Validation of a new in vitro Sun Protection Factor method to include a wide range of sunscreen product emulsion types

In 2017, Cosmetics Europe performed a double-blinded ring test of 24 emulsion-type sunscreen products, across 3 in vivo test laboratories and 3 in vitro test laboratories, using a new candidate in vitro SPF test method. Based on the results of this work, an article was published showing how data derived from a new lead candidate method conform to new International Standards (ISO) acceptance criteria for alternative SPF test methods (Any alternative method should consider the matrix effect and if required, specify the matrix applicability of the method; Criterion 1a: Systematic differences between methods should be negligible: 95% of all individual results of an alternative method are within the range of ±2× reproducibility standard deviation of the in vivo method, that is overall bias must be below 0.5× reproducibility standard deviation of the in vivo method; Criterion 1b: Measurement uncertainty of an alternative method should be below the measurement uncertainty of the in vivo method. Candidate method predicted values must fall within the full ‘funnel’ (SPF 6-50+) limits proposed by Cosmetics Europe (derived from the same minimum test design, that is using the ISO24444 Method to measure at least 24 products across at least 3 laboratories using at least 5 test subjects/laboratory, in a blinded fashion).). Of the 24 sunscreen products tested, the majority of emulsions were of the oil-in-water (O/W) type, whereas only one was water-in-oil (W/O) and there were no products with a mineral-only sun filter system. In order to confirm the scope of this method, therefore, a new study was conducted that included 73 W/O (12 mineral + organic, 44 mineral only and 17 organic only) and 3 O/W mineral-only, emulsion-type sunscreen products (a total of 76 new sunscreen products). When combined with the previous 24 products (tested in 3 different laboratories), this yielded a new data set comprising a total of 100 emulsion-type sunscreen products, with SPF values ranging from 6 to 50+ (with a total of 148 data points). These products were tested using the double-plate in vitro SPF test method and compared with the ISO TC217/WG7 acceptance criteria for alternative SPF test methods. Over 95% of paired in vitro: in vivo SPF values lay within the upper and lower limits of the ISO acceptance criteria funnel, with no bias. This new in vitro SPF test method, therefore, meets the minimum requirements for an alternative SPF test method to ISO24444:2010, for emulsion-type sunscreen products (which make up the majority of marketed sunscreen products).