Distribution—In Vitro Test: Protein Binding

Drugs can bind to proteins in the blood, in extracellular fluids, and in tissues. Measuring the unbound fraction of a drug is necessary for accurate prediction of the pharmacokinetic behavior of a drug. Various in vitro methods have been developed to measure the extent of drug-protein binding in plasma or other tissue matrices such as the microsomal fraction. The most commonly used assays to measure protein binding include equilibrium dialysis, ultrafiltration, and ultracentrifugation. Other methods are utilized based on the specific characteristics of the drug. Typical assay conditions, advantages and disadvantages of various in vitro methods, types of plasma proteins to which drugs commonly bind, factors that affect plasma protein binding, and equations utilized to account for the unbound fraction of the drug in prediction of pharmacokinetic parameters have been discussed in this chapter.