Drug distribution is the process by which the drug is delivered from the systemic circulation to body tissues and fluids. Distribu-tion of an absorbed drug within the body depends on several fac-tors:
· blood flow
· protein binding.
After a drug has reached the bloodstream, its distribution in the body depends on blood flow. The drug is quickly distributed to or-gans with a large supply of blood. These organs include the:
Distribution to other internal organs, skin, fat, and muscle is slower.
The ability of a drug to cross a cell membrane depends on whether the drug is water or lipid (fat) soluble. Lipid-soluble drugs easily cross through cell membranes; water-soluble drugs can’t.
Lipid-soluble drugs can also cross the blood-brain barrier and enter the brain.
As a drug travels through the body, it comes in contact with pro-teins such as the plasma protein albumin. The drug can remain free or bind to the protein. The portion of a drug that’s bound to a protein is inactive and can’t exert a therapeutic effect. Only the free, or unbound, portion remains active.
A drug is said to be highly protein-bound if more than 80% of the drug is bound to protein.