RNAi as New Class of Nanomedicines - Harnessing Biomaterials in Nanomedicine: Preparation, Toxicity, and Applications - page 172

of α-Tocopherol (Vitamin E) to the 5 ′ terminus of the guide strand
of a Dicer substrate siRNA (an elongated double-stranded RNA
molecule (~27-mer) that is processed by Dicer into an active siRNA)
facilitated the delivery of an ApoB siRNA into mice hepatocytes
[112]. The transferrin receptor, which is overexpressed on the
surface of many tumor types, has been the target for a variety of
delivery strategies including antibody-mediated (see above) and
ligand mediated delivery. In one iteration, systemic administration
of nanoparticles composed of cyclodextrin coated with transferrin
(Tf) and containing siRNAs targeting the EWS-FLI1 fusion gene,
eff ectively inhibited tumor growth in a mouse model of Ewing's
sarcoma [69]. Similarly, lipoplexes coated with Tf facilitated siRNA
delivery to primary cortical neuron in vitro and in vivo after injection
into the striatum of mice [17, 18]. Clinical studies have begun to test
the safety and efficacy of targeted nanoparticles for the treatment
of therapy-refractory solid tumors. Davis and colleagues used
nanoparticles composed of a linear, cyclodextrin-based polymer and
a hydrophilic polymer, PEG, and coated with Tf to deliver siRNAs
to transferrin receptor bearing tumors [26]. This formulation
had been shown previously to be well tolerated in non-human
primates [63]. Transmission electron microscopy analysis of tumor
biopsies showed accumulation of the nanoparticles in the tumor
tissue. In addition, quantitative real-time PCR and western blot
analysis demonstrated a significant reduction in the M2 subunit of
ribonucleotide reductase (RRM2) (the target of the siRNA) mRNA
and protein, respectively, in treated tumors compared with the
pretreatment biopsy samples. Delivered siRNAs have been shown
to engage the RNAi machinery since the siRNA cleavage site on the
RRM2 mRNA could be identified using 5 ′ RLM-RACE analysis in the
treated tumor samples [26]. Other ligands that have been used for
siRNA delivery, include N- acetyl galactosamine and galactose, which
bind asialoglycoprotein receptors, and apolipoprotein A-1, which
binds the scavenger receptor class B type 1 [84, 128].
Most viruses take advantage of endogenous cell surface molecules
to infect target cells. The rabies virus glycoprotein (RVG) binds to the
nicotine acetylcholine receptor present on the surface of neuronal
cells. The fusion of a peptide derived from the RVG to a polyarginine
tract (nona-arginine peptide, 9dR) capable of non-covalently binding
siRNAs specifically targeted neuronal cells for siRNA uptake [92].
Treatment with an siRNA targeting Japanese encephalitis virus (JEV)