Size-Limited Penetration of Nanoparticles into Porcine Respiratory Mucus after Aerosol Deposition

Abstract

We investigated the rheological properties and the penetration of differently sized carboxylated nanoparticles in pig pulmonary mucus, on different distance and time scales. Nanoparticles were either mechanically mixed into the mucus samples or deposited as an aerosol, the latter resembling a more physiologically relevant delivery scenario. After mechanical dispersion, 500 nm particles were locally trapped; a fraction of carboxylated tracer particles of 100 or 200 nm in diameter could however freely diffuse in these networks over distances of approximately 20 μm. In contrast, after aerosol deposition on top of the mucus layer only particles with a size of 100 nm were able to penetrate into mucus, suggesting the presence of smaller pores at the air-mucus interface compared to within mucus. These findings are relevant to an understanding of the fate of potentially harmful aerosol particles, such as pathogens, pollutants, and other nanomaterials after incidental inhalation, as well as for the design of pulmonary drug delivery systems.

The Mucus Barrier to Inhaled Gene Therapy

“…Using MPT, Murgia et al, showed that 100 and 200 nm PS exhibited similar mobile fractions, 43 and 51% respectively, but based on FRAP analysis, the mobile fraction of 100 nm PS NP was twofold less than that of 200 nm PS NP. 98 These results suggest differences in short-versus long-time diffusion behavior of mucoadhesive PS NP in porcine airway mucus. Conversely, prior studies showed that nonmucoadhesive gene vectors that exhibited greater diffusion rates in human CF mucus, as assessed by MPT, also provided more widespread distribution in vivo throughout mouse airways following inhalation.…”

“…73 Similarly, 76% of 100 nm, 46% of 200 nm, and 86% of 500 nm PS NP were immobilized in ex vivo porcine airway mucus collected from excised tracheas. 98 FRAP is particularly useful in the assessment of the diffusion behavior of small NP with low fluorescence intensity and/or NP undergoing diffusion that is so rapid that reliable individual particle tracking (discussed in Section "Particle tracking") is not feasible. In addition, diffusion over physiologically-relevant distances (i.e., several microns) can be assessed using FRAP.…”

“…Mucus from rats 125 and pigs 98 has been collected by carefully scraping the surface of the larynx and/ or trachea, which are excised following animal sacrifice. Airway mucus has also been directly collected from anesthetitized dogs, 126 ferrets, 127 and horses 128 by placing a cytology brush in the trachea of animals onto which mucus accumulates as a result of MCC.…”

“…In contrast, 50-60% of the same NP were diffusive when mechanically mixed into porcine airway mucus. 98 This finding suggests that the mucus gel layer may possess smaller pores at the air-mucus interface compared with deeper within the mucus layer.…”

“…To characterize the barrier properties of airway mucus, MPT has been used to quantify the diffusion of PS NP possessing various physicochemical properties in porcine airway mucus, 98 as well as human airway mucus from healthy individuals 23 and individuals with CF. 65,78,83,112 These studies have collectively demonstrated the effects of NP size and surface chemistry on the extent of hindrance to mucus penetration where they have shown NP must possess diameters less than the characteristic mucus mesh size (100-200 nm) and be nonadhesive to mucus in order to efficiently penetrate through the mucus barrier.…”