Developing a method for the study of perfusion effects in topical product pharmacokinetics.

Topical drug products are delivered to skin structures to treat numerous skin diseases. Due to the complexities of the skin environment and barrier, topical drug pharmacokinetics are difficult to determine, especially in vivo, as most pharmacokinetic assessment methods can only be performed ex vivo. Notably, in vivo conditions include perfusion via dermal capillaries, which influences topical drug uptake by acting as a clearance route and a "sink" driving permeation through the skin. In this study, we develop a method to examine the effects of perfusion on topical drug uptake in vivo using stimulated Raman scattering (SRS) microscopy, a chemically-specific imaging modality ideal for visualizing topical drug permeation over time. In this pilot study, we imaged the in vivo and ex vivo uptake of tazarotene in 70/30 v/v Transcutol:EtOH in paired mouse ear skin, comparing the effects of perfusion on tazarotene concentration (linearly proportional to SRS signal intensity) over time and pharmacokinetic parameters (Tmax, Cmax, AUC) in lipid-rich and lipid-poor regions in the stratum corneum and sebaceous glands. Obvious variations in SRS signal-time trends and statistically significant differences in stratum corneum pharmacokinetic parameters comparing uptake in lipid-rich and lipid-poor regions in vivo and ex vivo indicated slowed tazarotene flux through ex vivo skin in the absence of perfusion. The observed permeation differences in lipid-rich and lipid-poor regions in perfused and non-perfused skin reflects increased tazarotene permeation rate and removal in the presence of perfusion (in vivo) and decreased permeation rate and lack of elimination route in the absence of perfusion (ex vivo). Our method is demonstrated to be effective in assessing in vivo perfusion effects on topical drug uptake, promoting a better understanding of the influence of perfusion on topical drug delivery.