Page 140 - Interactive Theoritical Notes of Bioharmaceutics and pharamcokinetics.docx compressed

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

In circulating blood, there is a constant ratio between the bound and free

fractions of the drug.

When the concentration of the free drug falls due to redistribution, metabolism or

excretion, the free: bound ratio is maintained

by dissociation of the bound form of the drug. Thus plasma protein binding

mainly serves as a reservoir, which supplies free drug whenever required.

Every drug binds to plasma proteins to a different extent.

Drug binding is rapidly reversible, so some portion is always available as free

drug for distribution.

The association and dissociation process between the bound and unbound states is

very rapid.

Although only the unbound portion of drug exerts its pharmacologic effect, most

drug assays measure total drug concentration-both bound and unbound drug.

Protein binding in plasma can range from 0 to >99% of the total drug in the plasma

and varies with different drugs. Every drug bind to plasma proteins to a different
extent.

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