Local anesthetic cream prepared from lidocaine-tetracaine eutectic mixture.

Local anesthetic creams for the clinical treatment of conditions such as postherpetic neuralgia were prepared as an in-house formulation from the eutectic mixture of lidocaine-tetracaine (LT cream) using two eutectic mixtures of local anesthetic (EMLA) type bases. The LT formulation was compared with a lidocaine-prilocaine (LP cream) eutectic mixture formulated using the same base as EMLA. The chemical stability of lidocaine was examined in advance and was found to be stable for more than 3 months either in LT cream or in LP cream. The release rate of lidocaine from the formulated creams was examined using a cellulose ester membrane. The release rate of lidocaine from LT cream was similar to that from LP cream. The release rate of tetracaine was slightly slower than that of lidocaine in LT cream reflecting the larger molecular size of tetracaine. The penetration rate was examined in vitro using a Yucatan micropig skin. The penetration rate of lidocaine was similar between LT and LP creams. Infiltration anesthesia action examined in guinea pigs indicated that the difference between the two creams was statistically insignificant. The present study suggests the equivalence of the LT and LP creams as a local anesthetic and the potential of LT cream for clinical use either in the easy formulation or in the low-cost formulation.