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Neosaxitoxin showed an effective local anesthetic effect when injected in the subcutaneous plane. The efficacy of a 50-microg dose of neosaxitoxin was shown. This is the first report of neosaxitoxin as a local anesthetic in a human trial.

Local anesthetics effectively block and relieve pain, but with a relatively short duration of action, limiting its analgesic effectiveness. Therefore, a long-acting local anesthetic would improve the management of pain, but no such agent is yet available for clinical use. The aim of this study is to evaluate the potentiation of the anesthetic effect of neosaxitoxin, with bupivacaine or epinephrine in a randomized double-blind clinical trial. Ten healthy males were subcutaneously injected into the left and right forearms with a randomized pair of the following treatments: (i) bupivacaine (5 mg); (ii) neosaxitoxin (10 microg); (iii) neosaxitoxin (10 microg) plus bupivacaine (5 mg), and (iv) neosaxitoxin (10 microg) plus epinephrine (1:100.000), but all participant received all four formulations (in 2 ml; s.c.), with 1 month elapsing between the two round of experiments. A validated sensory and pain paradigm was used for evaluating the effect of the treatment 0-72 h after the injections, measuring sensory, pain, and mechanical touch perception threshold. The duration of the effect produced by combined treatments was longer than that by the single drugs. In conclusion, bupivacaine and epinephrine potentiate the local anesthetic effect of neosaxitoxin in humans when co-injected subcutaneously. The present results support the idea that neosaxitoxin is a new long-acting local pain blocker, with highly potential clinical use.

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