Functional selectivity is the ligand-dependent selectivity for certain signal transduction pathways relative to a reference ligand 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. A number of biased agonists, such as those at muscarinic M2 receptors tested as analgesics or antiproliferative drugs, or 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. Studies within the chemokine receptor system also suggest that GPCR biased agonism is physiologically relevant. For example, a beta-arrestin biased agonist of the chemokine receptor CXCR3 induced greater chemotaxis of T cells relative to a G protein biased agonist.
Functional vs. traditional selectivity
Functional selectivity has been proposed to broaden conventional definitions of pharmacology. Traditional pharmacology posits that a ligand can be either classified as an agonist, antagonist or more recently an inverse agonist through a specific receptor subtype, and that this characteristic will be consistent with all effector 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. Functional selectivity posits that a ligand may inherently produce a mix of the classic characteristics through a single receptor isoform depending on the effector pathway coupled to that receptor. For instance, a ligand can not easily be classified as an agonist or antagonist, because it can be a little of both, depending on its preferred signal transduction pathways. Thus, such ligands must instead be classified on the basis of their individual effects in the cell, instead of being either an agonist or antagonist to a receptor. It is also important to note that these observations were made in a number of different expression systems and therefore functional selectivity is not just an epiphenomenon of one particular expression system.
Examples
One notable example of functional selectivity occurs with the 5-HT2A receptor, as well as the 5-HT2C receptor. Serotonin, the main endogenous ligand of 5-HT receptors, is a functionally selective agonist at this receptor, activating phospholipase C, but does not activate phospholipase A2, which would result in arachidonic acid signalling. However, the other endogenous compound dimethyltryptamine activates arachidonic acid signaling at the 5-HT2A receptor, as do many exogenous hallucinogens such as DOB and lysergic acid diethylamide. Notably, LSD does not activate IP3 signaling through this receptor to any significant extent. Oligomers; specifically 5-HT2A– heteromers mediate this effect. This may explain why some direct 5-HT2receptor agonists have psychedelic effects, whereas compounds that indirectly increase serotonin signalling at the 5-HT2 receptors, such as selective serotonin reuptake inhibitors and monoamine oxidase inhibitors, generally do not; nor do 5HT2A receptor agonists without constitutive activity at the mGluR dimer, such as lisuride. Tianeptine, an atypical antidepressant, is thought to exhibit functional selectivity at the μ-opioid receptor to mediate its antidepressant effects. Oliceridine is a mu opioid receptor agonist that has been described to be functionally selective towards G protein and away from β-arrestin2 pathways. However, recent reports highlight that, rather than functional selectivity or 'G protein bias', this agonist has low intrinsic efficacy. In vivo, it has been reported to mediate pain relief without tolerance nor gastrointestinal side effects. The delta opioid receptor agonists SNC80 and ARM390 demonstrate functional selectivity that is thought to be due to their differing capacity to cause receptor internalization. While SNC80 causes delta opioid receptors to internalize, ARM390 causes very little receptor internalization. Functionally, that means that the effects of SNC80 do not occur when a subsequent dose follows the first, whereas the effects of ARM390 persist. However, tolerance to ARM390's analgesia still occurs eventually after multiple doses, though through a mechanism that does not involve receptor internalization. Interestingly, the other effects of ARM390 persist after tolerance to its analgesic effects has occurred.
