ABSTRACT
Topically applied products may target sites in one or more different skin layers, the skin appendages, and underlying tissues. Identifying the specific target region and formulating to deliver the therapeutic compound to that target is a key objective in topical product design. The skin surface is the target for cosmetics, sunscreens, and insect repellents, so the aim is to target actives to the skin surface with minimal absorption to deeper skin layers. The key target region for the majority of topically applied products is the viable epidermis and dermis, and sites targeted include nerves, keratinocytes, melanocytes, Langerhans cells, and hair follicles. Deeper tissues associated with muscles and joints may also be targeted, such as topical products for musculoskeletal pain and inflammation. Transdermal products target the systemic circulation. This chapter focuses on the rationale, mechanism, and application of the formulation approaches to topical and transdermal drug delivery.
