Routes of exposure: Topical and transdermal

The dermal route of administration poses a number of scientific, technical, and regulatory challenges to those charged with characterizing the safety of new excipients for topical or transdermal use. Many of these challenges will be discussed in this chapter. The goal of safety testing is the protection of patients from potential harm caused by a new excipient that will be applied to the skin. This chapter highlights scientific, technical, and some regulatory nuances specific to the dermal route of administration for the safety assessment of new pharmaceutical excipients. Reliance on dermal drug-testing strategies, which are not always straightforward, may result in a scope of testing that exceeds that necessary for an excipient. The most difficult aspects of developing a preclinical (nonclinical) toxicology strategy for a new pharmaceutical excipient are (a) to determine if certain testing is necessary or relevant, and (b) to successfully use the results of the toxicology studies to help direct clinical study designs. Recently, the Safety Committee of the International Pharmaceutical Excipients Council (IPEC) published proposed guidelines for the safety testing of new pharmaceutical excipients (1).