Conotoxin - an overview | ScienceDirect Topics

Chapters and Articles

You might find these chapters and articles relevant to this topic.

3.7 Conotoxins

Conotoxins also called conopeptides are small disulfide-rich peptides-based toxins found in the venoms of marine cone snails, which are carnivorous and predatory gastropod molluscs that include Conus geographus, Conus catus, Conus aulicus, Conus omaria, and Conus textile. Conotoxins have a wide range of targets. They are able to affect various neurotransmitter receptors (nicotinic, adrenergic, NMDA, and serotonergic) and ion channels (sodium, potassium, and calcium) in the body (Daly and Craik2009; Schroeder and Lewis,2006). Venomous cone snails use potent cocktail of venom components to defend themselves against foes and to capture and digest preys. Cone snails could be: piscivorous, molluscivorous, or vermivorous according to their diet. They have developed successful defense strategy, including a hollow, harpoon-like, radular tooth related to a venom gland, which secretes potent toxins targeted to the nervous system and musculature of the prey. The injected venom rapidly enters the circulatory system and causes paralysis in a short time (Peng etal.,2016). Cone snails are deadly to humans in which they provoke respiratory arrest by muscle paralysis of the diaphragm. A sting by cone snails can cause numerous symptoms, such as faintness, poor coordination, areflexia, paresthesia, blurred/double vision, speech difficulties, difficulty swallowing, weakness, nausea, and respiratory arrest. Death can occur within 1–5h and treatments consist of monitoring vital signs, respiratory support, and intubation (Anderson and Bokor,2012).

Read full chapter

URL: https://www.sciencedirect.com/science/article/pii/B9780128114421000031

Discovery of Conotoxins

The first conotoxins were purified and characterized from the venom of Conus geographus, a dangerous species of Conus that has caused human fatality. The first peptides investigated caused paralysis in either fish or mice and were shown to target ion channels that are important for neuromuscular transmission.21 The discovery by Craig Clark, then a 19-year-old student at the University of Utah that the different components of C. geographus venom elicited different behavioral phenotypes when injected into the central nervous system of mice led to the characterization of a larger group of Conus peptides, mostly from the venoms of fish-hunting cone snails. These had biological activity on mammalian neurons,22 and some were subsequently developed for direct therapeutic application. One peptide from Conus magus, ω-conotoxin MVIIA, was purified because it caused tremors in mice; it was approved as a drug for severe pain in December 2004 (commercial name Prialt).19,34

Initially, the discovery of each new conotoxin required biochemical purification from venom. Unusual posttranslationally modified amino acids were identified in the first group of conotoxins characterized.7 As more cDNA clones encoding conotoxins were elucidated, a molecular cloning/PCR approach for identifying novel conotoxins became feasible; the first venom peptide characterized solely on the basis of the predicted sequence from a cDNA clone was ω-conotoxin MVIIC.12 However, a problem with this approach is that if the conotoxin is posttranslationally modified, knowing the encoding DNA sequence does not necessarily permit an accurate prediction of which amino acids are posttranslationally modified.

As detailed below, it is highly probable that there are well >50,000 different peptides in the venoms of living cone snails, with an ever accelerating rate of discovery of new peptides. At this time, there are probably amino acid sequences of >2500 Conus peptides available, but of these, only a small minority have been biochemically characterized, and the mechanism of action of a miniscule fraction of those has been elucidated.

Read full chapter

URL: https://www.sciencedirect.com/science/article/pii/B9780123850959000610

INTRODUCTION

The primary biologic activity of conotoxins occurs through interactions with specific ion channels. Most commonly, these peptides modulate voltage-gated channels and inhibit the entrance of Na+, K+, and Ca2+ ions into cells. Conotoxins are among estimated 100,000 small, disulfide-rich peptide venoms produced by predatory cone snails (genus Conus). The remarkable diversity of pharmacologic structure, function, and utility has been recently reviewed.1 The typical physiologic role of voltage-gated ion channels is the production, modulation, and transduction of the electrical signals. Thus, the potential clinical utility of these agents would be alteration of biologic processes that depend upon electrical signaling. Human disease states that have been explored as conotoxin targets include pain, stroke, brain injury, mental illness, addiction, and epilepsy.

At present, only a single conotoxin has received approval from the U.S. Food and Drug Adminstration (FDA) for therapeutic use. Ziconotide, a synthetic form of ? conotoxin MVIIa, is marketed by Élan pharmaceuticals under the name Prialt.2 This compound was originally isolated from the venom of the cone snail Conus magus. This drug was also known as SNX-111 when it was an investigational pharmaceutical by Neurex Corporation. The indication for this agent is severe chronic pain in patients for whom intrathecal therapy is warranted, and who are intolerant of or refractory to other treatments, such as systemic analgesics, adjunctive therapies, or intrathecal morphine.3 The majority of this chapter will focus on the use of the medication with a smaller discussion reserved for conotoxins in clinical development.

Ziconotide is a 25 amino acid peptide that blocks a neural specific calcium channel on small myelinated and unmyelinated nociceptive afferents that are primarly localized in the superficial Rexed laminae (I and II).4 The analgesic effect of ziconotide is produced when this channel blockade results in diminishing neurotransmitter release from these primary nociceptive afferents. Thus, the transmission of the pain signal never crosses the synaptic cleft from the primary afferents to the second-order neurons.

One of the clinical challenges with ziconotide use is need for chronic spinal exposure of the agent in order to achieve therapeutic efficacy. Oral intake of ziconotide results in rapid breakdown into inactive peptides. Vascular exposure of ziconotide produces hypotension in rats and humans, probably due to cross-reactivity to vascular calcium channels. Intravenous administration of ziconotide was also found to be neuroprotective in brain ischemia models which lead to the some early investigations for this agent in acute stroke and head injury.5 Epidural administration of this agent did not result in consistent therapeutic efficacy. Thus, ziconotide is indicated for use only via an intrathecal delivery system. This agent has been formally evaluated with both temporary (CADD-Micro ambulatory infusion pump, Smiths Medical MD, Inc., St. Paul, MN) and permanent (SynchroMed EL and SynchroMed II Infusion System, Medtronic Inc., Minneapolis, MN) intrathecal pumps.5

Ziconotide's role in chronic pain management is evolving. This agent is entirely different from all other medications that are routinely used for intrathecal therapy. While several agents are commonly used,6,7 only three agents currently have FDA approval for intrathecal use: baclofen, morphine and ziconotide. Baclofen, a GABA-B agonist, is the mainstay of intrathecal therapy for spasticity and has FDA approval for spasticity of both cerebral and spinal origin. Morphine, considered by many clinicians to be the first-line agent for intrathecal pain management, has been approved by the FDA for clinical use for more than decades. Intraspinal opiates, such as morphine and hydromorphone, are thought to exert their therapeutic effect presynaptically by inhibiting calcium ion influx and postsynaptically by increasing potassium flux.8 Other commonly used agents for chronic intraspinal infusion include bupivacaine and clonidine. Treatment algorithms for use of these agents, both in monotherapy and combination therapy, are far from standardized. The highest level of evidence-based decision making for intrathecal pain therapy is consensus statements of experienced clinicians. When the last consensus statement was produced in 2004, ziconotide was still an investigational agent. Accordingly, ziconotide was not placed within the treatment algorithm.6 In January 2007, a consensus of experts again convened to review the current medical literature and put forth updated guidelines for intrathecal pain therapy. The 2007 consensus guidelines have recently been published with ziconotide monotherapy being put forth as a first-line agent with combination therapy of ziconotide and other agents attaining secondary indications.9

There are two major clinical trials that support a positive therapeutic effect of intrathecal ziconotide. Staats and colleagues11 explored the use of this agent in 111 patients with pain associated with cancer or acquired immunodeficiency syndrome (AIDS) in a randomized, controlled, double-blinded trial. The pain diagnoses included in this trial were variable for both cancer-related pain (neuropathy, postherpetic neuralgia, pathologic fractures and complications of radiation therapy) and AIDS-related (peripheral neuropathy, Kaposi sarcoma and postherpetic neuralgia) pain syndromes. The study population is notable for a relatively high percentage of neuropathic pain, which is considered relatively challenging to treat with intrathecal therapy. Patients were required to either have a permanent SynchroMed intrathecal infusion system implanted or have a temporary external intrathecal infusion system. Individuals who were treated with ziconotide started at either 0.1 µg per hour or 0.4 µg per hour. Patients had to rate their pain at least 50 mm of 100 mm line on a Visual Analog Score of Pain Intensity (VASPI). Dosing adjustments occurred every 12 to 24 hours based on the appearance of either adverse effects or pain relief. Intrathecal ziconotide exposure continued for up to 2 weeks. Patients were allowed to adjust their systemic pain regimens as needed but could not use any other intrathecal agent.

At the conclusion of the trial, the mean VASPI improved 53% for the ziconotide-treated group compared with 19% for the placebo group. Positive responders, defined as a 30% improvement in VASPI compared with baseline, was achieved in 50% of the active treatment group and in 17.5% in the placebo-treated group. Moderate to complete pain relief as measured by the Category Pain Relief Scale (CPRS) was achieved in 52.9 % of ziconotide-treated patients compared with 17.5 % of placebo-treated patients. Oral opioid consumption decreased 9.9% in the ziconotide-treated group, whrereas the placebo-treated group increased oral opioid consumption by 5.1%. Following the rapid titration phase of this study, patients treated with ziconotide were permitted to enter an extension maintenance phase, and placebo-treated patients were allowed to cross over to ziconotide therapy. All of the patients who continued ziconotide therapy maintained therapeutic efficacy, resulting in a 69.2% decrement in VASPI scores. Pain relief continued without dose escalation, which suggests that tolerance does not occur with this agent. Initial placebo patients experienced a 45% decrement in VASPI at the conclusion of the crossover phase. The positive therapeutic effect of ziconotide in this trial was statistically significant for both starting-dose subgroups (< 0.1 µg/h and > 0.1 µg/h) as well as for either disease subgroups (those with cancer or AIDS). No statistical significance in pain relief was seen with respect to age, gender or prior intrathecal therapy.

The adverse event profile for ziconotide therapy during this trial was significant. Seventy-one patients were initially assigned to receive ziconotide. Twenty-seven of these 71 individuals (38%) discontinued ziconotide therapy, in which 12 were discontinued due to adverse events (17%). The dropout rate for the placebo group was less compared with the active treatment group. Nine of the 40 subjects who were assigned to the placebo group dropped (22%), out of which 4 (10%) dropped out due to adverse events. During the titration phase, 22 of 72 patients (30.6%) exposed to ziconotide experienced a total of 31 serious adverse effects. In the opinion of the investigators, 14 of these 31 events were considered related to ziconotide treatment. All of these serious adverse events involved the nervous system with confusion, somnolence, and urinary retention being reported most commonly. Other common but less serious adverse events included dizziness, vomiting, postural hypotension, and fever. An interesting observation on these adverse events was that the appearance of adverse events occurred 2 to 3 days following a dosing adjustment. The central nervous system adverse events had a median time to resolution of 4 days (range 0 to 58 days) following a dosing decrement. These time lags supports the concept that ziconotide must penetrate neural tissue to exert an effect. A total of 15 patients died during or immediately after the study period. Death rates were not significantly different between the ziconotide- and placebo-treated groups. Twelve of the 15 deaths were due to the incident neoplasm. There were no reports of anaphylaxis or hypersensitivity to ziconotide.11

The other major controlled trial that explored the therapeutic benefit of ziconotide was performed Rauck and colleagues.12 This study reported on 220 patients with nonmalignant pain who were randomized to receive either ziconotide (112 subjects) or placebo (108 subjects). Similarly, the pain diagnoses were variable, but a high percentage of neuropathic pain patents were again enrolled (76% for the ziconotide group and 71% for the placebo group). In this study, patients were required to have an implanted SynchroMed infusion system before enrollment in the trial. Subjects were weaned from prior intrathecal medications before being exposed to ziconotide. Patients were required to have a VASPI score of greater than 50 mm at the conclusion of intrathecal weaning and to have a stabilized systemic analgesic regimen. The starting dose of ziconotide was 0.1 µg/h. The titration phase of this trial lasted for 3 weeks, during which dosing adjustments upward of 0.05 or 0.1 µg/h were performed until patients attained analgesia or reported intolerable side effects. At least 24 hours was required between each dosing increase, but downward adjustments were allowed at any time point to improve tolerability. Again, patients were allowed to adjust systemic medications as needed but could not use any intrathecal agent other than ziconotide.

During this trial, the primary outcome measure was improvement in VASPI scores. At the conclusion of the 3 weeks of titration, ziconotide-treated subjects demonstrated a statistically significant improvement in their VAPSI scores (14.7%) compared with placebo-treated subjects (7.2%). The onset of pain relief was observed among ziconotide-treated patients as early as Week 1, with a mean percent improvement on the VASPI of 16.6%, compared with a mean percent improvement of 5.0% among the placebo-treated patients. At Week 2, the mean percent improvement in VASPI scores for the ziconotide-treated group (13.8%) was greater than that for the placebo-treated group (8.2%), but this difference was not statistically significant. Another measure of treatment response was the portion of subjects who demonstrated at least a 30% improvement in VASPI score. For this outcome measure, there was no significant difference between the ziconotide-treated group (16.1%) and the placebo-treated group (12.0%). On the clinician's global impression (CGI) satisfaction scale, 28.4% of ziconotide-treated patients reported “a lot” or “complete” satisfaction compared with 12.1% of placebo-treated subjects. On the CGI Overall Pain Control measure, 11.9% of ziconotide-treated subjects reported “very good” or “excellent” pain control at study termination, compared with 0.9% of placebo-treated patients. Both of these CGI measures were statistically significant. The change in the Global McGill Pain Relief total score was significantly reduced in the ziconotide group compared with that of the placebo group. The Categorical Pain Relief Scale (CPRS) at study termination showed a trend in favor of ziconotide, but the difference did not reach statistical significance. With regard to oral opiate consumption, there was a 23.7% mean decrease in weekly opioid use (in morphine equivalents) from pretreatment stabilization to Week 3 for the ziconotide-treated group compared with a 17.3% decrease in the placebo group. Similarly, this result demonstrated a trend that did not achieve statistical significance.

The adverse event profile observed during this trial was also considerable, with the majority of subjects in both treatment arms experiencing side effects (92.9% of ziconotide subjects and 82.4% of placebo subjects). The severity of adverse events reported was primarily mild or moderate (ziconotide, 83.6%; placebo, 83.8%). The spectrum of adverse effects in this trial was similar to that of the cancer/AIDS trial. The side effects that occur with statistically greater frequency in the ziconotide group included dizziness (47.3% versus 13.0%), confusion (17.9% versus 4.6%), ataxia (16.1% versus 1.9%), abnormal gait (15.2% versus 1.9%) and memory impairment (11.6% versus 0.9%). The incidence of serious adverse event was similar for both the ziconotide-treated and placebo-treated groups (11.6% versus 9.3%). There were no reported deaths during this trial, which supports the hypothesis that the deaths noted in the earlier trial were due to the disease processes and were not directly related to ziconotide treatment. The onset and resolution of adverse effects were delayed in a fashion similar to that of the Staats study. The median time to onset for the most commonly reported ziconotide-related side effects (dizziness, nausea, confusion, ataxia, and asthenia) ranged from 3.0 to 9.5 days. The median time to resolution for most of these adverse events was within 1 to 2 weeks of drug discontinuation.12

As a result of the above-mentioned factors as well as other trials with ziconotide, this agent was approved by the FDA in December 2004. This event represents the first intrathecal agent for pain control that has successfully passed the high standards of the approval process. Morphine was approved more than a decade ago, but it was “grandfathered” in the FDA. Intrathecal baclofen is an FDA-approved intervention for spasticity. It is estimated that more than 1500 patients over 12 years have been exposed to ziconotide. During this exposure, there have been no permanent sequelae that have been unequivocally linked to the drug. Also, no withdrawal or abstinence syndrome following sudden cessation of drug delivery has been reported with this agent. This feature is distinctly different from intrathecal opiates, which can have a profound withdrawal syndrome.

One important observation during the clinical trials as well as after approval surveillance is that the incidence of ziconotide-related side effects corresponds not only to total daily dose but the titration rate at which a particular dose is achieved. Figure 37-1 demonstrates the adverse event profile during two early ziconotide clinical trials. The majority of side effects demonstrated an increased incidence during the “fast” trial (mean daily dose of 36 µg per day achieved in 5–6 days) compared qirh the “slow” trial (mean daily 7.2 µg per day achieved in 28 days). The current package insert describes a titration schedule as follows: starting dose of no more than 2.4 µg/day, upward titration by up to 2.4 µg/day at intervals of no more than 2 to 3 times per week, up to a recommended maximum of 19.2 µg/d (0.8 µg/h) by Day 21. Even this titration schedule, which is slower than the initial clinical trials, has been considered excessively rapid by experienced clinicians. One small group of experts has put forth the following titration recommendations: starting dosage of 0.5 µg/d, with increases of no more often than weekly.13

Another challenge with the use of ziconotide is the issue of stability. In contrast to opiates and other commonly used intrathecal agents, which have acceptable stability profiles,14 ziconotide is susceptible to breakdown at a rate that is clinically significant. All patients with implanted intrathecal delivery systems require refill of the pump reservoir to maintain a constant delivery of medication. Typically, refill intervals are between 2 and 6 months.10 The phyical stability of the ziconotide molecule can potentially interfere with refill frequency. Ziconotide degradation occurs primarily through oxidation to the methionine sulfoxide form of the drug. The breakdown products of ziconotide are both nontoxic and inactive. Two accelerants of the oxidation process are (1) heat and (2) the presence of dissolved oxygen in a solution. For these two reasons, ziconotide is stored in a refrigerator and is package in a methionine buffer. Using these strategies, ziconotide should be capable of maintaining acceptable stability for clinical use. Undiluted ziconotide solutions maintain greater than 90% stability for approximately 84 days.15

An increasing concern with long-term infusion of intrathecal agents is the development of inflammatory, noninfectious mass at the tip of the intrathecal cathter. Although the exact pathophysiology of these massses is unknown, there are several causally linked factors including the medication infused, catheter position, low cerebrospinal fluid volume, and the dose and concentration of the infused drug. Granuloma development is the most serious adverse effect associated with intrathecal pain therapy, with some clinicians considering this issue the most significantly barrier to patient access.9 To date, no granulomas have ever been detected in patients exposed to intrathecal ziconotide. Although this may represent a therapeutic advantage of this agent, it is important to acknowledge that the relatively low prevalence of ziconotide use and the rarity of granuloma development preclude a definite statement that use of this agent is not associated with granuloma development.

Many patients are exposed to ziconotide by converting the agents in their currrent intrathecal pump as opposed to having ziconotide as their initial intrathecal agent. If a patient has intrathecal opiates in his or her pump and a decision is made to attempt intrathecal ziconotide therapy, then weaning from the opiates should occur. Weaning of patients from intrathecal opioid therapy can be challenging. Many clinicians mistakenly adhere strictly to the belief that any decrease in the intrathecal opioid dose should be replaced with equianalgesic doses of systemic opioids. Various equianalgesic oral to intrathecal dosing ratios have been suggested, ranging from 12:1 to 300:1.16 These equianalgesic doses are typically inappropriate for weaning from chronic intrathecal opioid therapy, and patients can be weaned without strict adherence to equianalgesic conversion ratios. Patients may experience opioid withdrawal during the weaning process. Signs and symptoms may include lacrimation, rhinorrhea, yawning, insomnia, restlessness, mydriasis, nausea, vomiting, diarrhea, piloerection, abdominal cramps, anxiety, agitation, muscle twitching, diaphoresis, palpitations, flu-like symptoms, hypertonia, and increased pain.17 During some of the ziconotide trials described earlier, intrathecal opiates weaning periods varied from 2 to 8 weeks. A descriptive method for intrathecal opiate weaning has recently been published.18

Read full chapter

URL: https://www.sciencedirect.com/science/article/pii/B9781416049289000378

Publisher Summary

This chapter discusses ω-conotoxin and its receptors. ω-Conotoxin GVIA first described as a toxin of Conus geographus produces persistent shaking when administered intracranially to mice, but does not interact with acetylcholine receptors or voltage-sensitive sodium channels. ω-conotoxin is reported to block both N- and L-type voltage-sensitive calcium channels in nerve preparations. Recent electrophysiological reports, however, have shown that the toxin blocks only N-type channels and does not affect L-type channels in neurons. The reason for these discrepant reports can be because L-type channel action cannot be separated sufficiently in previous studies, or because the sensitivities of L-type channels to ω-conotoxin differ in different cells. There are differences in the primary structures of dihydropyridine receptors in skeletal and cardiac muscles. Therefore, the toxin blocks the L-type channels of non-neuronal tissue but does not block those in neuronal cells. The low-affinity binding site of ω-conotoxin in rat brain cannot be of L-type voltage-sensitive calcium channels or dihydropyridine binding sites, because the distributions of the two clearly differ in brain regions.

Read full chapter

URL: https://www.sciencedirect.com/science/article/pii/B9780121852665500208

Functional Experiments

ω-Conotoxin is a potent inhibitor of neurally evoked smooth muscle contractions in various mammalian tissues. Stimulated responses in the vas deferens, urinary bladder, stomach fundus, small intestine, uterus, and small mesenteric arteries were blocked by ω-conotoxin, whereas end-organ responses evoked by applied agonists were not affected by the toxin (25–28). In most of these functional experiments ω-conotoxin GVIA has turned out to be a very effective tool: concentrations of 1–10 nM ω-conotoxin GVIA prevented neurally evoked end-organ responses. These results indicate the release of noradrenaline and acetylcholine from postganglionic autonomic neurons to be predominantly controlled by N-type calcium channels.

Read full chapter

URL: https://www.sciencedirect.com/science/article/pii/B9780121852665500130

1.5.2.8 δ-Conotoxins and δ-Toxins From Spider Venom

The δ-conotoxins are another class of conotoxins that act on sodium channels, but unlike the μ and μO-conotoxins previously described, they cause slowing of NaV channel inactivation, similar to toxins that bind to receptor site 3 (Leipold, Hansel, Olivera, Terlau, & Heinemann, 2005). Despite similar activity to site 3 toxins, δ-conotoxins bind to an alternate neurotoxin site, designated site 6, which is made up of amino acid residues in DIV S4 (Fainzilber et al., 1994). Limited data are available on subtype selectivity of δ-conotoxins as they are highly hydrophobic and difficult to synthesize or express (Lewis et al., 2012); however, studies on δ-EVIA, which affects inactivation of NaV1.2, 1.3, and 1.6 but not 1.4 or NaV1.5, suggest that subtype-selective NaV modulation by the δ-conotoxins is possible (Barbier et al., 2004).

In addition, a recently described excitatory peptide from the vermivirous Conus suturatus, SuVIA, affects voltage dependence of activation at NaV1.7 with little effect on inactivation (Jin et al., 2015). This activity suggests a divergent binding site to other δ-conotoxins, and perhaps more appropriate classification of this peptide as an ιO-conotoxin based on its pharmacological effect and structural superfamily. Indeed, like ι-RXIA, SuVIA was most potent at NaV1.6, albeit full subtype selectivity remains to be determined.

Analogous to the δ-conotoxins are the δ-theraphotoxins, δ-actinopoditoxins, δ-ctenitoxins, and δ-hexatoxins from spider venoms. These peptides also act as gating modifier toxins that inhibit inactivation of mammalian voltage-gated sodium channels. Consistent with this activity, δ-toxins from spider venom are generally considered to act at site 3, interacting with amino acid residues located on the S1–2 and/or S3–4 linker of DIV, although experimental evidence for this has been obtained for only a few peptides (Corzo et al., 2003; de Lima et al., 2002; Gilles et al., 2002; Osteen et al., 2016). For example, Magi 4 (δ-hexatoxin-Mg1a) from Macrothele gigas displaced the radiolabeled site 3 toxin LqhαIT but not the site 6 toxin δ-conotoxin TxVIA (Corzo et al., 2003), as did δ-hexatoxin-Hv1a, albeit this toxin—in contrast to the α-scorpion toxins—does not distinguish between mammalian and cockroach channels but instead binds poorly to the locust NaV channel (Gilles et al., 2002). Interestingly, δ-hexatoxin-Hv1a and -Ar1a appear to stabilize distinct subconductance states as differential allosteric effects with the site 2 toxins batrachotoxin and veratridine were observed (Little, Wilson, et al., 1998; Little, Zappia, et al., 1998; Nicholson, Walsh, Little, & Tyler, 1998). At the organism level, δ-toxins from spider venom cause spastic paralysis of insects (Little, Wilson, et al., 1998; Nicholson, 2007), pain, as well as autonomic and somatic symptoms such as sweating, salivation, lacrimation, and muscle fasciculations that can be attributed to the prolongation of action potentials induced by these toxins (Nicholson, Little, & Birinyi-Strachan, 2004). Despite these profound effects on neuronal function, as well as interesting phyla-specific effects, detailed subtype selectivity studies have not been carried out systematically at the full NaV isoform panel. One notable exception is δ-theraphotoxin-Hm1a, which is a selective modulator of NaV1.1 channels (EC50 38 ± 6 nM) with at least fivefold selectivity over NaV1.2–NaV1.8 (Osteen et al., 2016).

Read full chapter

URL: https://www.sciencedirect.com/science/article/pii/S1054358917300042

V α-Conotoxins and the Nicotinic Acetylcholine Receptor Ion Channel

Although not targeted to ion channels, the α-conotoxins will be briefly discussed in the interest of covering all of the conotoxin classes. The α-conotoxins are the smallest of the conotoxin peptides known, comprising 13–15 amino acid residues, 4 of which are half-cystines involved in disulfide bonds. All of the known α-conotoxins are cationic, with formal charges of +1.5 to +3.5 at neutral pH. Sequences of the α-conotoxins are shown in Fig. 13. Conservative changes are tolerated at most positions without significant loss of activity, so that relatively little information on structure–function relationships can be derived from synthetic analogues. It is clear that the presence of two disulfides in such a short peptide imposes relatively severe constraints upon its three-dimensional structure. The proposed structure for α-conotoxin, based upon NMR measurements, includes β-turns at residues 5–8 and 9–12.

FIG. 13. Primary structures of α-conotoxins. The sequences are shown in the single-letter code, with invariant residues in boldface.

α-Conotoxins are directed at the nicotinic acetylcholine receptor channel and mechanistically act in the same manner as α-bungarotoxin. In fact, the most widely accepted model for α-conotoxin structure places several of its side-chain functional groups into positions very similar to those seen in the “active site” of bungarotoxin. The postsynaptic site of action of α-conotoxin is consistent with its observed ability to compete with other nAChR antagonists like d-tubocurarine and α-BgTX. On this basis, α-conotoxin is, strictly speaking, not truly directed against the ion channel, and will not be discussed further.

Read full chapter

URL: https://www.sciencedirect.com/science/article/pii/B9780126569704500348

Toxicokinetics

It is difficult to generalize about conotoxins because there are some 200 different conotoxins so far identified and it is estimated that there may be more than 50,000 in total. Each Conus species synthesizes 100–200 different conotoxins. However, in the clinical sense, effects of envenomation by conotoxins are generally felt within minutes. In survivors of severe envenomation, clinical signs may persist for weeks.

Read full chapter

URL: https://www.sciencedirect.com/science/article/pii/B9780124202276000220

ω-Conotoxins: Calcium Channel Blockers

The ω-conotoxins have six cysteine residues and belong to the O-superfamily, along with the ω-, δ- and μO-conotoxins. The ω-conotoxin GVIA (ω-GVIA) (also designated ω-CgTX in the neuroscience literature) isolated from Conus geographus57 is widely used as a pharmacologic tool, mainly to inhibit synaptic transmission; it is specific for N-type Ca2+ channels.58 In Conus magus venom, multiple ω-conotoxin isoforms were found: ω-conotoxin MVIIA (ω-MVIIA), which is highly specific for N-type Ca2+ channels (CaV2.2), and ω-conotoxin MVIIC (ω-MVIIC), which selectively targets the P/Q channels (CaV2.1).59,60 A characteristic feature of ω conotoxins is their high content of basic amino acid residues that have an essential role in the inhibition of Ca2+ channels.61 In addition to the positive charges, it is known that a tyrosine residue (Tyr-13 in ω-MVIIA and ω-MVIIC) is important for the binding to CaV2.2 and CaV2.1 channels.62-64 The ω-conotoxin CVID (ω-CVID) from Conus catus was shown to target an N-type Ca2+ channel in preganglionic nerve terminals.65 Another peptide, ω-conotoxin TxVII (ω-TxVII), targets the molluscan L-type Ca2+ channels.66 Several ω-conotoxins have been identified and functionally characterized; since this field has previously been reviewed comprehensively (for overviews, see references 67-69), the reader is referred to the earlier literature on ω-conotoxins.

Read full chapter

URL: https://www.sciencedirect.com/science/article/pii/B9781416049289000366

Conotoxins

Synthesizing Organism

Marine molluscs of the genus Conus (cone shells; cone snails), which includes more than 600 species. There are at least 70 species in Australian waters.

Distribution

Most cone shells are found in warm tropical waters with a few species adapted to temperate waters. Cone shells occur in the waters off both Australia and New Zealand but are more likely to be encountered in Australian waters. They are largely nocturnal but may be found in rock pools or burrowed in sand during the day.

Circumstances of Poisoning

Cone shells stab with a small chitin tooth of the radula, when picked up or disturbed. They are carnivorous and predatory, and use chemosensors on the siphon to detect potential prey. The cone shell fires the venom-loaded radula tooth into prey like a harpoon, by means of muscular contraction, and then retracts the radula to draw the prey in. Cone shells have multiple such “harpoons” and can “fire” in any direction. The attack is extremely rapid, occurring in milliseconds.

Accidental cone shell envenomation in domestic animals has not been reported in the peer-reviewed literature, but the most likely candidate would be a curious dog.

Toxicokinetics

It is difficult to generalize about conotoxins because there are some 200 different conotoxins so far identified and it is estimated that there may be more than 50,000 in total. Each Conus species synthesizes 100–200 different conotoxins. However, in the clinical sense, effects of envenomation by conotoxins are generally felt within minutes. In survivors of severe envenomation, clinical signs may persist for weeks.

Mode(s) of Action

The modes of action of conotoxins, which are small peptides are diverse, as a generalization they are inhibitors of ion channels, involved in the transmission of neuromuscular signals in animals, including calcium channels, sodium channels, and potassium channels.

Clinical Signs

Clinical signs reported by human victims of envenomation by cone shells include immediate sharp burning or stinging sensation, local numbness or paresthesia, oral or generalized paresthesia, nausea, blurred vision, weakness, dysphagia, areflexia, paralysis and apnea.

Diagnostic Aids

There are no specific diagnostic aids.

Treatment

There is no specific antivenom. Monitor and support vital functions. Airway maintenance and protection is vital, bearing in mind that the pharynx may be paralysed. Respiratory support may be required.

Prognosis

Likely to be guarded to poor in a significant envenomation.

Necropsy

No significant findings.

Public Health Considerations

People should be aware that they should never collect live Conus species, or handle them for any reason, even to move them out of the way of an inquisitive pet. The Conus radula can penetrate wetsuits, so even gloves are not protective.

Public awareness of the dangers of Conus species is generally high in Australia, but people in New Zealand tend to be surprised to learn that Conus species occur in New Zealand waters.

Prevention

Dog owners need to be vigilant when taking dogs to the beach or in the vicinity of rock pools.

Read full chapter

URL: https://www.sciencedirect.com/science/article/pii/B9780124202276000220