The distribution of drugs among various tissues in the body profoundly influences their efficacy and safety. Prediction of the distribution of a new chemical entity (NCE) in vivo can significantly aid in evaluating its potential as a successful drug candidate. Plasma protein binding of test compounds is assessed in vitro. The results predict the extent to which the compound will be bound to plasma proteins in vivo. This, in turn, affects the free concentration in the plasma, the rate and extent of distribution into extravascular tissues, and the susceptibility to breakdown and excretion via hepatic metabolism. Depending on the intended target of a drug and the desired duration of action, a high degree of plasma protein binding may or may not be a desirable feature. The concentration of a compound in target and non-target tissues is often of interest in the early stages of drug development. In situ organ perfusion methods, including in situ brain perfusion and in situ liver perfusion, can be used to determine tissue uptake rates. Tissue-to–plasma, e.g., brain-to-plasma, concentration ratios can be determined during the post-distribution phase of compound dosing to determine the total tissue concentration of compound. Tissue binding assays can then complete the picture by yielding data on the free fraction of compound likely present in the target tissue. Pharmacokinetic (PK) studies are the ultimate means of determining distribution in vivo.

One fundamental parameter of distribution is Vss, the volume of distribution under steady state conditions. This parameter in not a true “volume” in any physical sense, but it does represent the extent of distribution of the compound in the body. High volumes of distribution suggest high tissue uptake, whereas low volumes of distribution suggest that the compound is largely confined to the extracellular space. Intravenous infusions or bolus dosing are the preferred routes for introducing the compound into the body in order to determine the volume of distribution at steady state. Distribution parameters obtained from other routes of dosing, e.g., oral, are usually confounded by lack of knowledge of the fraction of the dose that actually enters the body and the rate of entry. If these parameters are available, then the distribution volumes can be extracted from the pharmacokinetic data.