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Fate of Drugs in Terrestrial Species

Absorption is described by determining the rate constant of drug passing into the bloodstream at successive time intervals after administration. Residues may be found in tissues after some or all of the drug has been systemically absorbed. Absorption is influenced by the properties of cell membranes, the physicochemical characteristics of the drug, and the route of drug administration (1). [Pg.11]

It has long been established that all cell membranes in the body are composed of a fundamental structure called plasma membrane. This boundary surrounds single cells such as epithelial cells. More complex membranes such as intestinal epithelium and skin, are composed of multiples of this fundamental structure, which has been visualized as a bimolecular layer of lipid molecules with a monolayer of protein adsorbed into each surface. Cell membranes are further interspersed with small pores that can be protein line channels through the lipid layer or, simply, spaces between the lipid molecules. In membranes composed of many cells, the spaces between the cells constimte another kind of membrane pores (2). [Pg.12]

Two other types of specialized transport mechanisms, pinocytosis and phagocytosis, may also account for the transmembrane movement of some macromolecules (2). In these complex processes, the cell engulfs a droplet of extracellular fluid or a particle of solid material such as a bacterium. The droplet or particle is completely surrounded by a portion of the cell membrane and the resulting vesicle becomes detached and moves into the cell cytoplasm. [Pg.13]

Apart from the properties of the cell membranes, certain physicochemical characteristics of the drugs can significantly influence the rate of their absorption. Most drugs are either weak acids or bases that exist in solution as a mixture of ionized and nonionized forms. Nonionized forms are more lipophilic whereas ionized forms more hydrophilic. Consequently, the nonionized forma are lipid soluble and able to permeate rapidly across cell membranes. This process is known as passive nonionic diffusion. [Pg.13]

On the other hand, the ionized forms, which tend to be less lipid soluble, cannot diffuse across tire lipid phase of the cell membrane. Ionized molecules may also repelled from the cell surface by groups with similar charge, or may be attracted to it and held there by groups with opposite charge. Ionized drug forms are, sometimes, unable to be filtered even through the aqueous pores of the membranes due to their own size or to the size they attain after the attraction of water molecules. [Pg.13]


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