Release and permeation kinetics of Melaleuca alternifolia (tea tree) essential oil bioactive compounds from cosmetic formulations

Melaleuca alternifolia essential oil (tea tree oil, TTO) is well known because of its antimicrobial, antifungal and anti-inflammatory properties [1]. Thanks to these properties, TTO is employed as active ingredient in several skin cares. This study aims to investigate and evaluate permeation and release kinetics of TTO bioactive marker compounds from several semisolid formulations (creams, ointments, gels) containing different percentages of TTO (from 5 to 30% p/p). In vitro permeation and release tests were carried out using Franz diffusion cells with synthetic and pig skin membranes at different times (from 10 minutes to 72 hours). Each test was repeated three times. The recovered receptor phases were analyzed by Headspace-Solid Phase Micro-Extraction (HS-SPME) in combination with GC-MS. Permeation and release kinetics were evaluated on some selected markers known to be responsible of the TTO biological activities, i.e. 1,8-cineole, 4-terpineol and α-terpineol. The results of permeation and release tests demonstrated that all compounds show the same kinetics, although lag time and amount of substances released are meaningfully different. Finally, the released markers were quantified by both standard addition approach and multiple headspace solid phase microextraction. Some preliminary experiments were carried out to determine the amount of the above markers accumulated into skin.