Thickening of hydrophilic/lipophilic and lipophilic/hydrophilic microemulsions II. Comparative study of the thickening influence on H/L and L/H microemulsions as enhancers for a lipophilic tracer

HL and L/H microemulsions were thickened using hydrophilic and lipophilic g elling agents in order to achieve improved cutaneous application. This work aimed at studying the cutaneous absorption of a lipophilic tracer from a m icroemulsion with a high water or high oil content, according to the percen tage content of gelling agent. alpha-tocopherol (lipophilic tracer) was cho sen for its properties as an antioxidant and free radical scavenger. Its co ncentration was 3% in all formulae and it was present either in the externa l phase (H/L microemulsion) or in the internal phase (L/H microemulsion). T ests were performed on excised young pig skin which was mounted on Frantz c ells. Levels of tracer were determined by HPLC in the skin layers 24 h afte r application, and in the receptor fluid 6 and 24 h after application. In t he H/L microemulsions without the gelling agent, the tracer was present (0. 3%) in the entire epidermis. With the gelling agent, the quantity of tracer in the entire epidermis increased from 0.4 to 1.36%, as a function of the concentration of gelling agent. Twenty-four hours after application, no alp ha-tocopherol could be detected in the receptor fluid. In the L/H microemul sions without gelling agent, the tracer was present (ata only 0.16%) in the entire epidermis. With the gelling agent, the quantity of tracer in the en tire epidermis increased from 0.29 to 1.03% as a function of the concentrat ion of gelling agent but to a lesser extent. Traces of alpha-tocopherol wer e found in the receptor fluid. In both cases, the gelling agent contributed to improving vehicle adhesion to the skin and increased the properties of the microemulsion as an enhancer of the lipophilic tracer. A higher level o f storage was observed in the epidermis with the H/L microemulsion, and eas ier percutaneous absorption was seen wit the L/H microemulsion.