Addressing technical challenges associated with the FDA's proposed rules for the UVA in vitro testing procedure

The proposed rules of the Food and Drug Administration (FDA) for the UVA in vitro testing procedure were applied to the evaluation of various sunscreen formulations and the following technical challenges were observed: when proposed roughened quartz substrates were used, the required coefficient-of-variation criteria were not met, and the dynamic ranges of the available transmittance analyzers were exceeded for sunscreens with high SPF values. In the proposed rules, the FDA requested comment regarding the suitability of other possible substrates. In this research, two modifications to the FDA's proposed rules were evaluated: (a) the use of an alternative substrate, Vitro Skin N-19 (IMS, Inc.) instead of roughened quartz substrate and (b) an increase in application time from 10 seconds to 30 seconds to ensure a uniform distribution of sunscreen product over the application area of the substrate. These two modifications allowed meeting the required coefficient-of-variation criteria without exceeding the dynamic ranges of the available transmittance analyzers. The modified test conditions were utilized for the evaluation of six commercial sunscreens, which fulfilled criteria of "medium" or "high" categories-based on their UVAI/UV ratios. These findings were in agreement with the statement in the proposed rules that the FDA is aware of the difficulty for current sunscreen formulations to meet the "highest" category and believes that allowing such a category will foster additional research and development in this area. To determine if it was possible to achieve a UVA rating greater than 0.95, two experimental sunscreen prototypes with bisoctrizole (USAN), bemotrizinol (USAN), avobenzone, and octocrylene were tested under the modified test conditions and attained the "highest" category. It should be noted that bisoctrizole and bemotrizinol are being evaluated by the FDA under TEA and are not permitted in the US at this time, but they are approved for use in the rest of the world.