tissue binding

Just as drugs cling to proteins in the blood, they also stick to components inside tissues — proteins, fats, nucleic acids and structural molecules. Tissue binding is this attachment of drug to tissue constituents, and it pulls drug out of the bloodstream and holds it in the body's nooks and crannies.

Strong tissue binding draws drug out of plasma, lowering blood levels while raising the total amount held in the body. This is one of the main reasons some drugs have a very large apparent volume of distribution. Bound drug in tissue, like bound drug in plasma, is generally inactive and in equilibrium with free drug; it is released slowly as free concentrations fall.

Tissue binding can prolong a drug's residence and its effect, and occasionally cause harm by accumulating where it is bound. Examples include digoxin binding to muscle, chloroquine and amiodarone accumulating in many tissues, and tetracyclines binding to calcium in growing bone and teeth, which can stain children's teeth.