Drugs from the Sea: 1. Cone snails and ziconotide

Cross-posted from Kayak Yak . . . Drug discovery from marine sources is an active area of research, and several drugs of marine origin have already reached regular clinical use. (There’s a whole journal dedicated to Marine Drugs. Open access, too). For quite some time, I’ve had the notion of doing a series of posts about these drugs, where they come from and what they do, and I’ve been tinkering with this first entry for about as long, trying to balance length with bio-geekery. So here goes.

In 200Conus geographicus, cone snail shell4, the pain-killing drug ziconotide (Prialt) was approved for marketing by the US FDA, some twenty years after Balmedro Olivera and Lourdes Cruz set out to find out how the sting from a 10 cm poisonous marine snail, Conus geographus, could kill a human many times its weight. C geographus (picture from Wikimedia commons) is a member of the cone snail family, so named for their distinctively shaped, colourful shells. C. geographus takes its name from its map-like shell pattern (photo from Wikimedia commons). It lives below the low tide mark in pockets of sand near the edges of coral reefs and atolls; on a map its distribution tracks a wide ribbon from Madagascar around the edges of the Indian Ocean, down through Indonesia, around the North coast of Australia and up the islands and atolls of the Pacific. To the biologist, it and its fellows are known as Class Gastropoda, Order Neogastropoda, Superfamily Toxoglossa, Family Conidae, while naturalists over the centuries have named the Apothecary Cone, Astrologers Cone, Hebrew Cone and Emaciated Cone. The dinosaurs were already extinct when the first cone shell was pressed into the fossil record, some 55 million years ago, but they now form a family around 700 species strong, which inhabits warm and tepid shallows around the globe. The more common species can be had for a pleasure-dive in reef waters, or a stroll along the ocean, while particularly rare and handsome specimens have been auctioned for more than the price of a painting (in 1798) or a family saloon car (1960). Occasionally, their price has been a life: 30 or so people are known to have died of cone snail stings, mainly from Conus geographus.

Cone snails’ dietary aspirations might seem, on first blush, overambitious: they are carnivorous, with a taste for worms, other molluscs … and fish. However, per Ecclestiates, the race is not to the swift . . . Cone snails generally use one of two hunting strategies, harpoon or net, and their secret weapon is poison, a cocktail of venoms tailored to hunting style and prey. The cocktails contains 50-200 individual toxins, and vary between species, so that there are an estimated 25 000 plus toxins across all the known cone snail species. The venom of the harpoon-wielding snail Conus purpurascens contains a mixture of fast-acting toxins that produce a nerve paralysis, and slower acting toxins that produce muscular paralysis. When a fish comes within range, the snail jabs at it with a venom-filled tooth held on the end of a proboscis. A stung fish can be paralyzed within two seconds, its body rigid and its fins standing out as though shocked. One toxin jams open the sodium channel involved in the propagation of the nerve impulse [footnote i], a channel that ordinarily would close immediately to allow the membrane to reset itself. Another jams closed those potassium channels which normally would open to quench the depolarization. The membrane depolarizes, rapidly and completely, and nerve conduction stops. Even as this happens, a second, slower-acting set of toxins, acting on calcium channels, starts to paralyze the fish’s muscles. They block sites to which signalling neurotransmitters bind, and sodium channels which would open in muscle contraction. Net-wielding snails, like Conus geographicus favour muscle paralytic toxins; since they first engulf their prey and then sting it, they can afford the slightly slower onset poison.

Contoxins are very small proteins, 10-35 amino acids long, and at their length would normally be a tumbling mixture of floppy conformations in solution rather than a fixed protein fold—Proteins depend on their ability to hold conformation to function. Conotoxins, however, take advantage of a property of the amino acid cysteine. Under the proper conditions, two cysteines in a peptide chain will link to each other, bringing their respective pieces of peptide chain into alignment. Conopeptides, small as they are, each have two or three pairs of cysteines, which cross-link to create a tight little package (3d structure of Ziconotide as the August 2006 molecule of the month at 3dchem). Since they are tightly folded, conotoxins waste neither time nor energy shifting into the right shape to bind to their target. There are two measures of quality of any interaction between molecules: how well a molecule discriminates between its own and all other binding sites, and how strongly it binds. By those measures, conotoxins are finely tuned, with certain conotoxins able to select between nerve cell sodium channels and muscle cell sodium channels, and others able to pick and chose between subtypes of calcium channel. Contoxins are several times more selective than peptides from snake and scorpion venoms. A measure of the strength of binding is the dissociation constant, a ratio of the amount of unbound toxins to the amount of bound toxin at a giving concentration; for the conotoxins those are of the order of 10(-9) or 0.000000001, or for every unbound toxin molecule, there are one trillion bound.

Drugs are often limited in their usefulness by side effects, some of which result from binding to molecular sites other than the target sites. For that reason, the conotoxin peptides, all 25 000-odd, with their strong, specific binding, are of great interest to scientists and several companies have investigated cone shell toxins as a source of drug candidates. Ziconotide (aka Prialt, from Elan Pharmaceuticals) is the first conotoxin-derived drug to pass successfully through all the stages of clinical drug development. It is a synthetic omega (calcium-channel binding) conotoxin from Conus magus, the Magician cone, which binds specifically to (calcium) channels in nerves in the spine which carry pain, and has proved effective in relieving pain for patients with intractable severe pain who either do not respond to or cannot tolerate other drugs. It’s not an opioid, therefore doesn’t produce tolerance, and can be combined with other drugs. But it’s still far from ideal: it has to be administered by intrathecal injection (directly into the fluid around the spine), which means it has to be given by an anaesthetist or by an implanted pump. The dose has to be increased slowly to decrease the risk of side effects, the onset of effect slow, response to dose changes is laggardly, and it can produce severe neurological and psychiatric side effects (Williams, 2008 PubMed abstract; Schmidtko, 2010 PubMed abstract).

To date, no other conotoxin-derived drugs have made it through clinical testing and into clinical use. Olivera in 2006 listed five in Phase I (first human) testing, mainly for pain, and their status in 2011 gives a snapshot of the vicissitudes of early phase drug development and the precarious life of small biotechnology companies: three of the companies involved appear to have either gone under or moved away from conotoxin development, and I am hard put to find evidence of progress on four of the compounds. The fifth (Xen-2174), which inhibits the uptake of the neurotransmitter norepinephrine, is in Phase II trials. Nevertheless, investigation continues; there are at least 24996 more conotoxins to go . . . A conotoxin derivative that can be taken orally has been developed. Conotoxins are being studied for their potential to protect brain tissue in stroke and heart muscle in heart attack, where part of the damage is known to be caused by uncontrolled leaks of ions across membranes [Twede et al, 2009 Full text]. And work with conopeptides has also identified other pathways involved in severe pain, leading to the development of non-conopeptide drugs directed at these pathways.

Footnotes and references.

[i] An aside on nerve conduction. It is orchestrated by the opening and closing of protein pores in the cell membrane of the neuron, which pass, according to their filter characteristics, sodium, potassium, or calcium ions. The sequence proceeds as follows: a trigger signal arrives, whether an electrical signal from another neuron, or a chemical signal. The membrane is held at a resting potential, a static voltage, with an excess of sodium outside the cell and potassium inside. When the impulse is triggered, sodium channels open, and sodium flows into the cell. The membrane depolarizes, losing its charge. Once the depolarization proceeds to a certain point, the sodium channels close, and potassium channels open, quenching the depolarization and allowing the membrane to reset to receive the next impulse. As each patch of nerve membrane is depolarized, it triggers depolarization of the next downstream; thus the impulse travels.