Physicochemical Factors In Transfer Of Drugs Across Membranes

The absorption, distribution, metabolism, and excretion of a drug all involve its passage across cell membranes (Figure 1—1).

The plasma membrane consists of a bilayer of amphipathic lipids with their hydrocarbon chains oriented inward to the center of the bilayer to form a continuous hydrophobic phase and their hydrophilic heads oriented outward. Individual lipid molecules in the bilayer vary according to the particular membrane and can move laterally and organize themselves with cholesterol (e.g., sphingolipids), endowing the membrane with fluidity, flexibility, organization, electrical resistance, and relative impermeability to highly polar molecules. Membrane proteins embedded in the bilayer serve as receptors, ion channels, and transporters to transduce electrical or chemical signaling pathways; many of these proteins are targets for drugs. Cell membranes are relatively permeable to water and bulk flow of water can carry with it small drug molecules ( <200 Da). Paracellular transport through intercellular gaps is sufficiently large that passage across most capillaries is limited by blood flow (e.g., glomerular filtration). Capillaries of the central nervous system (CNS) and a variety of epithelial tissues have tight intercellular junctions that limit paracellular transport.

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