Drug Distribution and Volume of Distribution

Drug distribution is the reversible movement of a drug from the bloodstream into the tissues of the body after absorption. The extent of distribution is summarised by the volume of distribution (Vd), a proportionality constant relating the total amount of drug in the body to its concentration in plasma. Distribution and Vd are core pharmacokinetic concepts that depend on a drug's binding to plasma proteins and tissues and on its physicochemical properties.

Definition

The volume of distribution is the apparent (theoretical) volume that would be required to contain the total amount of drug in the body at the same concentration as that measured in plasma; it characterises the extent to which a drug distributes out of plasma into tissues.

Scope

This topic covers how drugs partition between blood and tissues, the role of plasma protein and tissue binding, and the meaning, calculation, and interpretation of the apparent volume of distribution. It is presented as a pharmacokinetic and physicochemical topic and does not provide dosing guidance.

Core questions

• What determines how widely a drug distributes from blood into tissues?
• How is the apparent volume of distribution defined and interpreted?
• How do plasma protein binding and lipophilicity shape distribution?

Key concepts

• Apparent volume of distribution (Vd)
• Plasma protein binding
• Tissue binding
• Free (unbound) drug fraction
• Lipophilicity and tissue partitioning
• Total body water compartments
• Loading dose relationship (Vd x target concentration)

Mechanisms

After entering the blood, a drug distributes into tissues to an extent set by its ability to leave the plasma and bind elsewhere. A drug that is highly bound to plasma proteins such as albumin tends to be retained in the circulation and has a low volume of distribution, whereas a lipophilic drug that partitions extensively into tissues or binds tissue components has a large, sometimes very large, apparent volume of distribution that can exceed total body water. Because Vd is calculated from total plasma concentration, it is an apparent rather than a physical volume. Only the unbound (free) fraction is available to distribute, be metabolised, and act. Across hundreds of drugs the volume of distribution spans several orders of magnitude, reflecting this diversity of binding and partitioning behaviour (Obach, 2008; Rowland & Tozer, 2011).

Clinical relevance

The volume of distribution conceptually links the amount of drug in the body to its measurable plasma concentration and, together with clearance, helps explain a drug's half-life. It is also the parameter conceptually associated with a loading dose. This entry explains the concept and its determinants and is not a source of dosing recommendations or individualised advice.

Evidence & guidelines

Compendia of human pharmacokinetic parameters document the wide range of volumes of distribution observed across marketed drugs and relate them to physicochemical and binding properties, providing reference expectations for candidate characterisation (Obach, 2008). Standard pharmacokinetic texts formalise the definitions and the relationships among Vd, clearance, and half-life (Rowland & Tozer, 2011; Gibaldi & Perrier, 1982).

History

The volume of distribution arose from compartmental modelling of drug concentrations, formalised in mid-twentieth-century pharmacokinetics and codified in monographs such as Gibaldi and Perrier (1982). Recognition that Vd is an apparent parameter governed by plasma and tissue binding has remained central, and large empirical compilations have since mapped its range and physicochemical correlates across the drug landscape (Obach, 2008).

Key figures

• Malcolm Rowland
• Thomas Tozer
• Milo Gibaldi
• R. Scott Obach

Seminal works

• gibaldi-perrier-1982
• obach-2008

Frequently asked questions

What is the volume of distribution?

It is an apparent volume that relates the total amount of drug in the body to the concentration measured in plasma. A small value suggests the drug stays largely in the blood (often due to plasma protein binding), while a large value suggests extensive distribution into tissues.

Why can the volume of distribution be larger than the body's actual volume?

Because it is a calculated (apparent) parameter, not a physical space. When a drug partitions strongly into tissues, very little remains in plasma, so the calculated volume needed to account for the total amount can far exceed total body water.