In vitro permeation study of hinokitiol: effects of vehicles and enhancers

In vitro permeation of hinokitiol (HKL) through hairless mouse skin was investigated using a diffusion cell. Either propylene glycol (PG) or ethanol (EtOH) was used as a vehicle for HKL. After applying the HKL solutions of 0.5%. 1%, 2%, and 5% onto the skin, the amount of HKL transferred through the skin into the receptor solution, phosphate-buffered saline (PBS, pH7.4), was determined at a predetermined time intervals for 18 hr using a high performance chromatography (HPLC). EtOH was more effective than PG in terms of in vitro permeation of HKL. This is possibly because EtOH acts as a permeation enhancer. Another reason would be related to the higher thermodynamic activity of HKL in ethanol. To investigate the effect of an enhancer on the in vitro permeation, oleyl alcohol, 1-dodecyl-2-pyrrolidone (DP), and lauric acid were used as enhancers. Each was added to the HKL solution (1%) so that the concentration of the enhancer was 1%. Among the enhancers, DP was the most effective and it enhanced the permeation of HKL approximately 5-10 times.