Adaptive optics stimulated Raman scattering microscopy for topical product pharmacokinetic imaging

Topically applied drug products are used to treat various dermatological conditions, enabling local and targeted delivery of active pharmaceutical ingredients. To ensure appropriate topical drug efficacy and delivery, it is necessary to understand the pharmacokinetics of topical drug uptake. Stimulated Raman scattering (SRS) microscopy, a chemically specific imaging modality, is well-suited to quantitatively measure the spatiotemporal kinetics of topical drug permeation in skin. However, SRS imaging depth in skin is often limited due to skin heterogeneity and scattering effects, distorting the laser focus and reducing the SRS signal. Adaptive optics (AO) can help overcome and correct depth limitations by adjusting wavefront shape and improving laser focus with a programmable optical element, such as a deformable membrane mirror. Previous work has demonstrated the use of AO in optimizing coherent anti-Stokes Raman scattering (CARS) signal in tissue imaging. In this study, we evaluated the ability of AO to improve the SRS signal in tissue imaging using a deformable membrane mirror as the AO element, which utilized a simulated annealing algorithm to search for mirror shapes producing optimal SRS signal at different tissue imaging depths. AO SRS images had an 11% increase in SRS signal intensity and >350% increase in image contrast between high and low signal regions compared to non-AO SRS images in nude mouse ear skin at the same depth. Mirror shapes optimized at greater imaging depths produced increased SRS intensity and contrast for a range of imaging depths (up to 290 <i>μ</i>m in chicken muscle). Using AO SRS, we performed time-lapse imaging of ruxolitinib uptake in <i>ex vivo</i> human skin, demonstrating that AO SRS can be used to quantitatively measure topical drug pharmacokinetics in deeper skin structures (>100 <i>μ</i>m).