View source for Epibatidine

{{distinguish|Eptifibatide}}
{{Weasel|date=August 2009}}

{{Drugbox
| IUPAC_name        = (1''R'',2''R'',4''S'')-(+)-6-(6-chloro-3-pyridyl)-7-azabicyclo[2.2.1]heptane
| image             = (+)-Epibatidine Structural Formulae V.1.svg
| CAS_number        = 140111-52-0
| ATC_prefix        = 
| ATC_suffix        = 
| PubChem           = 11031065
| DrugBank          = 
| ChemSpiderID      = 746409
| chemical_formula  = C<sub>11</sub>H<sub>13</sub>ClN<sub>2</sub>
| molecular_weight  = 208.69 g/mol
| smiles            = C1CC2C(CC1N2)C3=CN=C(C=C3)Cl
| bioavailability   = 
| protein_bound     = 
| metabolism        = 
| elimination_half-life = 
| excretion         = 
| pregnancy_category= 
| legal_status      = 
| routes_of_administration = 
}}

'''Epibatidine''' is an [[alkaloid]] that originally is found in the skin of a neotropical poisonous [[frog]], [[Phantasmal poison frog|''Epipedobates tricolor'']], found in [[Ecuador]]. 

It was initially isolated by John Daly at the [[National Institutes of Health]], and was found to be a powerful analgesic, ~200 times the potency of [[morphine]].<ref name=Dowd>[http://www.phc.vcu.edu/Feature/oldfeature/epi/ Epibatidine] - A review by Matthew J. Dowd</ref>  

Several total syntheses have been devised due to the relative scarcity of epibatidine in nature.<ref>Olivo, Horacio F.; Hemenway, Michael S.  Recent syntheses of epibatidine. A review. ''Organic Preparations and Procedures International'' (2002), 34(1), 1-26.</ref>

== Structure ==

It has been attempted previously to breed these frogs in a domesticated environment so that the alkaloid can be extracted. Interestingly, it was documented that although it was possible to breed these frogs artificially, they did not produce isolable amounts of the desired alkaloid. 

Since epibatidine is extremely potent, only traces of this alkaloid are present on the skins of this breed of frog. 

Presumably the frog produces this poison for self-defense purposes, particularly in the face of [[adversity]] from larger predators.

Coupled to the low concentration (traces) of epibatidine in the skins of these frogs, the frogs themselves are an endangered species.

The above two considerations frustrated attempts to discover the chemical structure of this exotic alkaloid and the project had to be shelved until more sensitive analytical techniques could facilitate in this compounds identification.

It was not until the development of the now established and sensitive technique of [[NMR]] in the 1990s that the structure of epibatidine could finally be ascertained with any degree of accuracy.<ref name=Decker/>

Subsequent chemical syntheses of epibatidine also agree with the structure obtained from the natural sources.

== Medicinal Application ==

At first it was suspected that epibatidine might be opioidergic although application of the narcotic antagonist [[naloxone]] failed to either block or reverse its antinociceptive effects. Subsequently it was discovered that the compound is instead an agonist at nicotinic acetylcholine receptors since [[mecamylamine]] was able to block the actions of this drug. In this regard, the compound has a completely novel mechanism of action at blocking [[nociceptive]] stimuli.<ref name=Decker/>

Epibatidine is too toxic to use in clinical practice. In part, this is due to the fact that in addition to being an agonist at central nicotinic receptors, epibatidine is also believed to block neuromuscular junctions resulting in respiratory paralysis and death.<ref name=Decker/> 

It is rumored that [[Indigenous peoples of the Americas|Aboriginal tribes]] used to coat the tips of their arrows in the toxins secreted by the skins of these exotically colored frogs.

However, it is far too toxic to be used in the clinic. Indeed, [[sensitization]] to epibatidine occurs with repeat dosing, which might come as a surprise if it was expected that tolerance to its lethality occurs upon sustained exposure.

Nonetheless, this novel alkaloid served as an exciting lead in the search for novel [[analgesic]]s.<ref name=Decker>{{Cite pmid|14754452}}</ref><ref>{{Cite pmid|15050621}}</ref>

The aim of the [[drug design]] process is to dissociate the toxicity of this alkaloid from its [[antinociceptive]] properties which are hoped to provide a novel analgesic drug free from the dependence liability of classical narcotic analgesics acting on opiate receptors.

== Chemistry ==

Whereas popular drugs such as phenyltropane are semisynthetic, most of the designer epibatidine analogs so far have been totally synthetic.

The azabicyclic part of [[3-quinuclidinyl benzilate|BZ]] for example has been used to make novel nicotinic agonists.

It is too early at this stage to for any of the new designer compounds to have been fully characterized yet.

== Pharmacology ==

Epibatidine is a potent agonist at both the α4β2 and the α3β4 sub-types of nicotinic receptor in particular.<ref name=Dowd/> Epibatidine is relatively nonselective and is probably a strong agonist at most CNS [[nAChRs]].

Additionally, agonists at the α7 subunit are expected to have medicinal properties.<ref>Lightfoot AP, Kew JN, Skidmore J. Alpha7 nicotinic acetylcholine receptor agonists and positive allosteric modulators. Prog Med Chem. 2008;46:131-71. PMID 18381125</ref>

== Analogues==

Of the tested epibatidine derivatives, Abbott Labs' ABT-594 ([[Tebanicline]]) is the most promising reported to date. ABT-594 was discovered to be 50 times more potent than morphine, yet on animal tests, no paralysis or depression of muscle action was observed. It completed Phase II clinical trials in Europe,<ref>[http://www.chemsoc.org/ExemplarChem/entries/2004/icl_Pan/page5.html The New Morphine]</ref> but while it showed clinical efficacy for treating neuropathic pain in humans it was dropped from further development due to unacceptable incidence of gastrointestinal side effects.<ref>{{Cite pmid|16979678}}</ref> Further research in this area is ongoing.<ref>{{Cite pmid|17585748}}</ref>

[[Epiboxidine]] is another one of the relatively well respected epibatidine analogs. 

Epiboxidine is 1 tenth the potency of epibatidine but also said to be a lot less toxic.

Interestingly, epiboxidine is still regarded as too toxic for medicinal application, and its effects on man are undocumented.

== References ==
{{Reflist|2}}

== External links ==
* [http://www.chemsoc.org/ExemplarChem/entries/2004/icl_Pan/index.html# Epibatidine at chemsoc.org]


{{Stimulants}}
{{Analgesics}}
{{Poison frog alkaloids}}
{{Cholinergics}}

[[Category:Alkaloids]]
[[Category:Analgesics]]
[[Category:Halogen-containing alkaloids]]
[[Category:Organochlorides]]
[[Category:Pyridines]]
[[Category:Nicotinic agonists]]
[[Category:Toxins]]

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[[Category:2Fix]]