Essential oils bearing specialized metabolites with potential tyrosinase inhibitory activity: focus on dermal absorption from topical formulations

Downregulation of tyrosinase is a widely used approach to reduce excessive melanin production and prevent typical age-related skin diseases (e.g., freckles and solar lentigo). Plants are an interesting source of skin-whitening agents. So far, mainly phenolic compounds have been investigated, while terpenoids have been relatively little explored as tyrosinase inhibitors. In this study the tyrosinase inhibitory activity of sixty-nine essential oils (EOs) was first investigated in an in vitro enzymatic assay, with Kojic acid serving as a positive control. The chemical composition of the investigated EOs was also characterized by gas chromatography coupled with mass spectrometry. Among the essential oils studied, citral containing EOs (i.e., Cymbopogon schoenanthus Spreng (L.), Litsea cubeba (Lour.) Pers, Melissa officinalis L., and Verbena officinalis L.), and the β-myrcene containing EOs (i.e., Juniperus communis L. and Pinus mugo Turra EOs) were the most promising. The obtained results showed the advantages of using mixtures of compounds/EOs instead of single molecules and underlined the possible additive effect of two monoterpenoids (i.e., β-myrcene and citronellal) on the inhibitory activity of citral. Since citral is a fragrance that is also listed as an allergen among the 26 fragrances added to Annex III of the Cosmetics Directive by the 7th amendment (2003/15/ EC), this study aimed to blend different essential oils to reduce the amount of citral while maintaining tyrosinase inhibitory activity by exploiting the additive/synergistic effect of essential oils containing β-myrcene and citronellal. Two different formulations (oil/water emulsion and oil solution) containing a mixture of three EOs (i.e. Litsea cubeba, Pinus mugo, Cymbopogon winterianus) and applied to the skin both in non-occlusive and partially occlusive mode were evaluated. The amount of citral and other bioactive compounds (e.g., myrcene, citronellal) from the selected EOs delivered through the skin was evaluated depending on the type of formulation and the mode of application. For this purpose, solvent-free headspace sampling was used in combination with a fast GC-MS analysis using narrow bore columns. The results obtained indicate that an oil/water emulsion is preferable because it releases the bioactive compounds rapidly and minimises their evaporative loss due to their relatively high vapour pressure. In addition, semi-occluded conditions were required to prevent evaporation, resulting in limited availability of the bioactive compounds in the viable skin.