A Review on Terpenes as Skin Penetration Enhancers in Transdermal Drug Delivery

The skin itself hinders the widespread use of transdermal drug delivery (TDD) for administration of medication. Despite the different strategies devised and employed to reversibly overcome the skin barrier, this non-invasive delivery mode is restricted to potent, low molar mass therapeutic agents. The use of chemical penetration enhancers is the conventional approach to modify the skin structure and lower its resistance for increased drug permeation. Over the years, extensive screening and testing have identified different classes of chemicals as potential adjuvants. Among these, terpenes (Constituents of plant essential oils) have been widely investigated as skin penetration enhancers for both hydrophilic and hydrophobic drugs. In particular, monoterpenes were generally more efficacious probably due to their small molecular sizes. The main mechanisms of action of terpenes on the skin as determined by several analytical techniques were lipid extraction and phase separation. Terpenes such as limonene and menthol in experimental patch formulations were demonstrated to be safe on the skin and effective in improving drug delivery across the skin.