It is generally assumed that only the unbound drug can be transported across membranes and becomes subject to absorption, distribution, metabolism, and excretion (ADME) processes. Hence, plasma protein binding is an important parameter to understand the pharmacokinetic behavior of a drug as well as its pharmacological/toxicological effects. The binding of drug candidates in the plasma of humans, animals, and/or to isolated proteins such as human serum albumin or a-acid-glycoprotein is determined by equilibrium dialysis and expressed as an unbound fraction. Our in vitro assay may include an investigation of concentration dependent binding in all matrices.
The concentration ratio of drugs in whole blood and plasma provides an indication of drug binding to erythrocytes. This becomes important when clearance estimates are compared directly with organ blood flow rates e.g., in the liver, to obtain the extraction ratio of organ clearance.
Our whole blood distribution assay is designed to determine the blood-to-plasma ratio of drugs candidates in early development. Data can be used e.g., to scale from in vitro to in vivo clearances and will thus, allow an estimation of the hepatic first-pass extraction from plasma clearance, and to decide whether the bioanalysis for pharmacokinetic evaluation should be done in whole blood or in plasma.
Both in vitro distribution studies can be conducted with either radiolabeled or unlabeled test compounds applying liquid scintillation counting or tandem mass spectrometry for quantification, respectively.