Previous attempts to develop slow-release anesthetics have not been successful, primarily due to issues with conventional anesthetics causing toxicity to surrounding tissues. However, researchers at Children's Hospital Boston have - for the first time - packaged the potent anesthetic saxitoxin within specially designed fat-based particles called liposomes. They have shown that they can produce long-lasting local anesthesia in rats without causing toxicity to nerve or muscle cells. The researchers believe their work could revolutionize the management of pain during and after surgery. It may also give physicians a new tool to manage chronic pain.

As Daniel Kohane, M.D., Ph.D., of the Division of Critical Care Medicine in the Department of Anesthesiology at Children's, explains: "The idea was to have a single injection that could produce a nerve block lasting days, weeks, maybe even months. It would be useful for conditions like chronic pain where, rather than use narcotics, which are systemic and pose a risk of addiction, you could just put that piece of the body to sleep, so to speak."

In lab experiments, the researchers found that the most effective liposomes were those that contained a combination of saxitoxin and dexamethasone, a potent steroid known to augment the action of encapsulated anesthetics. With the dexamethasone, the liposomes produced nerve blocks lasting seven days, versus just two days with saxitoxin alone. By conducting cell culture experiments and analyzing tissue, they found that the formations were not toxic.

"If these long-acting, low-toxicity formulations of local anesthetics are shown to be effective in humans, they could have a major impact on the treatment of acute and chronic pain," says Alison Cole, Ph.D. of the NIH's National Institute of General Medical Sciences, which partially funded the work. "This slow-release technology may also have broader applications in drug delivery for the treatment of a variety of diseases."

Dr. Kohane is currently working on a formulation that would last even longer and believes a formulation will soon be ready for clinical trials.

News Release: Long-lasting nerve block could revolutionize pain management   www.sciencedaily.com    April 16, 2009