Opioids remain a mainstay of treatment for chronic and surgical pain, despite their side effects and risk for addiction and overdose.
While conventional local anesthetics block pain very effectively, they wear off quickly and can affect the heart and brain. Now, a study in rats offers up a possible alternative, involving an otherwise lethal pufferfish toxin.
In tiny amounts, in a slow-release formulation that efficiently penetrates nerves, the toxin provided a safe, highly targeted, long-lived nerve block, researchers report today in Nature Communications.
The study was led by Daniel Kohane, MD, Ph.D., director of the Laboratory for Biomaterials and Drug Delivery at Boston Children’s Hospital.
Kohane has long been interested in neurotoxins found in marine organisms like pufferfish and algae.
In small amounts, they can potentially provide potent pain relief, blocking the sodium channels that conduct pain messages.
Kohane’s lab has experimented with various ways of packaging and delivering these compounds in tiny particles, activating local drug release with ultrasound and near-infrared light, for example.
For the new study, the team chose tetrodotoxin, a potent, commercially available compound derived from pufferfish. (Tetrodotoxin is notorious for causing fugu poisoning from improperly prepared sashimi.)
Tetrodotoxin is a neurotoxin with potential analgesic activity.
Tetrodotoxin binds to the pores of fast voltage-gated fast sodium channels in nerve cell membranes, inhibiting nerve action potentials and blocking nerve transmission.
Although found in various species of fish (such as the pufferfish), newts, frogs, flatworms, and crabs, tetrodotoxin, for which there is no known antidote, is actually produced by bacteria such as Vibrio alginolyticus, Pseudoalteromonas tetraodonis, and other vibrio and pseudomonas bacterial species.
Illustrations: Some species of poisonous puffer fish. Green rough-backed puffer (Lagocephalus lunaris) (top); yellow fin puffer (Takifugu xanthopterus) (bottom) (courtesy of Agriculture, Fisheries and Conservation Department)
Tetrodotoxin is considered likely to be produced by marine bacteria that are often associated with marine animals.
In puffer fish, the distribution of tetrodotoxin is mainly in the ovaries (eggs), liver and skin. The flesh is normally free of toxin.
However, flesh of some toxic gobies such as Yongeichthys nebulosis also contains tetrodotoxin.
Tetrodotoxin is heat-stable and therefore likely to remain in fish tissue after food preparation steps (e.g. cooking and drying). Eating fish containing tetrodotoxin such as puffer fish and porcupine fish can therefore be hazardous.
Toxicity of Tetrodotoxin
In human, the lethal dose of tetrodotoxin is around 1 to 2 mg and the minimum dose necessary to cause symptoms has been estimated to be 0.2 mg.
The onset of symptoms of tetrodotoxin intoxication usually occurs from 10 to 45 minutes after ingestion, but may be delayed by three hours or more.
Paraesthesia appears in the face and extremities, which may be followed by dizziness or numbness.
Nausea, vomiting, diarrhoea and epigastric pain may also be present.
Later, respiratory symptoms such as rapid breathing may follow.
Low blood pressure, convulsions and irregular heart rate may occur.
In most instances, the patients retain consciousness until shortly before death, which usually takes place within the first six hours.
All humans are susceptible to tetrodotoxin poisoning.
There are currently no known antidotes or antitoxins to tetrodotoxin.
The treatment of symptoms is therefore supportive.
There are differences in toxicity and toxin distribution in tissues among different puffer fish species.
In addition, seasonal, individual and local variations of toxicity and toxin composition in puffer fish are occasionally observed.
Tetrodotoxin poisoning may be caused by ingestion of only a small amount of puffer fish.
Taming a lethal toxin
Rather than load tetrodotoxin into particles as before, the team bound it chemically to a polymer “backbone.”
The body very slowly degrades the bond between tetrodotoxin and the polymer via hydrolysis, the natural breaking of chemical bonds by water).
This releases the drug at a slow, safe rate.
“A lesson we learned is that with our previous delivery systems, the drug can leak out too quickly, leading to systemic toxicity,” says Kohane.
“In this system, we gave an amount of tetrodotoxin intravenously that would be enough to kill a rat several times over if given in the unbound state, and the animals didn’t even seem to notice it.”
Kohane’s fellows, Chao Zhao, Ph.D., and Andong Liu, Ph.D., experimented with different drug loadings and different polymer formulations to get the longest-possible nerve block with the least toxicity.
“We can modulate the polymer composition to control the release rate,” Zhao explains.
To further increase safety, the team paired the tetrodotoxin-polymer combination with a chemical penetration enhancer, a compound that made the nerve tissue more permeable.
This allowed them to use smaller amounts of tetrodotoxin but still achieve nerve block.
“With the enhancer, drug concentrations that are ineffective become effective, without increasing systemic toxicity,” says Kohane.
“Each bit of drug you put in packs the most punch possible.”
“We show that both the penetration enhancer and the reversible bonding of toxin to polymer are crucial to achieving such prolonged anesthesia,” adds Liu.
Good early results
When the researchers injected the combination near the sciatic nerve in rats, they achieved a nerve block for up to three days, with minimal local or systemic toxicity and no apparent sign of tissue injury.
In theory, nerve block in humans could last even longer, since one could administer it more safely than in rats, says Kohane. Using polymers with a longer retention time in tissue would also prolong effects.
“We could think about very long durations of nerve block for patients with cancer pain, for example,” he says. “Certainly for days, and maybe for weeks.”
Journal information: Nature Communications
Provided by Children’s Hospital Boston