Ligand bias
Functional selectivity (or “agonist trafficking”, “biased agonism”, “biased signalling”, "ligand bias" and “differential engagement”) is the ligand-dependent selectivity for certain signal transduction pathways relative to a reference ligand (often the endogenous hormone or peptide) at the same receptor. Functional selectivity can be present when a receptor has several possible signal transduction pathways. To which degree each pathway is activated thus depends on which ligand binds to the receptor. Functional selectivity, or biased signalling, is most extensively characterized at G protein coupled receptors (GPCRs). A number of biased agonists, such as those at opioid receptors that mediate pain, show potential at various receptor families to increase beneficial properties while reducing side effects. For example, pre-clinical studies with G protein biased agonists at the mu opioid receptor show equivalent efficacy for treating pain with reduced risk for addictive potential and respiratory depression.
Functional selectivity has been proposed to broaden conventional definitions of pharmacology.
Traditional pharmacology posits that a ligand can be either classified as an agonist (full or partial), antagonist or more recently an inverse agonist through a specific receptor subtype, and that this characteristic will be consistent with all effector (second messenger) systems coupled to that receptor. While this dogma has been the backbone of ligand-receptor interactions for decades now, more recent data indicates that this classic definition of ligand-protein associations does not hold true for a number of compounds; such compounds may be termed as mixed agonist-antagonists.
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