Carrier-mediated processes, transcellular route

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... 

Transport across the cell membrane may occur via different routes. Some of these transport processes are energy dependent and therefore termed active others are independent from energy, thus passive. Passive transport phenomena, for example, transcellular transport, are triggered by external driving forces, such as concentration differences, and do not require metabolic activity. However, generally, they are restricted to small lipophilic compounds. In contrast, active transport phenomena, such as active carrier-mediated transport or vesicular pathways, take course independent from external driving... 

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