Cannabinoid receptor type 1 (CB1), also known as cannabinoid receptor 1, is a G protein-coupled cannabinoid receptor that in humans is encoded by the CNR1 gene. The human CB1 receptor is expressed in the peripheral nervous system and central nervous system. It is activated by: endocannabinoids, a group of retrograde neurotransmitters that include anandamide and 2-arachidonoylglycerol (2-AG); plant phytocannabinoids, such as the compound THC which is an active ingredient of the psychoactive drug cannabis; and, synthetic analogs of THC. CB1 is antagonized by the phytocannabinoid tetrahydrocannabivarin (THCV).
A cannabinoid receptor antagonist, also known simply as a cannabinoid antagonist or as an anticannabinoid, is a type of cannabinoidergic drug that binds to cannabinoid receptors (CBR) and prevents their activation by endocannabinoids. They include antagonists, inverse agonists, and antibodies of CBRs. The discovery of the endocannabinoid system led to the development of CB1 receptor antagonists. The first CBR inverse agonist, rimonabant, was described in 1994. Rimonabant blocks the CB1 receptor selectively and has been shown to decrease food intake and regulate body-weight gain. The prevalence of obesity worldwide is increasing dramatically and has a great impact on public health. The lack of efficient and well-tolerated drugs to cure obesity has led to an increased interest in research and development of CBR antagonists. Cannabidiol (CBD), a naturally occurring cannabinoid, is a non-competitive CB1/CB2 receptor antagonist. And Δ9-tetrahydrocannabivarin (THCV), a naturally occurring cannabinoid, modulate the effects of THC via direct blockade of cannabinoid CB1 receptors, thus behaving like first-generation CB1 receptor inverse agonists, such as rimonabant. CBD is a very low-affinity CB1 ligand, that can nevertheless affect CB1 receptor activity in vivo in an indirect manner, while THCV is a high-affinity CB1 receptor ligand and potent antagonist in vitro and yet only occasionally produces effects in vivo resulting from CB1 receptor antagonism. THCV has also high affinity for CB2 receptors and signals as a partial agonist, differing from both CBD and rimonabant.
A-834,735 is a drug developed by Abbott Laboratories that acts as a potent cannabinoid receptor full agonist at both the CB1 and CB2 receptors, with a Ki of 12 nM at CB1 and 0.21 nM at CB2. Replacing the aromatic 3-benzoyl or 3-naphthoyl group found in most indole derived cannabinoids, with the 3-tetramethylcyclopropylmethanone group of A-834,735 and related compounds, imparts significant selectivity for CB2, with most compounds from this group found to be highly selective CB2 agonists with little affinity for CB1. However low nanomolar CB1 binding affinity is retained with certain heterocyclic 1-position substituents such as (N-methylpiperidin-2-yl)methyl (cf. AM-1220, AM-1248), or the (tetrahydropyran-4-yl)methyl substituent of A-834,735, resulting in compounds that still show significant affinity and efficacy at both receptors despite being CB2 selective overall.
AM-630 (6-Iodopravadoline) is a drug that acts as a potent and selective inverse agonist for the cannabinoid receptor CB2, with a Ki of 32.1 nM at CB2 and 165x selectivity over CB1, at which it acted as a weak partial agonist. It is used in the study of CB2 mediated responses and has been used to investigate the possible role of CB2 receptors in the brain. AM-630 is significant as one of the first indole derived cannabinoid ligands substituted on the 6-position of the indole ring, a position that has subsequently been found to be important in determining affinity and efficacy at both the CB1 and CB2 receptors, and has led to the development of many related derivatives.
AM-679 (part of the AM cannabinoid series) is a drug that acts as a moderately potent agonist for the cannabinoid receptors, with a Ki of 13.5 nM at CB1 and 49.5 nM at CB2. AM-679 was one of the first 3-(2-iodobenzoyl)indole derivatives that was found to have significant cannabinoid receptor affinity, and while AM-679 itself has only modest affinity for these receptors, it was subsequently used as a base to develop several more specialised cannabinoid ligands that are now widely used in research, including the potent CB1 agonists AM-694 and AM-2233, and the selective CB2 agonist AM-1241. AM-679 was first identified as having been sold as a cannabinoid designer drug in Hungary in 2011, along with another novel compound 1-pentyl-3-(1-adamantoyl)indole.
Org 28312 is a drug developed by Organon International which acts as a potent cannabinoid receptor full agonist at both the CB1 and CB2 receptors. It was developed with the aim of finding a water-soluble cannabinoid agonist suitable for intravenous use as an analgesic, but did not proceed to human trials, with the related compound Org 28611 chosen instead due to its better penetration into the brain. The structure-activity relationships of these compounds have subsequently been investigated further leading to the development of a number of more potent analogues, derived by cyclisation around the indole or piperazine rings.
AM-2233 is a drug that acts as a highly potent full agonist for the cannabinoid receptors, with a Ki of 1.8 nM at CB1 and 2.2 nM at CB2 as the active (R) enantiomer. It was developed as a selective radioligand for the cannabinoid receptors and has been used as its 131I derivative for mapping the distribution of the CB1 receptor in the brain. AM-2233 was found to fully substitute for THC in rats, with a potency lower than that of JWH-018 but higher than WIN 55,212-2.
AM-1248 is a drug that acts as a moderately potent agonist for both the cannabinoid receptors CB1 and CB2, but with some dispute between sources over its exact potency and selectivity. Replacing the 3-(1-naphthoyl) group found in many indole derived cannabinoid ligands, with an adamantoyl group, generally confers significant CB2 selectivity, but reasonable CB1 affinity and selectivity is retained when an N-methylpiperidin-2-ylmethyl substitution is used at the indole 1-position. The related compound 1-pentyl-3-(1-adamantoyl)indole was identified as having been sold as a cannabinoid designer drug in Hungary in 2011, along with another synthetic cannabinoid AM-679.
AM-1235 (1-(5-fluoropentyl)-3-(naphthalen-1-oyl)-6-nitroindole) is a drug that acts as a potent and reasonably selective agonist for the cannabinoid receptor CB1.
AM-1220 is a drug that acts as a potent and moderately selective agonist for the cannabinoid receptor CB1, with around 19 times selectivity for CB1 over the related CB2 receptor. It was originally invented in the early 1990s by a team led by Thomas D'Ambra at Sterling Winthrop, but has subsequently been researched by many others, most notably the team led by Alexandros Makriyannis at the University of Connecticut. The (piperidin-2-yl)methyl side chain of AM-1220 contains a stereocenter, so there are two enantiomers with quite different potency, the (R)-enantiomer having a Ki of 0.27 nM at CB1 while the (S)-enantiomer has a much weaker Ki of 217 nM.
AB-001 (1-pentyl-3-(1-adamantoyl)indole) is a designer drug that was found as an ingredient in synthetic cannabis smoking blends in Ireland in 2010 and Hungary and Germany in 2011. It is unclear who AB-001 was originally developed by, but it is structurally related to compounds such as AM-1248 and its corresponding 1-(tetrahydropyran-4-ylmethyl) analogue, which are known to be potent cannabinoid agonists with moderate to high selectivity for CB2 over CB1. The first published synthesis and pharmacological evaluation of AB-001 revealed that it acts as a full agonist at CB1 (EC50 = 35 nM) and CB2 receptors (EC50 = 48 nM). However, AB-001 was found to possess only weak cannabimimetic effects in rats at doses up to 30 mg/kg, making it less potent than the carboxamide analogue APICA, which possesses potent cannabimimetic activity at doses of 3 mg/kg.
MN-25 (UR-12) is a drug invented by Bristol-Myers Squibb, that acts as a reasonably selective agonist of peripheral cannabinoid receptors. It has moderate affinity for CB2 receptors with a Ki of 11 nM, but 22x lower affinity for the psychoactive CB1 receptors with a Ki of 245 nM. The indole 2-methyl derivative has the ratio of affinities reversed however, with a Ki of 8 nM at CB1 and 29 nM at CB2, which contrasts with the usual trend of 2-methyl derivatives having increased selectivity for CB2 (cf. JWH-018 vs JWH-007, JWH-081 vs JWH-098).
UR-144 (TMCP-018, KM-X1, MN-001, YX-17) is a drug invented by Abbott Laboratories, that acts as a selective full agonist of the peripheral cannabinoid receptor CB2, but with much lower affinity for the psychoactive CB1 receptor.
O-1269 is a drug that is a diarylpyrazole derivative, related to potent cannabinoid antagonist drugs such as rimonabant and surinabant. However O-1269 and several related drugs were unexpectedly found to act as full or partial agonists at the cannabinoid receptors rather than antagonists, and so produce the usual effects expected of cannabinoid agonists in animal tests, such as sedation and analgesic effects. The N-heptyl homolog O-1270 and the N-propyl homolog O-1399 also act as cannabinoid agonists with similar potency in vivo, despite weaker binding affinity at cannabinoid receptors compared to the pentyl homolog O-1269. Agonist-like and atypical cannabinoid activity has also been observed with a number of related compounds.
LBP-1 is a drug originally developed by Organon for the treatment of neuropathic pain, It acts as a potent and selective cannabinoid receptor agonist, with high potency at both the CB1 and CB2 receptors, but low penetration of the blood–brain barrier. This makes LBP-1 peripherally selective, and while it was effective in animal models of neuropathic pain and allodynia, it did not produce cannabinoid-appropriate responding suggestive of central effects, at any dose tested.
STS-135 (N-(adamantan-1-yl)-1-(5-fluoropentyl)-1H-indole-3-carboxamide, also called 5F-APICA) is a designer drug offered by online vendors as a cannabimimetic agent. The structure of STS-135 appears to use an understanding of structure-activity relationships within the indole class of cannabimimetics, although its design origins are unclear. STS-135 is the terminally-fluorinated analogue of SDB-001, just as AM-2201 is the terminally-fluorinated analogue of JWH-018, and XLR-11 is the terminally-fluorinated analogue of UR-144. STS-135 acts a potent cannabinoid receptor agonist in vitro, with an EC50 of 51 nM for human CB2 receptors, and 13 nM for human CB1 receptors. STS-135 produces bradycardia and hypothermia in rats at doses of 1–10 mg/kg, suggesting cannabinoid-like activity.
NNE1 (also known as NNEI, MN-24 and AM-6527) is an indole-based synthetic cannabinoid, representing a molecular hybrid of APICA and JWH-018 that is an agonist for the cannabinoid receptors, with Ki values of 60.09 nM at CB1 and 45.298 nM at CB2 and EC50 values of 9.481 nM at CB1 and 1.008 nM at CB2. It was invented by Abbott and has a CB1 receptor pEC50 of 8.9 with around 80x selectivity over the related CB2 receptor. It is suspected that metabolic hydrolysis of the amide group of NNE1 may release 1-naphthylamine, a known carcinogen, given the known metabolic liberation (and presence as an impurity) of amantadine in the related compound APINACA, and NNE1 was banned in New Zealand in 2012 as a temporary class drug to stop it being used as an ingredient in then-legal synthetic cannabis products. NNE1 was subsequently found to be responsible for the death of a man in Japan in 2014.
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