Drug concentration, binding, and effect in vivo.

It is an axiom of pharmacodynamics that drug effects are determined by drug concentration at the site of action. The link between concentration and effect can often be described by empirical models, but the ability to measure binding to the target tissue permits a more detailed description. The action and interaction of drugs at identifiable receptor sites can then be predicted from a knowledge of their binding properties and the law of mass action. The time course of drug effect is determined not only by drug disposition reflected in the blood circulation but also by the equilibration rate between blood and the effect site, and the steps linking the direct actions of the drug to their expression as an observable drug effect. Models encompassing these phenomena have been developed and have been applied in many situations to describe the kinetics of pharmacological response.