Drug Distribution

Abstract

The process of drug movement after absorption into various body compartments like the interstitial space and the intracellular space is called drug distribution. Distribution is an important process which results in exposure of the target organ to the drug. The initial phase is highly influenced by the amount of blood flow to various organs and is responsible for the acute onset action of the drugs. The second phase is a slower phase where the drug equilibrates with the muscle, skin, and fat. The second phase can be responsible for the termination of action for some of the drugs. The cardiac output, local regional blood flow, alterations in capillary permeability, plasma protein binding, the local pH, and affinity to tissue proteins are some of the factors that influence drug distribution. Increase in plasma protein binding results in lower volume of distribution, longer duration of action, and slower onset of action. Plasma protein binding is responsible for displacement reactions and drug interactions. Excessive tissue protein binding can result in organ toxicities. To some extent, distribution can be quantified by the apparent volume of distribution. The volume of distribution helps us determine the loading dose of drugs which are required in emergency settings.