A new local anesthetic microemulsion formulation for cosmetic painful application

A new lecithin based water in oil microemulsion system possessing a potentially improved skin bioavailability of local anesthetics were designed using lidocaine hydrochloride as a model drug. An egg lecithin, ethanol, Miglyol, olive oil and water were selected as a surfactant, cosurfactant, oil and aqueous phase, respectively.Pseudo ternary phase diagrams were constructed at fixed surfactant/cosurfactant ratios for microemulsion formation. The drug was added the inner aqueous phase of system. Particle size, the viscosity and flow diagram of microemulsions were determined. In-vitro permeation of lidocaine hydrochloride through dialysis membrane was studied using Franz diffusion cell. Moreover, the in-vivo efficacy of microemulsions developed and commercial products (EMLA and Betacaine Gel) containing lidocaine hydrochloride were investigated and compared in mice using radiant tail-flick test.Penetration studies and in vivo experiments showed that the permeation ability of of lidocaine hydrochloride was significantly improved by microemulsion composed Miglyol alone rather than combination of Miglyol and olive oil as oil phase. While microemulsions in a droplet sizes of 9.25 and 5.58 nm produced rapid topical analgesic effects in comparison to commercial product (Betacaine), microemulsion in a droplet size of 140.4 nm containing olive oil and EMLA did not elicit any effects in mice. Microemulsions exhibited a Newtonian behavior were almost stable at room temperature for at least six months.In conclusion, the permeating ability of lidocaine hydrochloride was significantly increased from the new microemulsion formulations and thus, they produce fast pain relief in comparison to gel type commercial product.Key words : Microemulsions; Topical anesthesia; Miglyol; Lecithin;tail-flick test; Lidocaine