For most the administration routes, the bloodstream is responsible for the distribution of the drug by the target tissues.

Plasma protein binding is a physico-chemical and biological property with implications for drug pharmacokinetics, distribution, metabolism, elimination, and toxicological and pharmacological parameters, with consequences for the overall pharmacological action.

The drug is bound to plasma proteins by electrostatic and hydrophobic interactions, which may be reversible or not. If protein-drug complex formation occurs, this macromolecule does not have the ability to pass through cell membranes, being confined to the bloodstream, thereby limiting the drug delivery to the target thereby affecting the desired pharmacological effect.