In vitro studies on release and human skin permeation of Australian tea tree oil (TTO) from topical formulations

Essential oils are widely used in pharmaceutical and cosmetic preparations e.g. as fragrance, active ingredient or penetration enhancer. However, reports on skin absorption are rare. Therefore, the aim of our study was to investigate the capability of terpinen-4-ol, the main compound of Australian tea tree oil (TTO), to permeate human skin. In static Franz diffusion cells permeation experiments with heat separated human epidermis were carried out using infinite dosing conditions and compared to liberation experiments. The flux values of three different semisolid preparations with 5% TTO showed the rank order semisolid O/W emulsion (0.067 μl/cm2 h) > white petrolatum (0.051 μl/cm2 h) > ambiphilic cream (0.022 μl/cm2 h). In comparison to the flux value obtained with the native TTO (0.26 μl/cm2 h), the flux values are remarkably reduced due to the lower amount of terpinen-4-ol. Papp values for cream (2.74 ± 0.06 × 10−7 cm/s) and native TTO (1.62 ± 0.12 × 10−7 cm/s) are comparable whereas white petrolatum (6.36 ± 0.21 × 10−7 cm/s) and semisolid O/W emulsion (8.41 ± 0.15 × 10−7 cm/s) demonstrated higher values indicating a penetration enhancement. No relationship between permeation and liberation was found.