Novel RNA Interference (RNAi)-Based Nanomedicines for Treating Viral Infections - Harnessing Biomaterials in Nanomedicine: Preparation, Toxicity, and Applications - page 204

Despite the promise of RNAi in in vitro studies, for RNAi-based
therapies to become viable in a clinical setting many hurdles need to
be overcome. These include but are not limited to (i) short in vivo half-
life of siRNA; (ii) toxicity due to off -targeting resulting in silencing of
unintended genes and/or activation of the innate immune system;
and (iii) targeted delivery to only the desired tissue/cell types and
across the plasma membrane and into the cytoplasm.
Unmodified siRNAs, also referred to as naked siRNAs, are known
to be extremely short-lived in vivo with a half-life of minutes [28, 106]
due to degradation by nucleases in the serum and rapid clearance
via kidney filtration, a result of the siRNA's small molecular weight
(~7 kilodaltons). However, the half-life can be significantly increased
by incorporating chemical modifications and by encapsulation in
carriers (Reviewed in [33]) (Figure 7.2). Chemical modifications
at the 2 ′ -position of the ribose backbone prevent degradation by
RNases [21]. Chemically conjugating siRNAs to cholesterol allows
for efficient uptake in cells in the absence of a transfection reagent
and significantly slows renal clearance of siRNAs [106]. Similarly,
encapsulation within nanocarriers also improves siRNA stability,
slows clearance, and improves biodistribution [40].
Off-target eff ects can never be completely avoided as a stretch of
just seven nucleotides is sufficient to achieve translational inhibition;
as is seen with targets of microRNAs. A careful design of inhibitory
RNAs to minimize unintended sequence matches is very important.
Additionally, off -targeting caused by incorporation of the passenger
strand, instead of the antisense strand of the siRNA into RISC can
be avoided by designing the siRNA ends to thermodynamically favor
RISC uptake of the active antisense strand. It has been experimentally
shown that certain nucleotide motifs are more likely to induce the
innate immune system [62]. Therefore, avoiding these motifs when
designing siRNAs can significantly reduce the likelihood of toxicity
due to activation of the innate immune response.
Controlled biodistribution is key to increasing efficacy by
concentrating RNAi mediators at the site of action and also reducing
toxicity of RNAi mediators in unintended cell types. Once the inhibitory
RNAs have been administered it is imperative that they encounter the
intended tissues/cells. Administering the inhibitory RNAs directly to