Abstract
The greatest hindrance for transdermal drug delivery (TDD) is the barrier property of skin, especially the stratum corneum (SC). Various methodologies have been investigated and developed to enhance the penetration of drugs through the skin. Among them, the most popular approach is the application of penetration enhancers (PEs), including natural terpenes, a very safe and effective class of PEs. In the present paper, we focused on terpenes as skin PEs for TDD. The mechanism of their action, the factors affecting their penetration enhancement effect, as well as their possible skin toxicity were discussed. Terpenes abundant in nature have great potential in the development of PEs. Compared to synthetic PEs, natural terpenes have been proved to possess higher enhancement activity. Interaction with SC intercellular lipids is the main mechanism of action for terpenes. The key factor affecting the enhancement effect is the lipophilicity of both terpenes and drug molecules. In addition, a lot of terpenes have also been proved to be much less toxic compared to azone, the classic synthetic PE. In summary, terpenes may be preferred over the chemically synthesized compounds as safe and effective PEs to promote the percutaneous absorption of drugs.
Highlights
Transdermal drug delivery (TDD) has become a viable alternative to conventional routes of drug administration since it can avoid the hepatic first pass effect, improve the compliance of patients, decrease the administration frequency, and reduce the gastrointestinal side effects
This review aims to give an overview of terpenes as penetration enhancers (PEs) for use in TDD, which will be helpful to researchers working on transdermal drug delivery systems (TDDS) in the selection of a suitable terpene
The results indicated that menthol and menthone could slightly interact with the lipophilic tails of skin lipids, which could contribute to the transdermal absorption of lipophilic drugs
Summary
Transdermal drug delivery (TDD) has become a viable alternative to conventional routes of drug administration since it can avoid the hepatic first pass effect, improve the compliance of patients, decrease the administration frequency, and reduce the gastrointestinal side effects. There are mainly three possible routes for percutaneous penetration of drug molecules, which include intracellular diffusion across the SC corneocytes, permeation through the SC intercellular lipid spaces, and penetration through skin appendages [6]. The basic chemical structure of terpenes consists of a number of repeated isoprene (C5 H8 ) units which are used to classify terpenes They are generally regarded to be safer compared to synthetic Pes which include surfactants, fatty acids/esters, and solvents [9]. This review aims to give an overview of terpenes as PEs for use in TDD, which will be helpful to researchers working on TDDS in the selection of a suitable terpene
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