RNAi Therapeutic Delivery by Exosomes - RNA Interference from Biology to Therapeutics - page 195

that is an extension of their natural function in mRNA and miRNA transport and
their association with RISC components. As such, it is likely that a greater propor-
tion of exosome cargo is delivered directly to the site of silencing, than is the case
with liposomes and viruses that have no such intrinsic intracellular delivery func-
tion. In addition, their ability to cross the BBB, as demonstrated in the Alvarez
study, makes them highly attractive for targeting CNS disorders following intrave-
nous administration. While it is not fully understood how exosomes enter target
cells and cross biological barriers from the bloodstream, where they are known to
be abundant and relatively stable, it has been proposed that exosomes may be inter-
nalized into MVBs of recipient cells then released again to be reinternalized into
MVBs of secondary recipient cells, and so on [ 80 ]. Therefore, by jumping from cell
to cell via the MVB compartment, exosomes could cross the multiple layers of the
BBB. However, it is also known that their unique membrane protein and lipid com-
position, not found on liposomes or viral capsules, is critical to their ability to enter
target cells. The membranes of exosomes are organized differently than the cellular
plasma membrane and are enriched in cholesterol, ganglioside GM3, sphingomy-
elins, phosphatidylcholines, phosphatidylethanolamines, phosphatidylserines, pro-
phatidylinositols, and lysophosphatidylcholines [ 81 ] . As phosphatidylserine is an
important docking site for proteins involved in signaling and fusogenesis, phos-
phatidylserine may be involved in exosome homing and entry into target cells. It has
also been shown that vesicles composed solely of lipids in the ratios encountered
in exosomes were unable to achieve fusion with cells, indicating that proteins on
exosomal membranes are as critical for their activity [ 81 ] .
In addition to delivery of cargo to the site of silencing and the innate ability to
cross biological barriers, exosomes appear to be relatively safe for in vivo use when
compared to liposomes or viral vectors. In the Alvarez study, there was no evidence
for immunogenicity or toxicity in animals even after repeated injections. There was
also little evidence both in the Alvarez study and in the Zhuang study for exosome
homing to the liver. It is likely that the immunological “inertness” of exosomes
depends greatly on their cellular origin. Indeed exosomes used in the Alvarez study
were derived from immature DCs that express minimal stimulatory molecules.
However, the natural lipid and protein composition of their membranes and their
rapid and efficient uptake into MVBs in host cells likely contribute to this immuno-
logical inertness and low toxicity.

9.5 Future Perspectives

The realization of the full potential of exosomes in drug delivery will be enabled by
the establishment of a scalable and long-term source of well-characterized exo-
somes. In this context, the use of embryonic stem cells (ESC) and induced pluripo-
tent stem cells (iPS) hold great promise. ESC cells can be differentiated into DCs,
while iPS cells can be derived from patient skin fibroblasts by well-established