However, it provides little information about the specific pattern of distribution. Each drug is uniquely distributed in the body. Some drugs go into fat, others remain in the ECF, and still others are bound avidly to specific tissues, commonly liver or kidney. Binding The extent of drug distribution into tissues depends on the extent of plasma protein and tissue binding. The ratio of bound to unbound drug in plasma is mainly determined by the reversible interaction between a drug and the plasma protein to which it binds, as governed by the law of mass action.
Many plasma proteins can interact with drugs.
Only unbound drug is thought to be available for passive diffusion to extravascular or tissue sites where pharmacologic effects occur. Plasma protein binding influences distribution and the apparent relationship between pharmacologic activity and total plasma drug concentration. At high drug concentrations, the amount of bound drug approaches an upper limit depending on the number of available binding sites, resulting in saturability. Binding may be very specific, as when chloroquine binds with nucleic acids. Binding usually occurs when a drug associates with a macromolecule in an aqueous environment but may occur when a drug is partitioned into body fat.