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. Author manuscript; available in PMC: 2010 Feb 27.
Published in final edited form as: Adv Drug Deliv Rev. 2008 Dec 13;61(2):158–171. doi: 10.1016/j.addr.2008.11.002

Figure 6.

Figure 6

Summary schematic illustrating the fate of mucus-penetrating particles (MPP) and conventional mucoadhesive particles (CP) administered to a mucosal surface. MPP readily penetrate the luminal mucus layer (LML) and enter the underlying adherent mucus layer (AML). In contrast, CP are largely immobilized in the LML. Because MPP can enter the AML and thus are in closer proximity to the cells, cells will be exposed to a greater dose of drug released from MPP compared to drug released from CP. As the LML layer is cleared, CP are removed along with the LML whereas MPP in the AML are retained, leading to prolonged residence time for MPP at the mucosal surface. Thus, at long times, there is almost no drug dosing to cells with CP, whereas MPP, because they are retained longer, will continue to release drugs to cells. Since MPP can penetrate both the LML and AML, a fraction may reach and bind to the underlying epithelia and further improve drug delivery. While this schematic reflects the mucosal physiology of the gastrointestinal and cervicovaginal tracts, the same behavior is expected for the respiratory airways. In the respiratory airways, CP are mostly immobilized in the luminal stirred mucus gel layer, whereas MPP penetrate the mucus gel and enter the underlying periciliary layer. Upon mucociliary clearance, a significant fraction of MPP remains in the periciliary layer, resulting in prolonged retention. This schematic does not depict the glycocalyx adjacent to the epithelial surface, which may contribute an additional steric barrier to cellular entry of MPP.