RNAi Therapeutic Delivery by Exosomes - RNA Interference from Biology to Therapeutics - page 197

observed that association of APO2L/TRAIL to the exosome-like liposomes
increased its bioactivity and resulted in more effective treatment of AIA [ 88 ] .
Other studies attempted to mimic the protein content of exosomes by including, for
example, connexins. These components of cellular gap junctions had previously
been identified on exosomes derived from various sources. Kaneda et al. showed
efficacy of liposomes displaying connexin 43 in delivering the anti-inflammatory
peptide, NEMO-binding domain peptide (NBDp) into Cx43-expressing U2OS cells
and also demonstrated that such delivery was dependent on Cx43 expression in the
recipient cells [ 89 ]. De La Pena et al. successfully coated liposomes consisting of
exosomal phospholipids with an optimized number of MHC class I peptide com-
plexes and other immunostimulatory ligands. These “exosome-like” liposomes
recapitulated the function of antigen-presenting cells [ 90 ] . These studies demon-
strate that artificial liposome-based delivery systems could be designed based on the
properties derived from exosomes and that further understanding of exosome biol-
ogy and trafficking will help the design and development of effective exosome-like
delivery vehicles.
Despite the exciting progress with the discovery of the drug delivery potential of
membrane vesicles, there exists a number of limiting factors in their clinical trans-
lation. First, membrane vesicles are known to play a role in pathology, for example,
in spread of HIV or prion particles [ 91, 92 ]. There is also evidence that membrane
vesicles may be involved in promoting tumor growth by inducing angiogenesis and
spreading oncogenes [ 93 ]. Citrullinated proteins common in rheumatoid arthritis
have been found in exosomes from synovial fluid and could be involved in stimulat-
ing the pathogenic autoimmune response [ 94 ]. It would be important to further
characterize membrane vesicles intended for drug delivery by proteomic studies in
order to identify any endogenous cargoes that may mediate potential unwanted side
effects. Furthermore, gene expression studies in exosome-treated cells would iden-
tify pathways other than those targeted by the siRNA cargo, which are altered by
exosome treatment. As well as eliminating potential pathological cargoes from
membrane vesicles, it would be important to produce clinical-grade vesicles with a
defined set of characteristics and a quantifiable level of drug cargo. However, this is
currently limited by lack of suitable and scalable nanotechnologies for the
purification, characterization, and loading of exosomes. Current ultracentrifugation
protocols produce a heterogeneous mix of exosomes, other cellular vesicles, and
macromolecular complexes. Therefore, novel purification methods based on the use
of specific desired markers, such as the expression of the targeting moiety on the
surface of exosomes, are required. In addition, siRNA loading into exosomes is
relatively inefficient and cost-ineffective, highlighting the need for the development
of novel transfection reagents tailored for nanoparticle applications. In summary,
the clinical translation of membrane vesicles for drug delivery hinges upon better
understanding of exosome biology and function in health and disease and the devel-
opment of nanotechnologies for the specific purification and characterization of
clinical-grade exosomes and their loading with a variety of therapeutic cargoes.