Plasma protein binding and blood-free concentrations: which studies are needed to develop a drug?

The plasma protein binding of drugs and metabolites is known to influence their pharmacokinetics and, therefore, their effects. Evaluating the extent and the linearity of protein binding is an essential piece of information that has to be generated during drug development. Blood cell partitioning has a similar relevance.

This paper summarizes the regulatory requirements and focuses particularly on two questions pertaining to the drug development process. The first of these questions asks when is it necessary to perform detailed clinical studies on protein binding while the second asks whether the in vitro studies presently performed in plasma produce biased information.

The authors propose that clinical ex vivo protein-binding studies should be performed on highly bound compounds (a definition of highly bound is suggested as > 95%). They also propose that in vitro studies, to measure the free drug, should be performed in whole blood, rather than in plasma, particularly if binding to proteins or blood cells is nonlinear.