RNAi Therapeutic Delivery by Exosomes - RNA Interference from Biology to Therapeutics - page 185

monoclonal antibodies (mAb), specific for insulin or transferrin receptors. By
recombinantly expressing various neuroprotective proteins, or segments of proteins,
as fusion proteins to the carboxyl terminus of the heavy chain of these mAbs, it has
been shown that the brain uptake increases up to 50- to 100-fold. This “Trojan
horse” concept has been exploited for the delivery of various macromolecular pro-
tein compounds including erythropoietin, TNF-a receptor decoys, ab -amyloid anti-
bodies, and GDNF to confer neuroprotection in various neuronal disorders including
Parkinson's disease, Alzheimer's disease, and stroke [ 24- 27 ] . Albeit a relatively
efficient strategy for BBB delivery of therapeutic proteins, this technology has
major limitations in several ways. First, this delivery system relies on cumbersome
expression and purification of fusion proteins that are needed in high quantities in
order to achieve activity, and, second, these large fusion proteins are novel antigens
and hence are potentially immunogenic upon repeated systemic administration.

9.2.3 RNAi Delivery Across the BBB

To our knowledge, there are only two synthetic nanoparticle-based systems reported
to convey nucleic acids across the BBB to date. One exploits the previously described
Trojan horse strategy by coating synthetic, nucleic acid-loaded liposomes with
BBB-homing antibodies [ 23 ], and the second system makes use of a chimeric pep-
tide comprising a polyarginine sequence for siRNA condensation and a rabies virus
glycoprotein (RVG) peptide sequence for BBB targeting [ 28 ]. RVG is a 29-mer
peptide derived from the rabies virus glycoprotein that specifically binds to acetyl-
choline receptors expressed on neuroendothelial cells as well as neuronal cells and
allows transvascular delivery to the nervous system [ 29 ]. Using the RVG-R9 chime-
ric peptide complexed with siRNA, the authors observed a strong RNAi response in
neuronal cells in vitro, and following systemic injection into mice, siRNAs local-
ized to neuronal cells that resulted in specific gene silencing within the brain.
Furthermore, intravenous treatment with RVG-R9-bound antiviral siRNA mediated
robust protection against fatal viral encephalitis in mice [ 28 ] .
Despite providing an efficient means of reaching the CNS, these systems are
hindered by the same major drawbacks as other polycation and liposome-based
nanoparticle vehicles. The fact that they are entirely synthetic makes them poten-
tially immunogenic and kidney-, liver-, and lung-related toxicities have been
reported for these and related delivery modalities [ 30, 31 ] . Thus, a major scienti fi c
challenge is to find enabling delivery technologies with a greatly enhanced ability
to target and traverse the BBB following systemic administration in a safe, nontoxic
manner and without triggering organ toxicity or immunological response upon
repeated administration. The exploitation of natural biological nanoparticles such as
exosomes could fulfill these criteria and provide a paradigm shift in technology for
BBB penetration and targeted macromolecular drug delivery to brain.