinetically controlled
mechanism provides multimolecular polydisperse particles that will eventually
aggregate further depending on the surrounding medium. The mechanism described
for entry into the cell is quite common [46] and is similar to the particle engulfment
by animals (chylomicrons) and pathogens (bacteria and viruses). It binds ubiquitous
transmembrane adhesion molecules to the syndecan receptor and utilizes the extra-
cellular matrix catabolism by the adherent cell.
The last stage of this catabolic process is lysosomal compartment, where endo-
somal trafficking occurs from the cell surface to the lysosomes, with acidification.
Due to the length of time, it does leave time for DNA complexes to escape. In an
attempt to prolong the endosomal trafficking time, PEI exploits its noncharged nitro-
gen. All amines in PEI are not protonated at physiological pH due to the proximity of
the nitrogen atoms within the structure. Thus, it provides a buffering action (proton
sponge) that will delay the acidification and fusion with the lysosome [45] .
The buffering provided by PEI due to acidic pH in the endosome causes proton
accumulation and subsequent influx of chloride ions into the vesicle. Further osmotic
swelling by the influx of water ruptures some of the endosomes, which allows the