Mucus as a barrier to drug delivery ­ understanding and mimicking the barrier properties.

Viscoelastic mucus lines all mucosal surfaces of the body and forms a potential barrier to mucosal drug delivery. Mucus is mainly composed of water and mucins; high molecular weight glycoproteins forming an entangled network. Consequently, mucus forms a steric barrier, and due to its negative charge and hydrophobic domains, the overall hydrophilic mucus also presents an interactive barrier limiting the free diffusion of components within and through the mucus. Furthermore, mucus is a dynamic barrier due to its continuous secretion and shedding from the mucosal surfaces. Mucus is thus a highly complex gel barrier to drug delivery. Current knowledge of mucus characteristics and barrier properties, as achieved by state-of-the-art methodologies, is the topic of this MiniReview emphasizing the gastrointestinal mucus and an overall focus on oral drug delivery. Cell culture-based in vitro models are well-established as essential tools in drug research and development, but traditionally, mucus-containing models have only rarely been applied. However, a number of mucus-containing in vitro models have recently been described in the literature, and their properties and applications will be reviewed and discussed. Finally, studies of peptide and protein drug diffusion in and through mucus and studies of mucus-penetrating nanoparticles are included to illustrate the mucus as a potentially important barrier to obtain sufficient bioavailability of orally administered drugs, and thus an important parameter to address in the development of future oral drug delivery systems.

Steric and interactive barrier properties of intestinal mucus elucidated by particle diffusion and peptide permeation.

The mucus lining of the gastrointestinal tract epithelium is recognized as a barrier to efficient oral drug delivery. Recently, a new in vitro model for assessment of drug permeation across intestinal mucosa was established by applying a biosimilar mucus matrix to the surface of Caco-2 cell monolayers. The aim of the present study was to gain more insight into the steric and interactive barrier properties of intestinal mucus by studying the permeation of peptides and model compounds across the biosimilar mucus as well as across porcine intestinal mucus (PIM). As PIM disrupted the Caco-2 cell monolayers, a cell-free mucus barrier model was implemented in the studies. Both the biosimilar mucus and the PIM reduced the permeation of the selected peptide drugs to varying degrees illustrating the interactive properties of both mucus matrices. The reduction in peptide permeation was decreased depending on the cationicity and H-bonding capacity of the permeant clearly demonstrated by using the biosimilar mucus, whereas the larger inter sample variation of the PIM matrix obstructed similarly clear conclusions. Thus, for mechanistic studies of permeation across mucus and mucosa the biosimilar mucus offers a relevant and reproducible alternative to native mucus.

Property profiling of biosimilar mucus in a novel mucus-containing in vitro model for assessment of intestinal drug absorption.

Oral delivery of drugs, including peptide and protein therapeutics, can be impeded by the presence of the mucus surface-lining the intestinal epithelium. The aim of the present project was to design and characterize biosimilar mucus compatible with Caco-2 cell monolayers cultured in vitro to establish a more representative in vitro model for the intestinal mucosa. The rheological profile of a biosimilar mucus mixture composed of purified gastric mucin, lipids and protein in buffer was optimized by supplementing with an anionic polymer to display viscoelastic properties and a microstructure comparable to freshly isolated porcine intestinal mucus (PIM). Further, this multicomponent biosimilar mucus mixture was optimized with regard to the lipid content in order to obtain cellular compatibility with well-differentiated Caco-2 cell monolayers. In contrast, PIM was found to severely disrupt the Caco-2 cell monolayer. When combined with the Caco-2 cell monolayers, the final biosimilar mucus was found to significantly affect the permeability profiles for hydrophobic and hydrophilic small and large model drug compounds in different ways. In conclusion, the present study describes an improvement of the biorelevance of the Caco-2 cell culture model by application of mucus, resulting in an in vitro model of oral mucosa suitable for future assessment of innovative drug delivery approaches.