Page 88 - Interactive Theoritical Notes of Bioharmaceutics and pharamcokinetics.docx compressed

PharmD clinical pharmacy program Level 3, Semester 2 Biopharmaceutics & Pharmacokinetics (PT608(

2. Carrier-Mediated Transport

A. Active Transport

B. Facilitated Diffusion
C. Carrier-Mediated Intestinal Transport

A. Active Transport

➢ Active transport is a carrier-mediated transmembrane process that plays an
important role in the GI absorption and both of renal and biliary secretion of

many drugs and metabolites.
➢ A carrier or membrane transporter binds to the drug and transports it across the

membrane.

Active transport is characterized by:

 Molecules can be transferred from a region of low concentration to one of higher

concentration (i.e. against a concentration gradient)

Therefore, energy is needed which arises from the hydrolysis of ATP or from the

transmembrane sodium gradient and/or electrical potential.

 It is a specialized process requiring a carrier that binds the drug to form a carrier–

drug complex that transfers the drug across the membrane and then dissociates

the drug on the other side of the membrane. The carrier molecule may be highly

selective for the drug molecule.

 If the drug structurally resembles a natural substrate that is actively

transported, then it is likely to be actively transported by the same carrier
mechanism. Therefore, drugs of similar structure may compete for sites of
adsorption on the carrier.

 It is a saturable process. Only a fixed number of carrier molecules are available,
all the binding sites on the carrier may become saturated if the drug concentration
gets very high.

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