Role of Penetration Enhancers in Transdermal Drug Delivery System

The transdermal route has been recognized as one of the highly potential routes of systemic drug delivery and provides the advantage of avoidance of the first-pass effect, ease of use and withdrawal (in case of side effects), and better patient compliance. However, the major limitation of this route is the difficulty of permeation of drug through the skin. The skin is very effective as a selective penetration barrier. The stratum corneum provides the greatest resistance to penetration, and it is the rate-limiting step in percutaneous absorption. Penetration enhancers are the substances that facilitate the absorption of penetrant through the skin by temporarily diminishing the impermeability of the skin. Ideally, these materials should be pharmacologically inert, nontoxic, nonirritating, nonallergenic, compatible with the drug and excipients, odorless, tasteless, colorless, and inexpensive and have good solvent properties. No single penetration enhancer can possess all the required properties. However, many enhancers exhibit many of these attributes, and they have been tested in clinics or in research laboratories. Several scientists are engaged in transdermal permeation studies using various enhancers for several drug moieties. The present review includes the classification of permeation enhancers and their mechanism of action; thus, it will help in the selection of a suitable enhancer(s) for improving the transdermal permeation of poorly absorbed drugs.