Determination of drug–receptor residence times by radioligand binding and functional assays: experimental strategies and physiological relevance

Abstract

Drug–receptor interactions were earlier quantified in terms of affinity and efficacy only but residence time has now also been recognized to affect clinical performance. Different approaches to measure drug binding kinetics are briefly presented and critically evaluated with the help of simulations. Based on the antagonist's ability to reduce agonist-evoked responses, two main methods are used in functional assays. Radiolabelled drug binding to cell membranes constitutes an alternative approach and provides the most direct information. Yet, due to distinct binding properties and the occurrence of rebinding phenomena, intact cell binding studies are likely to be more relevant from a physiological perspective. Indirect information can also be obtained for unlabelled drugs based on their ability to compete with radioligands. Here, attention should be paid to reversible partitioning phenomena. A special matter of concern is that some experimental observations that are commonly interpreted in terms of allosteric interactions can equally well point to long residence time and/or rebinding.