The role of excipients in promoting topical and transdermal delivery: Current limitations and future perspectives

Topical and transdermal delivery has historically offered an attractive and non-invasive route for administration of medicines. However, human skin is known to be a remarkably good barrier to the permeation of substances. The majority of dermatological drug products have been reported to only deliver a portion of the total dose applied, often resulting in low drug bio-availability at the site of action inside the skin. This insufficient formulation performance, coupled with the fact that percutaneous delivery is heavily influenced by the innate physicochemical properties of the active, pose limitations on effective treatment and prevention of diseases by using solely topical formulations. Generally, it is known that the rate and the extent of drug delivery to and through the skin is highly dependent on the formulation components. This work highlights the importance of the vehicle for the design of efficacious skin products, discusses current limitations in dermal delivery and explores recent advances for overcoming these challenges. Novel materials with penetration enhancing properties and innovative formulation strategies are also explored, together with future perspectives and outlooks. The emphasis here is on studies focused on passive skin transport because of clinical limitations associated with disrupting the skin barrier by physical methods. This information is believed to aid in the design and optimization of dermatological drug products for topical and transdermal delivery of actives.