Epibatidine is a chlorinated alkaloid that is secreted by the Ecuadoran frog Epipedobates anthonyi and poison dart frogs from the Ameerega genus.[1] It was discovered by John W. Daly in 1974, but its structure was not fully elucidated until 1992. Whether epibatidine occurs naturally 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.[2]
Epibatidine is toxic. Its toxicity stems from its ability to interact with nicotinic and muscarinic acetylcholine receptors. 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, it is no longer being researched for potential therapeutic uses.[3]
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.[2] Despite its common name - Anthony's Poison Arrow frog - suggesting that it was used by natives when hunting,[4] 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.[5]
The structure of epibatidine was elucidated in 1992, an effort hindered by E. anthonyi gaining IUCN protected status in 1984.[5] 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.[6] 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.[7] 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.[5]
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.[2] 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.[8]
Several total synthesis routes have been devised due to the relative scarcity of epibatidine in nature.[9]
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 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%.[10][11][12]
An enantioselective synthesis reported by E J Corey starting from chloronicotinaldehyde is outlined below:
A number of approaches to discovering structural analogs of epibatine that maintain analgesics effects, but without the toxicity, have been attempted.[15] For example, Abbott Laboratories has produced derivatives of epibatidine including tebanicline (ABT-594).[16] Tebanicline 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.[17] Other epibatidine analogs include ABT-418, epiboxidine, and their derivatives.[15][18][19][20][21] A synthesis of epibatidine, utilizing a microbial hydroxylation of an unactivated carbon in a 7-azanorbornane was published in 1999.[22]
Epibatidine has two mechanisms of action: it can bind to either nicotinic acetylcholine receptors (nAChR) or muscarinic acetylcholine receptors (mAChR). Specifically, the analgesic 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)[24] The rank order of affinities for the muscle nicotinic receptors is αε > αγ > αδ.[25]
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.[26] They propagate neurotransmission in the central and peripheral nervous system. 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 and norepinephrine, resulting in an antinociceptive effect on the organism.
The usual neurotransmitter for nAChR is acetylcholine. However, other substances (such as epibatidine and nicotine) 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 as well as efficacy (and thus also potency) are much higher than for nicotine.[10]
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 (-)-enantiomers of epibatidine are biologically active, and both have similar binding affinities to nAChRs[10] 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.[27]
Epibatidine has several toxic consequences. Empirically proven effects include splanchnicsympathetic nerve discharge and increased arterial pressure.[23] The nerve discharge effects can cause antinociception partially mediated by agonism of central nicotinic acetylcholine receptors at low doses of epibatidine; 5 μg/kg.[28] 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 and 13.98 μg/kg.[29] This makes epibatidine somewhat more toxic than dioxin (with an average LD50 of 22.8 μg/kg).[citation needed] 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 (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.[30]In vitro studies seem to suggest that epibatidine is hardly, if at all, metabolized in the human body.[31]
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.[10]
Epibatidine has a high analgesic potency, as stated above. Studies show it has a potency at least 200 times that of morphine.[10] As the compound was not addictive nor did it cause habituation,[citation needed], 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[28]), some epibatidine might bind to the muscarinic acetylcholine receptors and cause adverse effects, such as hypertension, bradycardia and muscular paresis.[23]
Compared to the gold standard in pain management, morphine, epibatidine needed only 2.5 μg/kg (11.98 nmol/kg) to initiate a pain-relieving effect whilst the same effect required approximately 10 mg/kg (35.05 μmol/kg) of morphine (approx. 2,900 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.[12]
The antidote to epibatidine is mecamylamine,[32] a nicotinic acetylcholine receptor antagonist that is non-selective and non-competitive.[33] Both the (+) and the (-) enantiomers of mecamylamine were seen to be efficient and both have the same affinity for nicotinic acetylcholine receptors.[34]
^Huang DF, Shen TY (1993). "A versatile total synthesis of epibatidine and analogs". Tetrahedron Lett. 34 (28): 4477–4480. doi:10.1016/0040-4039(93)88063-o.
^ abDowd MJ. "Epibatidine". Department of Medicinal Chemistry. Virginia Commonwealth University. Archived from the original on December 5, 2010.
^"Epibatidine". School of Chemistry. University of Bristol. Retrieved 2021年11月12日.
^Rizzi L, Dallanoce C, Matera C, Magrone P, Pucci L, Gotti C, et al. (August 2008). "Epiboxidine and novel-related analogues: a convenient synthetic approach and estimation of their affinity at neuronal nicotinic acetylcholine receptor subtypes". Bioorganic & Medicinal Chemistry Letters. 18 (16): 4651–4654. doi:10.1016/j.bmcl.2008年07月01日6. hdl:2434/59291. PMID18644719.
^Dallanoce C, Matera C, De Amici M, Rizzi L, Pucci L, Gotti C, et al. (July 2012). "The enantiomers of epiboxidine and of two related analogs: synthesis and estimation of their binding affinity at α4β2 and α7 neuronal nicotinic acetylcholine receptors". Chirality. 24 (7): 543–551. doi:10.1002/chir.22052. PMID22566097.
^Dallanoce C, Matera C, Pucci L, Gotti C, Clementi F, Amici MD, Micheli CD (January 2012). "Synthesis and binding affinity at α4β2 and α7 nicotinic acetylcholine receptors of new analogs of epibatidine and epiboxidine containing the 7-azabicyclo[2.2.1]hept-2-ene ring system". Bioorganic & Medicinal Chemistry Letters. 22 (2): 829–832. doi:10.1016/j.bmcl.2011年12月05日2. PMID22222032.
^Dallanoce C, Magrone P, Matera C, Lo Presti L, De Amici M, Riganti L, et al. (December 2010). "Synthesis of novel chiral Δ2-isoxazoline derivatives related to ABT-418 and estimation of their affinity at neuronal nicotinic acetylcholine receptor subtypes". European Journal of Medicinal Chemistry. 45 (12): 5594–5601. doi:10.1016/j.ejmech.201009009. PMID20932609.
^Olivo HF, Hemenway MS (November 1999). "Total Synthesis of (+/-)-Epibatidine Using a Biocatalytic Approach". The Journal of Organic Chemistry. 64 (24): 8968–8969. doi:10.1021/jo991141q. PMID11674810.
^Sullivan JP, Bannon AW (1996). "Epibatidine: Pharmacological Properties of a Novel Nicotinic Acetylcholine Receptor Agonist and Analgesic Agent". CNS Drug Reviews. 2 (1): 21–39. doi:10.1111/j.1527-3458.1996.tb00288.x.
^Watt AP, Hitzel L, Morrison D, Locker KL (October 2000). "Determination of the in vitro metabolism of (+)- and (-)-epibatidine". Journal of Chromatography A. 896 (1–2): 229–38. doi:10.1016/s0021-9673(00)00597-5. PMID11093658.
^Damaj MI, Creasy KR, Grove AD, Rosecrans JA, Martin BR (November 1994). "Pharmacological effects of epibatidine optical enantiomers". Brain Research. 664 (1–2): 34–40. doi:10.1016/0006-8993(94)91950-x. PMID7895043. S2CID46489298.
^Bacher I, Wu B, Shytle DR, George TP (November 2009). "Mecamylamine - a nicotinic acetylcholine receptor antagonist with potential for the treatment of neuropsychiatric disorders". Expert Opinion on Pharmacotherapy. 10 (16): 2709–2721. doi:10.1517/14656560903329102. PMID19874251. S2CID25690407.
