A new extended-release anesthetic can safely numb body parts for as long as a week, a new study in rats suggests. If the anesthetic has the same effect in humans, it might one day be used to manage chronic and surgery-induced pain, researchers say.

"Other studies have shown that it's possible to create nerve block [a numbing of the nerves to minimize pain] for days, but it damaged muscles and nerves," says study co-author Daniel Kohane, a pediatric intensive care doctor and anesthesiologist at Children's Hospital in Boston. But, he adds, "We have this really nice long block with no tissue damage."

Kohane's team took a potent numbing agent called saxitoxin and packaged it in tiny fat droplets called liposomes, which slowly leak the chemicals over time. They injected the liposomes into the tissue surrounding the sciatic nerve of rats, which runs from the lower back to the feet and is responsible for creating feeling in the back of the leg and the bottom of the foot in rats as well as in humans, according to Kohane. They then tested the animals' ability to respond to pain by placing their feet on a hot surface ("not agonizingly hot but uncomfortable," he says) and seeing if they withdrew their feet rapidly.

Their findings, published this week in the Proceedings of the National Academy of Sciences: rats injected with saxitoxin-packed liposomes lost sensation in their feet and legs for up to two days; those receiving a combo of saxitoxin and dexamethasone (a steroid that extends activity of local anesthetics for reasons not completely understood by scientists) were numb for an average of a week. When the researchers euthanized the animals to inspect their tissue, they found no evidence of damage to muscle or nerve cells.

If these anesthetic-packed liposomes had the same effect in humans, they could potentially be used to manage chronic pain associated with conditions such as phantom limb pain (a condition following amputation wherein the patient continues to perceive pain where the amputated foot, leg, hand or arm used to be), says Linda Le-Wendling, an anesthesiologist at the University of Florida in Gainesville, who was not involved in this study. "A single injected [nerve] block that lasts five to seven days would actually be beneficial for that type of patient."

The other potential medical application, Kohane says, is managing pain during and after certain types of surgery, such as a hernia repair, which involves a painful incision in the abdomen. Patients are often prescribed powerful narcotics post-surgery such as oxycodone, which resembles morphine and is highly addictive.

But Le-Wendling warns that if saxitoxin were to be used in humans, doctors would have to exercise great caution and have resuscitation devices on hand in the event of a slipped needle. The reason: it can cause paralysis of the breathing muscles if mistakenly injected into a blood vessel rather than the tissues surrounding the nerves.

Image of liposome containing local anesthetics. © Michael Lawlor