In vitro assessment of photostability and apparent UV protection enhancement in a multi-component organic UV filter system with synthetic and natural boosters.

Ultraviolet (UV) radiation is a major environmental factor contributing to skin damage, including erythema, oxidative stress, photoaging, and DNA damage. Organic UV filters are widely used in sunscreen formulations; however, their photoinstability remains a key limitation. This study aimed to evaluate the effect of combining synthetic and natural UV boosters on the photostability, apparent UV protection performance, and antioxidant activity of a multi-component organic UV filter system under in vitro conditions. Four formulations were prepared, including a booster-free control, synthetic-only, natural-only, and a dual-booster system. In vitro assessments were conducted on PMMA substrates under controlled UV irradiation. Photostability, spectral absorbance, in vitro UV protection metrics (including SPF and UVA protection factor, UVA-PF), and reactive oxygen species (ROS) suppression were evaluated. The dual-booster formulation demonstrated superior performance across all measured parameters, including enhanced photostability (UVA absorbance retention ~ 89%), improved durability of UV protection metrics (SPF retention ~ 88% and UVA-PF retention ~ 90%), and significant ROS suppression (~ 58%). Compared to the control, the incorporation of boosters resulted in a marked relative increase in apparent UV protection performance, along with improved spectral balance and measurement reproducibility. It is important to note that all SPF-related values were determined under in vitro conditions using PMMA substrates, which are known to overestimate absolute SPF compared to in vivo measurements. Therefore, these results should be interpreted as a comparative evaluation of formulation performance rather than a direct prediction of clinical SPF. Overall, the combined use of synthetic film-forming agents and natural antioxidants provides a synergistic strategy to improve photostability and functional performance of organic UV filter systems.