Epibatidine
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Epibatidine is a chlorinated
alkaloid Alkaloids are a class of basic, naturally occurring organic compounds that contain at least one nitrogen atom. This group also includes some related compounds with neutral and even weakly acidic properties. Some synthetic compounds of similar ...
that is secreted by the
Ecuador Ecuador ( ; ; Quechua: ''Ikwayur''; Shuar: ''Ecuador'' or ''Ekuatur''), officially the Republic of Ecuador ( es, República del Ecuador, which literally translates as "Republic of the Equator"; Quechua: ''Ikwadur Ripuwlika''; Shuar: ''Eku ...
an frog '' Epipedobates anthonyi'' and poison dart frogs from the Ameerega genus. It was discovered by John W. Daly in 1974, but its structure was not fully elucidated until 1992. Whether epibatidine is the first observed example of a chlorinated alkaloid remains controversial, due to challenges in conclusively identifying the compound from the limited samples collected by Daly. By the time that high-resolution spectrometry was used in 1991, there remained less than one milligram of extract from Daly's samples, raising concerns about possible contamination. Samples from other batches of the same species of frog failed to yield epibatidine. Epibatidine is toxic. Its toxicity stems from its ability to interact with
nicotinic Nicotinic acetylcholine receptors, or nAChRs, are receptor polypeptides that respond to the neurotransmitter acetylcholine. Nicotinic receptors also respond to drugs such as the agonist nicotine. They are found in the central and peripheral ner ...
and
muscarinic acetylcholine receptor Muscarinic acetylcholine receptors, or mAChRs, are acetylcholine receptors that form G protein-coupled receptor complexes in the cell membranes of certain neurons and other cells. They play several roles, including acting as the main end-rece ...
s. These receptors are involved in the transmission of painful sensations, and in movement, among other functions. Epibatidine then causes numbness, and, eventually, paralysis. Doses are lethal when the paralysis causes respiratory arrest. Originally, it was thought that epibatidine could be useful as a drug. However, because of its unacceptable
therapeutic index The therapeutic index (TI; also referred to as therapeutic ratio) is a quantitative measurement of the relative safety of a drug. It is a comparison of the amount of a therapeutic agent that causes the therapeutic effect to the amount that causes ...
, it is no longer being researched for potential therapeutic uses.


History

Epibatidine was discovered by John W. Daly in 1974. It was isolated from the skin of ''Epipedobates anthonyi'' frogs collected by Daly and colleague, Charles Myers. Between 1974 and 1979, Daly and Myers collected the skins of nearly 3000 frogs from various sites in Ecuador, after finding that a small injection of a preparation from their skin caused analgesic (painkilling) effects in mice that resembled those of an
opioid Opioids are substances that act on opioid receptors to produce morphine-like effects. Medically they are primarily used for pain relief, including anesthesia. Other medical uses include suppression of diarrhea, replacement therapy for opioid us ...
. Despite its common name - Anthony's Poison Arrow frog - suggesting that it was used by natives when hunting, a paper written by Daly in 2000 claimed that there was no local folklore or folk medicine surrounding the frogs and that they were considered largely unimportant by the locals. The structure of epibatidine was elucidated in 1992, an effort hindered by ''E. anthonyi'' gaining IUCN protected status in 1984. Furthermore, these frogs do not produce the toxin when bred and reared in captivity, because they do not synthesize epibatidine themselves. Like other poison dart frogs, they instead obtain it through their diet and then sequester it on their skin. Likely dietary sources are beetles, ants, mites, and flies. Daly and Charles noticed that epibatidine was produced from their diet due to their return trip to Ecuador in 1976 when they found that at one site, none of the frogs present produced alkaloids, such as epibatidine; they discovered that only the frogs at certain sites with the dietary means allowed these frogs to produce epibatidine. Overcoming the difficulties, the structure was eventually determined, and the first synthesis of epibatidine was completed in 1993. Many other synthesis methods have been developed since. Because of its analgesic effect, there was intense interest in epibatidine's use as a drug, because it was found not to be an opioid. This meant that it could potentially be used without fear of addiction. However, it was soon found that it cannot be used in humans because the dose resulting in toxic symptoms is too low for it to be safe.


Synthesis

Several total synthesis routes have been devised due to the relative scarcity of epibatidine in nature. After the discovery of the structure of epibatidine, more than fifty ways to synthesize it in the laboratory have been devised. In the first reported example, a nine-step procedure produces the substance as a
racemate In chemistry, a racemic mixture, or racemate (), is one that has equal amounts of left- and right-handed enantiomers of a chiral molecule or salt. Racemic mixtures are rare in nature, but many compounds are produced industrially as racemates. ...
(in contrast, the naturally occurring compound is the (+)-enantiomer; the (−)-enantiomer does not occur naturally). It was later determined that the (+) and (-) enantiomers had equivalent analgesic as well as toxic effects. The process has proven to be quite productive, with a yield of about 40%.Broka, C.A. Synthetic Approaches to Epibatidine. Med. Chem. Res. 1994, 4, 449-460. An
enantioselective In chemistry, an enantiomer ( /ɪˈnænti.əmər, ɛ-, -oʊ-/ ''ih-NAN-tee-ə-mər''; from Ancient Greek ἐνάντιος ''(enántios)'' 'opposite', and μέρος ''(méros)'' 'part') – also called optical isomer, antipode, or optical anti ...
synthesis reported by
E J Corey Elias James Corey (born July 12, 1928) is an American organic chemist. In 1990, he won the Nobel Prize in Chemistry "for his development of the theory and methodology of organic synthesis", specifically retrosynthetic analysis. Regarded by many a ...
starting from chloronicotinaldehyde is outlined below: In addition to Corey's method, other notable methods include those of Broka, Huang and Shen, and Clayton and Regan.


Synthetic analogs

A number of approaches to discovering
structural analog A structural analog (analogue in modern traditional English; Commonwealth English), also known as a chemical analog or simply an analog, is a compound having a structure similar to that of another compound, but differing from it in respect to a ce ...
s of epibatine that maintain analgesics effects, but without the toxicity, have been attempted. For example,
Abbott Laboratories Abbott Laboratories is an American multinational medical devices and health care company with headquarters in Abbott Park, Illinois, United States. The company was founded by Chicago physician Wallace Calvin Abbott in 1888 to formulate known dr ...
has produced derivatives of epibatidine including
ABT-594 Tebanicline (ebanicline, ABT-594) is a potent synthetic nicotinic (non-opioid) analgesic drug developed by Abbott. It was developed as a less toxic analog of the potent poison dart frog-derived compound epibatidine, which is about 200 times stron ...
. ABT-594 retains analgesic properties while avoiding paralysis by still binding to receptors that control pain perception and having a low affinity for muscle-type nicotinic acetylcholine receptors (nAChR) reducing its paralysis effect. Other epibatidine analogs include
ABT-418 ABT-418 is a drug developed by Abbott, that has nootropic, neuroprotective and anxiolytic effects, and has been researched for treatment of both Alzheimer's disease and ADHD. It acts as an agonist at neural nicotinic acetylcholine receptors, su ...
,
epiboxidine Epiboxidine is a chemical compound which acts as a partial agonist at neural nicotinic acetylcholine receptors, binding to both the α3β4 and the α4β2 subtypes. It was developed as a less toxic analogue of the potent frog-derived alkaloid e ...
and their derivatives.
A synthesis of epibatidine, utilizing a microbial hydroxylation of an unactivated carbon in a 7-azanorbornane was published in 1999.


Chemical structure

Epibatidine is a piperidine
pyridine Pyridine is a basic heterocyclic organic compound with the chemical formula . It is structurally related to benzene, with one methine group replaced by a nitrogen atom. It is a highly flammable, weakly alkaline, water-miscible liquid with a d ...
with a structure similar to that of
nicotine Nicotine is a natural product, naturally produced alkaloid in the nightshade family of plants (most predominantly in tobacco and ''Duboisia hopwoodii'') and is widely used recreational drug use, recreationally as a stimulant and anxiolytic. As ...
. It is a
hygroscopic Hygroscopy is the phenomenon of attracting and holding water molecules via either absorption or adsorption from the surrounding environment, which is usually at normal or room temperature. If water molecules become suspended among the substance ...
oily substance which is a base.


Biological effects


Mechanism of action

Epibatidine has two mechanisms of action: it can bind to either
nicotinic acetylcholine receptor Nicotinic acetylcholine receptors, or nAChRs, are receptor polypeptides that respond to the neurotransmitter acetylcholine. Nicotinic receptors also respond to drugs such as the agonist nicotine. They are found in the central and peripheral ne ...
s (nAChR) or
muscarinic acetylcholine receptor Muscarinic acetylcholine receptors, or mAChRs, are acetylcholine receptors that form G protein-coupled receptor complexes in the cell membranes of certain neurons and other cells. They play several roles, including acting as the main end-rece ...
s (mAChR). Specifically, the
analgesic An analgesic drug, also called simply an analgesic (American English), analgaesic (British English), pain reliever, or painkiller, is any member of the group of drugs used to achieve relief from pain (that is, analgesia or pain management). It ...
property of epibatidine is believed to take place by its binding to the α4/β2 subtype of nicotinic receptors. Epibatidine also binds to the α3/β4 subtype and to a much lesser extent α7 receptors (affinity 300-fold less than for α4/β2) The rank order of affinities for the muscle nicotinic receptors is αε > αγ > αδ. Nicotinic acetylcholine receptors are found in the post-synaptic membranes of nerve cells. These receptors are an example of ion gated channels where binding by a ligand causes a conformational change allowing ions to cross the membrane into the cell. They propagate neurotransmission in the
central Central is an adjective usually referring to being in the center of some place or (mathematical) object. Central may also refer to: Directions and generalised locations * Central Africa, a region in the centre of Africa continent, also known as ...
and
peripheral nervous system The peripheral nervous system (PNS) is one of two components that make up the nervous system of bilateral animals, with the other part being the central nervous system (CNS). The PNS consists of nerves and ganglia, which lie outside the brain ...
. When neurotransmitters bind to these receptors, ion channels open, allowing Na+ and Ca2+ ions to move across the membrane. This depolarizes the post-synaptic membrane, inducing an action potential that propagates the signal. This signal will ultimately induce release of
dopamine Dopamine (DA, a contraction of 3,4-dihydroxyphenethylamine) is a neuromodulatory molecule that plays several important roles in cells. It is an organic compound, organic chemical of the catecholamine and phenethylamine families. Dopamine const ...
and
norepinephrine Norepinephrine (NE), also called noradrenaline (NA) or noradrenalin, is an organic chemical in the catecholamine family that functions in the brain and body as both a hormone and neurotransmitter. The name "noradrenaline" (from Latin '' ad'', ...
, resulting in an antinociceptive effect on the organism. The usual neurotransmitter for nAChR is acetylcholine. However, other substances (such as epibatidine and
nicotine Nicotine is a natural product, naturally produced alkaloid in the nightshade family of plants (most predominantly in tobacco and ''Duboisia hopwoodii'') and is widely used recreational drug use, recreationally as a stimulant and anxiolytic. As ...
) are also able to bind to the receptor and induce a similar, if not identical, response. Epibatidine has an extremely high affinity for nAChRs, depending on the receptor subtype, from 0.05 nM at the α4β2 subtype to 22 nM at the α7 subtype.
Affinity Affinity may refer to: Commerce, finance and law * Affinity (law), kinship by marriage * Affinity analysis, a market research and business management technique * Affinity Credit Union, a Saskatchewan-based credit union * Affinity Equity Par ...
as well as
efficacy Efficacy is the ability to perform a task to a satisfactory or expected degree. The word comes from the same roots as ''effectiveness'', and it has often been used synonymously, although in pharmacology a pragmatic clinical trial#Efficacy versu ...
(and thus also
potency Potency may refer to: * Potency (pharmacology), a measure of the activity of a drug in a biological system * Virility * Cell potency, a measure of the differentiation potential of stem cells * In homeopathic dilutions, potency is a measure of how ...
) are much higher than for nicotine. The
paralytic Paralysis (also known as plegia) is a loss of motor function in one or more muscles. Paralysis can also be accompanied by a loss of feeling (sensory loss) in the affected area if there is sensory damage. In the United States, roughly 1 in 50 ...
property of epibatidine takes place after its binding to
muscle-type nicotinic receptor The muscle-type nicotinic receptor is a type of nicotinic acetylcholine receptor consisting of the subunit combination (α1)2β1δε (adult receptor) or (α1)2β1δγ (fetal receptor). These receptors are found in neuromuscular junctions, where acti ...
s. Low doses of epibatidine will only affect the nAChRs, due to a higher affinity to nAChRs than to mAChRs. Higher doses, however, will cause epibatidine to bind to the mAChRs. Both (+)- and (-)-
enantiomer In chemistry, an enantiomer ( /ɪˈnænti.əmər, ɛ-, -oʊ-/ ''ih-NAN-tee-ə-mər''; from Ancient Greek ἐνάντιος ''(enántios)'' 'opposite', and μέρος ''(méros)'' 'part') – also called optical isomer, antipode, or optical ant ...
s of epibatidine are biologically active, and both have similar binding affinities to nAChRs Only the (+)-enantiomer does not induce tolerance. While this may be a potential therapeutic advantage over morphine, epibatidine has not entered clinical trials because even very small doses are lethal to rodents.


Symptoms

Epibatidine has several toxic consequences. Empirically proven effects include
splanchnic Splanchnic is usually used to describe organs in the abdominal cavity. It is used when describing: * Splanchnic tissue * Splanchnic organs - including the stomach, small intestine, large intestine, pancreas, spleen, liver, and may also include the ...
sympathetic nerve discharge and increased arterial pressure. The nerve discharge effects can cause antinociception partially mediated by agonism of central nicotinic acetylcholine receptors at low doses of epibatidine; 5 µg/kg.Badio B, Daly JW. Epibatidine, a potent analgetic and nicotinic agonist, Molecular Pharmacology 1994; 45: 563-569 At higher doses, however, epibatidine will cause paralysis and loss of consciousness, coma and eventually death. The
median lethal dose In toxicology, the median lethal dose, LD50 (abbreviation for "lethal dose, 50%"), LC50 (lethal concentration, 50%) or LCt50 is a toxic unit that measures the lethal dose of a toxin, radiation, or pathogen. The value of LD50 for a substance is the ...
(LD50) of epibatidine lies between 1.46 µg/kg and 13.98 µg/kg. This makes epibatidine somewhat more toxic than
dioxin Dioxin may refer to: * 1,2-Dioxin or 1,4-Dioxin, two unsaturated heterocyclic 6-membered rings where two carbon atoms have been replaced by oxygen atoms, giving the molecular formula C4H4O2 *Dibenzo-1,4-dioxin, the parent compound also known as ...
(with an average LD50 of 22.8 µg/kg). Due to the small difference between its toxic concentration and antinociceptive concentration, its therapeutic uses are very limited. In research on mice, administration of doses greater than 5 μg/kg of epibatidine caused a dose-dependent paralyzing effect on the organism. With doses over 5 μg/kg, symptoms included
hypertension Hypertension (HTN or HT), also known as high blood pressure (HBP), is a long-term medical condition in which the blood pressure in the arteries is persistently elevated. High blood pressure usually does not cause symptoms. Long-term high bl ...
(increased blood pressure), paralysis in the
respiratory The respiratory system (also respiratory apparatus, ventilatory system) is a biological system consisting of specific organs and structures used for gas exchange in animals and plants. The anatomy and physiology that make this happen varies grea ...
system, seizures, and, ultimately, death. The symptoms do, however, change drastically when lower doses are given. Mice became resistant to pain and heat with none of the negative effects of higher doses.


Pharmacology

Epibatidine most effectively enters the body through injection.
In vitro ''In vitro'' (meaning in glass, or ''in the glass'') studies are performed with microorganisms, cells, or biological molecules outside their normal biological context. Colloquially called "test-tube experiments", these studies in biology an ...
studies seem to suggest that epibatidine is hardly, if at all, metabolized in the human body. Also there is currently little information on the path of clearance from the body. Maximum concentration in the brain is reached at about 30 minutes after entering the body.


Potential medical uses

Epibatidine has a high analgesic potency, as stated above. Studies show it has a potency at least 200 times that of
morphine Morphine is a strong opiate that is found naturally in opium, a dark brown resin in poppies (''Papaver somniferum''). It is mainly used as a analgesic, pain medication, and is also commonly used recreational drug, recreationally, or to make ...
. As the compound was not addictive nor did it cause
habituation Habituation is a form of non-associative learning in which an innate (non-reinforced) response to a stimulus decreases after repeated or prolonged presentations of that stimulus. Responses that habituate include those that involve the intact org ...
,, it was initially thought to be very promising to replace morphine as a painkiller. However, the therapeutic concentration is very close to the toxic concentration. This means that even at a therapeutic dose (5 µg/kg), some epibatidine might bind to the
muscarinic acetylcholine receptors Muscarinic acetylcholine receptors, or mAChRs, are acetylcholine receptors that form G protein-coupled receptor complexes in the cell membranes of certain neurons and other cells. They play several roles, including acting as the main end-rece ...
and cause adverse effects, such as
hypertension Hypertension (HTN or HT), also known as high blood pressure (HBP), is a long-term medical condition in which the blood pressure in the arteries is persistently elevated. High blood pressure usually does not cause symptoms. Long-term high bl ...
,
bradycardia Bradycardia (also sinus bradycardia) is a slow resting heart rate, commonly under 60 beats per minute (BPM) as determined by an electrocardiogram. It is considered to be a normal heart rate during sleep, in young and healthy or elderly adults, a ...
and muscular
paresis In medicine, paresis () is a condition typified by a weakness of voluntary movement, or by partial loss of voluntary movement or by impaired movement. When used without qualifiers, it usually refers to the limbs, but it can also be used to desc ...
. Compared to the gold standard in pain management,
morphine Morphine is a strong opiate that is found naturally in opium, a dark brown resin in poppies (''Papaver somniferum''). It is mainly used as a analgesic, pain medication, and is also commonly used recreational drug, recreationally, or to make ...
, epibatidine needed only 2.5 μg/kg to initiate a pain-relieving effect whilst the same effect required approximately 10 mg/kg of morphine (4,000 times the efficacy.) Currently, only rudimentary research into epibatidine's effects has yet been performed; the drug has been administered only to
rodents Rodents (from Latin , 'to gnaw') are mammals of the order Rodentia (), which are characterized by a single pair of continuously growing incisors in each of the upper and lower jaws. About 40% of all mammal species are rodents. They are nat ...
for analysis at this time.


Antidote

The antidote to epibatidine is
mecamylamine Mecamylamine ( INN, BAN; or mecamylamine hydrochloride (USAN); brand names Inversine, Vecamyl) is a non-selective, non-competitive antagonist of the nicotinic acetylcholine receptors (nAChRs) that was introduced in the 1950s as an antihypertensi ...
, a nicotinic acetylcholine receptor antagonist that is non-selective and non-competitive. Both the (+) and the (-) enantiomers of mecamylamine were seen to be efficient and both have the same affinity for nicotinic acetylcholine receptors.


See also

* 6-Chloronicotine *
Arrow poison Arrow poisons are used to poison arrow heads or darts for the purposes of hunting and warfare. They have been used by indigenous peoples worldwide and are still in use in areas of South America, Africa and Asia. Notable examples are the poisons se ...
*
Batrachotoxin Batrachotoxin (BTX) is an extremely potent cardio- and neurotoxic steroidal alkaloid found in certain species of beetles, birds, and frogs. The name is from the Greek word grc, βάτραχος, bátrachos, frog, label=none. Structurally-relate ...
(and closely related
homobatrachotoxin Batrachotoxin (BTX) is an extremely potent cardio- and neurotoxic steroidal alkaloid found in certain species of beetles, birds, and frogs. The name is from the Greek word grc, βάτραχος, bátrachos, frog, label=none. Structurally-related ...
) * Nemertelline, neurotoxin closely related to
nicotelline Nicotelline is an alkaloid first identified in 1914 as a chemical constituent of tobacco plants (''Nicotiana tabacum''). The chemical structure of nicotelline wasn't elucidated until 1956, when it was determined that nicotelline is a terpyridin ...
* Phantasmidine *
Tetrodotoxin Tetrodotoxin (TTX) is a potent neurotoxin. Its name derives from Tetraodontiformes, an order that includes pufferfish, porcupinefish, ocean sunfish, and triggerfish; several of these species carry the toxin. Although tetrodotoxin was discovered ...


References


External links


Epibatidine at chemsoc.org
{{Neurotoxins Analgesics Nicotinic agonists Vertebrate toxins Pyridine alkaloids Halogen-containing alkaloids Chloropyridines Neurotoxins Halogen-containing natural products Chlorine-containing natural products