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 signaling, is most extensively characterized at G protein coupled receptors (GPCRs). 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 μ-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.
The 5-HT2A receptor is a subtype of the 5-HT2 receptor that belongs to the serotonin receptor family and is a G protein-coupled receptor (GPCR). The 5-HT2A receptor is a cell surface receptor, but has several intracellular locations.
A serotonin receptor agonist is an agonist of one or more serotonin receptors. They activate serotonin receptors in a manner similar to that of serotonin, a neurotransmitter and hormone and the endogenous ligand of the serotonin receptors.
TCB-2 is a hallucinogen discovered in 2006 by Thomas McLean working in the lab of David Nichols at Purdue University. It is a conformationally-restricted derivative of the phenethylamine 2C-B, also a hallucinogen, and acts as a potent agonist for the 5-HT2A and 5-HT2C receptors with a Ki of 0.26 nM at the human 5-HT2A receptor. In drug-substitution experiments in rats, TCB-2 was found to be of similar potency to both LSD and Bromo-DragonFLY, ranking it among the most potent phenethylamine hallucinogens yet discovered. This high potency and selectivity has made TCB-2 useful for distinguishing 5-HT2A mediated responses from those produced by other similar receptors. TCB-2 has similar but not identical effects in animals to related phenethylamine hallucinogens such as DOI, and has been used for studying how the function of the 5-HT2A receptor differs from that of other serotonin receptors in a number of animal models, such as studies of cocaine addiction and neuropathic pain.
25I-NBOMe is a novel synthetic psychoactive substance with strong hallucinogenic properties, synthesized in 2003 for research purposes. Since 2010, it has circulated in the recreational drug scene, often misrepresented as LSD. The recreational usage of 25I is associated with severe intoxication and deaths in humans.
25B-NBOMe is a derivative of the phenethylamine psychedelic 2C-B, discovered in 2004 by Ralf Heim at the Free University of Berlin. It acts as a potent full agonist for the 5HT2A receptor. Anecdotal reports from users suggest 25B-NBOMe to be an active hallucinogen at a dose of as little as 250–500 µg, making it a similar potency to other phenethylamine derived hallucinogens such as Bromo-DragonFLY. Duration of effects lasts about 12–16 hours, although the parent compound is rapidly cleared from the blood when used in the radiolabeled form in tracer doses. Recently, Custodio et al (2019) evaluated the potential involvement of dysregulated dopaminergic system, neuroadaptation, and brain wave changes which may contribute to the rewarding and reinforcing properties of 25B-NBOMe in rodents.
25I-NBF is a derivative of the phenethylamine hallucinogen 2C-I, which acts as a highly potent partial agonist for the human 5-HT2A receptor, with bias towards the β-arrestin 2 coupled signalling pathway. It has been studied in its 11C radiolabelled form as a potential ligand for mapping the distribution of 5-HT2A receptors in the brain, using positron emission tomography (PET).
25TFM-NBOMe is a derivative of the phenethylamine hallucinogen 2C-TFM, discovered in 2004 by Ralf Heim at the Free University of Berlin. It acts as a potent partial agonist for the 5-HT2A receptor, though its relative potency is disputed, with some studies finding it to be of lower potency than 25I-NBOMe, while others show it to be of similar or higher potency, possibly because of differences in the assay used. 2C-TFM-NB2OMe can be taken to produce psychedelic effects similar to 2C-I-NB2OMe and 2C-D-NB2OMe.
25C-NBOMe is a psychedelic drug and derivative of the psychedelic phenethylamine 2C-C. 25C-NBOMe appeared on online vendor sites in 2010 but was not reported in the literature until 2011. It acts as a potent agonist of the 5-HT2A receptor, and has been studied in its 11C radiolabelled form as a potential ligand for mapping the distribution of 5-HT2A receptors in the brain, using positron emission tomography (PET). Multiple deaths have occurred from usage of 25C-NBOMe due to the ease of accidental overdose. The long-term toxic effects of the drug have not been researched.
25D-NBOMe is a derivative of the phenethylamine derived hallucinogen 2C-D. It acts in a similar manner to related compounds such as 25I-NBOMe, which is a potent agonist at the 5-HT2A receptor. 25D-NBOMe has been sold as a street drug since 2010 and produces similar effects in humans to related compounds such as 25I-NBOMe and 25C-NBOMe. It was banned as a Temporary Class Drug in the UK on 10 June 2013 after concerns about its recreational use.
25N-NBOMe is a derivative of the hallucinogen 2C-N. The pharmacological properties of 25N-NBOMe have not been described in the scientific literature, but it is believed to act in a similar manner to related compounds such as 25I-NBOMe and 25C-NBOMe, which are potent agonists at the 5HT2A receptor. 25N-NBOMe has been sold as a street drug and has only been described in the literature in terms of identification by forensic analysis.
25E-NBOMe is a derivative of the phenethylamine 2C-E. It acts in a similar manner to related compounds such as 25I-NBOMe, which are potent agonists at the 5-HT2A receptor. 25E-NBOMe has been sold as a drug and produces similar effects in humans to related compounds such as 25I-NBOMe and 25C-NBOMe.
25G-NBOMe (NBOMe-2C-G) is a derivative of the phenethylamine hallucinogen 2C-G, which acts as a highly potent agonist for the human 5-HT2A receptor.
25H-NBOMe (NBOMe-2C-H) is a derivative of the phenethylamine hallucinogen 2C-H, which acts as a highly potent full agonist for the human 5-HT2A receptor.
The 25-NB (25x-NBx) series, sometimes alternatively referred to as the NBOMe compounds, is a family of serotonergic psychedelics. They are substituted phenethylamines and were derived from the 2C family. They act as selective agonists of the serotonin 5-HT2A receptor. The 25-NB family is unique relative to other classes of psychedelics in that they are, generally speaking, extremely potent and relatively selective for the 5-HT2A receptor. Use of NBOMe series drugs has caused many deaths and hospitalisations since the drugs popularisation in the 2010s. This is primarily due to their high potency, unpredictable pharmacokinetics, and sellers passing off the compounds in the series as LSD.
25T7-NBOMe is a substituted phenethylamine derivative from the 25-NB family. It acts as an agonist at the 5-HT2A and 5-HT2C serotonin receptors, has psychedelic effects and has been sold as a designer drug.
(R)-69 (3IQ) is a tetrahydropyridine derivative which acts as a 5-HT2A receptor agonist, with 4.6x selectivity over 5-HT2B and 49x selectivity over 5-HT2C. It has a 5-HT2A Ki of 680nM and an EC50 of 41nM. (R)-69 is a biased agonist selective for activation of the Gq coupled signalling pathway, with much weaker activation of the β-arrestin 2 coupled pathway. In animal studies it produces antidepressant-like activity but without producing the head-twitch response associated with psychedelic effects.
25O-NBcP (NBcPr-2C-O) is a phenethylamine derivative from the 25-NB class. It acts as a potent agonist at the 5-HT2A receptor with weaker activity at 5-HT2B and 5-HT2C, and unlike the parent compound 2C-O, 25O-NBcP produces a head-twitch response in animal studies which often correlates with potential for psychedelic effects in humans.
2C2-NBOMe (NBOMe-MMDPEA-2) is a phenethylamine derivative from the 25-NB class. It acts as a potent agonist at the 5-HT2A receptor with weaker activity at 5-HT2B and 5-HT2C, and produces a head-twitch response in animal studies which often correlates with potential for psychedelic effects in humans. It is related in structure to psychedelic amphetamine derivatives such as MMDA-2 and is the first phenethylamine derivative with a methylenedioxy substitution on the phenyl ring but no alkyl substitution on the alpha carbon, that has been shown to produce psychedelic-appropriate responding in animals.
25N-N1-Nap is a phenethylamine derivative from the 25-NB class, which acts as a potent agonist at the 5-HT2A receptor with weaker activity at 5-HT2B and 5-HT2C. 25N-N1-Nap is a biased agonist, producing robust activation of 5-HT2A coupled signalling pathways mediated by beta arrestin 2, but with little or no activation of pathways mediated via Gq. In animal studies it produces antipsychotic effects but without producing the head-twitch response associated with psychedelic activity.
