From the Cover: Molecular diffusion in the human nail measured by stimulated Raman scattering microscopy

The effective treatment of diseases of the nail remains an important unmet medical need, primarily because of poor drug delivery. To address this challenge, the diffusion, in real time, of topically applied chemicals into the human nail has been visualized and characterized using stimulated Raman scattering (SRS) microscopy. Deuterated water (D₂O), propylene glycol (PG-d₈), and dimethyl sulphoxide (DMSO-d₆) were separately applied to the dorsal surface of human nail samples. SRS microscopy was used to image D₂O, PG-d₈/DMSO-d₆, and the nail through the O-D, -CD₂, and -CH₂ bond stretching Raman signals, respectively. Signal intensities obtained were measured as functions of time and of depth into the nail. It was observed that the diffusion of D₂O was more than an order of magnitude faster than that of PG-d₈ and DMSO-d₆. Normalization of the Raman signals, to correct in part for scattering and absorption, permitted semiquantitative analysis of the permeation profiles and strongly suggested that solvent diffusion diverged from classical behavior and that derived diffusivities may be concentration dependent. It appeared that the uptake of solvent progressively undermined the integrity of the nail. This previously unreported application of SRS has permitted, therefore, direct visualization and semiquantitation of solvent penetration into the human nail. The kinetics of uptake of the three chemicals studied demonstrated that each altered its own diffusion in the nail in an apparently concentration-dependent fashion. The scale of the unexpected behavior observed may prove beneficial in the design and optimization of drug formulations to treat recalcitrant nail disease.The effective treatment of nail disease requires efficient drug delivery into and through the barrier. However, the tightly woven keratin network of the nail plate means that poor drug uptake following topical administration is common. Despite considerable effort to improve formulations and to enhance drug delivery to the nail, progress has been slow at best. In general, the approaches adopted have failed to elucidate the complex interplay between drug, formulation components (including solvents), and the nail. For example, although it is quite clear that drug uptake from typical “lacquer” formulations (comprising the active, a film-forming polymer, and a volatile organic solvent) is intimately linked to the disposition of the solvent and effectively stops once the solvent has gone, there has been little effort to characterize the transport of these key vehicle components into and across the nail. Only the diffusion of water has received attention, its overall time-dependent uptake having been measured by various techniques (1–3); otherwise, apart from some information on the concentration-depth profiles of water and dimethyl sulphoxide (DMSO) in the very superficial, outermost 20 μm of the nail, there are essentially no time- and position-dependent data on the movement of chemicals into the nail.Stimulated Raman scattering (SRS) microscopy is a label-free imaging technique that offers a solution to this challenge. This method has been applied in a range of biomedical and pharmaceutical studies involving, for example, visualization in living cells (4), characterization of cortical vasculature morphology (5), imaging the constituents of solid, oral dosage forms (6), and tracking the pharmacokinetics of drugs and excipients in mammalian skin (7–9). In this paper, the first application to our knowledge of SRS microscopy to trace and visualize the diffusion of three pharmaceutically relevant solvents, water, propylene glycol (PG), and DMSO, as a function of depth and in real time in human nail is presented. The use of deuterated solvents provides unique Raman-active molecular vibrations that are easily distinguished spectroscopically from those originating in the nail, resulting in excellent, and label-free, image contrast. Because of the linear relationship between the SRS signal and the concentration of the chemical, the spectroscopic signature of which is being monitored, a semiquantitative analysis of solvent diffusion across the nail is possible and offers heretofore-unknown insight into the transport process.