Congratulations to Dr. Bill Clarke and Dr. Kelly Berg for most read article

William Clarke
William Clarke, Ph.D.

International Journal of Neuropsychopharmacology





Congratulations to Dr. Bill Clarke and Dr. Kelly Berg for having their paper on biased signaling in the top 10 most-viewed papers over the last 12 months!

Making Sense of Pharmacology: Inverse Agonism and Functional Selectivity

Dr. Bill Clarke –

“It is difficult to overestimate the importance of pharmacology for medicine and research. In medicine, drugs are essential components of a physician’s toolbox to treat disease. In fact, drugs have been used as medicines to treat disease since the beginning of recorded history. In research, drugs are used to perturb physiological and cellular systems to gain understanding of how these systems function. In response to experimental evidence accumulated over the past 20 to 30 years, major changes in our traditional understanding of drug-receptor interactions have occurred. This article highlights 2 of the major changes to pharmacology: inverse agonism and functional selectivity.”


Dr. Kelly Berg
William Clarke
William Clarke, Ph.D.

Constitutive receptor activity/inverse agonism and functional selectivity/biased agonism are 2 concepts in contemporary pharmacology that have major implications for the use of drugs in medicine and research as well as for the processes of new drug development. Traditional receptor theory postulated that receptors in a population are quiescent unless activated by a ligand. Within this framework ligands could act as agonists with various degrees of intrinsic efficacy, or as antagonists with zero intrinsic efficacy. We now know that receptors can be active without an activating ligand and thus display “constitutive” activity. As a result, a new class of ligand was discovered that can reduce the constitutive activity of a receptor. These ligands produce the opposite effect of an agonist and are called inverse agonists. The second topic discussed is functional selectivity, also commonly referred to as biased agonism. Traditional receptor theory also posited that intrinsic efficacy is a single drug property independent of the system in which the drug acts. However, we now know that a drug, acting at a single receptor subtype, can have multiple intrinsic efficacies that differ depending on which of the multiple responses coupled to a receptor is measured. Thus, a drug can be simultaneously an agonist, an antagonist, and an inverse agonist acting at the same receptor. This means that drugs have an additional level of selectivity (signaling selectivity or “functional selectivity”) beyond the traditional receptor selectivity. Both inverse agonism and functional selectivity need to be considered when drugs are used as medicines or as research tools.

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