Absorption, drug across nasal epithelium

The physicochemical properties of a molecule which affect its absorption across the nasal epithelium are broadly the same as those affecting transepithelial absorption at any site and have been discussed extensively in Section 1.3.4. These factors influence the mechanism and rate of drug absorption through the nasal epithelium. 

This refers to the transport across the epithelial cells, which can occur by passive diffusion, carrier-mediated transport, and/or endocytic processes (e.g., transcytosis). Traditionally, the transcellular route of nasal mucosa has been simply viewed as primarily crossing the lipoidal barrier, in which the absorption of a drug is determined by the magnitude of its partition coefficient and molecular size. However, several investigators have reported the lack of linear correlation between penetrant lipophilicity and permeability [9], which implies that cell membranes of nasal epithelium cannot be regarded as a simple lipoidal barrier. Recently, compounds whose transport could not be fully explained by passive simple diffusion have been investigated to test if they could be utilized as specific substrates for various transporters which have been identified in the... 

The implications of this for drag absorption are that administration of a drug as drops may only be suitable if the drag molecule is rapidly absorbed. Those drug molecules which diffuse across the nasal epithelium more slowly will need a longer contact time and may be better administered as sprays. The absorption rate of certain drags may be so slow that therapeutically active plasma levels are not attained. 

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