RESUMEN
Dermatologic diseases can present in varying forms and severity, ranging from the individual lesion and up to almost total skin involvement. Pharmacokinetic assessment of topical drug products has previously been plagued by bioanalytical assay limitations and the lack of a standardized study design. Since the mid-1990's the US Food and Drug Administration has developed and implemented a pharmacokinetic maximal usage trial (MUsT) design to help address these issues. The MUsT design takes into account the following elements: the enrollment of patients rather than normal volunteers, the frequency of dosing, duration of dosing, use of highest proposed strength, total involved surface area to be treated at one time, amount applied per square centimeter, application method and site preparation, product formulation, and use of a sensitive bioanalytical method that has been properly validated. This paper provides a perspective of pre-MUsT study designs and a discussion of the individual elements that make up a MUsT.