ABSTRACT
Introduction .................................................................................................................................... 159 Structural Classification .................................................................................................................. 160 Structure-Activity Relationships..................................................................................................... 162 Improving Permeation Enhancement Ability of Terpenes ........................................................... 163 Mechanism of Action of Terpenes ................................................................................................. 165 Terpene Derivatives as Enhancers................................................................................................. 168 Conclusions .................................................................................................................................... 171 References....................................................................................................................................... 171
One of the most common approaches to drug penetration enhancement is the use of chemical agents which modify the skin barrier properties. For such an enhancer to be acceptable, the following properties are desired1: It should be nontoxic, nonirritating, exhibit no pharmacological property of its own, exert reversible effects on the skin, and be cosmetically acceptable. While a number of chemicals such as sulfoxides, alcohols, pyrrolidones, fatty acids, Azone have been studied, none of them have been proven to be outstanding often due to safety concerns. Terpenes may offer advantages over such enhancers because of their natural origin as well as Generally Regarded As Safe (GRAS) status. They are carbon, hydrogen, and oxygen containing nonaromatic compounds found in essential oils, extracted from flowers, fruits, and other natural products. These compounds have been used for a long time as fragrances and flavoring agents in commercial preparations (sweets, toothpastes, cigarettes). Table 13.1 provides examples of essential oils which have various terpenes or terpenoids as their main constituents.
