Method to quantify the in vivo skin penetration of topically applied materials based on confocal Raman spectroscopy

Abstract

This article describes a unique noninvasive capability to determine the concentration (in mg/cm3) and total amount of topically applied materials in the skin (in μg/cm2 of skin surface). It is based on in vivo confocal Raman spectroscopy. A theoretical derivation is given of a general method to calculate a concentration ratio from a Raman spectrum of a material in a medium, which can be a solvent or other matrix, such as the skin. A practical implementation of the method is then presented along with a clarification of the assumptions used and applied to a quantitative analysis of the in vivo skin penetration of trans‐retinol and propylene glycol (PG). A comparison was made between the concentrations profiles of retinol and PG found in the skin and the concentrations of retinol and PG that had been applied to the skin. Determination of the amount of these materials in the skin at different timepoints after topical application also enabled a straightforward calculation of the flux of materials into the skin (in μg cm−2 h).

Franz Cell Diffusion Testing and Quantitative Confocal Raman Spectroscopy: In Vitro-In Vivo Correlation

“…These wavelengths were used to record spectra in the high wavenumber (HWN) (2500–4000 cm −1 ) and the fingerprint (FP) (400–1800 cm −1 ) region, respectively. On the day of the experiment, a National Institute of Standards and Technology (NIST) glass calibration standard was positioned over the microscope objective, and the instrument was calibrated as described elsewhere [ 24 ]. The Raman spectra of human skin were taken in both the FP and the HWN region.…”

“…The lack of quantifiable concentration profiles of actives has thus limited the use of CRS to date in studying the delivery of molecules to the skin in vivo [ 17 ]. Recently, Caspers et al [ 24 ] proposed a method to quantify the permeation of compounds following CRS analysis. These researchers demonstrated a mathematical procedure for the calculation of the mass ratio of the permeant (mg) per protein (g) based on the corresponding Raman signal intensity ratio.…”

Blueprint for mechanistic, data‐rich early phase clinical pharmacology studies in dermatology

“…The latter triggers more attention since matrix‐assisted laser desorption ionization‐time‐of‐flight (MALDI‐TOF) imaging techniques have become more established, enabling profiling of quantitative skin distribution 8 . An interesting alternative for dermal PK assessments is the noninvasive confocal Raman spectroscopy whereby the first validation results hold promise to wider application and quantification in vivo 9,10 . We should also note the rapidly expanding field of minimally invasive techniques for systemic PK profiling, including the only recently reported dry blood spot analysis for biologics (e.g., infliximab and adalimumab) 11,12 .…”

A New Method for In-Situ Skin Penetration Analysis by Confocal Raman Microscopy

“…As the SC is about 15–20 µm thick, which is comparable to the 21.6 µm PET film that was measured, an error of <1 µm across the whole thickness of the SC is deemed acceptable. This shows, that the setup is able to obtain valid depth profiles with a fairly high special resolution, compared to the 5 µm resolution of other instruments [ 13 ]. Figure 3 shows the results of in-situ measurements of caffeine penetration during 6 h of incubation with a 2% solution.…”

“…For measuring deeper penetrations, spatially offset Raman spectroscopy (SORS) is described as an effective method to obtain stronger signals in even deeper sample layers, than in this study [ 11 , 12 ]. Direct drug quantification is possible because of the linear correlation of molecule concentration and Raman scattering [ 13 ]. So far, CRM has been used in many studies to acquire penetration profiles of several drugs in vivo and in vitro [ 14 , 15 , 16 , 17 ].…”