Nanomedicines for Systemic Delivery of RNAi Therapeutics - RNA Interference from Biology to Therapeutics - page 132

Cholesterol-ApoB-siRNA conjugate as well as a -tocopherol [ 59 ] (Fig. 6.2b ) and
lithocholic acid or lauric acid conjugated to ApoB-siRNA [ 60 ] reduced serum cho-
lesterol and ApoB mRNA levels in the liver.
Aptamer-siRNA chimeras (Fig. 6.2c ) are RNA-based particles for specific deliv-
ery of siRNAs. This approach relies only on the fact that structured RNAs are capa-
ble of binding a variety of proteins with high affinity and specificity. The chimera
includes both a targeting moiety, the aptamer, and an RNA-silencing moiety, the
siRNA. The aptamer-siRNA chimeras have demonstrated specific binding and
delivery of siRNAs into a xenograft model of prostate cancer. The aptamer portion
of the chimeras mediates binding to Prostate-speci fi c membrane antigen (PSMA),
a cell-surface receptor overexpressed in prostate cancer cells and tumour vascular
endothelium, whereas the siRNA reduce the expression of survival genes [ 61 ] . This
approach eliminates various side effects, hence aptamers and siRNAs exhibit low
immunogenicity. Additionally advantages are the possibility to synthesise large
quantities at a relatively low cost and the smaller size of aptamers compared with
that of antibodies (<15 versus 150 kDa), which promotes better tissue penetration.
A similar strategy of conjugation is the dynamic polyconjugates [ 62 ] (Fig. 6.2d ).
This delivery approach includes membrane-active polymers whose activity is
masked until reaching the acidic environment of the endosomes. Due to the employ-
ment of N -acetylgalactosamine, which binds to the asialoglycoprotein receptor,
they target hepatocytes. Like the SNALPs, these particles decreased ApoB mRNA
levels in the liver when ApoB-siRNA is used.
Polyethylenimine (PEI) is a cationic polymer used to condense nucleic acids into
polyplexes and is endosomolytic due to its buffering capacity at the endosomal pH.
PEI polyplexes incorporating siRNAs have demonstrated functional silencing in
subcutaneously transplanted tumours in nude mice. Particles composed of RGD
(Arg-Gly-Asp) peptide coupled via PEG-PEI allow prolonged circulatory half-
life, reduced immunogenicity and active targeting. When complexed with siRNA,
RGD-PEG-PEI molecules form a polyplex, with the positively charged RGD-PEG
components exposed on its surface. The targeting ability of this particle is based on
the overexpression of a v integrins, which RGD peptides bind in certain cancers and
in tumour vasculature [ 63 ]. Similar to the previous two examples, cyclodextrin-
containing polycation (CDP) particles have been successfully used for siRNAs
delivery into murine subcutaneous tumours [ 64 ] (Fig. 6.2g ). CDP is a polymer with
a cyclic oligomeric glucose backbone that complexes with siRNAs to assemble into
a colloidal 50-70 nm particle. To achieve targeting, transferrin-coupled PEG is
attached to the surface of the particles exploiting the upregulation of transferrin (Tf)
receptors in cancers. However, despite considered less toxic than conventional
cationic polymers (such as PEI), safety evaluation in non-human primates revealed
that intravenous injection of these particles induced elevation in blood urea (that
might indicate kidney toxicity), mild increase in liver enzyme levels and a mild
increase in IL-6 levels at high concentrations. Multiple injections of the particles
induced antibodies to human-Tf. Despite these disadvantages, Tf-coupled CDP
containing siRNAs for melanoma cancer treatment has been taken into clinical
trials [ 65 ] .