Stratum corneum biomarkers after in vivo repeated exposure to sub‐erythemal dosages of ultraviolet radiation in unprotected and sunscreen (SPF 50+) protected skin

Background There is a need for non‐invasive biomarkers to assess in vivo efficacy of protective measures aiming at reducing ultraviolet radiation (UVR) exposure. Stratum corneum (SC) biomarkers showed to be promising markers for internal UVR dose and immune response. Purpose To establish a dose‐response relationship for SC biomarkers and explore their suitability for in vivo assessment of the blocking effect of two sunscreens with a high sun protection factor (SPF) (50+). Methods Twelve volunteers were exposed to a broad‐spectrum UVB (280‐320 nm), five times a week, during one week. Unprotected back skin was irradiated with 0.24, 0.48, 0.72 and 1.44 standard erythema dose (SED) and sunscreen‐protected skin with 3.6 SED. SC samples for determination of the relative amount of cis‐urocanic acid (cUCA) and thirteen immunological makers including cytokines and matrix metalloproteinases (MMP) were collected after each irradiation. Results cUCA sharply increased after the first irradiation in a dose‐dependent fashion. However, it levelled‐off after subsequent exposures and reached a plateau for the highest UV‐dose after the third irradiation. None of the immunological markers showed dose‐dependency. However, MMP‐9, IL‐1β and CCL27 increased gradually from baseline during repetitive exposures to the highest UV‐dose. Assessed from cUCA, both sunscreens blocked >98% of the applied UV‐dose. Conclusions cUCA is a sensitive, non‐invasive marker of the internal UVR dose enabling in vivo assessment of the blocking effect of high SPF sunscreens in the UVB‐region. Immunological SC markers show low sensitivity in detecting immune response at sub‐erythemal UVR dosages, suggesting they might be suitable only at higher and/or repetitive UVR exposure.