Polymeric Micelles for siRNA Delivery - RNA Interference from Biology to Therapeutics - page 167

PIC micelles formed with PEG-PLL, and the regulated release of free DNA from
the polymeric micelle was reported to be triggered through replacement by the
counterpolyanion [ 5 ] .
Similarly to the micellar assembly of the PIC micelles with DNA, block copoly-
mers consisting of PEG-polycation segments lead to the self-assembly of a structure
with a core-shell architecture, when mixed with siRNA. The first passive targeting
PIC micelle prepared for siRNA delivery comprised of a PEG-block catiomer car-
rying diamine side chain with distinctive p K a [ 60 ] . PEG-poly(3-[(3-aminopropyl)
amino] propyl aspartamide (PEG-DPT) was synthesized by a side chain aminolysis
reaction of PEG-poly(b -benzyl L-aspartate) block copolymer (PEG-PBLA) with
dipropylene triamine (DPT). The resulted block copolymer could efficiently incor-
porate the siRNA to form micelles and showed luciferase gene knockdown.
A siRNA nanocarrier system for active targeting was successfully reported by
Schiffelers and colleagues [ 33 ]. In this research, the authors utilized a copolymer
comprising PEI as the core-forming agent, PEG for steric stabilization, and peptide
ligand containing a disulfide-stabilized Arg-Gly-Asp (RGD) motif at the distal end
of PEG to provide tumor selectivity due to its ability to target integrins expressed on
activated endothelial cells in tumor vasculature. The self-assembly of the RGD-
PEG-PEI conjugates with siRNA was found to form sterically stabilized “layered”
PIC micelles with exposed RGD ligands. This work suggested that the introduction
of the targeting ligand resulted in a selective uptake of the polyplexes in the tumor
vasculature after intravenous administration.
Besides the electrostatic force, alternative approaches to provide higher stability
for the PIC micelles are being studied. This includes the use of additional forces
such as hydrophobic interactions through the incorporation of hydrophobic groups
for facilitated core formation, the chemical conjugation of siRNA to PEG, and inor-
ganic/organic interactions.
In an attempt to improve the stability of polymeric micelles in the blood, Kim
et al. prepared siRNA covalently bond to PEG segment via disulfide linkage [ 61 ] .
The micelles were assembled with the addition of a core-forming polycation poly-
ethylenimine (PEI) to produce a more stable core-shell structure. The hydrophilic
PEG segment covalently attached to the siRNA forms a corona-type surface layer,
while the core is formed by charge neutralization between siRNA and cationic
agents. This system formed micelles with a hydrodynamic diameter of ~99 nm and
was evaluated for in vivo delivery of antiangiogenic siRNA to female nude mice
bearing subcutaneous prostate tumors. Intravenously injected PEG-siRNA/PEI
micelles were two times more effective than siRNA/PEI complexes by the reduction
of angiogenic vascular endothelium growth factor (VEGF) protein in the tumor.
Interestingly, micelles accumulated passively in the tumor by the EPR effect, most
likely taking advantage of the protective PEG corona.
An alternative approach to design micelles for siRNA delivery based on the con-
jugate PEG-siRNA was reported by Oishi et al. [ 31 ] . Remarkably, enhanced gene
silencing was achieved by PIC micelles composed of lactosylated-PEG-siRNA con-
jugate bearing acid-labile b-thiopropionate linkage and poly(L-lysine) (Fig. 8.4 ).
PIC micelles were formed by the interaction of the polycation poly(L -lysine) and