Protein binding refers to the reversible association of a drug with plasma proteins, predominantly albumin and alpha-1-acid glycoprotein. Only the unbound or free fraction of a drug is pharmacologically active, because only free drug can cross cell membranes to reach target receptors, undergo metabolism, and be excreted. The bound fraction serves as a circulating depot that releases free drug as it is eliminated, maintaining equilibrium and prolonging the drug’s presence in the body.
Albumin and Alpha-1-Acid Glycoprotein
Albumin is the most abundant plasma protein, synthesized by the liver and present at concentrations of approximately 35 to 50 g per liter. Albumin binds primarily acidic drugs such as warfarin, phenytoin, salicylates, and many nonsteroidal anti-inflammatory drugs. It has multiple binding sites with different affinities, and the binding is typically of moderate to high affinity but limited capacity. The albumin molecule can undergo conformational changes that affect its binding properties, and its concentration can decrease in liver disease, nephrotic syndrome, and malnutrition.
Alpha-1-acid glycoprotein (AAG) , also known as orosomucoid, is an acute-phase reactant whose concentration increases during inflammation, infection, malignancy, and stress. AAG binds primarily basic drugs such as lidocaine, propranolol, quinidine, and many antidepressants. Because AAG concentrations rise in disease states, the free fraction of basic drugs can decrease during acute illness, potentially altering their pharmacological response.
Binding Affinity and Capacity
The extent of protein binding depends on the affinity of the drug for the protein and the capacity of the binding sites. A drug with high affinity and a favorable binding site concentration will be highly protein bound, meaning less than 10% of the total drug concentration is free. Such drugs are said to be restrictively cleared, as only the free fraction is available for glomerular filtration. Highly protein-bound drugs also tend to have smaller volumes of distribution because the bound drug is confined to the plasma compartment.
Drugs can compete for the same binding sites on proteins, leading to displacement interactions. When a highly bound drug is coadministered with another drug that has higher affinity for the same binding site, the displaced drug transiently increases its free fraction. This can be clinically significant if the displaced drug has a narrow therapeutic index, as in the case of phenytoin displaced by valproic acid. However, the increase in free drug concentration also makes more drug available for clearance, so a new steady state is eventually reached, often with the same free concentration but a lower total concentration.
Effect on Volume of Distribution
The apparent volume of distribution (Vd) is influenced by protein binding because bound drug is largely restricted to the plasma space. A highly protein-bound drug will have a smaller Vd, often approximating the plasma volume of approximately 3 L. Conversely, a drug that is extensively bound to tissue proteins will have a large Vd, even if it is also highly bound to plasma proteins. The tissue-to-plasma partition coefficient captures this balance.
Changes in protein binding can alter the Vd and the elimination half-life. In hypoalbuminemia, the free fraction of albumin-bound drugs increases, which can increase the Vd as more drug distributes out of the plasma. The clinical significance depends on whether the drug is normally highly bound and whether it has a narrow therapeutic window.
Clinical Significance
The clinical relevance of protein binding depends on several factors. For drugs that are less than 90% protein bound, changes in binding usually do not produce clinically meaningful effects because the free fraction remains relatively small even if it doubles. For highly bound drugs, however, small changes in binding can substantially alter the free concentration. The interpretation of total drug concentrations in therapeutic drug monitoring must account for protein binding status. A patient with low albumin may have a subtherapeutic total phenytoin concentration but a normal free phenytoin concentration, leading to inappropriate dose escalation if only total levels are measured.
Understanding protein binding helps clinicians anticipate drug interactions, interpret drug concentration measurements, and adjust therapy in patients with altered protein levels due to disease or physiological state.
