Gelsemine

Chemistry

Gelsemine was isolated from G. sempervirens Ait., in 1876. Its chemical formula was determined to be C₂₀H₂₂N₂O₂, thus with a molecular weight of 322.44 g/mol. Its structure was finally determined, by X-ray crystallographic analysis and by nuclear magnetic resonance (NMR) spectroscopy, in 1959.

It is an monoterpenoid type of indole alkaloid, and a close relative of the natural product gelseminine, which is also present from the same natural sources. The gelsemine class of alkaloids are some of a wide variety of the alkaloid and other natural products that have been isolated from this genus of plants.

Gelsemine's biosynthesis, as of 1998, is thought to proceed from 3α(S)-strictosidine (isovincoside), the common precursor for essentially all monoterpenoid indole alkaloids—itself deriving directly from mevalonic acid-derived secologanin and tryptamine. From strictosidine, the biosynthesis proceeds through five intermediates—including koumicine (akkuammidine), koumidine, vobasindiol, anhydrovobasindiol, and gelsenidine (humantienine-type). The related alkaloids koumine and gelsemicine also derive from this pathway (koumine from anhydrovobasindiol via oxidation and rearrangement, and gelsemicine from gelsemine itself, via aromatic oxidation and O-methylation). For the chemical synthesis (natural product synthesis, studies and total synthesis), see the separate section below.

Summary of biological activities

Full sections in following are devoted to specific activities of gelsemine. Noted are the facts that it is a highly toxic compound, where exposure can result in paralysis and death. It is reported to be a glycine receptor agonist with significantly higher binding affinity for some of these receptors than its native agonst, glycine. In addition, it has been shown to have effects on pathways/systems in model animals (rat, rabbit), related to xenobiotic- or diet-induced oxidative stress, and in the treatment of anxiety and other conditions.

History

Gelsimium extracts, and so gelsemine, indirectly, have been the subject of serious scientific study for over a hundred years. On the medical side, gelsemium tinctures were used in the treatment of neuralgia by physicians in England, in the late 19th century; Arthur Conan Doyle, the noted author who first trained as a physician, after observing the successful of such treatments, ingested increasing doses of a tincture daily, to “ascertain how far one might go in taking the drug, and what the primary symptoms of an overdose might be,” submitting his first career publication on this in the British Medical Journal.^{[primary source]} On the chemistry side, the December 1910 meeting of the Division of Pharmaceutical Chemistry, of the American Chemical Society, reports among the papers read, the "Assay of Gelsemium" by L.E. Sayre.^{[primary source]}

Mechanisms of action

Gelsemine is an agonist for the glycine receptor (GlyR) with a much greater affinity for studied examples of this receptor than glycine. When glycine receptors are activated, chloride ions enter the neuron causing an inhibitory postsynaptic potential, which, systemically, leads to muscle relaxation.^{[primary source]}

Toxicity and toxicology

The plant Gelsemium sempervirens is known to be toxic, and there are serious safety concerns with regard exposure to it. Exposure resulting in toxicity can be via ingestion or injection (subcutaneous or intraperitoneal). It has been shown to have LD₅₀s of 49 mg/kg intraperitoneal in mice, and of 0.1 mg/kg subcutaneous in rabbits. Its high toxicity has prevented study to determine its LD₅₀ in humans.‹See TfD› The sap of the plant may cause skin irritation in sensitive individuals, and there are reports that inhalation from the flowers alone may, in some cases, lead to human poisoning (see below, where insect death at such flowers is likewise reported).

The plant's herbage is known to contain several toxic alkaloids, and while there is report of its feeding to pigs, it is generally considered to be an abortifacient and lethal poison when livestock or other animals feed on its leaves. It has been reportedly used as a fish poison as well, e.g., on the island of Borneo.

Human poisonings are known, including pediatric and adult cases, and the case of adults, both accidental and intentional poisonings. At lower doses in humans, the inhibitory postsynaptic potential induced by gelsemine action at the glycine receptor can result in nausea, diarrhea and muscle spasms caused by loss of involuntary muscle control; at higher doses, vision impairment or blindness, paralysis, and death can occur. Children, mistaking the flower of G. sempervirens for honeysuckle, have been poisoned by sucking the nectar from the flower; its ingestion has been associated with honey bee (but not bumble bee) fatalities as well (e.g., in the southeast U.S.). It has been reportedly used, via ingestion or smoking, as a poison in cases of suicide, in China, Vietnam, and Borneo.

Treatment

Gelsemine is a highly toxic and therefore possibly fatal substance for which there is no antidote, but the symptoms can be managed in low dose intoxications. In the case of an oral exposure a gastric lavage is performed, which must be done within approximately one hour of ingestion. Activated charcoal is then administered to bind the free toxin in the gastrointestinal tract to prevent absorption. Benzodiazepine or phenobarbital is also generally administered to help control seizing, and atropine can be used to treat bradycardia. Electrolyte and nutrient levels are monitored and controlled.

In the case of a skin exposure, the area is washed wish soap and water for 15 minutes to avoid skin damage.

While there is no current treatment to reverse the effects of gelsemine poisoning, preliminary research has suggested that strychnine has potential therapeutic applications due to its antagonistic effects at the glycine receptor.^{[primary source]}

Chemical synthesis

The chemical synthesis of gelsemine has been an active target of interest since the early 1990s, given its place among the alkaloids, and its complex structure (seven contiguous stereocenters and six rings). Its first racemic total synthesis was in 1994, by W.N. Speckamp's group, with a remarkable first yield of 0.83% (given the subsequent range, prior to 2104, of 0.02-1.2%).

Eight further total syntheses have been reported in the literature, including from the groups of A.P. Johnson in 1994, T. Fukuyama in 1996 and again in 2000, D.J. Hart in 1997, L.E. Overman in 1999, S.J. Danishefsky in 2002, and Y. Qin in 2012, with the latter Fukuyama group synthesis (31 steps, 0.86%) and the Qin group synthesis (25 steps, 1%) being asymmetric. A further asymmetric synthesis using an organocatalytic Diels–Alder approach from the F.G. Qiu and H. Zhai groups in China, reporting a remarkable 12 steps and a 5% yield, was reported in 2015.

Modern medical utility

Pharmacological research has suggested gelsemine activities to have potential related to the treatment of anxiety, and in treatments of conditions involving oxidative stress. In addition, gelsemine has been noted to have anti-inflammatory and anti-cancer activities. Recent research on gelsemine has included investigations aimed at developing safer gelsemine analogs and derivatives that might allow safe application of the compounds beneficial effects.

The identified anti-anxiolytic effects of preparations derived from Gelsemium sempervirens are believed due to in largest part to the presence of gelsemine in such preparations. Based on a rat study, use of gelsemine has been reported as being potentially effective, where the comparison was to treatment with Diazepam.^{[primary source]}

Gelsemine has been suggested to have potential in offering protective effects against oxidative stress. In a small rat study, the off-target effects of cisplatin—nephrotoxicity arising from its induction of pathways that generate reactive oxygen species, a factor impacting its use in cancer treatment—were examined, and gelsemine was found to significantly attenuate cisplatin-induced damage to DNA, and further general damage due to oxidative mechanisms. Inhibition of xanthine oxidase and lipid peroxidation activities were noted, along with "increased production and/or activity of anti-oxidants, both enzymatic... and non-enzymatic...".^{[primary source]}

In a small rabbit study, the impact of gelsemine administration on parameters relating to diet-induced hyperlipidemia was examined, where gelsemine was observed to improve lipid profile parameters associated with hyperlipidemia to a significant extent, as well as to "decreas[e] hyperlipidemia-induced oxidative stress in a dose-dependent manner," as determined by altered activities of a number of relevant metabolite and enzyme activity levels. The results, taken together, led the study authors to conclude that supplements of gelsemine to animals exposed to high fat diets may be of use in reversing the effects and in protecting tissues from oxidative stress resulting from such diets.^{[primary source]}

Gelsemine has been observed to have anti-inflammatory activity.^{[primary source]}

Gelsemine has been observed to have anti-cancer activity.^{[primary source]}

Traditional medical uses

Preparations from the plant from which gelsemine derives, Gelsemium sempervirens, have been used as treatment for a variety of ailments, for instance, through use of Gelsemium tinctures. Applications have included treatment of acne, anxiety, ear pain, migraine, and more generally with diseases associated with an inflammatory response, and in cases of abnormal nervous function (paralysis, “pins and needles” feeling, neuralgia, etc.).^{[primary source]}

Popular culture

Gelsemine is used indirectly, via use of "yellow jasmine", in the 1927 Agatha Christie novel, The Big Four, where an injection of this natural preparation is used to kill the character, "Mr Paynter." It is then used directly, in 2013, as gelsemine, in series 13 of the ITV series, Agatha Christie's Poirot, as the agent to immobilize the character "Stephen Paynter" (played by Steven Pacey) before his being burnt to death, thus implicating character "Madame Olivier," a research neuroscientist (played by Patricia Hodge); and also, directly, to paralyze and immobilise Olivier and another character after their kidnappings.