1. Pharmacokinetics: Drug Absorption
Course Coordinator
Jamaluddin Shaikh, Ph.D.
School of Pharmacy, University of Nizwa
Lecture-5
February 19, 2012

2. Pharmacokinetics

3. Mechanism of Drug Absorption
Except for IV route, all other routes require that the drug be transported from the site of administration into the systemic circulation. The transport of drugs across membrane involves one or more of the following:
Passive transport
Passive diffusion
Facilitated diffusion
Active transport
Filtration
Ion-pair transport
Endocytosis

4. Passive Diffusion
Separated by two compartments, the drug moves from a region of high concentration to a lower concentration
Does not involve a carrier
Lipid-soluble drugs readily move
across biologic membranes due to
solubility in the membrane bilayers
Water-soluble drugs penetrate the
cell membrane through aqueous
channels or pores

5. Facilitated Diffusion
A protein carrier–mediated transport system
Dose not require energy
Concentration dependent, molecule transported from regions of higher to regions of lower concentrations 5

6. Active Transport
Specific carrier proteins needed that span the membrane
Energy-dependent and is driven by the hydrolysis of ATP
Capable of moving drugs against a concentration gradient; that is, from a region of low drug concentration to one of higher drug concentration

7. Filtration
Depends both on the existence of a pressure gradient as a driving force and on the size of the compound relative to the size of the pore through which it is to be filtered
In biological systems, the passage of many small water-soluble solutes through aqueous channels in the membrane is accomplished by filtration
Membrane

8. Ion-pair Transport
Absorption of some highly ionized compounds e.g., sulfonic acids and quaternary ammonium compounds from the gastrointestinal tract follow this mechanism
Highly lipophobic drugs combine reversibly with such endogenous compounds as mucin in the gastrointestinal lumen, forming neutral ion pair complexes; it is this neutral complex that penetrates the lipid membrane by passive diffusion
Membrane

9. Endocytosis
This type of drug delivery transports drugs of exceptionally large size across the cell membrane
Involves engulfment of a drug molecule by the cell membrane and transport into the cell
Exocytosis is the reverse of endocytosis and is used by cells to secrete many substances
by a similar vesicle formation
process, e.g., neurotransmitters
are released by exocytosis

10. Passive Vs Active Transport