Chemical penetration enhancers terpenes

Terpenes are widely used chemical penetration enhancers (Williams and Barry, 1991b Barry and Williams, 1993). Structural formulas of terpaies used as penetration enhancers are presented in Figure 12.6. Terpenes are of low cutaneous irritancy, provide excellent enhancement ability, and appear to be promising candidates for pharmaceutical formulations (El-Kattan etal., 2001 Gao and Singh, 1998 Zhao and Singh, 1999). Table 12.4 lists the in vitro percutaneous absorption enhancement of drugs using terpenes. 

Terpenes have been utilized for a number of therapeutic purposes, such as in antispasmodics, carminatives, and perfumery. They have been found to be useful when incorporated into topical and transdermal pharmaceutical formulations as they act as chemical penetration enhancers facilitating the permeation of drugs through the skin barrier, both healthy and diseased in comprehensive reviews [26, 27]. 

In the design of TDDS, the skin permeability often needs to be enhanced by various approaches including the use of skin permeation enhancers to reduce the barrier property of the skin. In the absence of permeation enhancers, systemic delivery of most drugs through the skin is limited, primarily because of the barrier function of the stratum corneum. Certain enhancers including some terpenes have a tendency to form eutectics when mixed with certain chemical entities. Terpenes are a group of chiral skin penetration enhancers that are derived from plant essential oils and are widely used as pharmaceutical excipients with various drugs. Terpenes are... 

Terpenes continue to be a popular choice as experimental enhancers for delivering materials across skin membranes. For example, L-menthol facilitated in vitro permeation of morphine hydrochloride through hairless rat skin [37], imipramine hydrochloride across rat skin [59], and hydrocortisone through hairless mouse skin [60]. Recently, niaouli oil was found to be the most effective of six essential oils in promoting estradiol penetration through hairless mouse skin [61]. It is noteworthy that there is currently little control on the topical use of most terpenes, and many aromatherapy oils and formulations contain appreciable quantities of these chemicals. Their excessive use offers potential for permeation of hazardous compounds from the same formulations into the skin some terpenes also have pharmacological activity. 

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