Biomedical Engineering Reference
In-Depth Information
Fig. 3 Activation of membrane-destabilizing component by acidic pH, resulting in endosomal
escape into the cytosol
charges in contrast to siRNA with 42 negative charges) has to be considered. For
example, the intracellular fates of medium-sized LPEI (22 kDa) and brPEI (25 kDa)
were compared using confocal fluorescence microscopy and FRET technology
[
208
]. The LPEI polyplexes were found to be more effective but less stable;
when reaching the cytosol, they release pDNA much faster than the more stable
brPEI polyplexes. The same research group [
209
] discovered that insufficient
decondensation of PEI/pDNA polyplexes in the nucleus was a major limiting factor
for gene expression. In this respect, polymers with lower pDNA affinity seem to be
preferable in transfections in vitro.
On the other hand, pDNA/PEI polyplexes were found to be not stable enough
in the extracellular in vivo environment. Unpackaging of PEI and PEG-PEI
polyplexes was observed [
64
,
65
,
81
], for example by serum proteins, soluble
glycosaminoglycans, or extracellular matrix components. The situation is even
worse in the case of siRNA polyplexes, where PEI polyplexes are dissociated in
full human serum, as monitored by fluorescence fluctuation spectroscopy [
66
,
67
].
Bioresponsive polymers are one logical solution for this “polyplex dilemma,”
i.e., the insufficient stability of polyplexes outside the cell, but too-high a stability
inside the cell. Strategies include the design of biodegradable high molecular
weight polymers that intracellularly degrade into low molecular weight nontoxic
fragments. Cleavable bonds include acetal bonds that degrade in the acidic envi-
ronment of endosomes [
200
,
210
], disulfide bonds that are reduced in the cytosol
[
207
,
211
,
212
], or hydrolyzable esters [
213
-
215
]. Polyplexes can also be
reversibly stabilized by caging [
50
-
52
,
109
,
216
,
217
], i.e., chemical crosslinking
