About the impact of water movement on the permeation behaviour of nanoparticles in mucus

Abstract

The purpose of this study was to establish a method to evaluate the diffusion behaviour of nanoparticles (NP) in mucus taking also the water movement into account. For this purpose, NP based on different chitosan derivatives, either thiolated or not, and marked with fluorescein diacetate were prepared by ionotropic gelation with hyaluronan. NP size and polydispersity were in the respective intervals 363.5 ± 33.3–385.7 ± 36.5 nm, and 0.35 ± 0.11–0.39 ± 0.10. An in vitro study of water-assisted NP transport through mucus was realized by filling the barrel of a syringe kept in vertical position, tip down, with mucus. Then a bottom-to-surface PBS flow across the mucus layer was realized by connecting the tip of the syringe to the bottom of a vertical cylindrical vessel by a flexible tubing, filling the vessel with PBS, level with the surface of the mucus layer in the syringe, and dripping PBS into the vessel without causing any phase mixing. Although the mucoadhesive NP interact more strongly with the mucus, yet they are able to overcome this barrier with the aid of the water movement from lumen to epithelium. This new method promises to be more predictive of in vivo NP transport across the mucus than already reported methods, as it takes into account the water movement and regulates its contribution to the physiologic value.

Introduction

Mucoadhesive polymeric nanoparticulate systems (NP) have raised interest as vehicles for drug delivery by the oral route for their potential ability to improve the bioavailability of drugs with low mucosal permeability and/or poor chemical stability in the gastrointestinal environment (Dünnhaupt et al., 2011, Ponchel et al., 1997, Sakuma et al., 2002, Sarmento et al., 2007, Takeuchi et al., 2001). To be absorbed across the intestinal epithelium into the systemic circulation, however, the NP will have to cross the layer of stagnant mucus adjacent to the intestinal membrane. Mucus is a viscoelastic gel layer primarily composed of crosslinked and entangled mucin fibers, continuously secreted and renewed by goblet cells and submucosal glands. The viscoelastic properties of GI mucus are essential for its protective and lubricating properties. In this light, mucus represents a barrier that can affect the transit of most orally administered particles. This layer is also crossed by the electrolytes and water that are absorbed by it (Hastewell et al., 1991), thereby generating pores that can be travelled by NP. These are thus given the possibility to reach the intestinal epithelium and cross it by transcytosis. In this context the tendency of NP to adhere to the mucus gel plays an important role since on the one hand it opposes the physiologic transit of the delivery system through the GI tract away from the absorption site, thus favouring drug absorption, whereas, on the other hand, it hampers the water-driven transport of NP from luminal to epithelial side of the mucus layer, thus slowing down absorption (Grießinger et al., 2015). Indeed, Maisel et al. (2015) on the basis of their results obtained in vivo with mice, stated that water absorption by the intestine increases in the fed state, and non-mucoadhesive NP can take advantage of this, for advective transport and hence, efficient and uniform drug delivery. In contrast mucoadhesive NP stick to the outer layers of mucus and aggregate in clumps far from the epithelial surface. In the light of the above information, it could be hypothesized that the water movement in the intestinal mucus could influence the distribution of mucoadhesive NP in the GI tract. For this reason, the purpose of the present work has been to develop and evaluate a simple in vitro system allowing the investigation of the movement of NP, either mucoadhesive or not, across a layer of porcine mucus concurrently crossed by a flow of phosphate buffered saline pH 6.8, 0.13 M (isotonic) (PBS), that could be adjusted to the physiologic flux value calculated from literature data for the human intestine (Lennernäs et al., 1994).Water-soluble quaternary ammonium-chitosan conjugates were used to prepare the NP (Zambito et al., 2008). To differentiate between more and less mucoadhesive NP these were prepared from chitosan derivatives containing thiol groups on their chains (more mucoadhesive) or thiol-free ones (less mucoadhesive) (Zambito et al., 2013).