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| Other names | C-Toxiferine I C-Toxiferin I Toxiferine I |
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| Formula | C40H46N4O2 |
| Molar mass | 614.834 g·mol−1 |
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Toxiferine, also known asc-toxiferine I, is one of the most toxic plantalkaloids known. It is derived from several plant species, includingStrychnos toxifera. Historically, it has been used as anarrow poison by indigenous peoples in South America for itsneuromuscular blocking properties, allowing them to paralyze animals during hunting, but also possibly kill due to paralysis of therespiratory muscles.[1] Toxiferine functions as anacetylcholine receptor (AChR)antagonist. The paralysis caused by toxiferine can in turn be antagonized byneostigmine.[2]
Toxiferine is the most important component incalabashcurare. Curare poisons contain many different toxins with similar properties of toxiferine. The most well known component of curare istubocurarine. The paralysis caused by toxiferine is very similar to that caused by tubocurarine, however toxiferine is ~170 times as potent.[3] The preparation of curare poisons involves complex rituals wherein the tribes extract toxins from various plants.
Curare was discovered in 1595, however toxiferine was only first isolated and characterized in 1941 byWieland, Bähr andWitkop.[4] They managed to produce only a couple micrograms of this compound as it is quite hard to isolate in large enough quantities to study. This is due to the complexity of curare as it is composed of many differentalkaloids. In 1949 King was able to isolate 12 different types of toxiferines (I to XII).[5] In 1951 some of these toxiferine types were analyzed for theirtoxicological andpharmacological properties. These types were found to differ slightly in structure and theirpotency.[3]
Curares liketubocurarine were later used asanesthetics in medical procedures, but were replaced in the 1960s by synthetic curare-like drugs likealcuronium,pancuronium,atracurium andvecuronium. These drugs were safer to use as they had a shorterduration of action and less side effects.[6]
After the replacement of curares by these synthetic alternatives, research on toxiferine declined as it was hard to isolate from calabash curare and better alternatives to curares had been found thus decreasing the interest in researching this specific compound. However, curares as a whole have been (and still are) extensively researched.[6]
Toxiferine is most commonly known for its use as anarrow poison alongside othercurares bysouth american tribes. It is extracted from plants, likestrychnos toxifera andchondrodendron tomentosum. It is hard to extract in large quantities. It is very toxic however, so small quantities will suffice in paralyzing or killing animals while hunting.[3] It could be used as ananesthetic in medical procedures, however it has a very longduration of action which doesn't make it suitable for such procedures. It is also unstable in solution, which further prevents its use in medical settings. Synthetic alternatives likealcuronium can and are still used in anesthetics due to their relatively shorter duration of action and fewer side effects.[7]
Toxiferine is especially useful as an arrow poison because of its very minimal absorption throughoral ingestion. Which is why it is safe to eat the animal after it has been shot with an arrow covered in toxiferine. It is also believed that because of its activity as muscle paralyzer, it can retainglycogen andATP from releasing after death and by this delayrigor mortis. This makes the meat more tender for longer and maintains its flavor.[8]
Toxiferine I competes withacetylcholine, aneurotransmitter, for binding to thenicotinic acetylcholine receptors on thepost-synaptic membrane of theneuromuscular junction. By binding to these receptors, toxiferine I prevents acetylcholine from attaching to them. This inhibition blocks the ion channels associated with these receptors from opening, thereby preventing the influx ofsodium ions into the muscle cell. The prevention of sodium influx leads to an inhibition ofdepolarization of the post-synaptic membrane, which is a necessary step formuscle contraction. Without depolarization, the muscle fiber cannot generate anaction potential, resulting in muscle paralysis.
Toxiferine I is a potentantagonist for severalacetylcholine receptors, but especially potent formuscle-type nAChR:[9]
| Compound | nicotinic acetylcholine receptor (muscle-type nAChR) Ki (nM) | alpha-7 nicotinic receptor (α7 nAChR) IC50 (nM) | muscarinic acetylcholine receptor M2 (allosteric site) EC0.5,diss (nM) | |||
|---|---|---|---|---|---|---|
| toxiferine I | 14 | 9500 | 96 | |||
| alcuronium | 234 | 4100 | 2 | |||
Similar to alcuronium, toxiferine is classified as anon-depolarizing neuromuscular-blocking drug. These are drugs that inhibitsignal transduction bycompetitive inhibition of in this case mainly muscle-type nAChRs.[10] Toxiferine though binds 17 times stronger to muscle-type nAChRs than its pharmacological analogue alcuronium.[9] Thequaternary ammonium salt that toxiferine and its analogues share withacetylcholine is thought to be the reason for the binding affinity to the AChRs. The exact reason for the especially high binding affinity of toxiferine to for example muscle-type nAChRs is unknown. There have been attempts at understanding the exact binding of toxiferine in nAChRs, but the models are dated.[11][12]
Neostigmine is known to be effective at reversing the competitive inhibition of toxiferine and its analogues.[13] Neostigmine works byinhibiting acetylcholinesterase, increasing the acetylcholine concentrations so it can compete more with the non-depolarizing neuromuscular-blocking drug. By this toxiferine can be freed into the circulation for excretion.[14]
Toxiferine I is anindole alkaloid derived fromtryptamine. It has adimeric structure with eachmonomer containing aquaternary ammonium salt. The parent structure, without counter ions, has the molecular formula C40H46N4O22+,[15] while the dichloride salt has the molecular formula C40H46N4O2Cl2. Alkaloids are naturally occurring compounds that are basic and contain at least one nitrogen atom.[16] Toxiferine is classified as a dimeric bisindole alkaloid because it is symmetrically constructed from two identical monomeric units, each containing an indole ring.[17][18]
A dozen different types of toxiferine were found to exist (designated as toxiferine I to XII),[5] but the different structures of toxiferine II till XII have not been studied in great detail. Toxiferine does have multiple analogues which are researched extensively. Some of these include: bisnortoxiferine, caracurine V and alcuronium. Caracurine V is, like toxiferine, another naturally occurring curare toxin from the strychnos toxifera.[19] Caracurine V, unlike the other analogues, has a closed ring formed between thehydroxyl groups and the middle ring. Though the main difference between the analogues are the side groups attached to the positive nitrogen atom, also called thequaternary ammonium ion. The main target of toxiferine and its analogues areacetylcholine receptors. It is this quaternary ammonium ion that both toxiferine and its analogues share withacetylcholine that gives them their specific affinity for these receptors. The difference in hydroxyl side groups together with most importantly the quaternary ammonium side groups is believed to cause the variability inreceptor affinity andmetabolic activity and with this a variability in the toxic properties of these analogues.[9] From this it can be concluded that the side groups of toxiferine, mainly those attached to the quaternary ammonium have large effect over its reactivity and affinity, but to this day no research exists that can reliably explain the structural reactivity of toxiferine.
Toxiferine is an indole alkaloid. Indole alkaloids are the largest among the alkaloids. They are primarily synthesized usingtryptophan.[20][21] The dimeric subunits of toxiferine bear high similarity tostrychnine and may be a related product of its biosynthesis. Especially the intermediateWieland-Gumlich aldehyde is very similar to the dimeric subunits of toxiferine, though this lacks the quaternary ammonium ion. Thebiosynthesis of strychnine was solved in 2022.[22] It is also possible toartificially synthesise strychnine. The exact biosynthetic and possible artificial ways to synthesise toxiferine are not known.
It is believed though that toxiferine may bebiosynthetically derived from themonoterpenoid indole alkaloidstrictosidine. Strictosidine in turn is derived from the alkaloidtryptamine and the terpenesecologanin through the action ofstrictosidine synthase.[23]
This describes theADME of toxiferine.
Toxiferine is known to enter the body in two different ways: eitherorally by ingestion orintravenously while applied to a sharp tip for killing purposes. When orally ingested, toxiferine is only absorbed minimally into theplasma and is not known to be dangerous even though it has a very high potency. Intravenous absorption is thus the only way of effective administration.[6]
The distribution of toxiferine has been researched inrats. Toxiferine distributes through the body in a way that is similar to other non-depolarizing curare alkaloids. Toxiferine is a highlywater soluble substance and because of this also generally notlipophilic. Toxiferine does not easily pass theblood-brain barrier. It is mostly retained inmotor endplates and thesciatic nerve where it binds to specific receptors. It was also found that toxiferine is distributed to tissues with a high acidicmucopolysaccharide content likeintervertebral discs andcartilage of the ribs.[24] Alkaloids are known to have affinity for such polysaccharides at acidic extracellular pH.[25]
Toxiferine does not have any knownmetabolic pathways. This may be due to its low lipophilicity. Bis-quaternary nitrogen compounds like toxiferine have shown to be dependent on their lipophilicity to be transported to sites in theliver.[26] The liver, which is the main site of metabolic activity, can thus not or hardly be reached. When comparing the metabolic activity of toxiferine to that of maybe its most important analogue alcuronium, it is observed that theallylic side chains ofalcuronium make it more potent forbiotransformation than toxiferine with itsmethyl side chains. This makes theduration of action of toxiferine significantly longer when compared to alcuronium.[27]
Toxiferine is mainly excreted in theurine even thoughelimination of toxiferine by thekidney is very poor relative to its analogue alcuronium. Toxiferine has a strong receptor affinity, which together with the poor excretion makes it accumulate in the body rapidly after repeated administration.[24][28] This is another reason why toxiferine has an especially long duration of action in the body.
The effects of toxiferine have been studied in multiple organisms likerhesus monkeys,guinea pigs andmice byintravenous (IV) andintramuscular (IM) injections of different doses of toxiferine. The data on rhesus monkeys likely resembles human effects more closely.
| ED50 (μg/kg) | LD50 (μg/kg) | Therapeutic index (LD50/ED50) | Margin of safety (LD1/ED99) | Time till onset (min) | |
|---|---|---|---|---|---|
| IV | 5.5 | 8.9 | 1.61 | 1.33 | <5 |
| IM | 6.5 | 17.8 | 2.74 | 1.89 | <15 |
It has to be said that the duration of paralysis varies a lot between different individual monkeys and doses. It could be as short as 6 minutes but also as long as 85 minutes.[29] In mice the LD100 was determined to be 23 μg/kg with a duration of paralysis around 12 minutes.[7]
