Drug Distribution and Protein Binding

Drug distribution is the reversible movement of a drug from the systemic circulation into tissues and back. How widely a drug distributes is shaped by its lipophilicity, its binding to plasma proteins, its binding to tissue components, and tissue perfusion; the net effect is summarised by the apparent volume of distribution.

Definition

Drug distribution is the reversible transfer of a drug between the systemic circulation and the tissues; protein binding is the reversible association of a drug with plasma or tissue proteins, which influences the fraction of drug that is free and therefore available to distribute and to act.

Scope

The entry covers the determinants of distribution, the concept of the apparent volume of distribution, the binding of drugs to plasma proteins such as albumin and alpha-1-acid glycoprotein, and the free-drug principle. It is a conceptual reference and does not give dosing advice.

Core questions

• What physiological and physicochemical factors determine how widely a drug distributes into tissues?
• What does the apparent volume of distribution represent, and why can it exceed total body water?
• How does binding to plasma proteins affect the free (unbound) drug concentration?
• When do changes in protein binding meaningfully alter drug exposure?

Key concepts

• Apparent volume of distribution
• Plasma protein binding (albumin, alpha-1-acid glycoprotein)
• Free (unbound) fraction
• Tissue binding and partitioning
• Perfusion- vs permeability-limited distribution
• Blood-brain barrier and restricted distribution
• Redistribution

Key theories

Free-drug (unbound) hypothesis

The principle that only unbound drug can cross membranes, distribute into tissues, be eliminated, and produce an effect, so the unbound concentration — not the total concentration — is the pharmacologically meaningful quantity; this also explains why protein-binding changes often have smaller net effects than the change in total concentration suggests.

Mechanisms

After entering the blood, a drug distributes between plasma and tissues according to perfusion, its ability to cross membranes, and its relative affinity for tissue and plasma components. Many drugs bind reversibly to plasma proteins — acidic and neutral drugs chiefly to albumin, basic drugs to alpha-1-acid glycoprotein — and only the unbound fraction can leave the circulation, reach its site of action, and be eliminated. The apparent volume of distribution relates the amount of drug in the body to the plasma concentration; extensive tissue binding makes it large (sometimes far exceeding body water), while extensive plasma binding tends to keep it small. Because clearance and unbound concentration adjust, a change in plasma-protein binding usually shifts total concentration more than it shifts the free concentration that governs effect.

Clinical relevance

Distribution and protein binding shape how drug exposure relates to plasma concentration and where in the body a drug accumulates, which is part of interpreting concentration measurements. This entry describes those principles as a reference and is not a basis for individual dosing decisions.

Evidence & guidelines

The free-drug principle and the role of protein binding are codified in standard clinical-pharmacokinetics texts and discussed in the clinical literature; the International Transporter Consortium's work additionally frames how transporters can govern tissue-specific distribution in drug development.

History

The apparent volume of distribution arose from compartmental analysis of plasma concentration-time data as a way to relate dose to concentration. Recognition that only unbound drug is active and eliminable — the free-drug principle — clarified the limited net consequence of many protein-binding changes, correcting an earlier tendency to over-interpret shifts in total concentration. Later work on membrane transporters showed that distribution into specific tissues can be carrier-mediated rather than purely passive.

Debates

How clinically important are protein-binding displacement interactions?

Displacement from plasma proteins transiently raises the free fraction, but compensatory distribution and elimination usually return free concentrations toward baseline, so the clinical significance of most binding-displacement interactions is more limited than once assumed.

Key figures

• Malcolm Rowland
• Thomas N. Tozer
• Leslie Z. Benet
• Jason A. Roberts

Seminal works

• roberts-2012
• rowland-tozer-2011

Frequently asked questions

Why can the volume of distribution be larger than the body itself?

The volume of distribution is an apparent (mathematical) volume relating the amount of drug in the body to the plasma concentration; when a drug binds extensively to tissues, very little remains in plasma, so the calculated volume can be many times total body water.

Does only the bound or the unbound drug produce an effect?

Only unbound (free) drug can leave the circulation, reach its target, and be eliminated, so the unbound concentration is the pharmacologically relevant one; protein-bound drug acts as a reversible reservoir.