Evaluating the Pharmacokinetics of Topically Applied Small Molecule Drugs in Skin using Stimulated Raman Scattering Microscopy

Understanding the pharmacokinetics of therapeutic compounds is crucial to the development of safe and effective therapeutics, including topical compounds, which are applied to the skin and other exterior sites of the body. Topical drugs are used to treat numerous dermatological conditions, ranging from cosmetic concerns to autoimmune conditions. Topical drug efficacy depends on topical drug pharmacokinetics in the skin, but cutaneous pharmacokinetics can be especially challenging to evaluate due to the barrier function and complex structure of the skin. Because of this, several aspects of cutaneous pharmacokinetics are not well understood, such as drug uptake to specific skin regions over time. While a variety of methods can be used to measure or visualize topical drug uptake in skin, many of these lack sufficient spatiotemporal resolution to evaluate cutaneous pharmacokinetics over time, especially in specific skin regions. Previous works have shown the value of using stimulated Raman scattering (SRS) microscopy as a tool to study topical drug uptake, since SRS imaging is rapid, chemically-specific, non-invasive, and SRS signal is proportional to drug concentration. This work describes the development of SRS imaging methods to evaluate topical drug pharmacokinetics, helping reveal previously poorly understood aspects of topical drug uptake and cutaneous pharmacokinetics, such as the effects of perfusion on cutaneous pharmacokinetics, how pharmacokinetics in the stratum corneum relate to pharmacokinetics in deeper skin layers, and the temporal dynamics of topical drug delivery to the sebaceous glands. Measuring uptake of tazarotene with SRS in in vivo and ex vivo mouse ears in a paired experiment revealed that differences in pharmacokinetics were observed in the presence of perfusion. Using adaptive optics to correct for wavefront aberrations and overcome SRS imaging depth limitations allowed the capture of ruxolitinib uptake data in the stratum corneum, viable epidermis, and dermis. Finally, developing an SRS imaging method to detect drug uptake in sebaceous glands and performing corrections to account for light attenuation variations and lens effects enabled the measurement of tazarotene uptake in sebaceous glands, representing a novel approach of assessing topical drug delivery over time to structures located deep in the skin. The methods developed here present promising avenues for future studies and further illumination of cutaneous pharmacokinetics and topical drug uptake.