Despite side effects and risk for addiction and overdose, opioids remain the drug of choice for chronic and surgical pain. However, the current opioid crisis has ignited a heightened urgency for the discovery of alternative methods of pain relief. In particular, anesthetics that have prolonged duration from single injections as conventional local anesthetics wear off quickly and can affect the heart and brain. Now, a study in rats offers up a possible alternative, with an infamous compound that comes from a relatively unusual source – the pufferfish.

In a new paper titled, “Polymer-tetrodotoxin conjugates to induce prolonged duration local anesthesia with minimal toxicity,” published in Nature Communications, a group from Boston Children’s Hospital and Harvard Medical School report a system that results in the slow release of tetrodotoxin (TTX.) The release occurs as a result of the hydrolysis of ester linkages that conjugate the TTX to a biocompatible and biodegradable polymer, poly(triol dicarboxylic acid)-co-poly(ethylene glycol) (TDP.)

Pufferfish (also known as fugu, bok, blowfish, globefish, swellfish, balloonfish, or sea squab) is best known as the most dangerous choice off of the menu at a sushi restaurant, due to the TTX production. And, if the fish is not prepared correctly, ingesting it can cause severe illness and even death. Symptoms, which include include tingling of the lips and mouth, dizziness, tingling in the extremities, problems with speaking, balance, muscle weakness and paralysis, vomiting, can start within minutes after eating the toxic fish. The toxin cannot be destroyed by cooking or freezing.

In tiny amounts, in this slow-release formulation that efficiently penetrates nerves, the toxin provided a safe, highly targeted, long-lived nerve block. Daniel Kohane, MD, PhD, Professor of anaesthesia at Harvard Medical School has long been interested in neurotoxins found in marine organisms. 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.

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.”

Chao Zhao, PhD, and Andong Liu, PhD, two postdoctoral fellows in the Kohane lab, 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 explained.

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,” said 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,” added Liu.

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 said. “Certainly for days, and maybe for weeks.”