ABSTRACT
Inhaled particles deposited in the central airways are mainly cleared by mucociliary clearance, whereas those deposited in the alveoli are cleared by alveolar macrophages. In addition, free drug is absorbed into the bronchial and/or pulmonary circulation. Epithelial transport can involve various mechanisms such as passive diffusion, vesicle-mediated endocytosis/transcytosis, or transporter-mediated absorption and efflux. Membrane transporters are either passive (that is, they do not require metabolic energy to facilitate the diffusion of solutes across biological membranes along their concentration gradient) or active. The latter requires metabolic energy to transport the substrates with or against the concentration gradient. The two-main superfamilies of membrane transporters are the ATP-binding cassette (ABC) family and the solute carrier (SLC) family. Transporters belonging to the ABC family are mainly involved in extruding their relevant substrates out of cells in an ATP-dependent mechanism, whereas those belonging to the SLC family mediate the uptake or efflux of small molecules by facilitated diffusion or via secondary active processes.
