Introduction to Absorption of Drugs.



  • Movement of drug molecules from site of administration to systemic circulation in the unchanged from.

  • The rate and extent at which the drug gets absorbed determines its therapeutic success rate.

Major routes of Drug Administration:

  1. The Enteral Route:

  • This includes peroral i.e. gastrointestinal, sublingual/buccal and rectal routes. 

  • The GI route is the most common route of administration.

  1. The Parenteral Route:

  • This  includes all routes of administration through or under one or more layers of skin. 

  • In case of intravenous route as drug is being poured directly in blood there is no question of absorption but,, it is necessary for extravascular parenteral routes like the subcutaneous and the intramuscular routes.

3. The Topical Route:

  • This includes skin, eyes or other specific membranes. 

  • The intranasal, inhalation, intravaginal and transdermal routes may be considered enteral or topical according to different definitions.

Gastrointestinal Absorption of Drugs.

  • GIT represents the most commonly used route of absorption of drugs.

Mechanisms of Drug Absorption:

  • Transport mechanisms involved in absorption of drugs can be broadly divided into three categories like,

    • Transcellular / Intracellular transport.

    • Paracellular / Intercellular Transport.

    • Vesicular Transport.

  1. Transcellular / Intracellular transport:

  • It is defined as the passage of drugs across the GI epithelium. 

  • It is the most common pathway for drug transport. 

  • The 3 steps involved in transcellular transport of or rugs are 

    • (i) Permeation of GI epithelial cell membrane, a lipoidal barrier -this is the major obstacle to drug absorption.

    • (ii) Movement across the intracellular space (cytosol).

    • (iii) Permeation of the lateral or basolateral membrane-this is of secondary importance. 

  • The various transcellular transport processes involved in drug absorption are:

  1. Passive Transport Processes: 

  • These transport processes do not require energy to pass through the lipid bilayer. 

  • Passive transport processes can be further classified into following types

    • (a) Passive diffusion.

    • (b) Pore transport.

    • (c) Ion-pair transport.

    • (d) Facilitated- or mediated-diffusion.

  1. Active Transport Processes: 

  • These transport processes require energy from ATP to move drug molecules from extracellular to intracellular and opposite. 

  • They are of two types

    • 1) Primary active transport. 

    • 2) Secondary active transport-this process is further subdivided into two

      • (i) Symport (co-transport)

      • (ii) Antiport (countertransport)

2. Paracellular / Intercellular Transport:

  • It is defined as the transport of drugs through the junctions between the GI epithelial cells.

  • Two types,

    • 1. Permeation through tight junctions of epithelial cells: this process basically occurs through openings which are little bigger than the aqueous pores. Compounds such as insulin and cardiac glycosides are taken up by this mechanism.

    • 2. Persorption is the permeation of drugs through temporary openings formed by shedding of two neighboring epithelial cells into the lumen.

  • Paracellular transport differs from pore transport.

3. Vesicular or Corpuscular Transport (Endocytosis): 

  • Like active transport, these are also energy dependent processes.

  • Since the mechanism involves transport across the cell membrane, the process can also be classified as transcellular. 

  • Vesicular transport of drugs can be classed into two categories

    • 1. Pinocytosis: Drinking of cells.

    • 2. Phagocytosis: Eating of cells.

Commonly asked questions.

  1. Define the process of drug absorption.

  2. Define absorption and discuss various mechanisms of drug absorption.

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