In biochemistry and pharmacology, a ligand is a substance that forms a complex with a biomolecule to serve a biological purpose. The etymology stems from Latin ligare, which means 'to bind'. In protein-ligand binding, the ligand is usually a molecule which produces a signal by binding to a site on a target protein. The binding typically results in a change of conformational isomerism (conformation) of the target protein. In DNA-ligand binding studies, the ligand can be a small molecule, ion, or protein which binds to the DNA double helix. The relationship between ligand and binding partner is a function of charge, hydrophobicity, and molecular structure.
Herkinorin is an opioid analgesic that is an analogue of the natural product salvinorin A. It was discovered in 2005 during structure-activity relationship studies into neoclerodane diterpenes, the family of chemical compounds of which salvinorin A is a member.
The adenosine A2B receptor, also known as ADORA2B, is a G-protein coupled adenosine receptor, and also denotes the human adenosine A2b receptor gene which encodes it.
N-Arachidonyl glycine receptor, also known as G protein-coupled receptor 18 (GPR18), is a protein that in humans is encoded by the GPR18 gene. Along with the other previously "orphan" receptors GPR55 and GPR119, GPR18 has been found to be a receptor for endogenous lipid neurotransmitters, several of which also bind to cannabinoid receptors. It has been found to be involved in the regulation of intraocular pressure.
The 5HT6 receptor is a subtype of 5HT receptor that binds the endogenous neurotransmitter serotonin (5-hydroxytryptamine, 5HT). It is a G protein-coupled receptor (GPCR) that is coupled to Gs and mediates excitatory neurotransmission. HTR6 denotes the human gene encoding for the receptor.
G protein-coupled receptor 55 also known as GPR55 is a G protein-coupled receptor that in humans is encoded by the GPR55 gene.
Dopamine receptor D3 is a protein that in humans is encoded by the DRD3 gene.
The 5-HT7 receptor is a member of the GPCR superfamily of cell surface receptors and is activated by the neurotransmitter serotonin (5-hydroxytryptamine, 5-HT). The 5-HT7 receptor is coupled to Gs (stimulates the production of the intracellular signaling molecule cAMP) and is expressed in a variety of human tissues, particularly in the brain, the gastrointestinal tract, and in various blood vessels. This receptor has been a drug development target for the treatment of several clinical disorders. The 5-HT7 receptor is encoded by the HTR7 gene, which in humans is transcribed into 3 different splice variants.
2-Ethyl-5-methoxy-N,N-dimethyltryptamine (EMDT) is a tryptamine derivative which is used in scientific research. It acts as a selective 5-HT6 receptor agonist, with a Ki of 16 nM, and was one of the first selective agonists developed for this receptor. EMDT inhibits both short- and long-term memory formation in animal studies, and this effect can be reversed by the selective 5-HT6 antagonist SB-399,885. Additionally, it is active in the tail suspension test, suggesting that it could be an effective antidepressant.
MS-245 is a tryptamine derivative used in scientific research. It acts as a selective 5-HT6 receptor antagonist with a Ki of 2.3 nM, and was derived through structure-activity relationship development of the selective 5-HT6 agonist EMDT. It has been used as a lead compound for further development of tryptamine-derived 5-HT6 antagonists. In animal studies it has been shown to boost the activity of, but not substitute for, both amphetamine and nicotine.
PSB-10 is a drug which acts as a selective antagonist for the adenosine A3 receptor (ki value at human A3 receptor is 0.44 nM), with high selectivity over the other three adenosine receptor subtypes (ki values at human A1, A2A and A2B receptors are 4.1, 3.3 and 30 μM). Further pharmacological experiments in a [35S]GTPγS binding assay using hA3-CHO-cells indicated that PSB-10 acts as an inverse agonist (IC50 = 4 nM). It has been shown to produce antiinflammatory effects in animal studies. Simple xanthine derivatives such as caffeine and DPCPX have generally low affinity for the A3 subtype and must be extended by expanding the ring system and adding an aromatic group to give high A3 affinity and selectivity. The affinity towards adenosine A3 subtype was measured against the radioligand PSB-11.
WAY-100135 is a serotonergic drug of the phenylpiperazine family which is used in scientific research. It acts as potent 5-HT1A receptor antagonist, and was originally believed to be highly selective, but further studies have demonstrated that it also acts as a partial agonist of the 5-HT1D receptor (pKi = 7.58; virtually the same affinity for 5-HT1A), and to a much lesser extent, of the 5-HT1B receptor (pKi = 5.82). These findings may have prompted the development of the related compound WAY-100635, another purportedly selective and even more potent 5-HT1A antagonist, which was synthesized shortly thereafter. However, WAY-100635 turned out to be non-selective as well, having been shown to act additionally as a potent D4 receptor agonist later on.
6-Br-APB is a synthetic compound that acts as a selective D1 agonist, with the (R)-enantiomer being a potent full agonist, while the (S) enantiomer retains its D1 selectivity but is a weak partial agonist. (R)-6-Br-APB and similar D1-selective full agonists like SKF-81,297 and SKF-82,958 produce characteristic anorectic effects, stereotyped behaviour and self-administration in animals, with a similar but not identical profile to that of dopaminergic stimulants such as amphetamine.
SB-206553 is a drug which acts as a mixed antagonist for the 5-HT2B and 5-HT2C serotonin receptors. It has anxiolytic properties in animal studies and interacts with a range of other drugs. It has also been shown to act as a positive allosteric modulator of α7 nicotinic acetylcholine receptors. Modified derivatives of SB-206553 have been used to probe the structure of the 5-HT2B receptor.
JTE 7-31 is a selective cannabinoid receptor agonist invented by Japan Tobacco. It is a reasonably highly selective CB2 agonist, but still retains appreciable affinity at CB1, with a Ki of 0.088nM at CB2 vs 11nM at CB1.
O-1602 is a synthetic compound most closely related to abnormal cannabidiol, and more distantly related in structure to cannabinoid drugs such as THC. O-1602 does not bind to the classical cannabinoid receptors CB1 or CB2 with any significant affinity, but instead is an agonist at several other receptors which appear to be related to the cannabinoid receptors, particularly GPR18 and GPR55. These previously orphan receptors have been found to be targets for a number of endogenous and synthetic cannabinoid compounds, and are thought to be responsible for most of the non-CB1, non-CB2 mediated effects that have become evident in the course of cannabinoid research. O-1602 produces some effects shared with classical cannabinoid compounds such as analgesic and antiinflammatory effects and appetite stimulation, but it does not produce sedation or psychoactive effects, and has several actions in the gut and brain that are not shared with typical cannabinoid agonists.
PSB-SB-1202 is a coumarin derivative which is an agonist at the cannabinoid receptors CB1 and CB2, with a CB1 Ki of 32nM and a CB2 Ki of 49nM. It is also a weak antagonist at the related receptor GPR55, with an IC50 of 6350nM, but has no significant affinity for GPR18.
CID16020046 is a compound which acts as an inverse agonist at the former orphan receptor GPR55, and may be the first selective inverse agonist characterised for this receptor. It was found to block a number of GPR55 mediated responses such as wound healing and activation of immune system T-cells and B-cells, as well as showing inverse agonist activity in the absence of GPR55 agonist stimulation. However while it was found to have good selectivity over the related CB1 and CB2 cannabinoid receptors as well as a number of other targets, CID16020046 has not yet been tested against another related receptor GPR18, so its selectivity for GPR55 over this target has not been established. It has antiinflammatory actions, has been used to study the interaction between GPR55 mediated and CB1 mediated activity, and research using this compound has revealed a role for GPR55 in learning and memory.
PSB-CB5 (CID-85469571) is a compound which acts as an antagonist at the former orphan receptor GPR18, and is the first selective antagonist characterised for this receptor, with an IC50 of 279nM, and good selectivity over related receptors (over 36x selectivity vs CB1 and GPR55, and 14x vs CB2.) As all previously known antagonists for GPR18 also antagonise GPR55, it has been difficult to separate the effects of these two receptor targets, so the discovery of a selective GPR18 antagonist is expected to be useful in research into the actions of this receptor.
SN-22 is a chemical compound which acts as a moderately selective agonist at the 5-HT2 family of serotonin receptors, with a Ki of 19 nM at 5-HT2 subtypes versus 514 nM at 5-HT1A receptors. Many related derivatives are known, most of which are ligands for 5-HT1A, 5-HT6 or dopamine D2 receptors or show SSRI activity.
