Topical delivery of active principles: the field of dermatological research

To be effective an active drug or principle must cross the stratum corneum barrier; this process can be influenced to obtain better functional and therapeutical effects. In spite of the wide variety of the methods studied in order to improve the transdermal transfer to obtain systemic effects, the applicability is limited in this field. Attention to the epidermal barrier and penetration of active principles has been reported mostly in studies concerning dermocosmetics. Studies regarding methods of penetration are gaining experimental and clinical interest. Cutaneous bioavailability of most commercially available dermatological formulations is low. Increase of intradermal delivery can relate to chemical, biochemical, or physical manipulations. Chemical enhancers have been adopted to: (a) increase the diffusibility of the substance across the barrier; (b) increase product solubility in the vehicle; (c) improve the partition coefficient. Moreover methods of interference with the biosynthesis of some lipids allow the modification of the structure of the barrier to increase the penetration. The main physical techniques that increase cutaneous penetration of substances are: iontophoresis (that increases the penetration of ionized substances), electroporation (that electrically induces penetration through the barrier), and sonophoresis, based on 20 to 25 KHz ultrasound that induces alterations of the horny barrier, allowing penetration of active principles. Recent development of these methods are here reported and underline the importance and role of vehicles and other factors that determine effects of partition and diffusion, crucial to absorption.