Nasal mucociliary clearance as a factor in nasal drug delivery.
The nasal mucociliary clearance system transports the mucus layer that covers the nasal epithelium towards the nasopharynx by ciliary beating. Its function is to protect the respiratory system from damage by inhaled substances. Impairment of nasal mucociliary clearance can result in diseases of the upper airways. Therefore, it is important to study the effects of drugs and drug excipients on nasal mucociliary clearance. A large number of methods are used to assess mucociliary clearance. These methods study the effects of drug and excipients on the mucociliary system in vitro or in vivo in animals and humans. In some cases, the results of different in vitro and in vivo measurements do not correlate well. In vitro methods, especially ciliary beat frequency measurements, have been demonstrated to be valuable tools for toxicity screening. However, in vivo studies are essential to confirm the safety of nasal drug formulations. Nasal mucociliary clearance also has implications for nasal drug absorption. Drugs are cleared rapidly from the nasal cavity after intranasal administration, resulting in fast systemic drug absorption. Several approaches are discussed to increase the residence time of drug formulations in the nasal cavity, resulting in improved nasal drug absorption. However, more experimental evidence is needed to support the conclusion that this improved absorption is caused by a longer residence time of the nasal drug formulation.