Anatoxin
also known as Very Fast Death Factor
By Neil Edwards
The Chemical Laboratories
School of Chemistry, Physics
& Environmental Science
University of Sussex at Brighton
Chime enhanced version VRML version Chemsymphony version
Introduction
Anatoxin is a potent alkaloid toxin derived from a species of cyanobacteria called Anabaena flos-aquae. The first published report of the potentially lethal effects of microorganisms known as blue-green algae appeared in Nature in 1878. George Francis described an algal bloom that had formed in the estuary of the Murray River, in Australia, as "a thick scum like green oil paint, some two to six inches thick." The water at this point of the river was rendered toxic to wildlife, with animals drinking it becoming rapidly ill and dying terrible deaths. The effect of microalgal toxins, both in marine and freshwater environments, has increased in severity in recent years, and poisoning episodes are becoming more common and more widespread. For example, in the midwestern United States, the consumption of contaminated water has resulted in the deaths of ducks and geese by the thousands.
Concern for wildlife and also issues related to public health led to much investigation into the causes of these mass animal mortalities, and in the 1950s and 1960s Paul Gorham and his co-workers at the National Research Council in Ottawa established cultures for Anabaena flos-aquae, which allowed them to isolate the poisonous compounds which it produces. Anatoxin is perhaps one of the most toxic of the cyanobacterial toxins in this group, since the effects of ingestion can be lethal within 4 minutes, depending on the quantity consumed. This led to the compound being dubbed "Very Fast Death Factor." The chemical structure is shown (right) and is an interesting bicyclic alkaloid, and it was hoped that knowledge of this structure would enable scientists to discover the mode of action of the toxin.
Anatoxin is a severe neurotoxin, and as such affects the functioning of the nervous system, often causing death due to paralysis of the respiratory muscles. It is known that it acts as a mimic of the neurotransmitter, acetylcholine and irreversibly binds the nicotinic acetylcholine receptor (NAChR). Normal neuromuscular action involves the release of acetylcholine, which binds its receptor, leading to the opening of a related sodium channel. The resulting movement of sodium ions produces the action potential causing the muscles to contract. At this point, an enzyme called acetylcholinesterase then cleaves the neurotransmitter, allowing the sodium channel to return eventually to its resting state, and hence the muscle can relax. Anatoxin also binds the NAChR to produce an action potential, but cannot be cleaved by the enzyme. The sodium channel is essentially locked open, and the muscles become over-stimulated and become fatigued and then paralysed. When respiratory muscles become affected, convulsions occur due to a lack of oxygen supply to the brain. Suffocation is the final result a few minutes after ingestion of the toxin.
Anatoxin & its Scientific & Medical Applications
Despite its poisonous nature, however, anatoxin and many related man-made analogues have found widespread use in medicine and for pharmacological applications. Since it binds the nicotinic acetylcholine receptor irreversibly, it is an excellent means of studying this receptor, and also the mechanisms of neuromuscular action. Modified analogues are being used in order to further elucidate the receptor sub-types, and this research may lead to the development of new drugs which have none of the toxicity associated with anatoxin itself, but which act merely as acetylcholine replacement candidates. For example, the neurodegenerative disorder, Alzheimer's disease is associated with an inability of neurons to produce acetylcholine. Using the neurotransmitter itself as a therapy would not work since it is not long-lived enough. A more stable (and non-toxic!) agonist similar to anatoxin may work very well indeed.
Chemical Synthesis
There have been many total syntheses of anatoxin in both racemic and in optically pure form. The syntheses can be grouped into six general categories.
1. Ring-Expansion of Tropanes
Edwards and his co-workers utilised a photochemical ring-expansion method to access the anatoxin skeleton. The full report of this work can be found in the primary literature (Can. J. Chem. 1977, 55, 1372).
2. Cyclisation of Cyclooctenes
A very commonly employed method for creating the anatoxin skeleton is by a transannular cyclisation of a suitably substituted cyclooctene. This approach was taken by the Wiseman group (J. Org. Chem. 1986, 51, 2485).
A similar protocol was developed by Danheiser and co-workers (J. Am. Chem. Soc. 1985, 107, 8066). The cyclooctene was synthesised by ring-expansion of a cyclopropyl cycloheptanamine.
3. Cyclisation of Iminium Salts
Rapoport's synthesis of anatoxin relied on the formation of a reactive iminium ion, which was generated by a Lewis acid-induced decarboxylation. Intramolecular trapping produced the anatoxin skeleton (J. Am. Chem. Soc. 1976, 98, 7448). In fact, Rapoport has synthesised a whole range of related analogues using this type of chemistry in order to probe the requirements of the nicotinic acetylcholine receptor.
The Speckamp group (Tetrahedron Lett. 1986, 27, 4799) and the Somfai group (Tetrahedron Lett. 1992, 33, 3791) have both used similar strategies for their research on the total synthesis of anatoxin.
4. Cycloaddition of Nitrones
A very elegant approach to anatoxin developed by Tufariello used a nitrone cycloaddition to produce the core structure. Reduction of the N-O bond and subsequent elaboration allowed the total synthesis to be completed.
This is the only example of a nitrone cycloaddition approach to anatoxin in the literature to date (J. Am. Chem. Soc. 1984, 106, 7979).
Cyclisation of Allenes
A new synthetic strategy for the synthesis of an early intermediate in the Gallagher route to anatoxin employed the cyclisation of allenes using silver tetrafluoroborate (J. Chem. Soc,. Perkin Trans. 1 1991, 145).
This early intermediate was then further elaborated, culminating the total synthesis of anatoxin.
6. Tandem Reactions
A very impressive synthesis of anatoxin was provided by the Parsons group (J. Chem. Soc., Chem. Commun. 1995, 1461) who utilised a tandem sequence of reactions in one pot, to add a nucleophile to a beta-lactam. Ring-opening of this beta lactam gave a new nitrogen nucleophile, which was able to undergo a transannular cyclisation via epoxide opening.
The total synthesis was completed using methodology developed in the Rapoport group.
Friday, February 18, 2005
HAZARD CODES
some thing I learnt today
B
C
E
F+
F
Xn
Xi
N
O
R
T
T+
Biohazard
Corrosive
Explosive
Extremely Flammable
Highly Flammable
Harmful
Irritant
Dangerous for the enviroment
Oxidizing
Radioactive
Toxic
Very Toxic
B
C
E
F+
F
Xn
Xi
N
O
R
T
T+
Biohazard
Corrosive
Explosive
Extremely Flammable
Highly Flammable
Harmful
Irritant
Dangerous for the enviroment
Oxidizing
Radioactive
Toxic
Very Toxic
NEW PAIN Killers from plants /snails and frogs
as a physician who has to face patients with ch pain and some malingerers addicted to pain medications I am always interested in finding out new developments .
I recently read about ZICONITIDE a conotoxin from a snail ,this is approved for infusion in to the spinal column for ch pain relief .
very few studies about 2000 patients allover the world have had the treatment in experiments so far .
another aspect is the socalled Nicotinic receptors in the brain some of theses are responsible for cognition ( remember alzheimers) and analgesia ( does somoking reduce pain ?
the article excerpt follows an alkaloid from a frog (may be from frog food !)
interesting reading
Epibatidine
--------------------------------------------------------------------------------
published on the net by
Matthew J. Dowd, Graduate Student
Department of Medicinal Chemistry
Virginia Commonwealth University
Richmond, VA 23298-0540 USA
Click here for the CHIME-enhanced version of this article. Also, click on any structure to view a 3-D version of the molecule.
Within the rainforest of Ecuador resides a small, colorful, seemingly harmless amphibian called Epipedobates tricolor (Figure 1). This frog first introduced itself to the scientific world in 1974. It was then that Dr. John Daly of the National Institutes of Health isolated from the frog a compound initially called alkaloid 208/210 (its MW from mass spectrometry) [1]. Daly demonstrated that this new alkaloid was a potent analgesic (as measured in the Straub-tail response when injected into mice). Even after subsequent trips to South America, too little of the compound was isolated to make a structural determination.[2,3] Because of this lack of compound, for both scientific and political reasons, the remaining sample, about 750 micrograms, was kept in storage for several years. During the early 1990's, when NMR instruments and methods became more sensitive and sophisticated, Daly's group was able to determine the structure of alkaloid 208/210, which was renamed epibatidine (1)(1R, 2R, 4S exo-2-(6-chloro-3-pyridyl)-7-azabicyclo[2.2.1]heptane).
As stated above, epibatidine was shown to be a potent analgetic (about 200 times more potent than morphine). The truly exciting discovery was that epibatidine's mechanism of action appeared to be non-opioid. Many potent pain relieving drugs are opiates, morphine (2) being a very familiar example. Morhpine is an effective and potent analgesic; however, the potential for addiction and the development of morphine tolerance are major drawbacks to its use. Several major pharmaceutical companies have focused their efforts on discovering better analgesics. When Daly showed that epibatidine's effect was not blocked by naloxone, an opioid antagonist, this revelation produced much enthusiasm in the hope for a better drug.
If epibatidine did not exert its analgesic effect through opioid receptors, how then did it produce the pain relief? Shortly after the publication of the structure of epibatidine, several research groups, including Daly's, determined the answer by examining epibatidine's interaction with nicotinic acetylcholine receptors (AChRs), a type of ligand-gated ion channel whose endogenous ligand is acetylcholine (3) [4-6]. S-(-)-Nicotine (4) also activates these receptors - hence their name. Not only did epibatidine bind to and activate these receptors, it did so at extremely low concentrations (Ki=0.043-0.055 nM or about 55 pM). The finding that the analgesic effects of epibatidine are blocked by mecamylamine (a noncompetitive nicotinic antagonist), along with previous research illustrating potentially beneficial effects of nicotine [7,8], sparked a resurgence in the medicinal chemistry of nicotine and nicotinic analogues.
Medicinal chemists have attempted to define the structural and chemical features that are important for epibatidine's high affinity. With many biologically active compounds, the chirality, or absolute spatial configuration of the molecule, often influences its activity. For example, S-(-)-nicotine (Ki = 1-2 nM), the naturally occurring stereoisomer, has about 20-fold higher affinity than its enantiomer, R-(+)-nicotine (4) (Ki = 25 nM). 1R, 2R, 4S-(-)-Epibatidine is the natural stereoisomer excreted by the frog. Its enantiomer has been synthesized, tested for receptor affinity, and shown to have the same affinity as the natural isomer. A molecular modeling study by Dukat et al. rationalized the nonstereospecificity of epibatidine [5]. The enantiomers of nicotine appear to occupy different volumes in spaces, whereas the enantiomers of epibatidine occupy the same molecular volume.
Another early insight into the binding of epibatidine resulted from a comparison of the structures of nicotine and epibatidine. Both contain a six-membered pyridine ring; both contain a basic nitrogen linked to the pyridine ring by one or two carbons; both basic nitrogens are part of a five-membered ring (in epibatidine, the five membered ring is part of the azabicycloheptane structure). In fact, Dukat et al. showed that energy-minimized molecular models of the compounds could be overlayed such that major structural features are in similar positions in space (Figure 2) [5]. This modeling experiment was the first to show in a three dimensional fashion that epibatidine and nicotine may interact with similar receptor features.
Fig 2. Superposition of nicotine (cyan) and epibatidine (red). Nitrogens are blue; chlorine is green.
When studying biologically active compounds, one goal of medicinal chemists is to define a pharmacophore - the optimal three dimensional arrangement of chemical and structural molecular features required by a certain receptor. In the past, several research groups have proposed pharmacophores for the nicotinic receptor. Beers and Reich [9], Barlow and Johnson [10], and Sheridan and coworkers [11] have suggested nicotine pharmacophores. With some simplification, the models all contain a hyrogen bond acceptor atom (e.g., pyridine N or carbonyl O) and a center of positive charge (e.g., protonated basic nitrogen), separated by a distance of approximately 4.8 A. This distance is often referred to as the "internitrogen distance" because most, although not all, nicotinic analogues contain a pyridine nitrogen and a more basic nitrogen. With the emergence of epibatidine as a high affinity nicotinic agonist, Glennon and his group reevaluated the nicotinic pharmacophore and produced a model which indicated an optimal internitrogen distance of 5.1-5.5 A [12]. In 1996, a research group at Abbott Laboratories pubished work in which they synthesized a series of pyridyl ether compounds which are nicotinic agonists, some as potent as epibatidine. Molecular modeling studies incorporating these new agents suggested that a internitrogen distance closer to 6.1 A may be optimal for interaction at the nicotinic receptor [13]. There is still much work to be done before a precise nicotine pharmacophore can be agreed upon by the medicinal chemists.
With such a brief overview, it is impossible to review or mention all the research and scientists that have contributed to our understanding of epibatidine. Also, many of the complexities of the chemistry and biology have been omitted for brevity's sake. One area of complexity concerns receptor subtypes and populations. The nicotinic acetylcholine receptor, as stated above, is a ligand-gated ion channel, composed of five individual subunits: (alpha), (beta), (gamma), (delta), and (epsilon). There are, however, several different subtypes of receptor, each with a different composition of subunits, and different pharmacological properties. To date, nine alpha subunits (1 - 9) and four beta (1 - 4) subunits have been discovered. The muscle type nAChR, the most studied ligand-gated ion channel, is composed of (1)2 1 . Neuronal nAChRs are composed of various combinations of and subunits. The binding affinity and the pharmacological effects of a particular ligand are dependent upon the subunit composition of the nAChR. In a recent report from Dr. Luetje's lab, the affinity of epibatidine for several different subtypes of the nAChR was reported [14]. The results are shown in Table 1. As can be observed, there is a somewhat significant change in affinity when the subunits are changed (e.g., 30-fold difference in affinity when 3 2 is changed to 3 4). For more details, please refer to any of the excellent reviews of epibatidine[15-17], nicotinic ligands [18-20], and nicotinic receptors [7,8,21,22].
Table 1. Epibatidine Affinity at Neuronal nAChR subtypes.
Receptor Affinity (pM)
2 2
10.3 3 2
13.6 4 2
30.0
2 4
86.8 3 4
303 4 4
84.7
Synthetic organic chemists have also shown intense interest in epibatidine. Epibatidine's azabicycloheptane system is not common in natural products. E.J. Corey [23], T.Y. Chen [24], Broka [25], and Clayton and Regan [26] were the among the first to report total syntheses of epibatidine. Many other synthetic routes were later reported (See references 27-29 for reviews). More recently, Aoyagi reported a total sythesis in which the key reaction was an asymmetric Diels-Alder reaction with a chiral N-acylnitroso compound as the dieneophile [30]. Also, Sirisoma and Johnson described their synthetic route, which utilized an -iodocycloalkenone in a modified Stille reaction [31]. Because of the rather simple but intriguing structure of epibatine, chemists are certain to devise additional avenues to its synthesis.
So what does the future hold for epibatidine? The chance of epibatidine ever being used as a medicinal agent is quite low because of its high toxicity. However, new analogues of epibatidine have been and are still being synthesized. One interesting analogue is epiboxidine (6), a hybrid between epibatidine and ABT-418 (5) [32]. ABT-418, an isosteric analogue of nicotine, has analgesic and cognitive-enhancing properties in certain test systems. ABT-418 was designed by replacing the pyridine ring of nicotine with a methylisoxazole ring. Daly used this same isosteric replacement in epibatidine, replacing the chloropyridine ring with the methylisoxazole ring, producing epiboxidine. Although not as potent as epibatidine, epiboxidine (Ki = 0.6 nM) has higher affinity to the nAChR than nicotine (Ki = 1.01 nM) and ABT-418 (Ki = 10 nM). In addition, epiboxidine is 20-fold less toxic than epibatidine.
Several analogues of epibatidine, in which the azabicycloheptane ring has been altered, have been synthesized and tested. These include homoepibatidine (7), bis-homoepibatidine (8), and the azabicyclooctane analogue 9 [33-36]. Interestingly, compound 7 was shown to have analgesic potency comparable to that of epibatidine [34]. The diazabicyclic pyrazine DBO-83 (10) is another high affinity (Ki = 4 nM) nicotinic ligand that has some structural similarity to epibatidine [37, 38]. The idea for this compound originated partly from research aimed at discovering analgesics that were selective for the mu-opioid receptor.
Another puzzle to solve is the source of epibatidine. Researchers first thought that the frog produced the compound biochemically. However, when Daly raised some E. tricolor frogs in captivity, he could not isolate or detect any epibatidine[2,3]. The common presumption is that the frog obtains epibatidine, or some biological precursor, from a dietary source. Insects are one suspected source. On the other hand, because of the structural similarity of epibatidine and nicotine, a plant-derived alkaloid, a floral source may be a possibility. Whichever the answer, identifying the producer of this potent nicotinic agonist may provide an abundant source of epibatidine.
I recently read about ZICONITIDE a conotoxin from a snail ,this is approved for infusion in to the spinal column for ch pain relief .
very few studies about 2000 patients allover the world have had the treatment in experiments so far .
another aspect is the socalled Nicotinic receptors in the brain some of theses are responsible for cognition ( remember alzheimers) and analgesia ( does somoking reduce pain ?
the article excerpt follows an alkaloid from a frog (may be from frog food !)
interesting reading
Epibatidine
--------------------------------------------------------------------------------
published on the net by
Matthew J. Dowd, Graduate Student
Department of Medicinal Chemistry
Virginia Commonwealth University
Richmond, VA 23298-0540 USA
Click here for the CHIME-enhanced version of this article. Also, click on any structure to view a 3-D version of the molecule.
Within the rainforest of Ecuador resides a small, colorful, seemingly harmless amphibian called Epipedobates tricolor (Figure 1). This frog first introduced itself to the scientific world in 1974. It was then that Dr. John Daly of the National Institutes of Health isolated from the frog a compound initially called alkaloid 208/210 (its MW from mass spectrometry) [1]. Daly demonstrated that this new alkaloid was a potent analgesic (as measured in the Straub-tail response when injected into mice). Even after subsequent trips to South America, too little of the compound was isolated to make a structural determination.[2,3] Because of this lack of compound, for both scientific and political reasons, the remaining sample, about 750 micrograms, was kept in storage for several years. During the early 1990's, when NMR instruments and methods became more sensitive and sophisticated, Daly's group was able to determine the structure of alkaloid 208/210, which was renamed epibatidine (1)(1R, 2R, 4S exo-2-(6-chloro-3-pyridyl)-7-azabicyclo[2.2.1]heptane).
As stated above, epibatidine was shown to be a potent analgetic (about 200 times more potent than morphine). The truly exciting discovery was that epibatidine's mechanism of action appeared to be non-opioid. Many potent pain relieving drugs are opiates, morphine (2) being a very familiar example. Morhpine is an effective and potent analgesic; however, the potential for addiction and the development of morphine tolerance are major drawbacks to its use. Several major pharmaceutical companies have focused their efforts on discovering better analgesics. When Daly showed that epibatidine's effect was not blocked by naloxone, an opioid antagonist, this revelation produced much enthusiasm in the hope for a better drug.
If epibatidine did not exert its analgesic effect through opioid receptors, how then did it produce the pain relief? Shortly after the publication of the structure of epibatidine, several research groups, including Daly's, determined the answer by examining epibatidine's interaction with nicotinic acetylcholine receptors (AChRs), a type of ligand-gated ion channel whose endogenous ligand is acetylcholine (3) [4-6]. S-(-)-Nicotine (4) also activates these receptors - hence their name. Not only did epibatidine bind to and activate these receptors, it did so at extremely low concentrations (Ki=0.043-0.055 nM or about 55 pM). The finding that the analgesic effects of epibatidine are blocked by mecamylamine (a noncompetitive nicotinic antagonist), along with previous research illustrating potentially beneficial effects of nicotine [7,8], sparked a resurgence in the medicinal chemistry of nicotine and nicotinic analogues.
Medicinal chemists have attempted to define the structural and chemical features that are important for epibatidine's high affinity. With many biologically active compounds, the chirality, or absolute spatial configuration of the molecule, often influences its activity. For example, S-(-)-nicotine (Ki = 1-2 nM), the naturally occurring stereoisomer, has about 20-fold higher affinity than its enantiomer, R-(+)-nicotine (4) (Ki = 25 nM). 1R, 2R, 4S-(-)-Epibatidine is the natural stereoisomer excreted by the frog. Its enantiomer has been synthesized, tested for receptor affinity, and shown to have the same affinity as the natural isomer. A molecular modeling study by Dukat et al. rationalized the nonstereospecificity of epibatidine [5]. The enantiomers of nicotine appear to occupy different volumes in spaces, whereas the enantiomers of epibatidine occupy the same molecular volume.
Another early insight into the binding of epibatidine resulted from a comparison of the structures of nicotine and epibatidine. Both contain a six-membered pyridine ring; both contain a basic nitrogen linked to the pyridine ring by one or two carbons; both basic nitrogens are part of a five-membered ring (in epibatidine, the five membered ring is part of the azabicycloheptane structure). In fact, Dukat et al. showed that energy-minimized molecular models of the compounds could be overlayed such that major structural features are in similar positions in space (Figure 2) [5]. This modeling experiment was the first to show in a three dimensional fashion that epibatidine and nicotine may interact with similar receptor features.
Fig 2. Superposition of nicotine (cyan) and epibatidine (red). Nitrogens are blue; chlorine is green.
When studying biologically active compounds, one goal of medicinal chemists is to define a pharmacophore - the optimal three dimensional arrangement of chemical and structural molecular features required by a certain receptor. In the past, several research groups have proposed pharmacophores for the nicotinic receptor. Beers and Reich [9], Barlow and Johnson [10], and Sheridan and coworkers [11] have suggested nicotine pharmacophores. With some simplification, the models all contain a hyrogen bond acceptor atom (e.g., pyridine N or carbonyl O) and a center of positive charge (e.g., protonated basic nitrogen), separated by a distance of approximately 4.8 A. This distance is often referred to as the "internitrogen distance" because most, although not all, nicotinic analogues contain a pyridine nitrogen and a more basic nitrogen. With the emergence of epibatidine as a high affinity nicotinic agonist, Glennon and his group reevaluated the nicotinic pharmacophore and produced a model which indicated an optimal internitrogen distance of 5.1-5.5 A [12]. In 1996, a research group at Abbott Laboratories pubished work in which they synthesized a series of pyridyl ether compounds which are nicotinic agonists, some as potent as epibatidine. Molecular modeling studies incorporating these new agents suggested that a internitrogen distance closer to 6.1 A may be optimal for interaction at the nicotinic receptor [13]. There is still much work to be done before a precise nicotine pharmacophore can be agreed upon by the medicinal chemists.
With such a brief overview, it is impossible to review or mention all the research and scientists that have contributed to our understanding of epibatidine. Also, many of the complexities of the chemistry and biology have been omitted for brevity's sake. One area of complexity concerns receptor subtypes and populations. The nicotinic acetylcholine receptor, as stated above, is a ligand-gated ion channel, composed of five individual subunits: (alpha), (beta), (gamma), (delta), and (epsilon). There are, however, several different subtypes of receptor, each with a different composition of subunits, and different pharmacological properties. To date, nine alpha subunits (1 - 9) and four beta (1 - 4) subunits have been discovered. The muscle type nAChR, the most studied ligand-gated ion channel, is composed of (1)2 1 . Neuronal nAChRs are composed of various combinations of and subunits. The binding affinity and the pharmacological effects of a particular ligand are dependent upon the subunit composition of the nAChR. In a recent report from Dr. Luetje's lab, the affinity of epibatidine for several different subtypes of the nAChR was reported [14]. The results are shown in Table 1. As can be observed, there is a somewhat significant change in affinity when the subunits are changed (e.g., 30-fold difference in affinity when 3 2 is changed to 3 4). For more details, please refer to any of the excellent reviews of epibatidine[15-17], nicotinic ligands [18-20], and nicotinic receptors [7,8,21,22].
Table 1. Epibatidine Affinity at Neuronal nAChR subtypes.
Receptor Affinity (pM)
2 2
10.3 3 2
13.6 4 2
30.0
2 4
86.8 3 4
303 4 4
84.7
Synthetic organic chemists have also shown intense interest in epibatidine. Epibatidine's azabicycloheptane system is not common in natural products. E.J. Corey [23], T.Y. Chen [24], Broka [25], and Clayton and Regan [26] were the among the first to report total syntheses of epibatidine. Many other synthetic routes were later reported (See references 27-29 for reviews). More recently, Aoyagi reported a total sythesis in which the key reaction was an asymmetric Diels-Alder reaction with a chiral N-acylnitroso compound as the dieneophile [30]. Also, Sirisoma and Johnson described their synthetic route, which utilized an -iodocycloalkenone in a modified Stille reaction [31]. Because of the rather simple but intriguing structure of epibatine, chemists are certain to devise additional avenues to its synthesis.
So what does the future hold for epibatidine? The chance of epibatidine ever being used as a medicinal agent is quite low because of its high toxicity. However, new analogues of epibatidine have been and are still being synthesized. One interesting analogue is epiboxidine (6), a hybrid between epibatidine and ABT-418 (5) [32]. ABT-418, an isosteric analogue of nicotine, has analgesic and cognitive-enhancing properties in certain test systems. ABT-418 was designed by replacing the pyridine ring of nicotine with a methylisoxazole ring. Daly used this same isosteric replacement in epibatidine, replacing the chloropyridine ring with the methylisoxazole ring, producing epiboxidine. Although not as potent as epibatidine, epiboxidine (Ki = 0.6 nM) has higher affinity to the nAChR than nicotine (Ki = 1.01 nM) and ABT-418 (Ki = 10 nM). In addition, epiboxidine is 20-fold less toxic than epibatidine.
Several analogues of epibatidine, in which the azabicycloheptane ring has been altered, have been synthesized and tested. These include homoepibatidine (7), bis-homoepibatidine (8), and the azabicyclooctane analogue 9 [33-36]. Interestingly, compound 7 was shown to have analgesic potency comparable to that of epibatidine [34]. The diazabicyclic pyrazine DBO-83 (10) is another high affinity (Ki = 4 nM) nicotinic ligand that has some structural similarity to epibatidine [37, 38]. The idea for this compound originated partly from research aimed at discovering analgesics that were selective for the mu-opioid receptor.
Another puzzle to solve is the source of epibatidine. Researchers first thought that the frog produced the compound biochemically. However, when Daly raised some E. tricolor frogs in captivity, he could not isolate or detect any epibatidine[2,3]. The common presumption is that the frog obtains epibatidine, or some biological precursor, from a dietary source. Insects are one suspected source. On the other hand, because of the structural similarity of epibatidine and nicotine, a plant-derived alkaloid, a floral source may be a possibility. Whichever the answer, identifying the producer of this potent nicotinic agonist may provide an abundant source of epibatidine.
Subscribe to:
Posts (Atom)