Gastrointestinal Absorption of Drugs

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Chapter: Biopharmaceutics and Pharmacokinetics : Absorption of Drugs

The oral route of drug administration is the most common for systemically acting drugs and therefore, more emphasis will be given to gastrointestinal (GI) absorption of drugs.


GASTROINTESTINAL ABSORPTION OF DRUGS

The oral route of drug administration is the most common for systemically acting drugs and therefore, more emphasis will be given to gastrointestinal (GI) absorption of drugs. Moreover, it covers all the aspects of variability observed in drug absorption. Before proceeding to discuss absorption aspects, a brief description of cell membrane structure and physiology is necessary.

Cell Membrane: Structure and Physiology

For a drug to be absorbed and distributed into organs and tissues and eliminated from the body, it must pass through one or more biological membranes/barriers at various locations. Such a movement of drug across the membrane is called as drug transport.

The basic structure of cell membrane is shown in Fig. 2.2.


The cellular membrane consists of a double layer of amphiphilic phospholipid molecules arranged in such a fashion that their hydrocarbon chains are oriented inwards to form the hydrophobic or lipophilic phase and their polar heads oriented to form the outer and inner hydrophilic boundaries of the cellular membrane that face the surrounding aqueous environment. Globular protein molecules are associated on either side of these hydrophilic boundaries and also interspersed within the membrane structure. In short, the membrane is a mayonnaise sandwich where a bimolecular layer of lipids is contained between two parallel monomolecular layers of proteins. The hydrophobic core of the membrane is responsible for the relative impermeability of polar molecules. Aqueous filled pores or perforations of 4 to 10 Å in diameter are also present in the membrane structure through which inorganic ions and small organic water-soluble molecules like urea can pass. In general, the biomembrane acts like a semipermeable barrier permitting rapid and limited passage of some compounds while restricting that of others.

The GI lining constituting the absorption barrier allows most nutrients like glucose, amino acids, fatty acids, vitamins, etc. to pass rapidly through it into the systemic circulation but prevents the entry of certain toxins and medicaments. Thus, for a drug to get absorbed after oral administration, it must first pass through this biological barrier.

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