Penetration enhancers and other factors governing percutaneous local anaesthesia with lidocaine

The percutaneous penetration of the local anaestetic lidocaine was investigated in the guinea-pig. Three different types of composition were employed: lidocaine hydrochloride in aqueous solution, lidocaine base in an aqueous alcoholic solvent mixture and lidocaine base in aqueous solutions of dipolar aprotic solvents. The latter solvents included simple tertiary aliphatic amides, amides related to dimethylacetamide, some cyclic amides as well as a number of miscellaneous compounds. The degree of dermal anaesthesia was noted in each case. In addition, the uptake and distribution of lidocaine in the skin and its absorption into the blood were studied using tritium-labelled drug. The results show that the percutaneous penetration of lidocaine is dependent on the concentration of the agent, the time of epicutaneous application of the composition, whether the agent was used as salt or free base, and the nature of the solvent medium. Lidocaine base in aqueous dimethylacetamide was most effective in producing percutaneous local anaesthesia.