Mucosal Delivery of RNAi Therapeutics - RNA Interference from Biology to Therapeutics - page 101

and respiratory regions in which site variations in both structure and cell composition
pose specific regional challenges to siRNA delivery.
The main role of the upper respiratory tract is to filter and conduct air to the
lower respiratory segment. As a consequence, its anatomical and cellular features
restrict material adsorption that includes naked siRNA or nanoparticle-based deliv-
ery systems. The trachea divides into the two primary bronchi at the carina, after
which, heavy branching from the lobular bronchi occurs and then onto continuously
narrowing tubes to the respiratory segment beginning with the respiratory bron-
chioles and the alveolar ducts and sacs. Delivery of siRNA to the lungs, whether
naked or incorporated in a particle, needs to address the branching of airways and
the mucus layer covering the conducting segments. Ciliated cells are abundant in
the nasal cavity and trachea, with apical cilia working in coordinated sweeps to
transport mucus along with trapped material towards the oesophagus. The constant
removal of mucus by the ciliated cells, termed the “mucociliary escalator” plays a
critical role in preventing inhaled particulates and pathogens from residing within
the trachea and the upper bronchiolar tree. The respiratory mucus consists of an
outer luminal layer and an inner layer (termed periciliary liquid) in direct contact
with the cilia. Under normal physiological conditions, the luminal mucus layer is
refreshed every 10-20 min, whereas renewal of the underlying layer is cleared
much slower [ 38 ]. The mucus layer is swept away and replenished continually
requiring trapped material or nanoparticles to diffuse across a current gradient in
order to reach the epithelial surface [ 17 ] .
Deeper into the lung, the mucus layer diminishes, but the passageways narrow.
This restricts the transit of particle-based delivery systems into the alveoli. If admin-
istered as an aerosol, inertia determines whether or not particles will impact on the
epithelium walls, in which case they will be cleared by the mucociliary escalator.
Furthermore, surfactant that covers the deeper regions to prevent collapse of the
respiratory sections during exhalation may interfere with particle integrity [ 57 ] ,
leaving the siRNA exposed to enzymatic degradation. Alveoli macrophages that
compensate for the lack of mucus protection are able to scavenge foreign material
by extending processes into the lumen of the alveolus. This could limit the effective-
ness of nanoparticle-based RNAi therapeutics; however, subsequent macrophage
migration may offer a mechanism for systemic delivery of the nanoparticles.

5.3.2 Naked siRNA Delivery

There is an ongoing discussion on whether non-formulated naked siRNA is
sufficient or a particle formulation is needed for effective pulmonary siRNA delivery.
Both approaches have been used (Table 5.1 ).
Non-formulated siRNA administered by intranasal or intratracheal instillation
have been able to mediate a reduction in target gene expression [ 58- 62, 74 ] or viral
titres [ 63, 65, 76 ] in mice and non-human primates [ 64 ]. In an interesting study from
2005, Bitko et al. demonstrated that naked phosphoprotein-specific anti-RSV siRNA
(70 mg single dose) performed near equally as siRNA complexed with the commercial