Mucosal Delivery of RNAi Therapeutics - RNA Interference from Biology to Therapeutics - page 98

(~110-160 m m) [ 20 ]. In the eyes, the thickness of the mucus layer has been reported
to be 30-40 m m [ 21, 22 ], in the airway 7-30 m m [ 23- 25 ] and in the bronchiole
~55 m m [ 26 ]. The most accessible mucosal surfaces are found in the nasal region
[ 27 ] due to a very thin layer of mucus, and in the deep lung where the epithelial lin-
ing is devoid of mucus and instead contains surfactants which reduces the surface
tension and potentiates gas transfer in the alveoli [ 28 ]. Relevant to mucosal thera-
peutic delivery is the thickness and integrity of the mucus layer may be compro-
mised under various pathological conditions. For example increase in mucus
thickness has been observed in asthma [ 29 ] , cystic fi brosis [ 26 ] and chronic obstruc-
tive pulmonary disease [ 30 ], whilst a decrease in thickness has been observed in
ulcerative colitis [ 20 ]. This variation in mucus thickness can lead to a predisposition
for disease or be induced by pathological consequences of the disease such as loss
of epithelial integrity as seen in ulcerative colitis.
Despite the existence of a steady layer just above the epithelial cells, mucus is a
dynamic substance which undergoes continuous renewal by secreting goblet cells
dispersed throughout the epithelial layer. This produces an outward moving barrier to
any entity that aims to reach the epithelial layer and determines the timeframe allowed
for particles to penetrate the epithelium before clearance. Renewal rates are tissue
dependent which has implications for designing therapeutic strategies. Mucus in the
nasal cavity is replaced approximately every 20 min [ 31 ] and between 10 and 20 min
in the respiratory tract [ 32 ] compared to a clearance rate of 4-6 h in the gastrointesti-
nal tract [ 33 ] as found in rats but values have not been fully determined in humans.
In addition to thickness and rate of renewal, the pH value varies at different sites,
with the lung and nasal mucus being nearly neutral [ 34, 35 ], the eye possessing a
weak basic pH [ 21 ] and the mucus of the stomach having a pH gradient from pH
1-2 at the luminal side to approximately pH 7 at the surface of the epithelial cells
underlying the mucus [ 36, 37 ]. These pH variations could be utilised for pH-respon-
sive delivery systems which release their cargo at specific mucosal sites.

5.2.1.2 Mucus Penetration

The protective properties of mucus pose a barrier to RNAi-based therapeutics both
in naked or nanoparticle form. Mucus constituents such as the glycoproteins
(mucins), cellular debris and lipids form a heterogeneous environment through
which drugs and/or drug carriers need to diffuse to reach their target [ 38 ] . Glycosylated
domains of the mucin fibres possess a negative charge under physiological condi-
tions, and hence mucus selectively controls the diffusion of particles not only through
particle physical parameters such as size, but also by their chemical surface proper-
ties. Based on the net negative charge of mucins, one could speculate that naked
siRNA might be repelled by the mucin fibres. Multivalent interactions between par-
ticles and the mucus network are main determinants of particle diffusion. Both elec-
trostatic and hydrophobic interactions occur, and the possibility of making large
numbers of non-specific hydrophobic interactions together with the more ther-
mal stable electrostatic interactions enables mucus to trap particles. Several studies
have demonstrated the efficacy of hydrophobic interactions to immobilise particles