Delivery of RNAi Therapeutics to the Airways—From Bench to Bedside

Qiu, Yingshan; Lam, Jenny K. W.; Leung, Susan W. S.; Liang, Wanling

Apart from the physical barriers, the RNAi molecules may undergo enzymatic degradation following pulmonary delivery. RNA is extremely susceptible to nuclease activity. Naked siRNA has a short half-life of less than 15 min in serum [47] . Due to the lack of serum in the lungs, the half-life of unmodified siRNA in the airways can reach up to three hours in mouse [48] . RNAi molecules may also be taken up and removed by alveolar macrophages upon lung deposition. The alveolar macrophages are responsible for engulfing and digesting foreign particles in the lower airways via phagocytosis. Unless the target of RNAi molecules is located inside the macrophages, such as Mycobacterium tuberculosis which is the causative agent of tuberculosis that typically reside in the alveolar macrophages [49] , otherwise the RNAi molecules are subjected to degradation inside the macrophages before reaching their target sites. The presence of fluid layers in the airways, including the mucus and the pulmonary surfactant, also creates a barrier to the delivery of drug molecules to the lungs. Mucus is a gel of three-dimensional network structure. It acts as a sieve that filters out large molecules (>500 nm) [38] . Mucins, which are the major components of mucus, contain the hydrophilic glycosylated residues rich in serine and threonine and the hydrophobic non-glycosylated cysteine-rich domains. This characteristic allows electrostatic, hydrophilic and hydrophobic interactions between mucus and drug particles [39] . Since mucus is continuously produced, shed and replaced, and together with the mucociliary clearance action, the fast turnover rate of mucus leads to the rapid clearance of the entrapped drug molecules, preventing them from reaching the epithelium [40, 41] . Moreover, cough clearance that occurs when the mucus reaches the throat is another removal mechanism of RNAi molecules from the airways. On the other hand, the role of pulmonary surfactant in nucleic acid delivery is controversial. Pulmonary surfactant is composed of approximately 90% lipids and 10% proteins [42] . It has been suggested that the lipids and proteins in the lung surfactant interact with the non-viral cationic lipid-based delivery system, leading to the premature release of nucleic acids [43, 44] . Conversely, polymer-based delivery systems were found to be more compatible with pulmonary surfactant. Commercial pulmonary surfactant was employed in the preparation of some polymeric nanoparticles to facilitate the cellular uptake of siRNA to improve gene silencing effect [45, 46] .