Volumes of distribution.

Volumes of distribution are proportionality constants between total amount of drug in the body and plasma concentrations. As snapshot plasma drug concentrations may be measured in different conditions (at equilibrium, under pseudo-equilibrium condition,...), several volumes of distribution have been defined. The two most relevant are the volume of distribution at equilibrium (V(ss)), and the volume of distribution during pseudo-equilibrium (V(area)). Volumes of distribution are used to compute a loading dose (V(ss)) or the residual amount of drug in the body knowing plasma concentrations (V(area)). Volume of distribution may be interpreted in terms of drug distribution having recourse to physiological models involving drug binding to plasma and tissues. Volumes of distribution should be determined early in drug development programmes and those having a large volume of distribution may be selected to obtain a long terminal half-life even for drugs having a relatively high clearance.