The binding pattern of the drug compound to the plasma proteins are a major contributing factor to the distribution and overall efficacy of the drug. Generally, the drug concentration at the site of action is limited when the binding between plasma protein and drug compound is sturdy. Hence, the efficacy of the corresponding compound also reduces. Thus, understanding the impact of free drug in the plasma or plasma protein binding is necessary for drug discovery as assists in finding the associated in vivo efficacy of the drug product.
The degree of plasma protein binding of a drug compound can be determined using Rapid Equilibrium Dialysis (RED). Plasma spiked with the drug is placed in the central chamber of the RED device, and the isotonic sodium phosphate solution is placed in the peripheral chamber. The sample is incubated at 37 °C for 4 hours, and the free compound equilibrium is attained by diffusing unbound compound through the dialysis membrane of the RED device.