Cellular level investigation of skin permeation using stimulated Raman scattering microscopy and in-silico modelling

This study presents a novel approach combining stimulated Raman scattering (SRS) microscopy with mechanistic modelling to quantify the disposition and permeation of topically applied compounds in skin. The dermal delivery of 4-cyanophenol and caffeine was investigated using SRS microscopy, generating high-resolution images resolving their distribution within the stratum corneum (SC) lipid matrix and corneocytes. The imaging data revealed distinct permeation pathways, with 4-cyanophenol preferentially localising within the intercellular lipid matrix and caffeine within the corneocyte-rich regions. Depth-resolved SRS imaging provided concentration profiles across the SC, showing a monotonic decrease in 4-cyanophenol concentration, whereas caffeine exhibited depth-dependent transport behaviour. These experimental concentration profiles were modelled using a two-dimensional mechanistic model with calibration to reveal the diffusion, partition and deposition behaviour within the lipid and corneocyte domains. For 4-cyanophenol, the optimised permeability was consistent with literature-reported values and baseline model predictions, while for caffeine, optimisation substantially improved permeability predictions. Simulations further captured formulation-dependent effects predicting 1.3-fold and 1.5-fold increases in SC uptake of 4-cyanophenol and caffeine, respectively, in propylene glycol relative to aqueous vehicles. Overall, this work demonstrates the potential of combining SRS microscopy with inverse mechanistic modelling to derive transport parameters directly from skin imaging data and reveal dermal drug disposition and permeation. To our knowledge, this represents the first application of SRS-derived depth profiles to inform mechanistic modelling of skin permeation.