Mucosal Delivery of RNAi Therapeutics - RNA Interference from Biology to Therapeutics - page 110

Since the GI tract epithelium is covered by mucus, drugs must diffuse through this
lubricant and protective layer in order to reach the absorptive surface. Therefore, the
uptake of a therapeutic compound will depend upon the interactions of the drugs with
the mucus components as well as the thickness of this layer, which in experimental
animal models has been shown to vary along the gastrointestinal tract [ 113, 114 ] .
Whilst paracellular transport across the GI epithelium is limited to ions and small
hydrophilic molecules that can diffuse across tight junctions, the hydrophobic nature
of the cell membranes impede the diffusion of most polar and charged molecules
[ 110, 115 ]. Consequently, macromolecular siRNA absorption across the epithelia is
restricted, although binding of specific ligands may facilitate uptake as demon-
strated in different cell types [ 6, 116- 118 ]. The capability to attach different chemi-
cal components by simple nucleic acid chemistry could promote this approach.
Nanoparticle-based carriers are taken up by adsorptive or receptor-mediated
endocytosis across enterocytes dependent on surface moieties. The level of uptake is
thought to be low, although penetration enhancers may potentiate paracellular deliv-
ery. An important consideration is delivery and breakdown in the liver due to the
first-pass effect commonly encountered by absorbed drugs. An alternative route
through the gut-associated lymphoid tissue (GALT) has been exploited for the deliv-
ery of micro and nanoparticles [ 119- 122 ] . The overlying follicle-associated epithe-
lium (FAE) contains specialised cells termed M-cells [ 115 ] that are anatomically
designed to sample luminal particles as part of the mucosal immune response. The
lymphoid follicle domes are highly populated with macrophages, which have been
shown to capture material. Systemic dissemination of these macrophages has been
proposed as a mechanism of transport to peripheral tissue. Although particle trans-
port through M-cells may be augmented by increasing particle-surface hydropho-
bicity or attachment of specific targeting ligands [ 123, 124 ], it is important to bear in
mind when utilising this route for intestinal absorption that GALT only constitutes a
small fraction of the GI tract, with the numbers deceasing with age. Recent attention
has focused on transport across the epithelial barrier directly mediated by dendritic
cells [ 125 ]. These phagocytic cells, widespread throughout the epithelia, have been
shown to disrupt tight junction and sample luminal content through the projection of
dendrites, providing an exciting opportunity for the design of oral vaccines.
Inter-species differences exist between humans and the animal models com-
monly used for the in vivo evaluation of oral drug administration. For example in
contrast to humans, mice and rats exhibit a less acidic stomach pH (~4 vs. ~1.7) and
lower mean intestinal pH [ 126 ]. These are important considerations when assessing
clinical translation.

5.4.2 Oral Studies

A number of studies have used the oral route for siRNA delivery (Table 5.2 ). A high
profi le study was reported in 2009 by Aouadi et al. [ 88 ]. In this study, porous b -1,
3-D-glucan shells were loaded with siRNA targeting expression of tumour necrosis
factor-alpha (TNF-a) or mitogen-activated protein kinase 4 (Map4k4) in mice. The