HDRS - Hybrid Diffuse Reflectance Spectroscopy: Non-Erythemal In Vivo Driven SPF and UVA-PF Testing.

Background/aims: In order to define a label SPF of topically applied sunscreens, in vivo test methods like ISO 24444, FDA Guideline, and the Australian Standard are used worldwide. The basis of all these methods is to induce an erythemal skin reaction by UV irradiation to find the level of MEDu and MEDp (Minimal Erythmal Dose unprotected and protected). In vitro methods replacing the human skin by any kind of nonhuman material are still not available. Thus, offering the new hybrid diffuse reflectance spectroscopy (HDRS) technique that can maintain an in vivo level for SPF testing while neglecting the UV-dose-related erythemal skin reaction is a perfect combination to take care of sun protection and any ethical concerns in SPF testing nowadays.

Methods: HDRS is a combination of in vivo diffuse reflectance spectroscopy measurements on the skin and in vitro transmission measurements of a sunscreen on a roughened polymethylmethacrylate plate. By this technique, the in vivo behavior of the investigated sunscreen on the skin is measured as well as the UVB absorption, which is still nonvisible in the reflectance technique. In order to establish an alternative method for in vivo SPF and UVA-PF testing, a huge number of sunscreens (250 samples) were measured by HDRS and compared with the worldwide accepted standards ISO 24444, ISO 24442, and ISO 24443. The variety of sunscreens measured reflect a wide range of different types of formulations as well as a wide range of SPFs (5-120) to validate this new alternative SPF testing procedure.

Results: Far-reaching statistical data analyses show an excellent link between the new nonerythemal-driven HDRS-SPF technique and ISO 24444 results. In the same way, HDRS-UVA-PF results can be correlated with UVA-PF values calculated from ISO 24442 as well as from ISO 24443. The importance of the inclusion of a spectral ratio of photodegradation is shown in the comparison of photostable and photounstable products.

Conclusion: Owing to the elimination of any erythemal-relevant UVB and UVA doses, absolutely no skin reaction occurs during the HDRS experiment. Consequently, there is no need to define an MED anymore. For the first time, an alternative way to arriving at SPF and UVA-PF values is shown, without any ethical concerns of SPF testing in vivo and/or any restriction of SPF testing in vitro. Regardless of the type of formulation or the level of protection, an excellent correlation between SPFHDRSand SPF24444as for sunscreen labeling could be found. By this new alternative nonerythemal technique, not only SPF values can be measured but also UVA-PF values can be calculated with a linear correlation to ISO 24442 as well as to ISO 24443 from the same set of data. By this a robust alternative test method of SPF and UVA-PF values is described, taking into account the interaction of sunscreen formulation and skin.