Navigating transdermal diffusion with multiphoton fluorescence lifetime imaging

We demonstrate the potential of fluorescence lifetime imaging by time-correlated single-photon counting as a method for monitoring the transdermal diffusion pathway and diffusion rate of pharmaceuticals in human skin. The current application relies on observing subtle changes in the fluorescence lifetime of the intrinsic fluorophores present in the intracellular region between corneocytes of the stratum corneum. We have comprehensively characterized the measured fluorescence lifetimes from intracorneocyte junctions in three skin section types (dermatomed skin, epidermal membranes and stratum corneum) revealing statistically significant differences of the short lifetime component between each of the types, which we attribute to the sample preparation and imaging method. We show using epidermal membrane sections that application of a drug/solvent formulation consisting of ethinyl estradiol and spectroscopic grade ethanol to the surface gives rise to a slight but statistically significant shortening of the fluorescence lifetime of the long-lived emitting species present in the sample, from approximately 2.8 ns to 2.5 ns. The method may be useful for future studies where the kinetics and pathways of a variety of applied formulations could be investigated.