In vivo evaluation of the protective capacity of sunscreen by monitoring urocanic acid isomer in the stratum corneum using Raman spectroscopy.

Background/purpose: Urocanic acid (UCA) is a major ultraviolet (UV) ray-absorbing component in the epidermis, and it isomerizes from the trans to the cis form upon exposure to UV radiation. Continuous measurement of UCA isomers in the skin at the same area is not available using conventional methods. This study aimed to evaluate the protective capacity of sunscreen by non-invasively monitoring the trans-UCA (t-UCA) amount in the stratum corneum (SC) by confocal Raman spectroscopy.

Methods: In vivo Raman spectra of the skin at the cheek or volar forearm were obtained from 27 healthy Japanese volunteers of different ages (age range, 22-53 years) throughout a whole year using confocal Raman spectroscopy. Eighteen healthy male Japanese volunteers (age range, 25-52 years) were enrolled for the evaluation of the protective capacity of sunscreen. The concentration depth profile of t-UCA relative to keratin was calculated from the Raman spectra in the 400-2200 cm(-1) region. Then, the integrated amount within the depth of 0-12 mum was calculated, which represented the total amount of t-UCA in the SC.

Results: The integrated amount of t-UCA in the cheek skin was significantly lower than that in the volar forearm skin throughout a year. The amount of it in the volar forearm skin was significantly the lowest in summer, but not in the cheek skin. The amount of t-UCA decreased immediately after UV exposure even below 1 minimal erythema dose, remained low for 1 week, and gradually increased up to the initial level about 2 weeks after UV exposure. The decrease in the t-UCA amount was hindered by the application of sunscreen on the skin surface. There were no statistical differences in response to UV exposure between the erythema-positive and erythema-negative groups.

Conclusions: The monitoring of the amount of t-UCA in the SC by confocal Raman spectroscopy is a good method to assess the efficacy of sun protective substances.