Bioresponsive Nanoparticles for the Intracellular Delivery of RNAi Therapeutics - Nanotechnology for the Delivery of Therapeutic Nucleic Acids - page 132

Knockdown of nucleus-restricted transcripts [29, 30] through
the involvement of DNA methylation [31, 32] has been reported that
may lead to an alternative class of transcriptional gene silencing
(TGS) therapeutics. In order to engage the nuclear RNAi pathways,
pri-, pre-miRNA, and TGS therapeutics must be delivered across the
nuclear membrane into the nucleus.
Plasmid [33] and viral-driven [34] intracellular expression of
short hairpin siRNA (shRNA) is an alternative to using synthetic
duplexes. However, despite the shorter knockdown duration
observed with synthetic duplexes, they allow accurate dosage
control and the ability to install stabilisation and reduce non-specific
effects through chemical modification. RNAi-based therapies are,
therefore, dependent on enabling technologies for cellular entry and
intracellular trafficking to engage in either the nuclear or cytoplasmic
RNAi machinery (e.g. Drosha, DICER, and RISC).

6.3 Nanoparticle-Based Delivery of

RNAi Therapeutics

Unique properties at the nanoscale have fuelled the development
of nanoparticles for delivery of RNAi therapeutics [11, 35]. These
include the predisposition of nanoscale particles to circumvent the
renal clearance mechanism yet permitting size-mediated transfer
across vasculature endothelium into tissue and cellular entry by
the process of endocytosis. Furthermore, they have a capacity to
incorporate a high siRNA payload that is protected from nuclease
degradation, in addition to a large surface to volume ratio that
maximises surface functionalisation effects. On the flip-side
endocytotic uptake results in endosomal capture and consequent
lysosomal breakdown. Moreover, nanoparticle association requires
release of the siRNA payload as a prelude to interactions with the
RNAi machinery.

6.3.1 Polycation-Based Nanoparticles

A versatile class of nanoparticles termed polyplexes are those
based on entropy-driven particle self-assembly between siRNA
and polycations [10, 11]. The formation of submicron spherical
particles (~50-300 nm) results from electrostatic interaction