The elusive nature of intrinsic efficacy.

In the discipline of pharmacology, drugs (ligands) are used as tools to elucidate the processes of biological systems. Because of this, pharmacologists strive to delineate all characteristics of drugs. Decades of research have resulted in the proposal that ligands possess two properties that are intrinsic to the ligand and are invariant of the system in which their effects are investigated. These properties are affinity (the capacity of a drug to bind to a receptor) and intrinsic efficacy (the capacity of a drug to activate or inactivate a receptor). Although affinity is a relatively easy parameter to measure with a variety of techniques, ways of quantifying intrinsic efficacy have remained elusive ever since its inception. Furthermore, recent evidence suggests that intrinsic efficacy might not be a single, ligand-dependent parameter but that agonists might have multiple intrinsic efficacies. William Clarke and Richard Bond discuss several reasons why the claim that intrinsic efficacy is a ligand-dependent parameter should be questioned, and the possible impact of these findings.