Prodrug strategy for enhancing drug delivery via skin

Skin as a route for drug delivery has been extensively investigated. However, because of the predominant barrier function of stratum corneum in skin, the clinical application is limited. One strategy to solve this problem of drug permeation via skin is the use of prodrugs. Prodrugs are inactive compounds which are metabolized either chemically or enzymatically in a controlled or predictable manner to its parent active drug. Prodrugs can enhance dermal/transdermal drug delivery via different mechanisms, including increased skin partitioning, increased aqueous solubility, and reduced crystallization, etc. Besides the prodrug itself, the optimization of vehicle is important as well. The prodrug partitioning between skin and vehicle as well as prodrug-vehicle interaction may influence the enhancing efficacy on skin permeation. This review explores the synthesis and enhancing mechanisms of prodrugs for topical drug delivery. The prodrugs categorized by the therapeutic use of the parent drugs, including anticancer drugs, analgesics, anti-inflammatory drugs and vitamins, are systemically introduced in this review.