Liposomes Deliver Longer Lasting Local Anesthetics

A liposomal formulation of a local anesthetic shows promise for long-lasting pain relief with minimal toxicity, reports a research group led by Daniel S. Kohane of Harvard Medical School (Proc. Natl. Acad. Sci. USA, DOI: 10.1073/pnas.0900598106). Longer lasting local anesthetics could relieve surgical or chronic pain without the need for systemic and potentially addictive narcotics. Efforts to create extended-release anesthetic formulations, however, have been hampered by limited duration of pain relief, negative reactions in local tissues, and systemic toxicity. One class of anesthetics, called site 1 sodium-channel blockers, have little local toxicity, but their hydrophilicity makes them difficult to encapsulate into typical extended-release polymeric particles. Kohane and colleagues found that liposomes containing one of the sodium-channel blockers, saxitoxin (an algal compound that can be lethal when delivered systemically), provided nerve blocks in rats for up to two days. Similar liposomes containing saxitoxin and the steroid dexamethasone, which is known to augment the action of encapsulated anesthetics, provided nerve-blocking effects for 7.5 days. Tissue and gene analyses and cell-culture experiments showed little evidence of inflammation or toxicity from the liposomal formulations.