ABSTRACT
A concentration is only worth measuring if it produces an effect! The derivation of pharmacokinetic (PK) parameters from plasma concentration data and how these parameters relate to the disposition of the compound under study has been discussed earlier. However, PK studies serve little purpose if the compound is biologically inert. PK studies are performed because it is assumed that the parameters obtained are relevant to the in vivo activity of the compound. Perhaps the duration of action can be determined from the half-life? Perhaps the volume of distribution can be related to access to a receptor? In reality these relationships may not be straightforward. A short half-life does not necessarily equate to a short duration of action. A large volume of distribution does not ensure access to intracellular receptors. For PK parameters to be related to biological activity the relationship between the plasma concentrations (PK) and the effects (pharmacodynamics or PD) must be established through PK/PD modelling.
