Formulation and efficacy studies of new topical anesthetic creams.

Local anesthetics (lidocaine or tetracaine) spontaneously melted at 25 degrees C when mixed with thymol and aqueous isopropyl alcohol solution (IPA) at proper ratios and formed novel two-phase melt systems (TMS). The TMS consisted of a homogeneous oil phase containing primarily a local anesthetic agent (lidocaine or tetracaine) and thymol, and a homogeneous aqueous phase containing primarily IPA and pH 9.2 buffer. The relationship between melting of the solid components and system composition was determined from the phase diagram obtained by a titration method. A select TMS of a local anesthetic agent (lidocaine or tetracaine) was directly emulsified to prepare an O/W cream and tested for the anesthetic efficacy on intact human skin. While both lidocaine (6%) and tetracaine (4%) creams were highly effective for dermal anesthesia with a similar onset time, the tetracaine cream exhibited a significantly longer duration of action than the lidocaine cream. An accelerated stability study indicated that lidocaine was significantly more stable than tetracaine in the creams.