Radioligand dissociation measurements: potential interference of rebinding and allosteric mechanisms and physiological relevance of the biological model systems.

INTRODUCTION: In many situations, optimal drug therapy requires continuing high levels of target occupancy and this notion has led pharmacologists to focus their attention on the rate by which drug candidates dissociate from their target. To this end, radioligand dissociation experiments are often carried out on in vitro models, such as intact cells and the membranes thereof, but the interpretation of the collected data is sometimes ambiguous. AREAS COVERED: Pharmacodynamics is concerned about what the drug does to the target and, in this respect, allosteric modulation constitutes a quite novel, very promising research topic. The ability of unlabeled drugs to accelerate radioligand dissociation is often advocated to be a hallmark of such mechanism. Yet, the present computerized simulations reveal that competitive drugs produce the same effect by preventing hindered diffusion- and "forced proximity"-related rebinding of the radioligand. Herein, the authors provide hints to discern among those mechanisms. EXPERT OPINION: A critical, but constructive appraisal of radioligand dissociation binding data leads to the viewpoint that, from a physiological perspective, dissociation from confluent target-expressing plated cells, when in a naïve medium, is likely to provide the most pertinent insight in that ligand's in vivo residence time.