Biomedical Engineering Reference
In-Depth Information
micelles or any higher order structures at concentrations from 0.1 to
10 mg mL
-1
at pH 7.4.
92
Then, a new generation of siRNA delivery polymers
based on this design was developed which exhibited enhanced transfection
efficiency and low cytotoxicity. This design incorporated a longer endosomo-
lytic block with increased hydrophobic content to induce micelle formation.
These polymers spontaneously formed spherical micelles in the size range of
40 nm with CMC values of approximately 2 mgmL
-1
. siRNA binding to the
cationic shell block did not perturb micelle stability or significantly increase
particle size. The self-assembly of the diblock copolymers into particles was
shown to provide a significant enhancement in mRNA knockdown at siRNA
concentrations as low as 12.5 nM.
93
Based on the same linear polymer, Palanca-Wessels et al. further developed a
targeted delivery system comprising (i) an internalizing streptavidin-conjugated
monoclonal antibody (mAb-SA) directed against CD22 and (ii) a biotinylated
diblock copolymer, poly(DMAEMA)
m
-b-(BMA
x
-co-DMAEMA
y
-co-PAA
z
)
n
.
Treatment of lymphoma DoHH2 cells with CD22-targeted polymeric micelles
exhibited enhanced siRNA uptake and gene knockdown, with a 70% reduction in
gene expression at the 15 nmol L
-1
siRNA concentration. Thus, this CD22-
targeted polymer carrier might be useful for siRNA delivery to lymphoma cells.
94
For sustained local delivery of siRNA, Nelson et al. provided a means to package
and protect siRNA within the pH-responsive, endosomolytic micellar nanopar-
ticles (si-NPs) formed by poly(DMAEMA)
m
-b-(BMA
x
-co-DMAEMA
y
-co-
PAA
z
)
n
that can be incorporated into nontoxic, biodegradable, and injectable
polyurethane (PU) tissue scaffolds. The si-NPs were homogeneously incorpo-
rated throughout the porous PUR scaffolds, and they were shown to be released
via a diffusion-based mechanism for over three weeks. The PUR scaffold
releasate collected in vitro in PBS at 37 uC for 1-4 days was able to achieve dose-
dependent siRNA-mediated silencing with approximately 50% silencing of the
model gene GAPDH achieved in NIH3T3 mouse fibroblasts.
95
Elsabahy et al. developed pH-responsive polyion complex micelles (PICMs)
consisting of a poly(amidoamine) (PAMAM) dendrimer-nucleic acid core and a
detachable poly(ethylene glycol)-block-poly(propyl methacrylate-co-methacrylic
acid) [PEG-b-P(PrMA-co-MAA)] shell. The micelles displayed a mean
hydrodynamic diameter ranging from 50 to 70 nm, a narrow size distribution,
and a nearly neutral surface charge. The resulting nanocomplexes could
accommodate siRNA in their core. Anti-CD71 Fab9 was conjugated at the
extremity of the PEG segment via a disulfide linkage to promote entry of the
micelles into cancer cells mediated by specific ligand-receptor recognition. Upon
cellular uptake, the acidic pH in the endosomal compartment protonates the
carboxylate groups of the MAA, thus causing the displacement of PEG-b-
P(PrMA-co-MAA) from the PICM and the remaining unshielded PAMAM
nucleic acid core can then promote endosomal escape (Figure 7.7). When these
pH-responsive targeted PICMs were loaded with siRNA targeting the
oncoprotein Bcl-2, they exhibited a greater transfection activity than non-
targeted PICMs or commercial PAMAM dendrimers.
96
d
n
4
y
3
n
g
|
8
Because the coupling
