Epibatidine

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Not to be confused with Eptifibatide.
Epibatidine
Epibatidine structure.png
Systematic (IUPAC) name
(1R,2R,4S)-(+)-6-(6-chloro-3-pyridyl)-7-azabicyclo[2.2.1]heptane
Clinical data
Legal status ?
Identifiers
CAS number 140111-52-0 YesY
ATC code ?
PubChem CID 11031065
DrugBank DB07720
ChemSpider 10399316 YesY
KEGG C11690 N
ChEMBL CHEMBL298826 YesY
Chemical data
Formula C11H13ClN2 
Mol. mass 208.69 g/mol
 N (what is this?)  (verify)

Epibatidine is a potent poisonous alkaloid found on the skin of the Epipedobates anthonyi (formally named Epipedobates tricolor)[1] 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.[2] 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.[3] 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.

History[edit]

Many Amerindian tribes are known to have used tree frog alkaloids on their blowdarts to paralyze game.

A photo of a Epipedobates tricolor on a leaf
Epipedobates tricolor on a leaf

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,[4][5] 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.[6] E. tricolor frogs are now endangered and the population was unstable, which does not allow scientists to extract sufficient epibatidine for research.[7]

In 1992, when technology finally allowed the determination of the compound's structure, epibatidine was found to be a new class of alkaloid. [8] 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.[5] 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.[9]

Synthesis[edit]

The chemical synthesis of epibatidine after Corey

Several total syntheses routes have been devised due to the relative scarcity of epibatidine in nature.[10]

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%.[5][11][12]

Synthetic analogues[edit]

Abbott Laboratories has produced derivatives of epibatidine, like ABT-594, in search of a less toxic analgesic with less side effects than opioids.[13]

There have also been successful attempts to create epibatidine-like molecules, like ABT-418 and epiboxidine. This was done in order to find an analgesic with less adverse effects.[6]

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.[6] To date these efforts remain unsuccessful.

Chemical Structure[edit]

Epibatidine (C11H13ClN2 208.6901) is a piperidine alkaloid with a structure similar to that of nicotine.[14] It is a hygroscopic oily substance which is a base, due to its nitrogen.

Pharmacology and toxicology[edit]

Epibatidine has two mechanisms of action: it can bind either the nicotinic acetylcholine receptors (nAChR) or the muscarinic acetylcholine receptors (mAChR)[5]

  • 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)[15] The rank order of affinities is αε > αγ > αδ.[16]

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.[5]

  • 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.[17] 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.[5]

Efficacy and toxicity[edit]

Epibatidine has a high analgesic potency, as stated above. Studies show it has a potency at least 200 times that of morphine.[5] As the compound was not addictive nor did it suffer the effects of habituation,[citation needed] 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[18]) some of the epibatidine might bind the muscarinic acetylcholine receptors and cause adverse effects, such as hypertension, bradycardia and muscular paresis.[14]

Epibatidine has several toxic consequences. Empirically proven effects include splanchnic sympathetic nerve discharge and increased arterial pressure.[14] 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.[18] 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.[19] 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. 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.[12]

See also[edit]

References[edit]

  1. ^ Fitch, R. W.; Spande, T. F.; Garraffo, H. M.; Yeh, H. J. C.; Daly, J. W. (2010). "Phantasmidine: An Epibatidine Congener from the Ecuadorian Poison Frog Epipedobates anthonyi⊥". Journal of Natural Products 73 (3): 331. doi:10.1021/np900727e. PMID 20337496.  edit
  2. ^ Daly, J.W., Gusovsky, F., Myers, C.W., Yotsuyamashita, M., and Yasumoto, T. (1994). "1st Occurrence of Tetrodotoxin in a Dendrobatid Frog (Colostethus-Inguinalis), with Further Reports for the Bufonid Genus Atelopus.". Toxicon 32 (3): 279–285. doi:10.1016/0041-0101(94)90081-7. PMID 8016850. 
  3. ^ Prince and Sine; Sine, SM (2008). "Epibatidine activates muscle acetylcholine receptors with unique site selectivity". Biophysical Journal (Biophysical Journal) 75 (4): 1817–27. doi:10.1016/S0006-3495(98)77623-4. PMC 1299853. PMID 9746523. Retrieved 2008-10-10. 
  4. ^ "Pain - Pt 10". Accessexcellence.org. Retrieved 2014-03-12. 
  5. ^ a b c d e f g "Epibatidine and pain" 81. British Journal of Anaesthesia 1998. pp. 69–76. Retrieved 2014-03-12. 
  6. ^ a b c [1][dead link]
  7. ^ "Epipedobates tricolor". Iucnredlist.org. 2004-04-30. Retrieved 2014-03-12. 
  8. ^ "sjbr/strong/1998". Hem.csustan.edu. Retrieved 2014-03-12. 
  9. ^ Diana L. Donnelly-Roberts, Pamela S. Puttfarcken, Theresa A. Kuntzweiler, Clark A. Briggs, David J. Anderson, Jeffrey E. Campbell, Marietta Piattoni-Kaplan, David G. Mckenna, James T. Wasicak, Mark W. Holladay, Michael Williams and Stephen P. Arneric. 1998. ABT-594 [(R)-5-(2-Azetidinylmethoxy)-2-Chloropyridine]: A Novel, Orally Effective Analgesic Acting via Neuronal Nicotinic Acetylcholine Receptors: I. In VitroCharacterization Retrieved December 24, 2010.
  10. ^ Olivo, Horacio F.; Hemenway, Michael S. (2002). "Recent syntheses of epibatidine. A review". Organic Preparations and Procedures International 34 (1): 1–26. doi:10.1080/00304940209355744. 
  11. ^ Clayton, S.C.; Regan, A.C. Total Synthesis of (+/-)-Epibatidine. Tetrahedron Lett. 1993,34, 7493-7496.
  12. ^ a b Broka, C.A. Synthetic Approaches to Epibatidine. Med. Chem. Res. 1994, 4, 449-460.
  13. ^ "Deriving a non-opiate painkiller [ABT-594] from Epipedobates tricolor". Mongabay.com. Retrieved 2014-03-12. 
  14. ^ a b c Fisher M, Huangfu D, Shen TY, Guyenet PG (1994). "Epibatidine, an alkaloid from the poison frog Epipedobates tricolor, is a powerful ganglionic depolarizing agent.". J Pharmacol Exp Ther 270 (2): 702–7. PMID 8071862. 
  15. ^ British Journal of Anaesthesia, 1998; 81: 69-76
  16. ^ Richard J. Prince and Steven M. Sine (1998-04-03). "Epibatidine Binds with Unique Site and State Selectivity to Muscle Nicotinic Acetylcholine Receptors". Jbc.org. Retrieved 2014-03-12. 
  17. ^ Watt A. P., Hitzel L., Morrison D., Locker K. L., Determination of the in vitro metabolism of (1)- and (2)-epibatidine, Journal of Chromatography A; 896: 229–238, 2000.
  18. ^ a b Badio B, Daly JW. Epibatidine,a potent analgetic and nicotinic agonist, Molecular Pharmacology 1994; 45: 563-569
  19. ^ Sihver, Acta (2002). "Neurologica Scandinavica, Ligands for in vivo imaging on nicotinic receptor subtypes in Alzheimer brain". Interscience.wiley.com. Retrieved 2014-03-12. 

External links[edit]