A nicotinic agonist is a drug that mimics the action of acetylcholine (ACh) at nicotinic acetylcholine receptors (nAChRs). The nAChR is named for its affinity for nicotine.
Examples include nicotine (by definition), acetylcholine (the endogenous agonist of nAChRs), choline, epibatidine, lobeline, varenicline and cytisine.[1]
History
editNicotine has been known for centuries for its intoxicating effect. It was first isolated in 1828 from the tobacco plant by German chemists Posselt and Reimann.[2]
The discovery of positive effects from nicotine on animal memory was discovered by in vivo researches in the mid 1980s. Those researches led to a new era in studies of nicotinic acetylcholine receptor (nAChR) and their stimulation but until then the focus had mainly been on nicotine addiction.[3][4] The development of nAChR agonists began in the early 1990s after the discovery of nicotine's positive effects. Some research showed a possible therapy option in preclinical researches. ABT-418 was one of the first in a series of nAChR agonists and it was designed by Abbott Labs.[4] ABT-418 showed significant increase of delayed matching-to-sample (DMTS) performance in matured macaque apes of different species and sex.[5] ABT-418 has also been examined as a possible treatment to Alzheimer's disease, Parkinson's disease and attention-deficit hyperactivity disorder: those experiments showed positive outcomes.[4]
One of the first nAChR active compounds, besides nicotine, that was marketed as a drug was galantamine, a plant alkaloid that works as a weak cholinesterase inhibitor (IC50 = 5
Nicotinic acetylcholine receptors and their signaling system
editSignaling system
editIn the human nervous system nicotinic cholinergic signals are extended throughout the system, where the neurotransmitter acetylcholine (ACh) plays a key role in activating ligand-gated ion channels.[7] The cholinergic system is a vital nervous pathway, where cholinergic neurons synthesize, store and release the neurotransmitter ACh. The main receptors that convert the ACh messages are the cholinergic muscarinic acetylcholine receptors, neuronal and muscular nAChRs. When looking back at evolutionary history, ACh is considered to be the oldest transmitter molecule and became present before the nervous cell. In the nervous system cholinergic stimulation mediated through nAChRs controls pathways such as release of transmitters and cell sensitivity, which can influence physiological activity including sleep, anxiety, processing of pain and cognitive functions.[8]
Nicotinic acetylcholine receptors
editnAChRs are cholinergic receptors found in the central nervous system (CNS), peripheral nervous systems (PNS) and skeletal muscles, these receptors are ligand-gated ion channels with binding sites for acetylcholine and other molecules. When ACh or other agonists bind to the receptors it stabilizes the open state of the ion channel allowing influx of cations such as potassium, calcium and sodium ions. The nAChRs are made up by different subunits which determine the quaternary structure of the receptor, those subunits are
Binding site
editThere are two binding sites on heteromeric nAChRs; to stabilize the open form of nAChRs, both binding sites must be occupied by agonist, such as nicotine or ACh.[11]
The ACh binding site of nAChR is made up by six loops, termed A–F. The A, B and C loops of the binding site are part of the
Mechanism of action
editα 4β2 receptor agonists
edit
α 7 receptor agonists
edit
Muscle type receptor agonists
editnAChR are found in the neuromuscular junction on skeletal muscles. Two different receptors have been found, one of which has primarily been found in adults contains two
Binding
editACh binds to nAChR because of charge difference between the molecule and the surface of the receptor. When binding to nAChR ACh fits into a binding pocket shaped by loops A, B and C which belong to
Drug design
editDrugs that influence nAChRs can be agonists, partial agonists or antagonists.[1] Agonists, e.g. nicotine, can however act as depolarizing agents when encountered to nAChRs for some time (seconds or minutes, depending on concentration and nAChR subtype), chronic exposure to agonist can also lead to long lasting functional deactivation because of rapid and persistent desensitization. Partial nAChR agonists have been studied since they seem to be helpful in smoking cessation. The partial agonists are believed to bind to the nAChRs and stimulate the release of dopamine in smaller portions than the agonists and therefore compensate for the absence of nicotine.[18]
The lack of specificity among some of the nicotinic agonists is well known and is a potential problem when using them to treat illnesses that require targeting a specific subtype of nAChRs. Among these nonspecific agonists are for example ACh, nicotine and epibatidine that all target more than one subtype of nAChRs.[19][20]
Pharmacophore
editThe development of nAChR agonist pharmacophore started in 1970 when it was proposed that the binding of the agonists to a receptor was dependent on a positively charged nitrogen atom and a hydrogen bond forming from carbonyl oxygen atom in acetylcholine or a nitrogen atom in (S)-nicotine. Since then it has been shown that a cationic center, atoms that are electronegative and able to form hydrogen bonds along with the center of the pyridine ring in (S)-nicotine are favorable. Stereochemistry is a part of the pharmacophore as is clearly seen with (S)- and (R)- nicotine where the (S)-enantiomer is 10-100 times more potent. The azabicyclic ring of epibatidine is another example of favorable steric interactions to the receptors. It has been suggested that a specific internitrogen distance, N+-N, is important for agonist affinity but debate has arisen over its influence. A newer theory is that a distance of 7-8 Å between points that complement the protonated nitrogen atom and hydrogen bond acceptor will enhance the potency. Low electronic density close to the protonated nitrogen and higher electron density close to the pyridine ring is favored in protonated nicotine ligands containing pyridine ring. In later years researchers have taken more interest in the
Structure-activity relationships
editStructure-activity relationships: Muscle nAChR agonists
editVarious models have been run where the affinity of nAChR agonists to the receptor subtype are tested to help identify the molecules, groups and steric conformation that are vital to greater affinity. By using a nAChR muscle receptor subtype (
- anatoxin > epibatidine > acetylcholine > DMPP >> cytisine > pyrantel > nicotine > coniine > tubocurare > lobeline,
where anatoxin had the highest activity efficacy and tubocurare the lowest. Acetylcholine on the other hand induced a much longer opening time of the receptor though anatoxin is more potent. The results suggest that anatoxin derivatives would be helpful in understanding structure-activity relationships (SAR) for muscle nAChRs.[22]
Succinylcholine chloride, which is a drug that's already on the market, is a bischoline ester and a short acting muscle relaxant. Bischoline esters are compounds that can act as a competitive agonist on muscle type nAChRs and have been used in SAR studies. In a Torpedo (
2 units and is lower for both fewer CH
2 units and more.[23]
Structure-activity relationships: α 4β2 nAChR agonists
edit
Combination of structural elements of ACh and nicotine as well as reducing the conformational flexibility by using a cyclopropane ring has led to the discovery of potent and selective
Structure-activity relationships: α 7 nAChR agonists
edit
The search for selective and potent
Various cyclic amine groups can act as the basic moiety and potency stays relatively unchanged for example aryl piperazine, piperidine and morpholine. An acyclic tertiary amine is tolerated as the basic moiety but larger steric groups are less tolerated.[15]
Many derivatives of quinuclidine such as quinuclidine amide are known to be
Drug development
editThe development of nicotinic acetylcholine receptor agonists began in the early 1990s after the discovery of nicotine's positive effects on animal memory.[3][4] The development of nicotinic acetylcholine receptor agonists has come a long way since then. Nicotinic acetylcholine receptor agonists are gaining increasing attention as drug candidates for multiple central nervous system disorders such as Alzheimer's disease, schizophrenia, attention-deficit hyperactivity disorder (ADHD) and nicotine addiction.[26][27] Nicotinic acetylcholine receptors are receptors found in the central nervous system, the peripheral nervous systems and skeletal muscles. They are ligand-gated ion channels with binding sites for acetylcholine as well as other agonists. When agonists bind to a receptor it stabilizes the open state of the ion channel allowing influx of cations.[9]
In 2009 there were at least five drugs on the market that affect the nicotinic acetylcholine receptors.
Quinuclidine carbamates | Quinuclidine amides | Quinuclidine ethers |
Products of nicotinic agonist
editActive ingredient | Product name | Chemical name | Pharmaceutical form | Pharmacodynamic properties | Therapeutic use | Structure |
---|---|---|---|---|---|---|
Varenicline tartrate | Champix, Chantix | 7,8,9,10-tetrahydro-6,10-methano-6H-pyrazino[2,3-h][3]benzazepine[27] | Film coated tablet | Partial agonist of the nicotinic acetylcholine receptor, subtype |
Treatment of tobacco dependence[28] | |
Galantamine hydrobromide | Reminyl, Nivalin, Razadyne and Razadyn ER | 4a,5,9,10,11,12-hexahydro-3-methoxy-11-methyl-6H-benzofuro[3a,3,2-ef][2]-benzazepin-6-ol[29] | Sustained release capsule, film coated tablet, oral solution | Cholinesterase inhibitor and a noncompetitive agonist of the nicotinic acetylcholine receptor[4] | Treatment of dementia caused by Alzheimer's disease[30] | |
Nicotine | Nicorette, Nicotinell, Niquitin, Boots NicAssist, Commit, Habitrol, Nicoderm CQ, Nicotrol, Thrive | 3-[(2S)-1-methylpyrrolidine-2-yl]pyridine | Transdermal patch, gum, inhaler, nasal spray, lozenge, microtab, and is naturally found in tobacco | Agonist of the nicotinic receptor,[31] both Ganglion type and |
Treatment of tobacco dependence[33] | |
Carbachol | Miostat | 2-[(aminocarbonyl)oxy]-N,N,N-trimethylethanaminium | Intraocular solution | Cholinergic agonist[34] | Treatment of glaucoma | |
Suxamethonium chloride (Succinylcholine chloride) | Anectine, Quelicin Suxamethonium Chloride | 2,2'-[(1,4-dioxobutane-1,4-diyl)bis(oxy)]bis(N,N,N-trimethylethanaminium) | Intravenous or intramuscular injection | Depolarizing neuromuscular blocking agent[35] | Short acting muscle relaxant[36] | |
Epibatidine | Not listed | 2-(6-chloropyridin-3-yl)-7-azabicyclo[2.2.1]heptane | Not listed | Agonist of the nicotinic acetylcholine receptor[37] | Not used as a drug |
Other nicotinic agonists, albeit generally with limited clinical use, include:
- lobeline, an agonist on Ganglion type nicotinic receptors and also affects sensory nerve terminals[32]
- epibatidine, an agonist on Ganglion type,
α 4β2 andα 7 receptors[32] - decamethonium causes depolarization block on muscle type receptors, similarly to suxamethonium[32]
Nicotinic versus muscarinic activity
editComparison of cholinergic agonists[38] | ||||
---|---|---|---|---|
Substance | Receptor specificity | Hydrolysis by acetylcholinesterase |
Comments | |
Muscarinic | Nicotinic | |||
Choline | +++ | +++ | ++ | Essential nutrient |
Acetylcholine | +++ | +++ | +++ | Endogenous ligand |
Carbachol | ++ | +++ | - | Used in the treatment of glaucoma |
Methacholine | +++ | + | ++ | |
Bethanechol | +++ | - | - | Used in bladder and gastrointestinal hypotonia. |
Muscarine | +++ | - | - | Natural alkaloid found in certain mushrooms. Cause of mushroom poisoning |
Nicotine | - | +++ | - | Natural alkaloid found in the tobacco plant. |
Pilocarpine | ++ | - | - | Used in glaucoma |
Oxotremorine | ++ | - | - |
Current status
editCurrently nicotine receptor agonist research and drug designing is aimed for treatment of multiple diseases and disorders of the CNS.[39]
Targacept has three drug candidates that are in clinical trials; AZD3480 (TC-1734) for ADHD which is currently in phase II clinical trials, AZD1446 (TC-6683) for Alzheimer's disease in collaboration with AstraZeneca and TC-5619 for cognitive dysfunctions in schizophrenia.
Memory pharmaceuticals with its partner Roche has one drug candidate, MEM 3454 (RG3487), a partial agonist of the nicotinic
Abbott Laboratories in partnership with NeuroSearch have two drug candidates in clinical trials, ABT-894, a selective
EnVivo pharmaceuticals has one drug candidate in clinical trials, EVP-6124, a selective
See also
editReferences
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External links
editMedia related to Nicotinic agonists at Wikimedia Commons
- nicotinic+agonists at the U.S. National Library of Medicine Medical Subject Headings (MeSH)
- MeSH list of agents 82018722