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Epibatidine is a potent poisonous alkaloid found on the skin of the Epipedobates anthonyi (formally named Epipedobates tricolor) species of poison dart frog. These frogs, like other poison dart frogs, are best known for their ability to sequester poisons from their prey and secrete these poisons onto their backs. Epibatidine is unique to Epipedobates tricolor and is not found on other poison frogs in the family dendrobatidae.
Epibatidine, was discovered in 1974, but not isolated and identified until 1992. The molecule was found to be a very powerful analgesic. However, the therapeutic dose was very near the fatal dose. This gives the compound a very small therapeutic index. A lower toxicity derivative, ABT-594 developed by Abbott Laboratories, reached Phase 2 clinical trials, but was discontinued due to gastrointestinal side-effects.
John W. Daly, working for the National Institute of Health, collected samples of skin secretions from the frog in 1974. Although it was shown to work as an effective analgesic in mice with a potency 200 times that of morphine, technology at the time was not able to discern the chemical's structure because the preserved sample was too small. In addition, Daly's research was hindered by the Convention on International Trade in Endangered Species of Wild Fauna and Flora, which put heavy restrictions on the collection of Epipedobates tricolor.
E. tricolor frogs do not naturally possess epibatidine, but obtain epibatidine or its precursor from their surroundings. It has been suspected that the frog obtains epibatidine or a precursor molecule from a flowery plant, since the similar toxin nicotine is also derived from this kind of source. E. tricolor frogs are now endangered and the population was unstable, which does not allow scientists to extract sufficient epibatidine for research.
In 1992, when technology finally allowed the determination of the compound's structure, epibatidine was found to be a new class of alkaloid.  There was great excitement when epibatidine was found to be non-opioid. This was proven by Daly's experiment: when administration of naloxone, an opioid antagonist, did not reverse epibatidine's effect. Being a non-opioid suggested that it might have been a non-addictive compound with very few adverse effects. However, because of the small difference between therapeutic and toxic dose, epibatidine has never been used in humans.
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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Pharmacology and toxicology
- 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.
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.
Efficacy and toxicity
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.
|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.
Its antidote is mecamylamine (Inversine).
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. 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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