Volume of Distribution and Protein Binding

Volume of distribution (Vd) is one of the most fundamental pharmacokinetic parameters, describing the apparent space in the body into which a drug distributes. It is calculated using the simple equation: Vd = Dose / Cp, where Dose is the amount of drug administered intravenously and Cp is the initial plasma concentration immediately after administration. The term "apparent" is crucial — Vd is not a real physiological volume but a mathematical construct that relates the total amount of drug in the body to the concentration measured in plasma.

Physiological fluid compartments provide context for interpreting Vd values. Plasma volume is approximately 3–4 L, interstitial fluid approximately 12 L, intracellular fluid approximately 25 L, and total body water approximately 42 L in a 70 kg adult. A drug with a Vd of 3–5 L is largely confined to plasma (e.g., warfarin heavily bound to plasma albumin). A Vd of 15–20 L suggests distribution into extracellular fluid. A Vd exceeding 40 L indicates significant intracellular or tissue accumulation. Some lipophilic drugs have Vd values of hundreds or thousands of litres — chloroquine, for example, has a Vd of approximately 200–800 L/kg, reflecting enormous sequestration in tissue compartments despite low plasma concentrations.

Plasma protein binding is the primary determinant of Vd for many drugs. Albumin is the principal binding protein for acidic drugs (phenytoin, warfarin, diazepam, salicylates), while alpha-1 acid glycoprotein (AAG) predominantly binds basic drugs (lignocaine, propranolol, amitriptyline). Only the unbound (free) fraction of a drug is pharmacologically active and available for distribution, metabolism, and excretion. This is the free drug hypothesis, which underpins therapeutic drug monitoring.

The degree of protein binding has several important clinical implications. First, changes in binding alter the free fraction. In hypoalbuminaemia (liver disease, malnutrition, nephrotic syndrome), total drug concentrations may appear low, but the free fraction is increased — leading to greater effect and potential toxicity if doses are adjusted based on total levels alone. Phenytoin is the classic example: a patient with albumin of 20 g/L may have a total phenytoin level of 40 µmol/L (apparently subtherapeutic) but a normal or elevated free concentration. The Sheiner-Tozer equation corrects total phenytoin for albumin: adjusted phenytoin = measured phenytoin / (0.2 × albumin [g/L] / 40 + 0.1 × non-albumin fraction). Second, protein binding displacement interactions can transiently increase free drug levels when two highly protein-bound drugs compete for albumin binding sites, though the clinical significance is often limited because the displaced drug is also more rapidly cleared.

Factors affecting Vd include lipophilicity (high lipophilicity → high Vd), molecular size (large molecules confined to plasma), tissue binding (sequestration in fat, muscle, or organs), ionisation state (pH-dependent), and protein binding. Understanding Vd guides loading dose calculations: Loading Dose = Vd × target plasma concentration / F, where F is bioavailability. In New Zealand, Pharmac-funded drugs such as phenytoin, digoxin, and amiodarone each demonstrate how Vd profoundly influences dosing strategy and monitoring requirements.