RNAi Therapeutic Delivery by Exosomes - RNA Interference from Biology to Therapeutics - page 183

basic research tool to investigate gene function and as potential therapeutics to
silence disease-causing genes (reviewed in [ 4 ]). In contrast to conventional single-
stranded antisense oligonucleotides (ASOs), siRNAs are considered significantly
more potent since only few siRNA effectors are needed to promote complete RNAi
in a given cell. Furthermore, improved sequence design and incorporation of chemi-
cally modified nucleotides at selected positions within siRNA duplexes have dra-
matically improved the potency further. However, the large size and hydrophilic
nature of siRNAs make them essentially impermeable for cells. For that reason,
various delivery vectors, some of which were initially developed for plasmids and
ASOs, have been used to improve the bioavailability of siRNAs. The majority of
these are based on polycations or cationic lipids that form nanocomplexes with
siRNAs that are subsequently taken up by cells through endocytosis [ 4- 6 ] . In addi-
tion to cationic liposomes, a class of peptide-based vectors referred to as cell-pene-
trating peptides (CPPs) have been utilized for nanoparticle formation with siRNA
with subsequent RNAi induction in various cell types in vitro as well as in vivo fol-
lowing systemic delivery [ 7- 9 ] .
Different vectors have been developed that hold clinical potential, in particular,
for RNAi-based treatment of liver-related malignancies [ 6, 10 ] ; however, only a
very small number of mechanistic-based treatments against CNS diseases exist in
the clinic, compared to diseases of other biological tissues. Major advances have
been made in the last decade in understanding the molecular mechanisms, genetic
basis, and pathogenesis underlying the pathology of major neurological disorders,
including Huntington's disease, Parkinson's disease, Alzheimer's disease, spinocer-
ebellar ataxias, stroke, and brain cancers. However, this advanced understanding of
disease and identification of therapeutic targets in the context of CNS pathology
have not resulted in successful development of CNS-targeting therapies, due pri-
marily to the impermeability of the BBB to most therapeutic agents. There is an
urgent need, therefore, for the development of novel approaches for efficient and
safe delivery of macromolecular cargoes across the BBB. One plausible strategy
would be to exploit one of nature's own cell-to-cell information transmitters such as
membrane vesicles, in particular exosomes.
This chapter provides an overview of existing methods for delivery of therapeu-
tics to brain, with focus on siRNA, and the challenges associated with delivery
across the BBB. Furthermore, membrane vesicles and their role in exchange of
genetic information between different cells will be discussed with emphasis on
endogenous exosome-mediated mRNA and microRNA (miRNA) transport and how
this system could be exploited for therapeutic RNAi delivery to the brain.

9.2 Drug Delivery to Brain

9.2.1 The Blood-Brain Barrier

The BBB is a tightly regulated biological barrier composed of several cell types that
cooperatively act to separate the blood from the brain parenchyma. The neurovascular