Pulmonary delivery of LNP-mRNAs aerosolised by vibrating mesh nebulizer: An emphasis on variations and in-depth analyses of physicochemical properties

The delivery of lipid nanoparticle (LNP)-mRNAs to the lungs attracts fast increasing interests for vaccination, as the mucosal immunity in the airway can prevent the establishment of an infection rather than only reduce the level of infection associated with systemic immunity triggered via intramuscular injection. The vibrating mesh nebuliser was well utilized to atomize inhalation solutions/suspensions for pulmonary delivery hence employed in this study for aerosolising LNP-mRNAs. In comparison with pre-aerosolised LNP-mRNAs, the post-aerosolised vectors demonstrated a significant increase (t-test, unpaired, p < 0.05) in particle size (215-363 nm vs. 116-130 nm), polydispersity index (PDI: > 0.33 vs. < 0.27), zeta potential (ZP: 11-14 mV vs. 2.6-7.7 mV), and encapsulation efficiency (EE: ∼99 % w/w vs. ∼91 % w/w), indicating a structural alteration upon high-frequency mesh vibration (HFMV). The particle sizes of LNP-mRNAs were further enlarged upon inertial impaction, and the size increments were dependent on the velocities of airflow for impaction and the N/P ratios. The aerosolised mists were fine, with >54 % w/w deposited in lower respiratory tract and >28.5 % w/w further delivered to alveolar regions. Further, a model was created to elucidate the variations of physicochemical properties for LNP-mRNAs upon HFMV and inertial impaction, and it disclosed that the fluidity and shear-induced fusion of LNPs were the fundamental reasons to cause these unfavourable changes particularly the size enlargement. These insights reveal that the effective development of inhaled LNP-mRNAs will rely on shear-less devices, formulation optimizations, inhalable dry powders, and their potential combinations.