The purpose of this funding opportunity is to support the research and development necessary to advance non-invasive (e.g., quantitative tomography-based) technologies, including the development of apparatus, methods, study designs, and methods of data analysis, to characterize and compare the rate and extent to which a topically applied drug becomes available at or near a site of action within the skin in vivo. The expectation is that the funded work will produce an accurate, sensitive and reproducible approach that rapidly measures the (relative) amount of drug present in the skin at a series of depths below the skin surface, which can be utilized to monitor the cutaneous pharmacokinetics (PK) of the drug at selected depths (e.g., in the epidermis) by repeated, serial measurements over time. The intent is to support the eventual development of an alternative, scientifically valid, in vivo cutaneous PK-based approach that can be used to efficiently demonstrate the bioequivalence (BE) of topical products.