The influence of pre-irradiation on the predictability of in vivo UVA protection with a new in vitro method

Background/Purpose: UVA protection of sunscreen formulations is becoming increasingly important especially because of recent investigations on the long-term skin damage associated with UVA light. The development of a new in vitro method to measure UVA protection performance made it possible to predict reliably the in vivo UVA protection performance of representative sunscreen formulations found presently in the European and US market (1). This study was performed in order to determine the applicability of the method developed by Wendel et al. (1) to photostable and photolabile filter combinations and in order to measure the influence of sample pre-irradiation on predicting the in vivo performance. This was done by subjecting six photostable and six photolabile filter combinations to a standard irradiation. Then the in vitro UVA protection afforded by each combination was measured and compared with the persistent pigment darkening (PPD) values determined in vivo . Results: The results clearly showed that pre-irradiation does not affect the in vitro PPD factor of the photostable and photolabile samples in the same way. Almost identical values were determined for the stable filter combinations with and without pre-irradiation, whereas distinct reductions in the in vitro factors by as much as 93% were observed after irradiation in the group of less stable filter combinations. Comparison of the in vivo and in vitro PPD factors showed that all 12 samples comprise a homogeneous distribution with identical factors before irradiation. After pre-irradiation only the factors for the six less stable products were selectively reduced. The correlation with the data determined on the skin was clearly poorer for these products after irradiation. Conclusion: Overall, the results showed that pre-irradiation should not to be used for the assessment of UVA protection using this method. Furthermore, it can be assumed that normalizing the in vitro absorbance curves to the labelled SPF of the sunscreen will adequately take into account the photochemical behaviour of UV filters on the skin during sun exposure.