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Epibatidine is a strong toxic alkaloid that is secreted Epipedobates anthonyi (also known as Epipedobates tricolor) which live in central and southern Ecuador. It was discovered by John W. Daly in 1974 and its structure was elucidated in 1992.
Epibatidine's toxicity stems from its ability to bind and turn on nicotinic and muscarine acetylcholine receptors. These receptors are involved in the transmission of painful sensation and in movement, among other functions. By binding and turning on specific receptors, epibatidine 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 but because it can be deadly even at very low doses it is no longer being researched for potential therapeutic uses.
Epibatidine was discovered by John W. Daly in the 1970s. It was isolated from the skin of Epipdobates anthonyi frogs collected by Daly and a colleague of his, 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) symptoms in mice with symptoms that resembled those of an opioid. In spite of 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.
It took until 1992 for epibatidine’s structure to be determined, an effort hindered by E. anthonyi gaining protected status by the Convention on International Trade in Endangered Species in 1984. Furthermore, it wasn’t possible to breed the frogs to harvest more of the compound because there is no epibatidine present on the skins of E. anthonyi raised in captivity. The frogs do not produce the chemical themselves; instead they isolate it from their diet and then sequester it on their skin. The likely source is beetles, ants or mites that they eat. In spite of these difficulties the structure was eventually determined and the first synthesis of epibatidine was completed soon after (in 1993). There have been numerous other syntheses have been developed since.
There was intense interest in epibatidine’s use as a drug because while it functioned as a painkiller it was found not to be an opioid. This meant that it could be used without the danger of habituation that accompanies the use of painkillers like morphine. However it cannot be used in humans because the dose resulting in toxic symptoms is too low for it to be safe.
Several total syntheses routes have been devised due to the relative scarcity of epibatidine in nature.
After the discovery of epibatidine's structure, more than fifty ways to synthesize epibatidine in the lab have been devised. A nine-step procedure produces the substance as a racemate (in contrast, the naturally occurring compound is the (+)-enantiomer; the (−)-enantiomer is not naturally occurring). 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%.
There has been a significant amount of research towards the creation of a derivative of epibatidine characterized by reduced toxicity, whilst retaining the powerful analgesic effect. To date these efforts remain unsuccessful.
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Mechanism of Action
- The analgesic property of epibatidine is believed to pass through by binding to α4/β2 nicotinic receptors. Epibatidine also binds to α3/β4 and to much lesser extent α7 receptors (affinity 300-fold less than for α4/β2) The rank order of affinities is αε > αγ > αδ.
Nicotinic acetylcholine receptors are positioned in the postsynaptic membrane of nerve cells. They propagate neurotransmission in the central and peripheral nervous system. When neurotransmitters bind to these receptors the ion channel opens allowing Na+ and Ca2+ ions to move across the membrane. This depolarizes the post synaptic membrane inducing an action potential that will propagate the signal. This signal will ultimately induce a release of several substances among which are dopamine and norepinephrine, resulting in an antinociceptive effect on the organism. The usual neurotransmitter for nAChR is acetylcholine yet other substances like epibatidine and nicotine are able to bind the receptor as well and induce a similar, if not identical, response. Epibatidine has an extremely high affinity for nAChR’s and will induce a response at concentrations of ~10 µM. This is a 1000x lower concentration when compared to a nicotine induced response.
- The paralytic property of epibatidine works by binding muscarinic acetylcholine receptors (mAChR)
These receptors are G-protein coupled of five subtypes. M2 and M4 are coupled to an inhibitory protein which impedes the functioning of adenylyl cyclase. This enzyme catalyses the reaction of ATP into cAMP, which is an important second messenger in a cell. M1, M3 and M5 are coupled to a Gq-protein, which will activate the phosphatidylinositol 3-kinases (PI3K). These enzymes will, when activated, catalyze the reaction of Phosphatidylinositol 4,5-bisphosphate (PIP2)into diacylglycerol (DAG) and inositol-1,4,5-triphosphate (IP3). These second messengers can affect several processes in the cell, such as (sarco) endoplasmic reticulum calcium ATP-ase (SERCA).
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 and cause the paralytic effects.
Both (+)- and (-)enantiomers of epibatidine are biologically active, and both have similar binding affinities to nAChRs
|This article is missing information about only the (+)-enantiomer does not induce tolerance, its chief advantage over morphine, with obvious implications for synthesis and pharmaceutical application. (March 2014)|
Epibatidine has several toxic consequences. Empirically proven effects include splanchnic 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−1. At higher doses, however, epibatidine will cause paralysis and loss of consciousness, coma and eventually death. The median lethal dose (LD50) of epibatidine lies between 1,46 µg kg−1 and 13,98 µg kg−1. This makes epibatidine somewhat more toxic than dioxin (with an average LD50 of 22,8 µg kg−1). Due to the little difference between its toxic concentration and antinociceptive concentration, its therapeutic uses are very limited.
In research on mice, administration of doses over 5 μg kg-1 of epibatidine caused a dose-dependent paralyzing effect on the organism. With doses over 5 μg kg-1, symptoms included hypertension (increased blood pressure), paralysis in the respiratory 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.
Epibatidine most effectively enters the body through injection. Oddly enough, in vitro 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. Maximum concentration in the brain is about 30 minutes after injection and epibatidine is still present after 4 hours. This shows that the clearance has a low rate.
Epibatidine has a high analgesic potency, as stated above. Studies show it has a potency at least 200 times that of morphine. As the compound was not addictive nor did it suffer the effects of habituation, it was very promising to replace morphine as a painkiller. Unfortunately for its therapeutic uses, the therapeutic concentration is very close to the toxic concentration. This means that even at a therapeutic dose (5 µg kg−1) some of the epibatidine might bind the muscarinic acetylcholine receptors and cause adverse effects, such as hypertension, bradycardia and muscular paresis.
Compared to the gold standard in pain management, morphine, epibatidine needed only 2.5 μg kg−1 to initiate a pain-relieving effect whilst the same effect required approximately 10 mg kg−1 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 for analysis at this time.
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