Limonene and Related Compounds as Potential Skin Penetration Promoters

Abstract

A transdermal therapeutic system can be developed by promoting the skin permeability of drugs with some materials. In this study, the promoting effect of terpenes present in essential oils on the percutaneous absorption of indomethacin from alcoholic hydrogels was investigated in rats in vivo. The enhancing activity of terpenes was expressed quantitatively as a quadratic function of lipophilicities of the terpenes, measured as a lipophilic index. Further increase in the significant level was observed when the molecular weight of terpenes was included in this function as the second factor. The synergism of ethanol with d-limonene, selected from among the other terpenes, on the promoting activity was statistically investigated. It was evident that both d-limonene and ethanol were important factors for promoting the percutaneous absorption of indomethacin. A significant synergism of d-limonene with ethanol was also observed. As a possible mechanism for enhancement action of d-limonene and ethanol, it was considered that, at first, d-limonene penetrates into the skin under coexistence with ethanol and may change the barrier structure of the stratum corneum. The transfer of ethanol to the skin is thereby enhanced under the coexistence with d-limonene in the skin. Thus, the permeation of indomethacin can be promoted due to its affinity with ethanol.