Comparison of essential oil from Mentha haplocalyx and menthol used as penetration enhancers

The aim of this paper was to investigate and compare the penetration-enhancing characteristics of menthol and essential oil from Mentha haplocalyx(M.haplocalyx oil) on the transdermal absorption of the complex traditional Chinese medicine(TCM) components. A series of TCM components were selected as model drugs based on their lipophilicity (logP value), namely osthole(OT, logP=3.85), tetramethylpyrazine(TMP, logP=2.34), ferulic acid(FA, logP=1.26), puerarin(PR, logP=-0.35) and geniposide(GP, logP=-1.01), in order to simply and characterize the TCM complex components system. Transdermal experiment in vitro was employed to investigate and compare the penetration-enhancing characteristics of menthol and M.haplocalyx oil on the transdermal absorption of these model drugs. Meanwhile, Fourier transform infrared spectroscopy(FTIR) was used to further compare the effect of menthol and M. haplocalyx oil on the molecular structure of stratum corneum(SC). The results showed that both of menthol and M.haplocalyx oil at proper concentration could promote the transdermal absorption of the selected model drugs. After application of menthol, the drug logP values gradually tended to have negative linear relationship with the logarithm of penetration enhancement ratio(ER); while after application of M.haplocalyx oil, the logP values tended to have parabolic relationship with the logarithm of ER. However, both menthol and M.haplocalyx oil exhibited higher efficiency for the drugs with relative low lgP value(ie hydrophilic drugs), with similar penetration-enhancing characteristics between these two. Infrared spectroscopy results showed that menthol and M.haplocalyx oil could affect the skin barrier functions mainly via stratum corneum lipids, with similar effect intensity, and this was consistent with the results of transdermal experiment in vitro. Thus, Menthol had similar penetration-enhancing characteristics with M.haplocalyx oil, and had same effect on the SC molecular structure. Therefore, as transdermal penetration enhancer, the menthol with single composition could be considered to replace M.haplocalyx oil with complex compositions.