Rapid skin anesthesia using a new topical amethocaine formulation: a preclinical study

We developed a fast-acting topical amethocaine emulsion and tested its analgesic activity against heat or mechanically induced pain in a rat paw model. The first experiment was performed in rats made hyperalgesic or allodynic after carrageenan-induced inflammation. Rats were distributed in five subgroups, each receiving topically one of the following: amethocaine microemulsion, amethocaine gel (Ametopgel), EMLA (Eutectic Mixture of Local Anesthetics) cream, amethocaine infiltration, or nothing (controls). The second experiment was conducted on healthy, selected heat- or touch-hypersensitive rats, which were distributed as in the first experiment. Paw withdrawal time from a heat and a mechanical stimulus was used as a pain index. In the first experiment, antihyperalgesic activity appeared at 4.2, 13.8, and 14 min after amethocaine microemulsion, gel, or EMLA cream, respectively. Amethocaine microemulsion was the only topical formulation with an antiallodynic effects, although less than with amethocaine infiltration. In healthy rats (second experiment), all topical formulations produced similar analgesic effects in heat-induced pain of the ipsilateral paw. Activity in the contralateral paw appeared earlier with amethocaine microemulsion, which was also the only one that increased touch-induced withdrawal time in the ipsi- and contralateral paws. Therefore, the microemulsion could be valuable for improving amethocaine skin penetration and thus bringing rapid pain relief.

Topical anesthetics are used in several painful clinical procedures, but they tend to have a slow onset time. A new amethocaine microemulsion with a faster onset of analgesia than commercial formulations was developed and its activity tested in pain states induced by heat or mechanical stimulus in inflamed and healthy rat paws.