Mucosal Delivery of RNAi Therapeutics - RNA Interference from Biology to Therapeutics - page 99

within mucus [ 38- 41 ] and Ribbeck and co-workers have shown that particle surface
charge, density and pH of mucin hydrogels can alter particle diffusion [ 42 ] . Particles
with a PEGylated surface possessing a neutral charge had a greater diffusion rate
compared to its negative or positive charged counterpart [ 42 ] . Two approaches gov-
ern the design of particles for mucosal delivery (1) the mucoadhesive and (2) mucus
penetrative approach.
Regarding the mucoadhesive strategy, much attention has been given to the
design of particles which associate with the mucus barrier and hence lower the
clearance rate. The essence of this strategy lies with the previously mentioned bi-
layered structure of mucus. Association of particles with the lower undisturbed
layer will avoid clearance and enhance the bioavailability of the bioactive entity.
Another positive effect from mucus association is increased viscosity due to greater
cross-linking of mucus fibres, which in turn may lower the clearance rate. Materials
of choice have been the so-called mucoadhesive materials. A common characteris-
tic of these materials is their adherence to mucus through various forces. A widely
used mucoadhesive polymer is chitosan [ 43 ], which have been utilised to form par-
ticles with siRNA and exhibit mucosal silencing [ 7 ] . Thiolation of polymers have
been shown to enhance mucus interaction through formation of disulfide linkages
with mucins [ 44 ] and various thiolated chitosan's have been synthesised [ 45, 46 ] .
Not only does the mucoadhesiveness prolong the bioavailability of particles at
mucosal surfaces, but it can also alter the structure of mucus so that it becomes more
permeable to siRNA-loaded particles [ 47 ] . As an alternative, mucus-penetrating
particle with limited interaction with mucus and increased diffusion rates is an
exciting approach. Coating with PEG has been demonstrated to mediate such sur-
face properties on latex particles (200-500 nm) [ 48 ] and nanoparticles composed of
a biodegradable di-block copolymer of poly (sebacic acid) and PEG [ 49 ] . It has
been further shown that both molecular weight and degree of surface coverage
determined the mobility of the coated particles [ 50 ]. Reports, however, have previ-
ously classified PEG as a mucoadhesive polymer [ 51- 53 ] . As speculated by Lai
et al. the contradicting reports might be attributable to variations in type of
PEGylation used, but the use of PEG-coated particles have yet to demonstrate intra-
cellular delivery of nucleic acids to the respiratory or gastrointestinal tract.
Interestingly, the design of mucus-penetrating particles builds on lessons learned
from nature where viruses with an equal surface density of positive and negative
charges readily penetrate mucus barriers [ 54 ] . Thus, surface chemistry together
with size appear to be determining factors for mucus penetration and evidence
points towards neutral surfaces for effective diffusion.

5.2.2 Epithelial Cell Barrier

An ordered array of closely packed epithelial cells overlaying a basement mem-
brane constitutes the mucosal epithelium. Cell type, morphology and arrangement
differ dependent on site and function. For example the small intestine comprises
of single-layered enterocytes assembled into structured villi that increase the